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

Optoelectronics – Devices and Applications

Optoelectronics - Devices and Applications is the second part of an edited anthology on the multifaced areas of optoelectronics by a selected group of authors including promising novices to experts in the field. Photonics and optoelectronics are making an impact multiple times as the semiconductor revolution made on the quality of our life. In telecommunication, entertainment devices, computational techniques, clean energy harvesting, medical instrumentation, materials and device characterization and scores of other areas of R&D the science of optics and electronics get coupled by fine technology advances to make incredibly large strides. The technology of light has advanced to a stage where disciplines sans boundaries are finding it indispensable. New design concepts are fast emerging and being tested and applications developed in an unimaginable pace and speed. The wide spectrum of topics related to optoelectronics and photonics presented here is sure to make this collection of essays extremely useful to students and other stake holders in the field such as researchers and device designers

Croissance épitaxiale des matériaux semi-conducteurs III-V et IV sur graphène pour des applications optoélectroniques

L’hétéroépitaxie conventionnelle permet la croissance épitaxiale de couches cristallines de haute qualité sur des substrats compatibles. Cependant, en raison des désaccords de maille/thermique, la formation de défauts cristallins tels que les dislocations dans les couches hétéroépitaxiales entrave fortement l’hétérointégration de divers semi-conducteurs et le développement de dispositifs de haute performance. Alternativement, l’épitaxie Van der Waals (VdWE), un nouveau paradigme de croissance épitaxiale permet la croissance des semiconducteurs cristallins sur des matériaux 2D sans les contraintes susmentionnées. Ce nouveau concept d’hétéroépitaxie mettant en œuvre de faibles interactions de type VdW est principalement dicté par les propriétés de la surface des matériaux 2D. Le graphène, combinant la flexibilité mécanique et la faible liaison de surface, s’est avéré être l’un des matériaux 2D les plus populaires pour la croissance épitaxiale VdW. Ce concept a suscité un grand intérêt dans la communauté scientifique, mais sa mise en œuvre est entravée par la compréhension incomplète et immature des processus gouvernant l’étape de nucléation. Ainsi, cette question reste ouverte, car aucune solution viable n’a encore été proposée et il n’y a pas de vision claire sur cette étape fondamentale de la croissance. En effet, la nucléation étant un processus éphémère est très difficile à observer en détail avec les techniques d’analyse conventionnelles post-croissance. En outre, les chercheurs n’ont pas pu fournir de preuves tangibles pour les nombreuses spéculations proposées sur la cinétique des premiers stades de la croissance des semi-conducteurs à 3D sur le graphène (2D). Ce défi a soulevé la question suivante : « Le graphène est-il le substrat ultime pour résoudre les défis fondamentaux de l’hétérointégration des matériaux ayant des désaccords de maille? ». Pour cette raison, les mécanismes gouvernant la nucléation des matériaux sur graphène font l’objet de débats depuis plus d’une décennie. Dans cette thèse, nous avons démontré pour la première fois l’épitaxie VdW de Ge sur monocouche suspendue de graphène (S-SLG) par observation directe à l’intérieur d’un microscope électronique en transmission (TEM). Pour ce faire, la synthèse de couches de graphène de haute qualité a été développée. Le graphène a été synthétisé sur le Ge (100) par CVD en utilisant une voie compatible avec l’intégration verticale avec les semi-conducteurs. Lors de cette étude, nous avons démontré que l’état de surface du substrat joue un rôle crucial dans la qualité du graphène déposé. Ainsi, un traitement de surface efficace, basé sur l’acide bromhydrique (HBr) a été mis au point. En effet, des couches de graphène de haute qualité avec un ratio ID/IG aussi faible que 0,2 ont été obtenues sur substrat de Ge (100). Pour mieux contrôler la nucléation qui est la phase centrale de la croissance cristalline, nous proposons de nouvelles perspectives à partir des observations en temps réel de la croissance in situ de Ge sur graphène en utilisant un TEM à haute résolution (HR). Nous avons étudié les mécanismes clés régissant la nucléation et la croissance du matériau 3D sur la surface du graphène. Alors que la faible énergie de surface du graphène rend difficile la nucléation de Ge sur la surface propre sans défaut, une stratégie en deux étapes a été proposée : Nucléation à basse température (220 °C) et recuit à des températures plus élevées afin d’améliorer la qualité cristalline des cristaux de Ge. À travers les images HRTEM, nous avons déterminé une taille critique de 0,7-1 nm2 permettant la nucléation de Ge sur SLG. Cette taille critique, qui n’a jamais été rapportée avant ce rapport, est cohérente avec celle prédite par la théorie classique de la nucléation. En outre, les données en temps réel ont révélé que, en raison de faibles interactions vdW, les germes Ge peuvent flotter librement à la surface du substrat de graphène, et leuriv coalescence est gouverné par un processus de murissement d’Ostwald extrême rapide. Notre observation majeure, cependant, a été l’implémentation des doubles hétérostructures de Ge/SLG/Ge (3D/2D/3D). Dans les conditions expérimentales employées, nous avons découvert une diffusion verticale (DV) des particules de Ge à travers le réseau hexagonal compact du graphène. Un tel phénomène intrigant ne peut être observé à l’aide des méthodes conventionnelles d’analyse ex situ, qui sont normalement réalisées après la croissance. Ces résultats ont mis en évidence le mécanisme de croissance des semi-conducteurs sur les monocouches de graphène et ont offert une