Croissance du GaInAs par épitaxie hybride et investigation de l’effet mémoire du germanium dans un réacteur III-V CBE pour des applications optoélectroniques

Au cours des dernières années, les énergies renouvelables et les technologies vertes se sont répandues afin de répondre au besoin de l’Homme tout en préservant l’environnement et en en contribuant a une nouvelle prosperité économique. Plusieurs domaines, dont l’optoélectronique, se sont développés allant du secteur industriel au grand public. L’essor de la technologie durable et verte a ainsi permis à l’optoélectronique d’envahir de nombreux domaines entre autres celui des télécommunications, de l’électronique militaire et du photovoltaïque. Ces développements sont rendus possibles par le biais de matériaux/alliages semi-conducteurs III-V et IV et du procédé de fabrication utilisée pour fabriquer le dispositif. Les matériaux III-V et IV présentent des propriétés intrinsèques uniques qui sont intéressantes pour de nombreux dispositifs qui utilisent les semiconducteurs comme par exemple : les cellules solaires à hautes performances et des récepteurs optiques tels que les photodiodes à avalanches, en télécommunication. De telles propriétés de matériaux sont étroitement liées à la technique de croissance (épitaxie) employée pour développer ces matériaux/alliages cristallins. Par exemple, le GaInAs présente une forte sensibilité aux conditions de croissance. Sa pureté nécessaire pour le bon fonctionnement des photodiodes à avalanche dépend donc de la technique de croissance utilisée. Par ailleurs en ce qui concerne les cellules solaires, l’utilisation de lumière concentrée se veut une méthode pour réduire les coûts de l’électricité. Cette réduction est toujours en cours, car elle est fortement dépendante de l’efficacité des cellules solaires sous lumière concentrée. Dans le cadre de cette thèse, nous avons évalué le portentiel d’une nouvelle technique d’épitaxie dite épitaxie hybride. Celle-ci est obtenue en modifiant notre réacteur CBE, ce qui a permis de croître sur InP une haute qualité et pureté de ternaires III-V (GaInAs et AlInAs). Ceci suggère ainsi l’implémentation de ces alliages dans des hétérostructures épitaxiales de photodiodes à avalanche à haute performance. La technique d’épitaxie hybride du GaInAs sur InP utilise l’arsine comme source d’arsenic, puis l’indium solide et le TEGa, respectivement comme précurseur d’indium et de gallium. La comparaison de cette technique avec des techniques conventionnelles comme la CBE, la MOCVD et la MBE atteste qu’un intérêt particulier devrait être porté à rendre mature cette technique pour une plus large utilisation. L’utilisation simultanée du groupe IV (Ge) et du groupe III-V dans notre réacteur III-V CBE a démontré un effet mémoire du Ge dans les alliages III-V. Les méthodes developpées pour limiter cet effet mémoire ont permis de croître du Ge de haute qualité et du GaAs pour les cellules solaires et les transistors bipolaires. Et enfin, une fabrication de cellule solaire double jonction GaInP/GaAs a été faite et ses performances sous lumière concentrée ont confirmé aussi bien la bonne qualité de l’hétérostructure de la cellule que du procédé de fabrication. Cette dernière ouvre ainsi la voie vers son intégration dans une cellule solaire quatre jonctions GaInP/GaAs/SiGeSn/Ge pour de plus hautes performances sous lumière concentrée.Abstract : In recent years, green energy and sustainable technologies have spread to meet human needs while preserving the environment and ensuring a good economy. Several sectors including optoelectronic have been developed, extending from the industrial sector to the public. The rise of sustainable technology has thus enabled optoelectronics to invade many fields such as telecommunications, military electronics, photovoltaics and many others. These developments are made possible by III-V and Group IV semiconductor materials/alloys whose unique intrinsic properties are attractive for high-performance solar cells and optical receivers such as avalanche photodiodes in telecommunications. Such properties are closely related to the growth technique (epitaxy) used to develop these materials/alloys. For example, GaInAs has a high sensitivity to growth conditions and its purity required for the proper functioning of avalanche photodiodes is therefore dependent on the growth technique used. In addition to the material properties, the fabrication technique also has a direct impact on the performance of the optoelectronic devices, including solar cells under concentrated light. In fact, the use of concentrated light is intended to be an approach to reduce electricity costs. This reduction is still ongoing because it is highly dependent on the efficiency of solar cells under concentrated light. In this research, we have developed a new epitaxy technique called hybrid epitaxy obtained by customizing our CBE reactor. It has allowed the growth of high quality and purity of III-V ternary alloys (GaInAs and AlInAs) on InP substrates. The latter permits the implementation of these alloys in epitaxial heterostructures of high-performance avalanche photodiodes. The hybrid epitaxy technique of GaInAs on InP substrates uses arsine as a source of arsenic, followed by solid indium and Triethylgallium (TEGa) as a precursor of indium and gallium, respectively. This approach uniquely combines solid sources and metalorganic sources. We demonstrate that the solid sources have the advantage of producing lower doping backgrounds. Comparison of this technique with conventional techniques such as CBE, MOCVD and MBE indicates that interest should be given to maturing this technique for wider use. Furthermore, the simultaneous use of group IV (Ge) and group III-V in our III-V CBE reactor has demonstrated a memory effect of Ge in III-V alloys, and the processes used to limit this memory effect have established the possibility of growing high-quality Ge and GaAs for solar cells and bipolar transistors. Finally, a GaInP/GaAs double junction solar cell has been fabricated. Its performance under concentrated light confirmed both the good quality of the heterostructure of the cell and of the fabrication process. This opens the way to its integration into a four-junction GaInP/GaAs/SiGeSn/Ge solar cell for higher performance under concentrated light

