Seite - 75 - in Photovoltaic Materials and Electronic Devices

electronic transitions, Lorentz oscillators representing IR phonon modes, and a constant real additive term ε8 to account for dispersion from absorption features outside the measured spectral range from 0.04 to 5 eV with parameters given in Table 4. The near IR to near UV range shows only small absorption below the lowest direct transitionat3.364eVasexpectedfordirectbandgapZnO[50]. Phononmodes forwurtziteZnOareΓopt =1A1 +2B1 +1E1 +2E2,withA1 andE1 modesIR-active. Onlyonecharacteristic transverseoptical (TO)modeforZnOwithE1 symmetryat 0.0501eV (404.08cm´1) is resolvedfor this sample [51–53]. Weakabsorptionbands in the spectral region from 0.134 to 0.264 eV (1080 to 2130 cm´1) have been observed andareoftenassociatedwithhydrogen-associatedbendingmodes; stretchingmodes of hydrogen bonded to heavier elements like zinc; and various carbon, oxygen, and nitrogen-related stretching modes not involving hydrogen [54]. These types of peaks are analogous to those found in the absorbance spectra from traditional unpolarized FTIR measurements, which lack sensitivity to discerning thickness and thefullcomplexopticalpropertiessimultaneously—acapabilityofSEmeasurements. Table 3. Parameters describing εand structure for a ZnO film deposited on Ag and the ZnO + Ag interface formed. Experimental ellipsometric spectra were collected in situ after deposition at room temperature in the spectral range from 0.734 to 5.0 eV and fit using least squares regression analysis with an unweighted estimator errorfunction,σ=7ˆ10´3. Parametersdescribing ε forAgwerefixedfromTable2. ForZnO, theparameterizationof εconsistedof twoCPPBoscillators,aSellmeier oscillator, and ε8. For theZnO +Aginterface, theparameterization of εconsisted ofaDrudeoscillator,aLorentzoscillator,and ε8. Layer Oscillators ZnO db =3060˘3Å ds =80˘1Å CPPB(µ= 0.5) ε8= 2.27˘0.01 A(Unitless) Γ (eV) En (eV) Materials  2016,  9,  128  9  of  23  Table  3.  Parameters  describing  ε  and  structure  for  a  ZnO  film  deposited  on  Ag  and  the  ZnO  +  Ag  interface  formed.  Experimental  ellipsometric  spectra  were  collected  in  situ  after  deposition  at  room  temperature  in  the  spectral  range  from  0.734  to  5.0  eV  an   fit  using  least  squares  r gression  analysis  with  an  unweighted  estim tor  error  function,     =  7  ×  10−3.  Parameters  describing  ε  for  Ag  were  fix d  from  Table  2.  For  ZnO,  the  parameterization  of  ε  consisted  of  two  CPPB  oscillators,  a  Sellmeier  oscillator,  and  ε .  For  the  ZnO  +  Ag  interface,  the  parameterization  of  ε  consisted  of  a  Drude  oscillator,  a  Lorentz  oscillator,  and  ε .    Layer  Oscillators  ZnO  db  =  3060  ±  3  Å  ds  =  80  ±  1  Å  CPPB  (μ  =  0.5)  ε =  2.27  ±  0.01  A  (Unitless)     (eV)  En  (eV)  Ө  (degrees)  2.63  ±  0.02  0.199  ±  0.002  3.363  ±  0.001   20.1  ±  0.5  1.41  ±  0.02  3.83  ±  0.08  4.36  ±  0.03  0  (fixed)  Sellmeier  A  (eV2)     (eV)  En  (eV)  0.080  ±  0.002  ‐  0  ZnO/Ag  Interface  =  108  ±  11  Å  Lorentz  ε =  1  A  (Unitless)     (eV)  E0  (eV)  2.8  ±  0.2  0.57  ±  0.05  2.83  ±  0.01  Drude     (   cm)     (fs)  3.7  ±  0.5  x10−5  2.7  ±  0.3  3.1.2.  Phonon  Modes  in  ZnO  The  analysis  was  extended  to  the  IR  by  fitting  parameters  defining  ε  for  ZnO  only  and  fixing  those  defining  ε  for  Ag  and  the  ZnO  +  Ag  interface  as  well  as  the  interface  layer  thickness.  This  analysis  approach  was  chosen  because  free  carrier  absorption  represented  by  the  Drude  feature  dominates  the  IR  response  of  Ag  and  the  ZnO  +  Ag  interface  layers  and  is  already  established  from  near  IR  to  UV  spectral  range  analysis.  A  common  parameterization  of  ε  for  the  ZnO  was  applied  for  the  data  collected  from  the  two  instruments  with  spectral  ranges  from  0.04  to  0.734  eV  and  0.734  to    5.0  eV,  respectively,  although  the  bulk  ZnO  layer  thickness  was  allowed  to  vary  for  the  ellipsometric  spectra  collected  from  each  respective  instrument  to  account  for  measurement  on  different  spots  over  the  sample  surface.  A  common  surface  roughness  thickness  between  the  two  sets  of  measured  spectra  was  obtained,  as  this  effect  will  vary  less  with  non‐uniformity  than  the  overall  bulk  layer  thickness.  Figure  3  shows  ε  for  ZnO  represented  by  a  combination  of  CPPB  oscillators  for  electronic  egre s) 2.63˘0.02 0.199˘0.002 3. 63˘0.001 ´ 0.1˘0.5 1.41˘0.02 3.83˘0.08 4.36˘0.03 0 (fixed) Sellmeier A(eV2) Γ (eV) En (eV) 0.080˘0.002 - 0 ZnO/Ag Interface=108˘11Å Lorentz ε8= 1 A(Unitless) Γ (eV) E0 (eV) 2.8˘0.2 0.57˘0.05 2.83˘0.01 Drude ρ (Ωcm) τ (fs) 3.7˘0.5x10´5 2.7˘0.3 75