Page 8 - Carrier Recombination Activity and Structural Properties of Small-Angle Grain Boundaries in Multicrystalline Silicon

Jpn. J. Appl. Phys., Vol. 46, No. 10A (2007) J. CHEN and T. SEKIGUCHI

SE EBSD EBIC_300 K
(a) (b) (c)

4.5° 6°

<1° 2°

2.5° 1.5° Σ3
200 µm

(d)

Fig. 9. SE, EBSD, and room temperature EBIC images (a–c) of SA-GBs in mc-Si with Fe contamination at 1000 C followed by
air cooling. The changes in dislocation distance and EBIC contrast of contaminated SA-GBs with respect to the tilt angle are plotted
in (d).

dislocations at a smaller tilt angle and by boundary contamination levels. 10) Adding the results of SA-GBs, the
reconstruction at a larger tilt angle. classification table is updated and shown in Fig. 10. The
amount of contamination was categorized into four levles:

3.3 Effect of Fe contamination on SA-GBs clean, light (annealing at 800 C), moderate (900–1000 C),

Figures 9(a)–9(c) shows SE, EBSD, and EBIC images, and heavy (1100 C). In the clean mc-Si, the EBIC contrast
respectively, of a mc-Si sample with moderate Fe contam- of SA-GBs varied from 0 to 30%, while that of LA-GBs was

ination induced by 1000 C annealing. SA-GBs with tilt less than 2%. In the Fe-contaminated mc-Si, the EBIC

angles from <1 to 6 exist in this sample. In the EBIC image contrast of both SA- and LA-GBs increased. At each

taken at 300 K, all the SA-GBs except for SA <1 were contamination level, SA-GBs (except SA <1 ) always
observed as dark lines with strong EBIC contrast of about showed stronger EBIC contrast than LA-GBs, indicating

25–40%. SA <1 showed relatively weak contrast (<10%). that the SA-GBs are much more effective gettering sites for
The above results suggest that the SA-GBs become electri- Fe atoms than the LA-GBs. The high density dislocations at
cally active after Fe contamination and the predominant SA-GBs act as strong gettering sites. In contrast, the smooth
defect energy levels change from shallow to deep. The boundaries of LA-GBs (particularly 3) are well recon-
variation of 300 K EBIC contrast reflects the gettering ability structed and there are fewer defects acting as impurity
of impurities. The changes in dislocation distance and EBIC gettering sites.
contrast with respect to the tilt angle of SA-GBs are plotted
in Fig. 9(d). The distance is calculated from the dislocation 3.4 Effect of H passivation on SA-GBs
model. The EBIC contrast is plotted using experimental Figure 11 shows EBIC images of SA-GBs in clean mc-Si
data. It is clear that the EBIC contrast is strongly related to before and after H passivation. In the as-grown state, some
the distance between boundary dislocations and becomes SA-GBs already show strong contrast (5–15%) at 300 K and

saturated from 1.5 . For SA <1 , the distance between are denoted as ‘‘strong SA’’, while others with no obvious
boundary dislocations is sufficiently large for each to be contrast at 300 K are denoted as ‘‘weak SA’’. At 100 K, all

treated as an individual dislocation, while for SA >1 , the the SA-GBs showed strong EBIC contrast. The intragranular
dislocations tend to become closely spaced. The latter may defects also became visible at 100 K.
have much greater ability at gettering impurity atoms. After H passivation, the EBIC image at 300 K [Fig. 11(b)]
In the previous paper, we established a classification table did not change significantly except that the contrast of the
determining the EBIC contrast of LA-GBs with for Fe strong SA was reduced slightly. However, the EBIC image
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