Surface Processes in the Growth of Silicon on (111) Silicon in Ultrahigh Vacuum

A replication technique for electron microscopy has been developed which is capable of detecting 3 Å steps on surfaces. This technique was used to observe the growth of silicon on a (111) silicon surface in ultrahigh vacuum. On a substrate surface that is ``clean'' by current standards of silicon surface study, three‐dimensional growth centers developed. However, on a cleaner surface consisting of a continuous layer of freshly deposited silicon, growth proceeded by a step‐motion mechanism. By the use of a simple model to calculate the critical supersaturation ratio and the equilibrium adatom concentration, and from the observed absence of nucleation on atomically flat terraces between steps, a surface self‐diffusion coefficient of 10⁻³ cm³/sec at 800°C was estimated. Straight steps which propagate in the $\text{〈1̄1̄2〉}$ direction were observed, in contradiction of the simple bond model of step edges which predicts straight steps that propagate in the$\text{〈112̄〉}$ direction. Step arrays and step separation were observed and interpreted by the recent theory of adatom capture at steps by Schwoebel and Shipsey.