Preparation of Microcrystalline Silicon with the Layer-by-Layer Technique at Various Plasma Excitation Frequencies
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ABSTRACT For application as nucleation layer in thin film devices, microcrystalline silicon was deposited with the layer-by-layer technique using plasma excitation frequencies between 27 and 95 MHz, various hydrogen treatment times and various film thicknesses per layer. An optimum phase transformation is found at an intermediate plasma excitation frequency, i.e. at this frequency the shortest hydrogen annealing time is necessary for an effective amorphous-to-crystalline phase transformation. INTRODUCTION The preparation of microcrystalline silicon (pc-Si:H) for use in thin film devices requires a controlled nucleation of crystalline seeds which in general is critically influenced by the type and morphology of the substrate and which is usually different from growth conditions for bulk material. For the preparation of pc-Si:H with plasma enhanced chemical vapour deposition (PECVD) it was found that the increase of the plasma excitation frequency Vex beyond the standard frequency of 13.56 MHz has a beneficial effect on the growth and structural properties of "bulk" pc-Si:H material in terms of increased deposition rate, increased crystalline volume fraction and grain sizes and less disorder between and within individual grains [1-3]. On the other hand an increased density of voids at the film-substrate interface on glass and SiO 2 was observed at high vex [4]. For an optimum device performance - which is strongly affected by the interface properties - and also for an optimum bulk material growth, it is desirable to have a large area, compact and homogeneous crystalline layer in the beginning of the growth. One possibility to achieve this could be to deposit microcrystalline silicon with the layer-by-layer technique (LbL), where deposited a-Si:H is treated by a hydrogen plasma and under certain conditions transformed into crystalline material [5-9]. As it was also observed that at high vx there is an improved growth of the crystalline phase on a-Si:H substrate material - which was attributed to a phase transformation similar to that in the LbL process [10] - it is of interest to study the influence of vex in the LbL process in some more detail. This is the topic of the present paper. EXPERIMENTAL The films are prepared in a conventional diode-type reactor with Very High Frequency Plasma Enhanced CVD processes using excitation frequencies between 27 and 95 MHz. As process gases we used silane and hydrogen at a 1:1 ratio during the deposition. For the doping of the a-Si:H layers a gas mixture of 10 ppm PH 3 diluted in H2 was used. For the H-treatment a pure hydrogen plasma with the same Vex as for the deposition process was applied. The plasma input power was 100 mW/cm2 for all depositions and hydrogen treatments, the reflected power was below 10%. The entire process is computer controlled and runs automatically. The change between the deposition and the H-treatment step is done by switching the gas flows without cutting the plasma. Typical response times of the mass flow controllers are 3 s. All samples are deposited
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