Short heat processes in nondegenerate semiconductors

Full Text: The electron and phonon transient temperature distributions arising because of the surface absorption of a rectangular laser pulse of an arbitrary duration are analytically studied in nondegenerate semiconductors. The last are characterized by the relatively great phonon thermal conductivity in comparison with the electron thermal conductivity and by the small thermal diffusivity in comparison with the electron one. It is shown that the lattice heat diffusion is the main energy relaxation mechanism in the phonon subsystem. This mechanism depends on the ratio of the sample length l to the electron cooling length lε in the electron gas. The energy relaxation takes place by means of the electron thermal diffusion in the short samples (l << lε), whereas the energy electron-phonon interaction dominates in the long ones (l >> lε). The characteristic relaxation times and the criteria of the long and short heat pulses are obtained in all cases. It is shown that the duration of the laser pulse can be simultaneously different with respect to the electrons and phonons due to the essentially different scale of the electron and phonon thermal diffusivities. For example it can be long with respect to the electron gas and the short with respect to the phonon one. This circumstance comes to the quantitatively different temperature distributions of the quasiparticles. The relaxation regions of the transient temperatures are examined in detail and it is shown that phonon temperature relaxation is described by the unique characteristic time τp. The electron temperature relaxation has the more compound nature, and it is described by the two characteristic times: τe which is much shorter than τp and the already indicated time τp. It is shown the possibility of the electron and phonon heat parameters detection as well as the determination of the electron-phonon energy relaxation rate. (author)