Step Faceting at the (001) Surface of Boron Doped Silicon

We have used low energy electron microscopy to study the energetics of steps on the (001) surface of heavily boron doped Si. When the surface is cooled below 980 °C, the $S_B$ step restructures dramatically, assuming a sawtooth profile consisting of triangular facets. The apex angle of the facets is strongly temperature dependent. By quantitative analysis of thermal step fluctuations and 2D island shapes we show that the free energy of isolated $S_A$ steps decreases sharply as the temperature is lowered. Using the Wulff construction, we demonstrate that the observed faceting is a direct consequence of the vanishing $S_A$ step free energy.