Conducting surface layers formed by hydrogenation of O-implanted β-Ga2O3

Sardar A., Isaacs-Smith T., Lawson J., Asel T., Comes R.B., Merrett J.N., Dhar S.

This work demonstrates the advantage of carrying out silicon ion (Si+) implantation at high temperatures for forming controlled heavily doped regions in gallium oxide. Room temperature (RT, 25 °C) and high temperature (HT, 600 °C) Si implants were carried out into MBE grown (010) β-Ga2O3 films to form ∼350 nm deep Si-doped layers with average concentrations up to ∼1.2 × 1020 cm−3. For such high concentrations, the RT sample was too resistive for measurement, but the HT samples had 82.1% Si dopant activation efficiency with a high sheet electron concentration of 3.3 × 1015 cm−2 and an excellent mobility of 92.8 cm2/V·s at room temperature. X-ray diffraction measurements indicate that HT implantation prevents the formation of other Ga2O3 phases and results in reduced structural defects and lattice damage. These results are highly encouraging for achieving ultra-low resistance heavily doped Ga2O3 layers using ion implantation.

García-Fernández J., Kjeldby S.B., Nguyen P.D., Karlsen O.B., Vines L., Prytz Ø.

Ion implantation induced phase transformation and the crystal structure of a series of ion implanted β-Ga2O3 samples were studied using electron diffraction, high resolution transmission electron microscopy, and scanning transmission electron microscopy. In contrast to previous reports suggesting an ion implantation induced transformation to the orthorhombic κ-phase, we show that for 28Si+, 58Ni+, and stoichiometric 69Ga+/16O+-implantations, the monoclinic β-phase transforms to the cubic γ-phase. The γ-phase was confirmed for implantations over a range of fluences from 1014 to 1016 ions/cm2, indicating that the transformation is a general phenomenon for β-Ga2O3 due to strain accumulation and/or γ-Ga2O3 being energetically preferred over highly defective β-Ga2O3.

Petkov A., Cherns D., Chen W., Liu J., Blevins J., Gambin V., Li M., Liu D., Kuball M.

β-Ga2O3 was suggested to have excellent irradiation hardness which makes β-Ga2O3-based devices extremely attractive for nuclear and space applications. To discern the fundamental nano-scale structural changes with irradiation, an in situ irradiation experiment in a transmission electron microscope was carried out using 400 keV Ar ions of fluences up to 8 × 1015 cm−2 (equivalent to four displacements per atom). Contrary to previous works, which indicate a phase transition of β-Ga2O3 into the κ polymorph, the β-Ga2O3 structure was found to remain intact throughout except for (i) anisotropic lattice distortions, which are most significant at low levels of irradiation, and (ii) the appearance of additional weak reflections above 2 dpa irradiation. The origin of the extra reflections is discussed.

Azarov A., Venkatachalapathy V., Karaseov P., Titov A., Karabeshkin K., Struchkov A., Kuznetsov A.

Ion irradiation is a powerful tool to tune properties of semiconductors and, in particular, of gallium oxide (Ga2O3) which is a promising ultra-wide bandgap semiconductor exhibiting phase instability for high enough strain/disorder levels. In the present paper we observed an interesting interplay between the disorder and strain in monoclinic β-Ga2O3 single crystals by comparing atomic and cluster ion irradiations as well as atomic ions co-implants. The results obtained by a combination of the channeling technique, X-ray diffraction and theoretical calculations show that the disorder accumulation in β-Ga2O3 exhibits superlinear behavior as a function of the collision cascade density. Moreover, the level of strain in the implanted region can be engineered by changing the disorder conditions in the near surface layer. The results can be used for better understanding of the radiation effects in β-Ga2O3 and imply that disorder/strain interplay provides an additional degree of freedom to maintain desirable strain in Ga2O3, potentially applicable to modify the rate of the polymorphic transitions in this material.

Yoo T., Xia X., Ren F., Jacobs A., Tadjer M.J., Pearton S., Kim H.

