Ab initio approach for thermodynamic surface phases with full consideration of anharmonic effects -- the example of hydrogen at Si(100)

TL;DR

This paper introduces a first-principles method to accurately predict surface phase diagrams in reactive environments, accounting for anharmonic effects, and applies it to hydrogen on Si(100), revealing many new stable phases.

Contribution

It develops a novel REGC-MD approach for surface thermodynamics that incorporates anharmonic effects, improving upon existing methods.

Findings

Identified 25 stable surface phases of Si(100) with hydrogen from 300 to 1000 K.

Discovered phases and phase transitions not observed experimentally.

Revealed significant differences from traditional atomistic thermodynamics predictions.

Abstract

A reliable description of surfaces structures in a reactive environment is crucial to understand materials functions. We present a first-principles theory of replica-exchange grand-canonical-ensemble molecular dynamics (REGC-MD) and apply it to evaluate phase equilibria of surfaces in reactive gas-phase environment. We identify the different surface phases and locate phase boundaries including triple and critical points. The approach is demonstrated by addressing open questions for the Si(100) surface in contact with a hydrogen atmosphere. In the range from 300 to 1 000 K, we find 25 distinct thermodynamically stable surface phases, for which we also provide microscopic descriptions. Most of the identified phases, including few order-disorder phase transitions, have not yet been observed experimentally. The REGC-MD-derived phase diagram shows significant, qualitative differences to the description by the state-of-the-art "ab initio atomistic thermodynamics" approach.