Interdiffusion in Coherent Si 0.90 Ge 0.10 /Si 0.95 Ge 0.05 Superlattices

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U5.14.1

Interdiffusion in Coherent Si0.90Ge0.10/Si0.95Ge0.05 Superlattices D. B. Aubertine, P.C. McIntyre Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT We present the use of x-ray scattering from Si1-XGeX/Si1-YGeY superlattices as a tool for measuring the concentration dependence of interdiffusivity in Si/SiGe epitaxial thin films. Although x-ray scattering from compositionally modulated films is an ultra-high-sensitivity technique for measuring interdiffusion in a variety of systems, the concentration dependence of Si/SiGe interdiffusion complicates its interpretation. We show that these complications can be avoided using Si1-XGeX/Si1-YGeY superlattices with a small compositional modulation. This strategy is assessed using numerical simulations of both interdiffusion and dynamical x-ray diffraction. We demonstrate its effectiveness by measuring the activation enthalpy and exponential prefactor for interdiffusion in compressively strained Si0.925Ge0.075. The results are 4.38 ± 0.05 eV and 36 ± 9 cm2/s respectively. Introduction As SiGe based device technology grows in importance, it is becoming increasingly clear that interdiffusion at Si/SiGe interfaces is a significant problem. This is especially true for strained channel MOSFET devices, which typically utilize an ultra thin, epitaxial, strained Si channel grown onto a relaxed SiGe layer. For such structures, out-diffusion of Ge from the SiGe layer poses an even more stringent limit on practical thermal exposure than strain relaxation.1,2 Several groups have shown, for example, that a 10 nm strained Si channel grown on a Si0.70.8Ge0.3-0.2 relaxed buffer layer experiences significant intermixing after a 30 second anneal at 1000°C.2-4 Due to its dependence on Ge concentration, interdiffusion at Si/SiGe interfaces can not be treated with linear diffusion calculations.5 Both the exponential prefactor and activation enthalpy for interdiffusion are concentration dependent6-9 and the interdiffusivity may be modified by film strain, 7,8,10 which is also a function of Ge concentration. Before interdiffusion at Si/SiGe interfaces can be accurately modeled, each of these couplings between Ge concentration and interdiffusivity must be isolated. Experimental technique In this letter, we present the use of x-ray diffraction from a Si1-XGeX/Si1-YGeY superlattice with small concentration amplitude to determine the interdiffusivity at a specific Ge concentration and strain state. X-ray diffraction from concentration modulated films is an established ultra-high-sensitivity probe of interdiffusion, but care must be taken when applying it to concentration dependent interdiffusion.11,12 At any given location in a Si/SiGe superlattice, the interdiffusivity is a function of the local Ge concentration, giving rise to a large range of diffusivity values throughout the film. To minimize this problem, we limited the range of concentrations present in our superlattice structure, which is illustrated schematically in Fig.

U5.14.2

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