β-Ga2O3 is an emerging ultra-wide bandgap semiconductor, holding a tremendous potential for power-switching devices for next-generation high power electronics. The performance of such devices strongly relies on the precise control of electrical properties of β-Ga2O3, which can be achieved by implantation of dopant ions. However, a detailed understanding of the impact of ion implantation on the structure of β-Ga2O3 remains elusive. Here, using aberration-corrected scanning transmission electron microscopy, we investigate the nature of structural damage in ion-implanted β-Ga2O3 and its recovery upon heat treatment with the atomic-scale spatial resolution. We reveal that upon Sn ion implantation, Ga2O3 films undergo a phase transformation from the monoclinic β-phase to the defective cubic spinel [Formula: see text]-phase, which contains high-density antiphase boundaries. Using the planar defect models proposed for the [Formula: see text]-Al2O3, which has the same space group as β-Ga2O3, and atomic-resolution microscopy images, we identify that the observed antiphase boundaries are the {100}1/4 ⟨110⟩ type in cubic structure. We show that post-implantation annealing at 1100 °C under the N2 atmosphere effectively recovers the β-phase; however, nano-sized voids retained within the β-phase structure and a [Formula: see text]-phase surface layer are identified as remanent damage. Our results offer an atomic-scale insight into the structural evolution of β-Ga2O3 under ion implantation and high-temperature annealing, which is key to the optimization of semiconductor processing conditions for relevant device design and the theoretical understanding of defect formation and phase stability.

Polyakov A.Y., Nikolaev V.I., Meshkov I.N., Siemek K., Lagov P.B., Yakimov E.B., Pechnikov A.I., Orlov O.S., Sidorin A.A., Stepanov S.I., Shchemerov I.V., Vasilev A.A., Chernykh A.V., Losev A.A., Miliachenko A.D., et. al.

Films of α-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 1015 cm−2 and with 7 MeV C4+ ions with a fluence of 1.3 × 1013 cm−2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at Ec-0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies VGa–VO, and oxygen vacancies VO. Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of VGa and VGa–VO.

Zhang J., Dong P., Dang K., Zhang Y., Yan Q., Xiang H., Su J., Liu Z., Si M., Gao J., Kong M., Zhou H., Hao Y.

Ultra-wide bandgap semiconductor Ga2O3 based electronic devices are expected to perform beyond wide bandgap counterparts GaN and SiC. However, the reported power figure-of-merit hardly can exceed, which is far below the projected Ga2O3 material limit. Major obstacles are high breakdown voltage requires low doping material and PN junction termination, contradicting with low specific on-resistance and simultaneous achieving of n- and p-type doping, respectively. In this work, we demonstrate that Ga2O3 heterojunction PN diodes can overcome above challenges. By implementing the holes injection in the Ga2O3, bipolar transport can induce conductivity modulation and low resistance in a low doping Ga2O3 material. Therefore, breakdown voltage of 8.32 kV, specific on-resistance of 5.24 mΩ⋅cm2, power figure-of-merit of 13.2 GW/cm2, and turn-on voltage of 1.8 V are achieved. The power figure-of-merit value surpasses the 1-D unipolar limit of GaN and SiC. Those Ga2O3 power diodes demonstrate their great potential for next-generation power electronics applications. The simultaneous achievement of high breakdown voltage and low resistance is a contradictory point because it would require high and low doping simultaneously. Here, Zhou et al. achieve a power figure-of-merit of 13.2 GW/cm2 through hole injection and conductivity modulation effect.

Titov A.I., Karabeshkin K.V., Struchkov A.I., Nikolaev V.I., Azarov A., Gogova D.S., Karaseov P.A.

The mechanisms of ion-induced defect formation and physical characteristics promoting radiation tolerance of wide and ultra-wide bandgap semiconductors are not well-studied and understood. In contrast to gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) can be crystallized in several polymorphs having different crystal structures and physical properties. In the preset paper, the damage buildup in wurtzite GaN as well as in corundum ( α -) and monoclinic ( β -) Ga 2 O 3 polymorphs bombarded at room temperature with 40 keV P + ions is studied by Rutherford backscattering/channeling spectrometry. We demonstrate that ion-beam-induced damage formation in Ga 2 O 3 is different from that observed in GaN and dramatically depends on the polymorph type. Both Ga 2 O 3 polymorphs cannot be rendered amorphous and exhibit considerably higher damage saturation at ∼90% of the full amorphization as compared to that of GaN. Intriguing enough the metastable α -Ga 2 O 3 demonstrates considerably higher radiation resistance as compared to the most thermodynamically stable β -Ga 2 O 3 polymorph. Furthermore, our results indicate that the sample surface and dynamic annealing play a significant role in the ion-induced damage formation processes in all Ga-based compounds studied. • Kinetics of ion irradiation damage accumulation in α- and β-Ga 2 O 3 and GaN is studied. • α -Ga 2 O 3 is more susceptible to radiation damage than GaN. • α -Ga 2 O 3 is considerably higher radiation resistant then the stable β -Ga 2 O 3 . • Mechanisms of radiation damage formation in the α - and β -Ga 2 O 3 are different.

