Atomic scale images of acceptors in III-V semiconductors

Abstract

This thesis reports measurements of single dopant atoms in III-V semiconduc-tors with low temperature Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS). It investigates the anisotropic spatial distribution of acceptor induced tunneling processes at the {110} cleavage planes. Two dif-ferent tunneling processes are identified: conventional imaging of the squared acceptor wave function and resonant tunneling at the charged acceptor. A thor-ough analysis of the tip induced space charge layers identifies characteristic bias windows for each tunnel process. The symmetry of the host crystal s band struc-ture determines the spatial distribution of the tunneling paths for both processes. Symmetry reducing effects at the surface are responsible for a pronounced asymmetry of the acceptor contrasts along the principal [001] axis. Uniaxial strain fields due to surface relaxation and spin orbit interaction of the tip induced elec-tric field are discussed on the basis of band structure calculations. High-resolution STS studies of acceptor atoms in an operating p-i-n diode confirm that an electric field indeed changes the acceptor contrasts. In conclusion, the aniso-tropic contrasts of acceptors are created by the host crystal s band structure and concomitant symmetry reduction effects at the surface.