Ion assisted MBE growth of SiGe nanostructures

The bombardment of thin SiGe buffers with 1-keV Si+ ions during molecular b eam epitaxial growth is a possible way for the injection of point defects i n order to promote the relaxation and to reduce the dislocation density. Fo r this purpose, the e-beam evaporator was optimized by increasing the emiss ion current and decreasing the energy of the impinging electrons to create a high density Si+ ion flux in our MBE system. To the isolated substrate ho lder a potential up to several kilovolts can be applied to direct, focus an d accelerate Si+ ions. A high efficiency Ge effusion cell ensures stable an d controllable Ge fluxes for growth rates up to 2.5 Angstrom/s. Under these conditions, several sets of thin SiGe layers (65-300 nm) containing from 2 3 to 100% of Ge were grown and investigated comparatively with reference sa mples deposited without ions at 650 degrees C. By the 'ion growth program', after the deposition of Si buffers, SiGe layers were grown in three stages . The first part of the layer (e.g. 1/8 of the nominal thickness) and the l ast one (e.g. 5/8 of the thickness) were grown without ion bombardment. The second part (e.g. 2/8 of the thickness) was deposited under 1-keV accelera ted Si+ ion bombardment. Ge content was kept constant during all three stag es. Sharp interfaces and uniform Ge profiles were shown by SIMS. Strain rel axation in the thicker layers is nearly 100% as proven by XRD. In thin pseu domorphic layers with low. Ge content, a bombardment may result in nucleati on of stacking faults shown by TEM. AFM and preferential chemical etching o f relaxed ion bombarded layers have shown higher surface smoothness and a r eduction of etch pit densities. (C) 1998 Elsevier Science S.A. All rights r eserved.