Dissertations / Theses on the topic 'Silicon quantum dots'

Under excitation by visible light, alkylated-silicon quantum dots emit an orange- coloured luminescence, peaking at around 650 nm. Following continuous illumination, a decay of the luminescence over a time-scale of 5-10 minutes was monitored concur- rently with a photo current generated by ejection of electrons from the dots. The photoluminescence and current both decayed to non-zero, steady-state values during irradiation by visible laser light at incident intensities in the range 0.25-0.3 ± 0.01 kW /cm2; on cessation, the non-conducting photoluminescent state was substantially regained. These observations are consistent with a model in which the decay is as- cribed to autoionization of the alkylated-silicon quantum dots with a mean lifetime {Ta), depending on particle size, and recovery of luminescence to electron-hole re- combination characterized by a mean lifetime {Teh). Values of {Ta) = 1.08 ± 0.03 sand {Teh) = 770 ± 300 s were extracted from nonlinear least squares fitting to the time dependence of the photoluminescence intensity. The temporal behaviour of the transient photocurrent was found to be quantitatively consistent with a one- dimensional model of diffusion of charge carriers between quantum dots. Integration of the time-dependence of the photo current response coupled with an estimate of the volume irradiated by the laser light suggests ionization of one electron per quantum dot during photon irradiation. Measurements of the time-resolved decay of orange-band emission over a time scale of tens of microseconds and of the dependence on applied intensity of luminescence from the quantum dots were performed using pulsed laser sources. The dependence of luminescence on time was found to be strongly non-exponential and was optimally ac- counted for by a probability density function which describes a continuous distribution of two decay times: the temporal behaviour is characteristic of a pair of elementary steps connected with light emission within a distribution of local environments, or a single rate process supported by two environments. Non-linear least-squares fits to the time dependent luminescence formulated on this basis with a Gaussian, Lorentzian or log-normal distribution of rates returned most probable lifetimes T1 = 21 ± 1 μS and T2 = 3.7 ± 0.8 μS. The widths of the distributions vary between σ1 = 0.01-D.03μs-1 and az = 0.14-1.1 μs-1 associated with 1/T1 and 1/T2 respectively. The intensity of luminescence displays a linear power dependence on the intensity of the applied field, from which an exponent n = 0.94±0.02 commensurate with single-photon absorption was derived.

Bulk silicon as an indirect bandgap semiconductor is a poor light emitter. In contrast, silicon nanocrystals (Si NCs) exhibit strong emission even at room temperature, discovered initially at 1990 for porous silicon by Leigh Canham. This can be explained by the indirect to quasi-direct bandgap modification of nano-sized silicon according to the already well-established model of quantum confinement. In the absence of deep understanding of numerous fundamental optical properties of Si NCs, it is essential to study their photoluminescence (PL) characteristics at the single-dot level. This thesis presents new experimental results on various photoluminescence mechanisms in single silicon quantum dots (Si QDs). The visible and near infrared emission of Si NCs are believed to originate from the band-to-band recombination of quantum confined excitons. However, the mechanism of such process is not well understood yet. Through time-resolved PL decay spectroscopy of well-separated single Si QDs, we first quantitatively established that the PL decay character varies from dot-to-dot and the individual lifetime dispersion results in the stretched exponential decays of ensembles. We then explained the possible origin of such variations by studying radiative and non-radiative decay channels in single Si QDs. For this aim the temperature dependence of the PL decay were studied. We further demonstrated a model based on resonance tunneling of the excited carriers to adjacent trap sites in single Si QDs which explains the well-known thermal quenching effect. Despite the long PL lifetime of Si NCs, which limits them for optoelectronics applications, they are ideal candidates for biomedical imaging, diagnostic purposes, and phosphorescence applications, due to the non-toxicity, biocompability and material abundance of silicon. Therefore, measuring quantum efficiency of Si NCs is of great importance, while a consistency in the reported values is still missing. By direct measurements of the optical absorption cross-section for single Si QDs, we estimated a more precise value of internal quantum efficiency (IQE) for single dots in the current study. Moreover, we verified IQE of ligand-passivated Si NCs to be close to 100%, due to the results obtained from spectrally-resolved PL decay studies. Thus, ligand-passivated silicon nanocrystals appear to differ substantially from oxide-encapsulated particles, where any value from 0 % to 100 % could be measured. Therefore, further investigation on passivation parameters is strongly suggested to optimize the efficiency of silicon nanocrystals systems.

QC 201501001

Quantum physics applied to computing is predicted to lead to revolutionary enhancements in computational speed and power. The interest in the implementation of an impurity spin based qubit in silicon for quantum computation is motivated by exceedingly long coherence times of the order of seconds, an advantage of silicon's low spin orbit coupling and its ability to be isotopically enriched to the nuclear spin zero form. In addition, the donor spin in silicon is tunable, its nuclear spin is available to be employed as a quantum memory, and there are major advantages to working with silicon in terms of infrastructure and scalability. In contrast, lithographically patterned artificial atoms called quantum dots have the complementary advantages of fast electrical operations and tunability. Here I present our attempts to develop a scalable quantum computation architecture in silicon, based on a coupled quantum dot and dopant system. I explore industry-compatible as well as industrial foundry-fabricated devices in silicon as hosts for few-electron quantum dots and utilise a high-sensitivity readout and charge sensing technique, gate-based radiofrequency reflectometry, for this purpose. I show few-electron quantum dot measurements in this device architecture, leading to a charge qubit with a novel multi-regime Landau-Zener interferometry signature, with possible applications for readout sensitivity. I also present spin-to-charge conversion measurements of a chalcogen donor atom in silicon. Lastly, I perform measurements on a foundry-fabricated silicon device showing a coupling between a donor atom and a quantum dot. I probe the relevant charge dynamics of the charge qubit, as well as observe Pauli spin blockade in the hybrid spin system, opening up the possibility to operate this coupled double quantum dot as a singlet-triplet qubit or to transfer a coherent spin state between the quantum dot and the donor electron and nucleus.

Au cours des dernières années le silicium est apparu comme un matériau hôte prometteur pour les qubits de spin. Grâce à la microélectronique moderne, la technologie du silicium a connu un formidable développement. Réaliser des qubits utilisant la technologie bien établie de fabrication CMOS de semi-conducteurs favoriserait clairement leur intégration à grande échelle.Dans cette thèse nous présentons les travaux effectués dans une perspective des qubits CMOS. En particulier, nous avons abordé les problèmes de confinement des charges et des spins dans les boîtes quantiques, la manipulation des spins et la lecture des charges et des spins.Nous avons exploré les différentes propriétés de confinement de charge et de spin dans des échantillons de tailles et de géométries différentes. Les MOSFETs de taille extrêmement réduites montrent du blocage de Coulomb jusqu'à température ambiante, avec des énergies de charges jusqu'à 200meV. Les dispositifs multi-grilles avec des dimensions géométriques plus grandes ont été utilisés pour confiner les spins et lire leur état par blocage de spin, en réalisant ainsi une conversion spin / charge.La manipulation des spins est réalisée au moyen d'un dipôle électronique induisant la résonance de spin (EDSR). Les deux plus basses vallées de la bande de conduction du silicium sont visibles sous forme de transitions de spin intra et inter-vallées. Nous observons une levée de dégénérescence de vallée d'amplitude 36μeV. La résonance de spin que l'on observe résulte de la géométrie spécifique de l'échantillon, de la physique des vallées et de l'interaction spin-orbite de type Rashba. Des signatures de manipulation cohérente, sous forme d'oscillations de Rabi, ont été mesurées, avec une fréquence de Rabi de 6MHz. Nous discutons également de la lecture rapide des charges et des spins effectuée par réflectométrie dispersive couplée à la grille. Nous montrons comment l'utiliser pour reconstruire le diagramme de stabilité de charge du dispositif et le signal attendu pour un système à double boîte isolé. La tension de polarisation finie modifie la réponse du système et nous l'avons utilisée pour sonder les états excités et leur dynamique
In the recent years, silicon has emerged as a promising host material for spin qubits. Thanks to its widespread use in modern microelectronics, silicon technology has seen a tremendous development. Realizing qubit devices using well-established complementary metal-oxide-semiconductor (CMOS) fabrication technology would clearly favor their large scale integration.In this thesis we present a detailed study on CMOS devices in a perspective of qubit operability.In particular we tackled the problems of charge and spin confinement in quantum dots, spin manipulation and charge and spin readout.We explored the different charge and spin confinement capabilities of samples with different sizes and geometries. Ultrascaled MOSFETs show Coulomb blockade up to room temperature with charging energies up to 200meV. Multigate devices with larger geometrical dimensions have been used to confine spins and read their states through spin-blockade as a way to perform spin to charge conversion.Spin manipulation is achieved by means of Electron Dipole induced Spin Resonance (EDSR). The two lowest valleys of silicon's conduction band originate as intra and inter-valley spin transitions; we probe a valley splitting of 36μeV. The origin of this spin resonance is explained as an effect of the specific geometry of the sample combined with valley physics and Rashba spin-orbit interaction. Signatures of coherent Rabi oscillations have been measured, with a Rabi frequency of 6MHz. We also discuss fast charge and spin readout performed by dispersive gate-coupled reflectometry. We show how to use it to recover the complete charge stability diagram of the device and the expected signal for an isolated double dot system. Finite bias changes the response of the system and we used it to probe excited states and their dynamics

Silicon quantum dots (QDs) in SiO2 superlattices were fabricated by alternate deposition of silicon oxide (SiO2) and silicon-rich oxide (SRO), i.e. SiOx (x<2), and followed by high temperature annealing. A deposited SRO film is thermodynamically unstable below 1173oC and phase separation and diffusion of Si atoms in the amorphous SiO2 matrix creates nano-scaled Si quantum dots. The quantum-confined energy gap was measured by static photoluminescence (PL) using an Argon ion laser operating at 514.5 nm. The measured energy band gaps of crystalline Si QDs in SiO2 matrix at room temperature (300 K) show that the emission energies from 1.32 eV to 1.65 eV originating Si dot sizes from 6.0 nm to 3.4 nm, respectively. There is a strong blue-shift of the PL energy peak position with decreasing the quantum dot size and this shows the evidence of quantum confinement of our fabricated Si QDs in SiO2 matrix. The PL results indicate that the fabricated Si QDs in SiO2 matrix could be suitable for the device application such as top cell material for all-silicon tandem solar cells. Silicon QD superlattices in nitride matrix were fabricated by alternate deposition of silicon nitride (Si3N4) and silicon-rich nitride (SRN) by PECVD or co-sputtering of Si and Si3N4 targets. High temperature furnace annealing under a nitrogen atmosphere was required to form nano-scaled silicon quantum dots in the nitride matrix. The band gap of silicon QD superlattice in nitride matrix (3.6- 7.0 nm sized dots) is observed in the energy range of 1.35- 1.98 eV. It is about 0.3- 0.4 eV blue-shifted from the band gap of the same sized quantum dots in silicon oxide. It is believed that the increased band gap is caused by a silicon nitride passivation effect. Silicon-rich carbide (SRC, i.e. Si1-xCx) thin films with varying atomic ratio of the Si to C were fabricated by using magnetron co-sputtering from a combined Si and C or SiC targets. Off-stoichiometric Si1-xCx is of interest as a precursor to realize Si QDs in SiC matrix, because it is thermodynamically metastable when the composition fraction is in the range 0 < x < 0.5. Si nanocrystals are therefore able to precipitate during a post-annealing process. SiC quantum dot superlattices in SiC matrix were fabricated by alternate deposition of thin layers of carbon-rich silicon carbide (CRC) and SRC using a layer by layer deposition technique. CRC layers were deposited by reactive co-sputtering of Si and SiC targets with CH4. The PL energy band gap (2.0 eV at 620 nm) from 5.0 nm SRC layers could be from the nanocrystalline ??-SiC with Si-O bonds and the PL energy band gap (1.86 eV at 665 nm) from 6.0 nm SRC layers could be from the nanocrystalline ??-SiC with amorphous SiC clusters, respectively. The dielectric material for an all-silicon tandem cell is preferably silicon oxide, silicon nitride or silicon carbide. It is found that for carrier mobility, dot spacing for a given Bloch mobility is in the order: SiC > Si3N4 > SiO2. By ab-initio simulation and PL results, the band gap for a given dot size is in the order: SiC > Si3N4 > SiO2. However, the PL intensity for a given dot size is in the order: SiC < Si3N4 < SiO2.

Silicon quantum dots have a multitude of potential uses within the biomedical and optoelectronic industries due to the outstanding optical properties they exhibit. However, they are susceptible to oxidation, which may have a significant impact on the yield and emission wavelength. These effects may be traced to the growth of a Si02 capping layer, or the inclusion of surface defects that yield optically active mid-gap states. Therefore the results of density functional calculations examining important oxidation processes for silicon quantum dots are presented within this thesis. The results have been obtained using the AIMPRO (Ab Initio Modelling PROgram) DFT code. First, a discussion is included concerning the thermodynamic and kinetic preferences for oxygen migration at the surface of a quantum dot, a key process in the growth of a Si02 layer, in particular examining the effects of local surface bonding and charging. Following on from this analysis, the progressive oxidation of a quantum dot through the formation of Si-O-Si bonds or the oxidation of surface SiH bonds has been examined to determine the most energetically favourable route for oxidation and the effect on the optical response of the system. Finally, the silanone surface defect (Si=O) has often been cited in the literature as a potential source of optical shifts upon exposure to an oxidising source. However the stability of this structure has only been analysed in a few idealised circumstances, and a more general and significantly extended study is included.

