Page 2 - Carrier Recombination Activity and Structural Properties of Small-Angle Grain Boundaries in Multicrystalline Silicon

Japanese Journal of Applied Physics
Vol. 46, No. 10A, 2007, pp. 6489–6497
#2007 The Japan Society of Applied Physics

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

Jun CHEN and Takashi SEKIGUCHI
Advanced Electronic Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
(Received June 5, 2007; accepted July 17, 2007; published online October 9, 2007)
The carrier recombination activity and structural properties of small-angle (SA) grain boundaries (GBs) in multicrystalline Si
(mc-Si) were systematically investigated by electron-beam-induced current (EBIC) and by transmission electron microscopy

(TEM). At 300 K, SA-GBs with a tilt angle from 0 to 10 generally showed weak EBIC contrast (0–10%) with a maximum

contrast appearing at 2 , while some special SA-GBs with a tilt angle of 2–3 showed particularly strong contrast (30%). At
a low temperature (100 K), all the SA-GBs showed strong EBIC contrast despite the tilt angle. Possible explanations for the
variation of the EBIC contrast are discussed in terms of the interaction and reconstruction of the boundary dislocations. When
decorated with a metal impurity such as Fe, the SA-GBs showed much stronger EBIC contrast than the large-angle GBs,
indicating that SA-GBs are effective gettering sites for impurities due to their particular boundary dislocation structures.
[DOI: 10.1143/JJAP.46.6489]
KEYWORDS: small-angle, grain boundaries, mc-Si, EBIC

however, we have found that the recombination activity of
1. Introduction 14)
some SA-GBs was also weak at 300 K. Such a difference in
It is well known that crystallographic defects such as results probably originated from the difference in the boun-
dislocations and grain boundaries (GBs) are harmful to the dary structure or in the contamination level of the impurity.
performance of semiconductor devices since these defects The difference in the boundary structure, i.e., SA-GBs
act as the recombination centers of minority carriers. Early with different structures may result in different electrical
reviews on GBs in semiconductors can be found in the activity. In contrast with the well-reconstructed LA-GBs,
literature. 1,2) According to the previous studies, GBs may act SA-GBs are composed of tilt or twist boundary dislocations
as carrier recombination centers of shallow or deep levels. as well as secondary defects. It is also speculated that broken
The GB character and impurity decoration affect the bonds may exist at the boundary. The different boundary
recombination activity of GBs. 3–9) Studies on the recombi- defects result in different electrical properties. Thus, to
nation activity of GBs in semiconductors are prompted by elucidate the effect of the boundary structure, it is necessary
the rapid development of multicrystalline silicon (mc-Si) as to examine different SA-GBs with tilt or twist components
a promising photovoltaic material. within a certain range of misorientation angles. In this study,
Recently, we have presented new results to contribute to avoid the extrinsic effect on the recombination activity of
to the ongoing discussion on the recombination activity of SA-GBs, high purity mc-Si ingots were used. The recombi-
GBs in Si. 10,11) By investigating different GBs in differently nation activity and structural properties of SA-GBs with a

Fe-contaminated mc-Si materials using an electron-beam- misorientation angle of 0 –10 were systematically inves-
induced current (EBIC), it was found that in clean samples, tigated as reported in §3.1.
despite the GB character, the EBIC contrast of large-angle On the other hand, most SA-GBs found in mc-Si are tilt
(LA) GBs (including special and random boundaries) boundaries. With the development of the direct Si wafer-
was rather weak at 300 K. This suggests that the intrinsic bonding technique, artificial boundaries with tilt or twist
recombination activity of LA-GBs is weak due to the factors can be fabricated free from metallic contamination.
boundary reconstruction. However, when decorated with Fe, The interfaces of directly bonded Si wafers can be regarded
the room-temperature EBIC contrast of LA-GBs increased as ideal boundaries. 15,16) Artificial small-angle boundaries
with respect to the GB character. Random and high- fabricated by Si wafer-bonding were also characterized for
boundaries exhibited stronger EBIC contrast than low- comparison. The recombination activity and structural
boundaries. The impurity-gettering ability was in the order properties of artificial small-angle boundaries are discussed
of random > high- > low-. In addition, the boundary in §3.2.
plane also played an important role in the recombination The contamination level of the impurity also plays an
strength of GBs in contaminated mc-Si. For example, among important role in the electrical properties of GBs. Fe is the
the 3 boundaries, 3 with {111} boundary planes was less most frequently found and detrimental impurity in mc-Si.
electrically active than 3 with other boundary planes. For Previous studies have found that LA-GBs with an Fe
3
LA-GBs, it is clear that LA-GBs with perfect periodicity and contamination of as low as about 10 13 atoms/cm showed
without any impurity contamination should be electrically significant recombination activity at 300 K. 10,17) For SA-
inactive. GBs, it is also necessary to investigate the impact of Fe
In contrast with LA-GBs, the recombination activity of contamination. By intentionally inducing Fe contamination
small-angle (SA) GBs has not been systematically inves- through thermal annealing at different temperatures, the
tigated. Some researchers have reported that the SA-GBs impact of Fe on the recombination activity of different
exhibited high recombination strength at 300 K. 12,13) Recently SA-GBs was studied and is reported §3.3. A comparison
between the recombination activity of SA- and LA-GBs

E-mail address: CHEN.Jun@nims.go.jp under different Fe-contamination levels is also given in §3.3.
6489