Microsrtuctures and Electrical Characterization of CVD-W and MO Films as Contacts in Photocells
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ABSTRACT Tungsten and molybdenum hexacarbonyls were used as precursors in chemical vapour deposition process for preparation of W and Mo thin films. Pyrolitical decomposition of these precursors proceeds at temperatures of 250-400°C. Thin films with thicknesses in the range of 0,02-1 pm were deposited on different substrates - bare or covered with CdTe glass, and monocrystalline Si. Microstructural studies performed by Reflection High Energy Electron Diffraction (RHEED) method showed that films deposited tend to grow textured. This is discussed as probably due to differences in the growth rate for various crystal planes. The sheet resistances of the as-deposited W and Mo films are in the range of 20-30 /O for thicknesses of 0.15 pm. After thermal annealing the resistance of W films drops to about 2 Q/O1 and for Mo films to about 9Q2/0. Decreasing in the resistivity of
the
films is tightly connected with the decreasing
in
the impurities
concentration. These impurities are considered to be in the base of the observed behaviour of the temperature dependence of the electrical resistance of the films. The CVD-W and Mo films are studied as back contacts on CdTe layer in CdS/CdTe photocells. In the paper some preliminary results are presented for the sheet and contact resistances when CVD W and Mo films are deposited at lower temperatures on the surface of CdTe layers, deposited by closespaced sublimation method. The thin film materials, produced by CVD technology look promising with respect to the required high deposition rates and extremely wide deposition areas in the mass production of solar cells.
INTRODUCTION Tungsten and Molybdenum thin films produced by CVD have been intensively studied with respect to applications in microelectronics - for metallization in the integrated circuits [1] and as contacts in photocells [2,3].W and Mo, as refractory metals possess high work function and they can be used for forming ohmic back contact in photocells. For deposition of W and Mo films different precursors have been used and investigated - WCL6, 119 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
MoCL5 and especially WF6 for the microelectronics purposes [4]. This technology may be regarded as a well-established technology, but the fluoride process involves temperatures of decomposition in the range of 500-800'C. These high temperatures and the existence of HF lead to some corrosive processes. To avoid some of these disadvantages carbonyl process has been suggested as an alternative technique [5]. It is of interest to study W and Mo CVD carbonyl films with respect to their application as back contacts in photocells. Since the carbonyl process can proceed at lower temperatures and there are no corrosive species, it seems that it is quite promissive technique. Previous experience in this field shows that films obtained at substrate temperatures exceeding 400-450'C for W and 300 -350'C for Mo [4,6] yield blanket deposition with smooth film surfaces. Applying of W and Mo films as contacts in photocel
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