Page - 86 - in Photovoltaic Materials and Electronic Devices

3.3.1. PhononModeVariations inZnO Differences in ε for theZnOareexpectedwhenover-coatedwitha-Si:H.PECVD of a-Si:H raises the temperature of ZnO to 200˝C and exposes it to hydrogen in the plasma. There are many studies on the growth and various effects of annealing on the optical and structural properties of ZnO layers [65–70]. It is well known that the properties of ZnO layers are strongly affected by not only the deposition conditions but also the post-deposition annealing conditions or temperature treatments. Annealing has a large effect on the crystallinity of the layers in termsofgrainsize, residualstrain,andthedefectdensityascomparedto as-depositedfilms. AsnotedinTable5, theamplitudeof theCPPBoscillatorsofZnO over-coatedwitha-Si:Harefittoaccountforchangesin εoccurringduringPECVDof the a-Si:H layers. The increase in amplitude for higher energy absorption features in εandthedecrease infilmthicknesscomparedtothesamplewithouta-Si:Hcoatingat T = 200˝C indicate that the as-deposited ZnO film densifies and increases the degree of crystallinityafter annealingat thea-Si:H depositiontemperature. Thesevariations are generally consistent with literature [69,70], in that the imaginary component of the optical response related to electronic transitions increases in amplitude and sharpens. However, Liu et al. reports a decrease in the real part of the complex index of refraction in the transparent region,whichtheyattribute tovoidformingalong withlargercrystallinegrains. Inoursamples,voidformationisnotobservedoptically, however thedecrease in thickness implies thatcrystallitegrowthhasoccurredwhich coupled with the higher observed real part of ε indicates that this film is now overall moredenselypacked. Thecomparisonofdifferentphononmodes inZnOwithand withouta-Si:HcoatingisshowninFigure9. ThecharacteristicTOmodeswithA1 and E1 symmetryat0.0467eV(376.66cm´1)and0.0506eV(408.12cm´1)arepresentand able toberesolved[51–53]. Thevibrationalmodeat0.0847eV(683.15cm´1) canbe attributed to longitudinal optical (LO) mode with E1 symmetry [52]. The orientation ofgrains inthefilmcouldbeareasonforshiftingofmodestoslightlyhigheror lower wavenumbers. The splitting of the peak observed at 404 cm´1 for the uncoated ZnO into twoexpectedpeaksat377and408cm´1 forZnOover-coatedwitha-Si:Hand appearanceofthe683cm´1 modeislikelyduetograinsizeincreasesorareductionin defectdensityfromannealingatthea-Si:Hdepositiontemperatureof200˝C[68]. The increase in amplitude for ε2 of phonon mode at 408 cm´1 also supports the idea that grain restructuring and material densification occurs. The presence of an additional absorptionmodeat1306.62cm´1 (0.162eV)canbeassociatedwithoxygen-hydrogen (O-H) bonds in the thin film, such as the formation of zinc hydroxide or absorbed water or stretching modes of hydrogen bonded to heavier elements like zinc [54]. The large broadening of this absorption peak could be due to the modification or damageto theZnOasaresultofexposure tohydrogenin theplasma. 86