<p>  </p> <p>Research that seeks to understand and develop the growth of III-nitride materials by molecular beam epitaxy (MBE) is beneficial to a broad range of the device community. MBE and the III-nitrides have been used to develop transistors, diodes, electroacoustic devices, solar cells, LEDs, LDs, intersubband devices, and quantum-cascade lasers. In this work we focus on the growth of III-nitride materials specifically for applications in near-infrared intersubband (NIR ISB) optical devices, however all this work is broadly applicable. </p> <p><br></p> <p>We begin by investigating the reduced indium incorporation in non-polar m-plane InGaN films. We find that InGaN grown on m-plane GaN has an effective activation energy for thermal decomposition of 1 eV, nearly half that reported for similar c-plane films. We produce high quality m-plane In0.16Ga0.84N and utilize it in AlGaN/InGaN devices designed for near-infrared ISB absorption measurements. We continue this work by exploring the growth of low-temperature AlGaN, necessary for these devices. We find that the utilization of an indium surfactant during low-temperature AlGaN growth enhances adatom diffusion, resulting in smoother surface morphologies, sharper interfaces, and reduced defects within the material. This growth method also prevents the anomalous suppression of the AlGaN growth rate, which we link to a reduction in the formation of high-aluminum containing defects. These investigations result in the demonstration of an Al0.24Ga0.76N/In0.16Ga0.84N heterostructure with a conduction band offset large enough to enable NIR ISB transitions.</p> <p><br></p> <p>Lastly, we explore the novel material ScAlN. This material’s large bandgap, large spontaneous polarization, ferroelectricity, and ability to be lattice matched to GaN at ~18% scandium composition make it an ideal candidate for a variety of devices, including NIR ISB devices. We investigate the reported temperature dependence of ScAlN’s <em>c</em>-lattice constant and confirm this dependence is present for high growth-temperature ScxAl1-xN with 0.11 < x < 0.23. We find that this temperature dependence is no longer present below a certain composition-dependent growth temperature. This finding, coupled with observations that samples grown at lower temperatures exhibit lower defect densities, smoother surfaces, and homogeneous chemical compositions suggest that high growth temperatures lead to defect generation that may cause the observed change in lattice parameters. We demonstrate lattice-matched, 50 repeat Sc0.18Al1-xN/GaN heterostructures with ISB absorption in excess of 500 meV with FWHM as little as 45 meV. </p>

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Festkörper-basierte Kühlkreisläufe, die auf dem elektrokalorischen Effekt beruhen, sind in den vergangenen Jahren in den Mittelpunkt aktueller Forschungen gerückt, da für den direkten Betrieb keine klimaschädlichen Treibhausgase erforderlich sind und da sie das Potential für eine hohe Energieeffizienz aufweisen. Der elektrokalorische Effekt (EKE) beschreibt eine reversible adiabatische Temperaturänderung in polaren Materialien, die durch die Änderung eines äußeren elektrischen Feldes induziert wird. Besonders stark aus-geprägte elektrokalorische Eigenschaften treten für ferroelektrische Materialien im Bereich der Umwandlung zwischen ferro- und paraelektrischer Phase auf. Zudem verstärkt sich der EKE mit zunehmender Feldstärkeänderung. Ferroelektrische Dünnschichten, an die im All-gemeinen hohe elektrische Felder angelegt werden können, zeigen daher gute elektrokalo-rische Eigenschaften. Für das Materialsystem BaZrxTi1-xO3 (BZT) wurde in der Literatur beschrieben, dass Massivproben in Hinblick auf den EKE ein günstiges Eigenschaftsprofil aufweisen. In dieser Arbeit werden BZT–Dünnschichten hergestellt, um die vielverspre-chenden Eigenschaften dieses Materialsystems näher zu untersuchen und um ein besseres Verständnis der zugrundeliegenden physikalischen Vorgänge zu erlangen. Dazu wird ein epitaktisches Schichtwachstum angestrebt, um ein möglichst klar definiertes Gefüge zu erhalten und so den Zusammenhang zwischen mikrostrukturellen, ferroelektrischen und elektrokalorischen Eigenschaften untersuchen zu können. Durch eine Optimierung der Herstellungsbedingungen werden mit Hilfe der gepulsten Laserdeposition epitaktische BZT-Dünnschichten auf (001)-orientierten einkristallinen SrTiO3-Substraten abgeschieden. Dabei werden die hergestellten Proben mit Röntgenbeugungs-, Elektronenmikroskop und-Die durch den EKE induzierte adiabatische Temperaturänderung wird auf Basis einer thermodynamischen Analyse von feld- und temperaturabhängigen Polarisationsmessungen indirekt bestimmt. Extrinsische Einflüsse wie Leckströme oder Randschichteffekte können zu Deformationen der Polarisationhysterese führen und daher eine fehlerhafte Abschätzung des EKE verursachen. Es werden daher zwei Ansätze für eine direkte Charakterisierung des EKE in Dünnschichten beschrieben.

