Physical Properties of P and B Doped Microcrystalline Si:H Deposited by PECVD
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PHYSICAL PROPERTIES OF P AND B DOPED MICROCRYSTALLINE SI:H DEPOSITED BY PECVD
A. RUBINO*, M.L. ADDONIZIO*, G. CONTE*, G. NOBILE*, E. TERZINI*, A. MADAN** *ENEA - Centro Ricerche Fotovoltaiche, P.O.Box 32, 80055 Portici (Napoli), Italy **MVSystems, Inc., 327 Lamb Lane, Golden, CO 80401, USA ABSTRACT Boron and phosphorus doped high conductivity microcrystalline thin films were deposited in a PECVD reactor. We report conductivities as high as 3 and 41 S/cm for B and P doped materials respectively on films deposited at 210 'C. The conductivity as well as the microcrystalline fraction increase for the n layer with decreasing RF power, while, for the p material, an increase of power is needed to improve the film characteristics. The conductivity prefactor (yo) as well as the conductivity ((5) itself as a function of the activation energy (Ea) show a slope inversion for both n and p materials at an activation energy of about 40 meV and 80 meV respectively. Different possible transport mechanisms are examined in order to explain the experimental data. INTRODUCTION Phosphorus or boron doped microcrystalline hydrogenated silicon (Si:H) thin films have achieved a noteworthy attention in the last years due to their high conductivity and the various possibilities of technological application, for example solar cells and other electronic devices [1]. Doped microcrystalline materials can be utilized as front and back contacts in p-i-n devices for photovoltaic applications; in this case the low activation energies can contribute to an increase in the built-in potential, while the high conductivity can reduce the series resistance in the devices. Finally, the grain structure would have an effect of texturization with enhanced light trapping effect and a possible increase of the total efficiency. Although high conductivity films (100 S/cm) have been obtained [2], questions are still open about the criteria for optimizing the deposition conditions in order to obtain as high a conductivity as possible. Th is work provides evidence of some differences among the optimal growth conditions for phosphorus or boron doped films. We have concentrated on preparation conditions which are compatible with the deposition parameters used in the process of fabrication of electronic devices. A detailed description is given of both electrical and structural properties of the films, with particular regard to the analysis of the conductivity properties. We also provide a possible interpretation of the temperature dependence of the conductivity. EXPERIMENTAL All the samples were deposited on Corning 7059 glass substrates in a 13.56 MHz capacitively coupled PECVD three chamber reactor. Dopant contamination has been avoided by depositing p+ and n+ samples in dedicated chambers. Details of the deposition system are described elsewhere [3]. Substrate temperature has been kept fixed at 210 'C. Different values of pressures have been explored: as reported in [4], the highest values of conductivity are obtained at relatively low pressures. Hydrogen diluted dib
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