Ga2O3 layers with thickness from 10 to 86 µm were grown by halide vapor phase epitaxy (HVPE) on GaN(0001)/sapphire templates in a hot wall reactor at 570°C, with the growth rate of about 3‑4 μm/h. The grown layers consisted of pure (001)-oriented κ-Ga2O3 polymorph with no admixture of β-Ga2O3 or α-Ga2O3 phases. The narrowest (004) x-ray rocking curves were observed for 13-20 µm thick κ-Ga2O3 layers. A further increase in thickness results in deterioration of the crystal quality which is indicated by the broadening of rocking curves. Electrical measurements of the thick layers revealed that they were n-type, with the concentration of shallow donors gradually decreasing from ~1016 cm-3 to ~1015 cm‑3. Deep level transient spectroscopy (DLTS) measurements revealed the presence of deep traps with levels near Ec-0.3 eV, Ec-0.6 eV, Ec-0.7 eV, Ec-0.8 eV, Ec-1 eV, with the Ec-0.8 eV being predominant.

Gallium oxide Ga 2 O 3 has at least five known polymorphic forms commonly denoted as α, β, γ, δ, and ε(κ). [1] According to recent findings, ε-Ga 2 O 3 is not a true polymorph of hexagonal P6 3 mc symmetry; rather, it is composed of twinned rotational domains of orthorhombic κ-Ga 2 O 3 of Pna2 1 space group. [2] All Ga 2 O 3 polymorphs are ultrawide-bandgap semiconductors and are promising for applications in power and sensor electronics. There is growing interest in the research of metastable polymorphs, including the pseudohexagonal ε-Ga 2 O 3 form.Pseudohexagonal ε-Ga 2 O 3 is stable up to the temperature of T ¼ 700 C and is epitaxially compatible with other semiconducting materials with a hexagonal or pseudohexagonal structure such as AlN, GaN, 6H-SiC, sapphire, and some other metal oxides. [3,4] The reported bandgap energy of ε-Ga 2 O 3 is close to that of β-Ga 2 O 3 and varies from 4.5 to 5.0 eV. [5][6][7][8][9] The ε-Ga 2 O 3 phase exhibits ferroelectric properties, [3,6,10,11] enabling the development of high-electron-mobility transistors (HEMTs) based on this material. [12] Solar-blind UV detectors based on ε-Ga 2 O 3 demonstrate record characteristics [5,[13][14][15][16] due to the high symmetry of this polymorph.Previously, we studied the effect of H 2 on the electrical conductivity and gas-sensing properties of the Pt-contacted doublephase ε-Ga 2 O 3 /α-Ga 2 O 3 :Sn structures grown by halide vaporphase epitaxy (HVPE) on patterned sapphire substrates (PSS). [17] These structures showed response to H 2 starting from room temperature (RT). The minimum detectable H 2 concentration was 54 ppm at T ¼ 125 C. At low applied voltages U < 7.5 V, the double-phase structures showed no response to CH 4 , O 2 , CO, and NH 3 . At U ¼ 7.5-150 V, these structures showed a significant response to O 2 and NH 3 . The authors concluded that the H 2 sensitivity of Pt-contacted double-phase α-Ga 2 O 3 /ε-Ga 2 O 3 structures was caused by the change of the energy barrier height at the interface between the catalytically active Pt contact and ε-Ga 2 O 3 . The reduction of Sn impurity concentration in ε-Ga 2 O 3 / α-Ga 2 O 3 from %4 Â 10 18 to %1.5 Â 10 17 cm À3 led to increase in sensitivity to O 2 at T ¼ 180-220 C and U ≤ 7.5 V. [18] The exposure to oxygen caused a reversible decrease in the current I through the structure. The gas-sensing effect was manifested in modulation of I due to the chemisorption of O 2 on the

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