Rozprawy doktorskie na temat „Si Quantum Dot”

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.

Self-assembled III-V quantum dots (QDs) attract intense research interest and effort due to their unique physical properties arising from the three-dimensional confinement of carriers and discrete density of states. Semiconductor III-V QD laser structures exhibit dramatically improved device performance in comparison with their quantum well (QW) counterparts, notably their ultra low threshold current density, less sensitivity to defects and outstanding thermal stability. Therefore, integrating a high-quality QD laser structure onto silicon-based platform could potentially constitute a hybrid technology for the realization of optical inter-chip communications. This thesis is devoted to the development of high-performance InAs/GaAs QD lasers directly grown on silicon substrates and germanium substrates for silicon photonics. In the integration of III-V on silicon, direct GaAs heteroepitaxy on silicon is extremely challenging due to the substantial lattice and thermal expansion mismatch between GaAs and Si. The inherent high-density propagating dislocations can degrade the performance of III-V based lasers on silicon substrates. To enhance the device performance, QW dislocation filters are used here to create a strain field, which bends the propagating dislocations back towards the substrate. Here, we report the first operation of an electrically-pumped 1.3-\mu m InAs/GaAs QD laser epitaxially grown on Si (100) substrate. A threshold current density of 725 A/cm2 and an output power of 26 mW has been achieved for broad-area lasers with as-cleaved facets at room temperature. To avoid the formation of high-density threading dislocations (TDs), an alternative to direct growth of GaAs on silicon substrate is to use an intermediate material, which has a similar lattice constant to GaAs with fewer defects. Germanium appears to be the ideal candidate for a virtual substrate for GaAs growth, because germanium is almost lattice-matched to GaAs (only 0.08% mismatch). In the last 20 years, the fabrication of germanium-on-silicon (Ge/Si) virtual substrates has been intensely investigated with the demonstration of high-quality Ge/Si virtual substrates. The main challenge for the growth of GaAs on Ge/Si virtual substrate is to avoid the formation of anti-phase domains due to the polar/non-polar interface between GaAs and germanium. A new growth technique was invented for suppressing the formation of anti-phase domains for the growth of GaAs on germanium substrates at UCL. Based on this technique, lasing at a wavelength of 1305nm with a threshold current density of 55.2A/cm2 was observed for InAs/GaAs QD laser grown on germanium substrate under continuous-wave current drive at room temperature. The results suggest that long-wavelength InAs/GaAs QD lasers on silicon substrates can be realized by epitaxial growth on Ge/Si substrates. Studies in this thesis are an essential step towards the monolithic integration of long-wavelength InAs/GaAs QD lasers on a silicon substrate, as well as the integration of other III-V devices through fabricating III-V devices on silicon substrates.

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.

The physics and technology of the heterojunction infrared photodetectors having different material systems have been studied extensively. Devices used in this study have been characterized by using mainly optical methods, and electrical measurements have been used as an auxiliary method. The theory of internal photoemission in semiconductor heterojunctions has been investigated and the existing model has been extended by incorporating the effects of the difference in the effective masses in the active region and the substrate, nonspherical-nonparabolic bands, and the energy loss per collisions. The barrier heights (correspondingly the cut-off wavelengths) of SiGe/Si samples have been found from their internal photoemission spectrums by using the complete model which has the wavelength and doping concentration dependent free carrier absorption parameters. A qualitative model describing the mechanisms of photocurrent generation in SiGe/Si HIP devices has been presented. It has been shown that the performance of our devices depends significantly on the applied bias and the operating temperature. Properties of internal photoemission in a PtSi/Si Schottky type infrared detector have also been studied. InGaAs/InP quantum well photodetectors that covers both near and mid-infrared spectral regions by means of interband and intersubband transitions have been studied. To understand the high responsivity values observed at high biases, the gain and avalanche multiplication processes have been investigated. Finally, the results of a detailed characterization study on a systematic set of InAs/GaAs self-assembled quantum dot infrared photodetectors have been presented. A simple physical picture has also been discussed to account for the main observed features.

In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation.

Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics. In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge. Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented. In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed. In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.

Quantum dots (QDs) are a semiconductor nanostructure in which a small island of one type of semiconductor material is contained within a larger bulk of a different one. These structure are interesting for a wide range of applications, including highly efficient LASERs, high-density novel memory devices, quantum computing and more. In order to understand the nature of QDs, electrical characterisation techniques such as capacitance-voltage (CV) profiling and deep-level transient spectroscopy (DLTS) are used to probe the nature of the carrier capture and emission processes. This is limited, however, by the nature of QD formation which results in a spread of sizes which directly affects the energy structure of the QDs. In this work, I sought to overcome this by using Si substrates patterned with a focused ion beam (FIB) to grow an array of identically-sized Ge dots. Although I was ultimately unsuccessful, I feel this approach has great merit for future applications.In addition, this thesis describes several unusual characteristics observed in InAs QDs in a GaAs bulk (grown by molecular beam epitaxy-MBE). Using conventional and Laplace DLTS, I have been able to isolate a single emission transient. I further show an inverted relation between the emission rate and the temperature under high field (emissions increase at lower temperatures). I attribute this to a rapid capture to and emission from excited states in the QD. In addition, I examine a metastable charging effect that results from the application of a sustained reverse bias and decreases the apparent emission rate from the dots. I believe this to be the result of a GaAs defect with a metastable state which acts as a screen, inhibiting emission from the dots due to an accumulation of charge in the metastable state. These unusual characteristics of QDs require further intensive work to fully understand. In this work I have sought to describe the phenomena fully and to provide hypotheses as to their origin.

