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An X-ray multiple diffraction technique has been used to investigate the effect on the lattice of single-crystal germanium of the introduction of arsenic at a concentration of approximately 1020 atoms cm-3. Two samples, of similar resistivity, have been examined: one in which arsenic was introduced via the melt during Czochralski growth, and one into which arsenic was diffused after growth. X-ray fluorescence spectroscopy revealed that the latter had an arsenic concentration two or three times that of the `alloy'. The lattice parameter at 25 °C of the Ge-As alloy was found to be 5.65795 Å, that of the As-diffused Ge, 5.65820 Å, compared with 5.65750 Å as determined for pure Ge, the experimental errors being ±0.00003 Å. The As-diffused sample was also found to exhibit a considerably higher degree of inhomogeneous strain than the alloy. A brief discussion is given of the possible correlations between arsenic concentration and the observed expansion and strain content of the germanium lattice.