Molecular Beam Epitaxy of β-(InxGa1-x)2O3 on β-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement

In this work, we investigate the growth of monoclinic β-(InxGa1-x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1-x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (10 2 ̅ ) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Study of β-Ga2O3 Ceramics Synthesized under Powerful Electron Beam

The synthesis of β-Ga2O3 ceramic was achieved using high-energy electron beams for the first time. The irradiation of gallium oxide powder in a copper crucible using a 1.4 MeV electron beam resulted in a monolithic ceramic structure, eliminating powder particles and imperfections. The synthesized β-Ga2O3 ceramic exhibited a close-to-ideal composition of O/Ga in a 3:2 ratio. X-ray diffraction analysis confirmed a monoclinic structure (space group C2/m) that matched the reference diagram before and after annealing. Photoluminescence spectra revealed multiple luminescence peaks at blue (~2.7 eV) and UV (3.3, 3.4, 3.8 eV) wavelengths for the synthesized ceramic and commercial crystals. Raman spectroscopy confirmed the bonding modes in the synthesized ceramic. The electron beam-assisted method offers a rapid and cost-effective approach for β-Ga2O3 ceramic production without requiring additional equipment or complex manipulations. This method holds promise for fabricating refractory ceramics with high melting points, both doped and undoped.

Structure and Thermal Stability of ε/κ-Ga2O3 Films Deposited by Liquid-Injection MOCVD

We report on crystal structure and thermal stability of epitaxial ε/κ-Ga2O3 thin films grown by liquid-injection metal−organic chemical vapor deposition (LI-MOCVD). Si-doped Ga2O3 films with a thickness of 120 nm and root mean square surface roughness of ~1 nm were grown using gallium-tetramethylheptanedionate (Ga(thd)3) and tetraethyl orthosilicate (TEOS) as Ga and Si precursor, respectively, on c-plane sapphire substrates at 600 °C. In particular, the possibility to discriminate between ε and κ-phase Ga2O3 using X-ray diffraction (XRD) φ-scan analysis or electron diffraction analysis using conventional TEM was investigated. It is shown that the hexagonal ε-phase can be unambiguously identified by XRD or TEM only in the case that the orthorhombic κ-phase is completely suppressed. Additionally, thermal stability of prepared ε/κ-Ga2O3 films was studied by in situ and ex situ XRD analysis and atomic force microscopy. The films were found to preserve their crystal structure at temperatures as high as 1100 °C for 5 min or annealing at 900 °C for 10 min in vacuum ambient (<1 mBar). Prolonged annealing at these temperatures led to partial transformation to β-phase Ga2O3 and possible amorphization of the films.

State-of-the-Art β-Ga2O3 Field-Effect Transistors for Power Electronics

Due to the emergence of electric vehicles, power electronics have become the new focal point of research. Compared to commercialized semiconductors, such as Si, GaN, and SiC, power devices based on β-Ga₂O₃ are capable of handling high voltages in smaller dimensions and with higher efficiencies, because of the ultrawide bandgap (4.9 eV) and large breakdown electric field (8 MV cm^-1). Furthermore, the β-Ga₂O₃ bulk crystals can be synthesized by the relatively low-cost melt growth methods, making the single-crystal substrates and epitaxial layers readily accessible for fabricating high-performance power devices. In this article, we first provide a comprehensive review on the material properties, crystal growth, and deposition methods of β-Ga₂O₃, and then focus on the state-of-the-art depletion mode, enhancement mode, and nanomembrane field-effect transistors (FETs) based on β-Ga₂O₃ for high-power switching and high-frequency amplification applications. In the meantime, device-level approaches to cope with the two main issues of β-Ga₂O₃, namely, the lack of p-type doping and the relatively low thermal conductivity, will be discussed and compared.

