Nucleation of p-Type Microcrystalline Silicon on Amorphous Silicon for n-i-p Solar Cells Using B(CH 3 ) 3 And BF 3 Dopan
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layer of thickness di. The dielectric function of the bulk layer is also determined in the analysis. In this study, we use the evolution of ds and db, as well as the dielectric function of the bulk p-layer after -120 A, to assess the microstructural evolution and final film crystallinity and void density. RESULTS Figure 1 shows the dielectric function spectra for intended ýtc-Si:H p-layers prepared on H 2 plasma treated i-layers at the lower plasma power level of 230 mW/cm 2 and at [B(CH3 )3 ]/[SiH 4 ] ratios of D=0, 0.01, and 0.05. These spectra and those following (Figs. 2-4) have been obtained at similar bulk layer thicknesses in the range 114-120 A and are characteristic of the substrate temperature of 200'C. The dielectric function spectra in Fig. 1 reveal a clear optimum in the film structure at D=0.01. Here, the p-layer is dense single-phase gtc-Si:H as indicated by the strong dielectric amplitudes, the shoulder and peak in C2 near 3.4 and 4.2 eV, and a well-defined optical absorption onset near 2.40 eV. For D=0, the film exhibits a mixture of microcrystalline and amorphous phases; however, its low dielectric amplitude is attributed to a void density of 40 vol.%, measured relative to the film prepared with D=0.01. The film with D=0.05 is amorphous as indicated by its optical gap (2.00 eV) and the lack of discernible features near 3.4 and 4.2 eV. The corresponding set of data is shown in Fig. 2 for p-layers prepared on H 2 -plasma treated i-layers at the lower power level of 230 mW/cm' and at [BF 3]/[SiH 4 ] ratios of D=0, 0.015, and 0.10. In this case, the trend with D is different than that for B(CH 3 )3 in Fig. 1. For BF 3 , there is no optimum in the film structure at low doping level, and the dielectric amplitudes continue to increase to D=0.10. This increase is attributed to a reduction in the void content from 28 vol.% for D=0 to 26 vol.% for D=0.015, relative to the best film prepared with D=0. 10. All films in Fig. 2 are microcrystalline; however, there appears to be a larger amorphous component in these films prepared from BF 3 compared to the optimum film prepared from B(CH 3 )3 with D=0.01 in Fig 1. Next, we consider variations in the rf plasma power at the optimum doping levels in Figs. 1 and 2 for the p-layers doped using B(CH 3)3 and BF 3 gases. For B(CH 3 )3 (D=0.01), an increase in rf power from 230 to 700 mW/cm2 leads to a degradation of the p-layer microstructure,
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FIG. I Dielectric function spectra at 2000 C for -120 A thick intended pc-Si:H p-layers prepared from B(CH 3)3 at the lower power level of 230mW/cm and at [B(CH 3) 3]/ [SiH 4 ] ratios of 0, 0.01, and 0.05. The ilayer was treated with a 1-H2 plasma prior to p-layer growth in all cases.
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FIG. 2 Dielectric function spectra at 200'C for -120 A thick [.c-Si:H p-layers prepared from BF 3 at the lower
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