Electronic and optical properties of Ga_3-xIn _5+xSn₂O₁₆: An experimental and theoretical study

Experimental measurements of optical and electronic properties and local-density approximation (LDA) calculations on polycrystalline Ga _3-xIn_5+xSn₂O₁₆ - the so-called "T-phase" in the Ga₂O₃-In₂O ₃-SnO₂ ternary system - have revealed it to be a good candidate for n-type transparent conducting oxide applications, particularly in the replacement of tin-doped indium oxide as a transparent electrode in organic photovoltaics. Room temperature conductivity of over 1000 S cm^-1 was measured in polycrystalline bulk samples. Band structure calculations reveal a highly dispersed conduction band, corresponding to an electron effective mass of about 0.2 m_e. Normalized carrier mobility and concentration trends indicate that conductivity changes in T-phase are attributable to changes in carrier concentration, with mobility remaining relatively constant through the range of processing conditions and sample composition. Screened exchange LDA calculations yield a fundamental band gap of about 2.60 eV. A relatively constant optical band gap in the range of 2.9-3.0 eV along the range of T-phase composition was measured by diffuse reflectance of bulk samples, whereas ab-initio simulations predict a decreasing fundamental band gap with increasing In-to-Ga ratio. This is attributed to an increasing Burstein-Moss shift - corresponding to increasing free electron concentration - with increasing In-to-Ga ratio.