Dietary isothiocyanates (ITCs) from cruciferous vegetables (CVs) have been shown to possess chemopreventive and chemotherapeutic effects in many cellular and animal studies but with only limited success in humans. The aim of this thesis is to further evaluate the bioactivities of ITCs and the mechanisms behind, and the potential of multi-functional nano-conjugates to maximize the beneficial effects of ITCs in cancer therapy. The effects of sulforaphane (SFN), one of the most studied ITCs, were examined on human hepatocytes (HHL5) and hepatocarcinoma (HepG2) cells. Results showed that SFN was more toxic towards HHL5 than HepG2, and that the high basal levels of Nrf2/GSH/ROS enabled HepG2 cells to benefit from the protective effects of SFN against H2O2-induced cell death, apoptosis and DNA damage. Three of the metabolites of SFN were also examined in terms of their anticancer activities, and were demonstrated to exhibit similar cytoprotective activity, but weaker cytotoxic effects than SFN. Allyl isothiocyanate (AITC), another common dietary ITC, showed biphasic effects on cell viability, DNA damage and migration in HepG2 cells, and on endothelial cell tube formation in a 3D model. siRNA knockdown of Nrf2 and GSH inhibition abolished the stimulatory effects of low dose AITC on cell migration as well as low dose AITC induced protection against DNA damage. The lack of selectivity and the biphasic effects of ITCs could present undesirable risks in the context for cancer prevention and treatment. The bioactivities of novel AITC-conjugated silicon quantum dots (AITC-SiQDs) were compared with AITC, and their cellular uptake was monitored by detecting the intrinsic fluorescence of SiQDs. AITC-SiQDs demonstrated similar activity as AITC at high doses whilst lacking the low dose stimulatory effects. In addition, AITC-SiQDs induced a long-lasting activation of Nrf2 via translocation into the nucleus, which correlated positively with their cellular uptake. ROS were involved in the anticancer effects of AITC-SiQDs. Taken together, these data provide novel insights into the anticancer properties of ITCs and highlight the possibility of application of nanotechnology to optimize their potential in cancer treatment.

Introduction It is undisputed that the silicon became the material most widely used in electronics in recent decades[1,2]. The qualities of silicon are well known, from its abundance and low cost to its ability to easily combine with oxides, so that the material has become essential in integrated electronic circuits and CMOS technology. A step further, though, is the idea of integrating electronics and photonics on the same silicon-based technology[3]. However, new strategies are needed to overcome the two principal obstacles of a possible bulk Si photonics: the indirect band gap and the band gap amplitude, suitable for operation only in the infrared range. Due to the quantum confinement of electric charges in silicon quantum dots (Si QDs)[4], the value of the energy gap of the material increases as the size of QD decreases, giving values greater than bulk Si and making Si QDs good candidates for tunable-band gap devices. Several applications have been developed in recent years using these new properties, from light-emitting devices [5] to solar cell tandem type [6] or other opto-electronic devices [7]. Objectives of this Thesis This Thesis initiated a collaboration between our group and the group of Prof. Ossicini of the University of Modena and Reggio Emilia, who has been modeling Si QDs for the last five years[8-14]. In this context, we contributed with the capacity to study the transport properties of these models by taking advantage of mixing two different techniques, Transfer Hamiltonian (TH) and Density Functional Theory (DFT). Thus, the aim of this work was to develop an approach to study transport in nanostructures by taking advantage of the atomistic information that ab initio methods can provide. In particular, the transport through a single Si QD embedded in a SiO2 dielectric matrix and the influence of the Si QD size, the amorphization level, and the doping were studied. Results and conclusions About the size of QDs In the case of embedded QDs of few nanometers, the strong non-planar interface between Si and SiO2 require a different treatment with respect to common planar Si/SiO2 devices. In this PhD Thesis, we have shown that, for small QD sizes, the particle-in-a-box model cannot describe accurately DOS and band offset, because of the large contribution of interface states. In this regime an ab initio approach is necessary to take into account the atomistic detail of the interface between the Si QD and the first shell of O atoms surrounding it. Regarding the electronic transport in Si QDs, a correlation between electron (hole) barrier value and electron (hole) current was found, obtaining larger current values for smaller energy barriers. Specifically, a contrary dependence on Si QD size and amorphization level is found for electron and hole current. On one hand, electron (hole) current is higher for large (small) Si QDs, and, on the other hand, it is enhanced for amorphous (crystalline) systems. On the effects of doping Finally, the DFT-TH technique was used to study the influence of impurity atoms, B for p-doping and P for n-doping, in embedded Si QDs. It is remarkable that this study is one of the first attempts to model with DFT the inclusion of impurity atoms in embedded Si QDs, after the wide knowledge of ab initio works on free-standing Si QDs of the last years. The principal features that we found were: • The impurity positions with lower formation energy are inside the dot for P-doping (P-dot) and at the interface for B-doping (B-int). • Relation between shift of the Fermi energy and improvement of conductivity in doped systems (due to the change in energy barriers). • Improvement of the conductivity for the most energetically favorable position of P-doping (P-dot) but not for the position of B-doping (B-int). • Change in the conductivity between doped and undoped is higher for P-doping than B-doping for a given Si QD size and impurity position, and decreases with QD size for a given specie and impurity position. Bibliography [1] M. Segal. Material history: Learning from silicon. Nature, 483, S43 (2012). [2] H. J. Leamy and J. H.Wernick. Semiconductor silicon: the extraordinary made ordinary. MRS Bulletin, 22, 47 (1997). [3] X. Hao, E.-C. Cho, G. Scardera, Y. Shen, E. Bellet-Amalric, D. Bellet, G. Conibeer and M. Green. Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells. Sol. Energ. Mat. Sol. C, 93, 1524 (2009). [4] J. P. Proot, C. Delerue and G. Allan. Electronic structure and optical properties of silicon crystallites: Application to porous silicon. Appl. Phys. Lett., 61, 1948 (1992). [5] Y. Berencen, J. M. Ramirez, O. Jambois, C. Dominguez, J. A. Rodriguez and B. Garrido. Correlation between charge transport and electrolumi-nescence properties of Si-rich oxide/nitride/oxide-based light emitting capacitors. J. Appl. Phys., 112, 033114 (2012). [6] G. Conibeer, I. Perez-Wur, X. Hao, D. Di and D. Lin. Si solid-state quantum dot-based materials for tandem solar cells. Nanoscale Res. Lett., 7, 193 (2012). [7] L. Pavesi, L. D. Negro, C. Mazzoleni, G. Franzo and F. Priolo. Optical gain in silicon nanocrystals. Nature, 408, 440 (2000). [8] M. Luppi and S. Ossicini. Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2: Beyond the quantum confinement effect. Phys. Rev. B, 71, 035340 (2005). [9] R. Guerra, I. Marri, R. Magri, L. Martin-Samos, O. Pulci, E. Degoli and S. Ossicini. Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size. Phys. Rev. B, 79, 155320 (2009). [10] R. Guerra, E. Degoli and S. Ossicini. Size, oxidation, and strain in small Si/SiO2 nanocrystals. Phys. Rev. B, 80, 155332 (2009). [11] R. Guerra and S. Ossicini. High luminescence in small Si/SiO2 nanocrystals: A theoretical study. Phys. Rev. B, 81, 245307 (2010). [12] R. Guerra, E. Degoli, M. Marsili, O. Pulci and S. Ossicini. Local-fields and disorder effects in free-standing and embedded Si nanocrystallites. Phys. Status Solidi B, 247, 2113 (2010). [13] R. Guerra, M. Marsili, O. Pulci and S. Ossicini. Local-field effects in silicon nanoclusters. Phys. Rev. B, 84, 075342 (2011). [14] M. Govoni, I. Marri and S. Ossicini. Auger recombination in Si and GaAs semiconductors: Ab initio results. Phys. Rev. B, 84, 075215 (2011).
Les nanopartícules de silici (silicon quantum dots, Si QDs, en anglès) són interessants materials que es proposen com a candidats per a la tercera generació de cel•les solars. Degut al confinement quàntic de les càrregues elèctriques dins del QD, el valor de l'energia de gap del material augmenta a mesura que la mida del QD disminueix, donant valors més gran que el Si bulk i fent que els QDs de Si siguin uns bons candidats per a dispositius amb valors de l'energia de gap modificables. En aquesta Tesi Doctoral proposem un marc teòric per estudiar el transport electrònic en nanoestructures aportant una descripció ab initio dels estats electrònics, basant-se en l'ús conjunt de dues tècniques: la Teoria del Funcional de la Densitat (Density Funcional Theory, DFT, en anglès) pel modelatge de la densitat d'estats del dispositiu i el Hamiltonià de Transferència (Transfer Hamiltonian, TH, en anglès) per la descripció del transport electrònic. Les principals conclusions d’aquesta Tesi Doctoral són: • En el cas de QDs de Si de pocs nanometres dins de matrius dielèctriques, la interfície fortament no-planar entre el Si i el SiO2 requereix un tractament diferent de la communtment utilitzada en l'heterojunció planar Si/SiO2. En aquesta Tesi Doctoral hem observat que, per Si QDs de mida petita, el model de partícula-dins-d'una-caixa no descriu les densitats d'estats i les barrers de potencial d'una forma acurada. Això és degut a què aquest model no recull l'efecte de la interfície, propietat que sembla ser essencial en la mida nanomètrica. • Respecte el transport electrònic en QDs de Si, Per una banda, el corrent d'electrons (forats) és més gran per a QDs DE Si de mida més gran (petita), i, per l'altra banda, el corrent d'electrons (forats) és més important per a sistemes amorfs (cristal•lins). • Les principals influències de dopatge tipus p (amb B) i tipus n (amb P) és (1) les configuracions de més baixa energia de formació són dins del QD quan dopem amb P, i a la interfície entre el QD i la primera capa d'oxígens quan dopem amb B, i (2) hi ha un millora en la conductivitat per la posició energètica més favorable pel dopatge amb P però no per la posició pel dopatge amb B.

This dissertation focuses on the characterisation of double quantum dots in silicon nano-transistors fabricated with complementary metal-oxide-semiconductor (CMOS) compliant technology. The progressive reduction of the dimension of electronic components searched by microelectronics industry and the research on quantum dots for quantum information processing (QIP) have been regarded as independent fields. In the last years an approach combining this two aspects has gained interest: the fabrication of semiconductor nano-devices in CMOS compliant and preindustrial technology for the study of quantum mechanic effects. The core material used in this approach is silicon: it is a standard material in classical electronics and is characterized by long coherence times. In particular double quantum dot system are appealing for QIP because of their possible implementation in different quantum bit architectures. This thesis reports on the work done at Laboratorio MDM-IMM-CNR (Agrate Brianza) and for three months at Hitachi Cambridge Laboratory during the three years Nanostructures and Nanotechnology PhD course of Università di Milano-Bicocca. I first describe the formation of a double quantum dot in a single gate nano-transistor. The quantum dots are located at the corners of the channel but the presence of a single gate doesn't allow for controlling the system. Nevertheless, one of them is hybridized with a single donor in strong coupling with the leads. The conservation of valley parity index during tunneling influences transport processes both at first and second order. The Kondo-perturbed regime manifests in the first spin-valley shell. Subsequently, I report the electrical characterization of multigate T-shaped devices. Here a single electron transistor is used to charge sense nearby quantum dots. Such architecture allows for tuning the number of the quantum dots, which at low filling are disorder-assisted, and a double quantum dot can be studied both from charge sensing and single charge dynamics measurements. I then investigate an alternative readout technique. The rf-reflectometry, by connecting a resonator to one of the gates, enables to study double quantum dots in split-gate nano-transistors, exploiting more compact and simple device architectures. In addition, the increased sensitivity with respect to standard DC-measurements can be exploited to investigate the few-electron regime. Finally, I report on the design and characterisation of a cryogenic printed circuit boards (PCBs) set for the broadband characterisation of multigate devices. One PCB hosts a custom CMOS transimpedance amplifier with selectable gain, maximum bandwidth of 250 kHz, minimum equivalent input noise of 4 pA rms. The high frequency lines are designed to ensure the transmission of ns pulses with low crosstalk up to few GHz in order to perform single charge manipulation.

This thesis describes experiments on quantum dots made by locally gating one-dimensional quantum wires. The first experiment studies a double quantum dot device formed in a Ge/Si core/shell nanowire. In addition to measuring transport through the double dot, we detect changes in the charge occupancy of the double dot by capacitively coupling it to a third quantum dot on a separate nanowire using a floating gate. We demonstrate tunable tunnel coupling of the double dot and quantify the strength of the tunneling using the charge sensor. The second set of experiments concerns carbon nanotube double quantum dots. In the first nanotube experiment, spin-dependent transport through the double dot is compared in two sets of devices. The first set is made with carbon containing the natural abundance of \(^{12}C\) (99%) and \(^{13}C\) (1%), the second set with the 99% \(^{13}C\) and 1% \(^{12}C\). In the devices with predominantly \(^{13}C\), we find evidence in spin-dependent transport of the interaction between the electron spins and the \(^{13}C\) nuclear spins that was much stronger than expected and not present in the \(^{12}C\) devices. In the second nanotube experiment, pulsed gate experiments are used to measure the timescales of spin relaxation and dephasing in a two-electron double quantum dot. The relaxation time is longest at zero magnetic field and goes through a minimum at higher field, consistent with the spin-orbit-modified electronic spectrum of carbon nanotubes. We measure a short dephasing time consistent with the anomalously strong electron-nuclear interaction inferred from the first nanotube experiment.
Physics

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others).

