Ion assisted MBE growth of SiGe nanostructures

The bombardment of thin SiGe buffers with 1-keV Si+ ions during molecular beam epitaxial growth is a possible way for the injection of point defects in order to promote the relaxation and to reduce the dislocation density. For this purpose, the e-beam evaporator was optimized by increasing the emission 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 holder a potential up to several kilovolts can be applied to direct, focus and accelerate Si+ ions. A high efficiency Ge effusion cell ensures stable and 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 23 to 100% of Ge were grown and investigated comparatively with reference samples 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 last 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 accelerated Si+ ion bombardment. Ge content was kept constant during all three stages. Sharp interfaces and uniform Ge profiles were shown by SIMS. Strain relaxation in the thicker layers is nearly 100% as proven by XRD. In thin pseudomorphic layers with low. Ge content, a bombardment may result in nucleation of stacking faults shown by TEM. AFM and preferential chemical etching of relaxed ion bombarded layers have shown higher surface smoothness and a reduction of etch pit densities. (C) 1998 Elsevier Science S.A. All rights reserved.