Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3

Feng Z., Anhar Uddin Bhuiyan A.F., Karim M.R., Zhao H.

Record-high electron mobilities were achieved for silicon-doped (010) β-Ga2O3 homoepitaxial films grown via metalorganic chemical vapor deposition (MOCVD). Key growth parameters were investigated to reduce the background doping and compensation concentration. Controllable n-type Si doping was achieved as low as low-1016 cm−3. Record carrier mobilities of 184 cm2/V s at room temperature and 4984 cm2/V s at low temperature (45 K) were measured for β-Ga2O3 thin films with room-temperature doping concentrations of 2.5 × 1016 and 2.75 × 1016 cm−3, respectively. Analysis of temperature-dependent Hall mobility and carrier concentration data revealed a low compensation concentration of 9.4 × 1014 cm−3. Using the two-donor model, Si on the tetrahedrally coordinated Ga(I) site represented the primary shallow donor state, and the secondary donor state was found to possess an activation energy of 120 meV. The demonstration of high-purity and high-quality β-Ga2O3 thin films with uniform and smooth surface morphology via MOCVD will harness its advantages as an ultrawide-bandgap semiconductor for power electronic and short-wavelength optoelectronic device applications.

Islam M.M., Rana D., Hernandez A., Haseman M., Selim F.A.

Electronic defects with shallow and deep levels in β-Ga2O3 single crystals were investigated by thermoluminescence (TL) spectroscopy. Undoped, Fe-doped, Sn-doped, and Mg-doped β-Ga2O3 single crystals grown by different methods were studied, and thermal activation energies of defects were calculated using the initial rise method. Hall-effect measurements and optical absorption spectroscopy were performed to determine the electrical transport properties and optical bandgaps. It was found that the dopants do not have any effect on the bandgap energy, which is important for comparing the trap levels in the samples. Three deep trap levels were found in the undoped crystals; the activation energy, ED, and concentration of defect centers for all of them have slightly changed after doping with Fe and Mg. Fe doping induced an additional defect center with activation energy of 0.62 eV. The measurements revealed the absence of TL emission in Sn doped crystals indicating that Sn doping may quench luminescence centers or modified some original electronic defects to inactive electron traps. The second interpretation “decrease of traps” may align with the successful incorporation of Sn as a donor and the high conductivity of Sn doped crystals revealed from Hall-effect measurements. This work also illustrates that the semi-insulating characteristics of Fe and Mg doped Ga2O3 are associated with the increase of the concentration of original traps in the crystal as well as the formation of new electron traps acting as deep acceptors. Recombination centers in all crystals are assumed to be associated with iron impurities.

Zhang Y., Alema F., Mauze A., Koksaldi O.S., Miller R., Osinsky A., Speck J.S.

In this work, we report record electron mobility values in unintentionally doped β-Ga2O3 films grown by metal-organic chemical vapor deposition. Using degenerately Sn-doped regrown n+ β-Ga2O3 contact layers, we were able to maintain Ohmic contact to the β-Ga2O3 films down to 40 K, allowing for reliable temperature-dependent Hall measurement. An electron mobility of 176 cm2/V s and 3481 cm2/V s were measured at room temperature and 54 K, respectively. The room and low temperature mobilities are both among the highest reported values in a bulk β-Ga2O3 film. A low net background charge concentration of 7.4 × 1015 cm−3 was confirmed by both temperature dependent Hall measurement and capacitance-voltage measurement. The feasibility of achieving low background impurity concentration and high electron mobility paves the road for the demonstration of high performance power electronics with high breakdown voltages and low on-resistances.

Neal A.T., Mou S., Rafique S., Zhao H., Ahmadi E., Speck J.S., Stevens K.T., Blevins J.D., Thomson D.B., Moser N., Chabak K.D., Jessen G.H.

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in $\beta$-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed (EFG), Czochralski (CZ), molecular beam epitaxy (MBE), and low pressure chemical vapor deposition (LPCVD). Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in $\beta$-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. High temperature Hall effect measurement of Fe doped $\beta$-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity.

Weiser P., Stavola M., Fowler W.B., Qin Y., Pearton S.

Hydrogen has a strong influence on the electrical properties of transparent conducting oxides where it can give rise to shallow donors and can passivate deep compensating defects. We have carried out infrared absorption experiments on H- and D-doped β-Ga2O3 that involve temperature- and polarization-dependent effects as well as relative H- and D-concentrations to probe the defect structures that hydrogen can form. The results of analysis of these data, coupled with detailed theoretical calculations, show that the dominant O-H vibrational line observed at 3437 cm−1 for hydrogenated Ga2O3 is due to a relaxed VGa-2H center.

