Transition state redox during dynamical processes in semiconductors and insulators

TL;DR

This paper reveals that transition states in semiconductors and insulators can change charge states during dynamical processes, significantly affecting activation barriers and Fermi level dependence, challenging traditional fixed-charge assumptions.

Contribution

It introduces a method to account for transition state redox effects, showing their importance in accurately calculating activation barriers in semiconductors and insulators.

Findings

Transition states can have different charge states from initial states.

Activation barriers are lowered when redox effects are included.

Barrier variation with Fermi level is continuous, not step-like.

Abstract

Activation barriers associated with ion diffusion and chemical reactions are vital to understand and predict a wide range of phenomena, such as material growth, ion transport, and catalysis. In the calculation of activation barriers for non-redox processes in semiconductors and insulators, it has been widely assumed that the charge state remains fixed to that of the initial electronic ground state throughout a dynamical process. In this work, we demonstrate that this assumption is generally inaccurate and that a rate-limiting transition state can have a different charge state from the initial ground state. This phenomenon can significantly lower the activation barrier of dynamical process that depends strongly on charge state, such as carbon vacancy diffusion in 4H-SiC. With inclusion of such transition state redox, the activation barrier varies continuously with Fermi level, in contrast to the step-line feature predicted by the traditional fixed-charge assumption. In this study, a straightforward approach to include the transition state redox effect is provided, the typical situations where the effect plays a significant role are identified, and the relevant electron dynamics are discussed.