<p>Preparing silicon wafers is a tedious multi-step process that includes etching, polishing, and cleaning. The minimum wafer thickness attainable in current high volume wafer production processes is generally 160 to 300 μm, and the kerf loss for these processes is up to 40% of the total volume. Thin silicon wafers (~30 to 100μm) are very expensive to produce and the wafering process is not cost effective due to the high amount of material loss (more than 80% at these dimensions) during the process and the risk of breakage of the wafers during wafering. In this thesis, a new method called Vapour-Liquid Interface Growth (VLIG) is proposed. VLIG is capable of directly growing a sheet of single crystal silicon without wafering with a thickness of about 30 to 50μm. The features of the process are 1) low temperature operation; 2) the resulting silicon sheet is easily detachable and self-supporting; 3) the resulting sheet has uniform thickness and is single crystal. The system operates in a supersaturated growth solution of an indium-silicon melt. A seed line in a substrate facing down is employed. A layer of single crystal silicon grows on the seed line at the melt surface due to surface segregation during the super cooling process. The grown silicon can grow laterally due to the limited thickness of the melt depth that minimizes growth in the vertical growth direction. The grown silicon can be easily peeled off from the seed line substrate due to the presence of a gap between the grown silicon sheet and the oxide layer on the seed line substrate. The self-supporting silicon sheet now comprises a very thin silicon substrate or sheet.</p> <p>VLIG silicon sheet is characterized by X-ray diffraction to determine the crystallinity. Hall Effect measurements are performed to measure the electrical properties. VLIG silicon sheet is (111) oriented single crystal and it exhibits the same orientation as the substrate. The growth temperature is from 975 to 850^oC, and the VLIG silicon is p-type doped with indium. The resistivity is 4.181x10^-3 ohm-cm, and the doping level is around 5.3.0x10¹⁸ /cm3. The measured mobility is ranging from 280 cm²/V.s. In this study, VLIG demonstrates the potential of growing thin sheet of single crystal silicon with qualities that feasible for photovoltaic application.</p> / Master of Applied Science (MASc)

𝛼-Ga₂O₃ ist ein Halbleiter mit extrem großer Bandlücke, der als solar-blinder Ultraviolett (UV) Photodetektor eingesetzt werden kann. Seine rhomboedrische Kristallstruktur ermöglicht es Legierungen mit isostrukturellem 𝛼-Al₂O₃ und verschiedenen Übergangsmetall-Sesquioxiden wie z.B. 𝛼-Cr₂O₃ und 𝛼-V₂O₃ zu bilden. Damit kann eine Variation der Bandlücke über einen noch nie dagewesenen Spektralbereich vom Infraroten bis zum tiefen UV erreichtwerden. In der vorliegenden Arbeit wird demonstriert, wie räumlich adressierbare 𝛼-(Ga,Al,Cr,V,Ti)₂O₃ Materialbibliotheken durch kombinatorische gepulste Laserabscheidung (c-PLD) realisiert und ihre physikalischen Eigenschaften mittels anschließender Hochdurchsatzmessungen bestimmt werden können. Im ersten Teil dieser Arbeit wird die Entwicklung eines umfassenden Modells für die numerische Beschreibung der lateralen Kompositions- und Schichtdickenverteilung, die bei der c-PLD entstehen, vorgestellt. Durch Identifikation und Korrektur eines Fehlers in einem etablierten Modell zur adiabatischen Expansion des Plasmas, wird erstmals eine realistische Beschreibung von PLD-Prozessen ermöglicht und das Modell für verschiedene Materialien verifiziert. Im zweiten Teil dieser Arbeit wird das Wachstum von 𝛼-Ga₂O₃ Dünnschichten mittels PLD vorgestellt. Dazu wird ein umfassendes