Step bunching process induced by the flow of steps at the sublimated crystal surface

TL;DR

This paper investigates how step bunching occurs on sublimating GaN surfaces, showing that step-flow anisotropy and Schwoebel barriers significantly influence surface pattern evolution during high-temperature sublimation.

Contribution

It combines kinetic Monte Carlo simulations with analytical models to elucidate the mechanisms of step bunching driven by step-flow anisotropy and Schwoebel barriers.

Findings

Low sublimation rates produce well-defined step bunches.

High Schwoebel barriers lead to surface roughening and step fracture.

Abstract

Stepped GaN(0001) surface is studied by the kinetic Monte Carlo method and compared with the model based on Burton-Carbera-Frank equations. Successive stages of surface pattern evolution during high temperature sublimation process are discussed. At low sublimation rates clear, well defined step bunches form. The process happens in the absence or for very low Schwoebel barriers at the ideal surface. Bunches of several steps are well separated, move slowly and are rather stiff. Character of the process changes for more rapid sublimation process where double step formations become dominant and together with meanders and local bunches assemble into the less ordered surface pattern. Solution of the analytic equations written for one dimensional system confirms that step bunching is induced by the particle advection caused by step-flow anisotropy. This anisotropy becomes important when due to the low Schwoebel barrier both sides of step are symmetric. Simulations show that in the opposite limit of very high Schwoebel barrier steps fracture and rough surface builds up.