Development of telecom wavelength InAs Quantum Dot lasers by MOCVD

The subject of this thesis is to develop quantum dot lasers around the telecom wavelengths of 1300nm and 1550nm for optical fibre communications. Quantum dots (QD) were grown by Metal-Organic Vapour Deposition (MOCVD) utilising both the conventional Stranski-Krastanov (S.K) method and a novel droplet epitaxy (DE) approach. The first section compares 1.1 µm InAs/GaAs QD lasers grown on the on-axis GaAs(100) substrates and substrates offcut 3° towards (110). QD lasers on the off-axis substrates had a lower threshold current density (Jth) and higher gain. An ~20% increase in the QD density for the 3-degree off-axis sample was found compared to the on-axis samples. The higher QD density is related to the change of morphology of the GaAs spacer layer, with more steps formed on the surface of the off-axis GaAs, providing a favourable nucleation site for QDs. These 1.1µm QDs are the first step towards the future realisation of 1.3µm QD lasers by MOCVD in comparison to MBE literature of 1310nm QDs. The longer term aim of the research is to incorporate these QD lasers on Silicon substrate for photonic integration. The second section presents the design, growth and characterisation of 1550nm InAs QD lasers grown by Droplet Epitaxy. The DE approach has several potential advantages, including removing the influence of the well-known wetting layer seen in the SK growth of QDs. The QD structure with the InP waveguide layer blue-shifts the QDs' wavelength to 1530nm, close to the target wavelength of ~1550nm. An approximately five times increase in emission intensity from QD samples was achieved when the Zn doping was reduced. The waveguide material was changed to InP, which significantly improved the carrier injection into the QDs. A high tail of diffused arsenic and a high oxygen concentration observed on the QD & QW samples may be preventing lasing. A new Quantum Well structure incorporating AlInGaAs as waveguide layer material was also introduced and formed the basis of further optimisation in future work to achieve room temperature lasing of these DE QD lasers

