Spontaneous surface roughening, stress relaxation and composition re-distribution are coupled fundamental processes during the growth of strained epitaxial Si1−xGex / Si (100) alloy layers. In this work, we will present both modeling and experimental approaches to investigate the inter-relationships among these mechanisms. Stress distributions in undulated layers were calculated via the Finite Element Method by assuming a sinusoidal surface geometry. They were investigated as functions of undulation wavelength and amplitude between the trough and peak regions. For example, with a 50 nm layer of x = 0.3 having undulations of 250 nm wavelength and 40 nm amplitude, the compressive stress is twice as high in the trough regions than in the peak regions even without compositional redistribution. To explore compositional redistribution in response to these laterally varying strain fields, an experimental approach has been developed to determine local germanium concentrations for such undulated alloyolayers. An etchant consisting of HNO3 (70 wt %): H2O: HF (0.5 wt %), 25: 35: 5, at 28 oC etches Si1−xGex / Si (100) alloy layers at a rate of several nanometers per minute. The etching rate increases linearly with increasing Ge concentration in the alloy layer. Such etching experiments can thus be applied to etch alloy layers with compositionally varying undulations on the free surface and utilized to quantify the local germanium concentrations. For an alloy layer with x = 0.3, the Ge concentration from this analysis is estimated to be about 20 % enhanced / depleted in the peak / trough regions, respectively.
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This work is funded by the NSF - DMR (Grant # 0075116). We acknowledge Dr. Alain Portavoce (UVa) for valuable discussions. The samples were grown by Dr. J.C. Bean (UVa).
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Wu, CC., Hull, R. Nano-scale Stress and Compositional Analysis of Epitaxial Si1−xGex / Si (100) Undulated Films. MRS Online Proceedings Library 854, U3.8 (2004). https://doi.org/10.1557/PROC-854-U3.8