Despite significant efforts have been directed toward reducing waste generation and encouraging alternative waste management strategies, landfills still remain the main option for Municipal Solid Waste (MSW) disposal in many countries. Hence, landfills and related impacts on the surroundings are still current issues throughout the world. Actually, the major concerns are related to the potential emissions of leachate and landfill gas into the environment, that pose a threat to public health, surface and groundwater pollution, soil contamination and global warming effects. To ensure environmental protection and enhance landfill sustainability, modern sanitary landfills are equipped with several engineered systems with different functions. For instance, the installation of containment systems, such as bottom liner and multi-layers capping systems, is aimed at reducing leachate seepage and water infiltration into the landfill body as well as gas migration, while eventually mitigating methane emissions through the placement of active oxidation layers (biocovers). Leachate collection and removal systems are designed to minimize water head forming on the bottom section of the landfill and consequent seepages through the liner system. Finally, gas extraction and utilization systems, allow to recover energy from landfill gas while reducing explosion and fire risks associated with methane accumulation, even though much depends on gas collection efficiency achieved in the field (range: 60-90% Spokas et al., 2006; Huitric and Kong, 2006). Hence, impacts on the surrounding environment caused by the polluting substances released from the deposited waste through liquid and gas emissions can be potentially mitigated by a proper design of technical barriers and collection/extraction systems at the landfill site. Nevertheless, the long-term performance of containment systems to limit the landfill emissions is highly uncertain and is strongly dependent on site-specific conditions such as climate, vegetative covers, containment systems, leachate quality and applied stress. Furthermore, the design and operation of leachate collection and treatment systems, of landfill gas extraction and utilization projects, as well as the assessment of appropriate methane reduction strategies (biocovers), require reliable emission forecasts for the assessment of system feasibility and to ensure environmental compliance. To this end, landfill simulation models can represent an useful supporting tool for a better design of leachate/gas collection and treatment systems and can provide valuable information for the evaluation of best options for containment systems depending on their performances under the site-specific conditions. The capability in predicting future emissions levels at a landfill site can also be improved by combining simulation models with field observations at full-scale landfills and/or with experimental studies resembling landfill conditions. Indeed, this kind of data may allow to identify the main parameters and processes governing leachate and gas generation and can provide useful information for model refinement. In view of such need, the present research study was initially addressed to develop a new landfill screening model that, based on simplified mathematical and empirical equations, provides quantitative estimation of leachate and gas production over time, taking into account for site-specific conditions, waste properties and main landfill characteristics and processes. In order to evaluate the applicability of the developed model and the accuracy of emissions forecast, several simulations on four full-scale landfills, currently in operative management stage, were carried out. The results of these case studies showed a good correspondence of leachate estimations with monthly trend observed in the field and revealed that the reliability of model predictions is strongly influenced by the quality of input data. In particular, the initial waste moisture content and the waste compression index, which are usually data not available from a standard characterisation, were identified as the key unknown parameters affecting leachate production. Furthermore, the applicability of the model to closed landfills was evaluated by simulating different alternative capping systems and by comparing the results with those returned by the Hydrological Evaluation of Landfill Performance (HELP), which is the most worldwide used model for comparative analysis of composite liner systems. Despite the simplified approach of the developed model, simulated values of infiltration and leakage rates through the analysed cover systems were in line with those of HELP. However, it should be highlighted that the developed model provides an assessment of leachate and biogas production only from a quantitative point of view. The leachate and biogas composition was indeed not included in the forecast model, as strongly linked to the type of waste that makes the prediction in a screening phase poorly representative of what could be expected in the field. Hence, for a qualitative analysis of leachate and gas emissions over time, a laboratory methodology including different type of lab-scale tests was applied to a particular waste material. Specifically, the research was focused on mechanically biologically treated (MBT) wastes which, after the introduction of the European Landfill Directive 1999/31/EC (European Commission, 1999) that imposes member states to dispose of in landfills only wastes that have been preliminary subjected to treatment, are becoming the main flow waste landfilled in new Italian facilities. However, due to the relatively recent introduction of the MBT plants within the waste management system, very few data on leachate and gas emissions from MBT waste in landfills are available and, hence, the current knowledge mainly results from laboratory studies. Nevertheless, the assessment of the leaching characteristics of MBT materials and the evaluation of how the environmental conditions may affect the heavy metals mobility are still poorly investigated in literature. To gain deeper insight on the fundamental mechanisms governing the constituents release from MBT wastes, several leaching experiments were performed on MBT samples collected from an Italian MBT plant and the experimental results were modelled to obtain information on the long-term leachate emissions. Namely, a combination of experimental leaching tests were performed on fully-characterized MBT waste samples and the effect of different parameters, mainly pH and liquid to solid ratio (L/S,) on the compounds release was investigated by combining pH static-batch test, pH dependent tests and dynamic up-flow column percolation experiments. The obtained results showed that, even though MBT wastes were characterized by relatively high heavy metals content, only a limited amount was actually soluble and thus bioavailable. Furthermore, the information provided by the different tests highlighted the existence of a strong linear correlation between the release pattern of dissolved organic carbon (DOC) and several metals (Co, Cr, Cu, Ni, V, Zn), suggesting that complexation to DOC is the leaching controlling mechanism of these elements. Thus, combining the results of batch and up-flow column percolation tests, partition coefficients between DOC and metals concentration were derived. These data, coupled with a simplified screening model for DOC release, allowed to get a very good prediction of metal release during the experiments and may provide useful indications for the evaluation of long-term emissions from this type of waste in a landfill disposal scenario. In order to complete the study on the MBT waste environmental behaviour, gas emissions from MBT waste were examined by performing different anaerobic tests. The main purpose of this study was to evaluate the potential gas generation capacity of wastes and to assess possible implications on gas generation resulting from the different environmental conditions expected in the field. To this end, anaerobic batch tests were performed at a wide range of water contents (26-43 %w/w up to 75 %w/w on wet weight) and temperatures (from 20-25 °C up to 55 °C) in order to simulate different landfill management options (dry tomb or bioreactor landfills). In nearly all test conditions, a quite long lag-phase was observed (several months) due to the inhibition effects resulting from high concentrations of volatile fatty acids (VFAs) and ammonia that highlighted a poor stability degree of the analysed material. Furthermore, experimental results showed that the initial waste water content is the key factor limiting the anaerobic biological process. Indeed, when the waste moisture was lower than 32 %w/w the methanogenic microbial activity was completely inhibited. Overall, the obtained results indicated that the operative conditions drastically affect the gas generation from MBT waste, in terms of both gas yield and generation rate. This suggests that particular caution should be paid when using the results of lab-scale tests for the evaluation of long-term behaviour expected in the field, where the boundary conditions change continuously and vary significantly depending on the climate, the landfill operative management strategies in place (e.g. leachate recirculation, waste disposal methods), the hydraulic characteristics of buried waste, the presence and type of temporary and final cover systems.

