Scanning tunneling microscope. images of. lattice-matched InGaAs/InP structures were investigated using autocorrelation analysis. Correlation lengths and correlation amplitudes were calculated from constant-current empty-state images. Theoretical STM images were calculated from a model which only considered surface displacements due to strain relaxation. By comparing model and experimental correlation lengths and amplitudes it is concluded that contrast variations. in constant-current images are dominated by strain relaxation effects. Changes in probe tip geometry and. applications of this technique to study clustering in III-V alloys are also discussed. (C) 2003 American Vacuum Society.