Step-height-tripling transition on vicinal Si(111)

We have used low-energy electron diffraction to characterize the temperature dependence of the structure of vicinal Si(111) surfaces misoriented by 6^∘{} and 12^∘{} toward the [21{} 1{}] azimuthal direction. At high temperatures these surfaces contain a uniform density of steps of height close to the (111) double-layer spacing. As the temperature is lowered the step structure changes abruptly, and reversibly, at approximately 860 ^∘{}C for both angles of misorientation. The changes in step structure occur simultaneously with the appearance of diffracted beams characteristic of the (7×{}7) reconstruction of the Si(111) surface. The step structure at low temperatures has triple the period of that of the high-temperature surface. The diffraction features, however, are inconsistent with a simple array of steps of height equal to three times the double-layer spacing, as we show by considering a simple diffraction model. The effect of the reconstructive transition on these [21{} 1{}]-misoriented Si(111) surfaces is contrasted with previous observations on [2{} 11]- and [11{}0]-misoriented surfaces, where surface faceting occurs simultaneously with the appearance of the (7×{}7) reconstruction at a temperature which decreases with increasing angle of misorientation.