A Review on Gallium Oxide Materials from Solution Processes

Gallium oxide (Ga₂O₃) materials can be fabricated via various methods or processes. It is often mentioned that it possesses different polymorphs (α-, β-, γ-, δ- and ε-Ga₂O₃) and excellent physical and chemical properties. The basic properties, crystalline structure, band gap, density of states, and other properties of Ga₂O₃ will be discussed in this article. This article extensively discusses synthesis of pure Ga₂O₃, co-doped Ga₂O₃ and Ga₂O₃-metal oxide composite and Ga₂O₃/metal oxide heterostructure nanomaterials via solution-based methods mainly sol-gel, hydrothermal, chemical bath methods, solvothermal, forced hydrolysis, reflux condensation, and electrochemical deposition methods. The influence of the type of precursor solution and the synthesis conditions on the morphology, size, and properties of final products is thoroughly described. Furthermore, the applications of Ga₂O₃ will be introduced and discussed from these solution processes, such as deep ultraviolet photodetector, gas sensors, pH sensors, photocatalytic and photodegradation, and other applications. In addition, research progress and future outlook are identified.

Floating Particles in the Melt during the Growth of β-Ga2O3 Single Crystals Using the Czochralski Method

Floating particles often appear during the Czochralski (CZ) growth of β-Ga2O3 in the Ir crucible, thereby impeding the seeding process. Identifying the floating nanoparticles and then inhibiting or removing them is critical for growing high-quality β-Ga2O3 single crystals. We grew β-Ga2O3 crystals containing floating particles using the CZ method. It is indicated that the floating particles were composed of Ir with a face-centered cubic (fcc) structure. In addition, the β-Ga2O3/Ir interface was comprehensively characterized, showing sharp and straight configuration on the whole with small fluctuations at the nanoscale. Combined with density functional theory (DFT) calculation, we found that Ir-O bonding was responsible for stabilizing the interface. Accordingly, the atomic configuration of the interface with the stablest structure, including the relaxed one, was determined. Based on the formation mechanism of the floating particles, we propose three effective strategies, including blowing sufficient oxygen into the bottom of the Ir crucible, coating a protective layer on its inwall and equipping a mechanical arm for inhibiting or removing them.

β-Ga2O3: a potential high-temperature thermoelectric material

The thermoelectric properties of intrinsic n-type β-Ga₂O₃ are evaluated by first-principles calculations combined with Boltzmann transport theory and relaxation time approximation. The electron mobility is predicted by considering polar optical phonon scattering in β-Ga₂O₃. A temperature power law of T^-0.67 is obtained for the intrinsic electron mobility. Due to the ultra-wide band gap of 4.7-4.9 eV, β-Ga₂O₃ has a large Seebeck coefficient. As a result, a maximum power factor of 3.1 × 10^-3 W m^-1 K^-2 is obtained at 1600 K. A clear anisotropy in lattice thermal conductivity is observed, with the highest thermal conductivity of 23.1 W m^-1 K^-1 at 300 K along the [010] direction, and a lower value of 13.2 and 12.2 W m^-1 K^-1 along the [001] and [100] directions, respectively. A high ZT value of 1.07 at 1600 K can be obtained at the optimal carrier concentration of 2.4 × 10¹⁹ cm^-3, which is superior to that of most other oxides such as ZnO. In addition, the lattice thermal conductivity can be reduced by precisely adjusting the grain size, and the lattice thermal conductivity at 300 K (1600 K) can be reduced by 73% (39%) when the grain size is decreased to 10 nm. The excellent thermoelectric properties of β-Ga₂O₃ have promoted its potential application in the field of high temperature thermoelectric conversion.

α-Gallium Oxide Films on Microcavity-Embedded Sapphire Substrates Grown by Mist Chemical Vapor Deposition for High-Breakdown Voltage Schottky Diodes

