Low-Temperature Preparation of Poly-Si Thin-Films Having Giant Grains
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177 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
highest intensity losses its excess thermal energy Qo) per unit area and solidifies completely. Since heat flows vertically to the substrate by diffusion, Qex should be written approximately as Q., = Cý,(Tý - T,
FrD,,) t,
.'
x
(1)
i_
I _U
0
w%;L V-'sub,
Ir
'C,
Ir
A T_%
'sub a"'u
"sub
aL
+U U
uVW. IiCaI
[Si02 substrate
I
Fig.l Schematic view of lateral growth mechanism
capacitance per unit volume of the substrate, the critical temperature for crystallization, the substrate temperature and the diffusion constant of the substrate, respectively, and t is the time for complete solidification after the laser light irradiation, that is, the lateral growth time. Since the lateral-growth velocity v is on the order of lOm/s [8] at the most, the maximum lateral-growth length L is proportional to t. From these relationships, L is concluded proportional to Cub"2. Since heat capacitance per unit volume of matter is predominantly determined by number density of atoms, it can be reduced dramatically only by making many pores inside the substrate. Guided by these considerations, we 7. Transparentelectrode have proposed recently a new poly-Si thin6 film solar-cell structure shown in Fig.2 [5]. . Si epitaxial layer The active layer is a Si epi-layer grown on .. Si seed layer Si a thin Si seed layer. Since the grain size in 4.Low meltingpoint layer the active layer equals to that in the seed
1
3 Dense Si0 2 layer layer, the gain size of the seed layer plays a critical role for solar cell efficiency. ...... 2.PorousSi0 2 layer Combination of the phase-modulated 1.Substrate excimer-laser annealing (PMELA) method [7] and a porous Si0 2 substrate with porosity P is very effective to grow giant Fig.2 Proposed solar cell structure grains for the seed layer. There are also the low melting-point layer and the dense Si0 2 thin layer. The low melting-point layer plays another important role for enlargement of grain size by reduction of T, and resulting enlargement of v, but is not discussed in this paper. The dense Si0 2 thin layer is of a flat and high-quality layer, which is very effective for burying small pores on the porous Si0 2 surface and for making a barrier against mixing effects between the Si seed layer and the porous Si0 2 substrate during PMELA. Since this dense Si0 2 layer with thickness do, acts to store a part of Qex, thermal equilibrium equation for the structure shown in Fig.2 should be modified as Q,
=Cos(T -T+s+){do, +(1-P)(-do, +
f;DO-t)},
(2)
for the case dox
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