First-Principles Prediction of Optical Absorption Enhancement for Si Native Defect Clusters under Biaxial Strain

Abstract

We use density-functional theory calculations to qualitatively explore the effects of fourfold-coordinated vacancy ͑V 4 ͒ and interstitial ͑I 4 ͒ clusters on optical absorption spectra in crystalline Si ͑c-Si͒ under selected conditions of biaxial strain ͑ = −3, 0, and 3%͒. While both native defect clusters enhance c-Si absorption by redshifting the absorption edge, we observe additional enhancement from biaxial strain. Increased strain magnitude tends to increase the absorption enhancement effect, but the optimal sign of strain exhibits a complementary relationship: compressive strain most effectively enhances V 4 absorption, while tensile strain most effectively enhances I 4 absorption. The absorption redshift as a function of strain correlates well with effective bandgap reduction, including the appearance of an intermediate band under certain conditions ͑ = −3 and 0%͒ for V 4 . Our results suggest that manipulation of native defect distributions and their strain fields can be used to engineer the Si absorption spectra. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3511714͔ All rights reserved.Manuscript submitted August 31, 2010; revised manuscript received October 14, 2010. Published November 10, 2010 Improvement in the net efficiency of crystalline Si ͑c-Si͒ solar cells has been impeded by intrinsic properties including low absorption and a bandgap ͑E g ͒ precluding photogeneration of carriers in the infrared. Low optical absorption inflates solar module cost by requiring thick substrates ͑ϳ100 m͒ 1 and has consequently generated strong interest in derivative thin film materials, such as hydrogenated amorphous Si ͑a-Si:H͒, 2 which significantly reduce the requisite substrate thickness ͑ϳ1 m͒ 1,3 through a serendipitous redshift of the absorption coefficient that accompanies disintegration of long range order. Other techniques to improve c-Si absorption include transition metal doping to create intermediate bands ͑IBs͒ ͑Ref. 4 and 5͒ and growth of Si nanowire arrays that promote broadband absorption. 6Optical methods 1 provide a non-destructive means of semiconductor characterization because optical properties are derived from electronic structure. Variations in phase 3 and defect-content 7,8 of Si produce significant changes in the optical absorption spectra, which suggests the plausibility of defect engineering 9 the spectra through structural manipulation. While c-Si absorption 10 is negligible below the 1.11 eV experimental bandgap, 11 Si absorption at lower energies is attainable through the participation of band tail extended states ͑Urbach region͒ or at even lower energies in the presence of localized defects, such as native defect clusters, that can generate states near the Fermi level. 7 The recent theoretical work of Pan et al. 12 elucidates an atomistic topological relationship connecting short ͑long͒ Si-Si bonds with valence ͑conduction͒ band character in the electronic structure of amorphous Si ͑a-Si͒, which suggests that control of strain distributions can modify the joint density of...