Scanning tunneling microscopy of the filled and empty arsenic states on the GaAs(001)-(2 × 4) surface

Scanning tunneling microscopy and current spectroscopy have been used to investigate the p-type arsenic-terminated GaAs(001)-(2 × 4) surface. Images were collected at both positive and negative voltages applied to the sample with respect to the tip. The main features in the images taken at positive sample bias do not differ from those taken at negative bias. This is in contrast to the scanning tunneling microscopy results on the GaAs(110) surface, where, at negative and positive sample bias, the As and Ga atomic orbitals are imaged, respectively, and are, thus, complementary to each other. The different behavior observed on the GaAs(001) surface is attributed to the fact that, on this surface, the Ga atoms are in the second layer. Hence, at positive sample bias tunneling occurs predominantly into the empty As molecular orbital. This conjecture is supported by a calculation of the tunneling current spectrum that contains the contributions of tunneling into the valence band as well as into the conduction band.