Optical characterizations of heavily doped p-type AlxGa1−xAs and GaAs epitaxial films at terahertz frequencies

Abstract

The optical properties of p-type AlxGa1−xAs (x=0, 0.01, and 0.16) epitaxial films with different beryllium and carbon doping concentrations (1018–1019cm−3) were investigated by far-infrared reflectance spectroscopy in the 1.5–15-THz frequency range. The dielectric response functions of the film samples were expressed using the classical Lorentz–Drude model. Optical properties were obtained using a three-phase model (air∕film∕substrate) which agrees with the experimental reflectance spectral data. The effects of doping concentrations on the optical constants were studied in detail. The results indicate that the refractive index increases with the doping concentration in the low-frequency region (⩽5THz) where the free-carrier absorption plays an important role in the optical response. However, the extinction coefficient increases with the doping concentration in the entire frequency region. This indicates that the absorption coefficient increases with the doping concentration. The calculated plasma frequencies agree with the values obtained from the measured doping concentrations. The free-carrier scattering time is ∼1.39×10−14s. The longitudinal-optical phonon plasmon coupled modes of the AlxGa1−xAs films are presented. The upper coupled mode increases with the doping concentration and shows a transition from phononlike to plasmonlike behavior. A sublinear relationship between the absorption coefficient and the doping concentration for p-type AlxGa1−xAs epitaxial films was obtained at a frequency of 3.75THz (80μm). These results can be used to design and improve the performance of terahertz detectors.