Nanopatterning as a Probe of Unstable Growth on GaAs(001)*

We report on observations of unstable growth on nanopatterned GaAs(001) surfaces. For growth at 500$^{o}$C, 1 ML/sec and an As$_{2}$/Ga beam equivalent pressure ratio of 10:1, we find that grooves oriented at right angles to [110] produce a build up of ridges of GaAs at the upper edges, while for grooves oriented at right angles to [1\underline {1}0] no ridges form; instead cusps evolve at the bottoms of such grooves [1]. The cusp-forming grooves show a pronounced initial amplification of depth during growth which changes with length/width ratio, and become more narrow. The ridge-forming grooves instead broaden during growth. We compare these experimental observations with kinetic Monte Carlo simulations in which a small anisotropic Ehrlich-Schwoebel barrier is included. [1] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, \textit{Phys. Rev. Lett. }\textbf{97}, 126101 (2006)