Terahertz optical properties of carriers in silicon

Abstract

A system based on the optoelectronic generation and reception of a beam of subpicosecond terahertz pulses has been used for spectroscopic studies in the 0.1-2.0-THz range. By inserting a sample in the beam and comparing the propagated terahertz pulses with the original pulses, one is able to deduce its frequency-dependent absorption and dispersion. Using this procedure, the absorption coefficient and index of refraction of both n- and p-type 1-Ω-cm silicon have been measured at room temperature. The measurements extend from 0.1 to 2 THz and provide a direct view of the carrier dynamics of electrons and holes; measurements of ultrapure silicon (10 k Ω-cm) showed practically no absorption and dispersion. The plasma frequency and damping rate of the carriers were determined. For electrons and holes, damping rates of 0.64(4) and 1.51(8) THz, respectively, were found. A combination of the measured damping rates and the known effective carrier masses yields mobilities of 1680 cm2/V-s for the electrons and 500 cm2/V-s for the holes. Similar transmission experiments were performed for n- and p-type 10- and 0.1-Ω-cm silicon. All samples were measured at 293 and 80 K. The results reflect marked drops in damping rate and moderate decreases in plasma frequencies due to carrier trapping at the dopant.