Azarov A., Bazioti C., Venkatachalapathy V., Vajeeston P., Monakhov E., Kuznetsov A.

Polymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and, potentially, for similar polymorphic families in other materials.

Karjalainen A., Weiser P.M., Makkonen I., Reinertsen V.M., Vines L., Tuomisto F.

Positron annihilation spectroscopy, Fourier transform-infrared absorption spectroscopy, and secondary ion mass spectrometry have been used to study the behavior of gallium vacancy-related defects and hydrogen in deuterium (D) implanted and subsequently annealed β-Ga 2O 3 single crystals. The data suggest the implantation generates a plethora of V Ga-related species, including V Ga 1- and V Ga 2-type defects. The latter’s contribution to the positron signal was enhanced after an anneal at 300  °C, which is driven by the passivation of V Ga ib by hydrogen as seen from infrared measurements. Subsequent annealing near 600  °C returns the positron signal to levels similar to those in the as-received samples, which suggests that split V Ga-like defects are still present in the sample. The almost complete removal of the V Ga ib-2D vibrational line, the appearance of new weak O-D lines in the same spectral region, and the lack of D out-diffusion from the samples suggest that the 600  °C anneal promotes the formation of either D-containing, IR-inactive complexes or defect complexes between V Ga ib-2D and other implantation-induced defects. The degree of electrical compensation is found to be governed by the interactions between the Ga vacancies and hydrogen.

Wang Z., Chen X., Ren F., Gu S., Ye J.

Abstract As an ultrawide bandgap semiconductor, gallium oxide (Ga2O3) has superior physical properties and has been an emerging candidate in the applications of power electronics and deep-ultraviolet optoelectronics. Despite numerous efforts made in the aspect of material epitaxy and power devices based on β-Ga2O3 with rapid progresses, the fundamental understanding of defect chemistry in Ga2O3, in particular, acceptor dopants and carrier compensation effects, remains a key challenge. In this focused review, we revisited the principles of popular approaches for characterizing defects in semiconductors and summarized recent advances in the fundamental investigation of defect properties, carrier dynamics and optical transitions in Ga2O3. Theoretical and experimental investigations revealed the microstructures and possible origins of defects in β-Ga2O3 bulk single crystals, epitaxial films and metastable-phased α-Ga2O3 epilayers by the combined means of first-principle calculation, deep level transient spectroscopy and cathodoluminescence. In particular, defects induced by high-energy irradiation have been reviewed, which is essential for the identification of defect sources and the evaluation of device reliability operated in space and other harsh environments. This topic review may provide insight into the fundamental properties of defects in Ga2O3 to fully realize its promising potential in practical applications.

Anber E.A., Foley D., Lang A.C., Nathaniel J., Hart J.L., Tadjer M.J., Hobart K.D., Pearton S., Taheri M.L.

Ion implantation-induced effects were studied in Ge implanted β-Ga2O3 with the fluence and energy of 3 × 1013 cm−2/60 keV, 5 × 1013 cm−2/100 keV, and 7 × 1013 cm−2/200 keV using analytical electron microscopy via scanning/transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction via TopSpin. Imaging shows an isolated band of damage after Ge implantation, which extends ∼130 nm from the sample surface and corresponds to the projected range of the ions. Electron diffraction demonstrates that the entirety of the damage band is the κ phase, indicating an implantation-induced phase transition from β to κ-Ga2O3. Post-implantation annealing at 1150 °C for 60 s under the O2 atmosphere led to a back transformation of κ to β; however, an ∼17 nm damage zone remained at the sample surface. Despite the back transformation from κ to β with annealing, O K-edge spectra show changes in the fine structure between the pristine, implanted, and implanted-annealed samples, and topspin strain analysis shows a change in strain between the two samples. These data indicate differences in the electronic/chemical structure, where the change of the oxygen environment extended beyond the implantation zone (∼130 nm) due to the diffusion of Ge into the bulk material, which, in turn, causes a tensile strain of 0.5%. This work provides a foundation for understanding of the effects of ion implantation on defect/phase evolution in β-Ga2O3 and the related recovery mechanism, opening a window toward building a reliable device for targeted applications.

