Impurity Redistribution in Ion Implanted Si After Picosecond Nd Laser Pulse Irradiation
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ABSTRACT Impurity redistribution in Bi-implanted Si and in Asimplanted Si has been investigated after irradiation with 25 ps Nd(X=l.06 pm) laser pulse in the energy range 0.1-1.5 2 J/cm . Channeling effect in combination with 2.0 MeV He+ backscattering in glancing detection has been used to characterize the epitaxial crystallization, the impurity location and its depth distribution. The amorphous to single crystal transition occurs at an energy density of about 2 0.4 J/cm . Bi atoms are located after crystallization in substitutional lattice sites for the in depth part of the distribution. Part of the Bi atoms accumulated at the sample surface and the amount of segregation increases with the pulse energy density and depends on the substrate orientation. A computer model has been also developed to calculate several parameters of interest, as the melt threshold, the melt duration, the carrier temperature etc including a detailed description of the absorption and of the energy relaxation processes. The calculations indicate that the simple thermal description accounts quantitatively for the experimental data on melt duration and impurity segregation.
INTRODUCTION Several phenomena caused by irradiation of semiconductors with high power picosecond laser pulses have been recently investigated[l,2J. Crystal to amor phous transition occurs in silicon irradiated at 0.53 pm wavelength in the einer 2 gy density range 0.20-0.25 J/cm [3]. Detailed analysis of the number of emitted charged particles (electr6ns and ions) during irradiation has shown that above a well defined.energy density value, positive and negative particles are detected in equal amounts. Electron and lattice temperature remain then practically equal during irradiation and the energy relaxation time between electrons and phonons should be then lower than 10-11s. This estimate is also supported by time-resolved reflectivity measurements performed on Si single crystals both on nanosecond and on picosecond time scale. It has been found [4] f6r instance that the reflectivity measured with a probe delay of 100 ps
Mat. Res. Soc. Symp. Proc. Vol. 13 (1983) QElsevier Science Publishing Co.,
Inc.
274 and with 1.06 pm wavelength increases abruptely for an energy density sligtly 2 larger than 0.2 J/cm _0.53 pm pump pulse. The measured value of 0.76±0.03 is characteristic of molten silicon at the probe wavelength and the rise by itself indicates the occurrence of a first order transition. Effects associated to the overheating of the silicon are seen at larger energy density value but disappear with time due to the cool down of the liquid silicon layer. All these experiments point out that a thermal model remains valid also on this time scale, in spite of the large electron-hole concentration and of plasma effects. Other phenomena, mainly investigated in the nanosecond irradiation regime, are related to the impurity behaviour. Solidification of the molten layer occurs at a solid-liquid interface velocity of a few m/s, several orders of magnitude higher than conventio
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