Pearton S.J., Yang J., Cary P.H., Ren F., Kim J., Tadjer M.J., Mastro M.A.

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Chikoidze E., Fellous A., Perez-Tomas A., Sauthier G., Tchelidze T., Ton-That C., Huynh T.T., Phillips M., Russell S., Jennings M., Berini B., Jomard F., Dumont Y.

Wide-bandgap semiconductors (WBG) are expected to be applied to solid-state lighting and power devices, supporting a future energy-saving society. Here we present evidence of p-type conduction in the undoped WBG β-Ga2O3. Hole conduction, established by Hall and Seebeck measurements, is consistent with findings from photoemission and cathodoluminescence spectroscopies. The ionization energy of the acceptor level was measured to be 1.1eV above the valence band edge. The gallium vacancy was identified as a possible acceptor candidate based on thermodynamic equilibrium Ga2O3 (crystal) – O2 (gas) system calculations (Kroger theory) which revealed a window without oxygen vacancy compensation. The possibility of fabricating large diameter wafers of β-Ga2O3 of p and n type nature, provides new avenues for high power and deep UV-optoelectronic devices.

Qian Y.P., Guo D.Y., Chu X.L., Shi H.Z., Zhu W.K., Wang K., Huang X.K., Wang H., Wang S.L., Li P.G., Zhang X.H., Tang W.H.

The p-type high insulating thin films were obtained by doping Mg into β-Ga2O3. Thin films with various Mg concentrations were deposited on (0 0 0 1) c-plane Al2O3 substrate by radio frequency magnetron sputtering followed by post-annealing treatment. The crystal structure expanded due to the substitution of the trivalent Ga3+ with the divalent Mg2+ in a larger ion radius. The Fermi level (EF) of the Mg doped film is closer to the valence band, exhibiting a characteristic of weak p-type. The Mg doped Ga2O3 thin films were used to construct the metal/semiconductor/metal (MSM) structure, and the devices showed a high resistance (4.1 pA at 10 V), a high sensitivity (8.7 × 105%), a high responsivity (23.8 mA/W) and a short decay time (0.02 s) under 254 nm UV light irradiation, suggesting a potential application in solar-blind photodetector.

Helali Z., Jedidi A., Syzgantseva O.A., Calatayud M., Minot C.

The reducibility of bulk metal oxides in which the cation is in its highest oxidation state (MgO, Sc2O3, Y2O3, TiO2, m-ZrO2, m-HfO2, CeO2, V2O5, Nb2O5, Ta2O5, WO3, CrO3, Al2O3, β-Ga2O3, SiO2, SnO2 and ZnO) has been studied by standard periodic density functional theory. We have defined and calculated descriptors able to describe and quantify semi-quantitatively the extent of reduction: electronic band gap, oxygen vacancy formation energy and electronic localization. We find that there is no single criterion for characterizing the reducibility. We discuss the advantages and limitations of each method, and we apply them to classify the materials with the PBE+U and B3LYP functionals. Typical irreducible oxides such as MgO show a large band gap, high oxygen vacancy formation energy and electronic localization of the reduction electrons forming and F-center, with a diamagnetic singlet electronic state. Reducible oxides such as TiO2 present small band gaps, small oxygen vacancy formation energy and electron localization of the reduction electrons in the cations, decreasing their oxidation state and presenting open-shell electronic states. Intermediate or ambivalent behavior is found for ZrO2, HfO2, β-Ga2O3, ZnO and SnO2.

Mastro M.A., Kuramata A., Calkins J., Kim J., Ren F., Pearton S.J.

The β-polytype of Ga2O3 has a bandgap of ∼4.8 eV, can be grown in bulk form from melt sources, has a high breakdown field of ∼8 MV.cm−1 and is promising for power electronics and solar blind UV detectors, as well as extreme environment electronics (high temperature, high radiation, and high voltage (low power) switching. High quality bulk Ga2O3 is now commercially available from several sources and n-type epi structures are also coming onto the market. There are also significant efforts worldwide to grow more complex epi structures, including β-(AlxGa1x)2O3/Ga2O3 and β-(InxGa1−x)2O3/Ga2O3 heterostructures, and thus this materials system is poised to make rapid advances in devices. To fully exploit these advantages, advances in bulk and epitaxial crystal growth, device design and processing are needed. This article provides some perspectives on these needs.

Selim F.A., Khamehchi A., Winarski D., Agarwal S.

