Oxygen beam SIMS depth profiling of Si_1-xGe_x layers: Transient processes

Characterization of shallow dopant profiles near surfaces and interfaces in Si_1-xGe_x layers by secondary ion mass spectroscopy (SIMS) is a challenge in advanced Si-based technologies. In standard SIMS, quantification with confidence is possible only in the dynamic steady state of the measurement after completing a transient period. Therefore, we analyse the transient processes during oxygen bombardment and sputtering of epitaxial Si_1-xGe_x layers, which were grown by low-pressure chemical vapour deposition (LPCVD) with different compositions (0 < x < 0.75). We show that a drop of the Ge⁺ secondary ion yield near the end of the transient period in epitaxial Si_1-xGe_x layers extends this period in comparison with pure Si. We demonstrate that the drop is a function of layer composition and bombardment parameters, such as primary ion beam energy and erosion rate. We show that an interfacial transient period after crossing a Si/Si_1-xGe_x interface can be neglected if the formation of the oxygen-beam-induced altered layer is completed within the Si cap layer before reaching the interface between the cap and the Si_1-xGe_x layer. This allows reliable quantification of dopant spikes within a few nanometers near to interfaces. By means of computer modelling we describe secondary ion emission kinetics and simulate the dynamics of structural and compositional changes in the subsurface region of Si_1-xGe_x alloys caused by interaction with oxygen beams. Experimental and theoretical time dependencies for secondary ion yields are in good accordance; the dependence of the Ge⁺ intensity drop on the Ge concentration is simulated and experimentally proved.