Atomic-resolution study of steps and ridges on arsine-exposed vicinal Ge(100)

Scanning tunneling microscope images of steps on AsH₃-exposed vicinal Ge(100) show very clearly that As/Ge steps reconstruct. A variety of As/Ge structures based on a ``double-row'' step reconstruction has been observed. We present and discuss some examples, and propose a structural model for a representative two-layer double-row As/Ge step. Under some conditions, we find that atomic-scale ridges are formed at the edges of terraces. These ridges are formed when arsine etching preferentially removes terrace atoms, leaving the steps intact. The resulting ridges are therefore simply the remnants of etch-resistant steps. An atomic-scale model for this process is proposed, and some consequences of ridge formation are discussed.