An in situ investigation of Si(x)Ge(1-x) chemical vapor deposition by differential reflectance

An investigation of the surface kinetic processes of low pressure chemical vapor deposition (LPCVD) of Si, Ge, and Si_xGe_1-x was carried out using time-resolved differential reflectance measurements. The source gas (disilane, digermane, or mixtures of these two, diluted in a helium carrier) was delivered to a heated substrate by a fast-acting modulated molecular jet valve. Thin film growth was studied in the temperature range of 400- 500°C on Si and Ge (001) substrates. The kinetics of chemisorption and of by-product desorption were determined from the surface differential reflectance signal obtained using a p-polarized, high-stability HeNe probe laser. Both chemisorption and by-product desorption were found to obey first-order kinetics. Chemisorption of the parent molecules was found to be relatively efficient (~5%) and weakly temperature dependent. For pure Si and Ge, by-product desorption occurred through a single first-order reaction. The activation energy of this step, ~2.75 eV for Si and ~ 1.5-2.2 eV for Ge, suggests that for both surfaces the stable chemisorption by-product is the monohydride, which decomposes via the elimination of molecular hydrogen. Two first-order desorption steps were inferred for the Si_xGe_1-x alloy surfaces. These reactions are believed to be H₂ desorption from Si- like and Ge-like surface sites. However, the activation energy of the more rapid of these two steps actually decreases as the Si content of the film increases.

In the earlier portion of this work, Ge on Si and Ge on Ge LPCVD experiments were performed in a high-vacuum growth chamber. Later, a UHV system was used for heteroepitaxial growth of Si, Ge, and SixGe1-x thin films on Si substrates. The composition, morphology, and epitaxial and crystalline quality of the films grown in the UHV system were examined by Rutherford backscattering spectroscopy, x-ray diffraction, Raman spectroscopy, ellipsometry, and atomic force microscopy. Generally, these films were of high crystalline quality and were very well aligned with the substrate. Preferential incorporation of digermane into the film produced an alloy composition that was Ge-rich relative to the gas composition.

The primary accomplishment of this work is the demonstration that the active surface layer of the important Si_{xGe1-x system can be monitored in situ by an optical probe under typical LPCVD conditions. The results that we have obtained to date indicate that the rate-limiting step in Si or Ge LPCVD obeys simple first-order kinetics. Two first-order desorption steps were inferred for SixGe1-x alloy surfaces, but further work is needed to understand fully the rate-limiting surface reaction in SixGe1-x LPCVD.}