Electron-Hole Plasma Formation in Semiconductors Due to the Absorption of Pulsed CO sub 2 Laser Light

摘要

A calculation is presented to describe the modification of the carrier distribution due to the presence of high-intensity CO sub 2 laser radiation for several semiconductors. In the calculation the inter-carrier collision rates are assumed to be sufficiently large that the distribution can be described by a carrier temperature, which may be much larger than the lattice temperature. The steady-state carrier temperature is calculated as a function of the light intensity by equating the rate at which the carriers receive energy from the electromagnetic field and the rate at which the carriers lose energy to the lattice by the emission of phonons. For sufficiently high light intensities, a significant fraction of the total free carrier density can have a kinetic energy which exceeds the bandgap. These hot carriers are energetically capable of relaxing by undergoing an inelastic pair-producing scattering event. This impact ionization process can lead to the formation of a laser-induced plasma by the absorption of light well below the bandgap of the material. Explicit values are presented for the intensity thresholds at which the impact ionization processes become important for light in the 9 to 11 mu m region. Comparison of the results with transmission measurements indicates that this mechanism may be responsible for the nonlinear absorption observed in several semiconductors by high-intensity pulsed CO sub 2 laser light. 39 references, 12 figures. (ERA citation 09:050944)