High Mobility Sixfold Valley Degenerate Electrons on Silicon [111] Surfaces

The 111 surface of silicon is predicted to retain the sixfold valley degeneracy of the ideal bulk crystal. We have developed a method for fabricating field effect transistors using vacuum as a dielectric in order to study electron transport on the bare hydrogen-terminated surface, free from the complications created by intrinsic disorder at Si-Si$O_2$ interfaces. The resulting devices display very high mobilities (up to 110,000 $cm^2/Vs$ at 70mK, more than twice as large as the best silicon MOSFETs), enabling us to probe valley-dependent transport dynamics to a much greater degree than previously possible. Measurements made on a recent device over a density range of $n_s=0.7-7\times10^{11}/cm^2$ reveal considerable information about the nature of this degeneracy and its role in 2D transport. In particular, we find (at $n_s$=6.7) that 1) low field Shubnikov-de Haas oscillations reveal a clearly sixfold degenerate system and allow us to establish an upper bound on the valley splitting of 0.2K 2) longitudinal resistivity at B=0 displays a strong temperature dependence, consistent with predictions that large valley degeneracy should enhance screening[1] and 3) the Hall coefficient near B=0 is modified by the presence of multiple valleys, and we can use this correction to measure the intervalley Coulomb drag and its temperature dependence. [1] E. H. Hwang and S. Das Sarma. PRB 75, 073301 (2007)