Conductance gap anomaly in scanning tunneling spectra of MBE-Grown (0 0 1) surfaces of III–V compound semiconductors

A scanning-tunneling-spectroscopy (STS) study was performed on MBE-grown (0 0 1) surfaces of GaAs, Al0.3Ga0.7As and In0.53Ga0.47As in an ultrahigh vacuum (UHV) STS system to clarify microscopic behavior of surface states causing Fermi level pinning on these III–V compound semiconductor surfaces. On all the sample surfaces, there existed spots which showed anomalous STS spectra showing conductance gaps much larger than the energy gap of the material. The rates of finding such spots as well as the magnitudes of the anomalous conductance gap were strongly material-dependent, increasing in the order of InGaAs, GaAs and AlGaAs. Scanning tunneling microscope (STM) images under low-positive sample biases showed dark areas which gradually decreased with the increase of the positive sample bias, and correlated with the spatial variation of conductance gaps of the STS spectra. On the basis of a detailed computer simulation, the conductance gap anomaly is explained by a tip-induced local charging of surface states where the apparent gap width depends on surface state distribution shape and density. The result shows that an extremely high density of surface states exist on the AlGaAs surface, but not so much on the InGaAs surface with the GaAs surface in between.