Device Quality a-Si:H at High Deposition Rate (>10 nm/s) by Hot Wire Chemical Vapor Deposition Using TaC Coated Graphite Filaments

One of the possible issues in the Hot Wire Chemical Vapor Deposition (HWCVD) technique for the fabrication of silicon or carbon containing thin films and materials is the reaction of the precursor gasses with the filament material, usually tungsten or tantalum. The silicidation or carburization of the filament alters its reactive and mechanical properties which influence the quality of the deposited materials and decrease the lifetime of the filament. In this paper we use a graphite filament coated with TaC, in which these effects are expected to be reduced [1]. However, the issue with this filament material for the deposition of thin film silicon could be the desorption of carbon from the TaC which can deteriorate the quality of the deposited films. We performed a calibration of the filament temperature as a function of the electrical power supplied to the filament and found the emissivity of the TaC surface to be close to 0.43. This is close to the value reported by Ozaki et al. [2] A series of a-Si:H films was made in which the deposition rate was varied from 1 to 12 nm/s. At all of these deposition rates, device quality material was obtained. At a deposition rate of 11 nm/s the material has an activation energy for the dark conductivity of 0.879 eV, and a photo response of 7x105. The hydrogen concentration as obtained by FTIR ~5 at.-%. We are presently studying the suitability of these films for use in thin film Si solar cells. The obtained deposition rate with this new filament material is up to 12 times higher than for our a Si:H deposited with tantalum filaments [3], whereas the surface area of the filament is only 1.5 times that of the tantalum filaments and the filament temperature was the same in both cases. A possible explanation is that the TaC filament surface is less poisoned with gaseous species than a pure metallic surface, leading to a larger fraction of catalytically active surface than in the case of purely metallic filaments. These results show device quality a-Si:H can be obtained at very high deposition rates with TaC coated filaments and that the potential carbon desorption from the filaments has no detrimental influence on the material quality of amorphous silicon. [1] I. T. Martin, C. W. Teplin, M. Landry, M. Shub, R. C. Reedy, J. V. Portugal and J. T. Mariner, High rate hot-wire chemical vapor deposition of thin films using a stable TaC covered graphite filament, oral presentation at the 6th Int’l Conf. on Hot-wire Chemical Vapor Deposition (Cat-CVD) Process, Palaiseau, France, 2010 [2] Y. Ozaki, R.H. Zee, Mater. Sci. Eng. A202 (1995) 134. [3] M.K. Van Veen, R.E.I. Schropp, Beneficial Effect of a Low Deposition Temperature of Hot-Wire Deposited Intrinsic Amorphous Silicon for Solar Cells, J. Appl. Phys. 93 (2003) 121.