碩士
國立中央大學
物理研究所
99
In this paper, we have studied the optical properties of Ge/Si/Ge quantum dots (QDs) structure by using Photoluminescence (PL) spectroscopy.And we use rapid thermal annealing process to improve its light efficiency.Comparing the PL measurements of Ge/Si/Ge QDs structure with Ge/Si QDs structure, the structural difference effect on optical properties is studied. According to the emission energy of annealed samples in excitation-powerdependent PL measurements, we found that Ge/Si/Ge QDs structure has higher emission energy and lower carriers confinement depth due to atomic intermixing effect. According to the PL intensity with power sublinear relation at different temperature measurements, we suggest that the defect has negative effect on light efficiency because emitting light will be absorbed by the electrons confined in the defect. Finally, we found that the Ge/Si/Ge QDs structure has higher activation energy from Temperature-dependent PL measurements. Therefore, we point out that the holes in Ge/Si/Ge QDs structure probably can exist on nearby QDs by tunneling effect.

博士
國立交通大學
光電工程研究所
102
In order to resolve the critical issues of “Green House Effect” and “Energy Crisis” for humanity’s future, the accelerated developments of renewable energies are necessary. Among all of the renewable energies, solar cells (SCs) are highly considered as the most potential one. To ponder these key factors of efficiency, cost, and lifetime, undoubtedly, the Si-based SCs have the most advantages on popularized developments in the future. However, to successfully achieve high efficiency and low cost, also called the third generation SC, the tandem Si-based SCs with multi-bandgap is required to efficiently reduce the mismatched photon energy loss. Based on the unique properties of Si quantum dot (QD), we propose to develop the novel Si QD thin films by utilizing a gradient Si-rich oxide multilayer (GSRO-ML) structure and integrating with ZnO matrix material to overcome the bottlenecks of the largely limited carrier transport efficiency in the Si-based SCs integrating Si QDs. In the beginning of this dissertation, we talk about the importance and recent developments of SCs, and then, the advantages and challenges of SCs integrating Si QDs are discussed. After that, our motivations, fabrication process, and apparatus are also introduced in details. To achieve the formation of super-high density Si QD thin films, we forsake the traditional [SiO2/SRO]-ML structure and develop a new one, GSRO-ML. In our results, by utilizing the periodical variations in Si/O atomic concentration during deposition, the Si QDs with super-high density and good size control can be self-assembled from the uniform aggregations of Si-rich atoms during annealing. Besides, the considerable enhancements on photovoltaic properties are also obtained by using a GSRO-ML structure due to the improved carrier transport efficiency and larger optical absorption coefficient. To obtain the better carrier transport path for the Si QD thin films, we also develop a new matrix material, ZnO, because it has many desirable features, such as wide and direct bandgap, high transparency, and highly tunable electrical properties. In our results, though embedded with Si QDs, the optical properties of ZnO thin film can be preserved in the long- and short-wavelength ranges. In the middle-wavelength range, the significantly enhanced light absorption and the unusual PL emission peak, owing to embedding Si QDs, are observed. These results represent the sub-bandgap formation in ZnO thin film by utilizing Si QDs while maintaining the essential optical properties of ZnO matrix. In the electrical properties, the Si QD embedded ZnO thin film reveals the significantly higher conductivity than that using SiO2 matrix material. Besides, the carriers transport mainly via ZnO matrix, not through Si QDs, is clearly observed. This unique transport mechanism differing from those using the traditional Si-based dielectric matrix materials has great potential on leading to the much better carrier transport efficiency and electrical properties for SC applications. In this dissertation, we had demonstrated the proposed novel Si QD thin films, utilizing a GSRO-ML structure and integrating with ZnO matrix material, are more suitable and advantageous for the Si-based SCs integrating Si QDs. Therefore, the high-efficiency Si-based SCs integrating Si QDs can be most definitely expected using the novel Si QD thin films.

