Modeling step bunching formed on vicinal GaAs(001) annealed in ${\mathrm{AsH}}_3$ and hydrogen ambient

A detailed analysis of the surface morphology of step bunching formed by annealing in ${\mathrm{AsH}}_3$ and hydrogen has been carried out on substrates with various miscut directions. The results show that branches, by which we refer to step bunchings that do not run in the substrate miscut direction, form on most of the substrates and for most of the annealing conditions, suggesting that they have a relatively low free energy and compose an intrinsic component of this step bunching. Also these branches are not randomly located on the surface but have a strong tendency to align side by side. In order to understand these experimental results, we propose a qualitative model designated as the ``chain reaction model.'' This model is founded on the fact that during annealing steps can only move by exchanging step-edge atoms with other steps. By this model we can explain the main characteristics of this step bunching: why the step bunching does not grow infinitely in size and why branches have a strong tendency to align side by side.