Thermodynamic coupling effect and catalyst effect for the artificial diamond growth
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Thermodynamic coupling effect and catalyst effect for the artificial diamond growth Ji-Tao Wang Department of Electronic Engineering, Fudan University, 200433 Shanghai, China
Zhong-Qiang Huang Department of Applied Mathematics, Tongji University, 200092, Shanghai, China
Yong-Zhong Wan, David Wei Zhang, and Hong-Yong Jia Department of Electronic Engineering, Fudan University, 200433 Shanghai, China (Received 21 March 1996; accepted 16 October 1996)
The activated chemical vapor deposition (CVD) diamond process became one of the worldwide interesting projects in the 1980s. The basic question is why diamond can grow under activated low pressure conditions. The driving force of the transformation from graphite to diamond under low pressure is coming from a coupled reaction of the association of superequilibrium atomic hydrogen. The thermodynamic coupling effect in the activated CVD process is different from the catalyst effect in the high pressure, catalyst-assisted process for the artificial diamond growth. I. INTRODUCTION
Low pressure diamond growth has been found for several decades. However, it has long been difficult to find a satisfactory thermodynamic explanation. A chemical pump model was proposed in 19901 and later has been improved and renamed as a nonequilibrium thermodynamic coupling model.2–4 The key points of view of the nonequilibrium thermodynamic coupling model for the activated low pressure diamond growth (for instance, for the hot filament CVD process) from a carbon-hydrogen system can be expressed as follows. Hereinafter, DGj is the change of Gibbs free energy of reaction j. Csgraphited Csdiamondd, H?p 2 H2 , 1
DG1 . 0, sT, P % 105 Pad .
DG2 ! 0, sT ! Tactivated,
P % 105 Pad .
(1)
(2)
Here, P is the total pressure. H?p , superequilibrium atomic hydrogen (SAH), means atomic hydrogen with an equilibrium concentration of the filament temperature Tactivated, and then with a superequilibrium concentration at the substrate temperature T. DG2 is a big negative value. s3d s1d 1 x s2d , Csgraphited 1 xH?p
x 2 H2
1 Csdiamondd ,
(3)
DG3 DG1 1 x ? DG2 , 0 sT, P % 105 Pa and if coupling parameter, x, is not too smalld . Due to the thermodynamic coupling between Reactions (1) and (2), and the negative value of DG3 , graphite 1530
J. Mater. Res., Vol. 12, No. 6, Jun 1997
can react with SAH to produce molecular hydrogen and diamond under low pressure activated conditions. That is the thermodynamic reason why diamond can grow under low pressure and even why graphite can be etched away during the existence of SAH. Based on comparison with experiments of the hot filament process (the filament temperature 2400 K in this paper), an empirical value of 0.28 for x was taken. Nearly all experimental data (diamond growth conditions) reported in the literature could be quantitatively or semiquantitatively explained by this model. A unified barrier model was proposed for different artificial diamond processes in the literature.5 Graphite and diamond were a
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