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DESIGN CONCEPTS OF TF-SI SOLAR CELLS 347
800
600
(a) Before
processing
Al
Si (220)
Intensity
[CPS]
400
200
0
Al
2500
2000
0 20 40 60 80 100 120 140
2 q
[deg]
(b) After
processing
Si (220)
1500
Intensity
[CPS]
1000
500
Si (111)
Al
Al
0
0 20 40 60 80 100 120 140
2 q
[deg]
Figure 8.27 XRD spectrum of a 2-µm-thick HWCVD Si film, deposited on Al/Cr/glass at about
500 ◦ C (a) before optical processing; and (b) after optical processing at ∼480 ◦ C for 3 min. A significant
increase in Si (220) compared to Si (111) orientation can be seen in (b)
toward a-Si. From this figure, it is clear that 3 min of optical processing time was not
sufficient to crystallize the entire film at 460 ◦ C. Figure 8.29(b) is a TEM plan view of
the film showing distribution of grain sizes near the Si–Al interface.
Detailed profiling of Al across the crystallized films, done by SIMS (secondary
ion mass spectroscopy) and micro-X-ray, showed that in optically crystallized films the
Al concentration dropped by several orders of magnitude within about 1 µm from the
Al-Si interface [81, 83]. These results indicate that Al is required only for the initiation
of crystallization. After the nucleation, the crystallization front propagates into a-Si while
grain enhancement occurs in the crystallized film. It is believed that vacancy injection
from Al-Si interface is responsible for grain enhancement.
Investigation of the possibility of applying optically assisted MIC showed that
crystallization of a-Si can start at about 200 ◦ C in less than 3 min. This crystallization