Abstract
Scanning tunneling microscopy and spectroscopy is used to study GaAs multiple pn junction samples cleaved in ultrahigh vacuum. Direct topographic contrast over the pn junctions can be observed in the constant current imaging mode. The topographic height in the p-type regions appears much lower (by about 5 Å) than that in the n-type regions. Tunneling spectroscopy measurements show consistency with the assignment of the p- and n-type regions. We discuss a possible mechanism for the observed contrast.