QC 20120920

Silicon quantum dots were synthesised wet-chemically by three different methods based on reduction of silicon tetrachloride with the reduction agents potassium naphthalide, sodium cyclopentadiene and the alkalide of potassium. The purpose of these quantum dots was to deposit them on a substrate in order to use them as down converters on top of photovoltaic solar cells for enhanced solar cell efficiency. One possible method for the formation of down converting layers is to incorporate quantum dots into silica thin films by deposition of quantum dots in an ethanol based silica sol, followed by spin coating. It is believed that when the quantum dots are water dispersible, this will make it easier to bind them to the silica network, and a good dispersion in the film is facilitated. The different hydrophilic functionalisations investigated were pentenoxy capping, oxidised pentenoxy capping, oxidised acrylic acid capping and ethanolamine capping. Challenges were encountered during the synthesis of water dispersible quantum dots, the most important were related to agglomeration and purification of the quantum dots. This was believed to be due to the tendency of hydrophilic surface groups to attract each other, interaction with the polar solvent and similar solubility characteristics of the quantum dots and the byproduct salts.Si quantum dots with hydrophobic octoxy capping were also synthesised. Dispersions of these quantum dots were deposited onto solid substrates followed by solvent evaporation. This was done to see whether it was possible to deposit the synthesised quantum dots by this simple approach, to investigate the fundamentals upon evaporation, the degree of agglomeration and the byproducts present in the quantum dot dispersions. It was found that agglomeration was very pronounced after the solvent had evaporated and that quite large amounts of byproducts were present in the final quantum dot dispersions. The most important reasons to this were believed to be too weak steric repulsive forces between the particles, too fast evaporation of the solvent and an insufficient purification procedure. For the use of Si quantum dots synthesised wet-chemically as down converters in solar cells, improvements of the particles are needed.

In this study silicon-germanium quantum dots grown on silicon have been investigated. The aim of the work was to find quantum dots suitable for use as a thermistor material. The quantum dots were produced at KTH, Stockholm, using a RPCVD reactor that is designed for industrial production.

The techniques used to study the quantum dots were: HRSEM, AFM, HRXRD, FTPL, and Raman spectroscopy. Quantum dots have been produced in single and multilayer structures.

As a result of this work a multilayer structure with 5 layers of quantum dots was produced with a theoretical temperature coefficient of resistance of 4.1 %/K.

There are significant differences between experiment and theoretical calculations of the electronic structure of GaSb/GaAs self-assembled quantum dots. Using a multi-band effective mass approximation it is shown that the influence of size and geometry of quantum dots has little or no effect in determining the hydrostatic strain. Furthermore, the valenceband ground state energies of the quantum dots studied are surprisingly consistent. This apparent paradox attributed to the influence of biaxial strain in shaping the heavy-hole and light-hole potentials. Consequently, it is shown that a simple, hydrostatically derived potential is insufficient to accurately describe the electronic structure of such quantum dots. In addition, using the latest experimental results measuring the conductionband offset, it has been shown that much better experimental contact may be achieved for the magnitude of the transition energies derived compared to theoretically derived transition energies. The transition energies of Si/Ge self-assembled quantum dots has also been calculated. In particular, a range of quantum dot structures have been proposed that are predicted to have an optical response in the 3-5 micron range.

In the last decades, nanomaterials, and in particular semiconducting nanoparticles (or quantum dots), have gained increasing attention due to their controllable optical properties and potential applications. Silicon nanoparticles (also called silicon nanocrystals, SiNCs) have been extensively studied in the last years, due to their physical and chemical properties which render them a valid alternative to conventional quantum dots. During my PhD studies I have planned new synthetical routes to obtain SiNCs functionalised with molecules which could ameliorate the properties of the nanoparticle. However, this was certainly challenging, because SiNCs are very susceptible to many reagents and conditions that are often used in organic synthesis. They can be irreversibly quenched in the presence of alkalis, they can be damaged in the presence of oxidants, they can modify their optical properties in the presence of many nitrogen-containing compounds, metal complexes or simple organic molecules. If their surface is not well-passivated, the oxygen can introduce defect states, or they can aggregate and precipitate in several solvents. Therefore, I was able to functionalise SiNCs with different ligands: chromophores, amines, carboxylic acids, poly(ethylene)glycol, even ameliorating functionalisation strategies that already existed. This thesis will collect the experimental procedures used to synthesize silicon nanocrystals, the strategies adopted to functionalise effectively the nanoparticle with different types of organic molecules, and the characterisation of their surface, physical properties and luminescence (mostly photogenerated, but also electrochemigenerated). I also spent a period of 7 months in Leeds (UK), where I managed to learn how to synthesize other cadmium-free quantum dots made of copper, indium and sulphur (CIS QDs). During my last year of PhD, I focused on their functionalisation by ligand exchange techniques, yielding the first example of light-harvesting antenna based on those quantum dots. Part of this thesis is dedicated to them.

Blinking dynamics of CdSe/ZnS semiconductor quantum dots (QD) are characterized by (truncated) power law distributions exhibiting a wide dynamic range in probability densities and time scales both for off- and on-times. QDs were immobilized on silicon oxide surfaces with varying grades of hydroxylation and silanol group densities, respectively. While the off-time distributions remain unaffected by changing the surface properties of the silicon oxide, a deviation from the power law dependence is observed in the case of on-times. This deviation can be described by a superimposed single exponential function and depends critically on the local silanol group density. Furthermore, QDs in close proximity to silanol groups exhibit both high average photoluminescence intensities and large on-time fractions. The effect is attributed to an interaction between the QDs and the silanol groups which creates new or deepens already existing hole trap states within the ZnS shell. This interpretation is consistent with the trapping model introduced by Verberk et al. (R. Verberk, A. M. van Oijen and M. Orrit, Phys. Rev. B, 2002, 66, 233202).

The research presented throughout this dissertation demonstrates the potential of miniemulsion polymerization to co-encapsulate silicon quantum dots (SiQDs) within polymeric nanoparticles and shows that the optical properties of SiQDs can be manipulated by their co-encapsulation with metal nanoparticles. In Chapter 1, the current state of the art in the preparation of inorganic/polymer composite nanoparticles via miniemulsion polymerization, utilizing inorganic species ranging from semiconductor nanocrystal quantum dots, metal nanoparticles, magnetic and oxide nanoparticles, is reviewed. A brief discussion on the properties of SiQDs, which are the inorganic entities of interest in this work, is presented. Chapter 2 describes the synthesis and characterization of encapsulated alkylated SiQDs within polymer nanoparticles composed of the monomers styrene and 4-vinylbenzaldehyde via miniemulsion polymerization. It was shown that the polymer nanoparticle surfaces, which display aldehyde groups, can be further decorated with organic molecules through the formation of imine, oxime or hydrazone bonds. The preparation of two-component polymer composite nanoparticles encapsulating both SiQDs and gold nanoparticles (AuNPs) is presented in Chapter 3. These Au-Si polymer composite nanoparticles display plasmon-enhanced fluorescence of the SiQDs attributable to the localized surfaced plasmon resonance of AuNPs co-encapsulated within the polymer nanoparticles. A preliminary study of Au-Si polymer composite nanoparticles in inkjet printing is also discussed. Chapter 4 presents the development of multicomponent polymer composite nanoparticles which co-encapsulate SiQDs and Au-Ag alloys NPs encoded with Raman-active molecules within polymer nanoparticles. The multicomponent polymer composite nanoparticles exhibit the simultaneous effects of surface-enhanced Raman scattering and metal-enhanced fluorescence as a consequence of the close proximity of the co-encapsulated SiQDs and Raman-active Au-Ag NPs. In Chapter 5, the conclusions and future directions of this study are discussed. This research moves iii towards the development of novel inorganic/polymer composite nanoparticles which may offer potential as new chemical probes with applications in biology, sensing and anti-counterfeiting.

The increasing application of nanoparticles in medicine, particularly in imaging and therapy, is heavily reliant on fundamental understanding of their unique physicochemical properties. This thesis investigates the interactions that occur between nanoparticles and human cells, specifically their toxicity and the mechanism of their internalisation. Alkyl-capped silicon quantum dots (SiQDs) were chosen for this investigation because their physical properties have been well-characterised and they have very bright luminescence. These properties facilitate their detection inside cells by fluorescence microscopy and flow cytometry. Toxicity and uptake mechanisms for SiQDs in human cells were investigated. The intestinal cell line CACO-2 was used as a model for ingestion of nanoparticles. Uptake and accumulation of SiQS in CACO-2 cells was demonstrated by epi-fluorescence microscopy and confocal fluorescence microscopy. Cytotoxicity in CACO-2 cells was studied by oxidative stress measurement using a intracellular dye method (H2DCFDA) (ROS assay), cell viability determination (MTT and ATP assays) and DNA damage measurement (Comet assay). Exposure of CACO-2 to SiQDs resulted in low cytotoxicity with regard to cell viability and effects on ATP production. SiQDs did not induce intracellular ROS production or DNA strand breaks. Over time periods up to 14 days, SiQDs showed no evidence of acute or chronic cytotoxicity. Accumulation of SiQDs inside a selection of human cell lines (CACO-2, HeLa, HepG2, and Huh7) was studied in detail using flow cytometry. SiQDs were internalised by all four cell lines. The highest levels of accumulation were seen with HepG2 and HuH7 cells. Some evidence for a role for caveolin 2 in this process in HuH7 was obtained using inhibitors and by gene expression analysis. The reason why HepG2 cells showed the highest accumulation of SiQDs remains unclear as caveolin expression measured by quantitative reverse transcriptase-PCR in these cells is very low. Further work on a possible role for the protein clathrin in the endocytosis process in these cells is needed as the inhibitor used to investigate this was toxic to the cells and firm conclusions could not be obtained. iii In summary, SiQDs are a highly promising alternative to heavy-metal based quantum dots and as a nanoparticle model to study mechanisms of uptake. SiQDs appear non-toxic in CACO-2 cells. However, the internalisation of SiQDs is cell-line dependent and further studies on their toxicity in cells such as HuH7 which show high levels of internalisation and cells relevant to immune responses such as macrophages are needed.