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

Recent suggestions that hydrogen incorporation at the Ga(1) vacancy in β-Ga2O3 may have an impact on its electronic properties have led us to extend our earlier work on these defects. While our previous work provides strong evidence for one, two, and perhaps three or four H trapped into the shifted vacancy configurations introduced by Varley and Kyrtsos, the apparent experimental absence of several H trapped in the unshifted configuration is puzzling. While a structure of two hydrogen atoms trapped in the unshifted configuration is not favored energetically, structures of three or four hydrogens in the unshifted configuration are favored. We suggest that these structures are absent because there are no available pathways for the system to reach them by sequentially trapped H and, therefore, that three- or four-hydrogen defects will occur only in the shifted vacancy configurations.

Polyakov A.Y., Lee I., Miakonkikh A., Chernykh A.V., Smirnov N.B., Shchemerov I.V., Kochkova A.I., Vasilev A.A., Pearton S.J.

Bulk n-type β-Ga2O3 samples with orientation (−201) and (010) were exposed to a high density hydrogen plasma at 330 °C for 0.5 h. The effects were radically different for the two orientations. For the (−201) sample, H plasma exposure increased the net surface concentration of shallow donors from 2.7 × 1017 cm−3 to 2.6 × 1018 cm−3, with the shallow donors having an ionization energy close to 20 meV as deduced from the temperature dependence of the series resistance of Ni Schottky diodes. By sharp contrast, H plasma exposure of the (010) sample led to a strong decrease in the net shallow donor density from 3.2 × 1017 cm−3 to below 1015 cm−3 in the top 0.9 μm of the sample and to 3.2 × 1016 cm−3 near the edge of the space charge region at 0 V, with the total width of the region affected by plasma treatment being close to 1.1 μm. For both orientations, we observed a major decrease in the concentration of the dominant E2 traps near Ec-0.82 eV related to Fe acceptors. The deep trap spectra in hydrogenated samples were dominated by the E2* traps commonly ascribed to native defects in β-Ga2O3. The peak of these traps with a level near Ec-0.74 eV was masked in the starting samples by the peak of the E2 Fe acceptors present in high concentration, so that E2* only broadened the Fe peak on the low temperature side, but could be revealed by the modeling of the spectra. The concentration of the E2* center was not strongly affected in the hydrogen-treated samples with orientation (010), but in the (−201) samples, the concentration of the E2* peak was greatly enhanced. The results are discussed in conjunction with previous reports on hydrogen plasma treatment of β-Ga2O3 and on obtaining p-type conductivity in the surface layers of β-Ga2O3 crystals annealed in molecular hydrogen at high temperatures [Islam et al., Sci. Rep. 10, 6134 (2020)].

Islam M.M., Liedke M.O., Winarski D., Butterling M., Wagner A., Hosemann P., Wang Y., Uberuaga B., Selim F.A.

Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in β-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20 meV and resistivity of 10−4 Ω.cm. The conductivity was switched to p-type with acceptor ionization energy of 42 meV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy measurements confirm this finding and the interpretation of the experimental results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time, it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices.

Ratajczak R., Sarwar M., Kalita D., Jozwik P., Mieszczynski C., Matulewicz J., Wilczopolska M., Wozniak W., Kentsch U., Heller R., Guziewicz E.

AbstractRE-doped β-Ga2O3 seems attractive for future high-power LEDs operating in high irradiation environments. In this work, we pay special attention to the issue of radiation-induced defect anisotropy in β-Ga2O3, which is crucial for device manufacturing. Using the RBS/c technique, we have carefully studied the structural changes caused by implantation and post-implantation annealing in two of the most commonly used crystallographic orientations of β-Ga2O3, namely the (-201) and (010). The analysis was supported by advanced computer simulations using the McChasy code. Our studies reveal a strong dependence of the structural damage induced by Yb-ion implantation on the crystal orientation, with a significantly higher level of extended defects observed in the (-201) direction than for the (010). In contrast, the concentration and behavior of simple defects seem similar for both oriented crystals, although their evolution suggests the co-existence of two different types of defects in the implanted zone with their different sensitivity to both, radiation and annealing. It has also been found that Yb ions mostly occupy the interstitial positions in β-Ga2O3 crystals that remain unchanged after annealing. The location is independent of the crystal orientations. We believe that these studies noticeably extend the knowledge of the radiation-induced defect structure, because they dispel doubts about the differences in the damage level depending on crystal orientation, and are important for further practical applications.

