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

We have studied the strain state, him and surface morphology of SiGe virtua l substrates grown by gas-source molecular beam epitaxy (use of disilane an d germane). The macroscopic strain relaxation and the Ge composition of the se virtual substrates have been estimated in high resolution x-ray diffract ion, using either omega-2 theta scans or reciprocal space maps around the ( 004) and (224) orders. Typically, linearly graded Si0.67Ge0.33 virtual subs trates 2.5 mu m thick are 97% relaxed. From transmission electron microscop y, vie confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. T he surface roughness of the relaxed SiGe virtual substrates increases signi ficantly as the Ce concentration and/or the growth temperature exceeds 20%/ 600 degrees C. At 550 degrees 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.