Strain relief of metastable GeSi layers on Si(100)

Abstract

Highly metastable pseudomorphic Ge0.3Si0.7 layers 570 nm thick were grown on Si(100) at ∼300 °C by molecular-beam epitaxy. The relief of strain in such metastable layers upon ex situ thermal annealing in vacuum is investigated by double-crystal x-ray diffractometry and MeV 4He channeling spectrometry. Upon isochronal annealing of 30 min, the strain relieves sharply at (375±25) °C, and reaches the thermal equilibrium value above 400 °C. Under isothermal annealing between 300 and 400 °C, the time evolution of the strain relief has the characteristics of a nucleation and growth transformation. The strain relief is very slow initially, increases approximately linearly as the strain is partially relieved, and saturates upon approaching equilibrium strain state. Two important results are drawn from the experimental data. First, a deformation-mechanism map is constructed from which the strain relief rate of a metastable GeSi/Si can be extrapolated for given stress state and temperature. Second, the rate of the strain relief when the strain is partially relieved increases with rising temperature, and follows an Arrhenius behavior as a function of the inverse temperature with a slope of 2.1±0.2 eV. This value coincides with the activation energy for dislocation glide in Ge0.3Si0.7. Furthermore, the strain-relief equation of a plastic flow model is solved and fits well the experimental strain-time dependence. One of the two fitting parameters, the time constant, has an Arrhenius temperature dependence. The slope, 1.9±0.2 eV, is assumed to be the activation energy for dislocation motion, and agrees with the previous value extracted from the simple rate-temperature dependence. In addition, as the strain is relieved, the x-ray-diffraction peak from the layer broadens and the channeling yield increases, confirming that the generation of misfit dislocations associated with the strain relief is accompanied by the generation of threading dislocations in the layer.