Integer quantum Hall effect and valley splitting on a H-Si(111) surface

We have recently developed a high mobility two-dimensional electron system, where an electric field is applied through an encapsulated vacuum cavity and induces electrons on a clean and atomically flat hydrogen-passivated Si surface. Low temperature magneto-transport measurements (0$<$B$<$12T) made on such an inversion layer has led us to the first observation of the integer quantum Hall effect on a Si(111) surface. With a mobility of $\sim $10,000 cm$^{2}$/Vs at T=150 mK and n$_{s}$=6.75 x 10$^{11}$ cm$^{-2}$, we observed filling factors $\nu $=6, 4, 3, and 2. This is particularly interesting, because Si(111) is expected to have a six-fold valley degeneracy. Activation energy measurements within filling factors 6 and 2 coupled with analysis of Shubnikov-de Haas oscillations suggests that the six-fold valley degeneracy is broken asymmetrically into 2-fold and 4-fold at B=0. In addition, the observation of $\nu $=3 indicates that individual valleys are splitting at higher fields. We will discuss these results along with the possible origins of the breaking of the six-fold valley degeneracy at B=0.