Bandgap engineered high mobility indium oxide thin films for photovoltaic applications
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Bandgap engineered high mobility indium oxide thin films for photovoltaic applications R.K. Gupta, K. Ghosh, P.K. Kahol Department of physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO-65897 ABSTRACT Magnesium and titanium doped indium oxide (IMTO) thin films were grown using pulsed laser deposition technique. Magnesium was added to enhance the bandgap, whereas titanium was added to improve carrier concentrations and mobility of indium oxide films. The effect of growth temperature on structural, optical, and electrical properties were studied. It was observed that the optical transparency of the films strongly depends on growth temperature and increases with increase in growth temperature. The films grown at 600 oC showed optical transparency > 85%. We observed widening in bandgap of indium oxide by doping with magnesium and titanium. The bandgap of IMTO films increases with increase in growth temperature. The maximum bandgap of 3.9 eV was observed for film grown at 600 oC. It was observed that growth temperature strongly affects the electrical properties such as resistivity, carrier concentration, and mobility. The electrical resistivity and mobility of the films increases with increase in growth temperature. On the other hand, carrier concentration decreases with increase in growth temperature. Temperature dependence electrical resistivity measurements showed that films grown at low temperatures are semiconducting in nature, while films grown at high temperature showed transition from semiconducting to metallic behavior. These wide bandgap, highly transparent, and high mobility films could be used for photovoltaic applications. INTRODUCTION Indium oxide (In2O3) has attracted considerable attention in recent years for their wide applications as transparent conducting electrodes in photovoltaic and optoelectronic devices [1]. These transparent conducting oxides have potential application in flat panel display, organic light emitting diodes, and solar cell applications. Indium oxide is widely used for these applications because of its high transparency and electrical conductivity. Indium oxide is an n-type semiconductor with bandgap of ~ 3.5 eV [2]. The bandgap engineering of the metal oxides has attracted considerable attention. Chattopadhyay et. al. have reported that titanium doping could widen the bandgap of CdO films [3]. They observed the bandgap of 3.0 eV by addition of 5 % titanium in CdO film. Recently, we have shown the effect of substrate temperature of optical and electrical properties of aluminum doped CdO films [4]. It is observed that increase in growth temperature widen the bandgap of the films. Thin films of transparent conducting oxides have been deposited using different techniques such as reactive DC magnetron sputtering [5], RF magnetron sputtering [6], thermal reactive evaporation [7], high density plasma evaporation [8], electron beam evaporation [9], ultrasonic spray pyrolysis [10], sol-gel [11] and pulsed laser deposition [12]. Among these techniques, pulsed
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