碩士
國立中央大學
電機工程學系
102
This thesis continuously studied the formation of Ge QDs using thermal oxidation of poly-SiGe pillar/Si3N4/Si heterostructure in our lab, and further discussed the dependence of morphology and internal defects of Ge QD between oxidation and annealing conditions. Meanwhile, the author used this process on poly-SiGe pillar/ Si3N4/Si on insulator substrate in order to realize Ge/SiO2/Si on insulator heterostructure advantaging in the gate engineering of Ge MOSFET and phototransistor. This thesis used transmission electron microscopy (TEM), energy dispersive X-ray (EDX) mapping and Raman analysis to investigate the evolution of morphology, crystallinity, lattice orientation, defects, Ge concentration and stress of Ge QD and shell under different thermal budget by modifying oxidation and annealing conditions. The author found that Ge QD catalytically enhances the local oxidation of underlying Si3N4 and Si. The released Si atoms diffuse into Ge QD and make the migration of Ge QD in Si3N4 and Si. In addition, the quantity of Si inward Ge QD obviously changes the morphology of Ge QD. This thesis investigated the relationship between Ge QD/shell using selective oxidation and thermal budget. The demonstrated SiGe shell with uniform thickness, subject to compressive stress and stable (111) lattice orientation not only can be used in self-aligned Ge QD but also have good SiO2 quality between Ge QD and shell. Therefore, this heterostructure can be used as the application of gate stack and the channel of Ge MOSFET and phototransistor.

碩士
國立交通大學
光電工程研究所
102
In order to further reduce quantum dot (QD) separation, we had proposed and successfully developed the gradient Si-rich oxide multilayer (GSRO-ML) structure for the super-high density Si QD thin films with larger carrier tunneling probability. In this study, we investigate the B-doping and QD size effects on the super-high density Si QD thin films by using a GSRO-ML structure. Under B-doping effect, the preserved high crystallinity of Si QDs and the slightly reduced Eg with increasing PB are observed, besides, the electrical and PV properties are enhanced with increasing PB from 0 to 25W due to the increased active B-doped atoms but degraded at the higher PB than 30 W due to the increased inactive B-doped atoms and the interfacial over-diffusion of B-doped atoms. The decreased VOC with increasing PB due to the interfacial over-diffusion is efficiently improved by inserting the lowly B-doped GSRO thin films as buffer layers. Under QD size effect, the red-shift effect is clearly confirmed in the absorption band edge and quantum efficiency response with increasing NL thickness. Therefore, our results had demonstrated the feasibility and great potential for the higher efficiency Si-based solar cells integrating Si QDs by using a GSRO-ML structure.

碩士
國立交通大學
光電工程研究所
103
Recently, the solar cells (SCs) integrating nano-crystalline Si quantum dot (nc-Si QD) thin films have been researched widely due to the highly-tunable ability of bandgap (Eg). Si-QD SCs can overcome the issue for energy loss from the high-energy photon due to the larger Eg than monocrystalline and amorphous Si. The general structure is Si QD embedded in SiO2 matrix utilizing [Si dioxide/Si rich oxide] multilayer ([SiO2/SRO]-ML) thin film structure. However, the poor photovoltaic (PV) properties were obtained due to naturally highly resistive properties of SiO2 matrix. Therefore, the reduction of QD separation, the increasing of QD density and the heavily impurities (B, P atoms, etc.) doping are available methods to enhance PV properties of devices. In 2013, we have proposed a new structure, gradient Si-rich oxide mutilayer (GSRO-ML) structure and demonstrated a super-high density Si QD thin film to reduce the separation between Si QDs which leads to the higher probability of carrier tunneling. To further enhance the PV properties, the boron doping effect on the super-high density Si QD thin film have been studied and the obvious improvement on PV properties can be observed. In this thesis, we propose to dope P atoms into GSRO-ML thin films by means of thermal diffusion of phosphorus oxide trichloride (POCl3). The P-doped effect and the issue of defect reduced of the P-doped super-high density Si QD thin film will be investigated and discussed. Under P-doping effect, the preserved crystallinity of Si QDs and high absorption coefficient are maintained. In addition, the electrical and PV properties are enhanced with increasing POCl3 flow rate from 280 to 880 sccm, and the best performance is obtained at 880 sccm due to the largest active P-doped atoms but decreased at 1000 sccm due to the increased inactive P-doped atoms. However, a harmful material for Si QD thin films, PCl5, was produced during the high temperature P doping process. We raise the O2 flow rate for the PCl5 reacted absolutely. The clear effect of defect reduced is observed, and the declined PV properties of devices have been observed. The phenomenon means the defect in matrix is helpful for carrier transport. Therefore, our result demonstrated the P-doping effect in the Si QD thin films by using a GSRO-ML structure.

