Carrier removal rates and electron and hole trap densities in β-Ga2O3 films grown by hydride vapor phase epitaxy (HVPE) and irradiated with 18 MeV α-particles and 20 MeV protons were measured and compared to the results of modeling. The electron removal rates for proton and α-radiation were found to be close to the theoretical production rates of vacancies, whereas the concentrations of major electron and hole traps were much lower, suggesting that the main process responsible for carrier removal is the formation of neutral complexes between vacancies and shallow donors. There is a concurrent decrease in the diffusion length of nonequilibrium charge carriers after irradiation, which correlates with the increase in density of the main electron traps E2* at Ec − (0.75–0.78) eV, E3 at Ec − (0.95–1.05) eV, and E4 at Ec − 1.2 eV. The introduction rates of these traps are similar for the 18 MeV α-particles and 20 MeV protons and are much lower than the carrier removal rates.
The Fermi level in bulk semi-insulating β-Ga2O3 doped with Fe (∼5 × 1018 cm−3) is found to be pinned near Ec − 0.85 eV. At temperatures ≥400 K, Ni Schottky diodes showed good rectification and measurable low frequency capacitance, allowing the measurement of capacitance-frequency (C-f), capacitance-voltage (C-V), and capacitance-temperature (C-T) characteristics. The activation energy and the electron capture cross section obtained were (0.75–0.82) eV and (2–5) × 10−15 cm2, in good agreement with the reported signature of the E2 electron trap assigned to Fe. The concentration of the filled centers determined from C-V was close to the concentration of residual shallow donors in undoped materials. Photoinduced current transient spectroscopy measurements showed that Fe doping does not promote the generation of high densities of deep traps other than those related to Fe.
Epitaxial layers of α-Ga2O3 with different Sn doping levels were grown by halide vapor phase epitaxy on sapphire. The films had shallow donor concentrations ranging from 1017 to 4.8 × 1019 cm−3. Deep level transient spectroscopy of the lowest doped samples revealed dominant A traps with level Ec − 0.6 eV and B traps near Ec − 1.1 eV. With increasing shallow donor concentration, the density of the A traps increased, and new traps C (Ec − 0.85 eV) and D (Ec − 0.23 eV) emerged. Photocapacitance spectra showed the presence of deep traps with optical ionization energy of ∼2 and 2.7 eV and prominent persistent photocapacitance at low temperature, surviving heating to temperatures above room temperature. The diffusion length of nonequilibrium charge carriers was 0.15 µm, and microcathodoluminescence spectra showed peaks in the range 339–540 nm, but no band-edge emission.
Films of α-Ga2O3 doped with Sn were grown by halide vapor phase epitaxy (HVPE) on planar and patterned sapphire substrates. For planar substrates, with the same high Sn flow, the total concentration of donors was varying from 1017 cm−3 to high 1018 cm−3. The donor centers were shallow states with activation energies 35–60 meV, centers with levels near Ec–(0.1–0.14) eV (E1), and centers with levels near Ec–(0.35–0.4) eV (E2). Deeper electron traps with levels near Ec−0.6 eV (A), near Ec−0.8 eV (B), Ec−1 eV (C) were detected in capacitance or current transient spectroscopy measurements. Annealing of heavily compensated films in molecular hydrogen flow at 500 °C for 0.5 h strongly increased the concentration of the E1 states and increased the density of the E2 and A traps. For films grown on patterned substrates the growth started by the formation of the orthorhombic α-phase in the valleys of the sapphire pattern that was overgrown by the regions of laterally propagating α-phase. No improvement of the crystalline quality of the layers when using patterned substrates was detected. The electric properties, the deep traps spectra, and the effects of hydrogen treatment were similar to the case of planar samples.
Undoped epitaxial films of α-Ga2O3 were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O2 (O2-control growth regime), and with constant flow of O2 and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O2-control deposition, and these results appear to be the best so far reported for α-Ga2O3 films. All grown α-Ga2O3 epilayers were high-resistivity n-type, with the Fermi level pinned near Ec − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O2-control pulsed growth conditions of deep hole traps with levels near Ev + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga2O3 samples are close to the position of dominant electron and hole traps in well documented β-Ga2O3 crystals and films.
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