Evaluation of Band Alignment of Rutile-Sn_1-xGe_xO₂ by XPS

<b>Introduction:</b> An alloy of rutile-type SnO₂ and GeO₂ (Sn_1-<i>x</i>Ge<i>_x</i>O₂: SGO) is an ultrawide-gap oxide semiconductor. SGO is expected to be applicable for power electronic and ultraviolet optoelectronic devices because of its wide tunability of bandgap from ~3.7 eV to ~4.7 eV [1, 2]. A unique feature of SGO is a possibility of ambipolar doping, originating from shallow valence band maximum (VBM) of rutile GeO₂ (<i>x</i>=1), predicted by first-principles calculations [3, 4]. Although the band alignment (band edge position) of SGO is important for considering dopability in SGO as well as for designing SGO-based heterostructure devices, it has not been experimentally investigated yet. In this study, therefore, we evaluated the position of VBM of a series of SGO epitaxial thin films (<i>x</i>=0-0.6) using X-ray photoelectron spectroscopy (XPS) and determined its band alignment.<br/><b>Methods:</b> The (001)-oriented SGO thin films (<i>x</i>=0-0.6) were epitaxially grown on Nb:TiO₂ (001) substrates by the pulsed laser deposition [1]. Crystal structure and Ge/Sn ratio of the films were confirmed by X-ray diffraction and energy-dispersive X-ray spectroscopy, respectively. In the XPS measurements, valence band and O 2s spectra of the SGO films were measured at 300 K with an Al-Kα X-ray source. The variation of the VBM from the vacuum level as a function of the chemical composition was evaluated using the position of O 2s peak as a reference. Before the XPS measurement, the SGO thin films were annealed at 800 °C for 6 hours in air to remove Sn²⁺ component from the surface.<br/><b>Results:</b> VBM of the SGO films became shallower monotonically by 0.5 eV with increase of <i>x</i> from 0 to 0.6, which qualitatively agreed with the prediction by first-principles calculations [3]. On the other hand, the position of VBM was still deeper than the practical level of Fermi energy pinning level for p-type doping, requiring further increase of Ge to realize ambipolar doping. We also estimated the position of conduction band minimum (CBM) using the band gap of SGO determined from the optical absorption spectroscopy. As <i>x</i> increased from 0 to 0.6, CBM also became shallower by 0.8 eV. This rise of CBM position can explain the reduction of donor activation ratio with increasing <i>x</i>, observed in Ta-doped SGO thin films. These results also indicated that a heterostructure consisting of SGO thin films with different Sn/Ge ratio will become a type-II heterojunction.<br/><br/><b>References:</b><br/>[1] Y. Nagashima <i>et. al.</i>, <i>Chem. Mater.</i> <b>34</b>, 24, 10842–10848 (2022)<br/>[2] H. Takane <i>et. al.</i>, <i>Phys. Rev. Mater.</i> <b>6</b>, 8, 084604 (2022)<br/>[3] C. A. Niedermeier <i>et. al.</i>, <i>J. Phys. Chem. C</i> <b>124</b>, 47, 25721–25728 (2020)<br/>[4] S. Chae <i>et. al.</i>, <i>Appl. Phys. Lett.</i> <b>114</b>, 102104 (2019)