Thermal relaxation kinetics of strained Si/Si1−xGex heterostructures determined by direct measurement of mosaicity and lattice parameter variations

Abstract

The thermal stability of strained Si1−xGex/Si heterostructures grown by molecular-beam epitaxy was investigated. Alloy structures with Ge fractions between 5% and 23% and thicknesses from 160 to 200 nm, corresponding to excess stress from 112 to 712 MPa, were grown at 450 °C. X-ray high-resolution reciprocal lattice mapping (HRRLM) was employed as the major analysis tool to investigate the status of the strain and the material quality of the structures prior to and after high-temperature furnace annealing between 700 and 1000 °C. By this technique, the variations of the alloy lattice parameters parallel and perpendicular to the growth direction were independently determined as functions of the annealing temperature (Ta) within an accuracy ≊10−5. In addition, the annealing-induced variations in the shape of the x-ray scattering distributions were used to investigate the material quality and the changes in the mosaic character of the structures. For 700≤Ta≤800 °C, a strong increase in the strain relaxation with an activation energy of (2.3±0.2) eV was observed. The relaxation dependence on the excess stress was determined to follow a power of (1.9±0.2) law. In addition, strong broadening of the diffraction peaks was observed in the direction orthogonal to the scattering vector, demonstrating that the formation of strain-relieving dislocations introduces significant mosaicity in the structures. At Ta≳800 °C, the levels of strain relaxation and mosaicity were found to be high, but the rate of increase was clearly reduced. The contribution to this effect owing to the dislocation–dislocation interaction was evaluated and was shown to be an important mechanism in limiting the maximum attainable relaxation in the structures. Effects of Ge diffusion were estimated and were shown to be relevant at Ta=1000 °C. The relaxation was significantly higher for structures with a higher Ge fraction but, in a structure with 5.67% Ge, a low-intensity diffuse scattering probably related to local strain fields from the initial stages of relaxation and dislocation formation, was observed. The use of HRRLM in the characterization of this important heterostructure system appeared to be appropriate not only for providing an accurate determination of lattice parameter variations as a function of Ta but, in addition, for differentiating between the effects of strain character and those of defect-related contributions to Bragg reflections.