La mécanique quantique affiche déjà toute son étrangeté en considérant l’équation de Schrödingerdans un puits de potentiel à une dimension. L’effet tunnel qui en résulte, en est un exemple frappant.La possibilité de récréer cette situation dans un système matériel est un enjeu excitant et un grandpas vers le contrôle des effets quantiques.Le confinement spatial des spins électroniques a été suggéré comme une approche possible pour laréalisation d’un ordinateur quantique. Chaque spin formant un système à deux niveaux pouvant coderune bit élémentaire pour l’information quantique (spin qubit). Cette proposition par Loss etDiVincenzo a contribué à l’ouverture d’un domaine de recherche important dénommé spintroniquequantique. L’intérêt des qubits de spin s’appuie sur le fait que les états de spin ont des temps decohérence beaucoup plus long que les qubits de charge (états orbitaux).Un potentiel de confinement de spin peut être créé de différentes façons, comme par exemple enutilisant l’alignement des bandes d’énergies de semi-conducteurs de différentes natures. Cependant,les dimensions spatiales du système obtenu doivent toujours être inférieures à la longueur decohérence de phase des quasi-particules considérées afin de préserver leur comportement quantique.Jusqu’à présent, la plupart des progrès ont été réalisés en utilisant des hétérostructures semiconductricesà base d’arséniure de Gallium(GaAs). Dans de tels systèmes, lemouvement des porteursde charges est limité à un plan bidimensionnel et le confinement latéral peut être obtenu par destechniques de lithographie. De cette façon, des systèmes quasi-zéro-dimensionnels dont les étatsélectroniques sont parfaitement quantifiés (boîtes quantiques), sont réalisés.Diverses techniques utilisant des signaux hautes fréquences ont permis de manipuler et lire l’état despin de tels boîtes quantiques de GaAs et, il y a quelques années, les premiers qubits de spin ont étédémontrés. Cependant, ces systèmes ont montré des temps de cohérence relativement courts enraison de l’interaction hyperfine avec les spins nucléaires. En dépit de progrès significatifs sur lecontrôle de la polarisation, ce problème n’est toujours pas résolu.Au cours de ces dernières années, un effort croissant s’est donc concentré sur des systèmes à base dematériaux alternatifs pour lesquels l’interaction hyperfine est naturellement absente ou rendue trèsfaible par des techniques de purification. Même si le Silicium, qui est le matériau de base enmicroélectronique, remplit cette condition, il souffre d’une faible mobilité par rapport aux semiconducteursIII-V, ce qui pose problème pour la spintronique quantique. Les structures à base Silicium-Germanium (SiGe) offrent un moyen de contourner ce problème tout en gardant un matériaucompatible avec les procédés de fabrication standards.Durant mon travail de thèse, je me suis concentrée principalement sur l’étude des propriétésélectroniques d’îlots auto-assemblés (nanocristaux) de SiGe. Le manuscrit de thèse qui relate lesprincipaux aspects de cette étude est organisé en six chapitres. Dans le premier chapitre, je décris lesprincipaux concepts de la croissance cristalline d’îlots auto-assemblés de SiGe ainsi que les propriétésdu potentiel de confinement qu’ils définissent. Le chapitre 2 est consacré aux principes du transportélectronique dans de telles structures. Le chapitre 3 traite de la modulation électrique du facteur deLandé (g) des trous confinés dans les îlots en vu de la manipulation rapide des états de spin. Dans lechapitre 4, je présente les résultats théoriques et expérimentaux relatifs à la sélectivité en spin dansles nanocristaux de SiGe. Le chapitre 5 décrit les résultats sur la réalisation d’une pompe électroniqueobtenue à partir de nanofils d’InAs/InP. Enfin, le chapitre 6montre les progrès technologiques que j’aiobtenus vers la réalisation et l´étude de dispositifs couplés à base de nanocristaux de SiGe
Quantum mechanics displays all its exciting strangeness already by considering the Schrödingerequation in a one-dimensional square well potential; tunnelling events put this statement in evidence.To recreate this situation in a given material system is an inspiring playground and a big step towardstaking control of quantum mechanisms. For instance, it is now possible to confine electrons in solidstatedevices enabling amore efficient solar-cell technology. Confining individual electron spins has infact been suggested as a possible approach to the realization of a quantum computer. Each electronspin forms a natural two-level systems encoding an elementary bit of quantum information (a socalledspin qubit). This proposal, by Loss and DiVincenzo, has contributed to the opening of an activeresearch field referred to as quantum spintronics. Spin qubits rely on the fact that spin states canpreserve their coherence on much longer time scales than charge (i.e. orbital) states.A confinement potential can be created artificially in many different ways; producing constantmagnetic fields and spatially inhomogeneous electric fields, applying oscillating electric fields, usingconductive oxide layers, etc. To take advantage of the band-alignment of different semiconductors isamong these. The relevant dimensions of the considered system should still be smaller than the phasecoherence length of the confined particles in order that their quantum behaviour is preserved.So far, most of the progress has been achieved using GaAs-based semiconductor heterostructures. Insuch layered systems themotion of carriers is confined to a plane and further confinement is achievedbymeans of lithographic techniques, which allow lateral confinement to be achieved on a sub-100 nmlength scale. In this way, quasi-zero-dimensional systems whose electronic states are completelyquantized, i.e. quantum dots (QDs), can be devised.Various time-resolved techniques involving high-frequency electrical signals have been developed tomanipulate and read-out the spin state of confined electrons in GaAs QDs, and several years ago thefirst spin qubits were reported. In GaAs-based QDs, however, the quantum coherence of electronspins is lost on relatively short time scales due to the hyperfine interactionwith the nuclear spins (bothGa and As have non-zero nuclear spin moments). In spite of significant advances on controlling thenuclear polarization [3, 4], this problem remains unsolved.In the past few years an increasing effort is concentrating on alternative material systems in whichhyperfine interaction is naturally absent or at least very weak and, in principle, controllable by isotopepurification. While Si fulfils this requirement and it is the dominant material in modernmicroelectronics, it suffers from low mobility compared to III-V semiconductors, which obstructs itsapplication for quantum spintronics. SiGe structures offer a way to circumvent this problem that isstill compatible with standard silicon processes.I have focused mainly on the study of the electronic properties of SiGe self-assembled islands, alsocalled SiGe nanocrystals. This work, which condensates the main points of this study, is organized insix chapters. In the first chapter, I describe the basics of the growth of SiGe self-assembled islands andthe properties of the quasi-zero-dimensional confinement potential that they define. Chapter 2 isdevoted to the basics of electronic transport in these structures. Chapter 3 deals with the electricmodulation of the hole g-factor in SiGe islands, which would enable a fast manipulation of the spinstates. In Chapter 4 I present theoretical and experimental findings related to spin selectivity in SiGeQDs and Chapter 5 is dedicated to the realization of an electron pump in InAs nanowires based on thiseffect. Finally, Chapter 6 exhibits our progress towards the study of coupled SiGe QD devices

This thesis examines the optical properties of nanoscale silicon and the sensitization of Er with Si. In this context, it predominantly investigates the role of defects in limiting the luminescence of Si nanocrystals, and the removal of these defects by hydrogen passivation. The kinetics of the defect passivation process, for both molecular and atomic hydrogen, are studied in detail. Moreover, the optical absorption of Si nanocrystals and the effect of annealing environment (during nanocrystal synthesis) on the luminescence are investigated. The effect of annealing temperature and hydrogen passivation on the coupling (energy transfer) of Si nanocrystals to optically active centres (Er) is also examined.¶ The electronic structure of silicon-implanted silica slides is investigated through optical absorption measurements. Before and after annealing to form Si nanocrystals, optical absorption spectra from these samples show considerable structure that is characteristic of the particular implant fluence. This structure is shown to correlate with the transmittance of the samples as calculated from the modified refractive index profile for each implant. Due to the high absorption coefficient of Si at short wavelengths, extinction at these wavelengths is found to be dominated by absorption. As such, scattering losses are surprisingly insignificant. To eliminate interference effects, photothermal deflection spectroscopy is used to obtain data on the band structure of Si in these samples. This data shows little variance from bulk Si structure and thus little effect of quantum confinement. This is attributed to the dominance of large nanocrystals in the absorption measurements.¶ The effect of annealing environment on the photoluminescence (PL) from silicon nanocrystals synthesized in fused silica by ion implantation and thermal annealing is studied as a function of annealing temperature and time. Interestingly, the choice of annealing environment (Ar, N2, or 5 % H2 in N2) is found to affect the shape and intensity of luminescence emission spectra, an effect that is attributed both to variations in nanocrystal size and the density of defect states at the nanocrystal/oxide interface.¶ The passivation kinetics of luminescence-quenching defects, associated with Si nanocrystals in SiO2, during isothermal and isochronal annealing in molecular hydrogen are studied by time-resolved PL. The passivation of these defects is modeled using the Generalized Simple Thermal model of simultaneous passivation and desorption, proposed by Stesmans. Values for the reaction-rate parameters are determined for the first time and found to be in excellent agreement with values previously determined for paramagnetic Si dangling-bond defects (Pb type centers) found at planar Si/SiO2 interfaces; supporting the view that non-radiative recombination in Si nanocrystals is dominated by such defects.¶ The passivation kinetics of luminescence-quenching defects during isothermal and isochronal annealing in atomic hydrogen are studied by continuous and time-resolved PL. The kinetics are compared to those for standard passivation in molecular hydrogen and found to be significantly different. Atomic hydrogen is generated using the alneal process, through reactions between a deposited Al layer and H2O or –OH radicals in the SiO2. The passivation and desorption kinetics are shown to be consistent with the existence of two classes of nonradiative defects: one that reacts with both atomic and molecular hydrogen, and the other that reacts only with atomic hydrogen. A model incorporating a Gaussian spread in activation energies is presented that adequately describes the kinetics of atomic hydrogen passivation and dissociation for the samples.¶ The effect of annealing temperature and hydrogen passivation on the excitation cross-section and PL of erbium in silicon-rich silica is studied. Samples are prepared by co-implantation of Si and Er into SiO2 followed by a single thermal anneal at temperatures ranging from 800 to 1100 degrees C, and with or without hydrogen passivation performed at 500 degrees C. Using time-resolved PL, the effective erbium excitation cross-section is shown to increase by a factor of 3, while the number of optically active erbium ions decreases by a factor of 4 with increasing annealing temperature. Hydrogen passivation is shown to increase the luminescence intensity and to shorten the luminescence lifetime at 1.54 micron only in the presence of Si nanocrystals. The implications of these results for realizing a silicon-based optical amplifier are also discussed.

The research carried out here employed analytical and imaging transmission electron microscopy and scanning transmission electron microscopy to gain a good understanding of local structure and composition of semiconductor nanocrystals and quantum dots for electronics and optoelectronics applications. One of the world's most advanced analytical scanning transmission electron microscopes in the field, the Daresbury SuperSTEM, was used to scrutinise the structure and composition of the samples. Three nanostructure systems are investigated in this thesis: 1. Structures consisting of Ge-nanocrystals (NCs) in alumina. Here HRTEM suggests relaxed and twinned smaller NCs grown annealed at lower temperature compared to elongated non-faulty bigger NCs annealed at higher temperature. HRTEM also suggests a polycrystalline structure of the matrix. 2. With regards to the InAs/GaAs quantum dots (QD) the study aims in particular at elucidating QD formation by investigating samples grown with and without growth interrupt (GI). Diffraction contrast TEM shows formation of buried dots in the sample prepared with GI whereas for the sample without GI the immediate growth of GaAs after InAs inhibits diffusion and segregation of In adotoms, and no footprint of buried dots has been observed. HRTEM and HAADF show coherent QDs in the sample with GI and abrupt InAs/GaAs interfaces in the sample without GI. In executing energy electron loss spectroscopy (EELS) and geometric phase analysis (GPA) the distribution of In in InGaAs/GaAs QDs has been obtained in samples grown in the critical thickness regime for quantum dot formation. The highest In percentage achieved in the dots grown with a nominal fraction of 100% was ~70%. EELS shows variations in the In concentration within the QD structure and wetting layer 3. In the case of Er-doped Si-NCs in silica this research tries to provide an understanding of structure, composition and position of excess Si and Er in the silica matrix of materials prepared under different growth conditions and to correlate this information with the PL emission, all with the aim to find preparation routes for optimum optical efficiency for applications of this materials system in silicon photonics. High spatial correlation between Si-NCs, Er and O in the Er and Si co-implanted sample with strong indication of an Er-oxide/Si core-shell structure had been found. The lack of an Er-oxide plasmon indicates, however, that the shell structure and its interface with the SiNCs is highly defective and a likely cause for non-radiative recombination. The sample with similar excess Er and Si concentrations but prepared in a two-stage implantation and annealing process shows a 10 times improvement in the optical emission. Here no spatial correlation between Er and Si-NCs was found in core loss EELS. EELS and HAADF evidenced more highly, near-atomically dispersed Er in the matrix with no formation of a core-shell structure as compared to the co-implanted sample. No footprint of Er-silicide plasmon was observed by low loss valence band EELS investigation in the co-implanted sample.

Le confinement quantique dans le silicium, sous forme de boîtes quantiques de silicium de diamètre 5 nm, permet de contrôler le bandgap et donc l'émission de lumière. Cette ingénierie du bandgap des nanocristaux de silicium est utile pour les applications photovoltaïques avancées et présente l'avantage de conserver la compatibilité avec les technologies silicium existantes. Ces boîtes quantiques peuvent aider à réduire les pertes par thermalisation dans une cellule solaire homo-jonction. Ce travail se concentre sur la fabrication à grande échelle des nanocristaux de silicium dans SiO2 en utilisant le Dépôt Chimique en Phase Vapeur assisté par Plasma (PECVD), suivi d'un recuit à haute température. Des monocouches sont comparées avec des multicouches pour les propriétés morphologiques, électriques et optiques et des dispositifs avec ces différents couches sont comparés. Dans le cas d'une structure monocouche, l'épaisseur de la couche contrôle l'organisation des nanocristaux et permet de mettre en évidence l'amélioration de la conductivité électrique, avec cependant une réponse optique faible. Les multicouches montrent un bandgap du Si augmentée et controlee, avec une meilleure absorption dans la gamme bleu-vert visible, accompagnée d'une conductivité électrique faible. L'amélioration de ces propriétés optiques est un signe prometteur pour une potentielle intégration photovoltaïque.

Semiconductor nanostructures exhibit many remarkable electronic and optical properties. The key to designing and utilising semiconductor quantum structures is a physical understanding of the detailed excitation, transport and energy relaxation processes. Thus the nonequilibrium dynamics of semiconductor quantum structures have attracted extensive attention in recent years. Ultrafast spectroscopy has proven to be a versatile and powerful tool for investigating transient phenomena related to the relaxation and transport dynamics in semiconductors. In this thesis, we report investigations into the electronic and optical properties of various semiconductor quantum systems using a variety of ultrafast techniques, including up-conversion photoluminescence, pump-probe, photon echoes and four-wave mixing. The semiconductor quantum systems studied include ZnO/ZnMgO multiple quantum wells with oxygen ion implantation, InGaAs/GaAs self-assembled quantum dots with different doping, InGaAs/InP quantum wells with proton implantation, and silicon quantum dots. The spectra of these semiconductor nanostructures range from the ultraviolet region, through the visible, to the infrared. In the UV region we investigate excitons, biexcitons and oxygen implantation effects in ZnO/ZnMgO multi-quantum wells using four-wave mixing, pump-probe and photoluminescence techniques. Using time-resolved up-conversion photoluminescence, we investigate the relaxation dynamics and state filling effect in InGaAs self-assembled quantum dots with different doping, and the implantation effect in InGaAs/InP quantum wells. Finally, we study the optical properties of silicon quantum dots using time-resolved photoluminescence and photon echo spectroscopy on various time scales, ranging from microseconds to femtoseconds.