Molecular Beam Epitaxial (MBE) Growth of High Quality InP and Al0.48In0.52As

This thesis concerns the growth and characterisation of InP grown by MBE and MOMBE; and AlInAs grown by MBE. The growth of undoped InP epitaxial layers using solid sources was examined. The quality of the layers was assessed using the techniques of capacitance voltage electrochemical (CV) profiling, photoluminescence (PL) and Hall effect measurement. By controlling systematically the three growth parameters of substrate temperature, growth rate and flux ratio, materials of improved quality were produced as shown by narrower PL line-width, and higher Hall mobilities. A thorough study of n-type doping in InP using sulphur generated from an electrochemical Knudsen source has been performed. The experimental results are explained in terms of a kinetic model of sulphur incorporation and desorption in conjunction with a thermodynamic analysis of the effect of substrate temperature and flux ratios. The results show that sulphur is a controllable dopant with well defined behaviour in InP. An attempt has been made to produce p-type InP using magnesium doping. The electrical and optical properties of these doped layers were comparable to the undoped samples. However, the surface morphology of the doped samples was found to deteriorate as the magnesium flux was increased. However there was little effect on the electrical or optical characteristics. Epitaxial InP layers grown by MOMBE at BTRL were also characterised. Magnesium, sulphur and silicon were found to be the residual shallow impurities present in these samples. Deep Level Transient Spectroscopy (DLTS) measurements shows that all electron traps observed in the MOMBE material have already been observed in InP grown by solid sources. Impurity band conduction exists in these samples which dominates the Hall measurements at low temperatures. The MOMBE samples have comparable electrical and optical properties to the best InP layers grown by solid source MBE. The ternary compound of AlInAs grown lattice matched to InP was characterised. During growth it was observed that there is a region of substrate temperature which produces a rough surface. This region of rough surface growth was correlated with an increase in the PL line-width as well as the appearance of unique deep traps in the DLTS measurements. Growth at elevated temperatures led to an increase in the loss of indium leading to mismatched layers. The thesis concludes with a study of PL recombination mechanisms associated with the observed emission spectrum from AlInAs. Using a Diamond Anvil Cell (DAC) the PL dependence on hydrostatic pressure was determined

Development and Life Cycle Assessment of Advanced-Concept III-V Multijunction Photovoltaics

III-V semiconductors make for highly efficient solar cells, but are expensive to manufacture. However, there are many mechanisms for improving III-V photovoltaics in order to make them more competitive with other photovoltaic (PV) technologies. One possible method is to design cells for high efficiency under concentrated sunlight, effectively trading expensive III-V material for cheaper materials such as glass lenses. Another approach is to reduce the amount of III-V material necessary for the same power output, which can be achieved by removing the substrate and installing a reflector on the back of the cell, while also adding quantum structures to the cell to permit absorption of a greater portion of the solar spectrum. Regarding the first approach, this dissertation focused on the development of an InAlAsSb material for a mulitjunction design with the potential of achieving 52.8% efficiency under 500 suns. First, development of a single-junction InAlAs cell lattice-matched to InP was executed as a preliminary step. The InAlAs cell design was optimized via simulation, then grown via metal organic vapor phase epitaxy (MOVPE) and fabricated resulting in 17.9% efficiency under 1-sun AM1.5, which was unprecedented for the InAlAs material. Identical InAlAs cells were grown using alternative MOVPE precursors to study the effects of necessary precursors for InAlAsSb. Fits to experimental device results showed longer lifetimes when grown with the alternative aluminum precursor. InAlAsSb grown using these alternative precursors targeted a 1.8 eV bandgap required for the multijunction design. Ultimately, InAlAsSb material with the desired bandgap was confirmed by photoreflectance spectroscopy. For the second approach, this dissertation studied the integration of InAs quantum dots (QDs) in a GaAs solar cell in conjunction a back surface reflector (BSR). A quantum dot solar cell (QDSC) with a BSR has the potential to increase short-circuit current by 2.5 mA/cm2 and also increase open-circuit voltage due to photon recycling. In this study, multiple textured BSRs were fabricated by growing inverted QDSCs on epitaxial lift-off templates and then texturing the rear surface before removing the device from the substrate. Identical cells with a flat BSR served as controls. Optimization of inverted QDSC growth conditions was also performed via a cell design study. Device results showed increased open-circuit voltage with increasing optical path length, and the greatest improvement in sub-band current over a flat BSR control device was 40%. In the final chapter, a life cycle assessment (LCA) of these technologies was performed to identify the hypothetical optimum at which energy investments in cell performance (such as the two described above) no longer correspond to improvements in the overall life cycle performance of the PV system. Four cell designs with sequentially increasing efficiencies were compared using a functional unit of 1 kWp. The first is a commercially available and has been studied in previous LCAs. The second is the design containing InAlAsSb mentioned above. The third represents the most material-intensive option, which bonds two substrates to create a five-junction cell. The fourth is a six-junction cell that uses a metamorphic grade between subcells and represents the most energy-intensive option. A thorough literature review of existing LCAs of high-concentration photovoltaic (HCPV) systems was performed, which obviated the need for data on the manufacture of MOVPE precursors and substrates. LCAs for the most common III-V substrate (GaAs) and precursors were executed prior to conducting the HCPV system LCA, due to the absence of detailed information on the life cycle impacts of these compounds in literature. Ultimately, both the cumulative energy demand and greenhouse gas emissions of the HCPV system decreased proportionally with increasing cell efficiency, even for the most energy and material-intensive cell designs. It was found that the substrates and precursors corresponded to less than 2% of system impacts. This implies that current mechanisms to increase cell efficiency are environmentally viable in HCPV applications without the need for material reduction, and would make III-V HCPV more environmentally competitive with dominant silicon PV technologies

Growth and characterization of InAs quantum dots prepared by low-pressure metal-organic vapor phase epitaxy using N2 as carrier gas.

