Mössbauer effect of Ge73 in laser-processed Si and Ge crystals

The first observation of the 13.3-keV Mössbauer resonance of Ge73 nuclei located at substitutional sites in a single-crystal Si host lattice is reported. Mössbauer sources are made using laser melting to incorporate volatile As73 radioactive parent nuclei into (111) crystals of Si or Ge. Laser-induced diffusion (LID) of As into Si is found to be an order of magnitude more efficient than conventional oven diffusion. Possible lattice damage resulting from the laser processing is studied by Ge73 Mössbauer effect, Rutherford backscattering, and transmission electron microscopy (TEM). With the LID As73:Si source the Mössbauer resonance is broadened to 118 m/sec full width at half maximum, of which 106 m/sec is attributable to the laser processing of Si. Mössbauer evidence indicates that this broadening is due not to a distribution of isomer shifts, but to an electric field gradient (EFG) induced at most Ge daughter nuclei by bulk strain. TEM measurements indeed reveal a high density of dislocations. Bond-orbital calculations are reported of the EFG induced at substitutional Ge73 nuclei by deforming the Si host lattice in uniform tension in the plane of the (111) surface. The strains required are as large as the average bulk strain arising in the entire lattice contraction undergone by the Si as it cools from 1412°C to 20°C following pulsed laser melting, ll=0.56%. Even with this maximal assumed strain the calculated broadening is insufficient to fully account for the observed effect if the unknown Ge73 13.3-keV nuclear quadrupole moment Qe is near zero. Assuming maximal strain, the As73:Si data allow us to estimate the value Qe=-0.4(3) b for this moment. In addition, with simple scaling we are able to account for the EFG observed in two other independent Ge73 Mössbauer experiments involving stressed lattices. In one of these the strain deduced from Mössbauer broadening was directly confirmed by x-ray diffraction.