A microfluidic reactor for synthesizing cadmium selenide (CdSe) quantum dots (QDs) was synthesized out of a silicon wafer and Pyrex glass. Microfabrication techniques were used to etch channels into the silicon wafer. Holes were wet-drilled into the Pyrex glass using a diamond-tip drill bit. The Pyrex wafer was anodically bonded to the etched silicon wafer to enclose the microfluidic reactor. Conditions for anodic bonding were created by exposing the stacked substrates to 300V at ~350oC under 5.46N of force. A syringe containing a room temperature CdSe solution was interfaced to the microfluidic reactor by using Poly (dimethylsiloxane) (PDMS) as an interface. The reactor was placed on a hot plate at 225oC, creating thermodynamic conditions for the QD chemical reaction to occur within the etched channels. Tygon® tubing transported solutions in and out of the microfluidic reactor. The CdSe solution was injected into the reactor by a syringe pump at an injection rate of 5 mL/hr, with a channel length of 2.5 cm. While in the microfluidic channels, QD residence time of approximately 30 seconds was sufficient enough for nucleation and growth of QDs to occur. The QD size was characterized by fluorescence full-width-half-maximum (FWHM), which is directly proportional to size distribution. The FWHM of the QDs synthesized was 38 nm, with a peak wavelength of 492 nm. By controlling combinations of pump rate and channel length, a range of QD sizes was able to be consistently synthesized through the microfluidic reactor with significant repeatability and reproducibility.

In the rapidly evolving field of experimental quantum information processing, one important sub-field pursues a potentially scalable implementation that transports quantum information encoded in photons throughout “photonic circuits” fabricated in a silicon wafer. A key component is an efficient on-demand source of these single photons, and this dissertation aimed to assess the feasibility of one proposed realization of such a source by integrating few PbSe colloidal quantum dots (CQDs, demonstrated single photon emitters in nanoparticle form) into the mode volume of an optical microcavity designed to efficiently direct quantum dot emission into a silicon photonic circuit. Although no direct evidence of {\it single} photon emission was observed, results prompted a number of follow-up experiments and considerable theoretical modeling to understand this quantum dot, photonic circuit system. The methods of investigation included (1) temporally-, spectrally-, and spatially-resolved photoluminescence (PL) measurements of PbSe CQDs integrated into SOI PICs and relatable environments (solution, thick film, thin film), (2) temperature-dependent, air-exposure studies of PbSe CQD thick film PL, (3) development and application of kinetic and quantum mechanical cavity-coupled modeling that admit complete accounting of the photonic density of states, depolarization effects, and non-radiative decay, and (4) a photon coincidence test of single photon emission. The main findings of this work are: (1) while capture of cavity-enhanced PbSe CQD emission into a silicon photonic circuit was demonstrated, the overall photon generate rate is inadequate for any useful implementation, (2) the measured coupling rate can be modeled and explained in terms of system parameters extracted from auxiliary experimental results obtained with the PbSe CQDs in isolation, or on isolated microcavities, and (3) consistent results could only be obtained after nontrivial depolarization factors and non-radiative decay processes are properly accounted for. From this it is clear that the performance of PbSe CQDs in this configuration of a single photon source in silicon is currently limited by a long-lived trap state with a several microsecond lifetime, and large depolarization effects that inhibit emission. Although plausible future efforts may mitigate these effects substantially, performance may still be hindered by the intrinsic emission strength of PbSe CQDs.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

In this thesis a through study of nanostructured silicon rich oxide for advanced photovoltaic application is carried forward. Several structural parameters are studied, among which are: thickness, nanostructuration, stoichiometry, doping. Along with these, the effect of light is studied and an EQE measurements on an all silicon quantum dot based solar cell is presented.

La photonique sur silicium permet de palier au faible rendement et la consommation énergétique élevée des liens télécoms exploitant les câbles à paires torsadées ou les câbles coaxiaux. Cette technologie offre une versatilité exceptionnelle, de nouvelles fonctionnalités et des performances accrues pour les communications à haut-débit, les systèmes d’interconnexions optiques à courte portée et le déploiement de liaisons optiques d’une puce à une autre, d’une carte à une autre, ou d’un rack à un autre (datacom). Le silicium est un matériau semi-conducteur très efficace pour le guidage de la lumière, notamment en raison du fort contraste d’indice avec la silice. Cependant, sa bande interdite indirecte ne permet pas une émission radiative efficace. La réalisation de lasers repose donc sur des technologies hybrides de collage ou de report du matériau actif III- V (wafer-bonding, flip-chip) sur le silicium passif. Cependant, cette intégration hétérogène présente des inconvénients comme par exemple un coût élevé et une évolutivité limitée. Les lasers hybrides sur silicium sont aussi plus sensibles aux réflexions parasites provenant des transitions des différentes interfaces passives/actives. Un moyen permettant de surmonter ces inconvénients consiste à faire croître directement le matériau III-V sur le silicium. Dans ce contexte, les lasers à boîtes quantiques utilisant des atomes semi-conducteurs comme milieu de gain sont des candidats très prometteurs en raison de leur compacité, de leur grande stabilité thermique et d’une tolérance accrue aux défauts structuraux. Certaines applications comme les systèmes cohérents, les futures horloges atomiques intégrées sur puces et les radars où la sensibilité aux bruits de fréquence et d’intensité influe fortement le taux d’erreur binaire requièrent l’utilisation d’émetteurs optiques à très faible bruit. Dans une première partie, cette thèse révèle le potentiel de lasers à boîtes quantiques InAs/InP présentant une largeur de raie spectrale intrinsèque de 80 kHz et un bruit relatif d’intensité inférieur à -150 dB/Hz. A cet effet, il est montré qu’un faible couplage vertical entre les états liés est plus approprié pour une réduction du bruit d’intensité notamment grâce à la suppression du bruit de porteurs associée à l’état excité. Dans une deuxième partie, les propriétés dynamiques et non- linéaires des lasers à boîtes quantiques directement épitaxiés sur silicium sont étudiées. Comme susmentionné, les lasers intégrés de manière hétérogène sur le silicium sont plus sensibles aux réflexions parasites. Combinées à une rétroaction optique externe, la stabilité du laser peut s’en trouver fortement affectée. Sachant qu’il n’existe pas à ce jour d’isolateurs optiques intégrés sur puce ayant un taux d’isolation suffisant, le développement d’émetteurs insensibles aux rétroactions est un objectif majeur. Cette thèse présente notamment un résultat de transmission sans erreur à partir d’un laser à boîtes quantique directement épitaxié sur silicium soumis à une modulation externe à 10 Gb/s ainsi qu’à une rétroaction optique maximale de 100%. Cette insensibilité aux réflexions résulte de plusieurs propriétés remarquables comme un facteur d’élargissement spectral proche de zéro, un facteur d’amortissement élevé, un fort contraste entre les seuils d’émission des états liés, et une durée de vie des porteurs plus courte. Ces résultats permettent d’envisager le développement de futurs circuits intégrés photoniques sur silicium à haute performance et fonctionnant sans isolateur optique
Silicon photonics have been introduced to overcome low efficiency and high energy consumption of telecom links using twisted pairs or coaxial cables. This technology provides novel functionality and high performance for applications in high speed communication systems, short reach optical interconnects, and the deployment of optical links from chipto-chip, board-to-board or rack-to-rack (datacom). Silicon is known as a very efficient semiconductor material for waveguiding light in particular owing to the strong index contrast with silica. However, the indirect bandgap of silicon makes light emission from silicon inefficient, and other techniques such as wafer- or flipchip bonding must be investigated if light emission is to be realized. The drawbacks of such heterogeneous integration concentrate on the high cost and the limited scalability. Lasers heterogeneously integrated on silicon are also more sensitive to optical reflections originating from the transition between passive/active interfaces. The best way to overcome these drawbacks is to move on to direct epitaxial growth of IIIV materials on silicon for photonics integration. In this context, quantum dot lasers using semiconductor atoms as a gain medium are ideal because they enable smaller devices, amplification with large thermal stability and high tolerance to epitaxial defects. Ultra-low noise optical transmitters are required not only for the coherent systems but also for future chipscale atomic clocks and radar related applications because of the sensitivity to the frequency noise and intensity noise can strongly affect the bit error rates. To this end, the first part of the thesis reports an intrinsic spectral linewidth as low as 80 kHz and a relative intensity noise less than - 150 dB/Hz in InAs/InP quantum dot lasers. In particular, it is shown that a small vertical coupling is more suitable for low intensity noise operation due to the suppression of the carrier noise in the excited state. The second part of the thesis investigates the dynamic and nonlinear properties of epitaxial quantum dot lasers on silicon. As mentioned above, lasers heterogeneously integrated on silicon are more sensitive to parasitic reflections. When combined with external optical feedback, the laser stability can be dramatically affected. As no on-chip optical isolators integrated with lasers and having sufficient isolation ratio exist, the development of feedback insensitive transmitters remains a major objective. This thesis presents an error-free transmission of an epitaxial quantum dot laser on silicon externally modulated at 10 Gb/s and subjected to 100% optical feedback. Such remarkable feedback insensitivity directly results from the near-zero linewidth enhancement factor, the large damping factor, the strong contrast between the ground state and excited states and a shorter carrier lifetime. These results pave the way for future high-performance photonics integrated circuits on silicon operating without optical isolators

Cette thèse est dédiée à la mesure des pressions et des températures locales et à la comparaison de la génération de chaleur dans les contacts élastohydrodynamiques (EHD) de type tout acier et hybride (nitrure de silicium-acier). Le but ultime de ce travail est de développer une nouvelle technique in situ non-intrusive, exploitant la sensibilité de la photoluminescence (PL) des boîtes quantiques (QDs) de CdSe/CdS/ZnS aux variations de pression et température, afin de cartographier ces deux paramètres dans les contacts EHD. Dispersible à faible concentration dans les lubrifiants, il est montré que les QDs ne modifient pas le comportement rhéologique du fluide porteur et que le cisaillement n’est pas perturbateur à la réponse en PL. La calibration des QDs en suspension confirme la dépendance de la réponse en PL des QDs à la pression et à la température. Les mesures in situ sont effectuées en utilisant un banc d’essai bille-disque. La comparaison entre les mesures in situ de pression et de température et celles prédites à l'aide d'un modèle éléments finis TEHD interne montre une bonne concordance, ce qui démontre la faisabilité de la méthodologie proposée. Les effets du glissement et du chargement normal sur la pression, la température et la chaleur générée sont reportés. L’effet des propriétés thermiques des solides est souligné et la répartition de la chaleur générée entre les solides en contact est étudiée. L'équilibre énergétique entre l'énergie mécanique et l'énergie thermique interne générée par compression et cisaillement est démontré en comparant les pertes de puissance expérimentales et la chaleur générée issue du modèle numérique, pour des contacts acier-acier et hybrides
This thesis is dedicated to the measurement of local pressure and temperature and to compare the heat generation in all-steel and silicon nitride-steel (hybrid) elastohydrodynamic (EHD) contacts. The ultimate goal of this work is to develop a new non-intrusive in situ technique, exploiting the sensitivity of the photoluminescence (PL) of CdSe/CdS/ZnS quantum dots (QDs) to pressure and temperature. Dispersible in small concentration in lubricants, it is shown that the QDs doesn’t modify the rheological behavior of the carrier fluid and that shearing is not perturbative to the QDs PL response. The calibration of QDs in the suspension confirms the QDs PL dependence on temperature and pressure. The in situ measurements were conducted in EHD contacts using a ball-on-disc test rig. Comparisons between pressure and temperature measurements and predictions, using an in–house finite element thermal EHD model, showed a good agreement which demonstrates the feasibility of the proposed methodology. The effects of sliding and normal loading on pressure, temperature and heat generation are indicated. The effect of the thermal properties of the solid materials is underlined and the partition of the generated heat between the contacting solids is investigated. The energy equilibrium between the mechanical energy and the internal thermal energy generated by compression and shearing is demonstrated by comparing experimental power losses and numerical heat generation, in steel-steel and hybrid contacts