Yttrium aluminum garnet (YAG), Y3Al5O12 is one of the most important optical materials with many applications such as optical windows, laser host materials, detectors and phosphors. Nano YAG could offer many advantageous over bulk materials and large grain size phosphors. In this work Ce doped YAG (Ce:YAG) nanophosphors (NP) were synthesized using simple chemical methods and crystalized by annealing at 800, 900 and 1100°C. Luminescence was recorded in the range of 200 to 800 nm using X-ray induced luminescence technique to detect all emission centers in the sample and evaluate their relative intensities. The effect of annealing temperature and the use of different complexing and polymerization agents on the particle morphology and luminescence were investigated. Trapping phenomena were studied in Ce:YAG NP and bulk ceramics by thermally stimulated luminescence spectroscopy and a comparison was made between them and Ce:YAG single crystals. Measurements concluded that trapping is dominated by crystal defects in single crystals and by trapping sites at the grain boundaries in ceramics. Ce:YAG NPs -on the other hand- are free of traps, which seems to be characteristic of their small grain structure. This study illustrates the effect of chemical agents and annealing temperatures on the structural and optical properties of Ce:YAG nanophosphors and shed light on the nature and characteristics of traps in YAG, which greatly affect its performance in a wide range of applications. Furthermore it reveals that different trapping mechanisms take place among single crystals, bulk ceramics and NPs which could have impact on understanding the optical and scintillation properties of various luminescent materials.

Zhang F., Arita M., Wang X., Chen Z., Saito K., Tanaka T., Nishio M., Motooka T., Guo Q.

Precise control of dopant composition is critical for the production of semiconductor films with desired properties. Here, we present results on the electrical properties for Si doped Ga2O3 films grown by pulsed laser deposition technique (PLD). The Si composition in the films can be controlled by changing the target composition as observed from the secondary ion mass spectroscopy measurement. The carrier density of the films is varied from the order of 1015 to 1020 cm−3 while the conductivity from 10−4 to 1 S cm−1 as measured by Hall equipment. The carrier density of the films has been verified by Kelvin force microscopy, which shows an increased surface work function with the increase of carrier density. The results suggest that the carrier density of β-Ga2O3 films is controllable by Si doping by PLD, paving a way to develop the Ga2O3 film-based electronic devices.

Winarski D.J., Anwand W., Wagner A., Saadatkia P., Selim F.A., Allen M., Wenner B., Leedy K., Allen J., Tetlak S., Look D.C.

Undoped and Ga- and Al- doped ZnO films were synthesized using sol-gel and spin coating methods and characterized by X-ray diffraction, high-resolution scanning electron microscopy (SEM), optical spectroscopy and Hall-effect measurements. SEM measurements reveal an average grain size of 20 nm and distinct individual layer structure. Measurable conductivity was not detected in the unprocessed films; however, annealing in hydrogen or zinc environment induced significant conductivity (∼10−2 Ω.cm) in most films. Positron annihilation spectroscopy measurements provided strong evidence that the significant enhancement in conductivity was due to hydrogen passivation of Zn vacancy related defects or elimination of Zn vacancies by Zn interstitials which suppress their role as deep acceptors. Hydrogen passivation of cation vacancies is shown to play an important role in tuning the electrical conductivity of ZnO, similar to its role in passivation of defects at the Si/SiO2 interface that has been essential for the successful development of complementary metal–oxide–semiconductor (CMOS) devices. By comparison with hydrogen effect on other oxides, we suggest that hydrogen may play a universal role in oxides passivating cation vacancies and modifying their electronic properties.

Yadav S.K., Uberuaga B.P., Nikl M., Jiang C., Stanek C.R.

Complex doping schemes in R3Al5O12 (where R is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on Lu3B5O12 where B is Al, Ga, In, As, and Sb, and R3Al5O12, where R is Lu, Gd, Dy, and Er. We find that admixing with various R cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sbmore » decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. Furthermore, this approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.« less

Sturm C., Furthmüller J., Bechstedt F., Schmidt-Grund R., Grundmann M.

The dielectric tensor of $\beta$-Ga$_2$O$_3$ was determined by generalized spectroscopic ellipsometry in a wide spectral range from $0.5\,\mathrm{eV}$ to $8.5\,\mathrm{eV}$ as well as by calculation including quasiparticle bands and excitonic effects. The dielectric tensors obtained by both methods are in excellent agreement with each other and the observed transitions in the dielectric function are assigned to the corresponding valence bands. It is shown that the off-diagonal element of the dielectric tensor reaches values up to $|\varepsilon_{xz} | \approx 0.30 $ and cannot be neglected. Even in the transparent spectral range where it is quite small ($|\varepsilon_{xz} | < 0.02 $) it causes a rotation of the dielectric axes around the symmetry axis of up to $20^\circ$.