α-Gallium oxide, with its large band gap energy, is a promising material for utilization in power devices. Sapphire, which has the same crystal structure as α-Ga₂O₃, has been used as a substrate for α-Ga₂O₃ epitaxial growth. However, lattice and thermal expansion coefficient mismatches generate a high density of threading dislocations (TDs) and cracks in films. Here, we demonstrated the growth of α-Ga₂O₃ films with reduced TD density and residual stress on microcavity-embedded sapphire substrates (MESS). We fabricated the two types of substrates with microcavities: diameters of 1.5 and 2.2 μm, respectively. We confirmed that round conical-shaped cavities with smaller diameters are beneficial for the lateral overgrowth of α-Ga₂O₃ crystals with lower TD densities by mist chemical vapor deposition. We could obtain crack-free high-crystallinity α-Ga₂O₃ films on MESS, while the direct growth on a bare sapphire substrate resulted in an α-Ga₂O₃ film with a number of cracks. TD densities of α-Ga₂O₃ films on MESS with 1.5 and 2.2 μm cavities were measured to be 1.77 and 6.47 × 10⁸ cm^-2, respectively. Furthermore, cavities in MESS were certified to mitigate the residual stress via the redshifted Raman peaks of α-Ga₂O₃ films. Finally, we fabricated Schottky diodes based on α-Ga₂O₃ films grown on MESS with 1.5 and 2.2 μm cavities, which exhibited high breakdown voltages of 679 and 532 V, respectively. This research paves the way to fabricating Schottky diodes with high breakdown voltages based on high-quality α-Ga₂O₃ films.

Enhanced third harmonic generation in ultrathin free-standing β-Ga2O3 nanomembranes: study on surface and bulk contribution

Third harmonic generation (THG) has proven its value in surface and interface characterization, high-contrast bio-imaging, and sub-wavelength light manipulation. Although THG is observed widely in general solid and liquid substances, when laser pulses are focused at nanometer-level ultra-thin films, the bulk THG has been reported to play the dominant role. However, there are still third harmonics (TH) generated at the surface of the thin-films, not inside the bulk solid - so-called surface TH, whose relative contribution has not been quantitatively revealed to date. In this study, we quantitatively characterized the surface and bulk contributions of THG at ultra-thin β-Ga₂O₃ nanomembranes with control of both the laser and thin-nanomembranes parameters, including the laser peak power, polarization state, number of layers, and nanomembranes thicknesses. Their contributions were studied in detail by analyzing the TH from freestanding β-Ga₂O₃ nanomembranes compared with TH from β-Ga₂O₃ nanomembranes on glass substrates. The contribution of the TH field from the β-Ga₂O₃-air interface was found to be 5.12 times more efficient than that from the β-Ga₂O₃-glass interface, and also 1.09 times stronger than the TH excited at bulk 1-μm-thick β-Ga₂O₃. Besides, TH from the β-Ga₂O₃-air interface was found to be 20% more sensitive to the crystalline structure than that from the β-Ga₂O₃-glass interface. This research work deepens our understanding of surface and bulk THG from crystalline materials and provides new possibilities towards designing highly efficient nonlinear optical materials for bio-imaging, energy-harvesting, and ultrafast laser development.

Control and understanding of metal contacts to β-Ga2O3 single crystals: a review

Gallium oxide (Ga2O3) is a promising semiconductor for high power devices and solar blind ultraviolet photodetectors due to its large bandgap, a high breakdown field, and high thermal stability. Recently, a considerable achievement has been obtained for the growth of high-quality β-Ga2O3 and high performance β-Ga2O3 based devices. However, rapid advance in device performance can be limited by the critical issues of metal contacts to β-Ga2O3 such as barrier height, leakage current, ohmic contact, and surface, interfacial and deep states. This article aims to provide a review on the recent studies in the control and understanding of metal contacts to β-Ga2O3, particularly in terms of the barrier formation. This review suggests that understanding the current transport mechanisms of metal contacts to β-Ga2O3 more thoroughly is necessary to enhance the performance, stability and reliability of β-Ga2O3 based devices.

Deep-Level Traps Responsible for Persistent Photocurrent in Pulsed-Laser-Deposited β-Ga2O3 Thin Films

Gallium oxide (β-Ga2O3) is emerging as a promising wide-bandgap semiconductor for optoelectronic and high-power electronic devices. In this study, deep-level defects were investigated in pulsed-laser-deposited epitaxial films of β-Ga2O3. A deep ultraviolet photodetector (DUV) fabricated on β-Ga2O3 film showed a slow decay time of 1.58 s after switching off 250 nm wavelength illumination. Generally, β-Ga2O3 possesses various intentional and unintentional trap levels. Herein, these traps were investigated using the fractional emptying thermally stimulated current (TSC) method in the temperature range of 85 to 473 K. Broad peaks in the net TSC curve were observed and further resolved to identify the characteristic peak temperature of individual traps using the fractional emptying method. Several deep-level traps having activation energies in the range of 0.16 to 1.03 eV were identified. Among them, the trap with activation energy of 1.03 eV was found to be the most dominant trap level and it was possibly responsible for the persistent photocurrent in PLD-grown β-Ga2O3 thin films. The findings of this current work could pave the way for fabrication of high-performance DUV photodetectors.