Phasendiagramm für das Wachstum von Ga₂O₃ auf m-Saphir erstellt, das ein ausgeprägtes Wachstumsfenster für die metastabile 𝛼-Phase mit hoher struktureller Qualität aufzeigt. Darauf basierend wurden Materialbibliotheken aus (CrₓGa₁₋ₓ)₂O₃, (VₓGa₁₋ₓ)₂O₃ und (TiₓGa₁₋ₓ)₂O₃ mit kontinuierlicher Kompositionsverteilung durch c-PLD hergestellt. Ihre physikalischen Eigenschaften wurden durch lateral aufgelöste Röntgenbeugung (XRD), energiedispersive Röntgenspektroskopie und Transmissionsmessungen erfasst. Es werden Vergleiche zwischen der gemessenen Verteilung der Zusammensetzung der Dünnschichten und dem zuvor entwickelten c-PLD-Modell erörtert, die Aufschluss über die Wachstumskinetik der verschiedenen Sesquioxide geben. Sowohl für (CrₓGa₁₋ₓ)₂O₃ als auch für (VₓGa₁₋ₓ)₂O₃ wurde das phasenreineWachstum in der rhomboedrischen 𝛼-Phase durchXRD-Messungen über den untersuchten Zusammensetzungsbereich von 0,08< x(Cr) <0,54 und 0,07 < x(V)<0,62 bestätigt. Die Absorptionsenergie der 𝛼-(VₓGa₁₋ₓ)₂O₃-Dünnfilme zeigte eine systematische Verschiebung bei steigendem x(V) von 5,3 eV auf 2,9 eV. Damit wird zum ersten Mal eine Bandlückenverschiebung hin zu niedrigeren Energien innerhalb des rhomboedrischen Sesquioxid-Material-systems über einen derart breiten Spektralbereich demonstriert. Diese Ergebnisse sind vielversprechend für mögliche Anwendungen des Materialsystems, z.B. als wellenlängenselektiver Photodetektor,:1. Introduction 2. Theoretical Background 2.1 Material Properties 2.1.1 Rhombohedral 𝛼-Ga2O3 2.1.2 Other Ga2O3 Polymorphs 2.1.3 Ternary (Me,Ga)2O3 Alloys 2.1.4 Growth mechanisms of Ga2O3 2.2 Layer Thickness Distributions for PLD Growth 2.2.1 Approaches to Layer Thickness Distributions 2.2.2 Adiabatic Plasma Plume Expansion Model for Pulsed Laser Deposition 3. Experimental Methods 3.1 Growth methods and sample preparation 3.1.1 Pulsed Laser Deposition 3.1.2 Photolithography 3.2 Characterization Techniques 3.2.1 X-Ray Diffraction 3.2.2 Spectroscopic Ellipsometry 3.2.3 Transmission Measurements 3.2.4 Atomic Force Microscopy 3.2.5 Profilometry 3.2.6 Energy-Dispersive X-Ray Spectroscopy 4. Analysis of PLD-Thickness Distributions and Applications to High-Throughput Combinatorial PLD 4.1 Analytical Description of PLD Thickness Distributions 4.2 Analysis of Lateral Thickness Distributions of Sesquioxide thin films 4.3 Modelling of combinatorial PLD for arbitrary target segmentations 5 A Novel PLD-Control Software and FAIR-Data Management 5.1 Digital Data Management 5.2 A Digital Twin for PLD 5.3 cPLD - Software 6. Growth of Phase-Pure, Highly Crystalline 𝛼-Ga2O3 Thin Films by PLD 6.1 Influence of Substrate Orientation 6.2 Influence of Growth Temperature 6.3 Influence of Layer Thickness 6.4 Comprehensive 𝑝(O2)-𝑑-𝑇g-phase diagram for PLD of Ga2O3 on m-plane sapphire 6.5 Achieving thick 𝛼-Ga2O3 layers on m-plane sapphire 6.6 Structuring 𝛼-Ga2O3 by Sacrificial ZnO Layers 6.7 High-Throughput Electrical Property Screening 6.8 Intermediate Summary 7 Ternary Alloys of 𝛼-Ga2O3 and Transition Metal Sesquioxides 7.1 Preliminary investigations of binary Cr2O3 and Ti2O3 thin films 7.2 Investigations of ternary (Ga,TM)2O3 alloys 7.2.1 Characterization of (Ga,Cr)2O3 thin films 7.2.2 Characterization of (Ga,V)2O3 thin films 7.2.3 Characterization of (Ga,Ti)2O3 thin-films 7.3 Intermediate Summary 8 Summary and Outlook 8.1 Summary 8.2 Outlook Abbreviations List of Symbols List of Electronic Lab Book References List of Own and Contributed Articles Collaborations and third-party services Supervisors Institutes Bibliography Appendix Acknowledgement / 𝛼-Ga₂O₃ is an ultra-wide bandgap semiconductor, with potential applications as a solar blind ultraviolett (UV) photodetector. Due to its rhombohedral crystal structure, alloying to isostructural 𝛼-Al₂O₃ and various transition metal sesquioxides like e.g., 𝛼-Cr₂O₃ and 𝛼-V₂O₃, enables bandgap engineering over an unprecedented large spectral range from the infrared to the deep-UV. In the present work, the realization of spatially-addressable 𝛼-(Ga,Al,Cr,V,Ti)₂O₃ material libraries by combinatorial pulsed laser deposition (c-PLD) and the subsequent high-throughput screening of their physical properties are discussed. In the first part of this thesis the development of a comprehensive model for the numerical description of lateral composition and thickness distributions arising during c-PLD are presented. An error in a well-established adiabatic plasma plume expansion model is identified and corrected, such that a real-world description of PLD processes is feasible from now on. In the second part of this thesis, the growth of 𝛼-Ga2O3 thin films by PLD is presented. Therefore, for the first time a comprehensive phase diagram for the growth of Ga2O3 on m-plane sapphire is constructed, exhibiting a distinct growth window for metastable 𝛼-Ga₂O₃ thin films with up to now unprecedented structural quality. Based on optimized process parameters, material libraries of (CrₓGa₁₋ₓ)₂O₃, (VₓGa₁₋ₓ)₂O₃ and (TiₓGa₁₋ₓ)₂O₃ with continuous composition spread were deposited by c-PLD. Their physical properties were mapped by high throughput laterally resolved X-ray diffraction, energy-dispersive X-ray spectroscopy and transmission measurements. Comparisons of the measured compositional distribution of the thin films to the c-PLD model developed earlier are discussed, revealing insight into the growth kinetics of the different sesquioxides. For both, (CrₓGa₁₋ₓ)₂O₃ and (VₓGa₁₋ₓ)₂O₃, phase-pure growth in the rhombohedral 𝛼-phase was confirmed by XRD measurements over the investigated composition range of 0. 08<𝑥(Cr) <0. 54 and 0. 07<𝑥(V) <0. 62. The absorption onset energy of the 𝛼-(VₓGa₁₋ₓ)₂O₃ thin films showed a systematic shift for increasing 𝑥(V) from 5.3 eV to 2.9 eV. With that, bandgap engineering within the rhombohedral sesquioxide material system towards lower energies over a wide spectral range is demonstrated for the first time. These results are promising for possible applications of the material system as e.g., a wavelength selective photodetector.:1. Introduction 2. Theoretical Background 2.1 Material Properties 2.1.1 Rhombohedral 𝛼-Ga2O3 2.1.2 Other Ga2O3 Polymorphs 2.1.3 Ternary (Me,Ga)2O3 Alloys 2.1.4 Growth mechanisms of Ga2O3 2.2 Layer Thickness Distributions for PLD Growth 2.2.1 Approaches to Layer Thickness Distributions 2.2.2 Adiabatic Plasma Plume Expansion Model for Pulsed Laser Deposition 3. Experimental Methods 3.1 Growth methods and sample preparation 3.1.1 Pulsed Laser Deposition 3.1.2 Photolithography 3.2 Characterization Techniques 3.2.1 X-Ray Diffraction 3.2.2 Spectroscopic Ellipsometry 3.2.3 Transmission Measurements 3.2.4 Atomic Force Microscopy 3.2.5 Profilometry 3.2.6 Energy-Dispersive X-Ray Spectroscopy 4. Analysis of PLD-Thickness Distributions and Applications to High-Throughput Combinatorial PLD 4.1 Analytical Description of PLD Thickness Distributions 4.2 Analysis of Lateral Thickness Distributions of Sesquioxide thin films 4.3 Modelling of combinatorial PLD for arbitrary target segmentations 5 A Novel PLD-Control Software and FAIR-Data Management 5.1 Digital Data Management 5.2 A Digital Twin for PLD 5.3 cPLD - Software 6. Growth of Phase-Pure, Highly Crystalline 𝛼-Ga2O3 Thin Films by PLD 6.1 Influence of Substrate Orientation 6.2 Influence of Growth Temperature 6.3 Influence of Layer Thickness 6.4 Comprehensive 