Ph.D. Dissertation Defense (Feb 8th) - Nevran Ozguven

Nevran Ozguven nevran at stanford.edu
Sun Feb 4 16:46:09 PST 2007


  "Interdiffusion Studies in  Si-Ge Heterostructures and Selective 
Oxidation for Ge-on-Insulator"

Nevran Ozguven
Department of Materials Science and Engineering

Advisor : Professor Paul McIntyre

Date :    Thursday, February 8th, 2007
Time :    2PM (Refreshments served at 1:45 PM)
Place :   CISX 101 (Auditorium)
*
*Abstract:

    Recently, Si/Si_1-x Ge_x _ heterostructures have gained interest for 
use in MOS devices. These structures offer significant improvements in 
device performance relative to Si due to their enhanced carrier 
mobilities. Compared to other lattice mismatched systems, Si/Si_1-x Ge_x 
heterostructures are especially attractive due to their compatibility 
with mainstream CMOS processing technology.
    The Si/Si_1-x Ge_x _ system has been understood well enough to be 
implemented in commercial products.  However, there is still a need for 
fundamental research, especially regarding methods to prepare Ge-rich 
Si_1-x Ge_x of controlled strain and composition on Si substrates.  
During device fabrication, processing temperatures in the range of 
600-1000°C are typically used. It is therefore important to understand 
the temperature-dependent mechanisms of Si-Ge interdiffusion and strain 
relaxation in detail. In this study, two different conditions of 
interdiffusion in Si-Ge heterostructures were explored.
    We first present interdiffusion results of single-crystal Si_1-x 
Ge_x /Si_1-y Ge_y superlattices, with an epitaxial Si capping layer, 
grown by low-pressure chemical vapor deposition (LPCVD) on Si (001) 
substrates. During post-deposition dry oxidations, which inject 
interstitial defects, the Si cap was partially consumed. The effects of 
oxidation on Si-Ge interdiffusion were studied over the temperature 
range of 770-870°C. The interdiffusion kinetics, including Ge 
concentration effects, were measured via high-resolution x-ray 
diffraction (XRD). Experiments were performed on superlattice samples 
annealed in both inert and oxidizing ambients. We have observed 
enhancement in interdiffusion during oxidation. This indicates that the 
non-equilibrium point defect concentrations created during Si oxidation 
can significantly modify Si-Ge interdiffusion rates. These results allow 
us to place bounds on the interstitial- and vacancy-mediated 
contributions to Si-Ge interdiffusion.
    We also investigate the effects of direct oxidation of an 
LPCVD-grown Si_1-x Ge_x layer prepared on an SOI (100) substrate. In 
this case, thermal oxidation was carried out at temperatures below the 
Si_1-x Ge_x melting point. This is a possible means of preparing 
high-mobility Ge-rich films on insulator for future high-performance 
transistors.  During oxidation, the Ge atoms are rejected from the SiO_2 
layers, and their out-diffusion is suppressed by the top and buried 
oxides. As a result, the thickness of the Si_1-x Ge_x layer decreases, 
while its Ge concentration increases. The Ge fraction and the strain in 
the Si_1-x Ge_x layer as a function of oxidizing conditions were 
determined using XRD. Secondary ion mass spectrometry (SIMS) was used to 
observe Ge depth profiles, while transmission electron microscopy (TEM) 
was used to measure film thicknesses. We found that it is possible to 
retain the initial in-plane lattice spacing of the SOI layer after 
selective Si oxidation for final Si_1-x Ge_x layers with ~60% Ge 
composition and thicknesses exceeding critical layer thickness values.
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