CHARACTERIZATION OF SI_XGE_1-X LAYERS WITH GRAZING X-RAY REFLECTION. R.Treichler, H.Schäfer, W.Hösler and H.Göbel, Siemens AG, Corp. R&D, D-81730 Munich, Germany

Silicon bipolar technology is the most advanced in the VLSI field and conventional devices have reached impressive switching times. For further speed enhancement, the vertical device dimensions have to be shrunk to layer thicknesses down to a few nanometers. It is foreseeable that the standard homojunction devices will soon run into serious limits. Therefore the properties of Si_xGe_1-x layers for Si-based bandgap engineering have attracted wide attention. Epitaxial SiGe heterostructures provide large flexibility in designing new devices because parameters on both sides of a heterojunction can be varied with the prospect of highly improved electrical characteristics. The usage of Si_xGe_1-x as a base layer offers the potential of reduced doping levels, capacities, transit times and resistances. The success of modern CVD and MBE techniques to grow Si_xGe_1-x layers of only a few nanometers thickness with sufficient precision is closely tied to the availability of analytical tools which are able to monitor important layer parameters.

Our contribution shows that the grazing x-ray reflection technique (GXR) is well suited for a precise characterization of Si_xGe_1-x on Si with respect to layer thickness, density and roughness. We applied GXR and Rutherford backscattering (RBS) as supporting tool to determine these parameters on a series of layers with different thickness and Ge concentration. The pronounced strength of GXR is the capability to measure layer thicknesses with an accuracy of a few tenths of a nanometer. In addition we determined Ge concentrations with an accuracy of 5% from the critical angle of total reflection and the oscillation amplitudes. As the lattice constant of Ge (0.5657nm) and Si (0.543nm) differ by 4%, Si_xGe_1-x layers on Si are always strained, and therefore a critical layer thickness -depending on the Ge contents- exists, which should not be exceeded. The relaxation of Si_xGe_1-x shows up as the formation of dislocations, but already far below the critical layer thickness a micro interface roughness at the growth front develops. We compare this layer roughness as detected by GXR to AFM measurements and discuss it in terms of critical layer behaviour of pseudomorphic SiGe on Si. As advanced device designs require graded SiGe interfaces to diminish band gap discontinuities and increase the charge carrier mobility, we also applied GXR to gradings and discuss potential and limitations of the method.