Due to the higher lattice constant of SiGe compared to Silicon, an epitaxial SiGe layer deposited on Si will be stressed compressively. When a certain thickness value has been reached, called critical thickness, the layer will start to relax. This fact is well known since the early beginning of SiGe epitaxial growth. There were many investigations to calculate the critical thickness of combined semiconductor structures. Unfortunately, the results differ by more than one order of magnitude for Ge concentrations which are of interest for the semiconductor industry. Since the benefit of strain engineering by SiGe has come to the focus of scientific and industrial research, critical thickness, especially for SiGe, became an important topic again.

We have achieved new results describing the behaviour of LPCVD epitaxial grown SiGe layers on 200mm Si substrates. The SiGe layers deposited have various Germanium concentrations up to 45% as measured by AES and XRD. The measured values for critical thickness, i.e. where the onset of relaxation could be identified, show a good correspondence between the calculated values by Cohan-Solal. We also calculated the degree of relaxation by XRD measurements. We derived an equation which describes this value in dependence of the germanium concentration and layer thickness pretty accurately. Furthermore, when the SiGe layer starts to relax, its surface roughness as been measured by AFM increases drastically and typical crosshatch patterns can be observed on the surface. This is in particular true for small relaxation values.

The project was funded in line with the technology funding for regional development (EFRE) 2000-2006 of the European Union and by funds of the Freestate of Saxony.

Fig. 1. Degree of relaxation of Si_(1-x)Ge_x layers in dependence of layer thicknesses and Ge concentration. Comparison between experimental data and different theoretical descriptions

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