Ultra-Short Pulse Laser Annealing
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ULTRA-SHORT PULSE LASER ANNEALING MRKI*
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KENJI GAMO, KOUICHI MURAKAMI MITSUO KAWABE , SUSUMU NAMBA AND YOSHINOBU AOYAGI** Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan * Institute of Materials Science, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan •* The Institute of Physical and Chemical Research, Wako-shi Saitama 351, Japan
ABSTRACT A single picosecond pulse laser annealing of ion-implanted Si is reviewed as ultra-short pulse laser annealing, comparing them with nanosecond pulse and picosecond-pulse train annealing. In order to clarify the physical mechanism of pulsed laser annealing, the dynamic behavior of the amorphous to crystalline transition has been investigated by means of time-dependent optical reflectivity measurement at 0.63 pm (cw) and 1.06 pm (30-ps pulse itself) under the irradiation of the annealing beam of a single 30-ps laser pulse at 1.06 pm. A tentative model is proposed for explaining the results and further problems which remain to be resolved are discussed.
INTRODUCTION A lot of investigations [1] have been performed on ultra-short pulsed laser irradiation to semiconductors, but laser annealing using such ultra-short pulse with higher power densities has been investigated only by a few groups, i.e. from Osaka University [2-7], Harvard University [8, 9], Max Planck Institut [10] and University of Texas [11]. Why are the ultra-short pulses used for laser annealing? The reasons are as follows: (1) Physical mechanism of pulsed laser annealing of implanted Si is believed to be explained by the thermal melting model [12]. Up to now, however, the details of the dynamic behavior prior to melting have not been elucidated. On the other hand, a non-thermal model has also been proposed by Van Vechten (13]. Absorption and relaxation processes in the high power density more than -100 MW/ 2 cm are not as well understood as they are at lower power density. It is necessary, therefore, to study dynamics of the high density electron-hole(e-h) plasma excited and of the recrystallization induced by the pulsed laser. The 2 single picosecond pulse with extremely high power densities (10-100 GW/cm ) would be very useful for the study. (2) It is expected that an optimum pulse width should exist for pulsed-laser annealing. This will be mainly determined by the absorption processes, relaxation processes or, if the melting occurs, the maximum value of the meltfront velocity which is limited by the speed of the transition from the solid to liquid state, the optical absorption length and the thermal diffusivity of the hot liquid state. Therefore, it is necessary to investigate the influence of laser pulse width and shape on the annealing effects. (3) Even in the simplest experiment of picosecond single pulse annealing, several interesting effects are observed. Using a single 30-ps pulse at 1.06 jim,we have investigated the annealing effects and the dynamic behavior of recrystallization by the transient (timedependent) optical reflectivity and transmission measurements. In
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