Inhomogeneous broadening of phosphorus donor lines in the far-infrared spectra of single-crystalline SiGe

The origins of line broadening in the far-infrared spectrum of phosphorus donors in SiGe are investigated. Using a combination of Fourier transform infrared (FT-IR) spectroscopy and time-resolved pump-probe measurements, we show that the line shapes are dominated by inhomogenous broadening. Experimental FT-IR absorbance spectra measured in the temperature range 6-150 K are presented for three different Ge contents. Additional spectra of pure phosphorus doped silicon recorded under similar experimental conditions are presented and compared with the SiGe results. We propose a simple quantitative model to simulate the line broadening in our experimental spectra. Our model takes into account the compositional variations in the random SiGe binary alloy and its effect on the permittivity of the environment around each donor. We also show that the addition of small amounts Ge to Si single crystals has little detrimental effect on the lifetime of the excited infrared electronic energy levels, despite the observed line broadening.