The origins of line broadening in the far-infrared spectrum of phosphorus donors in SiGe are investigated.\udUsing a combination of Fourier transform infrared FT-IR spectroscopy and time-resolved pump-probe measurements,\udwe show that the line shapes are dominated by inhomogenous broadening. Experimental FT-IR\udabsorbance spectra measured in the temperature range 6âÂÂ150 K are presented for three different Ge contents.\udAdditional spectra of pure phosphorus doped silicon recorded under similar experimental conditions are presented\udand compared with the SiGe results. We propose a simple quantitative model to simulate the line\udbroadening in our experimental spectra. Our model takes into account the compositional variations in the\udrandom SiGe binary alloy and its effect on the permittivity of the environment around each donor. We also\udshow that the addition of small amounts Ge to Si single crystals has little detrimental effect on the lifetime of\udthe excited infrared electronic energy levels, despite the observed line broadening.
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