On the Origin of Certain Spirals of Large Step-Height Observed on Crystal Surfaces

Periodic bunching together of advancing or retreating atomic sheets frequently occurs on crystal surfaces inclined at small angles with densely packed atomic planes, and it causes microscopically observable steps to appear. Around a domed surface, in the case of growth, or around an etch pit, in the case of solution or evaporation, such steps usually form concentric rings. It is shown, however, that a simple ``mistake'' in the early stages of step formation will produce, instead of rings, a continuous spiral step. No screw dislocations, therefore, are required for the production of this spiral step.