This work focuses on the fabrication of AlN-based optical cavities for the realization of UV microsources. Using AlN epitaxial films grown on silicon substrates by NH3-assisted MBE, photonic crystal cavities (PCs) and microdisk (disk) resonators are fabricated. A bottom-up and a top-down fabrication process are developed and assessed. PCs exhibit similar optical quality factors (Q) whatever the fabrication process. However the top-down fabrication process gives a much better fabrication yield. Q values up to 4400 are measured in the near UV and a good control of the energy of cavity modes is demonstrated. On the other hand, disk resonators exhibit whispering gallery modes with Q factors above 7000 in the near UV. GaN/AlN quantum dots (QDs) turn out to be intense UV emitters but due to the strong internal electric field, their radiative lifetime is long and the number of photons feeding the cavity mode is likely too low for lasing. The growth mode of GaN/AlN QDs is studied and modified to obtain smaller dots in order to shorten their radiative lifetime. In a last part, new growth strategies are developed to improve and facilitate the growth of GaN on Si. An original surface treatment is studied to decrease the dislocation density in GaN layers. Also, growth on patterned silicon substrates is studied to overcome the cracking of GaN layers grown on silicon substrates
Cette étude porte sur la fabrication de cavités optiques à base d’AlN pour la réalisation de micro-sources UV. A partir de films d’AlN épitaxiés sur substrats silicium par EJM-NH3, des cavités à cristaux photoniques (CPs) et des microdisques ont été fabriqués. Deux procédés de fabrication de type « bottom-up » et « top-down » ont été développés et comparés. Quelque soit le procédé de fabrication, les CPs possèdent des facteurs de qualité (Q) comparables. Cependant, l’approche classique “top-down” permet d’obtenir un meilleur rendement de fabrication. Des valeurs de Q autour de 4400 sont mesurées dans le proche UV et on démontre un bon contrôle de l’énergie du mode de cavité en fonction des paramètres structuraux de la cavité. Dans les microdisques, on mesure des Q supérieurs à 7000 dans le proche UV. Les boîtes quantiques (BQs) GaN/AlN s’avèrent être des émetteurs efficaces dans l’UV mais à cause d’un fort champ électrique interne, leur durée de vie radiative est longue et le nombre de photons qui se trouve dans le mode de cavité est trop faible pour obtenir un effet laser. Nous avons modifié le mode de croissance des BQs GaN/AlN afin d’obtenir des BQs plus petites, et ainsi raccourcir leur temps de vie radiatif. Dans la dernière partie de l’étude, nous proposons des stratégies pour améliorer la croissance du GaN sur Si. En particulier, nous proposons un traitement de surface original qui permet de réduire la densité de dislocations dans le GaN. Enfin, une étude de la croissance sur substrats structurés indique que cette technique permet d’empêcher la fissuration du GaN sur Si

2012/2013
Riassunto (Abstract) La fabbricazione di punti quantici (quantum dots, QD) auto-assemblati è una tematica di particolare rilevanza a causa delle loro possibilità di applicazione in dispositivi optoelettronici. Nel presente lavoro, ci siamo prefissi di ottenere array di QD di semiconduttore altamente uniformi, con lo scopo di raggiungere un controllo completo sulla loro distribuzione spaziale, ed un’uniformità spettrale superiore, rispetto a QD auto-assemblati convenzionali. Il metodo consiste in un approccio combinato top-down ebottom-up: QD auto-assemblati vengono cresciuti tramite Epitassia a Fasci Molecolari (MolecularBeam Epitaxy, MBE) su una superficie pre-patternata con un array regolare di buchi. Nella prima parte di questo lavoro di tesi, abbiamo ottimizzato la crescita di punti di InAs su substrati patternati di GaAs. Nella maggior parte dei lavori precedenti, i substrati vengono patternati tramite litografia elettronica (Electron BeamLitography, EBL), che non è la tecnica ottimale per l’applicazione a dispositivi broad-area, a causa della sua natura seriale e degli alti costi. Il metodo più indicato per superare questa limitazione è la scelta di un approccio litografico alternativo come la litografia a nanostampa (Nanoimprint Lithography,NIL), che ha come vantaggi un alto volume di produzione e dei costi più ridotti. Comunque, esistono soltanto pochi studi sull’uso della NIL per il patterning di superfici di GaAs. Nella maggior parte dei casi, viene usata la variante dell’UV-NIL, che richiede l’uso addizionale di una maschera di SiO2, con conseguente complicazione del processo. In questo lavoro, abbiamo utilizzato la forma di NIL più semplice, che non richiede alcun processo addizionale (quali il trattamento in UV o uno strato di SiO2). L’attacco chimico del GaAs è stato effettuato tramite wetetcìhing, per ottenere superfici prive di difetti, ed è stato ottimizzato per ottenere nanopori con le dimensioni laterali e la profondità desiderate. I substrati patternati di GaAs, dopo un processo di pulizia, vengono usati per la crescita controllata di QD nell’MBE. Abbiamo ottimizzato i protocolli di crescita per migliorare le proprietà strutturali ed ottiche dei QD, con lo scopo di migliorare le prestazioni di dispositivi optoelettronici. Abbiamo esaminato l’effetto del desorbimento dell’ossido superficiale a bassa temperatura tramite fasci di gallio (tecnica già applicata con successo in precedenza)e l’effetto della crescita sul substratodi uno strato di GaAs sulla forma e dimensione dei buchi, per migliorare l’occupazione di QD singoli nei buchi stessi. Abbiamo adottato un approccio alternativo per rimuovere l’ossido superficiale usando fasci di indio. Questo approccio è stato adottato siccome l’eventuale indio in eccesso può essere desorbito facilmente dalla superficie di GaAs scaldando a temperature che non alterino il profilo dei buchi. Inoltre, abbiamo osservato che nei nostri array patternati il desorbimento tramite fasci di indio ha anche l’effetto benefico di preservare meglio la forma dei buchi dopo la deposizione dello strato di GaAs, il che è di grande aiuto per migliorare l’occupazione di QD singoli e l’uniformità dei QD. Abbiamo usato la spettroscopia di fotoluminescenza (photoluminescence, PL) a bassa temperatura per esaminare le proprietà ottiche dei punti di InAs/GaAs. I risultati sono confrontabili o migliori, rispetto ai pochi studi simili effettuati su punti cresciuti su substrati definiti con la NIL, ed hanno il potenziale di raggiungere l’allargamento spettrale non-omogeneo ottenuto su QD controllati ottimizzati realizzati con la tecnica EBL. Nella seconda parte della tesi, abbiamo esteso questa tecnica alla crescita selettiva di punti InAs/GaAs su silicio patternato. Il silicio è il materiale principale per i dispositivi a semiconduttore (90%). Comunque, il gap di energia indiretto del silicio limita la realizzazione di dispositivi optoelettronici efficienti. D’altra parte, a causa del loro gap in molti casi diretto, i semiconduttori III-V hanno proprietà ottiche ed optoelettroniche eccellenti. L’integrazione di semiconduttori composti III-V su Si ha una grande prospettiva per la realizzazione di circuiti fotonici integrati. Tuttavia, l’integrazione GaAs/Si è limitata fortemente dalla loro differenza del 4% nel parametro reticolare, che induce dislocazioni e difetti nel materiale cresciuto sul substrato di silicio e previene l’emissione di luce. Svariati approcci sono stati esplorati per l’integrazione III-V/Si, quali la crescita di nanofili, la fusione dei substrati, l’utilizzo di strati spessi di rilassamento e l’applicazione di composizioni di materiali per accomodare i parametri reticolari. La crescita di strati spessi aumenta la complessità dei processi ed il costo dei materiali. Un’alternativa sarebbe quindi la crescita diretta di nanostrutture III-V su silicio. La crescita di QD III-V su substrati Si patternati dovrebbe aiutare a ridurre i difetti a causa di effetti di taglia e rilassamento della tensione laterale. Tuttavia, ad oggi esistono solo pochi studi su substrati di Si patternati, e la maggior parte includono maschere di SiO2con grosse periodicità dei buchi (1 µm) e piccole aree patternate (1 mm2) definite tramite EBL. In questo lavoro abbiamo cresciuto nanostrutture III-V direttamente su substrati patternati di Si con array di buchi densi (periodo 300 nm) e grandi aree patternate attraverso la tecnica NIL. Abbiamo ottimizzato l’attacco chimico del SI e la passivazione della sua superficie tramite terminazione in idrogeno, allo scopo di facilitare il desorbimento dell’ossido nella camera MBE a temperature relativamente basse. Il desorbimento dell’ossido ed i protocolli di crescita sono stati ottimizzati allo scopo di ottenere array di punti uniformi. Abbiamo usato la spettroscopia EDS (Energy Dispersive X-rayspectroscopy) per verificare la presenza di InAs e GaAs solo in corrispondenza dei buchi. Verranno anche mostrati risultati preliminari di spettroscopia PL per controllare le proprietà ottiche dei punti InAs/GaAs.
Abstract The fabrication of self-assembled quantum dots (QDs) is a topic of high current interest due to their vast applications in optical devices. In this research work, our aim is to obtain highly uniform arrays of semiconductor QDs to reach a complete control on their spatial distribution and a superior spectral uniformity, with respect to conventional self-assembled dots. The method consists of a combined top-down and bottom-up approach: self-assembled QDs are grown by molecular beam epitaxy (MBE) on a pre-patterned surface with a regular array of holes. In the first part of this thesis work, we have optimized the growth of InAs dots on patterned GaAs substrates. In most of the research efforts, GaAs substrates are patterned through electron beam lithography (EBL), which is not the optimal technique for application in broad-area devices, due to its serial nature and high cost. The finest way to overcome this limitation can be through choosing an alternative lithographic approach like nanoimprint lithography (NIL) for patterning of the GaAs surfaces, which has the advantage of high throughput and low cost. However, there are only few studies available that have used NIL for the patterning of GaAs surfaces. In most of the cases, instead of NIL, UV-NIL is being used for patterning, that requires an additional layer of SiO2 for masking, which also complicates the process. In this work, we have tried to use the simplest form of NIL for patterning, which requires no additional processing (like UV treatment or SiO2 layer). Wet etching process is chosen for GaAs etching to get defect-free surfaces, and is optimized to get the nanopores with required lateral dimensions and depth. These patterned GaAs substrates after optimizing all cleaning procedures are used for further growth of site-controlled QDs in MBE. We have optimized the growth protocols to improve the structural and optical properties of the dots, with the aim of improving the performance of optoelectronic devices. We have examined the effect of the low-temperature oxide desorption by means of Ga beams (which was already applied successfully to patterned GaAs surfaces) and the effect of the GaAs buffer layer growth on the hole shape and size, to improve the single-dot occupancy of the patterned holes. We have adopted an alternative approach to remove the oxide layer using In beams. This approach have been adopted because excess In can be easily desorbed from GaAs surface just by heating it to temperatures that do not alter the hole profiles. Furthermore, we have observed that for our patterned arrays In-assisted desorption has also the beneficial effect to better preserve the hole shape after the growth of the GaAs buffer layer, which ultimately helps in improving the single-dot occupancy, as well as the structural uniformity of the dots. We have used low temperature photoluminescence (PL) spectroscopy to assess the optical properties of InAs/GaAs dots. The results compare favourably with the few similar dot arrays previously grown on NIL-defined patterns, and have the potential to match the inhomogeneous broadening reported for optimized site-controlled dots on EBL-defined patterns. In the second part of the thesis, we have extended this technique to the selective growth of InAs/GaAs QDs on patterned silicon. Silicon is the main material for semiconductor devices (90%). However, the indirect bandgap of silicon prevents the realization of efficient light emitting devices. On the other hand, due to their direct bandgap in many cases, III-V semiconductors have excellent optical properties and optoelectronic capabilities. Integration of III-V compound semiconductor with Si has a broad prospective for the realization of photonic integrated circuits. However, GaAs/Si integration is largely limited by their 4% lattice mismatch, which induces dislocations and defects in the grown material on the Si substrate and ultimately prevents light emission. Several approaches are under exploration for III-V/Si integration like nanowire growth, wafer fusion techniques, using thick relaxation layers and applying lattice matched material compositions. Growth of these buffer layers increases the process complexity and material cost. A perfect alternative would thus be the direct epitaxial growth of III-V nanostructures on silicon. Growth of III-V quantum dots on pre-patterned Si substrates should help to reduce defects because of size effect and effective lateral stress relaxation due to the presence of facet edges and side walls. However, there are limited research efforts available on patterned Si substrates including mostly on SiO2 as mask with large periods (1µm) and small patterned areas (1mm2) defined by EBL. In our work, we have grown III-V nanostructures directly on patterned Si substrates with dense hole arrays (period 300nm) and larger patterned areas through nanoimprint lithography. We optimized the dry etching of Si and its surface passivation with H-termination, in order to facilitate oxide desorption in the MBE at relatively low temperatures. Oxide desorption and growth protocols were optimized in order to obtain uniform dot arrays. We used energy-dispersive X-ray spectroscopy (EDS) analysis for the characterization of InAs/GaAs QDs to verify the presence of GaAs and InAs only at the hole location. We will also show preliminary results using photoluminescence spectroscopy to assess the optical properties of InAs/GaAs dots.
XXVI Ciclo
1976

