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Symposium A: Energy Beam-Solid Interactions and Transient Thermal Processing
Photon-Solid Interaction (Symposium A) H. Kurz, Harvard University The study of non-equilibrium phenomena by optical methods has contributed considerably to our understanding of the photon-solid interaction. With the advent of powerful laser sources, the interaction of intense optical coherent radiation with condensed, highly absorbing media has likewise drawn attention to practical processing technologies. The issue which has attracted considerable interest is the behavior of semiconductors under the irradiation of short (1 x lO-'-lO"13 s) intense laser pulses because of the fascinating technological prospects for the development of new electronic structures and devices. One of the most intense debates in recent years centers on the fundamental interpretation of pulsed laser heating and melting of semiconductor surfaces. Ample evidence is presented in this paper showing that, even on a picosecond time scale, simple thermal melting takes place. Although there is no complete microscopic model of the melting process itself, one may question in which form the absorbed laser energy resides at the time of the transition from the solid to the liquid state. Initially the laser energy is exclusively used to excite electrons in the semiconductor. The photon energy may exceed the energy gap between the electron energy in the unperturbed ground state and the lowest excited state possible in a semiconductor. Then each photon is said to create an electron-hole pair and to impart a large kinetic energy to them. Under irradiances normally encountered in laser processing with picosecond pulses, up to 1021 electronhole pairs per cm3 are created in a time of 10 picoseconds. At these high densities, the correlation between the single excited electron and the vacant bonding state (holes) is totally destroyed by rapid collisions among the electrons, among the holes, and between electrons and holes. An electronhole plasma is formed. The energy distribution of the electronic carriers in the optically generated plasma is determined by the classical Max well-Boltzmann distribution characterized by a plasma temperature Tc. However, this plasma is not isolated from the ions in the material. By time-resolved optical techniques, where a second weak picosecond pulse monitors, at a certain time delay, the changes in the optical properties of the exciting pulse, the coupling between plasma and lattice can be studied. Thus, by probing the transmission at different times after the exciting pulse, the temporal build-up of vibrational energy can be observed directly. This technique is far superior to the conventional Raman-scattering technique, where the vibrational modes are detected by the frequency shift of the scattered light. The Raman technique requires the superposition of several thousand laser shots to gain a reliable signal of scattered light and suffers from a series of other experimental complexities. As a result very confusing data have been reported in the pas
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