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ed to immediate nucleation of single-phase nanocrystalline silicon films which coalesced at thicknesses as low as -25-40 A [7]. In addition, these p-layers exhibited well-defined absorption onsets near 2.4 eV, characteristic of the presence of nanocrystals, and a low relative void volume fraction. In Ref. [7], the range of gas phase doping level D for optimum structure was found to be narrow for B(CH 3) 3, but very broad for BF 3 . We present results for these latter films owing to the relative insensitivity to the deposition process. For the tunnel junctions, two different n-layer conditions using R=50 (at 0.5 Torr pressure) and R=200 (at 0.9 Torr) were explored in these studies. The fixed conditions included a doping level of 2 D=[PH 3]/[SiH 4]=0.012 and a plasma power of 700 mW/cm . The latter conditions were obtained in optimization studies using H2-plasma-treated a-Si:H i-layer substrates as described in the next section. Amorphous n-layers 50 A thick used at the tunnel junction interface were prepared using R=0, D=0.02, a plasma power of 70 mW/cm 2, and a pressure of 0.1 Torr. Finally, ultrathin oxides 10 A thick used at the tunnel junction interface were obtained by exposing the completed p-layer to a pure 02 plasma for 2 mm using a flow rate of 5 sccm, a power of 70 mW/cm , and a pressure of 0.09 Torr. RTSE measurements were performed using a rotating compensator multichannel ellipsometer with a spectral range from 1.4 to 4.5 eV [8]. Spectra were collected during p-layer, interface layer (if used), and n-layer growth with acquisition and repetition times of I and 5 s, respectively. Realistic multilayer models were employed to analyze the RTSE data that incorporate the effects of substrate modification, substrate surface roughness filling, nucleation, and growth [3]. RESULTS The initial experiments performed in this study involved optimization of n-layers on a-Si:H i-layers for single-phase, high-density, n-type gc-Si:H. Both H 2-plasma treated and untreated i-layer substrates were used and n-layers were deposited with a range of R values (50-200), different doping levels (D=0.006, 0.012), and different plasma power levels (P=230, 700 mW/cm 2). Figure 1 shows results for the change in surface roughness layer thickness Ad, versus bulk layer thickness db for three n-layers prepared on H2-plasma treated a-Si:H i-layers at different R with the other parameters held constant (D=0.006, P=700 mW/cm 2). Because the initial roughness layer thickness depends on the deposition history of the underlying substrate, it is subtracted from the data. For R=50 and 100, the relatively stable surface with gradual smoothening in the initial stages of growth is attributed in this case to conformal coverage of the i-layer by a-Si:H:P. The abrupt roughening transitions after 75 A for R=50 and 40 A for R=100 are consistent with the development of nanocrystals which then propagate throughout the film with increasing thickness db [9]. The behavior for R=200 is the opposite; in the early stages of growth, d, gradually increases and then