Sn-alloying as a means of increasing the optical absorption of Ge at the C- and L-telecommunication bands

The optical properties of Ge_1-ySn_y alloys (y ∼ 0.02) grown by chemical vapor deposition on Si substrates have been studied using spectroscopic ellipsometry and photocurrent spectroscopy. The system shows a 10-fold increase in optical absorption, relative to pure Ge, at wavelengths corresponding to the C-telecommunication band (1550 nm) and a 20-fold increase at wavelengths corresponding to the L-band (1620 nm). Measurements on a series of samples with different thicknesses reveal nearly identical dielectric functions, from which the composition reproducibility of the growth method is estimated to be as good as 0.1%. It is shown that a model that includes excitonic effects reproduces the measured onset of absorption using the direct band gap E₀ as essentially the only adjustable parameter of the fit.