Download An x-ray diffraction study of concentration and strain dependent Si/SiGe interdiffusion

An x-ray diffraction study of concentration and strain dependent
Si/SiGe interdiffusion
Daniel B. Aubertine
Stanford University
Department of Materials Science and Engineering
SiGe alloys have become important materials for semiconductor device engineering.
They provide a means of tailoring the properties of the semiconductor, such as band-gap,
carrier mobility, and dopant solubility, at specific locations within a device. CVD growth
rates for single crystal SiGe are also much faster than for Si, allowing improved
throughput and decreased contamination. Further, all of these benefits are realized at a
relatively low cost owing to the high degree of compatibility between SiGe and Si
processing technologies.
As SiGe films are introduced into deeply scaled, ultra-fast MOS devices, it is
increasingly clear that interdiffusion at Si/SiGe interfaces is a significant problem.
Strained Si MOSFET’s typically utilize a thin, epitaxial, strained Si channel grown onto a
relaxed SiGe layer. For these structures, out-diffusion of Ge from the SiGe layer into the
Si channel is a factor limiting the practical thermal exposure during processing.
Predicting the degree of intermixing is difficult because the interdiffusion process is
influenced by the local Ge concentration, film strain, and non-equilibrium point defect
concentrations. Development of a robust model for Si/SiGe interdiffusion requires that
each of these effects be isolated and quantified.
Toward this end, I have employed x-ray diffraction from concentration modulated SiGe
films as a probe of both interdiffusion and strain relaxation. Although less commonly
applied to semiconductor diffusion than techniques that map out concentration profiles
directly, this technique has a long history as an ultra-high-sensitivity probe of both
interdiffusion and film strain. By growing and analyzing a series of films with varied
mean Ge concentrations, spatial modulation periods, and degrees of strain relaxation, I
have built up a model for concentration and strain-dependent Si/SiGe interdiffusion. The
model results have been successfully tested against x-ray measurements of interdiffusion
in large-amplitude Si/SiGe superlattices and SIMS measurements of intermixing at
Si/SiGe interfaces. Further, this model is readily applicable to predicting the thermal
stability of technologically important Si/SiGe interfaces during device processing.