Free-Carrier Absorption inn-Type GaP

The possible scattering mechanisms which can cause free-carrier absorption in $n$-type GaP are enumerated, and it is shown that, as a consequence of the many-valleyed nature of the conduction band, the dominant scattering is a combination of intravalley and intervalley scattering by phonons via the deformation-potential interaction. The calculated absorption for these scattering mechanisms has a wavelength dependence of ${\lambda }^p$ with $p\approx 1.7-1.8$, which is in accurate agreement with experimental measurements reported here and elsewhere. Several components of the theoretical absorption have identical wavelength dependences, making a unique decomposition impossible. We show, however, that it is possible to obtain an accurate quantitative fit to the data using reasonable values of the adjustable parameters. The values used were obtained from the literature and correspond to a conduction-band deformation potential ${E_1}^c\approx 10$ eV and energetic-mode coupling parameters $D_i\approx 8\times {10}^8$ eV ${\mathrm{cm}}^{-1\mathrm{}}$. By subtracting out the free-carrier absorption, we obtain a value 0.276±0.007 eV for the threshold of the $X_1\to X_3$ transition. This value is probably more accurate than those previously reported, since the experimental value obtained for this threshold is rather sensitive to the free-carrier-absorption correction.