Near-field optical microscopy has attracted remarkable attention, as it is the only technique that allows the investigation of local optical properties with a resolution far below the diffraction limit. Especially, the scattering-type near-field optical microscopy allows the nondestructive examination of surfaces without restrictions to the applicable wavelengths. However, its usability is limited by the availability of appropriate light sources. In the context of this work, this limit was overcome by the development of a scattering-type near-field microscope that uses a widely tunable free-electron laser as primary light source. In the theoretical part, it is shown that an optical near-field contrast can be expected when materials with different dielectric functions are combined. It is derived that these differences yield different scattering cross-sections for the coupled system of the probe and the sample. Those cross-sections define the strength of the near-field signal that can be measured for different materials. Hence, an optical contrast can be expected, when different scattering cross-sections are probed. This principle also applies to vertically stacked or even buried materials, as shown in this thesis experimentally for two sample systems. In the first example, the different dielectric functions were obtained by locally changing the carrier concentration in silicon by the implantation of boron. It is shown that the concentration of free charge-carriers can be deduced from the near-field contrast between implanted and pure silicon. For this purpose, two different experimental approaches were used, a non-interferometric one by using variable wavelengths and an interferometric one with a fixed wavelength. As those techniques yield complementary information, they can be used to quantitatively determine the effective carrier concentration. Both approaches yield consistent results for the carrier concentration, which excellently agrees with predictions from literature. While the structures of the first system were in the micrometer regime, the capability to probe buried nanostructures is demonstrated at a sample of indium arsenide quantum dots. Those dots are covered by a thick layer of gallium arsenide. For the first time ever, it is shown experimentally that transitions between electron states in single quantum dots can be investigated by near-field microscopy. By monitoring the near-field response of these quantum dots while scanning the wavelength of the incident light beam, it was possible to obtain characteristic near-field signatures of single dots. Near-field contrasts up to 30 % could be measured for resonant excitation of electrons in the conduction band of the indium arsenide dots
Die optische Nahfeldmikroskopie hat viel Beachtung auf sich gezogen, da sie die einzige Technologie ist, welche die Untersuchung lokaler optischer Eigenschaften mit Auflösungen unterhalb der Beugungsgrenze ermöglicht. Speziell die streuende Nahfeldmikroskopie erlaubt die zerstörungsfreie Untersuchung von Oberflächen ohne Einschränkung der verwendbaren Wellenlängen. Die Nutzung ist jedoch durch das Vorhandensein entsprechender Lichtquellen beschränkt. Im Rahmen dieser Arbeit wurde diese Beschränkung durch Entwicklung eines streuenden Nahfeldmikroskops überwunden, das einen weit stimmbaren Freie-Elektronen-Laser als primäre Lichtquelle benutzt. Im theoretischen Teil wird gezeigt, dass ein optischer Kontrast erwartet werden kann, wenn Materialien mit unterschiedlichen Dielektrizitätskonstanten kombiniert werden. Es wird hergeleitet, dass diese Unterschiede in unterschiedlichen Streuquerschnitten für das gekoppelte System aus Messkopf und Probe resultieren. Diese Streuquerschnitte definieren die Stärke des Nahfeldsignals, welches auf unterschiedlichen Materialien gemessen werden kann. Ein optischer Kontrast kann also erwartet werden, wenn unterschiedliche Streuquerschnitte untersucht werden. Dass dieses Prinzip auch auf übereinander geschichtete oder sogar verborgene Strukturen angewendet werden kann, wird in dieser Doktorarbeit an zwei Probensystemen experimentell gezeigt. Im ersten Beispiel wurden die unterschiedlichen Dielektrizitätskonstanten durch örtliches Ändern der Ladungsträgerdichte in Silizium durch Bor-Implantation erreicht. Es wird gezeigt, dass die Dichte der freien Ladungsträger an Hand des optischen Kontrastes zwischen implantiertem und reinem Silizium ermittelt werden kann. Zu diesem Zweck wurden zwei unterschiedliche Ansätze verwendet, ein nicht-interferometrischer mittels variabler Wellenlängen und ein interferometrischer mit einer konstanten Wellenlänge. Weil diese Techniken gegensätzliche Informationen liefern, können sie genutzt werden, um die effektive Ladungsträgerdichte quantitativ zu bestimmen. Beide Ansätze lieferten konsistente Resultate für die Trägerdichte, welche sehr gut mit den Vorhersagen der Literatur übereinstimmt. Während die Strukturen im ersten Beispiel im Mikrometer-Bereich lagen, wird die Möglichkeit, verborgene Nanostrukturen zu untersuchen, an Hand einer Probe mit Indiumarsenid Quantenpunkten demonstriert. Diese sind von einer dicken Schicht Galliumarsenid bedeckt. Zum ersten Mal wird experimentell gezeigt, dass Übergänge zwischen Elektronenzuständen in einzelnen Quantenpunkten mit Nahfeldmikroskopie untersucht werden können. Durch die Messung der Nahfeld-Antwort der Quantenpunkte unter Änderung der Wellenlänge des eingestrahlten Lichtes war es möglich, charakteristische Nahfeld-Signaturen der einzelnen Quantenpunkte zu erhalten. Nahfeld-Kontraste bis zu 30 Prozent konnten für die resonante Anregung der Elektronen im Leitungsband der Indiumarsenid Punkte beobachtet werden

La microélectronique rencontre des difficultés croissantes avec la miniaturisation des composants. La photonique sur silicium propose de les contourner en choisissant le photon comme vecteur de l'information, mais les sources de ces photons restent des verrous. Cette thèse s'est donc attachée à la réalisation par épitaxie par jets moléculaire en mode vapeur-liquide-solide et la caractérisation par spectroscopie de photoluminescence (PL) de boîtes quantiques en nanofils (NFs-BQ) d'InP/InAs crûs sur silicium orienté (111), dans le but d'une intégration monolithique de sources lumineuses. Des NFs d'InP de phase cristalline pure wurtzite ont d'abord été crûs verticaux sur Si(111), à partir d'un catalyseur sous forme de gouttelettes or-indium. La formation préalable d'un piédestal d'InP par la cristallisation de ces gouttelettes, ainsi que la migration de l'or au sommet de ce piédestal pour catalyser la croissance, ont été mise en évidence. Le diamètre de ces NFs a ensuite été augmenté pour qu'ils se comportent comme un matériau massif du point de vue des propriétés optomécaniques. Ils ont été soumis à une pression hydrostatique allant jusqu'à quelques GPa pour déterminer des paramètres mal connus de l'InP Wz. L'optimisation de la croissance du système NF-BQ d'InP/InAs a ensuite été réalisée. Des BQs de hauteur variable ont été obtenues, avec des interfaces très abruptes. Les études de PL sur un ensemble de NFs-BQ montrent des spectres plus ou moins complexes suivant la hauteur des BQs, ainsi qu'une polarisation de l'émission accordable avec cette hauteur. Le dernier objectif a été d'améliorer l'efficacité de l'émission des NFs-BQs d'InP/InAs grâce à l'effet photonique d'une coquille en silicium amorphe (a-Si). Les études de PL ont révélé une forte perte d'intensité de PL et la disparition de l'anisotropie de polarisation de l'émission des NFs-BQ après dépôt. Plusieurs raisons sont discutées pour expliquer ceci
Microelectronics encounter growing issues with components miniaturization. Silicon photonics offer to avoid them by taking the photon as the information carrier, but the sources are challenging to make. This thesis thus focused on the realization by vapor-liquid-solid assisted molecular beam epitaxy and the characterization by photoluminescence spectroscopy (PL) of InAs/InP quantum dots in nanowires (QD-NW) on (111) oriented silicon, with the aim of monolithic integration of light sources. Pure wurtzite InP NWs have first been vertically grown on Si(111) with a gold-indium droplet catalyst. The preliminary formation of InP pedestals by the crystallization of the droplets, and the migration of gold at the top of the pedestals to catalyze the growth, have been evidenced. The NWs diameter has then been increased so they behave as bulk InP regarding optomechanical properties. The NWs have been put under hydrostatic pressure to several GPa to determine little known InP wurtzite parameters. The growth optimization of the InAs/InP QD-NW system has then been realized. QDs with various height and very sharp interfaces have been obtained. PL studies show more or less complex spectra, according to the QDs' height, as well as a height-tunable polarization. The last goal was to enhance the efficiency of the InAs/InP QD-NWs thanks to the photonic effect brought by an amorphous silicon shell. PL studies revealed a high signal loss and the disappearance of the polarization anisotropy of the QD-NWs emission after deposition. Several hypothesis are discussed

Ce travail de thèse porte sur les propriétés structurales, optiques et électriques de nanostructures et alliages à base de GaP pour la photonique intégrée sur silicium. Parmi les méthodes d’intégration des semi-conducteurs III-V sur Si, l’intérêt de l’approche GaP/Si est tout d’abord discuté. Une étude de la croissance et du dopage de l’AlGaP est présentée afin d’assurer le confinement optique et l’injection électrique dans les structures lasers GaP. Les difficultés d’activation des dopants n sont mises en évidence. Ensuite, les propriétés de photoluminescence des boites quantiques InGaAs/GaP sont étudiées en fonction de la température et de la densité d’excitation. Les transitions optiques mises en jeu sont identifiées comme étant des transitions indirectes de type-I avec les électrons dans les niveaux Xxy et les trous dans les niveaux HH des boites quantiques InGaAs et de type-II avec les électrons dans les niveaux Xz du GaP contraint. Malgré une modification notable de la structure électronique de ces émetteurs, une transition optique directe et type I n’est pas obtenue ce qui reste le verrou majeur pour la promotion d’émetteurs GaP sur Si. La maitrise de l’interface GaP/Si et de l’injection électrique est par ailleurs validée par la démonstration de l’électroluminescence à température ambiante d’une LED GaPN sur Si. Si l’effet laser n’est pas obtenu dans les structures lasers rubans GaP, un possible début de remplissage de la bande Гdans les QDs est discuté. Enfin, l’adéquation des lasers à l’état de l’art avec les critères d’interconnections optiques sur puce est discutée
This PhD work focuses on the structural, optical, electrical properties of GaP-based nanostructures and alloys for integrated photonics on silicon. Amongst the integration approaches of III-V on Si, the interest of GaP/Si is firstly discussed. A study of the growth and the doping of AlGaP used as laser cladding layers (optical confinement and electrical injection) is presented. The activation complexity of n-dopants is highlighted. Then, the photoluminescence properties of InGaAs/GaP quantum dots are investigated as a function of temperature and optical density. The origin of the optical transitions involved are identified as (i) indirect type-I transition between electrons in Xxy states and holes in HH states of quantum dots InGaAs and (ii) indirect type-II with electrons in Xz states of strained GaP. Despite an effective modification in the electronic structure of these emitters, a direct type I optical transition is not demonstrated. This is the major bottleneck in the promotion of GaP based emitters on Si. This said, the control of the GaP/Si interface and electrical injection are confirmed by the demonstration of electroluminescence at room temperature on Si. If no laser effect is obtained in rib laser architectures, a possible beginning of Г band filling in QDs is discussed. Finally, the adequacy of state of the art integrated lasers with the development of on-chip optical interconnects is discussed

Dans cette thèse, je me suis interessé à la description ab initio du processus de génération de seconde harmonique (SHG), qui est une propriété optique non-linéaire des matériaux, et je me suis concentré sur les systèmes quantiques confinés, à base de silicium. Ces dernières années, les études ab initio ont suscité un grand intérêt pour l'interprétation et la prévision des propriétés des matériaux. Il est indispensable d'améliorer la connaissance des processus non-linéaires et de proposer une description de SHG, à partir des premiers principes. En raison de difficultés importantes, la description de l'optique non linéaire n'a pas encore atteint la précision des phénomènes linéaires. L'état de l'art des calculs ab initio SHG est représenté par l'inclusion des effets à plusieurs corps comme les champs locaux (LF) et l'interaction électron-trou, mais aujourd'hui, l'approche la plus utilisée est l'approximation de particules indépendantes (IPA), la seule en mesure d'aborder les calculs de structures complexes, tels que des surfaces et des interfaces. Alors que IPA peut être une bonne approximation pour les systèmes massifs, dans des matériaux discontinus d'autres effets peuvent être prédominants. L'objectif de ma thèse est de donner une analyse du processus de SHG dans des systèmes complexes comme les interfaces et les systèmes confinés à base de silicium, d'inférer de nouvelles connaissances sur le mécanisme physique mis en jeu et son lien avec la nature du système. J'utilise un formalisme fondé sur la théorie de la fonctionnelle de la densité dépendant du temps (TDDFT) où les effets à plusieurs corps sont inclus par un choix approprié des noyaux de la TDDFT. Le formalisme et le code ont été développés au cours de mon travail, permettant l'étude de matériaux complexes. Mes recherches ont porté sur l'étude de l'interface Si (111)/CaF2 (de type B,T4). Des études de convergence montrent l'importance du matériau semi-conducteur par rapport à l'isolant. La réponse est caractéristique d'une région profonde au-delà de l'interface Si, alors que CaF2 converge rapidement juste après l'interface. La réponse montre une sensibilité aux modifications électroniques, induites dans des états bien en-dessous de l'interface, et non à la structure ionique du silicium, qui retrouve rapidement la configuration du matériau massif. Une procédure de normalisation pour comparer avec l'expérience a été proposée. Les spectres de SHG ont été calculés en IPA, et en introduisant les interactions de champs locaux et excitoniques. De nouveaux comportements ont été observés par rapport aux processus SHG dans GaAs ou SiC, montrant l'importance des effets de champ locaux cristallins. Alors que IPA décrit la position des pics principaux de SHG et que les effets excitoniques modifient légèrement l'intensité totale, seuls les champs locaux reproduisent la forme spectrale et les intensités relatives des pics. Cela souligne combien les effets des différents acteurs dans le processus dépendent de la nature des matériaux. De nouvelles méthodes d'analyse de la réponse ont été proposées: en effet, le lien direct entre la position des pics et les énergies de transition est perdu dans les calculs de SHG : le signal provient d'une équation de Dyson du second ordre où les fonctions de réponse linéaires et non-linéaire pour des fréquences différentes sont mélangées. En outre, la complexité du matériau m'a permis d'obtenir des informations sur une grande variété de systèmes comme les multicouches et les couches de silicium confinées. Les résultats montrent un bon accord avec l'expérience, confirmant la structure de l'interface proposée. Cela souligne la précision du formalisme, la possibilité d'améliorer nos connaissances sur ces matériaux complexes. Les simulations ab-initio de SHG peuvent être utilisées comme une technique prédictive, pour soutenir et guider les expériences et les développements technologiques. Les résultats préliminaires sur les structures Si/Ge sont présentés
In this thesis I have dealt with the ab initio description of the second-harmonic generation (SHG) process, a nonlinear optical property of materials, focusing in particular on quantum confined, silicon-based systems. In the last decades, the accuracy and possibilities of ab initio studies have demonstrated a great relevance in both the interpretation and prediction of the materials properties. It is then mandatory to improve the knowledge of the nonlinear optical processes as well as the SHG first-principle description. Nowadays, due to nontrivial difficulties, nonlinear optics has not yet reached the accuracy and development of linear phenomena. In particular, the state of the art of ab initio SHG calculations is represented by the inclusion of many-body effects as crystal local fields (LF) and electron-hole interaction, but today, the mostly used approach is the independent particle approximation (IPA), the only one able to approach calculations of complex structures such as surfaces and interfaces. Whereas IPA can be a good approximation for bulk systems, in discontinuous materials other effects may be predominant. Hence their description is of great relevance although the lack of studies. My thesis tries to give a first analysis of the SHG process in more complex systems as the interfaces and the Si-confined systems, inferring new insights on the physical mechanism and its link with the nature of the system. I use an efficient formalism based on the Time Dependent Density Functional Theory (TDDFT) where many-body effects are included via an appropriate choice of the TDDFT kernels. Both the formalism and the code have been developed during the thesis work permitting the study complex materials. The research has been focused on the Si(111)/CaF2 (T4 B-type) interface case study. Convergence studies show the importance of the semiconductor material with respect to the insulator. The response is characteristic of a deep region beyond the Si interface whereas the CaF2 converges soon after the first interface layers. Moreover, the signal demonstrates to be sensitive to the electronic-states modifications that are induced far below the interface, and not to the Si ionic structure that recovers soon the bulk configuration. A normalization procedure to compare with the experiment has been proposed. The SHG spectra have been calculated in the IPA, introducing LF and excitonic interactions. New behaviors have been observed with respect to the SHG processes on strained silicon, GaAs or SiC showing in particular the importance of crystal local-field effects with respect to both the IPA and the excitons. Whereas IPA can describe the position of the SHG main peaks and the excitonic effects slightly modify the total intensity, only LF are able to correctly reproduce the spectral shape and the relative intensities of the peaks. This underlines how SHG and the different involved effects depends on the nature of the materials. New methods of analysis of the response have been proposed; actually, the direct link between the peaks position and the transition energies is lost in SHG calculations (i. E. The signal comes from a second order Dyson equation where linear and nonlinear response functions at different frequencies are mixed together). Furthermore, the complexity of the system allowed me to extend the study to a large variety of materials as the multilayers and the silicon confined slabs. The results show a good agreement with the experiment confirming the proposed T4 B-type interface structure. This underlines the accuracy of the formalism, the possibility of improving our knowledge on these complex materials going beyond the standard approaches, and confirms the possibility of SHG ab-initio simulations to be employed as a predictive technique, supporting and guiding experiments and technological developments. Preliminary results on Si/Ge superlattice are presented

