TUNNELING MICROSCOPY AND SPECTROSCOPY ON CROSS-SECTIONS OF MOLECULAR-BEAM-EPITAXY-GROWN (AI)GAAS MULTILAYERS

We report scanning tunneling microscopy (STM) observations of topography and spectroscopy on molecular-beam-epitaxy (MBE)-grown (Al)GaAs multilayers. The experiments were done on ultrahigh vacuum (UHV)-cleaved, MBE-grown GaAs-AlGaAs multilayer structures, exposing the (110) plane which is cross sectional to the <001> growth direction. A UHV scanning electron microscopy (SEM) is used for easy guidance towards the epitaxial structure. The atomic registry is clearly observed in the epitaxial GaAs, in the AlGaAs layer as well as in their interfaces. The transition between these materials, as seen by STM, takes place over a region of 1-2 unit cells. The GaAs layers have a very homogeneous appearance, in contrast to the AlGaAs layers, which show a patchy texture in the atomic resolution images. This is attributed to an inhomogeneous distribution of the local density of states in the AlGaAs. Furthermore, the current-voltage characteristics in the AlGaAs indicate variations in the position of conduction band with respect to the valence band. These features are indicative of composition fluctuations on the nanometer scale, which are directly observed by our tunneling electron probe. We present results from current-voltage spectra, taken over a GaAs/AlGaAs interface: the transition in valence band position is observed over a length scale of six unit cells (3.5 nm). A marked distinction between the GaAs and the AlGaAs material is the chemisorbed coverage, which takes place on the Al compound at a high rate, even under the UHV condition of < 5 X 10(-11) mbar.