Molecular dynamics simulations and photoluminescence measurements of annealed ZnO surfaces

TL;DR

This study combines molecular dynamics simulations and photoluminescence measurements to analyze how thermal annealing affects surface oxygen sublimation and defect formation in ZnO, revealing threshold temperatures for surface changes.

Contribution

It introduces a ReaxFF-based MD simulation approach to study surface atom sublimation and defect evolution in ZnO, aligning with experimental PL data.

Findings

Oxygen atoms sublimate from the oxygen-terminated surface above 700K

Formation of oxygen dimers on the surface during annealing

Photoluminescence intensity correlates with surface vacancy formation

Abstract

The effect of thermal annealing on wurtzite ZnO, terminated by two surfaces, (0 0 0 $\bar 1$) (which is oxygen-terminated) and (0 0 0 1) (which is Zn-terminated), is investigated via molecular dynamics simulation using reactive force field (ReaxFF). As a result of annealing at a threshold temperature range of 700~K $ < T_{\mbox{\small t}} \leq 800$~K, surface oxygen atoms begin to sublimate from the (0 0 0 $\bar 1$) surface, while no atom leaves the (0 0 0 1) surface. The ratio of oxygen leaving the surface increases with temperature $T$ (for $T \geq T_{\mbox{\small t}}$). The relative luminescence intensity of the secondary peak in the photoluminescence (PL) spectra, interpreted as a measurement of amount of vacancies on the sample surfaces, qualitatively agrees with the threshold behavior as found in the MD simulations. Our simulations have also revealed the formation of oxygen dimers on the surface and evolution of partial charge distribution during the annealing process. Our MD simulation based on the ReaxFF is consistent with experimental observations.