Effect of grain alignment on lateral carrier transport in aligned-crystalline silicon films on polycrystalline substrate
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Effect of grain alignment on lateral carrier transport in aligned-crystalline silicon films on polycrystalline substrates Woong Choia) and Alp T. Findikoglub) Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Manuel J. Romero and Mowafak Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 14 September 2006; accepted 13 December 2006)
We report the studies on the effect of grain alignment on lateral carrier transport in nominally 〈001〉-oriented aligned-crystalline silicon (ACSi) films on polycrystalline substrates. With improving grain alignment, energy barrier height at the grain boundaries was reduced from 150 to less than 1 meV, and both conductivity and Hall mobility became less sensitive to hydrogen passivation. This suggests that the dangling bonds in ACSi films are a major source of trapping sites, and that they become less dominant with improving grain alignment. These results demonstrate that improving grain alignment enhances the lateral carrier transport in small-grained (艋1 m) polycrystalline silicon films, by reducing dangling bond density at the grain boundaries.
Polycrystalline Si thin films on non-single-crystalline substrates have been of great interest for applications in thin-film transistors (TFTs) and solar cells.1,2 However, energy barriers induced by charge trapping at grain boundaries severely degrade lateral carrier transport in polycrystalline Si.3 While grain-boundary modification by either hydrogen passivation or grain recrystallization has been widely studied in research literature, little attention has been given to the effect of improving intergrain alignment on lateral carrier transport in polycrystalline Si. Recently, we reported a grain-alignment technique that uses ion-beam-assisted deposition texturing to grow nominally 〈001〉-oriented Si thin films, called aligned-crystalline Si (ACSi), on flexible polycrystalline metal tapes.4 For ACSi films with good grain alignment, an analysis based on Hall mobility measurements indicated negligible energy barrier at the grain boundaries.5 It also showed that energy barrier caused by charge trapping at the grain boundaries depended strongly on grain alignment.5 However, that study did not establish a a)
Present address: Applied Materials Inc., Sunnyvale, California 94085. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0105 J. Mater. Res., Vol. 22, No. 4, Apr 2007
relationship between the Hall mobility and the drift mobility (conductivity) in ACSi films, nor did it identify the origin of trapping sites at the grain boundaries. In the work presented here, we expand our earlier experimental work by providing measurement results of both Hall mobility and conductivity and their dependence on hydrogen passivation. Also, we discuss the origin of trapping sites at grain boundaries by comparing effects of hydrogen passivation on ACSi samples with different grainalignment. The ACSi films were grown on polycrystalline me
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