Rozprawy doktorskie na temat „Quantum Confinement Effect (QCE)”

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999.
Includes bibliographical references (p. 161-165).
The thermoelectric figure of merit (Z) determines the usefulness of a material for thermoelectric energy conversion applications. Since the 1960's, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at a temperature ofT = 300 K. The advancement of nano-scale technologies has opened up the possibility of engineering materials at nano-scale dimensions to achieve low-dimensional thermoelectric structures which may be superior to their bulk forms. In this thesis, I established the basis of the low dimensional thermoelectric transport principle in the Si/Si1-xGex quantum well superlattice (two-dimensional) system and in the Bi quantum wire (one-dimensional) system. In bulk form, Si1_xGex is a promising thermoelectric material for high temperature applications. The Si/Si1 _xGex quantum well superlattice structures are studied based on their electronic band structures using semiclassical transport theory. Detailed subband structures are considered in an infinite series of finite height quantum wells and barriers. A significant enhancement of the thermoelectric figure of merit is expected. Based on my calculations, experimental studies are designed and performed on MBE grown Si/Sii -xGex quantum well superlattice structures. The experimental results are found to be consistent with theoretical predictions and indicate a significant enhancement of Z within the quantum wells over bulk values. The bismuth quantum wire system is a one-dimensional (ID) thermoelectric system. Bismuth as a semimetal is not a good thermoelectric material in bulk form becamm of the approximate cancellation between the electron and hole contributions to the Seebeck coefficient. However, quantum confinement can be introduced by making Bi nanowires to yield a ID semiconductor. ID transport properties are calculated along the principal crystallographic directions. By carefully tailoring the Bi wire size and carrier concentration, substantial enhancement in Z is expected. A preliminary experimental study of Bi nanowire arrays is also presented.
by Xiangzhong Sun.
Ph.D.

Esta tese de doutorado tem por objetivo aprofundar as pesquisas realizadas no mestrado, a saber, da caracterização e estudo das propriedades estruturais e ópticas de filmes de oxinitreto de silício (SiOxNy:H) ricos em silício depositados pela técnica de deposição química a vapor assistida por plasma a baixa temperatura (PECVD). Os resultados obtidos no mestrado indicaram que os filmes de SiOxNy:H ricos em silício apresentam emissão luminescente na faixa do visível cuja intensidade e freqüência de emissão estão em correlação com o excesso de silício. Os resultados sugeriram que o excesso de silício na matriz do SiOxNy:H estava disposto na forma de aglomerados de silício de dimensões nanométricas responsáveis por efeitos de tamanho quântico bem como a estados radiativos na interface dos aglomerados com a matriz isolante. Neste trabalho a fim de avaliar o efeito da separação de fases, do tamanho quântico, e da interface, foram produzidos sistemas nanoestruturados a base de silício com total e parcial separação de fases para caracterizar e analisar suas propriedades ópticas e estruturais e compará-las com as dos filmes ricos em silício. Assim foram produzidas multicamadas de a-Si:H de poucos nanômetros de espessura com materiais dielétricos. Em algumas destas multicamadas foi promovida a mistura parcial das camadas por meio de bombardeamento iônico. O estudo nas estruturas de multicamadas permitiu caracterizar e analisar as propriedades estruturais e ópticas de materiais nanoestruturados com total e parcial separação de fases para posteriormente contrastá-los com as características dos filmes de oxinitreto de silício ricos em silício. A fim de analisar a influência da interface nas propriedades ópticas destes sistemas as multicamadas foram fabricadas com dois dielétricos diferentes: o óxido de silício e o ni treto de silício. A espessura das camadas dielétricas foi mantida fixa entanto que a das camadas de silício foi variada para avaliar efeitos de confinamento no silício. A caracterização foi feita utilizando técnicas de absorção óptica no UV-Vis, absorção no infravermelho (FTIR), espectroscopia Raman, fotoluminescência (PL), espectroscopia de absorção de raios X próximos 7 à borda do silício (XANES), e microscopia eletrônica de transmissão de alta resolução (HRTEM). Da análise dos resultados concluiu-se que o confinamento é fundamental para a existência da emissão luminescente embora o tipo de interface influencie a energia e a intensidade da emissão. A análise comparativa com as multicamadas permitiu verificar que os filmes de oxinitreto de silício ricos em silício apresentam, separação parcial de fases já como depositados, os tratamentos térmicos promovem a segregação do silício aumentando conseqüentemente a separação de fases.
