Silicon Systems

Si_1-xGe_x/Si Thin Films

Si-Ge alloys are grown on Si using Ultra High Vacuum Chemical Vapor Deposition (UHV-CVD). The CVD growth and characterization facility is part of the University of Wisconsin's Materials Research and Science Engineering Center. This system contains several in-situ probes. One of the unique in-situ characterization probes is a differentially pumped RHEED (Reflection High Energy Electron Diffraction) gun and energy analyzer that allows the surface structure and morphology to be followed dynamically at chamber pressures as high as 20 mTorr. The RHEED capability has allowed the monitoring of surface structure changes due to segregation or contamination. It has also proved invaluable in monitorng the appearance and overgrowth of SiGe clusters during the growth of quantum dot superlattices.

Additional in-situ probes include low resolution energy loss spectroscopy and Fourier transform infrared spectroscopy (FTIR). FTIR is performed on beveled wafers so that attenuated total reflection (ATR) can be used to increase the signal. One of the most recent results using FTIR shows that atomic hydrogen can induce a place exchange of surface Ge with surface Si when the surface is dosed at temperatures above 250° C but below the hydrogen desorption temperature. A scanning tunneling microscope will be added to an attached analysis chamber this year, and a spectroscopic ellipsometer will be added to the growth reactor. Ex-situ probes allow structural characterization and standard electrical characterization. These include high-resolution x-ray diffraction, cross-sectional TEM, and atomic force microscopy (AFM).

When SiGe is deposited on Si(001), strain is created on the surface because of the lattice mismatch between the SiGe and the Si(001). Research indicates that coherently strained, three-dimensional islands with {105} facets form to relieve this strain. Different island sizes and shapes have been observed depending on the specific conditions of the growth experiment. Several models have been proposed to describe specific aspects of specific experiments, yet no model describes all of the experimental results satisfactorily, nor has a comprehensive model been proposed. Current interest in SiGe 3-D islands as quantum-dot arrays necessitates a model which encompasses the various results and phenomena. Such a model will enable better engineering of future structures.

One current research project is a classical surface science study of Si(501) as a model system for the {105} facets bounding SiGe 3-D islands. The goal is to develop a basis for a comprehensive model of the structure of coherently strained, faceted 3-D islands.