Wang Hui.Thesis (M.Phil.)--Chinese University of Hong Kong, 2004.Includes bibliographical references (leaves 78-84).Abstracts in English and Chinese.ABSTRACT (ENGLISH) --- p.iABSTRACT (CHINESE) --- p.iiiACKNOWLEDGEMENTS --- p.ivCONTENTS --- p.vChapter CHAPTER 1: --- INTRODUCTION --- p.1Chapter 1.1 --- Motivation behind This Study --- p.1Chapter 1.2 --- Concept of Quantum Dots --- p.2Chapter 1.2.1 --- Introduction to SemiconductorsChapter 1.2.2 --- From Bulk Semiconductor to Quantum DotsChapter 1.3 --- Device Applications of Quantum Dots --- p.10Chapter 1.3.1 --- Laser DiodesChapter 1.3.2 --- Infrared PhotodetectorsChapter CHAPTER 2: --- METAL-ORGANIC VAPOR PHASE EPITAXY --- p.14Chapter 2.1 --- Introduction in Epitaxial Growth --- p.14Chapter 2.1.1 --- What's EpitaxyChapter 2.1.2 --- Heteroepitaxy TechniquesChapter 2.2 --- MOVPE Growth System and Principles --- p.15Chapter 2.2.1 --- What's MOVPEChapter 2.2.2 --- MOVPE Chemistry and Basic ConceptsChapter 2.2.3 --- Growth Regimes in MOVPE ProcessChapter CHAPTER 3: --- SELF-ASSEMBLED QUANTUM DOT GROWTH THEORY --- p.23Chapter 3.1 --- Strained Layer Heteroepitaxy --- p.23Chapter 3.2 --- Epitaxial Growth Modes --- p.24Chapter 3.2.1 --- IntroductionChapter 3.2.2 --- Frank-van-der-Merve ModeChapter 3.2.3 --- Stranski-Krastanow ModeChapter 3.2.4 --- Volmer-Weber ModeChapter 3.3 --- Self-assembly of Quantum Dots --- p.30Chapter 3.4 --- Current Issues and Problems --- p.30Chapter CHAPTER 4: --- EXPERIMENTAL METHODS --- p.34Chapter 4.1 --- Equipment --- p.34Chapter 4.2 --- Preparation of Sample --- p.34Chapter 4.3 --- Growth Rate and Composition Determination --- p.35Chapter 4.4 --- Characterization Techniques --- p.40Chapter 4.4.1 --- Atomic Force Microscopy (AFM)Chapter 4.4.2 --- Photoluminescence (PL)Chapter 4.4.3 --- Other TechniquesChapter CHAPTER 5: --- RESULTS AND DISCUSSION --- p.44Chapter 5.1 --- Introduction --- p.44Chapter 5.2 --- Formation Trends of In As QDs --- p.45Chapter 5.2.1 --- Experimental ProceduresChapter 5.2.2 --- Results and DiscussionChapter 5.2.2.1 --- Effect of Growth TemperatureChapter 5.2.2.2 --- Effect of Growth RateChapter 5.2.2.3 --- Effect of In As CoverageChapter 5.2.2.4 --- Effect of Buffer Layer MaterialChapter 5.2.3 --- SummaryChapter 5.3 --- Annealing of InAs QDs under Dissimilar Ambient Flux --- p.65Chapter 5.3.1 --- Experimental ProceduresChapter 5.3.2 --- Results and DiscussionChapter 5.3.3 --- SummaryChapter CHAPTER 6: --- CONCLUSIONS --- p.75Chapter 6.1 --- Summary --- p.75Chapter 6.2 --- Future Work --- p.77BIBLIOGRAPHY --- p.78PUBLICATION LIST --- p.84APPENDIX: Abbreviations --- p.8