The aim of this doctorate thesis is to enhance the knowledge in the research conducted along the Master degree based on the characterization and study of the structural and luminescent properties of silicon rich silicon oxynitride films (SiOxNy:H) deposited at low temperature by Plasma Enhanced Chemical Vapor Deposition (PECVD). The results of this study indicated that silicon rich SiOxNy:H films present luminescence in the visible spectra range with intensity and frequency in correlation with the silicon excess. The results suggested that the silicon excess in the SiOxNy:H matrix is confined in nanometric silicon clusters responsible for the to quantum size effects as well as for radiactive states at the interface of the silicon clusters with the insulating matrix. In the present work in order to evaluate the effect of phase separation, quantum size and interface effects si licon based nanostructured systems presenting total and partial phase separation were produced and their structural and optical properties were characterized in order to correlate them with the silicon rich films ones. In this way multilayers with few nanometers thick a-Si layers with dielectric materials were produced. The mixture of the layers was promoted by ion bombardment in some of these multilayers. The study of these structures permitted the characterization of structural and optical properties of materials with total and partial phase separation with the purpose of comparing them to the silicon-rich silicon oxynitride films characteristics. In order to analyze the interface influence in the optical properties, multilayers systems with two different dielectric materials, silicon oxide and silicon nitride, were fabricated. The dielectric layer thickness was kept constant while the silicon layer was varied in order to study the confinement effect. The characterization was done utilizing UV-Vis optical absorption, infrared absorption (FTIR), Raman spectroscopy, Photoluminescence (PL), X-ray absorption near edge spectroscopy (XANES) and high-resolution transmission electron microscopy (HRTEM) techniques. From the results analysis it was concluded that confinement is essent ial for the existence of luminescent 9 emission although the type of interface also influences the energy and intensity of the emission. The comparative analysis with the multilayers permitted to verify that the silicon-rich silicon oxynitride films present, as deposited, partial phase separation and that the thermal treatments promotes silicon aggregation thus increasing the phase separation.

Mis en évidence expérimentalement au début des années 2010, les couples de spin-orbite (SOTs) ont très rapidement suscité un très fort intérêt dans la communauté du magnétisme et de l'électronique de spin. En effet, ils permettent,dans un empilement de type métal lourd / métal ferromagnétique / oxyde (HM/FM/Ox), de manipuler l'aimantation de la couche ferromagnétique (FM) en injectant un courant dans le plan des couches. En remarquant que la bicouche FM/Ox correspond à la moitié d'un empilement typique utilisé dans les cellules mémoires de types MRAM (Magnetic Random Access Memory, mémoire magnétique à accès aléatoire), on comprend que ce mécanisme est très intéressant pour l'écriture de la couche libre de ces cellules. En effet, le courant d'écriture ne traverse plus la barrière tunnel, ce qui répond naturellement à certaines limitations des MRAM actuelles. L'interprétation physique de ces phénomènes s'est cependant révélée particulièrement complexe. Ces couples ont deux composantes, généralement appelée "Field-like", FL, et "Damping-like", DL. Si dans un premier temps, les études théoriques ont prédit que la composante DL provenait principalement d'un effet de volume dans la couche HM,et que la composante FL provenait principalement d'un effet d'interface, des études expérimentales plus récentes ont montré qu'il n'était pas si simple de séparer ces deux contributions.Dans ce travail de thèse, nous avons choisi une approche originale permettant de n'étudier qu'une seule des deux contributions. Pour ce faire, nous avons choisi de nous concentrer sur la contribution interfaciale en étudiant des échantillons de types Ox1/FM/Ox2. Nous avons ainsi pu mettre en évidence dans ces empilements la présence de SOTs ce qui n'était textit{a priori} pas si évident dans une structure ne contenant pas de métal lourd et présentant de surcroît une forte symétrie. D'autre part, nous avons pu montrer que seule la composante FL de ces couples était présente. Le comportement inattendu de ce FL-SOT en fonction de l'épaisseur de la couche FM, nous a conduit à proposer un modèle basé sur la combinaison d'un effet interfacial de type Rahsba et d'un effet de confinement quantique dû à la très faible épaisseur de matériau conducteur dans ces empilements
