Structural transition at the subsurface of few-layer Bi(110) film during the growth

Abstract

The structure and growth behavior of Bi(110) ultrathin films on Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ substrate are studied using real-time x-ray crystal truncation rod scattering measurements. The film grows in units of bilayers up to four atomic layers and shows a quasi--layer-by-layer growth above four layers in which even-layer-height domains are preferentially formed. It is revealed that the even-layer-height domains fully consist of black-phosphorus-like paired layers while the odd-layer-height domains have an additional bulk-Bi(110)-like layer beneath the top bilayer. This suggests that a structure change occurs at the subsurface when an additional single layer grows on the even or odd domains. Monte Carlo simulations indicate that a smaller free energy of the even structure than the odd structure leads to the preferential formation and that a small interface energy at the even/odd domain boundary allows the frequent transitions during the growth.