Mapping of GaAs wafers by quantitative infrared microscopy

Abstract

A near-infrared transmittance mapping system, previously described for quantitative macroscale (∼1 mm) mapping of the midgap EL2 defect in thin semi-insulating GaAs wafers, has been modified to provide such quantitative data on a finer scale (∼50 μm). This allows EL2 mapping on both macro- and microscales to be compared, quantitatively; and these also to be compared with dislocation micromaps and microphotographs. The well-known cellular structures for both EL2 and dislocations are resolved, and it is possible to measure the optically detectable neutral EL2 concentration inside cells (where dislocations are fewest) and in the cell walls (where they are most numerous). Comparisons of micromaps for various small parts of a wafer, and with a macromap showing the broad low-resolution variations, indicates that the local EL2 values inside cells and at cell walls vary across a wafer with the same spatial trend as given by a low-resolution macromap. This is consistent with the hypothesis that EL2 simply getters around any dislocation as a Cottrell atmosphere, as contrasted with expectations from an alternative model which postulates that EL2 (or the point defects from which EL2 is able to construct itself) will be produced by dislocation climb.