Laser damage mechanism and in situ observation of stacking fault relaxation in a β-Ga2O3 single crystal by the EFG method

We for the first time built up a laser damage mechanism and in situ observed stacking fault relaxation in a β-Ga₂O₃ single crystal.

Optimized growth of high length-to-diameter ratio Lu2O3 single crystal fibers by the LHPG method

Lu₂O₃ crystals have attracted intense attention due to their great potential in the field of high power solid-state lasers.

Investigation of the blue color center in β-Ga2O3 crystals by the EFG method

This work investigated the blue color center in β-Ga2O3 crystals grown by the EFG and obtained an effective method to eliminate it.

Numerical Analysis of Difficulties of Growing Large-Size Bulk β-Ga2O3 Single Crystals with the Czochralski Method

The difficulties in growing large-size bulk β-Ga2O3 single crystals with the Czochralski method were numerically analyzed. The flow and temperature fields for crystals that were four and six inches in diameter were studied. When the crystal diameter is large and the crucible space becomes small, the flow field near the crystal edge becomes poorly controlled, which results in an unreasonable temperature field, which makes the interface velocity very sensitive to the phase boundary shape. The effect of seed rotation with increasing crystal diameter was also studied. With the increase in crystal diameter, the effect of seed rotation causes more uneven temperature distribution. The difficulty of growing large-size bulk β-Ga2O3 single crystals with the Czochralski method is caused by spiral growth. By using dynamic mesh technology to update the crystal growth interface, the calculation results show that the solid–liquid interface of the four-inch crystal is slightly convex and the center is slightly concave. With the increase of crystal growth time, the symmetry of cylindrical crystal will be broken, which will lead to spiral growth. The numerical results of the six-inch crystal show that the whole solid–liquid interface is concave and unstable, which is not conducive to crystal growth.

Single-Domain and Atomically Flat Surface of κ-Ga2O3 Thin Films on FZ-Grown ε-GaFeO3 Substrates via Step-Flow Growth Mode

Herein, single-domain κ-Ga₂O₃ thin films were grown on FZ-grown ε-GaFeO₃ substrates via a step-flow growth mode. The ε-GaFeO₃ possessing the same crystal structure and similar lattice parameters as those of the orthorhombic κ-Ga₂O₃ facilitated the growth of κ-Ga₂O₃ thin films, as observed by the X-ray diffraction (XRD) analysis. Furthermore, the surface morphologies of the κ-Ga₂O₃ thin films exhibited a step-terrace and atomically flat structure. XRD φ-scan and transmission electron microscopy with selected area electron diffraction revealed that there is no occurrence of in-plane rotational domains in the κ-Ga₂O₃ thin films on ε-GaFeO₃ substrates and that the κ-Ga₂O₃ thin film comprised a single domain. TEM analysis revealed that there were no clear dislocations in the observation area. Moreover, high-resolution TEM observation showed that the atomic arrangements of the film and the substrate were continuous without the presence of an intermediate layer along the growth direction and were well-aligned in the in-plane direction.

Electron mobility and mode analysis of scattering for β-Ga2O3from first principles

The electrical transport properties of β-Ga2O3are studied by first-principles calculations. The calculated intrinsic electron Hall mobilities agree well with experiments, with intrinsic Hall factor decreasing monotonically from 1.54 at 100 K to 1.14 at 800 K. The anisotropy of electron mobility is weak due to the almost isotropic electron effective mass, which also results in nearly isotropic Seebeck coefficient and electronic contribution to the thermal conductivity. The mode analysis of phonon scattering reveals that the optical phonon scattering is almost entirely determined by the long-range polar interactions, whereas the acoustic phonon scattering also plays an important role especially at low temperatures. The intrinsic electron mobility is significantly overestimated even above room temperature by only considering the polar optical phonon scattering, in contrast to previous predictions from fitting of phenomenological models.

