Page - 77 - in Photovoltaic Materials and Electronic Devices

grown as a function of R in an effort to probe the subtlefluctuations expected as the material transitions from amorphous to nanocrystalline [31,55]. A distinct type of roughening transition is reported in which crystallites nucleate from the growing amorphousphase. Becauseof the lowcrystallitenucleationdensityasobservedby Fujiwara et al. andFerlauto et al. [4,31], thegrowthofcrystallineprotrusionsproduce a roughness layer that increases promptly when compared to increases in bulk layer thickness. Thus, the onset of roughening identifies a transition to mixed-phase amorphous+nanocrystalline (a+nc)-Si:H film growth accompanied by changes in the film optical properties. This behavior denotes the amorphous-to-mixed-phase [aÑ(a+nc)] transition. Simple roughening of the amorphous phase also tends to exhibit a lower increase in ds, accompanied by only minimal increases inσ, with accumulated bulk layer thickness. Crystallites nucleating from the amorphous phase grow preferentially over the surrounding material, until the point at which the crystallites cover the surface. The disappearance of the amorphous phase and coalescence of crystallites is denoted as the mixed-phase-to-single-phase nanocrystalline [(a+nc)Ñnc] transition. The aÑ(a+nc) and (a+nc)Ñnc transitions were detected using RTSE monitoring and data analysis in this work. The Si:H films prepared at low R remain in the amorphous growth regime throughout the deposited thickness. VIA was applied to RTSE data collected during growth for Si:H transitioningfromamorphous tonanocrystalline [31,56,57]. Figure 4 shows an example of the results of VIA applied to RTSE data to obtain the surface roughness thickness, nanocrystallite fraction, void fraction, and averagemeansquareerror (Equation(1))as functionsof the bulk layer thickness for a R = 50 i-layer on a BR over-coated with a 200 Å R =50n-layer. The VIA applied here utilizes spectra from 2.75 to 5.0 eV and ε for a-Si:H and nc-Si:H components as shown in Figure 5. Spectra in ε for nc-Si:H was obtained from the end of the respectivedepositionwhenthefilmisknowntobefullynanocrystalline, ~1150Å of a 1300 Å thick film using the same optical model as was used for the i-layer growthevolutiondiagram. In thismodel the freeparametersaredb andds. Spectra in ε for the amorphous phase was taken from the analysis of R =15 deposition correspondingtoatimewithinthefirst~200Åofbulkmaterialpriortothenucleation of nanocrystallites. There is strong optical contrast between the two sets of ε for Si:H, in that theamorphousphasehasonlyasinglebroadresonancewhile thatof nanocrystallitematerialhas twofeaturesrepresentativeofdampenedandbroadened critical point features found in single crystal silicon [58]. These reference spectra in ε for a-Si:H and nc-Si:H, along with that for void (ε= 1), were then used in a three component Bruggeman effective medium approximation [42,43] layer and a least-squaresregressionwithin theVIAwithds andtherelativenanocrystallite (fnc) and void (fvoid) fractions as free parameters andthe amorphous fraction constrained (fa =1´ fnc´ fvoid). 77