Strain relaxation mechanisms in Si_1−xGe_x layers grown by solid-phase epitaxy: Influence of the layer composition and growth temperature

Abstract

The influence of the composition and growth temperature on the strain and defect structure of Si_1−xGe_x layers of 0.21≤x≤0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The strain in the layers was measured by Raman spectroscopy and Rutherford backscattering spectrometry/channeling angular scans. The defects were analyzed using high resolution electron microscopy. Three different relaxation mechanisms have been identified and characterized. The first mechanism occurs at the layer-substrate interface of the samples by the introduction of isolated defects. It is found to be thermally activated with an activation energy of E_a=0.16 eV and a prefactor that depends on the Gecontent of the layer. This mechanism produces partial relaxation of the layers and hinders the growth of fully strained layers. The second relaxation mechanism emerges at a distance from the interface which depends on the stress in the crystallized portion of the layer. In this case, the strain relaxation is caused by stacking faults that nucleate when they are energetically feasible and propagate toward the surface of the sample during growth. At low growth temperatures, the defects are confined to the upper part of the epitaxial layers at a distance from the interface that agrees with the theoretical predictions based on the equilibrium critical layer thickness. The third relaxation mechanism is introduced at high growth temperature and is based on the gliding of the stacking faults toward the layer-substrate interface. As a result of this mechanism, the stress in the layers is reduced compared to the stress in the layers grown at lower temperatures and approaches the equilibrium value corresponding to the total layer thickness. This behavior indicates that the layers grown at low temperature, where the stacking faults are confined to the upper part, are to some extent metastable.