Page - 67 - in Photovoltaic Materials and Electronic Devices

ellipsometric spectra is analyzed and full understanding of the underlying structure is not required in the analysis procedure. When combined withΣσ-minimization approaches for structurally graded Si:H, VIA yields ε for both the nc-Si:H and a-Si:H components as well as the time and bulk layer thickness dependence of component material fractions in the overlayer of material accumulated between eachpseudo-substrateandsubsequentdatasetpair. These techniquesareusedto provideguidancefor thedepositionand in situcharacterizationofa-Si:H,nc-Si:H, andmixed-phase (a+nc)-Si:Hlayersduringgrowth indevicestructures. In addition to RTSE studies of material growth evolution limited to the near IR toUVspectral range,wealsohaveused exsitu, roomtemperature IR-extendedSE (IR-SE) from 0.04 to 0.75 eV using a Fourier transform IR ellipsometer operating over this spectral range. Thisextensionenablesspectra in ε forSi:HandBRcomponents layers to be determined from the mid-IR to UV wavelength range. Simultaneous analysisofellipsometricspectracollectedfrommultiplesamplesconsistingofBRand i-layer/n-layer/BRstacksdepositedonborosilicateglasssubstrateswasusedtoyield acommonε foreachlayerwhilestructuralparameterssuchasdb andds maybevaried separatelyas inRTSEdataanalysis [4,7,11,13–15,27,28]andsimilar inmethodology to thedividedspectral rangeapproach[32]. Acommonparameterizationof ε isused tofit exsituellipsometricspectracollectedfromseparatenear IR-UVmultichannel andFTIRellipsometers. Thisapproachyieldsacontinuoussetofspectra in ε foreach material, although theparticular beamspot location onthe samplesurface maynot be the same during measurement using each ellipsometer [33]. The results of the analysisof IR-SEdatawereusedtostudyabsorption in theBRcomponents,which can be used to extract electrical transport properties and phonon modes. In addition, εfor protocrystalline intrinsic a-Si:H extracted from IR-SE data is sensitive to the silicon-hydrogenbondingconfiguration. Comparisonofopticalabsorptionfeatures affords a method of assessing film structural and chemical character, which then suggestswaystoimprovematerialqualityandpotentiallydeviceperformance[34,35]. PVdevicesincorporatingoptimizationprinciplesbasedonIRspectroscopyandRTSE analyzed for each layer in the device configuration have exhibited relatively high performance [23,34]. InsituRTSEstudieshavebeenusedtoyieldgrowthevolutionphasediagrams of each doped and undoped Si:H layer in the n-i-p PV device configuration and structuralevolutionprofilesofcrystalliteandvoidfractions. The informationfrom growth evolution diagrams was used to design PV device structures, lacking the p-layerandincorporatingonly theamorphousphase, forcharacterizationusing ex situ IR-extended SE. Results of ex situ IR-extended SE combined with ex situ near IR-UV SE have been used to obtain spectra in ε from 0.04 to 5.0 eV for ZnO and a-Si:H in the BR/n-i-p a-Si:H solar cell configuration. Higher energy transitions related to electronic structure in Ag, ZnO, and a-Si:H; the band gap in ZnO and 67