Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE

We report ultra-high responsivity of epitaxial (Sn_xGa_1-x)₂O₃ (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (-201) oriented n-type β-Ga₂O₃ substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5×10⁴ A/W at -5V applied bias under 250nm illumination with sharp cutoff shorter than 280nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (2.20 eV) not seen in β-Ga₂O₃ along with enhancement of the blue emission-band (2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga₂O₃, V_Ga–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga₂O₃ devices by means of thermionic emission and electron tunneling. Regenerating the V_Ga–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity.