Infrared reflectivity measurements ͑200-5000 cm Ϫ1 ) and transmittance measurements ͑500-5000 cm Ϫ1 )have been carried out on heavily-doped GaAs:C films grown by molecular-beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000-3000 cm Ϫ1 region and the onephonon band near 270 cm Ϫ1 rides on a progressively increasing high-reflectivity background. An effectiveplasmon/one-phonon dielectric function with only two free parameters ͑plasma frequency p and damping constant ␥) gives a good description of the main features of both the reflectivity and transmittance spectra. The dependence of p 2 on hole concentration p is linear; at pϭ1.4ϫ10 20 cm Ϫ3 , p is 2150 cm Ϫ1 . At each doping, the damping constant ␥ is large and corresponds to an infrared hole mobility that is about half the Hall mobility. Secondary-ion mass spectroscopy and localized-vibrational-mode measurements indicate that the Hall-derived p is close to the carbon concentration and that the Hall factor is close to unity, so that the Hall mobility provides a good estimate of actual dc mobility. The observed dichotomy between the dc and infrared mobilities is real, not a statistical-averaging artifact. The explanation of the small infrared mobility resides in the influence of intervalence-band absorption on the effective-plasmon damping, which operationally determines that mobility. This is revealed by a comparison of the infrared absorption results to Braunstein's low-p p-GaAs spectra and to a k"p calculation extending Kane's theory to our high dopings. For n-GaAs, which lacks infrared interband absorption, the dc and infrared mobilities do not differ.

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By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4ϫ10 20 cm Ϫ3 , were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon ͑intraband͒ and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located ͑at high doping͒ at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband ͑IVB͒ absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters we obtain for highly doped p-GaAs yield an infrared mobility which ͑unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model͒ is in substantial agreement with the dc mobility.