碩士
元智大學
化學工程與材料科學學系
105
Amorphous silicon has been the main material for backplane technology for many years and has a variety of manufacturing methods,to improve its energy efficiency, refresh speeds, and the display’s viewing angle. Up to now, amorphous-Si (a-Si) displays still hold the smartphone display market approximately 37‒39 %. Although OLED growth is rapid, the next few years are double-digit growth, but TFT-LCD is still the mass stream. For mobile displays with a pixel density lower than 300 pixels per inch, this technology remains the preferable backplane of choice, mainly due to its low costs and relatively simple manufacturing process. However, when it comes to higher resolution displays and new technologies such as AMOLED, a-Si shows a significant display effect. Compared with a-Si TFT-LCD, AMOLED exerts more electrical stress on the transistors, thus facilitating the technique more current to each pixel. Furthermore, AMOLED pixel transistors take up more space, blocking the AMOLED display light emission, making a-Si rather unsuitable. And LTPS price is dropping, a-Si is losing its advantage. In 2015, LTPS already held 41% of the market, and it is believed that the percentage would exceed a-Si in 2017. As a result, new technologies and manufacturing processes have been developed to meet the increasing demands made of display panels over recent years. With industry trend and market demand for high definition and power-efficient display panels, metal oxides are rapidly becoming a suitable display technology. Indium gallium zinc oxide-thin-film transistor (IGZO-TFT) has the advantage of having power-efficient consumption than a-Si-TFT LCD panels. This technology is suitable for portable devices with high-resolution displays nonetheless. In LCD displays, Quantum dot (QD) technology is now offering a wide color gamut (WCG) that is consistently bigger than that offered by OLED displays and other LCD technologies. When coupled with high dynamic range (HDR), these displays offer the best picture performance on the market and are expected to be the main trend in the next wave of technology. This technical report will explore the metal oxide IGZO and quantum dot materials behind the technology, which can be used to manufacture LCD displays in various methods and spaces. The advantages and disadvantages of each method will be discussed in this report. Compared with the amorphous silicon, IGZO can reduce the size of the transistor about ½, so that the LCD panel pixel aperture rate increased, about twice the resolution. Its electron mobility is ten times faster while leakage current is much less than amorphous silicon, so the power consumption of mobile devices can be reduced to two-thirds. In term of performance, quantum dot with high-performance blue LED and traditional filters can be more than 50% of the traditional LCD performance, and the light efficiency and the color performance is also close to the OLED, but the process yield and material costs are far less than OLED. So the future display of quantum dot can be expected.

博士
國立中央大學
電機工程研究所
98
This thesis investigates the device physics and electrical characteristics of Ge-QD photodiodes (PDs) and hototransistors (PTs) fabricated in a complementary metal-oxide-semiconductor (CMOS) process. The main features encompass as follows. First, thermally oxidizing poly-Si0.87Ge0.13 onto a dielectric layer produces Ge QDs embedded in an SiO2 matrix to serve as an efficient absorption layer for visible to ultraviolet light. Secondly, we have successfully demonstrated the feasibility of MOS PDs including multi-layer Ge QDs in the gate oxide and elucidated the current–voltage characteristics in terms of zero, one and three Ge-QD layers. Ge-QD PDs exhibit dramatically enhanced current under light illumination in the inversion mode and amplified responsivity (quantum efficiency) from 4.64 (1.42%), through 482 (148%) to 812 (245%) mA/W, respectively, as the Ge-QD layer number increases from zero, through one to three. Shrinking Ge QD size from 9.1 nm to 5.1 nm reveals considerable blueshift in spectral peak energies, originating from the quantum confinement effect (QCE). The temperature and bias dependences on the dark current ascribe the charge transport mechanism to be percolation hopping. Thirdly, Ge-QD PTs are realized by double-gated thin-film transistors with Ge QDs in the top-gate dielectrics. Compared with the current in darkness, 405-450 nm light illumination strongly enhances drain current and improves the PTs’subthreshold characteristics, following from that only photo-excited holes within Ge QDs inject into the active channel via vertical electric field and contribute to photocurrent without the counterpart photo-generated electron-induced junction barrier lowering. Spectral responses of Ge-QD PTs are consistent with that of Ge-QD PDs, attributing the PTs’photo-absorption to QCE. Temperature-dependent and light pulse characterizations demonstrate Ge-QD PTs have great thermal stability and photo-absorption efficiency. These Ge-QD PDs and PTs offer a deterministic approach to integrate with Si-based electronics monolithically.

This thesis proposes a multi-physics model to couple the electrical, mechanical, thermal and quantum mechanical interactions in semiconductor and their heterostructures. Governing differential equations and constitutive relations among the coupled fields are derived from the principles of irreversible thermodynamics. Variational principle is applied to solve the problem numerically using finite element method. Boundary and interface conditions are derived consistently. In the first part of the thesis, semiconductor-solid interfaces are considered with the example of III-N thin films. The contributions of individual physical fields in the coupled field interactions on electronic band structures are studied in detail. The effect of various boundary conditions, defused interfaces and doping are analysed using AlN/GaN heterostructures in the present framework. In the second part, we study three-dimensional Si quantum dots/ nanocrystals embedded in SiO2 matrix. The purpose of this analysis is to introduce atomic continuum framework in the present scheme. The matrix embedded structure is first created using atomistic simulations based on molecular dynamics theory. The scheme is to create simulated annealing so that realistic Si/SiO2 interface is formed. The interface properties namely width, stoichiometry, point defects and their statistics are studied with respect to nanocrystal size. Residual strain due to thermal annealing is computed. The local properties are then applied in the proposed continuum framework via interpolation functions to calculate the electronic band structure of the nanocrystals. The electronic energy bandgap variation with quantum dot diameter is estimated and compared with reported literature. The corrections applied via local strain is observed to improve the accuracy of computation. In the final part of the thesis, we apply the multi-physics model to study semiconductor- fluid interfaces. Semiconductor nanostructures in microfluidic environment are emerging as frontiers in next generation of bioengineering and biomedical fields. The complexity and multi-disciplinary nature of the problem poses a highly challenging task in understanding and designing new designs. In this direction, our model aims at addressing coupled electrical, mechanical and quantum mechanical effects on fluid ow in microfluidic channels consisting of semiconductor nanostructures. To illustrate this, we analyse the effect of nanowire array on electric field and ow in microfluidic channels. A systematic study on the effect of geometry, orientations and inter-nanowire spacing on the physical fields are studied. Nanowire arrays of Si, Si/SiO2 and ZnO in microfluidic channel is considered as examples. Further, their implications on particle trajectories are discussed in the context of trapping and lysing of biological cells. Additionally, we study the effect of electrolytic fluid on the electronic band structure of ZnO nanowires by varying the diameter, tip pointedness and applied electric field. To summarize, the thesis proposes a fully coupled quantum-continuum multi-physics modelling framework and provide computational examples having potential applications in nanostructure-based devices. The contribution made in this thesis would be useful in advancing the current understanding of nano-scale phenomena involving electro-thermal mechanical interactions, quantum effect, nanostructure heterojunctions, semiconductor- fluid interfaces and several others, and towards developing better tools in designing new nano-electronic devices from concepts to operation.

