Tunable Ordering of 2D Tin on Silicon

The optical and electronic properties of binary alloys can depend on atomic scale, site-to-site correlations. This is clearly illustrated in group IV alloys of tin, silicon, and germanium, where a direct bandgap is theoretically predicted in the random alloy, but an indirect bandgap is experimentally observed and thought to result from short range order (SRO). The balance between the deposition rates, temperature, and templating by the substrate influences the atomic-scale organization within thin films in intertwined ways, which is doubly challenging due to the atomic-scale characterization required for visualization. Here, we combine the ability to deposit materials with sub-monolayer coverage, apply heat in a separate controlled step, and <i>in situ</i> atomic-scale imaging of each discrete step with scanning tunneling microscopy (STM), to visualize the growth of Sn:Si.<br/>The fundamental goal of our work is to systematically characterize SRO, to better understand its impact of electronic properties of a material. Existing STM work on Sn monolayers deposited on Si and Ge, in both 100 and 111 orientations, has shown the nucleation of different long-range surface reconstructions. How this influences growth is an open question, as Sn also surface segregates at moderate growth temperatures. By analyzing growth in discrete steps, comprised of Sn deposition, annealing, followed by Si deposition, and further annealing, using STM <i>in situ</i>, the dynamic nature of the thin film growth at the substrate interface can be better understood. This talk will focus on analyzing the STM data of different growth and heating combinations to reveal the forces driving ordering and surface segregation. This step-by-step mode of growth enables the creation of metastable configurations that manipulate the degree of order itself, which, in the future, can help in better understanding the relationship of order to optical and electronic properties.<br/><br/>SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525