nouvelle voie vers des dispositifs hybrides semiconducteurs/graphène à hautes performances. Finalement, dans l’ambition d’optimiser la croissance de matériaux III-V avec de bonnes propriétés pour la VdWE, une nouvelle approche d’épitaxie hybride a été mise au point et évaluée. Cette technique hybride d’épitaxie, utilisant à la fois des sources solides et gazeuses, est alternative aux techniques standards comme l’épitaxie par faisceaux moléculaires (MBE) et chimiques (CBE). Les résultats expérimentaux de la croissance d’AlInAs et de GaInAs sur substrats d’InP, ont démontré que cette nouvelle approche est efficace pour croitre des couches avec d’excellentes propriétés cristallines, optiques et à faible dopage résiduel.Abstract : Conventional heteroepitaxy allows epitaxial growth of highly crystalline layers onto compatible substrates. However, due to the lattice/thermal mismatch, the formation of crystalline defects such as dislocations in heteroepitaxial layers severely impede the heterointegration of various semiconductors with complementary properties and the development of high-performance devices. Alternatively, Van der Waals epitaxy (vdWE), a new paradigm of epitaxial growth can enable the growth of crystalline semiconductors on 2D materials without the aforementioned constraints. This new concept of heteroepitaxy, mediated by weak VdW interactions, is mainly dictated by the surface properties of emerging 2D materials, which also have many novel electrical, optical, thermal, and mechanical properties. Graphene, a 2D material that combines mechanical flexibility and weak surface bonding, is found to be one of the most popular for VdW epitaxial growth. This is currently the subject of intense research in the community, but tangible exploitation of such a compelling phenomenon is impeded by the immature and incomplete understanding of the nucleation and growth processes. Therefore, the question remains open as no viable solution was reached yet and there is no clear picture of the nucleation step. In fact, the nucleation process, which is a fleeting event is very difficult to observe in detail by using conventional ex situ analyses. Besides, researchers could not provide hard evidence for the several speculations involving the kinetics of threedimensional (3D) crystal growth over 2D graphene, at early stages of growth process. This challenge raised the following question: “Is graphene the ultimate substrate to solve fundamental challenges of heterointegration of mismatched materials?” For matters as such, governing mechanisms for nucleation on graphene has been debated for more than a decade now; however, despite the promises of VdW epitaxy, the lack of understanding of basic phenomena is still a major obstacle to the new applications. In this thesis, we demonstrated for the first time, in-situ TEM observation of VdWE of Ge on freestanding single layer graphene suspended graphene monolayers (S-SLG). To do this, the synthesis of high-quality graphene layers was developed. Using a compatible approach for the monolithic integration of 3D semiconductors on 2D materials, graphene was synthesized on germanium (Ge) (100) by CVD. In this study, we demonstrated that the physical and chemical surface state of the substrate play a crucial role in the quality of deposited graphene layers. Thus, an effective surface treatment, based on hydrobromic acid (HBr) has been developed. Highquality graphene layers with an ID/IG ratio as low as 0.2 were obtained on Ge (100). To better control the nucleation that is the central phase of crystalline growth, we offer new perspectives from real-time observations of Ge's in situ growth on graphene using a highresolution TEM. Through meticulous analysis of the collected video data, we investigated the key mechanisms governing the nucleation and the growth of sp3-bonded 3D material on the weakly interacting graphene surfaces. Whereas the low surface energy of the graphene layer prevented the nucleation of Ge over pristine and defect-free graphene, a two-step strategy consists of nucleating at low temperature (220 °C) and annealing at higher temperatures, resulted in growth of highly crystalline quality Ge. In view of the high-resolution TEM images, we determined a critical size of 0.7-1 nm2 enabling the nucleation of Ge on SLG. This critical size, which has never been reported prior to this report, is consistent with the one predicted by the classical nucleation theory. Moreover, the real-time data revealed that, due to weak VdW interactions, the Ge germs can freely float on the surface of graphene substrate,vi making the coarsening process of Ge layer to be dominated by a very fast Ostwald ripening process. Our major finding, however, was achieved by examining the 3D/2D/3D (2D/2D/2D) configuration of the Ge/graphene/Ge double heterostructures. Under the experimental conditions employed in our work, we recorded a vertical diffusion (VD) of Ge particles through the closely packed hexagonal ring of single layers of graphene. Such an intriguing and yet unexplored phenomenon cannot be obtained by means of the conventional ex situ analysis methods, which are normally carried out after the growth of material. These findings highlighted the growth mechanism of semiconductors on graphene monolayers and provided a new path to high-performance semiconductor/graphene hybrid devices. Finally, in the ambition to optimize the growth of III-V materials with good properties for VdWE, a new hybrid epitaxy approach has been proposed and evaluated. This hybrid epitaxy technique, using both solid and gaseous sources, is an alternative approach to standard techniques such as molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE). Experimental results of the growth of AlInAs and GaInAs on InP substrates showed that this new approach is effective in growing layers with excellent crystalline, optical properties, and low residual doping