High-energy x-ray radiation effects on the exfoliated quasi-two-dimensional β-Ga2O3 nanoflake field-effect transistors

The effects of x-ray irradiation on the mechanically exfoliated quasi-two-dimensional (quasi-2D) β-Ga₂O₃ nanoflake field-effect transistors (FETs) under the condition of biasing voltage were systematically investigated for the first time. It has been revealed that the device experienced two stages during irradiation. At low ionizing doses (<240 krad), the device performance is mainly influenced by the photo-effect and the subsequent persistent photocurrent (PPC) effect as a result of the pre-existing electron traps (e-trap) in the oxides far away from the SiO₂/β-Ga₂O₃ interface. At larger doses (>240 krad), the device characteristics are dominated by the radiation-induced structural or compositional deterioration. The newly-generated e-traps are found located at the SiO₂/β-Ga₂O₃ interface. This study shed light on the future radiation-tolerant device fabrication process development, paving a way towards the feasibility and practicability of β-Ga₂O₃-based devices in extreme-environment applications.

Fabrication of a Nb-Doped β-Ga2O3 Nanobelt Field-Effect Transistor and Its Low-Temperature Behavior

For the first time, we report the successful fabrication of well-behaved field-effect transistors based on Nb-doped β-Ga₂O₃ nanobelts mechanically exfoliated from bulk single crystals. The exfoliated β-Ga₂O₃ nanobelts were transferred onto a purified surface of the 110 nm SiO₂/Si substrate. These Nb-doped devices showed excellent electrical performance such as an ultrasmall cutoff current of ∼10 fA, a high current on/off ratio of >10⁸, and a quite steep subthreshold swing (SS, ∼120 mV/decade). Furthermore, we investigated the temperature dependence down to 200 K, providing insightful information for its operation in a harsh environment. This work lays a foundation for wider application of Nb-doped β-Ga₂O₃ in nano-electronics.

A study on the technical improvement and the crystalline quality optimization of columnar β-Ga2O3 crystal growth by an EFG method

Innovative technology assessment and crystalline quality optimization of columnar β-Ga₂O₃ crystal growth were performed via an EFG method.

Inhibition of volatilization and polycrystalline cracking, and the optical properties of β-Ga2O3 grown by the EFG method

Large-size β-Ga₂O₃ single crystals without sub-grain boundaries and cracks were grown by the optimized EFG technology.

Transport Properties and Finite Size Effects in β-Ga2O3 Thin Films

Thin films of the wide band gap semiconductor β-Ga₂O₃ have a high potential for applications in transparent electronics and high power devices. However, the role of interfaces remains to be explored. Here, we report on fundamental limits of transport properties in thin films. The conductivities, Hall densities and mobilities in thin homoepitaxially MOVPE grown (100)-orientated β-Ga₂O₃ films were measured as a function of temperature and film thickness. At room temperature, the electron mobilities ((115 ± 10) cm²/Vs) in thicker films (>150 nm) are comparable to the best of bulk. However, the mobility is strongly reduced by more than two orders of magnitude with decreasing film thickness ((5.5 ± 0.5) cm²/Vs for a 28 nm thin film). We find that the commonly applied classical Fuchs-Sondheimer model does not explain sufficiently the contribution of electron scattering at the film surfaces. Instead, by applying an electron wave model by Bergmann, a contribution to the mobility suppression due to the large de Broglie wavelength in β-Ga₂O₃ is proposed as a limiting quantum mechanical size effect.

