ICMCTF2006 Session D2-1: Diamond and Diamond-Like Carbon Materials

The effects of clustering on a nanometer scale of the carbon sp² phase are important in understanding the electronic properties of disordered amorphous carbon thin films. This conductive sp² phase is largely responsible for the optical and transport properties in the films. For films grown by plasma enhanced chemical vapour deposition, an increase in the unpaired electron spin density from 10¹⁷ to 10²⁰ cm^-3 is observed as the deposition self-bias is increased. At high deposition biases (negative self-bias), the Tauc gap, which is a measure of the mean sp² cluster size, and the spin resonance line width, both decrease. These effects are attributed to enhanced delocalisation of the electron wavefunction, which in turn is associated with the larger sp² clusters found at higher biases. We show that it is possible to describe the clustering of the sp² phase in terms of a structural disorder and a topological disorder¹. We also show that it is possible to tailor the electronic properties of films grown using pulsed laser ablation of graphite by varying the different background pressures of an inert gas during deposition, mimicking the self bias in a PECVD process. By increasing the background pressure, the surface morphology of the films change from atomically smooth (at low background pressure) to nodular and then filamentary, nanoporous films at the highest pressures used. Films grown at high background pressures exhibit room temperature photoluminescence (PL). A correlation with the results from visible Raman spectroscopy demonstrates that the intensity of the PL is directly related to clustering of the nanoscale sp² phase².

¹ J.D. Carey and S.R.P. Silva, Phys. Rev. 70, 235417 (2004).

² S.J. Henley, J.D Carey and S.R.P. Silva, Appl. Phys. Lett. 85, 6236 (2004).

Microwave plasma assisted CVD synthesis of diamond was first demonstrated during the early 1980s by Kamo et al [1] of Japan. Diamond synthesis was achieved in a small ( 2-4 cm ), tubular reactor where microwave energy was coupled into a quartz tube that was inserted through a waveguide. This reactor produced high radical densities and high quality diamond films and was inexpensive, simple to design, construct and operate. Hence it was utilized by many early diamond researchers to experimentally investigate and understand CVD diamond synthesis. However this reactor type had a number of inherent limitations such as deposition area, operating pressure regime, and input power there by limiting it commercial potential. Since these early investigations the significant potential of industrial application of CVD diamond synthesis has spurred numerous, innovative, microwave plasma reactor designs and associated CVD processes that have enabled the synthesis of a variety of high quality diamond materials, i.e. ultrananocrystalline diamond, polycrystalline diamond and single crystal diamond, at high rates (i.e. single crystal diamond > 50microns/hr), over large areas (i.e. > ten inch diameters ), and over a wide pressure regime(i.e. a few Torr to over 200Torr).

This paper will briefly review the historical development of microwave CVD diamond synthesis techniques and then will focus on state-of- the art of microwave plasma reactor technology as it currently applies to a variety of CVD diamond synthesis applications. In particular, technologies that yield large area, uniform, high rate, high quality and controllable deposition will be described. It is recognized that diamond applications such as diamond MEMs, optical windows and lenses, heat sinks, gem materials, coating for tools and wear surfaces, etc. are driving reactor design and associated process development. Thus microwave reactor performance will be discussed with respect to several potential synthesis processes/applications ranging from small grained nano-diamond, to poly diamond and single crystal diamond. The presentation will conclude with a summary of the reactor design and process development challenges that if achieved would enhance the commercialization of CVD diamond synthesis applications.