Gas-source molecular beam epitaxy of SiGe virtual substrates: II. Strain relaxation and surface morphology

Abstract

We have studied the strain state, film and surface morphology of SiGe virtual substrates grown by gas-source molecular beam epitaxy (use of disilane and germane). The macroscopic strain relaxation and the Ge composition of these virtual substrates have been estimated in high resolution x-ray diffraction, using either omega-2 theta scans or reciprocal space maps around the (004) and (224) orders. Typically, linearly graded Si0.67Ge0.33 virtual substrates 2.5 µm thick are 97% relaxed. From transmission electron microscopy, we confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. The surface roughness of the relaxed SiGe virtual substrates increases significantly as the Ge concentration and/or the growth temperature exceeds 20%/600 °C. At 550 °C, we find for the technologically important Ge concentration of 30% a surface root mean square roughness of 12 nm, with an undulation wavelength for the cross-hatch of the order of one micron.