The influence of stress on growth instabilities on Si substrates

Abstract

The growth instabilities that develop during growth on Si substrates lead to a sinusoidal-like morphology. In this paper we investigate the role of the two main parameters that influence the development of surface undulations: the surface atomic configuration of the substrate and the external stress applied to the growing film. We characterize the amplitude and the correlation length of the surface profiles by reflectivity measurements, high resolution electron microscopy (HREM) and atomic force microscopy (AFM). Concerning the role of the atomic configuration, we performed a series of experiments on various substrate misorientations (from Si(111) to high miscut angles). We show that a critical step density is necessary for the nucleation of the instability. Indeed, we find that both Si and Si 1− x Ge x deposits present a perfect 2D surface when grown on singular Si(111). In contrast, in the same experimental conditions, instabilities develop on vicinal substrates from a misorientation of 2° and amplify with the miscut angle up to 10° off. Concerning the effect of stress, we find that the biaxial compressive stress applied to the growing film during Si 1− x Ge x heteroepitaxy dramatically enhances the instability development. Indeed, if we compare the growth modes of Si and Si 1− x Ge x ( x=0.3) on 10° off Si(111) we find that a 10 nm thick Si 0.7Ge 0.3 layer (∼1.2% misfit) displays an undulation comparable to that obtained for a 500 nm thick Si film. HREM analysis shows that the undulation consists of a series of low energy facets created by a step bunching mechanism. We suggest that the onset of the instability could be attributed to a change in the nature of the interactions between steps at a critical step density, due to local stresses at the step edges. The evolution of the phenomenon is then kinetically controlled by various kinetic factors (growth temperature, local flux variations, doping level, presence of H…). Ultimately, the undulatory morphology which is a metastable state kinetically evolves towards a faceted equilibrium shape.