Laser CVD of A-Si:H: Film Properties and Mechanism
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LASER CVD OF a-Si:H: FILM PROPERTIES AND MECHANISM K. HESCH AND P. BESS Institute of Physical Chemistry, University ofHeidelberg, Im Neuenheimer Feld 253, D-6900 Heidelberg, F.R.G. H. OETZMANN AND C. SCHMIDT Asea Brown Boveri AG, Corporate Research Eppelheimer Strasse 82, D-6900 Heidelberg, F.R.G.
ABSTRACT Films of a- Si: Hwere deposited from Sill4 in a parallel configuration employing acw CO2 laser. Infrared spectra, photoconductivities and dark conductivities of the films were studied as a function of surface temperature between 250 0 C and400° C. The results are compared with data obtained from deposition experiments using Si 2 H6 and an ArF laser. Implications for the mechanism of LICVD are dis cussed. INTRODUCTION The laser-induced chemical vapor deposition (LICVD) of amorphous, hydrogenated silicon (a-Si:H) has found increasing interest recently. Especially the processes using a cw CO 2 laser and Sil 4 [1-4] and a pulsed ArF laser and Si 2H6 [5-7] in a parallel surface - beam configuration have been studied in more detail. From a thermodynamic point of view a-Si:His ametastable material, and therefore its properties and relative stability strongly depend on the conditions of preparation. It is generally believed that the laser methods are soft deposition methods, which allow an accurate control of the conditions during film growth. Therefore, the use of laser photons to initiate the deposition process may not only lead to a-Si:H films with optimized properties, but also to a better understanding of the mechanism and the chemical processes involved in the gas phase and at the surface. In this paper results are reported for the chemical composition and bonding configuration obtained by IR spectroscopy and the dependence of the photoconductivity and dark conductivity of the a-Si:H films on substrate temperature. These film properties measured for a - Si:H deposited from Sill4 with acw CO 2 laser are compared with the corresponding results observed for films grown from Si 2 H6 employing a pulsed ArF laser. EXPERIMENTAL Deposition of a-Si: H films was achieved by decomposing SiH4 with a cw CO2 laser beam in a configuration with the laser beam parallel to the substrate surface. More details of the experimental setup and the deposition conditions such as gas flow, pressure, laser power, etc. can be found elsewhere [3]. In the parallel configuration the laser radiation directly heats the gas above the surface and the surface temperature can be controlled more or less independently. Thus, the processes occurring in the gasphase, namely laser-induced chemistry, and theprocesses taking place at the surface determining the film structure and composition may be synergistic. The substrate surface temperature is of crucial importance for the film properties and often was not measured with sufficient accuracy in previous experiments. Therefore, the surface temperature was measured by an evaporated Ni sensor with an accuracy of ± 1K. This device is described in more detail in [4]. The films prepared for FTIR analysis were de
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