Structural characterisation of MBE grown Si and SiGe material
The application of advanced X-ray techniques, using both laboratory and synchrotron radiation sources, to the structural characterisation of semiconductor multilayer systems is described with reference to their use in understanding the growth by Molecular Beam Epitaxy of SiGe layers. These layers, prepared using VG V80 and VG V90 growth systems have been characterised for x-y uniformity and compositional reproducibility. The superlattice period dispersion, due to short term flux instabilities in the growth fluxes, has been measured by consideration of the relative heights of Pendellosung fringes present between the superlattice fringes in the X-ray diffraction rocking curve. This has allowed the period dispersion to be found to an accuracy of ± 0.3 nm. Interface roughness has been measured with X-ray reflectivity. This has shown significant (1.0±3 nm) long range (~70 nm) roughness at the Si1-xGex to Si interface for x=0.4 and x=0.6 superlattices. SiGe layers have been grown to well beyond the metastable critical thickness, and the residual strain remaining in the structures found from use of two reflections in double crystal X-ray diffraction. This work has demonstrated an empirical relationship between thickness/critical thickness and the degree of residual strain remaining in the layer. This relationship is found to be independent of the Ge content of the epitaxial layer. SiGe layers have also been grown beyond the equilibrium critical thickness but less than the metastable critical thickness. Standard anneals have been carried out on these layers allowing determination of how the layers relax with varying composition and thickness. The misfit dislocations created are found to be longer for layers with lower degrees of strain. By growing layers with longer misfit dislocations relaxed buffer layers have been produced with threading dislocation densities of ~10^6 cm^-2 and a surface roughness of ±3 nm. To avoid growing highly dislocated SiGe layers SiGe has been grown on a Si substrate which has been etched to provide "mesa" structures of size varying from 1500 μm to 1 μm in width. Mesa islands smaller than 15 microns across show no misfit dislocations and rectangular islands 15 microns wide and 100 microns long have misfit dislocations running on one direction only indicating that relaxation in the other direction has taken place by elastic rather than plastic processes. Characterisation of delta layers in Si using SIMS. is limited by cascade mixing to a depth resolution of 3 nm. Double axis X-ray diffraction using the interference effects between the Si below the delta layer and the Si above the delta layer has allowed the width of B delta layers to be determined to ±0.5 nm. This technique has also been used to measure the strin due to the B delta thus allowing determination of the degree of activation of the dopant. Thermal annealling of B delta layers has shown that anneals at 700°C for 1 hour causes measurable broadening of the delta layers. Sb delta layers have also been examined and their widths found to be less than 2 nm.