Step bunching caused by annealing vicinal GaAs(001) in ${\mathrm{AsH}}_3$ and hydrogen ambient in its stationary state

The growth of step bunches on vicinal GaAs(001) annealed in ${\mathrm{AsH}}_3{/\mathrm{H}}_2$ ambient stops after the step bunches reach a particular size. The surface has reached a stationary state with the ${\mathrm{AsH}}_3{/\mathrm{H}}_2$ ambient. In this paper, we report how the surface morphology of step bunches in the stationary state depends on the annealing conditions. The fact that step bunching always occurred when vicinal GaAs(001) substrates were annealed in ${\mathrm{AsH}}_3{/\mathrm{H}}_2$ ambient led us to conclude that ${\mathrm{AsH}}_3{/\mathrm{H}}_2$ is directly related to its cause. In order to understand the formation mechanism of this step bunching, we develop a microscopic theory that describes step dynamics during annealing. Based on this theory, we propose a formation mechanism that attributes the cause of step bunching to ${\mathrm{AsH}}_x$ attached to step edges. We assume that ${\mathrm{AsH}}_x$ attached to step edges induces irreversible detachment and incorporation processes for Ga atoms terminating step edges, generating a net upward mass transfer across step edges. This results in the formation of step bunches. By assuming a reasonable coverage of ${\mathrm{AsH}}_x$ at step edges the complicated dependence of the size of the stationary step bunches on annealing conditions can be explained.