Carrier multiplication in semiconductors induced by the absorption of high-intensity CO2 laser light

Abstract

A calculation is presented to describe the modification of the carrier distribution due to the presence of high-intensity CO2 laser radiation for several semiconductors. In the calculation the intercarrier 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 of energy which the carriers receive from the electromagnetic field and the rate of energy which the carriers lose to the lattice by the emission of phonons. For sufficiently high light intensities, a significant fraction of the total free carrier density can have kinetic energies which exceed the band gap. 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 band gap of the material. Explicit values are presented for the intensity thresholds at which the impact ionization process becomes important for light in the 9–11 μm region. Comparison of the results with transmission measurements indicate that this mechanism may be responsible for the nonlinear absorption observed in several semiconductors by high-intensity pulsed CO2 laser light.