Atom-Scale Mechanisms for Unstable Growth on Patterned GaAs(001)*

Molecular beam epitaxy on patterned GaAs(001) under standard conditions of temperature ($\sim $600\r{ }C), rate ($\sim $ 0.3 nm/s) and flux ratio (As$_{2}$/Ga$\sim $10:1) leads to a transient instability toward perturbation of the flat surface [1]. Lowering the temperature through approximately 540$^{o}$C, roughly coincident with the preroughening temperature changes the mode of this instability [2]; however, as we show in this talk, observations of the As$_{2}$ flux dependence rule out both preroughening and a reconstructive phase transition as driving the growth mode change. Instead, we find evidence that the change in unstable growth mode can be explained by a competition between decreased adatom collection rate on small terraces and a small anisotropic multi-step Ehrlich-Schwoebel barrier. We relate these effects to the up-down symmetry breaking term which commonly appears in continuum equations for growth. [1] H.-C. Kan, S. Shah, T. Tadayyon-Eslami and R.J. Phaneuf, Phys. Rev. Lett., \textbf{92}, 146101 (2004). [2] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, Phys. Rev. Lett., \textbf{97}, 126101 (2006).