Impact of Grain Boundary Character on Electrical Property in Polycrystalline Silicon
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ABSTRACT Grain boundaries in polycrystalline silicon are most likely to generate localized states in band gap. The localized states play a dominant role in determining the performance of solar cells by acting as traps or recombination center of carriers. In the present investigation, the scanning electron microscope - electron channeling pattern(SEM/ECP) method and SEM - electron back scattered diffraction pattern(SEM/EBSD) technique were applied to characterize the grain boundaries in p-type polycrystalline silicon with 99.999%(5N) in purity. Thereafter, temperature dependence of electrical activity of individual grain boundaries was measured by an electron beam induced current(EBIC) technique. It has been found that temperature dependence of EBIC contrast at grain boundaries can change, depending on the misorientation angle the orientation of the boundary plane. The results can be explained by the difference in the position of the localized state within the band gap on the basis of the Shockley-Read-Hall statistics. The 1111 }I 3 symmetrical tilt boundary has shallow states, while high I boundaries have deep states. Low angle boundaries reveal high electrical activities. The EBIC contrast at low angle boundaries was found to increase with increasing misorientation angle up to 20, followed by an almost constant value. High electrical activity at low angle boundaries is probably attributed to a stress field of primary dislocations forming low angle boundaries.
INTRODUCTION Recently, it has been demonstrated that the bulk properties of polycrystalline materials and service performance can be greatly improved by properly controlling grain boundaries[l]. In case of polycrystalline solar cell materials such as silicon, it is well known that the grain boundary can act as carrier recombination site reducing the efficiency of performance of photovoltaic energy conversion[2]. This is because in the photovoltaic materials, the double Schottky barriers are developed at grain boundaries, resulting in the minority carriers being trapped or recombined at defect level in the band gap at the boundary. However, it is not necessary that the individual grain boundaries in the material always have the same property. In turn, grain boundary related phenomena must depend on the grain boundary character and structure. For instance, the electrical properties of grain boundaries in silicon are reported to be affected by the crystallographic characters associated with the boundary[3-6]. From the reported studies of electrical property of grain boundary in silicon, we see that symmetrical twin boundaries such as 1111 }Y3 and {122119 coincidence boundaries with rotation axis, {150}113 and 11701 -25 ones with axis have no boundary level and no 163
Mat. Res. Soc. Symp. Proc. Vol. 586 ©2000 Materials Research Society
potential barrier in as-grown crystals[3,4]. However, those symmetrical boundaries, which are (I II 113, {122}X9 and ( 150}113, were not electrically active, and that the {170}X25 boundary was active after annealed in PoC13 atm
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