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The effect of strain on the exciton spectrum of germanium Glass, Alastair Malcolm


The intensity of optical absorption, near the absorption edge in germanium, is examined as a function of strain applied to the lattice at 90 °K. The results are interpreted in terms of the change of the band structure of the lattice with applied strain. The absorption edge in unstrained germanium shows a single sharp peak due to exciton formation, whereas the edge in the strained specimens shows two exciton peaks. The peak positions vary linearly with both compressional and tensional strain up to the maximum strains applied (0.1% deformation). The exciton binding energies in the strained germanium lattice, calculated within the effective mass approximation, are approximately equal and independent of the magnitude of the strain. The separation of the peaks gives for the shear deformation potentials, the values |b| = (2.7 ± 0.3)eV/unit <100> shear and |d| = (4.7 ± 0.5)eV/unit <111> shear. The positions of the peaks give the shift of the energy gap as - (10.3 ± 1) eV/unit dilatation. Lattice imperfections are found to have no effect on the interpretation of the results. The broad absorption edge observed in evaporated films of germanium is accounted for in terms of internal strains.

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