Velocity and Orientation Dependence of Solute Trapping in Si
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Laser and Electron-Beam Solid Interactions and Materials Processing
67
VELOCITY AND ORIENTATION DEPENDENCE OF SOLUTE TRAPPING IN SI
P. BAERI*, G. FOTI* AND J. M. POATE Bell Laboratories, Murray Hill, New Jersey 07974 S. U. CAMPISANO AND E. RIMINI Istituto di Struttura della Materia dell'Universita, corso Italia 57-Catania, Italy A. G. CULLIS Royal Signals and Radar Establishment, Malvern, Worcs WR143PS, England
ABSTRACT The segregation phenomena of In, Ga and Bi in Si have been investigated as a function of the liquid-solid interface velocity following laser irradiation. The crystallization velocity has been changed within the range 0.8-5 m/s by varying either the substrate temperature during irradiation or the laser pulse duration. The measured interfacial segregation coefficients depend critically on the velocity and on the crystal orientation of the solidifying plane.
INTRODUCTION Laser irradiation of ion implanted semiconductors has been used to produce supersaturated solid solutions of such dopants as As, Bi, Te, Sb and Ga [1,2,31. The occurrence of such supersaturated solutions has been explained in terms of kinetic trapping at the rapidly moving liquid-solid interface [4,5]. For standard irradiation conditions the solidification velocity is of the order of a few m/s and the quenching rate is of the order of 109 K/s. In earlier work we demonstrated experimentally that the amount of segregation of Pt [6] and In [71 in Si depends markedly on the velocity of the liquid-solid interface. In the present work we extend the measurements to Ga and Bi as a function of velocity and substrate orientation.
VELOCITY OF THE LIQUID-SOLID INTERFACE The velocity of the liquid-solid interface is given by the rate of dissipation of the latent heat: Lpv = K-oT (9z where L, p, and K are latent heat, density and thermal conductivity at the interface of the considered OT material and -T is the temperature gradient in the solid. This last parameter can be varied by az changing either the substrate temperature or the laser pulse duration. In the first case one has to take into account the dependence of K on temperature. It has been shown that for irradiation with 25 ns Nd pulses this velocity increases by a factor of 3 on going from 600 to 77 K [6]. On the other hand irradiation with different pulse lengths will produce different heat diffusion conditions and thus different * Permanent address, Istituto di Struttura della Materia dell'Universita, Catania
68 temperature gradients. Results of calculations [7] are shown in Fig. I where the liquid-solid interface velocity is reported as a function of the pulse energy density for 15 and 50 ns ruby laser pulses. Note that the energy density required to melt the same thickness of amorphous silicon depends on the pulse duration and therefore different interface velocities result.
0.9
RUBYLASER ON Si 1.1 1.3
1.5
1.7
6 E
z o
I..-
LAYER
0
D
1.3
0.1
0.1
t5
0.
0
1.7
1.9
j/cn2
043
0.4
MAX MOLTEN THICKNESS(/jm)
Fig. 1.
Calculated mean values of liquid-solid inte
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