Nonlinear Laser Melting of Indium Antimonide and Silicon
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MICHAEL P. HASSELBECK AND H. S. KWOK Department of Electrical and Computer Engineering State University of New York at Buffalo, Amherst, New York
14260
USA
ABSTRACT pulsed 10.6pm TEA CO 2 laser light has been used to melt the semiconductors silicon and InSb. Measurements indicate that generation of free carriers necessary for melting may take place by nonlinear processes such as two-photon absorpIf the semicontion or intraband avalanche ionization. ductor is sufficiently doped, melting may also result from In all cases, it appears linear free carrier absorption. that the molten depth exceeds several pm, which is much greater than obtained with lasers of shorter wavelength.
INTRODUCTION The physical mechanism by which a pulsed laser melts a semiconducting solid The transfer of laser energy to the lattice takes is now well established. place in an indirect process consisting of the simultaneous (1) generation of a dense electron-hole plasma, (2) excitation and heating of these free carriers by the laser field and (3) transfer of the energy from the free carriers to the condition has been achieved almost exclusively by direct lattice. The first single photon interband transitions, so that the laser photon of frequency V In a series where E is the semiconductor bandgap energy. must satisfy hv !Eg, of experiments, we discovered that this condition on the laser frequency can be circumvented for the CO 2 laser where the photon energy is only 0.12 eV. The important observation is that pulsed laser-induced melting occurs at At high intensities which are quite close to the material damage threshold. such high intensities, nonlinear carrier generation mechanisms can be very In addition, if the semiconductor effective in generating the free carriers. is heavily doped to begin with, melting can occur via absorption by these existing carriers. (1) We have demonstrated the above ideas for three semiconductor systems: intrinsic InSb, where the nonlinear process of two-photon absorption was used free carto generate the free carriers; (2) highly doped Si, where the initial riers are used to melt the solid; and (3) intrinsic Si, where the highly nonlinear process of avalanche ionization took place. LASER SYSTEM The CO 2 laser system used in the experiments is illustrated schematically in Fig. 1. A transversely excited, atmospheric pressure (TEA) double discharge laser head, and a longitudinally pulsed, low pressure (0,6torr) section provided polarized, temporally smooth, single longitudinal mode laser pulses of "670 ns duration and peak power of %8 MW. The oscillator is grating tuned and The spatially filtered for operation in the lowest order transverse mode. The P(20) 10.6pm CO 2 laser line was used exclusively in these experiments. picosecond pulses were generated by optical free induction decay (OFID) [1]. This method of active pulse shaping is based on the high frequency modulation of the laser pulse and the subsequent filtering by means of an optical resonant Mat. Res. Soc. Symp. Proc. Vol. 13 (1983) @Elsevier Science Publi
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