𝑝(O2)-𝑑-𝑇g-phase diagram for PLD of Ga2O3 on m-plane sapphire 6.5 Achieving thick 𝛼-Ga2O3 layers on m-plane sapphire 6.6 Structuring 𝛼-Ga2O3 by Sacrificial ZnO Layers 6.7 High-Throughput Electrical Property Screening 6.8 Intermediate Summary 7 Ternary Alloys of 𝛼-Ga2O3 and Transition Metal Sesquioxides 7.1 Preliminary investigations of binary Cr2O3 and Ti2O3 thin films 7.2 Investigations of ternary (Ga,TM)2O3 alloys 7.2.1 Characterization of (Ga,Cr)2O3 thin films 7.2.2 Characterization of (Ga,V)2O3 thin films 7.2.3 Characterization of (Ga,Ti)2O3 thin-films 7.3 Intermediate Summary 8 Summary and Outlook 8.1 Summary 8.2 Outlook Abbreviations List of Symbols List of Electronic Lab Book References List of Own and Contributed Articles Collaborations and third-party services Supervisors Institutes Bibliography Appendix Acknowledgement

The incredible technological advancements over the last century have been possible due to tiny trinkets designed using semiconducting crystalline materials, especially Silicon and III-V compounds. Silicon, a group IV element has become the first choice in developing microchips serving an ever-growing set of applications including, computation, RF communications, solar cells, power electronics, quantum computing and its periphery, optoelectronics, IOT sensors, and lately artificial intelligence. Billions of Si-based complementary transistors (CMOS) are present at the center of most computing devices used today such as HPC servers, compute farms, laptops, and smartphones. The astonishing rise in transistor count, performance, and functionality as well as the exponential reduction in cost has been possible over the past decades due to a singular idea: shrinking the device. However, this rule, also called Moore's Law has been slowing over the past two decades and has eventually come to a standstill in its traditional definition. Moore's law has since been sustained by ingenious innovations such as high-k gate dielectrics, vertical scaling, lattice strain engineering, novel material developments and, lately chiplets as well as multi-die vertical packaging. As conventional Si CMOS approaches a roadblock, this work presents research on Germanium-based multi-gate devices providing promise for faster and low-power operation. This work discusses how Ge grown on a GaAs substrate can be tuned and utilized to form a virtually defect-free channel for ultra-scaled multi-gate transistors. Calibrated solvers informed using in-house materials and devices as well as literature are used to predict device performance for advanced structures. Further, a hybrid CMOS system with the high hole mobility p-channel device formed using tensile strained Ge, and the high electron mobility n-channel device formed using the underlying InGaAs layer is proposed and simulated. As scaling approaches Gate-all-around Nanosheet FETs in 2024 and complementary-FETs (CFETs) around 2034, Ge-on-AlAs based transistors can offer unique process simplifications, defect reduction, yield improvement, and high-performance advantages showing promise for future IRDS nodes. The process design, material stack, device, and circuit performance for this novel Ge-based NSFET is presented in this work. The lack of large strain or strain relaxation in the NS multilayer starting stack is seen to be a great process advantage for the Ge-AlAs NSFET system. To a certain extent, Si seems omnipotent for all things electronics. However, one exception is on-chip light generation. A coherent electrically controllable on-chip light source is a central component critical for optoelectronics, quantum technologies, fiber communications, and sensing. Due to the