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WW6.07 - Combinatorial and In-Situ Experiments Combined with First-Principles Modeling Identify Non-Equilibrium Origin of Conductivity in ZnO: Ga Transparent Conductive Oxide 
April 24, 2014   10:30am - 10:45am

Transparent conductive oxides (TCOs) is a technologically significant class of materials that are commercially used in optoelectronic devices such as solar cells, flat-panel displays, light-emitting diodes. The prototypical compounds are In2O3:Sn (ITO), ZnO:Al (AZO) and SnO2:F (FTO). Despite the heavy industrial use of these materials, the physical origins of simultaneous high optical transparency and large electrical conductivity remain controversial. For example, undoped indium oxide thin films have been recently demonstrated to have thickness-dependent high conductivity that results from surface donor-type defects [1], challenging the traditional oxygen vacancy donor defect model.In this contribution we reveal the non-equilibrium origin of high electrical conduction in gallium zinc oxide ZnO:Ga (GZO) thin films using complementary contributions from high-throughput combinatorial experiments, first-principles modeling and in-situ electrical transport measurements as a function of temperature (T) and oxygen partial pressure (pO2) [2]. Specifically, gallium zinc oxide thin film sample libraries prepared using combinatorial pulsed laser deposition with temperature gradient across the glass substrate show that the maximum electrical conductivity occurs in low-pO2 and low-T regime. In contrast, constrained-equilibrium first-principles theoretical defect model suggests that that conductivity should increase with increasing temperature. The in-situ electrical measurements of high-quality ZnO:Ga layers on both amorphous and crystalline substrates reconcile these two contradicting observations by discovering a non-monotonic dependence of Ga:ZnO electrical conductivity on temperature. Starting from room temperature at ambient pressure, the conductivity first drops by 5 orders of magnitude up to 500C, but then the trend reverses and matches well the theoretically predicted dependence.The results of these studies indicate that high electrical conductivity of commercial-grade Ga:ZnO TCO occurs by virtue of its non-equilibrium state. This discovery calls for development of new theoretical and experimental tools for predictive design of metastable materials. This work also exemplifies the importance of complementary iteration between combinatorial experiments, theoretical modeling and in-situ characterization for materials by design.This research was supported by U.S. Department of Energy[1] PRL 108 016802 (2012)[2] APL, in press (2013)

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