碩士
國立交通大學
顯示科技研究所
101
So far, the Si-based solar cell is the highest global market share and the good development potential due to the plentiful materials and the well-developed fabrication technique. In order to achieve the goal of the third generation solar cells with high efficiency and low cost, all Si-based multiple-junction solar cell is widely investigated and developed nowadays. The nano-crystalline Si quantum dot (QD) thin film is one of the potential structures to overcome the bandgap limitation of Si-based materials. Silicon-rich oxide (SRO) single layer and [SRO/SiO2] multilayer (ML) thin films are the most commonly used deposition structures for Si QD thin films. However, the former is hard to control the QD's size and density simultaneously, the latter exists the QD’s separation limitation due to the SiO2 barrier layers inserted. Furthermore, the QD’s density of both structures is still not high enough for a better PV application. These result in the difficulty for good photo-generated carrier’s transportation and high conversion efficiency. Hence, to efficiently improve the carrier’s transportation properties is a critical issue for the high efficiency Si-based solar cells integrating Si QD thin film. In this study, we propose a more potential deposition structure by a gradient Si-rich oxide multilayer (GSRO-ML) structure for the QD size control and the high QD density. The nano-structure, crystalline, and optical properties of Si QD thin films using a GSRO-ML structure had been studied. It also shows the better photovoltaic properties than that using a [SRO/SiO2]-ML structure. A higher conversion efficiency of Si QD thin films utilizing a GSRO-ML structure can be highly expected by using a heavy doping concentration in the near future.

碩士
國立交通大學
光電工程學系
99
Researches of Si quantum dot (QD) thin film solar cell is fabricated by using nanocrystal silicon (nc-Si) QD embedded in the Si-based dielectric material. However the experimental results are still substantially lower than the theoretical value due to the unfavorable material characteristics of these Si-based matrices, that is, they are not electrically conductive. In this thesis, we propose to use Zinc oxide (ZnO) as the substitutable matrix material due to its many potential applications and unique features over other conventional wide band gap semiconductors such as high transparency, nice crystallinity, and easiness to control the electrical properties. Hence, ZnO is a suitable material to serve as the matrix for nano-crystalline Si thin film solar cells We deposited ZnO/Si multilayer thin films by radio-magnetron sputtering. At first part, the nc-Si QD formed after annealing by rapid thermal annealing thermal process. Then we investigated the nc-Si formation and crystalline quality of ZnO thin films by analyses of Raman spectra and XRD patterns. Optical-electrical properties of the multilayer thin films after annealing are also investigated. We observe that the sputtered Si atoms have more kinetic energy to aggregate together as Si nano-clusters under higher Si sputtering power. An obvious aggregation of the sputtered Si atoms during deposition is helpful for the formation of nc-Si and the better crystallization of the ZnO matrix in the nc-Si embedded ZnO thin films during the RTA process. At second part, we substitute RTA to furnace as our new thermal treatment equipment to solve some problems cause by RTA thermalprocess. Then, we observe good rectification ratio and better photon response after annealing by furnace. At the last part, we try to solve the film bending problem cause by the high stress interior the films after annealing. We observe film bending problem may be solved with a heated substrate during deposition.

碩士
國立成功大學
電機工程學系專班
92
GaN films were grown on Si(111) substrates by home made MOVPE system using a vertical reactor. The low-temperature AlN buffer layer,30 nm in thickness, was grown on 7000C. The high temperature GaN films were grown at 10600C. XRD observation showed that the films are well oriented hexagonal GaN. But SEM observation showed that there were many cracks and defects in the films. The PL measurement results showed the light response of the films were not clear. 　　We successfully fabricated a novel device: InGaN/GaN multi-quantum dot (MQD) p-n junction photodiodes, and discussed the characteristics of fabricated PDs. We achieved nanoscale InGaN self-assembled QDs in the well layers of the active region. It was found that the maximum responsivity of the fabricated MQD p-n junction PD was observed at 350 nm, and the responsivity was nearly a constant from 400 nm to 440 nm. It was also found that the minimum of spectral response was measured at 465 nm.

