Diamond is known as a crystalline material with exceptional properties. The relative rarity and limited size of natural diamonds has stimulated the development of their artificial production. Two approaches are currently utilized for this purpose: diamond crystallization from melted graphite under high pressure and high temperature (HPHT process) and formation of diamond films using chemical vapor deposition (CVD process).
While the dimensions of HPHT diamonds are approximately the same as natural diamond, the CVD process is able to fabricate relatively thin diamond films, but with a rather large surface area, that is very attractive for technical applications. The gaseous media required for CVD diamond growth comprises one of carbonaceous vapor or gases (preferably hydrocarbons) as the carbon source and hydrogen (or some of halogens) as a catalyst to activate the diamond structure formation. Other gaseous components may be used for process modification or, for example, diamond doping.
Diamond formation occurs due to the chemical reactions at the substrate surface in contact with the gaseous media, activated by thermal decomposition (‘hot filament’ CVD), gas discharge (direct current – DC, microwave – MW, etc. plasma enhanced CVD) or some other method. Depending on the activation method, as well as the nucleation and process parameters, the crystalline quality of CVD diamond films can be varied over a wide range; from perfect single crystal epitaxial to polycrystalline material, which is the most suitable for mass produciton and application. The van der Drift competitive growth model1 has been expanded to describe the formation of the polycrystalline films. According to the model, growing diamond crystallites compete with each other for access to the carbonaceous gas phase. Diamond films of different quality may be obtained depending on the CVD process parameters. Overall, the density of structural defects in the films is determined mainly by the diamond grain density and is normally expected to be reduced with an increase of the grain size. At the same time, the crystallinity of individual diamond grains is determined by other CVD process conditions and may be close to an ideal single crystal for a film with higher overall defect density. This circumstance has been revealed in the recent studies, where the smallest and less ordered fractions of material were removed from the polycrystalline diamond films through the use of thermal oxidation. The selective oxidation allows diamond crystallites on the micron scale to be obtained, with perfect pyramid shapes as resulted of the van der Drift competition2. These single diamond pyramids are attractive for different applications from micro-cutting tools to probes for atomic force microscopy3 and to quantum information processing.
This month’s cover image shows a scanning electron microscopy (SEM) plan-view micrograph of the microdiamonds obtained after selective oxidation of CVD film grown by DC CVD from methane-hydrogen gas mixture on an Si substrate. The pyramid shaped crystallites are perpendicular to the Si wafer plane with their apexes on the substrate surface. The rectangular basal planes of the pyramids ({100} faces) are on the outer film surface. In this particular case two crystallites were grown from almost identical nuclei located closely to each other. The specific shapes of the observed crystallites are determined by their growth competition and were exhibited by eliminating surrounding nanodiamonds and disordered carbon using selective oxidation.
REFERENCES
1. van der Drift, A., Philips Res Rep (1967) 22, 267.
2. Zolotukhin, A.A., et al., Diamond and Related Mater (2010) 19, 1007.
3. Obraztsov, A.N., et al., Review of Scientific Instr (2010) 81, 013703
This article was originally published in Materials Today (2012) 15(11), 519. To access past issues of Materials Today, and register for your free subscription to the magazine, just click here.
doi: 10.1016/S1369-7021(12)70221-3