The meandering of steps on GaAs(100)

The zincblende structure of GaAs gives rise to two types of steps on a GaAs(100) surface. In a model using abrupt bulk-termination, these steps would be labelled A or B, depending upon whether they are As or Ga terminated, and in the [011] or [011] directions, respectively. Since these steps are structurally and chemically different, one expects them to have different roles in epitaxy. We have used reflection high-energy electron diffraction (RHEED) to study the morphology of each of these two types of steps on surfaces prepared by molecular beam epitaxy (MBE). The results show large differences in the qualitative and quantitative structural disorder of these two step configurations. During static conditions of no growth, but with an As₄ flux provided, the A-steps were straight over long distances. However the terraces between them exhibit large fluctuations in length. Conversely, the B-steps are highly kinked, but are seperated by terraces which show little length variation. For the A-steps, the terrace length fluctuations are greatly reduced by changing the reconstruction from the 2×4 or 1×1 or by initiating growth. During growth, the difference between RHEED intensity oscillations on the two surfaces is striking. For identically prepared and simultaneously mounted wafers that had been oriented 2° from the (100), the A-surface showed intensity oscillations over a range in temperatures. By contrast, the B-surface exhibited only faint oscillations that were rapidly damped. In addition, during growth of GaAs on stepped Si(100) surfaces, growth on surfaces prepared to yield A-steps gave much smoother morphologies than on surfaces prepared to yield B-steps.