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Silicon-Germanium Alloys For Infrared Detection

We have prepared Si -Ge alloys in the range of 10 at.% Ge by Czochralski growth. The slow growth rates necessary to avoid cellular growth required long growth times, which permitted uptake of doping impurities from the quartz crucible. The highest resistivity obtained was 1,700 Q -cm. Even though our alloys had 1.06 }1m absorption coefficients near 50 cm-1 (in contrast to 14 cm-1 for Si), they were unsuitable for p -i -n detectors because their resistivities precluded achieving large enough depletion regions. We prepared some alloy crystals by a crucible -free process (which would lead to high resistivity) similar to float zoning. The maximum Ge content achieved was about 2.5 at. %. Even though this value could be increased by equipment modification and process development, a fundamental drawback of the method is an axial variation of the Ge content, since the Ge initially present in the zone is depleted. We also made In -doped Si -Ge alloys by the Czochralski technique. Variations in the energy level of the In acceptors with Ge content were observed, as described in a companion paper, providing a way to adjust the spectral response of an extrinsic detector.