Page 6 - 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

SA <1° SA 2.5°

SA 5° SA 9.5°

Σ3 R

Fig. 5. TEM images of SA (<1, 2.5, 5, and 9.5 ), 3 and R GBs in mc-Si.

interaction between two neighbor dislocation cores is 3.2 Artificial SA-GBs in bonded Si
thought to occur at a certain distance. This hypothesis is Figure 6 shows EBIC images of the interfaces (artificial
under detailed study. SA-GBs) of bonded Si with different tilt angles of 0 (1),

Third, for the particular strong EBIC contrast of SA2–3 0.4, 1, and 3 taken at 300 and 100 K. The interfaces are
at 300 K, we think there are two possible explanations. The perpendicular to the observation plane. The EBIC contrast of
most probable explanation is that it occurs because of the these artificial SA-GBs is weak at 300 K and strong at 100 K.
high density of defects and the strain field around SA-GBs. Figure 7 shows the average EBIC contrast at both temper-
Comparing the EBIC images in Figs. 2 and 3, it was found atures with respect to the tilt angle. The strongest EBIC

that the density of intragranular defects changed from grain contrast was found at the 1 boundary, with contrasts of 7%
to grain. For special SA-GBs with strong contrast at 300 K, at 300 K and 30% at 100 K. Figure 8 shows TEM images of
the strong contrast was always accompanied with a high these boundaries. The 1 boundary consists of a regularly
density of intragranular defects [see Figs. 3(d) and 3(h)]. arranged square network of screw dislocations due to the

The high density of intragranular defects was probably small twist component (0.3 ). The 0.4 and 1 boundaries are
created by the strong strain in these grains. It is speculated composed of a regular array of edge dislocations. The

that these grains were formed in the ingot after solidification. dislocations in the 0.4 boundary are not straight, probably

During the high-temperature and long-time cooling proce- due to the small twist component. The 3 boundary is also
dure, crystal defects may accumulate and react with each composed of an array of dislocations, but each dislocation
other to form sub-boundaries. Since these sub-boundaries are could hardly be distinguished due to the small spacing.
formed after solidification, the residual strain field may have The EBIC results of artificial SA-GBs in bond Si
not been fully relaxed and SA-GBs may possess strong corresponded very well to those of SA-GBs in the clean
electrical activity. The second explanation is related to the mc-Si. For example, the artificial SA-GBs also had strong
impurity contamination. Although our mc-Si material is of EBIC contrast at 100 K, indicating the existence of recom-
high purity, for some reason a source of impurities may still bination centers with high density, which originated from the
exist. Such impurities might diffuse out from the source and dislocation bundles at the boundaries. The EBIC contrast of

be gettered by SA-GBs after solidification. In the recombi- SA-GBs has a peak value at a certain tilt angle (2 in mc-Si

nation model of dislocations proposed by Kveder et al., 29) and 1 in bonded Si). Although only four artificial SA-GBs
it was suggested that the aggregation of small number of were observed, some tendencies of the variation of EBIC
impurity atoms at the dislocation core would lead to the contrast with tilt angle still exist, that are similar to those
marked enhancement of recombination activity at disloca- in the mc-Si. Thus, the discussion in §3.1 also holds for
tions. The SA-GBs with strong EBIC contrast at 300 K artificial SA-GBs. For SA-GBs, the EBIC contrast was
might be contaminated with impurity atoms, such as Fe. predominantly determined by the density of boundary
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