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 Ga_{2}O_{3} and, potentially, for similar polymorphic families in other materials. 

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Disorder-Induced Ordering in Gallium Oxide Polymorphs

Comparative study of radiation tolerance of GaN and Ga2O3 polymorphs

Silicon based power devices have limited capabilities in terms of voltage handling and switching speeds, leading to rampant research in the field of next generation wide bandgap semiconductors like SiC, GaN, and Ga 2 O 3 . Amongst these, gallium oxide with its ultra-wide bandgap of 4.6-4.9 eV and high breakdown field (approx. 8 MV cm -1 ) turns out to be a potential replacement. Availability of large size, high-quality wafers at moderate costs make it desirable even from industrial viewpoint. Ga 2 O 3 power diodes having breakdown voltages (V br ) of hundreds of Volts have been reported. However, they suffer from very high on-resistance (R on ) leading to increased switching losses and decreased switching speed. This timely review analyses the recent progress made in Ga 2 O 3 based power devices with detailed discussion on the basic parameters such as V br , R on and leakage current along with the factors critically affecting them. Special focus is laid on the impeccable value-additive extreme environment applications. Open challenges plaguing the field such as trade-off between achieving high V br and low R on simultaneously, shortcomings in the material itself and the need for new physics to explain the high energy carrier transport is also explored along with the future prospects required to achieve true power-saving and commercialization. • Physics behind power device parameters and effect on the performance is explained. • Extensive outlook is provided on the current status of Ga 2 O 3 based power devices. • Effects of temperature, defects, growth methods on power devices are discussed. • High radiation hardness of Ga 2 O 3 and its extreme environmental uses are emphasized. • Open challenges and future prospects of Ga 2 O 3 based power devices are elucidated. 

A Strategic Review on Gallium Oxide Based Power Electronics: Recent Progress and Future Prospects

Alpha (α)- and beta (β)-phase gallium oxide (Ga2O3), emerging as ultrawide-band gap semiconductors, have been paid a great deal of attention in optoelectronics and high-performance power semiconductor devices owing to their ultrawide band gap ranging from 4.4 to 5.3 eV. The hot-wall mist chemical vapor deposition (mist-CVD) method has been shown to be effective for the growth of pure α- and β-phase Ga2O3 thin films on the α-Al2O3 substrate. However, challenges to preserve their intrinsic properties at a critical growth temperature for robust applications still remain a concern. Here, we report a convenient route to grow a mixed α- and β-phase Ga2O3 ultrathin film on the α-Al2O3 substrate via mist-CVD using a mixture of the gallium precursor and oxygen gas at growth temperatures, ranging from 470 to 700 °C. The influence of growth temperature on the film characteristics was systematically investigated. The results revealed that the as-grown Ga2O3 film possesses a mixed α- and β-phase with an average value of dislocation density of 1010 cm–2 for all growth temperatures, indicating a high lattice mismatch between the film and the substrate. At 600 °C, the ultrathin and smooth Ga2O3 film exhibited a good surface roughness of 1.84 nm and an excellent optical band gap of 5.2 eV. The results here suggest that the mixed α- and β-phase Ga2O3 ultrathin film can have great potential in developing future high-power electronic devices.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.1c05859

Temperature Dependence of Ultrathin Mixed-Phase Ga2O3 Films Grown on the α-Al2O3 Substrate via Mist-CVD

Thick (23 µm) films of κ-Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on GaN/sapphire templates at 630 °C. X-ray analysis confirmed the formation of single-phase κ-Ga2O3 with half-widths of the high-resolution x-ray diffraction (004), (006), and (008) symmetric reflections of 4.5 arc min and asymmetric (027) reflection of 14 arc min. Orthorhombic κ-Ga2O3 polymorph formation was confirmed from analysis of the Kikuchi diffraction pattern in electron backscattering diffraction. Secondary electron imaging indicated a reasonably flat surface morphology with a few (area density ∼103 cm−2) approximately circular (diameter ∼50–100 µm) uncoalesced regions, containing κ-Ga2O3 columns with in-plane dimensions and a height of about 10 µm. Micro-cathodoluminescence (MCL) spectra showed a wide 2–3.5 eV band that could be deconvoluted into narrower bands peaked at 2.59, 2.66, 2.86, and 3.12 eV. Ni Schottky diodes prepared on the films showed good rectification but a high series resistance. The films had a thin near-surface region dominated by Ec − 0.7 eV deep centers and a deeper region (∼2 µm from the surface) dominated by shallow donors with concentrations of ≤1016 cm−3. Photocurrent and photocapacitance spectra showed the presence of deep compensating acceptors with optical ionization energies of ∼1.35 and 2.3 eV, the latter being close to the energy of one of the MCL bands. Deep level transient spectroscopy revealed deep traps with energies near 0.3, 0.6, 0.7, 0.8, and 1 eV from the conduction band edge. The results show the potential of HVPE to grow very thick κ-Ga2O3 on GaN/sapphire templates.

https://aip.scitation.org/doi/pdf/10.1063/5.0091653

Structural and electrical properties of thick κ-Ga2O3 grown on GaN/sapphire templates

Ion-beam modification of metastable gallium oxide polymorphs

Synthesis of nanoparticles in insulators attracts tremendous attention due to their unique electrical and optical properties. Here, the gallium (Ga) and gallium nitride (GaN) nanoclusters have been synthesized in the silicon nitride matrix by sequential ion implantation (gallium and nitrogen ions) followed by either furnace annealing (FA) or rapid thermal annealing (RTA). The presence of Ga and GaN nanoclusters has been confirmed by Fourier-transform infrared, Raman and X-ray photoelectron spectroscopy. Thereafter, the effect of RTA and FA on the conduction of charge carriers has been studied for the fabricated devices. It is found from the current–voltage measurements that the carrier transport is controlled by the space charge limited current conduction mechanism, and the observed values of parameter m (trap density and the distribution of localized state) for the FA and RTA devices are ~2 and ~4.1, respectively. This reveals that more defects are formed in the RTA device and that FA provides better performance than RTA from the viewpoint of opto- and nano-electronic applications.

Alexey Nezhdanov

Papers

Ion-beam modification of metastable gallium oxide polymorphs

Gallium nitride nanocrystal formation in Si 3 N 4 matrix by ion synthesis