The influence of stress on growth instabilities on Si substrates

The growth th instabilities that develop during growth on Si substrates lea d to a sinusoidal-like morphology. In this paper we investigate the role of the two main parameters that influence the development of surface undulati ons: the surface. atomic configuration of the substrate and the external st ress applied to the growing film. We characterize the amplitude and the cor relation length of the surface profiles by reflectivity measurements, high resolution electron microscopy (HREM) and atomic force microscopy (AFM). Co ncerning the role of the atomic configuration, we performed a series of exp eriments on various substrate misorientations (from Si(111) to high miscut angles). We show that a critical step density is necessary for the nucleati on of the instability. Indeed, we find that both Si and Si1-xGex deposits p resent a perfect 2D surface when grown on singular Si(111). In contrast, in the same experimental conditions, instabilities develop on vicinal substra tes from a misorientation of 2 degrees and amplify with the miscut angle up to 10 degrees off. Concerning the effect of stress, we find that the biaxi al compressive stress applied to the growing film during Si1-xGex heteroepi taxy dramatically enhances the instability development. Indeed, if we compa re the growth modes of Si and Si1-xGex (x = 0.3) on 10 degrees off Si(111) we find that a 10 nm thick Si0.7Gr(0.3) layer (similar to 1.2% misfit) disp lays 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 t he instability could be attributed to a change in the nature of the interac tions between steps at a critical step density, due to local stresses at th e step edges. The evolution of the phenomenon is then kinetically controlle d by various kinetic factors (growth temperature, local flux variations, do ping level, presence of H...). Ultimately, the undulatory morphology which is a metastable state kinetically evolves towards a faceted equilibrium sha pe. (C) 1998 Elsevier Science S.A, All rights reserved.