Deep Ultraviolet Transparent Electrode: Ta-Doped Rutile Sn_1–xGe_xO₂

Transparent conductive oxides (TCOs) are key materials for highly efficient optoelectronic devices such as light-emitting diodes (LEDs) and photovoltaic cells. While high-performance TCOs have been developed for use in the visible light spectrum, few materials are identified as TCOs feasible for deep ultraviolet (DUV) devices, especially at wavelengths shorter than 280 nm (UV-C region). Herein, we demonstrate that an alloy of rutile SnO2 and isostructural GeO2, rutile Sn1–xGexO2 (SGO), is a promising host semiconductor for a DUV-TCO. The optical band gap for (001)-oriented rutile SGO epitaxial thin films grown on Al2O3 (10–10) substrates increases monotonically from 3.79 eV (x = 0) to 4.09 eV (x = ∼0.7) with increasing GeO2 content. The rutile SGO thin films with a low Ge content (x ≤ 0.36) exhibit a low electrical resistivity of 2–3 × 10–4 Ω cm on doping with Ta5+ as a donor impurity. The increase in carrier electrons simultaneously enhances the DUV transparency of the films owing to the Burstein–Moss effect, resulting in an optical band gap of ∼4.2 eV for the film with x = ∼0.30. Notably, Ta-doped rutile SGO thin films can also be grown on AlN substrates, and their performance as DUV-TCOs is superior to that of conventional TCOs and competitive with state-of-the-art DUV-TCOs. These results indicate that Ta-doped rutile SGO thin films are promising candidates for transparent electrodes on III–V nitride semiconductor-based DUV-LEDs.