Structure Analysis of Sn Bilayer Films on Si (111)

Chemical doping is a well-established method for controlling the electronic properties of bulk semiconductors and, e.g, complex oxide materials. In this process, dopant atoms are located at substitutional lattice locations, from where they introduce free charge carriers to the host material. These carriers greatly improve the electrical conductivity of the host material and can even induce an insulator-metal transition at high doping levels. Dopants, however, also introduce scattering centers that are detrimental to conductivity, especially in low-dimensional systems such as nanowires and ultrathin films. These problems can be overcome by using a modulation doping approach in which the dopant atoms are spatially separated from the conducting layers via heterostructure engineering. In a recent study, Mulugeta and coworkers were able to induce a symmetry-breaking metal insulator transition in an atomic bilayer of Sn on Si(111), by substituting Si atoms in the substrate with boron. However, the structure of the Sn layer is still under dispute and it is not clear where precisely the boron atoms are located and whether the Sn bilayer structure is significantly altered by the boron. The resolve this issue, we performed structure studies on both doped and undoped Sn bilayers, using low-energy electron diffraction (LEED). The intensities of the diffracted beams were recorded as a function of beam voltage and the resulting I(V) curves served as input for the structural refinement with a dynamical diffraction structure code. Various structure models were tested by our collaborators at Penn State University. The structure model proposed by Ichikawa (REF) produces the best fit to the I(V) spectra, although further refinement remains necessary. Comparison of the I(V) spectra of the doped and undoped Sn layers strongly suggests that the boron atoms are indeed located below the Sn layer. Furthermore, it can be stated with great certainty that the doped and undoped Sn layers have identical structures. Hence, the metal-insulator transition observed by Mulugeta et al. can indeed be attributed to carrier doping.