Transport Properties and Finite Size Effects in β-Ga2O3 Thin Films

Thin films of the wide band gap semiconductor β-Ga₂O₃ have a high potential for applications in transparent electronics and high power devices. However, the role of interfaces remains to be explored. Here, we report on fundamental limits of transport properties in thin films. The conductivities, Hall densities and mobilities in thin homoepitaxially MOVPE grown (100)-orientated β-Ga₂O₃ films were measured as a function of temperature and film thickness. At room temperature, the electron mobilities ((115 ± 10) cm²/Vs) in thicker films (>150 nm) are comparable to the best of bulk. However, the mobility is strongly reduced by more than two orders of magnitude with decreasing film thickness ((5.5 ± 0.5) cm²/Vs for a 28 nm thin film). We find that the commonly applied classical Fuchs-Sondheimer model does not explain sufficiently the contribution of electron scattering at the film surfaces. Instead, by applying an electron wave model by Bergmann, a contribution to the mobility suppression due to the large de Broglie wavelength in β-Ga₂O₃ is proposed as a limiting quantum mechanical size effect.

Investigation of the Mechanism for Ohmic Contact Formation in Ti/Al/Ni/Au Contacts to β-Ga2O3 Nanobelt Field-Effect Transistors

The issue of contacts between the electrode and channel layer is crucial for wide-bandgap semiconductors, especially the β-Ga₂O₃ due to its ultra-large bandgap (4.6-4.9 eV). It affects the device performance greatly and thus needs special attention. In this work, the high-performance β-Ga₂O₃ nanobelt field-effect transistors with Ohmic contact between multilayer metal stack Ti/Al/Ni/Au (30/120/50/50 nm) and unintentionally doped β-Ga₂O₃ channel substrate have been fabricated. The formation mechanism of Ohmic contacts to β-Ga₂O₃ under different annealing temperatures in an N₂ ambient is systematically investigated by X-ray photoelectron spectroscopy. It is revealed that the oxygen vacancies at the interface of β-Ga₂O₃/intermetallic compounds formed during rapid thermal annealing are believed to induce the good Ohmic contacts with low resistance. The contact resistance (R_c) between electrodes and unintentionally doped β-Ga₂O₃ reduces to ∼9.3 Ω mm after annealing. This work points to the importance of contact engineering for future improved β-Ga₂O₃ device performance and lays a solid foundation for the wider application of β-Ga₂O₃ in electronics and optoelectronics.

Growth mode evolution during (100)-oriented β-Ga2O3 homoepitaxy

This work focuses on homoepitaxial growth of β-Ga₂O₃ on (100)-oriented substrates during molecular beam epitaxy. It provides a comprehensive study on the growth mode by combining in situ with ex situ tools. In situ reflection high-energy electron diffraction (RHEED) indicates 2D layer-by-layer mode accompanied by (1 × 1) surface reconstruction. The homoepitaxial layers are grown pseudomorphic with the substrate without in-plane strain as probed by in-plane azimuthal RHEED and out-of-plane synchrotron-based high resolution x-ray diffraction. In contrast to the substrate, stacking faults and twin domains are present in the layer.

Recent Advances in β-Ga2O3-Metal Contacts

Ultra-wide bandgap beta-gallium oxide (β-Ga₂O₃) has been attracting considerable attention as a promising semiconductor material for next-generation power electronics. It possesses excellent material properties such as a wide bandgap of 4.6-4.9 eV, a high breakdown electric field of 8 MV/cm, and exceptional Baliga's figure of merit (BFOM), along with superior chemical and thermal stability. These features suggest its great potential for future applications in power and optoelectronic devices. However, the critical issue of contacts between metal and Ga₂O₃ limits the performance of β-Ga₂O₃ devices. In this work, we have reviewed the advances on contacts of β-Ga₂O₃ MOSFETs. For improving contact properties, four main approaches are summarized and analyzed in details, including pre-treatment, post-treatment, multilayer metal electrode, and introducing an interlayer. By comparison, the latter two methods are being studied intensively and more favorable than the pre-treatment which would inevitably generate uncontrollable damages. Finally, conclusions and future perspectives for improving Ohmic contacts further are presented.

