SPECTROSCOPIC ELLIPSOMETRIC CHARACTERIZATION OF SI/SI1-XGEX STRAINED-LAYER SUPERLATTICES

Spectroscopic ellipsometry (SE) was employed to characterize Si/Si1-xGex strained-layer superlattices. An algorithm was developed, using the available optical constants measured at a number of fixed x values of Ge composition, to compute the dielectric function spectrum of Si1-xGex at an arbitrary x value in the spectral range 1.7 to 5.6 eV. The ellipsometrically determined superlattice thicknesses and alloy compositional fractions were in excellent agreement with results from high-resolution X-ray diffraction studies. The silicon surfaces of the superlattices were subjected to a 9: 1 HF cleaning prior to the SE measurements. The HF solution removed silicon oxides on the semiconductor surface, and terminated the Si surface with hydrogen-silicon bonds, which were monitored over a period of several weeks, after the HF cleaning, by SE measurements. An equivalent dielectric layer model was established to describe the hydrogen-terminated Si surface layer. The passivated Si surface remained unchanged for > 2 h, and very little surface oxidation took place even over 3 to 4 days.