Experimentally demonstrated in the early 2010's, spin-orbite torques (SOTs) very quickly generated a very strong interest in the magnetism and spin electronics community. Indeed, they allow, in a heavy metal / ferromagnetic metal / oxide (HM/FM/Ox) multilayer, to manipulate the magnetization of the ferromagnetic layer (FM) by injecting an in-plane current. Noting that the FM/Ox bilayer corresponds to half of a typical stack used in MRAM memory cells (Magnetic Random Access Memory), we understand that this mechanism is very interesting for writing the free layer of these cells. Indeed, the writing current no longer crosses the tunnel barrier, which naturally responds to some of the limitations of current MRAMs. However, the physical interpretation of these phenomena has proved to be particularly complex. These torques have two components, generally called "Field-like", FL, and "Damping-like", DL. While initially, theoretical studies predicted that the DL component was mainly due to a volume effect in the HM layer, and the FL component was mainly due to an interface effect, more recent experimental studies have shown that it is not so simple to separate these two contributions.In this thesis work, we have chosen an original approach that allows us to study only one of the two contributions. To do so, we have chosen to focus on the interfacial contribution by studying Ox1/FM/Ox2 samples. We were thus able to highlight in these stacks the presence of SOTs, which was not so obvious in a structure that did not contain heavy metal and also had a strong symmetry. On the other hand, we were able to show that only the FL component of these couples was present. The unexpected behaviour of this FL-SOT as a function of the thickness of the FM layer led us to propose a model based on the combination of a Rahsba interfacial effect and a quantum confinement effect due to the very thin thickness of conductive material in these multilayers

Nanotechnology is the science and engineering of creating functional materials by precise control of matter at nanometer (nm) length scale and exploring novel properties at that scale. It is vital to understand the quantum mechanical phenomena manifested at nanometer scale dimensions since that will enable us to precisely engineer quantum mechanical properties to realize novel device functionalities. This dissertation investigates optical and electronic properties of compound and transition metal doped compound semiconductor nanowires with a view to exploiting them for a wide range of applications in semiconductor electronic and optical devices. In this dissertation work, basic concepts of optical and electronic properties at low dimensional structures will be discussed in chapter 1. Chapter 2 discusses the nanofabrication technique employed to fabricate highly ordered nanowires. Using this method, which is based on electrochemical self-assembly techniques, we can fabricate highly ordered and size controlled nanowires and quantum dots of different materials. In Chapter 3, we report size dependent fluorescence spectroscopy of ZnSe and Mn doped ZnSe nanowires fabricated by the above method. The nanowires exhibit blue shift in the emission spectrum due to quantum confinement effect, which increases the effective bandgap of the semiconductor. We found that the fluorescence spectrum of Mn doped ZnSe nanowires shows high luminescence efficiency, which seems to increase with increasing Mn concentration. These results are highly encouraging for applications in multi spectral displays. Chapter 4 investigates field emission results of highly ordered 50 nm tapered ZnO nanowires that were also fabricated by electrochemical self-assembly. Subsequent to fabrication, the nanowires tips are exposed by chemical etching which renders the tips conical in shape. This tapered shape concentrates the electric field lines at the tip of the wires, and that, in turn, increases the emission current density while lowering the threshold field for the onset of field emission. Measurement of the Fowler-Nordheim tunneling current carried out in partial vacuum indicates that the threshold electric field for field emission in 50-nm diameter ZnO nanowires is 15 V/µm. In this study we identified the key constraint that can increase the threshold field and reduce emission current density. In Chapter 5 we report optical and magnetic measurement of Mn-doped ZnO nanowires. Hysterisis measurements carried out at various temperatures show a ferromagnetic behavior with a Curie temperature of ~ 200 K. We also studied Mn-doping of the ZnO nanowires. The room temperature fluorescence spectroscopy of Mn-doped ZnO nanowires shows a red-shift in the spectra compared to the undoped ZnO nanowires possibly due to strain introduced by the dopants in the nanowires. Finally, in Chapter 6, we report our study of the ensemble averaged transverse spin relaxation time (T2*) in InSb thin films and nanowires using electron spin resonance (ESR) measurement. Unfortunately, the nanowires contained too few spins to produce a detectable signal in our apparatus, but the thin films contained enough spins (> 109/cm2) to produce a measurable ESR signal. We found that the T2* decreases rapidly with increasing temperature between 3.5 K and 20 K, which indicates that spin-dephasing is primarily caused by spin-phonon interactions.

