Insulating Cs overlayer on InSb(110)

Cesium overlayers on room-temperature InSb(110) have been studied with scanning tunneling microscopy and spectroscopy. A two-dimensional (2D) overlayer is obesrved, consisting of four-atom Cs(110)-like planar clusters arranged in a c(2×{}6) superlattice. Interestingly, current-versus-voltage (I-V) spectra exhibit a band gap of {}0.6 eV, larger than the substrate band gap of {}0.15 eV. The I-V spectra are very similar to those observed on the similar 2D overlayer on GaAs(110), suggesting that the measured gap is a property of the 2D Cs film. The possible origins of this insulating behavior are discussed.