Two-dimensional electron gas of the In$_{2}$O$_{3}$ surface: Enhanced thermopower, electrical transport properties, and reduction by adsorbates or compensating acceptor doping

In$_{2}$O$_{3}$ is an n-type transparent semiconducting oxide possessing a surface electron accumulation layer (SEAL) like several other relevant semiconductors, such as InAs, InN, SnO$_{2}$, and ZnO. Even though the SEAL is within the core of the application of In$_{2}$O$_{3}$ in conductometric gas sensors, a consistent set of transport properties of this two-dimensional electron gas (2DEG) is missing in the present literature. To this end, we investigate high-quality single-crystalline as well as textured doped and undoped In$_{2}$O$_{3}$(111) films grown by plasma-assisted molecular beam epitaxy to extract transport properties of the SEAL by means of Hall effect measurements at room temperature while controlling the oxygen adsorbate coverage via illumination. The resulting sheet electron concentration and mobility of the SEAL are ≈1.5 × 10$^{13}$ cm$^{−2}$ and ≈ 150 cm$^{2}$/Vs, respectively, both of which are strongly reduced by oxygen-related surface adsorbates from the ambient air. Our transport measurements further demonstrate a systematic reduction of the SEAL by doping In$_{2}$O$_{3}$ with the deep compensating bulk acceptors Ni or Mg. This finding is supported by x-ray photoelectron spectroscopy (XPS) measurements of the surface band bending and SEAL electron emission. Quantitative analyses of these XPS results using self-consistent, coupled Schrödinger-Poisson calculations indicate the simultaneous formation of compensating bulk donor defects (likely oxygen vacancies), which almost completely compensate the bulk acceptors. Finally, an enhancement of the thermopower by reduced dimensionality is demonstrated in In$_{2}$O$_{3}$: Seebeck coefficient measurements of the surface 2DEG with partially reduced sheet electron concentrations between 3 × 10$^{12}$ and 7 × 10$^{12}$ cm$^{−2}$ (corresponding average volume electron concentration between 1 × 10$^{19}$ and 2.3 × 10$^{19}$ cm$^{−3}$) indicate a value enhanced by ≈80% compared to that of bulk Sn-doped In$_{2}$O$_{3}$ with comparable volume electron concentration.