  Low-temperature in-plane magnetotransport measurements have been performed on
adsorbate-induced electron systems formed at in-situ cleaved surfaces of p-type
InAs. The Ag-coverage dependence of the surface electron density strongly
supports a simple model based on a surface donor level lying above the
conduction band minimum. The observations of the quantized Hall resistance and
zero longitudinal resistivity demonstrate the perfect two-dimensionality of the
surface electron system. We also observed the Rashba effect due to the strong
asymmetry of the confining potential well.
