A spectroscopic investigation of nominally undoped semi-insulating InP prepared by high-temperature annealing

Optical spectroscopy reveals the incorporation of iron in InP wafers during high-temperature annealing. Depending on the calibration factor used, an Fe/sup 2+/ concentration of (1-2)*10/sup 15/ cm/sup -3/ is derived, which is at least a factor of two smaller than the net shallow donor concentration in the as-grown material. This discrepancy is definitely out of range of the experimental uncertainty. Moreover, the calibration of the Fe/sup 2+/ intracenter absorption is always based on the assumption of 100% electrical activity of the chemically detected iron content in n-type InP:Fe reference samples and, therefore, should be considered to yield an upper limit for the true concentration. Furthermore, all attempts to detect iron in the Fe/sup 3+/ charge state in any of the samples failed. Thus, it is concluded that other mechanisms must be considered to explain the high-resistivity properties of annealed InP. A new electronic transition at 2874.5 cm/sup -1/ observed in the low-temperature absorption spectra of annealed samples is tentatively assigned to a complex Fe-shallow donor pair defect.<>