Les etudes presentees dans ce manuscrit portent sur la photo- (pl) et l'electro-luminescence (el) de couches de silicium poreux obtenu par attaque electrochimique de substrat de silicium monocristallin. La premiere partie presente les principales caracteristiques du materiau et la description de ses proprietes de luminescence accompagnee des differents modeles avances pour en expliquer les mecanismes, en s'attachant tout particulierement a celui du confinement quantique des porteurs dans les cristallites qui forment la couche poreuse, qui est le plus generalement retenu. La deuxieme partie decrit les differents montages experimentaux permettant la formation des couches poreuses et leurs caracterisations. Dans une troisieme partie, nous presentons les resultats obtenus lors de l'etude par spectroscopie infrarouge de l'influence des especes presentes en surface des cristallites de silicium sur la pl de la couche poreuse. Nous montrons que la desorption de l'hydrogene present en surface apres formation du materiau s'accompagne d'une diminution de l'intensite de pl. L'analyse de la composition chimique d'une couche poreuse oxydee electrochimiquement nous permet de mettre en evidence la conservation de la passivation par l'hydrogene au cours de ce traitement et de confirmer que l'augmentation du rendement quantique de l'emission associee a l'oxydation electrochimique peut etre attribuee a une meilleure localisation des porteurs photogeneres dans les cristallites. La quatrieme partie est consacree aux effets d'une polarisation electrique sur les phenomenes de luminescence. Ce travail a ete mene sur des couches nanoporeuses formees sur substrat de type n, polarisees par l'intermediaire d'un contact electrolytique. On observe une extinction selective, progressive (mais reversible) de la pl quand la polarisation cathodique est augmentee. Parallelement, l'el observee en presence d'une espece oxydante dans l'electrolyte subit un deplacement spectral vers le bleu. L'etude detaillee des differentes caracteristiques de ces deux phenomenes nous permet de montrer qu'ils resultent de l'injection selective des electrons depuis le substrat dans les cristallites de plus en plus confinees au fur et a mesure que la polarisation augmente. On verifie par ailleurs que ce caractere selectif disparait progressivement au cours de l'oxydation electrochimique de la couche poreuse. L'extinction de la pl et de l'el sous forte polarisation cathodique peut etre attribuee a la recombinaison non radiative des porteurs de charges par un mecanisme de type auger

碩士
元智大學
光電工程研究所
97
High-density (3.0×10^10 cm^-2) GaN/InGaN multiple quantum wells (MQWs) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850 ℃ for 1 min. The diameters and heights of nanorods were estimated to be 100 to 150 nm and around 0.28 μm, respectively. In this study, the photoelectrochemical (PEC) methods were applied to oxidate the GaN/InGaN MQWs nanorods. The measured PL from the embedded GaN/InGaN MQWs shows an energy shift of 30 meV and a smaller QCSE, which are casused by quantum confinement effect and the strain relaxation of MQWs.

碩士
元智大學
先進能源研究所
99
In this study, the silicon nano-crystals were embedded into amorphous silicon thin films, leading to an increase in optical band gap due to the quantum confinement effect. A nc-Si:H film with the band gap of 1.95 eV was obtained by VHF (27.12 MHz) PECVD system and was used as a window layer for p-i-n a-Si:H thin film solar cells. The hetero-junction, p-layer nc-Si:H/i-layer a-Si:H, solar cell demonstrated an open-circuit voltage (Voc) of 0.88 V and a conversion efficiency of greater than 9%. This kind of cell structure has great potential for the development of the a-Si/uc-Si tandem cell and the conversion efficiency is expected to be 11%. In addition, ECR-CVD (microwave frequency: 2.45 GHz) technique is powerful for the deposition of silicon films with high deposition rate. In this study, we attempted to use the system to deposit silicon films. The PECVD process experiences were helpful to give reasonable deposition parameters for ECR-CVD. Finally, we give some suggestions base on the measurement results of the film deposited by ECR-CVD.

