Ion assisted MBE growth of SiGe nanostructures

Abstract

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 Å/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°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.