Spectroscopic ellipsometry and photoluminescence from Si1−xGex alloys grown by atmospheric-pressure chemical vapor deposition

Si1−xGex layers grown using atmospheric pressure chemical vapor deposition have been characterized using both room temperature spectroscopic ellipsometry (SE) and low-temperature photoluminescence (PL). Single layers of Si0.9Ge0.1, 30, 100, 1000, and 2000 nm thick on p+ Si(100) wafers were investigated to determine the effect of strain on the indirect and direct optical transitions. The thinner two layers were pseudomorphic and the thicker ones relaxed. The samples were examined by spectroscopic ellipsometry, which allowed the optical constants to be determined from the ultraviolet to near infrared (3.5–1.8 eV). Using optical constants available from the literature for cubic Si1−xGex, the thicknesses of the alloy layers were verified. From our optical constants and the published calibration curves for normal incidence reflectivity at 633 nm and for the energy of the E1 transition (both a function of x), we found that the average germanium concentration appeared to be significantly below the nominal 10% for these samples. Phonon-resolved PL spectra were observed at 2 K for the thicker three samples with the transition from strained to unstrained layers clearly visible in the shift of the Si1−xGex band gap as seen from the energy of the no-phonon line. Dislocation lines appeared only for the relaxed material and the no-phonon line widths were ~4 times smaller for the strained Si1−xGex material.