Experimental requirements for quantitative mapping of midgap flaw concentration in semi-insulating GaAs wafers by measurement of near-infrared transmittance

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Abstract
The experimental properties are critiqued that relate the midgap flaw concentration in semi‐insulating GaAs, and the resulting near‐infrared transmittance of a polished wafer. Since quantitative information of such flaw concentrations is desirable even for an optical thickness αt≪1, a highly stable and accurate experimental arrangement is described, which permits a meaningful evaluation even when αt≂0.01. (The transmittance is then almost Tmax, as set by the substantial reflectance losses.) This system permits mapping over a wafer’s area, by translation of the wafer with respect to the optical path. Calibration of absorption into flaw concentration is discussed for the midgap EL2 donor defect, and (in an appendix) for chromium‐doped GaAs. Representative wafer maps for EL2 are used as illustrations, some as mosaic grey‐scale matrix plots, and others as pseudo‐three‐dimensional contour plots.