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(I) The deposition rate of jtc-Si:H in standard PECVD processes with SiH 4 and H2 is much too low for deposition of active photovoltaic layers of 2-3 gim, not to speak of "thin" crystalline materials of 30 gm. Generally a correlation between crystallinity and suppressed growth rate is found [5]. Techniques providing high growth rates while maintaining good material quality are needed. A remarkable improvement in growth rate of quality jtc-Si:H is achieved in PECVD using fluorinated gases [6]. Also with hot wire CVD, much increase in deposition rate is achieved [7]. While with the latter technique a further post-deposition hydrogen passivation of grain boundary defects could be necessary, with fluorinated gases some additional experimental precautions might be needed. It is therefore of much interest that also PECVD processes with plasma excitation frequencies (Vex) higher than the standard 13.56 MHz lead to higher growth rates of good quality gc-Si:H [8-12]. (II) Related to the maintenance of good material quality at high growth rate is the control of the nucleation of gic-Si:H on various kinds of substrates as the resulting interfaces will strongly affect the performance of thin film devices. The substrate dependence of the nucleation behaviour of gc-Si:H has in the past been studied on a variety of substrate materials such as glass, metal, transparent conductive oxides, crystalline and amorphous silicon [13-19]. Generally nucleation depends on type and morphology of substrate. The differences are in terms of crystallinity, porosity, columnar structure, inhibition of crystallization especially on amorphous silicon as a substrate or an amorphous or porous interface layer. Thus nucleation has to be optimized for each substrate. We present results on gc-Si:H material prepared with PECVD in the frequency range 13.56 MHz - 116 MHz. The material is investigated with transmission electron microscopy (TEM), X725 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society

ray diffraction at grazing incidence (XRD), Raman spectroscopy, optical band-to-band and infrared absorption (IR), photoluminescence (PL), electron spin resonance (ESR) and electrical conductivity. It is shown that the plasma excitation frequency Vex strongly affects the growth and nucleation process of jic-Si:H and consequently the structural and electronic properties. The influences of Vex on the process plasma and on the jtc-Si:H film growth, which are related to a reduction of the plasma sheath potentials and sheath thickness, a higher dissociation of the process gases and a higher ion current density at the substrate [20-24], will be discussed. EXPERIMENTAL DETAILS The samples were prepared by PECVD with plasma excitation frequencies between 13.56 pressure 510-9 mbar MHz and 116 MHz at 200 'C in a commercial deposition system with base [25]. The reaction chamber is of standard diode type with 10*10 cm 2 substrates. We use gas mixtures of SiH 4 in H 2 with PH 3 as doping gas. For plasma excitation a broad band generator (10 - 125 MHz) and a