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 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
Deep levels studies on a set of n-GaN films grown by MOCVD and HVPE reveal the presence of electron traps with levels near E c -0.25 eV, E c -0.55 eV, E c -0.8 eV, E c -1 eV, hole traps with levels near E v +0.9 eV and a band of relatively shallow states in the lower half of the bandgap. The total density of these latter states was estimated to be some 10 16 cm -3 and they were tentatively associated with dislocations in GaN based on their high concentration and band-like character. None of the electron or hole traps could be unambiguously related with strong changes of diffusion lengths of minority carriers in various samples. It is proposed that such changes occur due to different surface recombination velocities. An important role of E c -0.55 eV traps in persistent photoconductivity phenomena in n-GaN has been demonstrated.
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