In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

博士
國立清華大學
材料科學工程學系
91
Enhanced growth of low-resistivity metal silicides and self-assembled NiSi and CoSi2 quantum dot arrays on epitaxial Si-Ge alloys have been studied by sheet resistance measurement, glancing incidence X-ray diffractometry (GIXRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), and energy dispersion analysis of X-ray (EDAX). The formation of Co silicides on (001) Si with an interposing Ni layer has been investigated. For Co(20 nm)/Ni(7 nm)/Si(001) samples, CoSi2 and NiSi phase were observed to form at 400-500 ºC. AES analysis indicated that Ni diffused from the Co/Si interface to disperse in CoSi2 layer during annealing. Above 700 ºC, (Co,Ni)Si2 was the only silicide phase present. The presence of Ni was found to decrease the formation temperature of CoSi2 by about 100 ºC, prevent oxygen contamination from the annealing ambient, and improve uniformity of CoSi2 compared to that without Ni interlayer. In addition, the resulting (Co,Ni)Si2 layer has good thermal stability up to 1000 ºC and has a preferential epitaxial orientation with respect to the Si substrate. Formation of Co silicides on Si0.7Ge0.3 alloys with a thin interposing Au layer and capping Ti layer has been investigated. CoSi2 was observed to be the only silicide phase in Si0.7Ge0.3 samples annealed at 650-950 ℃ with a thin interposing Au layer and capping Ti layer. The sequence of phase formation is the same as the reaction of Co with (001)Si. The presence of Au was found to decrease the formation temperature of CoSi2 by about 300 ℃ compared to that of Co(30nm)/Si0.7Ge0.3 samples. In addition, a thin capping Ti layer improves the uniformity and thermal stability of CoSi2 layer. For Ti(5nm)/Co(30nm)/Au(1nm)/Si0.7Ge0.3 system, the process window of CoSi2 was extended to 650-950 ℃. SIMS analysis indicated that a large amount of Au diffused from the Co/Si0.7Ge0.3 interface to disperse in the CoSi2 layer during annealing. Enhanced growth of low-resistivity self-aligned CoSi2, C54-TiSi2, and NiSi on epitaxial Si0.7Ge0.3 has been achieved with an interposing amorphous Si (a-Si) layer. The a-Si layer was used as a sacrificial layer with appropriate thickness to prevent Ge segregation, decrease the growth temperature, as well as maintain the interface flatness and morphological stability in forming CoSi2, C54-TiSi2, and NiSi on Si0.7Ge0.3 grown by molecular beam eptiaxy. The process promises to be applicable to the fabrication of high-speed Si-Ge devices. Self-assembled NiSi and CoSi2 quantum dot arrays have been grown on relaxed epitaxial Si0.7Ge0.3 on (001)Si. The formation of the one-dimensional ordered structure is attributed to the nucleation of NiSi nanodots on the surface undulations induced by step bunching on the surface of SiGe film owing to the miscut of the wafers from normal to the (001)Si direction. The two-dimensional, pseudo-hexagonal structure was achieved under the influence of repulsive stress between nanodots. Since the periodicity of surface bunching can be tuned with appropriate vicinality and misfit, the undulated templates promise to facilitate the growth of ordered silicide quantum dots with selected periodicity and size for utilization in nanodevices.

abstract: This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots (QDs). The first step of this project was to investigate the growth of Ag islands on Si(100). Two distinct families of Ag islands have been observed. “Big islands” are clearly faceted and have basal dimensions in the few hundred nm to μm range with a variety of basal shapes. “Small islands” are not clearly faceted and have basal diameters in the 10s of nm range. Big islands form via a nucleation and growth mechanism, and small islands form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the Stranski-Krastanov layer existing at room-temperature. The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer. Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer. The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.
Dissertation/Thesis
Doctoral Dissertation Physics 2015

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.
Source: Dissertation Abstracts International, Volume: 68-07, Section: B, page: 4759. Adviser: Joseph E. Greene. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

