Laser Quenching of Amorphous Si from the Melt Containing Dopants
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LASER QUENCHING OF AMORPHOUS Si FROM THE MELT CONTAINING DOPANTS S. U. Campisano*, D. C. Jacobson and J. M. Poate, AT&T Bell Laboratories, Murray Hill, NJ 07974; A. G. Cullis and N. G. Chew, Royal Signals and Radar Establishment, St. Andrews Rd., Malvern WRI4 3PS, England.
ABSTRACT" The formation of amorphous Si by the quench of a thin surface layer melted by fast UV laser irradiation has been investigated. The starting (Ill) surface layers were either pure or doped with As, Bi. In and Te by implantation. The asimplanted samples were recrystallized by ruby laser irradiation resulting in surface accumulation of Bi,In and Te. For the same UV irradiation condition, the amorphous layer formed in As, Bi, In or Te doped Si is about twice the thickness of the amorphous layer formed on pure Si. In the presence of the surface accumulation of Bi, In or Te, the amorphization results in an inward segregation of the dopant. For In, a very thin metal' layer -- 1I5A thick, is formed 150A beneath the amorphous surface. These results show that the amorphous-liquid interfacial segregation coefficients for Bi, In or Te are less than unity and that the amorphous solidification proceeds from the surface and bottom of the liquid layer. INTRODUCTION
The formation of thin amorphous silicon layers by ultrafast laser irradiation of Si single crystals has 3 2 41 35 been studied in detail.- ) Heat flow calculations " and in situ conductivity measurements have shown that the amorphous phase is obtained by quenching a liquid layer at solidification velocities exceeding 15 51 m/s and 12 m/s for (100) and (Ill) oriented Si respectively. Recent measurements have shown that the amorphous layer formed by picosecond laser irradiation in Bi doped Si is thicker than that formed in pure material. In this paper we report on two aspects of this amorphization process: firstly, the dependence of the amorphous thickness on dopant species within the liquid layer and, secondly, the segregation of impurities at the amorphous-liquid interface. EXPERIMENTAL 15
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Si(lll) crystals were implanted with In (120 keV, 2×10 at./cm ), Hi (150 keV, 1×10' at./cm ), As 5 2 5 2 4 2 (100 keV, 2×10' at./cm ) or Te (50 keV, 2×101 at./cm ; 50 keV 5×101 at./cm ) ions. The amorphous surface layers formed by the implantation process were transformed into single crystal by means of 30 ns, 2 ruby (A = 694 nm) laser pulses of 1.8-2.0 J/cm . These samples, together with unimplanted Si (Ill) wafers, were subsequently irradiated with frequency-doubled (A = 347 nm) pulses of 2.0 or 3.5 ns duration 2 to form amorphous surface layers. The energy density ranged from 0.1 to 0.4 3/cm with the homogenized laser beam being 0.3 cm in diameter. Rutherford backscattering and channeling and cross sectional transmission electron microscopy were used to analyse the irradiated layers. RESULTS AND DISCUSSION The high resolution channeling analysis of pure and Te doped Si(I 11) shown in Fig. I, indicates a rather large difference in the amorphized layer thickness. Both samples were irradiated wi
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