We have measured the interband optical properties of grain boundaries by a spatial-modulation technique which allows direct observation of the differences between the regular lattice and the grain boundary. These results are obtained by periodic motion of the grain boundary in a bicrystal in and out of a narrow light beam and phase-sensitive detection of the difference signal. The grain boundary is found to have an absorption edge which is broadened by the electric fields within the boundary as well as an exponential distribution of states below the direct edge. The results were independent of the tilt axis of the grain boundary (from 4° to 25°) and the data are found to be very similar to those observed for amorphous films of germanium. A 6° twist boundary gave identical results. While this paper is concerned with the properties of a planar defect, we believe the method will also prove useful for the study of point defects in crystals.

• Received 29 July 1968

DOI:https://doi.org/10.1103/PhysRev.178.1328