Seite - 73 - in Photovoltaic Materials and Electronic Devices

Table 2. Parameters describing complex dielectric function (ε = ε1 + iε2) and structure for a semi-infinite Ag film on a borosilicate glass over coated by Cr beforeZnOdeposition. Experimentalellipsometricspectrawerecollected in situ after deposition at room temperature in the spectral range from 0.734 to 5.88 eV andfitusing least squareregressionanalysiswithanunweightedestimatorerror function,σ= 5ˆ10´3. For bulk Ag, the parameterization of ε consisted of a Drude oscillator, two oscillators assuming critical point parabolic bands (CPPB), and a constant additive term to ε1 denoted ε8. Spectra in ε for the 30˘ 2 Å surface roughness layerwereparameterizedwith twoLorentzoscillatorsand ε8=1. AgSurfaceRoughness Oscillator A(Unitless) Γ (eV) E0 (eV) - - Lorentz 4.2˘0.2 2.5˘0.1 5.17˘0.02 - - Lorentz 1.0˘0.3 0.06˘0.03 3.61˘0.01 - - BulkAg Oscillator A(Unitless) Γ (eV) En (eV)   Figure  1.  Complex  dielectric  function  spectra,  ε  =  ε1  +  iε2,  (arrow  pointing  left  for  ε1  axis,  arrow  pointing  right  for  ε2  axis)  from  0.734  to  5.88  eV  for  a  semi‐infinite  Ag  film  parameterized  with  a  combination  of  a  Drude  oscillator  and  two  oscillators  assuming  critical  point  parabolic  bands  (CPPB)  with  parameters  listed  in  Table  2. Table  2.  Parameters  describing  complex  dielectric  function  (ε  =  ε1  +  iε2)  and  structure  for  a    semi‐infinite  Ag  film  on  a  borosilicate  glass  over  coated  by  Cr  before  ZnO  deposition.  Experimental  ellipsometric  spectra  were  collected  in  situ  after  deposition  at  room  temperature  in  the  spectral  range  from  0.734  to  5.88  eV  and  fit  using  least  square  regression  analysis  with  an  unweighted  estimator  error  function,     =  5  ×  10−3.  For  bulk  Ag,  the  parameterization  of  ε  consisted  of  a  Drude  oscillator,  two  oscillators  assuming  critical  point  parabolic  ban s  (CPPB),  and  a  constant  additive  term  to  ε1  denoted  ε .  Spectra  in  ε  for  the  30  ±  2  Å  surface  roughness  layer  were  parame erized  with  two  Lorentz  oscillators  and  ε   =  1.    Ag  Surface  Roughness Oscillator  A  (Unitless)  (eV) E0 (eV) ‐  ‐ Lorentz  4.2  ±  0.2 2.5  ±  0.1 5.17  ±  0.02 ‐  ‐ Lorentz  1.0  ±  0.3 0.06  ±  0.03 3.61  ±  0.01 ‐  ‐ Bulk  Ag Oscillator  A  (Unitless)  (eV) En ( ) Ө  (degrees)  μ CPPB  5.29  ±  0.09 0.70  ±  0.03 3.845  ±  0.008  180.306  ±  0.002  0.5 CPPB  10.39  ±  0.07 0.87  ±  0.01 4.025  ±  0.001  7.0  ±  0.4  0.5 Drude   (Ωcm)  (fs)  Constant  additive  term  to  ε1  3.02  ±  0.03  ×  10−6 16.7  ±  0.1  ε   1.632  ±  0.008 egre s) µ CPPB 5.29˘0.09 0.70˘0.03 3.845˘0.008 ´180.306˘0.002 0.5 CPPB 10.39˘0.07 0.87˘0.01 4.025˘0.001 ´7.0˘0.4 0.5 Drude ρ (Ωcm) τ (fs) Constantadditive termtoε1 3.02˘0.03ˆ10´6 16.7˘0.1 ε8 1.632˘0.008 The structural model for the ZnO/Ag BR in the energy range 0.734 to 5 eV consisted of a semi-infinite Ag metal layer deposited onto glass, a 108˘10 Å ZnO + Aginterfacial layer,a 3059˘3Å bulkZnOlayer,anda80˘1Åsurfaceroughness representedusingBruggemaneffectivemediumapproximationof0.5ZnOand0.5 voidvolumefractions. Parametricexpressionswereusedtodescribe ε forAg,ZnO, and the ZnO + Ag interface and are listed in Tables 2 and 3. Previous studies of ZnO/Ag interfaces in the BR of thin film n-i-p a-Si:H PV shows that the optically determined value of Ag surface roughness obtained from RTSE is very close to that measured with atomic force microscope (AFM) with ds,RTSE (Å) = 0.96 ds,AFM (Å) + 5 Å [28]. The ds,RTSE = 30˘2 Å for Ag corresponds to a ds,AFM =26Å. After depositionofZnO, theZnO/Aginterface layer thickness is reportedbyDahal et al. as di (Å) = 1.98 ds (Å) + 17.5 Å. The interface layer thickness predicted from the Ag surface roughness in this work is 76.9 Å as compared to that obtained in our parametricanalysisof108Å[28]. Ourparametricvalueslightlyoverestimates the prediction,howeverinDahaletal.[28]thesampleswithsimilarAgsurfaceroughness, 25–30 Å, also has an interface thickness of 75–110 Å which are greater than the linear prediction. Figure2showsthat thespectra in εobtainedfor theZnO+Aginterface is optically different than Ag and ZnO alone and can be modeled by a Lorentz oscillator and a Drude oscillator in the near IR to near UV range (0.734 to 5 eV) with ε8=1. TheZnO+Aginterfaceexhibitsaclear localizedparticleplasmonabsorption feature whichcan bemodeled using aLorentz oscillatorwith a resonanceenergy at 2.83˘0.01 eV [27,48]. A resistivity of 3.7˘0.5ˆ10´5Ωcm and a scattering time of 73