indirect bandgap, Si cannot produce light hence direct bandgap materials such as GaAs and GaN have been the primary choice for off-chip light sources integrable on the platform. Interestingly, Ge has a pseudo-direct bandgap, i.e., unlike Silicon, it can be manipulated to produce light using heavy doping, tensile strain, and Sn alloying. Similar to conventional III-V light sources, reduction in the dimensionality of the gain medium, i.e., Ge can enable a drastic reduction in the current required to produce light, among other performance considerations. This reduced dimensionality can be achieved by forming quantum wells and quantum dots. In this work, two new types of Ge-based quantum well lasers are introduced and analyzed along with qualitative and quantitive benchmarking. The first QW laser uses a small epitaxial biaxial tensile strain to improve the direct-ness of the Ge gain medium. The internal quantum efficiency, net gain, and threshold current can be improved drastically by choosing the right tensile strain while staying within a certain critical thickness value. For the first time, the impact of biaxial tensile strain on the optical properties of Ge is analyzed and reported through a systematic study of the dielectric spectra and optical constant using VASE. The changes in the band structure due to tensile strain are correlated with the critical points to uncover various optical transitions. An even better QW laser architecture is possible by utilizing a GeSn QW. This QW laser uses Sn-alloying to form a GeSn active region which is further lattice matched to the waveguide (InGaAs) and the optical confinement layers (InAlAs) around it. This completely lattice-matched laser structure can offer unique advantages such as virtually defect-free active region, tunability as well as improved efficiency and threshold current density. The absence of strain and consequently strain relaxation in the laser stack enables one to steer away from the critical thickness limitation while opening doors to designing multiple quantum well lasers among other complex architectures. The impact of Sn alloying on the atomic structure, lattice coherence, and relaxation is analyzed through XRD reciprocal space maps and rocking curves as a function of Sn concentration. Further, this lattice-matched system, GeSn-InGaAs-InAlAs has the potential to mirror the benefits of the mature GaAs-AlGaAs system which led to many great technological innovations over the past decades such as lasers and LEDs. / Doctor of Philosophy / This thesis introduces two transistor technologies to extend the scaling beyond conventional Si devices into the next decade, and two QW laser technologies for integrated photonics. Through calibrated numerical solvers, a high mobility Ge and InGaAs cointegrated CMOS system for 0.5 V is introduced, analyzed and benchmarked with literature. A lattice matched Ge-on-AlAs multilayer stack is shown to have great potential to form a novel CMOS system which uses Ge Nanosheets, providing process advantage and superior performance. The next part of the thesis introduces two types of Ge based quantum well lasers, one based on tensile strained Ge and the other based on lattice matched Ge. Both show large performance improvements over previous attempts in literature. Lasing from an indirect bandgap material such as Ge, the associated challenges and performance metrics are discussed. Lastly, the optical, dielectrics and CP properties of tensile strained are presented for the first time uncovering interesting trends. Ge samples with increasing tensile strain are grown using MBE and measured using VASE to elucidate the physical phenomenon.