Nanocrystalline Undoped Silicon Films Produce by Hot Wire Plasma Assisted Technique
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Nanocrystalline Undoped Silicon Films Produce by Hot Wire Plasma Assisted Technique Isabel M.M. Ferreira, Rodrigo F. P. Martins, Ana M. F. Cabrita, Elvira M. C. Fortunato, Paula Vilarinho1 CENIMAT, Departament of Materials Science of Faculty of Science and Technology of New University of Lisbon and CEMOP-UNINOVA, Quinta da Torre, 2825-114 Caparica (Portugal) 1 Department of Ceramic and Glass Engineering, UIMC, University Aveiro, 3810-193 Aveiro (Portugal). ABSTRACT In this work, we show results concerning electro-optical properties, composition and morphology of nanocrystalline hydrogenated undoped silicon (nc-Si:H) films produced by hot wire plasma assisted chemical vapour deposition process (HWPA-CVD) and exhibiting a compact granular structure, as revealed by SEM micrographs. This was also inferred by infrared spectra, which does not present the SiO vibration band located at 1050-1200 cm-1, even when samples have long atmospheric exposition. The photoconductivity measured at room temperature also does not change when samples have a long time exposition to the air or to the light irradiation. The influence of hydrogen dilution on the properties of the films was also investigated INTRODUCTION One of the main weak points of micro/nano-crystalline undoped silicon films produced by the hot wire technique is that the films present a porous like structure with a roughness surface. This type of structure leads to a fast oxidation of the films once exposed to air, which increases with the time of air exposition. On the other hand, the production of polycrystalline undoped silicon films by the conventional plasma enhanced chemical vapour deposition is very difficult it requires the use of high rf power [1] and/or high rf frequencies [2]. In order to overcome these difficulties and to produce high compact structures at high growth rates with the performances required to device application, we combine the plasma assisting the hot wire deposition technique to produce undoped silicon amorphous and nanocrystalline films [3]. The combination of hot wire with rf plasma to produce boron doped films was first reported by Chattopadhyay et al. in 1995 [4]. He compares the properties of films produced near the coils and away from the coil. In this work we show that using the HWPA-CVD technique we were able to produce high compact structures at moderate filament temperature (1800-1900ºC) and with growth rate above 9Å/s, presenting regular distributed grains with sizes in the range of nanometers, where postoxidation, and Stabler-Wronsky effect is prevented. The films produced also have good electronic properties for optolectronic device applications, EXPERIMENTAL The reactor used to produce undoped silicon films by HWPA-CVD technique is depicted in figure 1. Inside the system three Ta filaments 160 mm in length, 0.5 mm in diameter separated 30 mm from each other are placed 25 mm below the substrate holder and the heater. A22.4.1
A grid is placed about 1cm under the filaments for rf assisting the chemical vapour decomposition of Heater
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