Spontaneous lateral composition modulation as a consequence of the deposition of a(AlAs)n/(InAs)mshort period superlattice on an InP(001) substrate is examined. Transmission electron microscopy images show distinct composition modulation appearing as vertical regions of In- and Al-rich materials alternating in the [1¯10] projection. The periodicity of the modulation is 130 Å, and is asymmetric. The transmission electron and x-ray diffraction patterns from the structure exhibit distinct satellite spots which correspond to the lateral periodicity. Transmission electron microscopy images show that the individual superlattice layers possess cusplike undulations, which directly correlate with the composition modulation. Composition modulation in this sample appears to be coupled to morphological and compositional instabilities at the surface due to strain. ©1997 American Institute of Physics.