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348 THIN-FILM SILICON SOLAR CELLS
Si(111) intensity
[CPS]
1400
1200
1000
800
600
400
3-µm
10-µm
6-µm
200
450
700
600
460 470 480 490 500 510
Processing temperature
[°C]
(a)
10-µm
Si(220) intensity
[CPS]
500
400
300
3-µm
6-µm
200
100
450
460 470 480 490 500 510
Processing temperature
[°C]
(b)
Figure 8.28 Intensity of (a) Si (111) and (b) Si (220) peaks in the XRD spectra of processed
samples as functions of processing temperature. A “jump” of peak intensity around 490 ◦ C can be
observed for the 10-µm-thick sample
becomes much stronger at temperatures close to 450 ◦ C. By controlling the process
conditions, it is possible to confine Al to the vicinity of the Si–Al interface, leaving
the crystallized film (away from the interface) with a low Al concentration. In contrast,
thermally crystallized films have nearly uniform and high concentration of Al. Some films,
thermally processed at higher temperatures, may exhibit lateral nonuniformities because
of segregation into Si-rich and Al-rich phases.
The proposed explanation of the optically assisted, Al-induced crystallization and
grain enhancement of thick samples is as follows:
• Optical processing generates a nonuniform temperature distribution within the film
structure, especially at the Al-Si interface where energy can be locally absorbed, producing
a higher temperature spike.
• In high-temperature (>450 ◦ C) optical processing, although the monitored film surface
temperature is lower than the eutectic point of an Al-Si system, melting in the local