An Ultrawide Bandgap Transparent Conductor for Deep Ultraviolet—A-Doped Sn_1-xGe_xO₂ Thin Films

<b>Introduction:</b> In recent years, deep ultraviolet (DUV) optoelectronic devices, such as light emitting diodes (LEDs) and photosensors, have been extensively studied for applications in various fields of medicine, industry, agriculture, etc. High performance transparent electrode is one of the key components for improving their efficiency. However, conventional transparent conducting oxides (TCOs) are not compatible with DUV devices because of their insufficient optical gap (<i>E</i>_opt). In this study, we attempted to enlarge the <i>E</i>_opt of SnO₂ (~3.8 eV), a widely used host semiconductor of TCOs, by alloying with isostructural rutile GeO₂ (<i>E</i>_opt=~4.7 eV) for developing a DUV transparent conductor.<br/><b>Methods :</b> Undoped and Ta-doped Sn_1−<i>x</i>Ge<i>_x</i>O₂ (SGO) thin films were deposited on (001) plane of TiO₂, (10-10) plane of Al₂O₃ (m-Al₂O₃) and (0001) plane of AlN substrates by pulsed laser deposition. The substrate temperature <i>T</i>_s and the oxygen partial pressure <i>P</i>_O2 were varied as the growth parameters.<br/><b>Results:</b> Epitaxial growth of (001)-oriented rutile SGO thin films (0 ≤ <i>x </i>≤ 0.7) were confirmed on TiO₂ (001) and m-Al₂O₃ substrates at <i>T</i>_s = 500 °C. Both the <i>a</i>- and the <i>c</i>-axis length of the undoped SGO films linearly decreased with increasing GeO₂ content <i>x</i>. The lattice constants agreed well with those predicted from Vegard’s law, indicating the formation of solid solution. Furthermore, <i>E</i>_opt of the SGO thin films evaluated from Tauc plots assuming direct transition increased monotonically from 4.1 eV (<i>x</i>=0) to 4.45 eV (<i>x</i>=0.7). By ~2at% Ta-doping, the Sn_0.64Ge_0.36O₂ (001) thin films showed remarkably low electrical resistivity of 2.7×10^-4Ω cm as well as enlarged <i>E</i>_opt of 4.65 eV due to the Burstein-Moss shift, indicating that Ta:SGO is a promising DUV transparent conductor. We also tried epitaxial growth of Ta-doped SGO films on AlN substrates, considering the application as transparent electrodes in III-V nitride based DUV-LEDs. Although the Ta-doped SGO films deposited on bare AlN substrates became polycrystals, insertion of a thin seed layer of SnO₂ enabled epitaxial growth of (100)-oriented Ta-doped SGO films. The epitaxial Ta-doped SGO films grown on the AlN substrate also exhibited excellent electrical conductivity as well as high UV transparency.