GaAs-based nanowires are attractive building blocks for the development of future (opto)electronic devices owing to their excellent intrinsic material properties, such as the direct band gap and high electron mobility. A pre-requisite for the implementation of novel functionalities on a single Si chip is the monolithic integration of the nanowires on the well-established Si complementary-metal-oxide-semiconductor (CMOS) platform with precise control of the nanowire growth process. The self-catalyzed (Ga-assisted) growth of GaAs nanowires on Si(111) substrates using molecular beam epitaxy has offered the possibility to obtain vertical nanowires with predominant zinc blende structure, while potential contamination by external catalysts like Au is eliminated. Although the growth mechanism is fairly well understood, control of the nucleation stage, the nanowire number density and the crystal structure has been proven rather challenging. Moreover, conventional growth processes are typically performed at relatively high substrate temperatures in the range of 560-630 °C, which limit their application to the industrial Si platform. This thesis provides two original methods in order to tackle the aforementioned challenges in the conventional growth processes. In the first part of this thesis, a simple surface modification procedure (SMP) for the in situ preparation of native-SiOx/Si(111) substrates has been developed. Using a pre-growth treatment of the substrates with Ga droplets and two annealing cycles, the SMP enables highly synchronized nucleation of all nanowires on their substrate and thus, the growth of exceptionally uniform GaAs nanowire ensembles with sub-Poissonian length distributions. Moreover, the nanowire number density can be tuned within three orders of magnitude and independent of the nanowire dimensions without prior ex situ patterning of the substrate. This work delivers a fundamental understanding of the nucleation kinetics of Ga droplets on native-SiOx and their interaction with SiOx, and confirms theoretical predictions about the so-called nucleation antibunching, the temporal anti-correlation of consecutive nucleation events. In the second part of this thesis, an alternative method called droplet-confined alternate-pulsed epitaxy (DCAPE) for the self-catalyzed growth of GaAs nanowires and GaAs/AlxGa1-xAs axial nanowire heterostructures has been developed. DCAPE enables nanowire growth at unconventional, low temperatures in the range of 450-550 °C and is compatible with the standard Si-CMOS platform. The novel growth approach allows one to precisely control the crystal structure of the nanowires and, thus, to produce defect-free pure zinc blende GaAs-based nanowires. The strength of DCAPE is further highlighted by the controlled growth of GaAs/AlxGa1-xAs axial quantum well nanowires with abrupt interfaces and tunable thickness and Al-content of the AlxGa1-xAs sections. The GaAs/AlxGa1-xAs axial nanowire heterostructures are interesting for applications as single photon emitters with tunable emission wavelength, when they are overgrown with thick lattice-mismatched InxAl1-xAs layers in a core-shell fashion. All results presented in this thesis contribute to paving the way for a successful monolithic integration of highly uniform GaAs-based nanowires with controlled number density, dimensions and crystal structure on the mature Si platform.
GaAs-basierte Nanodrähte sind attraktive Bausteine für die Entwicklung von zukünftigen (opto)elektronischen Bauelementen dank ihrer exzellenten intrinsischen Materialeigenschaften wie zum Beispiel die direkte Bandlücke und die hohe Elektronenbeweglichkeit. Eine Voraussetzung für die Realisierung neuer Funktionalitäten auf einem einzelnen Si Chip ist die monolithische Integration der Nanodrähte auf der etablierten Si-Metall-Oxid-Halbleiter-Plattform (CMOS) mit präziser Kontrolle des Wachstumsprozesses der Nanodrähte. Das selbstkatalytische (Ga-unterstützte) Wachstum von GaAs Nanodrähten auf Si(111)-Substrat mittels Molekularstrahlepitaxie bietet die Möglichkeit vertikale Nanodrähte mit vorwiegend Zinkblende-Struktur herzustellen, während die potentielle Verunreinigung der Nanodrähte und des Substrats durch externe Katalysatoren wie Au vermieden wird. Obwohl der Wachstumsmechanismus gut verstanden ist, erweist sich die Kontrolle der Nukleationsphase, Anzahldichte und Kristallstruktur der Nanodrähte als sehr schwierig. Darüber hinaus sind relativ hohe Temperaturen im Bereich von 560-630 °C in konventionellen Wachstumsprozessen notwendig, die deren Anwendung auf der industriellen Si Plattform begrenzen. Die vorliegende Arbeit liefert zwei originelle Methoden um die bestehenden Herausforderungen in konventionellen Wachstumsprozessen zu bewältigen. Im ersten Teil dieser Arbeit wurde eine einfache Prozedur, bezeichnet als surface modification procedure (SMP), für die in situ Vorbehandlung von nativem-SiOx/Si(111)-Substrat entwickelt. Die Substratvorbehandlung mit Ga-Tröpfchen und zwei Hochtemperaturschritten vor dem Wachstumsprozess ermöglicht eine synchronisierte Nukleation aller Nanodrähte auf ihrem Substrat und folglich das Wachstum von sehr gleichförmigen GaAs Nanodraht-Ensembles mit einer sub-Poisson Verteilung der Nanodrahtlängen. Des Weiteren kann die Anzahldichte der Nanodrähte unabhängig von deren Abmessungen und ohne ex situ Vorstrukturierung des Substrats über drei Größenordnungen eingestellt werden. Diese Arbeit liefert außerdem ein grundlegendes Verständnis zur Nukleationskinetik von Ga-Tröpfchen auf nativem-SiOx und deren Wechselwirkung mit SiOx und bestätigt theoretische Voraussagen zum sogenannten Nukleations-Antibunching, dem Auftreten einer zeitlichen Anti-Korrelation aufeinanderfolgender Nukleationsereignisse. Im zweiten Teil dieser Arbeit wurde eine alternative Methode, bezeichnet als droplet-confined alternate-pulsed epitaxy (DCAPE), für das selbstkatalytische Wachstum von GaAs Nanodrähten und GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen entwickelt. DCAPE ermöglicht das Nanodrahtwachstum bei unkonventionell geringeren Temperaturen im Bereich von 450-550 °C und ist vollständig kompatibel mit der Standard-Si-CMOS-Plattform. Der neue Wachstumsansatz erlaubt eine präzise Kontrolle der Kristallstruktur der Nanodrähte und folglich das Wachstum von defektfreien Nanodrähten mit phasenreiner Zinkblende-Struktur. Die Stärke der DCAPE Methode wird des Weiteren durch das kontrollierte Wachstum von GaAs/AlxGa1-xAs axialen Quantentopf-Nanodrähten mit abrupten Grenzflächen und einstellbarer Dicke und Al-Anteil der AlxGa1-xAs-Segmente aufgezeigt. Die GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen sind interessant für den Einsatz als Einzelphotonen-Emitter mit einstellbarer Emissionswellenlänge, wenn diese mit gitterfehlangepassten InxAl1-xAs-Schichten in einer Kern-Hülle-Konfiguration überwachsen werden. Alle Ergebnisse dieser Arbeit tragen dazu bei, den Weg für eine erfolgreiche monolithische Integration von sehr gleichförmigen GaAs-basierten Nanodrähten mit kontrollierbarer Anzahldichte, Abmessungen und Kristallstruktur auf der industriell etablierten Si-Plattform zu ebnen.