ICCG-10: Tenth International Conference on Crystal Growth. Oral presentation abstracts

Oral presentation abstracts from the tenth International Conference on Crystal Growth (ICCG) (Aug. 16-21, 1992) are provided. Topics discussed at the conference include superconductors, semiconductors, nucleation, crystal growth mechanisms, and laser materials. Organizing committees, ICCG advisory board and officers, and sponsors of the conference are also included

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

Space Application of Novel Solar Cells

Multiple solar cell technologies from three different generations of photovoltaic cells were studied for space applications. The GaAsSb, crystalline first-generation solar cells were studied under low-intensity-low-temperature (LILT) space conditions. In order to examine performance of the cells in energetic irradiation environments this system was exposed to electron irradiation of 1 MeV. Improvement in the carrier extraction was observed in short wavelength regime in the external quantum efficiency (EQE) measurements after electron irradiation. Transmission electron microscope (TEM) images indicated enhanced crystallinity near the top of the cells, coinciding with the region that short wavelength incident light is absorbed. The Cu(InGa)Se2 (CIGS), thin film second generation solar cells were studied under space LILT conditions. These conditions included low Sun intensity at the outer planets in the solar system and their respective temperatures. The unencapsulated CIGS cells were irradiated with 1.5 MeV protons of varying fluence, resulting in significant damage to the performance of the cells. The damage rooted in irradiation induced defects which diffused toward the grain boundaries and resulted in reduced shunt resistance. The degradation manifested more pronounced at low temperatures of the outer planets. A Fresnel lens concentrating system was suggested for the CIGS solar cells to compensate for low light intensity of the outer planets. This concentrating system has a remarkable effect on improving the efficiency of the cells at the planets with dim conditions. Multiple structures of the perovskite solar cells, third generation photovoltaics, were studied in extreme space conditions of irradiation and temperature. Proton irradiation induced defect states inside the perovskite solar cells compromised the performance of the cells. Although, after two months of keeping samples in the dark we observed self-healing in the perovskite cells, resulting in performance of the cells to become very similar to the pre-irradiation conditions. We observed that for irradiation with 1 MeV and extreme fluence level of 4 × 1014 (1/cm2) the solar cell stopped functioning after a few months. Through modifications in architecture of the FACsPb(IBrCl)3 solar cells, very high temperature, 490 K (217 °C), performance of the perovskite solar cells was achieved with 70% of the room temperature efficiency. The modifications to the FACsPb(IBrCl)3 perovsite solar cell included: First, use of a double cation (FACs) composition which improves the stability of the perovskite absorber layer. Second, a transparent conductive back contact was used to prevent metal migration or iodine-metal corrosion. Finally, a alumina-based nanolaminate was applied on top of the cell to prevent thermal decomposition due to loss of volatile species. The investigation of these solar cell technologies under extreme space conditions resulted in finding the weak points and also hidden capacities of these systems. In the case of CIGS solar cells grain boundaries facilitate shunting after irradiation. In perovskite solar cells aside from the stability problem, samples showed very high radiation tolerance and extremely good performance at temperatures more than 200 °C

ICCG-10: Tenth International Conference on Crystal Growth. Poster presentation abstracts

Poster presentation abstracts from the tenth International Conference on Crystal Growth (ICCG) (Aug. 16-21, 1992) are provided. Topics discussed at the conference include crystal growth mechanisms, superconductors, semiconductors, laser materials, optical materials, and biomaterials. Organizing committees, ICCG advisory board and officers, and sponsors of the conference are also included

Development of Resonant Tunnelling Diode Terahertz Emitter

This thesis reports on the development of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for terahertz (THz) applications. A wide variety of characterisation techniques are employed to investigate the material properties and quality on the wafer level allowing future device and growth optimisation. An optical characterisation technique based on photoluminescence (PL) spectroscopy is developed to non-destructively map the doping and structural composition of the RTD on the wafer level. A new non-destructive optical characterisation technique to resolve the absolute energy level position of the first electron state of RTDs through low temperature PL spectroscopy is also reported. The absolute energy levels are resolved by a combination of type-I and type-II quantum well (QW) recombination, allowing an investigation into important scattering mechanisms affecting device performance, and monitor the QW alloy content and thickness non-destructively. Details of the growth process and the characterisation techniques are discussed. A new fabrication technique based on conventional i-line photolithography for micron scale high current density RTD devices is also developed with accurate control over the final device area (and hence characteristics). This is achieved by measuring the V-I characteristic of the RTD during the fabrication process, which has not been previously possible. This was made possible by guiding the emitter current through the full RTD structure by a large second contact electrode on the collector side and using an air-bridge contact to the collector. Important information about the RTD performance is extracted by using this method. Temperature dependent V-I characterisation is also carried out to investigate the valley current of the RTD. Details of the design, fabrication, and characterisation of a room temperature operating THz emitter in the 300 GHz band are reported