Kinetics, thermodynamics and microstructure of tungsten rods grown by thermal laser CVD

Single crystal and polycrystalline tungsten rods have been grown by thermal laser assisted chemical vapour deposition. The rods were grown from a reaction gas mixture of WF6 and H-2 by focusing an Ar+ laser on a tungsten wire substrate. The kinetics for the single crystal growth was investigated for different partial pressures. The growth rate was monitored in situ by inspection with a microscope and the deposition temperature on the tip of the rod was determined by a photoelectric pyrometric set-up. The activation energy was determined to 77 kJ mol(-1) for a H-2/WF6 ratio of 3 and to 50 kJ mol(-1) for a H-2/WF6 ratio of 5. The obtained reaction order of 3/2 with respect to the H-2 partial pressure is higher than the usual reported value of 1/2 for this process. The deposition rate showed no dependence on the WF6 partial pressure i.e. the reaction order was 0. Thermodynamic calculations were performed for the experimental conditions to confirm the etching behaviour of non-reduced WF6. Morphology and microstructure of single crystal and polycrystalline rods were investigated by scanning electron microscope and transmission electron microscope. The preferred growth direction for the single crystal rods was [1 0 0] while the polycrystalline rods showed a random distribution of directions. (C) 2002 Elsevier Science B.V. All rights reserved.