Moussa R., Semari F., Seksak Y., Meradji H., Khenata R., Bin-Omran S., Ahmed W., BakhtiarUlHaq, Abdiche A.

Liu Z., Qiu W., Kuang X., Li B., Sun C., Tu R., Zhang S.

Ganguly S., Nama Manjunatha K., Paul S.

AbstractThe traditional domination of silicon (Si) in device fabrication is increasingly infiltrated by state‐of‐the‐art wide bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC). However, the performance of these wide bandgap semiconductors has not yet exceeded the optical material limitation, which leaves ample room for further development. Gallium oxide (Ga2O3) has surfaced as the preferred material for next‐generation device fabrication, as it has a wider bandgap (≈4.5–5.7 eV), an estimated twofold greater breakdown field strength of 8 MV cm−1, and a higher Baliga's figure of merit(BFOM) (>3000) than SiC and GaN, therefore pushing the limit. In this review, the properties of gallium oxide, several methods for epitaxial growth, its energy band, and its broad spectrum of applications are discussed. Metals for achieving different types of contact and the influence of interfacial reactions are additionally assessed. Furthermore, defects and challenges such as p‐type doping, integration with heterostructures, the formation of superlattices, and thermal management associated with the use of this material are also reviewed.

Polyakov A.Y., Schemerov I.V., Vasilev A.A., Romanov A.A., Lagov P.B., Miakonkikh A.V., Chernykh A.V., Romanteeva E.P., Chernykh S.V., Rabinovich O.I., Pearton S.J.

The electrical properties and deep trap spectra of semi-insulating Ga2O3(Fe) implanted with Si ions and subsequently annealed at 1000 °C were investigated. A significant discrepancy was observed between the measured shallow donor concentration profile and the profile predicted by Stopping Power and Range of Ions in Matter simulations, indicating substantial compensation. Deep level transient spectroscopy revealed the presence of deep acceptors at Ec −0.5 eV with a concentration of ∼10¹⁷ cm−³, insufficient to fully account for the observed compensation. Photocapacitance spectroscopy identified additional deep acceptors with optical ionization thresholds near 2 and 2.8–3.1 eV, tentatively attributed to gallium vacancy-related defects. However, the combined concentration of these deep acceptors still fell short of explaining the observed donor deactivation, suggesting the formation of electrically neutral Si-vacancy complexes. Furthermore, the properties of Ga2O3 (Fe) implanted with Si and subjected to hydrogen plasma treatment at 330 °C were also examined. This material exhibited high resistivity with the Fermi level pinned near Ec –0.3 eV, similar to common radiation defects in proton-implanted Ga2O3. A prominent deep center near Ec −0.6 eV, consistent with the known E1 electron trap attributed to Si-H complexes, was also observed. These results highlight the challenges associated with Si implantation and activation in Ga2O3 and suggest that hydrogen plasma treatment, while effective for Ga-implanted Ga2O3 is less suitable for Si-implanted material due to the formation of compensating Si-H complexes.

Grivickas V., Ščajev P., Miasojedovas S., Voss L., Grivickas P.

Zhang C., Fu X., Wang H.

Qiao B., Zhang Z., Wang Y., Huang X., Zhang Z., Zheng Z., Sun X., Xie X., Li B., Chen X., Liu K., Liu L., Shen D.

Xu K., Yang Q., Liu W., Zhang R., Wang Z., Ye J.

Polyakov A.Y., Yakimov E.B., Shchemerov I.V., Vasilev A., Kochkova A.I., Nikolaev V., Pearton S.J.

Abstract The material system of ultra-wide bandgap Ga2O3 has already shown great promise in the field of solar-blind photodetectors with high photoresponsivity, high photoresponsivity gain and low dark current. These promising results have been achieved on Ga2O3 films of different polymorphs and by different methods, often not with particularly high crystalline quality. In fact, it would often seem the case that the lower the crystalline quality of the films, the higher the photosensitivity and its gain. This, however, is in most cases accompanied by unusually long photocurrent build-up and decay times. We show that the experimental results can be explained by models in which the high photosensitivity gain is related to the effects of holes being trapped by deep states, which, in Schottky diodes, results in a decrease of the Schottky barrier height with a consequent increase in the electron current, and in metal–semiconductor–metal (MSM) structures additionally gives rise to the usual gain increase due to the increased concentration and lifetime of electrons. We present and discuss models describing the effects in Ga2O3 Schottky diodes, MSM structures, and unipolar and bipolar heterojunctions, and we propose possible candidates for the role of the hole traps in different Ga2O3 polymorphs. We also discuss the existing results for the photocurrent build-up and decay times and offer possible explanations for the observed temperature dependences of the characteristic times where such data are present.