The photoluminescent properties of silicon quantum dots embedded in a stabilizing polymer matrix are relevant to a number of potential applications of these unique nanomaterials such as drug delivery, temperature sensing, and photovoltaics. Aspects of how these photoluminescent properties change with respect to variations in such parameters as excitation intensity, polymer interactions, particle size and particle polydispersity are investigated here. Improving the photostability and understanding the nature of how this is achieved will be critical for realizing the potential of silicon quantum dots in a number of applications. Improvements in photoluminescent stability related to fluorescence intermittency, radiative lifetime, emitted intensity, and wavelength shifts are shown to be due to decreased exposure to oxygen, increased particle packing, decreased temperature, and increased monodispersity of the quantum dots.

Les propriétés des quantum dots (QDs) en font des sondes adaptées à la reconnaissance moléculaire. Leur pic d’émission en fluorescence est très étroit et ajustable, tandis que la section efficace de leur spectre d’absorption est très large. En outre, ils sont très brillants et résistent mieux au photoblanchiment que les colorants organiques conventionnels.Notre objectif a été de concevoir un nouveau type de sondes fluorescentes pour une détection rapide à l’échelle de la molécule unique. L’utilisation d’agrégats contenant plusieurs milliers de QDs, plutôt que celle de QDs individuels, permet d’accroître le signal de fluorescence et de simplifier les modalités de détection. La morphologie et la chimie de surface des premiers agrégats préparés n’ont pas pu être contrôlées en les recouvrant avec des molécules de surfactants courts ou une couche de polymère en solution aqueuse. La stratégie centrale de ce manuscrit a permis d’assembler les QDs en nanobilles (NBs) monodisperses de quelques centaines de nanomètres de diamètre, encapsulées dans une couche de silice Stöber. Leur stabilité colloïdale et leur photostabilité ont ainsi été conservées. Un nouveau type d’hybride polymère-silane a été greffé sur la silice. Il présente des chaînes zwittérioniques, garantissant la solubilité en milieu aqueux et une adsorption non spécifique minimale, ainsi que des fonctions réactives pour la bioconjugaison. La réactivité de NBs fonctionnalisées par de la streptavidine avec des billes commerciales biotinylées a été démontrée. Nos résultats préliminaires ont également montré que les NBs peuvent être intégrées dans un dispositif microfluidique pour être comptées individuellement
Using nanotechnology for molecular diagnostics holds many advantages e.g. an improvement in the simplicity and the sensitivity of analysis. Semi-conductor nanocrystals or quantum dots (QDs) demonstrate several unique properties that make them suitable probes for biomolecular recognition. These QDs present narrow size-tunable emission spectra and a broad excitation spectrum; in addition, they offer higher photostability and brightness than conventional organic dyes. Our aim was to design a new diagnostic probe based on fluorescent nanobeads containing QDs, envi-sioned as a tool for fast and single-molecule detection. An even brighter fluorescence and easily detectable analytical signals could indeed be achieved by aggregating several thousand of QDs together, as compared to single QDs. Coating QD clusters with small surfactants or a polymer layer didn’t provide morphological control or a suitable surface chemistry for bioconjugation. The strategy that we developed consists in self-assembling QDs into monodisperse nanobeads of a few hundreds of nanometers in diameter, on top of which a silica shell was grown by a Stöber-inspired process. This allowed us to protect their colloidal and photo-stability. A new type of multidentate polymer-silane hybrid was subsequently grafted onto the silica shell, presenting a zwitterionic chain for water solubility and antifouling, as well as reactive functions for conjugation with biomolecules. We succeeded in reacting streptavidin-conjugated nanobeads with commercial biotinylated beads. Preliminary results have also shown that we can integrate the nanobeads into a microfluidic system for an efficient single-particle counting

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 107-116).
This thesis demonstrates and evaluates the potential application of luminescent quantum dot/polymer solutions on crystalline silicon photovoltaics. After spin coating the QD/polymer onto silicon photodiodes, an increase of 3% in current density was observed. This performance improvement was used to determine the impact application would have on the crystalline silicon photovoltaic supply chain. Supply chain costs were modeled to estimate the segment costs for Sharp's NUU230F3 230W module. The benefits realized by use of cells coated with the QD/polymer solution were then estimated at both the module and the cell segments. Finally, an installation cost model for the residential market was built to determine the impact an increase in efficiency had on total system costs.
by Garrett Bruer.
M.Eng.

We present the development of a novel, UHV-compatible device fabrication strategy for the realisation of nano- and atomic-scale devices in silicon by harnessing the atomic-resolution capability of a scanning tunnelling microscope (STM). We develop etched registration markers in the silicon substrate in combination with a custom-designed STM/ molecular beam epitaxy system (MBE) to solve one of the key problems in STM device fabrication ??? connecting devices, fabricated in UHV, to the outside world. Using hydrogen-based STM lithography in combination with phosphine, as a dopant source, and silicon MBE, we then go on to fabricate several planar Si:P devices on one chip, including control devices that demonstrate the efficiency of each stage of the fabrication process. We demonstrate that we can perform four terminal magnetoconductance measurements at cryogenic temperatures after ex-situ alignment of metal contacts to the buried device. Using this process, we demonstrate the lateral confinement of P dopants in a delta-doped plane to a line of width 90nm; and observe the cross-over from 2D to 1D magnetotransport. These measurements enable us to extract the wire width which is in excellent agreement with STM images of the patterned wire. We then create STM-patterned Si:P wires with widths from 90nm to 8nm that show ohmic conduction and low resistivities of 1 to 20 micro Ohm-cm respectively ??? some of the highest conductivity wires reported in silicon. We study the dominant scattering mechanisms in the wires and find that temperature-dependent magnetoconductance can be described by a combination of both 1D weak localisation and 1D electron-electron interaction theories with a potential crossover to strong localisation at lower temperatures. We present results from STM-patterned tunnel junctions with gap sizes of 50nm and 17nm exhibiting clean, non-linear characteristics. We also present preliminary conductance results from a 70nm long and 90nm wide dot between source-drain leads which show evidence of Coulomb blockade behaviour. The thesis demonstrates the viability of using STM lithography to make devices in silicon down to atomic-scale dimensions. In particular, we show the enormous potential of this technology to directly correlate images of the doped regions with ex-situ electrical device characteristics.

Nanomaterials have attracted considerable attention during the past decades due to their unique and fascinating properties. However, this class of materials is not an invention of modern age. People have been using nanomaterials for centuries, although unwittingly. Probably the most famous example for the usage of nanomaterials in ancient times is the Lycurgus Cup, a Roman glass cage cup created in the 4th century which changes the colour of its glass from green to ruby depending on the illumination conditions. The foundation for the development of the field of nanotechnology was laid by the speech of Feynman “There is plenty of room at the bottom” in 1959, in which he spoke about the principles of miniaturisation as low as to the atomic level. Today, modern nanotechnology made it its business to purposefully develop and synthesise nanomaterials as well as to face their applications in various fields, such as microelectronics, catalysis or biomedicine. However, the term “nanomaterials” does not solely involve the nanoparticulate units itself, but also their arrangement into two- or three-dimensional structures. Thereby, the maintenance of the nanoscale properties is one of the main challenges. This task was focussed by this work implied the preparation and macroscale arrangement of fluorescent QDs while preserving their optical properties. The main achievement of this work was the development of a novel aerogel material with non-quenching PL behaviour by using silica coated QDs as nanoparticulate building units. In comparison to other monolithic silica-QD structures or aerogels from pure QDs, a defined and controllable distance between the fluorescent QDs is provided in these structures by the silica shell. The spacing was shown to efficiently disable energy transfers so that no spectral shifts, lifetime shortening or PL QY losses are observed during the colloid to gel transition. The silica shell, established by a standard reverse microemulsion approach, was found to exhibit a certain porosity, which was proven by gas adsorption measurements. Existing cavities in the micro- and mesoporous range were found to allow small species such as metal ions to pass through the shell and interact with the QD core causing a detectable change of the PL intensity, which makes these materials suitable for future sensing applications. The gel preparation was based on a metal ion assisted complexation approach, which requires tetrazole functionalisation of the nanoparticulate building units. A major development in this work that permitted this gelation approach for silica-QDs was the development of a novel tetrazole-silane ligand. TMSPAMTz was specifically designed to bind to the silica surface of silica-QDs in aqueous solution and was prepared by a covalent coupling of an alkyl chained silane with a 5-subsituted tetrazole ring. Network formation is subsequently achieved by the interconnection of negatively charged tetrazole rings with metal ions, which allows for a broad spectrum of aerogel materials from different NP species as well as their mixtures as long as tetrazole capping is provided. Considering this diversity and the disabling of energy transfers, straightforward colour tuning was demonstrated herein by mixing differently emitting silica-QD species which gives great prospects for lighting applications. Furthermore, the possibility of plasmon enhanced emission was presented for mixed Au NP/silica-QD gels. With respect to future sensing applications, thin porous films from silica-QDs gels were prepared, which showed a promising concentration dependant PL quenching for the model analyst hydrogen peroxide. However, the film reproducibility of the applied drop-cast coating method was insufficient. As a suggestion to this, a LbL method was presented, wherein a gel is subsequently grown with the metal ion assisted complexation approach. In addition to the tetrazole ligands on the NP surface, tetrazole-silane ligands were used in this approach to functionalise the glass substrate surface. By this, homogeneous gel films of distinct thickness can be grown while the use of organic polymers can be completely avoided. Besides the preparation of NP assemblies, standard Cd-based QD materials as well as Au NPs of different sizes and shape, recent progresses in the synthesis of InP-based QDs were presented in this work. A thorough investigation and understanding of the growth influencing parameters allowed for the establishment of preparation routes for In(Zn)P/GaP/ZnS core/shell/shell QDs with emission wavelengths tuneable within a large range from 500 to 650 nm, narrow peak widths of 45 to 70 nm and PL QYs up to 60%. Successful incorporation of these QDs into salt matrices was further demonstrated. The resulting composite materials are very photostable and suitable as colour conversion materials for solid state lighting, as was clearly pointed out by a self-prepared WLED that met the standard commercial LEDs.