C.J. Tang

Influence of nucleation density on film quality, growth rate and morphology of thick CVD diamond films

Abstract The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH4/H2/O2 plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm−1, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (104–105 cm−2) and high nucleation densities (>1010 cm−2), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>1010 cm−2), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×104–105 cm−2) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.

A new elegant technique for polishing CVD diamond films

Abstract It is well known that the columnar growth nature of CVD diamond results in a very rough growth surface and the surface roughness steeply increases with film thickness, especially for thick CVD diamond films. In this paper, we report the successful implementation of a new elegant technique for polishing thick polycrystalline CVD diamond films at high polishing rate of up to 10 μm/h. This technique involves polishing the as-grown polycrystalline diamond films with another thick as-grown polycrystalline diamond film, which acts as a polishing abrasive. Two types of diamond films were prepared using microwave plasma CVD and then polished for 2 h using the new polishing technique. A stylus profilometer, scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure the surface roughness, characterize morphology and optical transmission of the samples before and after polishing, respectively. By polishing, thickness of 20–30 μm was removed from the top surface, and the mean surface roughness Ra of the films reduced significantly, e.g. for one film Ra reduced initially from 5.2 to 1.35 μm and the other from 3.2 to 0.55 μm. The principal advantages of this new polishing technique are simplicity, flexibility and time saving. This simple method can serve as ‘rough chipping’ to quickly remove the rough top surface and then combine with conventional polishing methods for precision machining to further reduce the surface roughness to a specific desired degree.

Infrared absorption study of hydrogen incorporation in thick nanocrystalline diamond films

We present an infrared (IR) optical absorbance study of hydrogen incorporation in nanocrystalline diamond films. The thick nanocrystalline diamond films were synthesized by microwave plasma-assisted chemical vapor deposition and a high growth rate about 3.0μm∕h was achieved. The morphology, phase quality, and hydrogen incorporation were assessed by means of scanning electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Large amount of hydrogen bonded to nanocrystalline diamond is clearly evidenced by the huge CH stretching band in the FTIR spectrum. The mechanism of hydrogen incorporation is discussed in light of the growth mechanism of nanocrystalline diamond. This suggests the potential of nanocrystalline diamond for IR electro-optical device applications.

Influence of nucleation on hydrogen incorporation in CVD diamond films

Abstract In this work, the effect of nucleation on hydrogen incorporation in Microwave Plasma CVD (MPCVD) deposited diamond films is studied for the first time. Free-standing diamond films with thickness of approximately 200 μm were grown on silicon wafers with different nucleation densities under the same MPCVD deposition. The bonded hydrogen content and incorporation forms were measured by Fourier Transformer Infrared Spectroscopy (FTIR). Our results show that the total hydrogen content and the intensity of the peak at 2828 cm−1 increases rapidly with decreasing nucleation density while the film quality decreases. The origin of the two peaks at 2818 and 2828 cm−1 is discussed.

The effect of oxygen and nitrogen additives on the growth of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films have been synthesized by using either nitrogen addition or oxygen addition to conventional CH4/H2 mixtures besides the most commonly used Ar/CH4 with or without H2 chemistry. However, the synthesis of NCD films using both nitrogen and oxygen addition simultaneously into CH4/H2 gases has not been reported thus far. In this work, we investigate the effect of simultaneous O2 and N2 addition to CH4/H2 plasma on the growth of nanocrystalline diamond (NCD) films, focusing particularly on the ratio between the amount of O2 and N2 additives into conventional CH4/H2 gas mixtures on the morphology, microstructure, texture, and crystalline quality of the NCD films. The NCD samples were produced by using a high microwave power (3 kW) in a microwave plasma-assisted chemical vapour deposition reactor with a maximum power of 5 kW on large silicon wafers, 2 inches in diameter, and characterized by high-resolution scanning electron microscopy, x-ray diffraction and micro-Raman spectroscopy. Our work demonstrates that, under the conditions investigated here, NCD films can be formed when the ratio of O2/N2 addition is increased from 0 through 1 up to 7/3 (at higher than 7/3, for example 4, a large-grained polycrystalline diamond film will form), and the crystalline quality is significantly enhanced with the increase of oxygen addition. The mechanism of O2 and N2 additives on the formation of NCD films is briefly studied.

Characterization of chemical vapour deposited diamond films: correlation between hydrogen incorporation and film morphology and quality

In order to tailor diamond synthesized through chemical vapour deposition (CVD) for different applications, many diamond films of different colours and variable quality were deposited by a 5 kW microwave plasma CVD reactor under different growth conditions. The morphology, quality and hydrogen incorporation of these films were characterized using scanning electron microscopy (SEM), Raman and Fourier-transform infrared (FTIR) spectroscopy, respectively. From this study, a general trend between hydrogen incorporation and film colour, morphology and quality was found. That is, as the films sorted by colour gradually become darker, ranging from white through grey to black, high magnification SEM images illustrate that the smoothness of the well defined crystalline facet gradually decreases and second nucleation starts to appear on it, indicating gradual degradation of the crystalline quality. Correspondingly, Raman spectra evidence that the diamond Raman peak at 1332 cm−1 becomes broader and the non-diamond carbon band around 1500 cm−1 starts to appear and becomes stronger, confirming increase of the non-diamond component and decrease of the phase purity of the film, while FTIR spectra show that the CH stretching band and the two CVD diamond specific peaks around 2830 cm−1 rise rapidly, and this indicates that the total amount of hydrogen incorporated into the film increases significantly.