acase@tulane.edu
Ever since the successful isolation of graphene, plenty of researches have been pursued to study fundamental physics in low-dimensional van der Waals materials, referred to as materials with the existence of out of plane vdW force. Not only graphene but also many other novel vdW materials start to emerge and play important roles in quantum physics. Due to the highly preserved crystal quality of the nanostructures achieved by micromechanical exfoliation, a variety of new phenomenon have been discovered in these novel materials. This dissertation focuses on the discovery and electronic properties study of new vdW materials both in 2D and 1D systems. Semiconducting transition metal dichalcogenides with layered structure have been viewed as the promising channel materials for field-effect transistors (FETs) in modern electronics. To characterize the performance, we have fabricated FETs based on multilayer WS2 thin crystals. By using gold as the contact metal and varying the thickness of the crystal, high-performance FETs with on/off ratio of 108 and mobility up to 234 cm2V-1s-1 at room temperature have been realized. The high performance is associated with the minimized Schottky barrier and a shallow impurity level below the conduction band. Elementary substance and binary compound crystals have limited members belong to 2D or 1D family. Thus, expanding the research to ternary compound materials is necessary. In this regard, we focused on a novel ternary compound 2D material Nb3SiTe6 and studied its magneto-transport. We have discovered that by using such a high crystalline 2D metal, we could study the inelastic electron-phonon (e-ph) interactions involved with reducing dimensions. From 3D bulk to 2D films with a rigid substrate, the weak antilocalization (WAL) signature is gradually enhanced according to our magnetoresistance (MR) measurements. Systematic studies of the temperature dependence of the dephasing rate in the crystal with various thicknesses suggest the suppression of electron-phonon interaction due to quantum confinement of the phonon spectrum. Our work shows great consistency with the long-standing predicted theory. We have successfully expanded the mechanical exfoliation method to 1D material group. As demonstrated by semiconducting quasi-1D materials, Ta2Pd3Se8 (TPdS) and Ta2Pt3Se8 (TPtS), the external force can efficiently break the weak vdW interactions between ribbons. In our work, we have produced ultrathin 1D TPdS and TPtS nanowires, and fabricated 1D FETs showing p-type and n-type transistor behavior respectively. Moreover, we have successfully built the functional logic NOT gate using these two different 1D FETs.
1
Xue Liu

Nanostructures often possess unique properties, which may lead to the development of new microelectronic and optoelectronic devices. They also provide an opportunity to test fundamental quantum mechanical concepts such as the role of quantum confinement. Considerable effort has been made to understand the electronic and structural properties of nanostructures, but many fundamental issues remain. In this work, the electronic and structural properties of nanostructures are examined using several new computational methods. The effect of dimensional confinement on quantum levels is investigated for hydrogenated Ge <110> using the plane-wave density-functional-theory pseudopotential method. We present a real-space pseudopotential method for calculating the electronic structure of one-dimensional periodic systems such as nanowires. As an application of this method, we examine H-passivated Si nanowires. The band structure and heat of formation of the Si nanowires are presented and compared to plane wave methods. Our method is able to offer the same accuracy as the traditional plane wave methods, but offers a number of computational advantages such as the ability to handle large systems and a better ease of implementation for highly parallel platforms. Doping is important to many potential applications of nano-regime semiconductors. A series of first-principles studies are conducted on the P-doped Si <110> nanowires by the real-space pseudopotential methods. Nanowires of varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the wires, energetics of P atoms in different doping positions and core-level shift of P atoms. Defect wave functions of P atoms are also analyzed. In addition, we study the electronic properties of phosphorus-doped silicon <111> nanofilms using the real-space pseudopotential method. Nanofilms with varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the films, and energetics of P atoms in different doping positions. Quantum confinement effects are compared with P-doped Si nanocrystals and as well as nanowires. We simulate the nanofilm STM images with P defects in varied film depths, and make a comparison with the experimental measurement.