Yan Y., Jin Z., Zhang H., Yang D.

In recent years, ultra-wide bandgap β-Ga2O3 has emerged as a fascinating semiconductor material due to its great potential in power and photoelectric devices. In semiconductor industrial, thermal treatment has been widely utilized as a convenient and effective approach for substrate property modulation and device fabrication. Thus, a thorough summary of β-Ga2O3 substrates and devices behaviors after high-temperature treatment should be significant. In this review, we present the recent advances in modulating properties of β-Ga2O3 substrates by thermal treatment, which include three major applications: (i) tuning surface electrical properties, (ii) modifying surface morphology, and (iii) oxidating films. Meanwhile, regulating electrical contacts and handling with radiation damage and ion implantation have also been discussed in device fabrication. In each category, universal annealing conditions were speculated to figure out the corresponding problems, and some unsolved questions were proposed clearly. This review could construct a systematic thermal treatment strategy for various purposes and applications of β-Ga2O3.

Zhu H., Tang Y., Zhong A., Wang L., Liu F., Zhao P., Liu J., Shu L., Wu Z., Li B.

Swift heavy Ta ions with an ultra-high energy of 2896 MeV are utilized for irradiation of β-Ga2O3 photodetectors. Noteworthy variations in device performance under different wavelengths are observed. Under 254 nm light illumination, the photocurrent of the devices exhibit degradation at low ion fluences but gradually recover and even surpass the performance of non-irradiated devices at the irradiation fluence of 1 × 1010 cm−2. Conversely, under 365 nm light illumination, photocurrent increases at low fluence but slightly decreases at the same high fluence of 1 × 1010 cm−2. Cathodoluminescence spectra and first-principles calculations elucidate the mechanism underlying the evolution of device performance with irradiation fluence. At low irradiation fluence, the introduction of point defects such as oxygen vacancies and gallium vacancies leads to an expansion of the bandgap, resulting in a decline in photocurrent under 254 nm light illumination. Additionally, deep defect levels are generated by these point defects, promoting an enhancement of photocurrent under 365 nm light illumination. Higher fluences transform these point defects into complex defects such as Ga–O pair vacancies, resulting in a reduction in the bandgap. Consequently, an increase in photocurrent is observed for devices illuminated with 254 nm light. However, at high irradiation fluences, charge recombination induced by the presence of deep defect levels becomes more significant, leading to a decrease in photocurrent when exposed to 365 nm light. No matter what, at 1 × 1010 cm−2 fluence, β-Ga2O3 photodetectors still maintain excellent performance, implying their strong radiation resistance and immense potential for application in space environments.

Sarwar M., Ratajczak R., Mieszczynski C., Wierzbicka A., Gieraltowska S., Heller R., Eisenwinder S., Wozniak W., Guziewicz E.

Radiation-induced crystal lattice damage and its recovery in wide bandgap oxides, in particular beta-gallium oxide (β-Ga2O3), is a complex process. This paper presents the detailed study of defect accumulation in the β-Ga2O3 single crystal implanted with Ytterbium (Yb) ions and the impact of Rapid Thermal Annealing (RTA) on the defects formed. The (2¯01)oriented β-Ga2O3 single crystals were implanted with eleven fluences of Yb ions ranging from 1 × 1012 to 5 × 1015 at/cm2. Channeling Rutherford Backscattering Spectrometry (RBS/c) was used to study the crystal lattice damage induced by ion implantation and the level of structure recovery after annealing. The quantitative and qualitative analyses of collected spectra were performed by computer simulations. As a result, we present the first defect accumulation curve of β-Ga2O3 implanted with rare earth ion that reveals a two-step damage process. In the first stage, the damage of the β-Ga2O3 is inconspicuous, but begins to grow rapidly from the fluence of 1 × 1013 at/cm2, reaching the saturation at the random level for the Yb ion fluence of 1 × 1014 at/cm2. Further irradiation causes the damage peak to become bimodal, indicating that at least two new defect forms develop for the higher ion fluence. These two damage zones differently react to annealing, suggesting that they could origin from two phases, the amorphization phase and the new crystalline phase of Ga2O3. High-resolution x-ray diffraction (HRXRD) demonstrates the presence of strain and the γ phase of Ga2O3 after implantation, which disappear after annealing.

