In situ-type infrared absorption measurements are carried out on bulk Si 1−x Ge x after implantation with protons or deuterons at temperatures below 20K. The infrared absorbance spectra recorded at 8K after implanting the samples with protons reveal two intense and well-resolved lines located at around 2000cm −1 . Spectra recorded at 8K after each annealing step in an isochronal annealing sequence show that the lines disappear when the samples are heated to about 175K. Such annealing behavior is very similar to that of the 1998cm −1 line observed in pure Si implanted with protons under similar conditions. The dependence of the frequency of lines on the composition of the alloy and their annealing behavior demonstrates that they are produced by the Si–H stretch modes of H BC + located between Si neighbors in two different next-neighbor configurations, where the higher frequency mode corresponds to a Si–H–Si site locally strained by a Ge atom bond to one of the Si atoms. Based on the experimental data, the formation and stability of the two defects are discussed.