Bias-voltage dependent inversion of contrast between GaAs and AlGaAs in cross-sectional scanning tunneling microscopy of heterostructures

Bias-voltage dependent STM-images of an n-type GaAs/AlGaAs superlattice are presented. It is observed that the contrast between the GaAs- and AlGaAs-layers in the STM image, inverts at small values of the bias voltage. This effect occurs for both empty- and filled-states images, although the effect is stronger for the filled-states imaging mode. Furthermore it is observed in the empty-states imaging mode, that the GaAs-layers of the superlattice appear broadened at a certain voltage-interval. This broadening effect is attributed to the fact that at this voltage interval, we image the confined electron-states of superlattice.