Shi Y., Meng J., Chen J., Wu R., Zhang L., Jiang J., Deng J., Yin Z., Zhang X.

As a common impurity, H plays a role in tuning the electrical properties of β-Ga2O3 and has attracted immense interest. Despite years of investigations, the influence of H-doping on electrical properties is not always clear due to the lack of comprehensive characterization on both micro- and macro scale. In this work, we investigate the effects of the H-plasma treatment on the electrical properties of β-Ga2O3 films by combining several techniques, from macroscale Hall and photoluminescence measurements to microscale conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM). The incorporation of H in β-Ga2O3 not only introduces shallow donor states such as Hi, but also passivates VGa defects by forming the VGa-4H complex. As a result, both the carrier concentration and mobility of H-doped β-Ga2O3 film are significantly increased, corresponding to an enhancement of conductivity by four orders of magnitude in comparison with the intrinsic one. These results correlate well with the local conductivity and surface potential mappings obtained from CAFM and KPFM. Moreover, we found that the work function of β-Ga2O3 thin films can also be tuned by the H-plasma treatment.

Polyakov A.Y., Yakimov E.B., Nikolaev V.I., Pechnikov A.I., Miakonkikh A.V., Azarov A., Lee I., Vasilev A.A., Kochkova A.I., Shchemerov I.V., Kuznetsov A., Pearton S.J.

In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects.

Polyakov A., Lee I., Nikolaev V., Pechnikov A., Miakonkikh A., Scheglov M., Yakimov E., Chikiryaka A., Vasilev A., Kochkova A., Shchemerov I., Chernykh A., Pearton S.

AbstractThe structural and electrical properties of undoped and Sn doped κ‐Ga2O3 layers grown by epitaxial lateral overgrowth on TiO2/sapphire substrates using stripe and point masks show that the crystalline structure of the films can be greatly improved relative to conventional planar growth. The undoped films are semi‐insulating, with the Fermi level pinned near EC‐0.7 eV, and deep electron traps at EC‐0.5 eV and EC‐0.3 eV are detectable in thermally stimulated current and photoinduced current transient spectra measurements. Low concentration Sn doping results in net donor concentrations of ≈ 1013 cm−3, and deep trap spectra determined by electron traps at EC‐0.5 eV, and deep acceptors with an optical ionization threshold near 2 and 3.1 eV. Treatment of the samples in hydrogen plasma at 330 °C increases the donor density near the surface to ≈ 1019 cm−3. Such samples show strong persistent photocapacitance and photoconductivity, indicating the possible DX‐like character of the centers involved. For thin (5 µm) κ‐Ga2O3 films grown on GaN/sapphire templates, p‐type‐like behavior is unexpectedly observed in electrical properties and  we  discuss the possible formation of a 2D hole gas at the κ‐Ga2O3/GaN interface.

Polyakov A.Y., Kuznetsov A., Azarov A., Miakonkikh A.V., Chernykh A.V., Vasilev A.A., Shchemerov I.V., Kochkova A.I., Matros N.R., Pearton S.J.

Defects created in lightly Sn-doped (2 × 1016 cm−3) (010)-oriented bulk β-Ga2O3 implanted with 1.2 MeV, 3 × 1015 cm−2 197Au+ ions before and after treatment in hydrogen plasmas at 330 °C were studied by X-ray measurements, Rutherford backscattering spectra, capacitance–voltage, current–voltage, admittance spectra and deep level transient spectroscopy. Au implantation creates defects that produce total depletion of carriers in the top 1.5 µm and introduces electron traps with energy levels at Ec-0.7 eV, Ec-1.05 eV, Ec-0.45 eV, and deep acceptors with optical ionization thresholds near 1.3 eV, 2.3 eV and 3.1 eV, similar to the centers dominating the spectra of deep traps in β-Ga2O3. Hydrogen plasma treatment greatly enhances the photocurrent and photo-capacitance and decreases the width of the insulating layer produced by Au implantation. The results can be explained by hydrogen passivation of the triply charged Ga vacancies and doubly charged split Ga vacancies acceptors in the implanted region, returning part of this region to n-type conductivity.