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Optical and electrical properties of Ti-doped β-Ga2O3 (Ti3+:β-Ga2O3) bulk crystals grown by floating zone method

While β-Ga2O3 has recently attracted a lot of attention for applications in UV detection and high-power electronics as an ultrawide-band gap semiconductor with a direct band gap Eg ≈ 4.8 eV, it has been also found that crystalline β-Ga2O3 doped with trivalence titanium (Ti3+:β-Ga2O3) is a potential material for ultrafast and tunable laser applications. As a semiconductor and as a laser medium, the optical and electrical properties are critical to its broad applications. In this paper we report the optical and electrical properties of intentionally, Ti-doped β‐Ga2O3 bulk crystals (TiGaO) grown by the float-zone method in comparison with their counterpart: the unintentionally doped ones. The optical and electrical properties of TiGaO crystals, both the as-grown and those annealed in 600 °C and 1000 °C, were investigated by transmittance and reflectance spectroscopy and by thermally stimulated current (TSC) spectroscopy. The optical constants were determined by our approximation-free approach, and the photocurrents (PCs) and electronic defects of the crystals due to titanium doping were analyzed and compared. We measured reflectance and transmittance on the double-side polished samples, and then derived closed-form expressions for the absorption and reflection coefficients, α and R, respectively, in terms of measured reflectance and transmittance, Rm and Tm. The real (η) and imaginary (κ) parts of the index of refraction (n = η + iκ) can be also obtained from α and R. Then, we used TSC spectroscopy to study the electrical properties and deep level defects in TiGaO crystals. The Ti-doped TiGaO crystals are resistive but annealing further increases the resistivity and shrinks slightly the bandgap. The as-grown samples have little photoresponse but after annealing they show negative PCs upon UV illumination, indicating annealing generates multiple recombination centers in the crystals. Nevertheless, they all have a dark current activation energy of 1.01 eV, which might be attributed to titanium doping that is different to the undoped ones which usually have an activation energy of 0.84 eV attributed to a trace amount of iron as is reported in literature.