The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.
The reduction of threading dislocation density due to their mutual interactions in GaSh thin films grown on (001) GaAs substrates by molecular beam epitaxy has been investigated. The effectiveness of several buffer layer schemes including GaSb/AlSb strained layer superlattice and In0 12Ga0 89SbfGaAs buffers for threading dislocation suppression was evaluated. High-resolution transmission electron microscopy shows that the GaSb/GaAs interface consists of a highly periodic network of 90° pure edge misfit dislocations, with an average spacing close to that for a fully relaxed system. This results in relatively low threading densities in the GaSb epilayer, despite their large lattice constant mismatch (8.2%). The threading dislocation density as a function of GaSb film thickness was determined by plan-view transmission electron microscopy and was found to decrease with film thickness due to mutual interactions among dislocations. It was found that a strained layer superlattice of GaSb/A1Sb, with each layer close to its critical thickness, is the most effective in threading density reduction.
We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.
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