Page - 69 - in Photovoltaic Materials and Electronic Devices

identical conditions for each sample to study how the Si:H layers grow in the deviceconfiguration. Table1. Deposition conditions for the individual layers in the a-Si:H n-i-p solar cell configuration deposited on 6”ˆ6” borosilicate glass substrates. The 5% dopant gas in H2 is by volume. Cr, Ag, and ZnO were sputtered at room temperature (RT). Layer Substrate TemperatureT (˝C) Pressurep (mTorr) Radio Frequency(rf) PowerP(W/cm2) GasFlow(sccm) Ar SiH4 5%PH3 or B2H6 inH2 H2 R=[H2]/[SiH4] Cr RT 5 0.92 10 - - - - Ag RT 5 0.92 10 - - - - ZnO RT 5 0.92 10 - - - - n 200 1500 0.032 - 2 0.5PH3 40–160 20–80 i 200 800 0.04 - 5 - 50–250 10–50 p 100 1500 0.066 - 2 0.5B2H6 100–400 50–200 RTSE was performed in situ at a single spot during deposition using a rotating-compensator multichannel ellipsometer (J. A. Woollam Company model M-2000) thatcanmeasureellipsometricspectra (in theformofN =cos2ψ,C=sin2ψ cos∆, S = sin 2ψ sin∆) from 0.734 to 5.88 eV with a minimum data acquisition time of 50 ms [38,39]. This type of instrument collects ellipsometric spectra at all photon energies in parallel by a combination of a 1-D linear detector array and serial pixel readout. Dual detectors are required to access this spectral range, and consist of a silicon based charged coupled device (CCD) and indium gallium arsenide photodiode array (PDA). RTSE measurements were collected at the respective deposition temperature at angles of incidence near 70˝ and spectra obtained from single optical cycles were averaged over 1.5 s intervals to increase the signal-to-noise ratio. Analysis of experimentally collected RTSE data was performed using J. A. Woollam Co. CompleteEASE software (Lincoln, NE, USA). The time evolution of db and ds as well as the spectroscopic εof the bulk Si:H layers were extracted from RTSE data using a globalΣσ-minimization procedure. For Si:H films, globalΣσ-minimization analysis of RTSE involves using test db and ds values for the Si:H layer being deposited on top of a pre-defined substrate stack to numerically solve for test εof the Si:H layer [29]. The test values of εare then used to fit other spectracollectedatdifferent timeswhenthefilmisrelativelyhomogeneous, typically near 100–200 Å in accumulated material thickness for Si:H films where structural transitionshavenotyethadtimetomature. Theapproach isapplied in theregime prior tocrystallitenucleation andis iterated in order to obtainnumerically inverted εyieldingthe lowestspectrallyandtime-averagederror,σ, over themultiple time measurements selected. The numerically inverted εminimizingσare taken to be the best representationof thea-Si:Hopticalpropertiesandare thenusedtodetermine structural parameter variations over the full set of RTSE data, with the nucleation of crystallites from the a-Si:H matrix identified by a sharp increase in the surface 69