Valley-degenerate two-dimensional electrons in the lowest Landau level

We report low temperature magnetotransport measurements on a high mobility (mu = 325 000 cm(2)/Vs) two-dimensional electron system on a H-terminated Si(111) surface. We observe the integral quantum Hall effect at all filling factors v <= 6 and find that v = 2 develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding v = 3/2, consistent with twofold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.