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E2.9.1

Carrier transport of extended and localized states in InGaO3(ZnO)5 Kenji Nomura1,2, Hiromichi Ohta2, Kazushige Ueda1, Toshio Kamiya1,2, Masahiro Hirano2 and Hideo Hosono1,2 1 Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan. 2 Hosono Transparent ElectroActive Materials, ERATO, JST, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan. ABSTRACT Carrier transport properties and electronic structure of an n-type transparent oxide semiconductor, InGaO3(ZnO)5, were investigated using single-crystalline thin films. Room-temperature Hall mobility strongly depends on carrier concentration, and rapidly increased from ~ 2 cm2 (Vs)-1 to > 10 cm2 (Vs)-1 around the carrier concentration (Nth) ~3 × 1018 cm-3. This change is associated with insulator-metal transition. These results are explained by a model similar to Anderson localization, in which shallow semi-localized states are formed originating from random distribution of Ga3+ and Zn2+ ions in the intrinsic crystal structure of InGaO3(ZnO)5. The present conclusion suggests that electron densities larger than Nth are necessary to attain high performances in drift carrier devices fabricated using InGaO3(ZnO)5. It was demonstrated that transparent filed-effect transistors exhibited good performances such as a “normally-off characteristics”, an on/off current ratios as large as 105, and a field-effect mobility ~80 cm2 (Vs)-1 when high-k material, amorphous HfOx, was used as a gate insulator. INTRODUCTION Transparent oxide semiconductors (TOSs) are promising for future opto-electronics devices such as “invisible circuit”, because they have a unique feature of optical transparency in visible region and high electronic conduction coexist [1]. So far, TOS-based opto-electronic devices including ultraviolet light-emitting diodes, transparent pn refractors, and transparent field-effect transistors (TFETs) have been developed [2, 3]. Although transparent FETs using conventional n-type TOSs, SnO2:Sb and ZnO, have been reported [4-6], practical performances such as a large field-effect mobility has not been achieved at good reproducibility due to the absence of high-quality single-crystalline TOS thin films. Furthermore these devices operated in a depletion mode (“normally-on” type) unintentionally: i.e., drain current flowed even when the gate voltage was not applied because most of TOSs show high electrical conductivity even in the undoped, as-prepared state, and it is hard to control carrier concentration down to less than ~ 1017 cm-3 without counter doping. It is therefore important to chose an appropriate TOS material for a channel layer and to fabricate high-quality single-crystalline TOS thin films to realize high performance transparent FETs operating in an enhancement mode (”normally-off” type). Especially, carrier transport properties in active channel materials strongly affect on device performances. Thus, it is important to know intrinsic carrier transport properties of TOSs. We chose a homologous series T