Faster Quenching by Silicon Pulsed Laser Annealing Under Water
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FASTER QUENCHING BY SILICON PULSED LASER ANNEALING UNDER WATER A.POLMAN, S. ROORDA, S.B. OGALE* and F.W. SARIS
FOM-Institutefor Atomic and Molecular Physics Kruislaan407,1098 SJ Amsterdam, The Netherlands ABSTRACT A novel method of pulsed laserprocessing of ion-implantedsilicon is presented, in which samples are irradiatedin water ambient. The water layer in contact with the silicon during irradiationhas a considerable influence on melting and solidificationdynamics. Still, perfect epitaxy of a thin amorphous layer can be obtainedusing this method. For epitaxy to occur on a sample irradiatedunder water, 40 % more absorbed energy is necessary thanfor a sample irradiatedin air.This indicates the occurrence of a considerable heat-flow from the silicon into the water layer during the laser pulse. From impurity redistributionafter irradiationit is found that by processing a sample under water liquid-phase diffusion is reduced.Diffusion theory arguments indicate thatthis can be due to a reduction in total melt duration by about afactor2-3. This can be due to faster cooling of the liquid silicon layer after the laser pulse whereas the melt-in time might be influenced as well. As a consequence, shallower impurity profiles can be obtained in crystalline silicon. No oxygen incorporationis detected and the surface morphology is not disturbed using this new process. 1. INTRODUCTION In recent years there has been considerable interest in pulsed laser processing of semiconductors for use in electronic or energy conversion devices 1' 2 . In numerous situations pulsed laser irradiation is applied on thin ion-implanted silicon surface layers for impurity incorporation and epitaxial regrowth. At present, melting and resolidification processes are commonly understood from the results of extensive work on heat-flow phenomena and phase transformations involved. Impurity redistribution occurring during melting and solidification
can generally be understood from mass diffusion and segregation arguments. Thus, by now, impurity profiles and structural state after irradiation can be predicted for given parameters such as laser energy density and pulse duration, laser wavelength absorption depth and
thermodynamical properties of the material, mass diffusion and segregation coefficients for the specific dopants etc. Many intriguing aspects of pulsed laser irradiation such as explosive crystallization are still being explored'. Pulsed laser irradiation is a promising technique to form shallow dopant profiles in low-energy implanted Si substrates due to the small laser skin depths and high quench rates. Irradiation at the threshold laser energy for melting through the amorphized layer results in epitaxial regrowth and in broadening of the dopant profile as a result of liquid-phase diffusion4 . The total broadening obtained after irradiation is determined by the integral melt duration. If this duration could be reduced for the same melt depth, it would be possible to obtain shallower dopant profiles in epitaxially crystallized material. In th
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