We report on growth and electrical properties of α-Ga2O3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr2O3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr2O3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr2O3 bandgap and a sharp MCL line near 700 nm due to the Cr+ intracenter transition. Ohmic contacts to Cr2O3 were made with both Ti/Au or Ni, producing linear current–voltage ( I– V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2- μm-thick α-Ga2O3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at EC of 0.3 eV. The I– V and capacitance–voltage ( C– V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr2O3 buffer indicated the presence of two face-to-face junctions, one between n-Ga2O3 and p-Cr2O3, the other due to the Ni Schottky diode with n-Ga2O3. The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga2O3 system.

Gallium oxide, and in particular its thermodynamically stable β-Ga2O3 phase, is within the most exciting materials in research and technology nowadays due to its unique properties. The very high breakdown electric field and the figure of merit rivaled only by diamond have tremendous potential for the next generation “green” electronics enabling efficient distribution, use, and conversion of electrical energy. Ion implantation is a traditional technological method used in these fields, and its well-known advantages can contribute greatly to the rapid development of physics and technology of Ga2O3-based materials and devices. Here, the status of ion implantation in β-Ga2O3 nowadays is reviewed. Attention is mainly paid to the results of experimental study of damage under ion irradiation and the properties of Ga2O3 layers doped by ion implantation. The results of ab initio theoretical calculations of the impurities and defect parameters are briefly presented, and the physical principles of a number of analytical methods used to study implanted gallium oxide layers are highlighted. The use of ion implantation in the development of Ga2O3-based devices, such as metal oxide field-effect transistors, Schottky barrier diodes, and solar-blind UV detectors, is described together with systematical analysis of the achieved values of their characteristics. Finally, the most important challenges to be overcome in this field of science and technology are discussed.

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

No abstract

High-temperature β-Ga2O3:Cr2O3-based sensors sensitive to oxygen- and hydrogen-containing gases have been developed and studied. Magnetron cosputtering is the method of choice for the thin film synthesis as an industry-compatible technique. The composition-structure-properties relationship has been revealed. An introduction of 0.04–0.14 wt. % Cr leads to a significant increase in the response of the O2 sensors over the temperature range 250–400 °C. The highest response in the above-mentioned temperature range has been achieved for a Cr addition of 0.14 wt. %. An increase in the Cr content from 0.04 to 0.22 wt. % leads to a decrease in the β-Ga2O3-based sensors’ response time, especially for low O2 concentrations (≤10 vol. %). Reliable control of the β-Ga2O3:Cr2O3-based sensors’ selectivity to industry-relevant reducing gases—hydrogen, carbon monoxide, and toluene—is demonstrated. β-Ga2O3 films with a Cr incorporation content of 0.04 and 0.06 wt. % have a high response to toluene at operating temperatures 300–500 °C, while the films with 0.14 and 0.22 wt. % Cr have a high response to H2 in the range 400–500 °C. Regardless of the Cr content in β-Ga2O3 thin films, all sensors considered demonstrate a weak response to CO within the operating temperature range 250–500 °C. The results attained are of certain technological importance, i.e., in terms of the development of cost-effective methods for the synthesis of materials and systems for monitoring and control of industry-relevant gases for an environmentally friendly and sustainable growth.