Crossover from incoherent to coherent electron tunneling between defects in MgO

Abstract

Long-range electron tunneling is a fundamental process that is critical to the performance of oxide materials in microelectronics, energy generation, and photocatalysis, but extremely challenging to probe experimentally. Here we devise a computational approach that allows one to probe the mechanism and calculate the rate of electron transfer (ET) in such materials from first principles. Application to ET between defects in MgO reveals that the activation energy for ET depends strongly on defect separation, an effect not usually taken into account in semiempirical models of ET processes in oxides. Importantly, for distances below a critical defect separation (6 \AA{}), the nature of ET changes from incoherent to coherent tunneling, suggesting that existing empirical models require essential modifications. These calculations extend first-principles modeling of ET in oxides to the regime of long-range incoherent transport, an outstanding problem important for modeling many processes of technological relevance.