Infrared studies of hole-plasmon excitations in heavily-doped p-type MBE-grown GaAs : C

Infrared reflectivity measurements (200-5000 cm(-1)) and transmittance meas urements (500-5000 cm(-1)) have been carried out on heavily-doped GaAs:C fi lms grown by molecular-beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000-3000 cm(-1) region and th e one-phonon band near 270 cm(-1) rides on a progressively increasing high- reflectivity background, An effective; plasmon/one-phonon dielectric functi on with only two free parameters (plasma frequency omega(p) and damping con stant gamma) gives a good description of the main features of both the refl ectivity and transmittance spectra. The dependence of omega(p)(2) on hole c oncentration p is linear; at p = 1.4 x 10(20) cm(-3), omega(p) is 2150 cm(- 1). At each doping, the damping constant gamma is large and corresponds to an infrared hole mobility that is about half the Hall mobility. Secondary-i on mass spectroscopy and localized-vibrational-mode measurements indicate t hat the Hall-derived p is close to the carbon concentration and that the Ha ll factor is dose to unity, so that the Hall mobility provides a good estim ate of actual de mobility. The observed dichotomy between the de and infrar ed mobilities is real, not a statistical-averaging artifact. The explanatio n of the small infrared mobility resides in the influence of intervalence-b and absorption on the effective-plasmon damping, which operationally determ ines that mobility. This is revealed by a comparison of the infrared absorp tion 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 infrar ed interband absorption, the de and infrared mobilities do not differ.