Electronic defects with shallow and deep levels in β-Ga2O3 single crystals were investigated by thermoluminescence (TL) spectroscopy. Undoped, Fe-doped, Sn-doped, and Mg-doped β-Ga2O3 single crystals grown by different methods were studied, and thermal activation energies of defects were calculated using the initial rise method. Hall-effect measurements and optical absorption spectroscopy were performed to determine the electrical transport properties and optical bandgaps. It was found that the dopants do not have any effect on the bandgap energy, which is important for comparing the trap levels in the samples. Three deep trap levels were found in the undoped crystals; the activation energy, ED, and concentration of defect centers for all of them have slightly changed after doping with Fe and Mg. Fe doping induced an additional defect center with activation energy of 0.62 eV. The measurements revealed the absence of TL emission in Sn doped crystals indicating that Sn doping may quench luminescence centers or modified some original electronic defects to inactive electron traps. The second interpretation “decrease of traps” may align with the successful incorporation of Sn as a donor and the high conductivity of Sn doped crystals revealed from Hall-effect measurements. This work also illustrates that the semi-insulating characteristics of Fe and Mg doped Ga2O3 are associated with the increase of the concentration of original traps in the crystal as well as the formation of new electron traps acting as deep acceptors. Recombination centers in all crystals are assumed to be associated with iron impurities.

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Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.