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碩士
臺灣大學
物理研究所
98
The stacking behaviors and out of plane magnetization of bilayer triangular Co nanoislands on Cu(111) were studied. Due to quantum confinement, the interference patterns and surface state shift can be resolved in this system by scanning tunneling microscopy and spectroscopy. We used the free electron model to fit the interference patterns with experimental data, and we discovered that the magnetic standing wave patterns can also be resolved by non spin-polarized STM. Furthermore, we observed some conjoint Co islands with anti-parallel spin alignment. From our statistical result and calculation, we deduced that dipolar interaction is strong enough to compete with ferromagnetic coupling to preserve the anti-parallel spin alignment in two conjoint Co nanoislands.

博士
國立臺灣大學
光電工程學研究所
102
All-optical amplification, modulation, and wavelength conversion have been demonstrated by using the Si-QD doped in SiOx/SiNx waveguide. In the first part, the bandgap energy and radiative recombination rate of the Si-QD are theoretically simulated by the finite potential well approximation. The luminescent mechanisms, including NOV, E''d defect and Si-QDs related spontaneous emission, in SiOx:Si-QD are clearly distinguished by combining the PL with TRPL analysis. The small-signal gain of the SiOx:Si-QD waveguide is demonstrated by injecting the modulated probe signal into the optically pumped Si-QDs. The small-signal gain of 14.7 dB/6.5 dB at 785-/650-nm are measured in the SiO1.24:Si-QD/SiO1.42:Si-QD waveguide amplifiers. By considering the power-dependent gain coefficient in the Si-QD based waveguide amplifier, the peak power gain and saturation power in the Si-QD based waveguide amplifiers are determined. In the second part, the free-carrier absorption in the Si-QD is utilized to demonstrate the all-optical FCA modulator. The FCA cross-section at 1550 nm is reduced from 2.8*10-17 cm2 to 8*10-18 cm2 with shrinking Si-QD size from 4.3 nm to 1.7 nm. Although the FCA cross-section is degraded when shrinking the Si-QD size, the free-carrier relaxation lifetime in SiOx:Si-QD waveguide is shortened from ~10 us to 0.48 us due to the increased momentum overlapping factor of electron-hole wave-functions in Si-QDs. The achievable bit rate of the inverted data modulation with RZ-OOK data format at 1550 nm has been significantly increased from 100 kbit/s to 2 Mbit/s by shrinking Si-QD size from 4.3 nm to 1.7 nm. In the rest part, the nonlinearity of the Si-QD doped in SiNx matrix and corresponding applications are discussed. Based on the Z-scan measurement, the nonlinear refractive index at 800 nm for Si-rich SiNx film is increased from 5.7*10-13 to 9.2*10-12 cm2/W when increasing the excessive Si concentration from 16.3% to 23.4%. Such optical nonlinearity enhancement in the Si-rich SiNx film can be attributed to the strong quantum confinement effect in the Si-QD doped in the SiNx matrix. Then, the FWM has been firstly demonstrated by using the SiNx:Si-QD channel waveguide. The maximum conversion efficiency of -46 dB with the 3-dB bandwidth of 18 nm has been measured in the 8-mm long Si-rich SiNx channel waveguide. Furthermore, the all-optical data modulation in the SiNx based ring resonator has been achieved by using the nonlinear Kerr effect of the Si-QD. The refractive index change induced by the nonlinear Kerr effect is increased from 2*10-5 to 1.6*10-4 for SiNx R0.5 to R0.9 ring resonator under the pump pulse excitation with peak power of 3W. It indicates that the nonlinear refractive index at ~1550 nm are increased from 1.4*10-14 to 1.6*10-13 cm2/W by increasing the excessive Si concentration in SiNx films from 16.3% (R0.5) to 23.4% (R0.9). Finally, the 12 Gbit/s all-optical data conversion and inversion with NRZ-OOK data format has been firstly demonstrated by nonlinear Kerr effect in the SiNx based ring resonator.