Qin X., Zhang J., Liu J., Zhao B., Li C., Wan Q., Jiang C., Wei J., Han W., Wang B., Lv L., Chen X., Wan H., Wang H.

Fan W., Li S., Ren W., Yang Y., Li Y., Liu G., Wang W.

Doping divalent metal cations into Ga2O3 films plays a key role in adjusting the conductive behavior of the film. N-type high-resistivity β-Ga2O3:Mg films were prepared using electron beam evaporation and subsequent postannealing processing. Various characterization methods (X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, etc.) revealed that the Mg content plays an important role in affecting the film quality. Specifically, when the Mg content in the film is 3.6%, the S2 film’s resistivity, carrier content, and carrier mobility are 59655.5 Ω·cm, 1.95 × 1014 cm3/C, and 0.53682 cm2/Vs. Also, the film exhibits a smoother surface, more refined grains, and higher self-trapped exciton emission efficiency. The Mg cation mainly substitutes the Ga+ cation at a tetrahedral site, acting as a trap for self-trapped holes.

Wang Y., Zhu M., Liu Y.

β-gallium oxide (β-Ga2O3), as the typical representative of the fourth generation of semiconductors, has attracted increasing attention owing to its ultra-wide bandgap, superior optical properties, and excellent tolerance to high temperature and radiation. Compared to the single crystals of other semiconductors, high-quality and large-size β-Ga2O3 single crystals can be grown with low-cost melting methods, making them highly competitive. In this review, the growth process, defects, and dopants of β-Ga2O3 are primarily discussed. Firstly, the growth process (e.g., decomposition, crucible corrosion, spiral growth, and development) of β-Ga2O3 single crystals are summarized and compared in detail. Then, the defects of β-Ga2O3 single crystals and the influence of defects on Schottky barrier diode (SBD) devices are emphatically discussed. Besides, the influences of impurities and intrinsic defects on the electronic and optical properties of β-Ga2O3 are also briefly discussed. Concluding this comprehensive analysis, the article offers a concise summary of the current state, challenges and prospects of β-Ga2O3 single crystals.

SASAKI K.

Abstract This review describes the progress of research on gallium oxide as a material for power devices, covering the development of bulk crystal growth through to epitaxial growth, defect evaluations, device processes, and development, all based on the author’s research experiences. During the last decade or so, the epi-wafer size has been expanded to 4–6 inches, and Schottky barrier diodes and field-effect transistors capable of ampere-class operations and with breakdown voltages of several kV have been demonstrated. On the other hand, challenges to the practical application of gallium oxide power devices, such as the cost of epi-wafers, killer defects, purity of epitaxial layer, etc., have also become apparent. This paper provides a comprehensive summary of the history of these developments, including not only papers but also patents and conference presentations, and gives my personal views on the prospects for this material’s continued development.

Venzie A., Stavola M., Fowler W.B., Glaser E.R., Tadjer M.J., Forbus J.I., Zvanut M.E., Pearton S.J.

Hydrogen in β-Ga2O3 passivates shallow impurities and deep-level defects and can have a strong effect on conductivity. More than a dozen O–D vibrational lines have been reported for β-Ga2O3 treated with the heavy isotope of hydrogen, deuterium. To explain the large number of O–D centers that have been observed, the involvement of additional nearby defects and impurities has been proposed. A few O–H centers have been associated with specific impurities that were introduced intentionally during crystal growth. However, definitive assignments of O–H and O–D vibrational lines associated with important adventitious impurities, such as Si and Fe, have been difficult. A set of well-characterized Si-doped β-Ga2O3 epitaxial layers with different layer thicknesses has been deuterated and investigated by vibrational spectroscopy to provide new evidence for the assignment of a line at 2577 cm−1 to an OD–Si complex. The vibrational properties of several of the reported OD-impurity complexes are consistent with the existence of a family of defects with a VGa1ic−D center at their core that is perturbed by a nearby impurity.

Angeloni L.A., Shan I.-., Leach J.H., Schroeder W.A.

The energetic positions of the two Fe dopant levels in the bandgap of β-Ga2O3 are determined to be at 3.05(±0.05) and 3.85(±0.05) eV below the conduction band minimum from transmission measurements employing a sub-picosecond tunable ultraviolet laser radiation source. A further measurement of the quantum efficiency of photoelectron emission from the Fe:Ga2O3(010) photocathode is consistent with the obtained absorption data and a dominant optical phonon assisted Franck–Condon emission mechanism, while also providing an estimate of 100 ps for the conduction band electron lifetime.