Ultrawide Band Gap β-Ga2O3 Nanomechanical Resonators with Spatially Visualized Multimode Motion

Beta gallium oxide (β-Ga₂O₃) is an emerging ultrawide band gap (4.5 eV-4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. β-Ga₂O₃ thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal β-Ga₂O₃ nanomechanical resonators using β-Ga₂O₃ nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating β-Ga₂O₃ circular drumhead structures, we demonstrate multimode nanoresonators up to the sixth mode in high and very high frequency (HF/VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Young's modulus of E_Y = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ∼40% upshift in frequency and ∼90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable β-Ga₂O₃ electronic, optoelectronic, and physical sensing devices.

Power Electronic Semiconductor Materials for Automotive and Energy Saving Applications - SiC, GaN, Ga2O3, and Diamond

Power electronics belongs to the future key technologies in order to increase system efficiency as well as performance in automotive and energy saving applications. Silicon is the major material for electronic switches since decades. Advanced fabrication processes and sophisticated electronic device designs have optimized the silicon electronic device performance almost to their theoretical limit. Therefore, to increase the system performance, new materials that exhibit physical and chemical properties beyond silicon need to be explored. A number of wide bandgap semiconductors like silicon carbide, gallium nitride, gallium oxide, and diamond exhibit outstanding characteristics that may pave the way to new performance levels. The review will introduce these materials by (i) highlighting their properties, (ii) introducing the challenges in materials growth, and (iii) outlining limits that need innovation steps in materials processing to outperform current technologies.

Incomplete Ionization of a 110 meV Unintentional Donor in β-Ga2O3 and its Effect on Power Devices

Understanding the origin of unintentional doping in Ga₂O₃ is key to increasing breakdown voltages of Ga₂O₃ based power devices. Therefore, transport and capacitance spectroscopy studies have been performed to better understand the origin of unintentional doping in Ga₂O₃. Previously unobserved unintentional donors in commercially available \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\bar{2}01)$$\end{document}(2¯01) Ga₂O₃ substrates have been electrically characterized via temperature dependent Hall effect measurements up to 1000 K and found to have a donor energy of 110 meV. The existence of the unintentional donor is confirmed by temperature dependent admittance spectroscopy, with an activation energy of 131 meV determined via that technique, in agreement with Hall effect measurements. With the concentration of this donor determined to be in the mid to high 10¹⁶ cm⁻³ range, elimination of this donor from the drift layer of Ga₂O₃ power electronics devices will be key to pushing the limits of device performance. Indeed, analytical assessment of the specific on-resistance (R_onsp) and breakdown voltage of Schottky diodes containing the 110 meV donor indicates that incomplete ionization increases R_onsp and decreases breakdown voltage as compared to Ga₂O₃ Schottky diodes containing only the shallow donor. The reduced performance due to incomplete ionization occurs in addition to the usual tradeoff between R_onsp and breakdown voltage.

One-step exfoliation of ultra-smooth β-Ga2O3 wafers from bulk crystal for photodetectors

High-quality bulk β-Ga₂O₃ single crystals have been grown by optimized edge-defined film-fed growth (EFG) method.

Theoretical and experimental investigation of optical absorption anisotropy in β-Ga2O3

The question of optical bandgap anisotropy in the monoclinic semiconductor β-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk β-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5-4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Γ point.

Electronic structure and optical property of metal-doped Ga2O3: a first principles study

A long list of main group and transition metals, even some lanthanides, have been examined based on first principles studies, to search for potential p-type dopants for β-Ga₂O₃.

Solar-blind photodetector based on Ga2O3 nanowires array film growth from inserted Al2O3 ultrathin interlayers for improving responsivity

We propose a method to obtain Ga₂O₃ nanowire films which combines the benefits of nanowires and thin films by alternative deposition of Ga₂O₃ and Al₂O₃ ultrathin layers. The nanowire film-based photodetectors exhibit much higher responsivities than smooth film-based ones.

Homo- and heteroepitaxial growth of Sn-doped β-Ga2O3 layers by MOVPE

For the first time, n-type homoepitaxial semiconducting β-Ga₂O₃ layers were attained by MOVPE.

Electronic materials with a wide band gap: recent developments

The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap E g = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (E g = 1.12 eV). This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider E g were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and β-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity.