The electrical conductivity of cubic (In1−x Ga x )2O3 films (x ≤ 0.18): native bulk point defects, Sn-doping, and the surface electron accumulation layer

Abstract

The alloying of the group-III transparent semiconducting sesquioxides In2O3 and Ga2O3 can lead to a modulation of the properties of the parent compounds, e.g. the shallow- and deep-donor character of the oxygen vacancy or the presence and absence of a surface electron accumulation layer, respectively. In this work, we investigate the effect of alloying on the electron transport properties of unintentionally-doped single-crystalline and textured bixbyite (In1−x Ga x )2O3 thin films annealed in oxygen and vacuum with Ga contents up to x = 0.18. Hall effect measurements demonstrate a surprising increase in electron density due to native defects with added Ga, possibly induced by Ga-related unit-cell distortions. Based on the measured electron mobility we assign this increase to oxygen vacancies rather than Ga-interstitials. The large electron density of >7 × 1019 cm−3 in vacuum-annealed, Ga-containing films would place the 2+/0 transition level of the O-vacancy in our films at least 340 meV above the conduction band minimum. A combined investigation based on hard and soft X-ray photoelectron spectroscopy measurements demonstrates the existence of the surface electron accumulation layer for all alloy films irrespective of annealing and, hence, no depletion up to x = 0.18. Comparison of Hall and Seebeck measurements confirms negligible contribution of the SEAL to the electron transport in the few-100 nm thick, unintentionally-doped films. Finally, we additionally demonstrate a single-crystalline (In0.92Ga0.08)2O3:Sn film, as a possible transparent conductive oxide with a wider band gap than that of (Sn-doped) In2O3, with transport properties similar to those of ITO.