The effect of carrier diffusion on laser heating of lightly damaged semiconductors

Abstract

It has been observed that the surface temperature obtained during laser heating of lightly damaged material is generally considerably lower than the temperature obtained in heavily damaged material. We develop a model which explains this observation by taking into acount the diffusion of the photoexcited carriers. The time evolution of temperature profiles was calculated for pulsed ruby and Nd : YAG illumination of silicon and ruby illumination of GaAs. The material, optical, and electronic properties were varied, according to the material quality. Surface recombination of the carriers was also taken into account. For the same laser power, the induced temperature was lower and the heating times were longer in good-quality materials than in heavily damaged materials. This is because of longer carrier difusion lengths in good-quality material. The effect of the damage on the heating was found to be much more pronounced for Nd : YAG than for ruby irradiation because a larger portion of the ruby laser energy (excess above band gap) is given rapidly to the lattice through quasithermalization inside the semiconductor energy bands. Thermal spikes are formed as the result of the preferential recombination of the carriers at the surface. For a sufficiently fast surface recombination, the temperature profile may develop a secondary maximum in the material interior.