• PDF / 384,353 Bytes
• 7 Pages / 420.48 x 639 pts Page_size
• 28 Downloads / 213 Views

DOWNLOAD

REPORT

---

INTRODUCTION Solid state laser annealing may be a way to measure atomic diffusion coefficients, as shown by MATSUMOTO ill for the cases of As and P in Si. This technique is in principle well adapted to the study of fast-diffusing impurities since it allows to choose short effective diffusion times teff. For the case of Fe in Si, first studied by STRUTHERS 121, interstitial diffusion is very fast (D = 10- 6 cm2s- 1 near 900 0 C), so that Fe diffusion in an oven for some minutes is currently used to obtain homogeneous Fe-doped wafers. Another analogous case might be that of intergranular diffusion, into polycrystals, of normally slowdiffusing impurities like Al, at least if we envisage a possible extrapolation at high temperatures of results recently obtained near 400 0 C 131. We have made measurements of D for Fe and Al in Si using semi-continuous laser annealing, i.e. a continuous laser chopped to give pulses of typically some milliseconds; impurity profiles were measured by SIMS and interpreted using a model that we derived following the line of GOLD and GIBBON's model 1hi. An important feature seeked from these experiments is the behaviour of electron lifetime killing impurities like Fe or Al in the growth of epitaxial solar cells. When using a cheap impure substrate, the quality of the CVD-grown active layer is determined by the presence of such impurities. Conversely, one can tolerate in the substrate an impurity content 10 times larger than the one acceptable near the surface of the active layer if the latter is grown in 20 minutes, for a diffusion coefficient of io-8cm2s-l : the exact knowledge of D as a function of temperature T is a prerequisite to define an acceptable purity for the substrate. It can also help tailoring the gettering step which may be used before CVD j51. This goal explains why we were particularly interested in the possible dependence of DFe on the boron content in the diffusion sample : large boron doping of the back of the CVD layer has been proposed to improve the cell 151; on the other hand it is known that the diffusion kinetics may be affected by donor-acceptor interactions or by the formation of complexes 161. The diffusion of Fe in Si raises a special problem, since two diffusion kinetics have been observed 171. MODELLIZATION OF LASER ANNEALING A knowledge of the temperature distribution inside the illuminated solid is Mat. Res.

Soc. Symp. Proc. Vol. 13 (1983) QElsevier science Publishing Co.,

Inc.

222 necessary to account for laser annealing

181

141 1Ii.

The thermal effects indu-

ced by a N laser are well described by the usual heat diffusion equation : pC(3T/Dt)

- V(K.VT)

= Q

(I)

Where Q is the absorbed radiant energy and K, p, C are the heat conductivity, volumic mass and thermal capacity of the solid. Equation (1) is solved after being linearized 191 1101 and integrated using the Green source function. To write the analytic solution we assume the silicon reflectivity R to be constantly equal to 0.4 1111. Furthermore, like NISSIM 1101we assume the thermal conductivi