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.

DSL Photoetching: Principles and Application to Study Nature of Defects in III-V Materials

“…The main difference between this model and the opinion commonly described in literature on the origin of Cottrell atmospheres of dislocation cell walls is then the sequence of events; it is assumed that gettering of As (and related defects e.g. EL2) on dislocations occurs during post-growth cooling [29][30][31][32], while in this model a local agglomeration of native defects constitutes the primary structural feature. An additional implication of the present model is that in the vicinity of dislocations a narrow depleted area of EL2 defects might be expected.…”

Infra-red imaging and EL2

“…Stirland [62] suggested that the microenvironnement of the dislocations would be richer in EL2 centres than the inner part of the cell. This point of view was largely accepted [37,44,63,64,65] in spite of arguments Fig. 6.…”

“…The exact nature of EL2 is not definitely established and the possible role of the anti site As' is still a matter of debate. The exact role of grown-in or strain induced dislocations in the generation or in the gettering process has also to be explained [3, 7,8,36,37,35]. These points will be emphazised through the analysis of transmission or tomography or luminescence images.…”