Experimental observation of six valleys and an anisotropic integer quantum Hall effect on H-Si(111) surfaces

We have recently developed a new high mobility 2DES on a clean and atomically flat hydrogen-passivated Si(111) surface, where electrons are gated through an encapsulated vacuum dielectric. Our devices have exhibited peak scattering times, $\tau \sim $5ps, which exceed even the best Si(100) MOSFETs. We will discuss magneto-transport measurements made as a function of tilted magnetic fields in such a 2DES. The Si(111) surface is unique in that it has been calculated to have six valleys of equal energy in its ground state. Measurements at T=150mK and n$_{s}$= 6.5 x 10$^{11}$cm$^{-2 }$show clear signatures of the integer quantum Hall effect where contrary to predictions filling factors less than 6 are observed (i.e. $\nu $=6, 5, 4, 3, and 2). In addition, we have observed anisotropy in R$_{xx}$ with respect to the crystal orientation in a magnetic field range of 0$\le $B$\le $12T, which is also unexpected for an equally occupied six-fold degenerate system. As a result, an application of a non-interacting model whereby two valleys have a greater population than the remaining four has general agreement with the Landau level crossings at specific tilt orientations and with the anisotropy of R$_{xx}$ at low fields. At high fields, the presence of $\nu $=3,4 and 5 indicates that individual valleys are splitting but what is more interesting is the anisotropy of R$_{xx}$ at $\nu $=3 {\&} 4 toward in-plane magnetic fields.