Reaction mechanisms and thin a-C:H film growth from low energy hydrocarbon radicals

Abstract

Molecular dynamics simulations using the Brenner potential have been performed to investigate reaction mechanisms of various hydrocarbon radicals with low kinetic energies on amorphous hydrogenated carbon (a-C:H) surfaces and to simulate thin a-C:H film growth. Experimental data from an expanding thermal plasma setup were used as input for the simulations. The hydrocarbon reaction mechanisms were studied both during growth of the films and on a set of surface sites specific for a-C:H surfaces. Thin film growth was studied using experimentally detected growth species. It is found that the reaction mechanisms and sticking coefficients are dependent on the specific surface sites, and the structural properties of the growth radicals. Furthermore, it is found that thin a-C:H films can be densified using an additional H-flux towards the substrate. a-C:H films with intermediate H-content (20-40%). Although these films have generally a lower sp 3 content, the C-C sp 3 network is more extensive as compared to PLCH films. Hence, these films are denser and harder. Hardness values of up to 20 GPa can be obtained [2]. They are often referred to as diamond-like a-C:H (DLCH). 3. ta-C:H, or hydrogenated tetrahedral amorphous carbon. They contain up to 70% sp 3 bonds, and a H-fraction of about 25%. These films have the highest density and hardness of all a-C:H's, with a hardness of up to 50 GPa [3]. 4. a-C:H with low H-content (< 20%). They have a high sp 2 content, and are referred to as graphitic a-C:H, or GLCH. Their hardness is usually only a few GPa [4]. In the following, the term a-C:H is used to simply denote a film composed of carbon and hydrogen, without any further specifications. Note that in the literature, the term DLC has often been used wrongly for this purpose.Obviously, the categories as given above are not defined by sharp boundaries. Furthermore, the overall structure is not necessarily homogeneous. For example, ta-C:H can locally contain crystalline fractions, embedded in a more amorphous matrix. DLCH can contain clusters of sp 2 carbons, embedded in a sp 3 matrix.The growth mechanism of a-C:H films depends on the deposition technique used. When hard films are desired, the key property is the sp 3 fraction. The sp 3 matrix of hard DLCH and ta-C:H forms a rigid, strongly cross-linked network, determining the mechanical properties of the film. The deposition process which promotes sp 3 bonding is a physical process: ion bombardment [5][6][7][8][9]. The deposition mechanism of these hard films is currently understood in terms of the so-called "subplantation model". Robertson proposed that the subplantation creates a metastable increase in density, leading to a local change in bonding to sp 3 [10,11]. Various simulations demonstrated the basic idea of subplantation, see e.g. [12][13][14][15]. Carbon ions in the energy range of 10-1000 eV can penetrate up to a few nm into the growing film, loosing their energy mainly by elastic collisions with the target atoms (nuclear stopping). Hence, the carbon ions penetrate th...

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