The role of hydrogen in diamond synthesis using a microwave plasma in a CO/H2 system

Abstract

In order to clarify the role of hydrogen in diamond synthesis using a microwave plasma in a CO/H2 system, carbon films were grown by varying hydrogen mole fractions in a CO/H2/He microwave plasma. The correlation between film properties and plasma species was investigated through film characterization and plasma emission spectroscopy. C and C2 were formed in the gas phase of the CO/He system and only sootlike carbon was deposited. Hydrogen additions to the CO/He system were found to enhance diamond growth by suppressing the formation of C and C2, which inhibited diamond growth by blocking the nucleation sites. The complicated structure of amorphous hydrogenated carbon, diamond microcrystallites having a diameter of 100 Å, and graphitic carbon was formed in the CO(5%)/H2 (30%)/He system, while columnar polycrystallites were grown in the CO(5%)/H2 system. Almost the same amount of atomic hydrogen in the ground state was found to exist in both systems, whereas a larger amount of electronically excited atomic hydrogen existed in the CO/H2/He system than the CO/H2 system. The atomic hydrogen in the ground state enhanced diamond growth by removing the amorphous carbon deposits, while the electronically excited atomic hydrogen exhibited no contribution to diamond growth and reacted with the amorphous carbon deposits to form the amorphous hydrogenated carbon. Finally, oxygen-containing species and hydrocarbons were suggested to be important precursors for diamond crystallization.

Real time spectroellipsometry for optimization of diamond film growth by microwave plasma-enhanced chemical vapor deposition from CO/H2 mixtures

“…Muranaka et al have proposed that CO on the diamond surface can react with excited molecular hydrogen from the plasma, leading to C incorporation in the diamond phase and H adsorbed on the diamond surface. 5,6 The general features of this simple model are consistent with the results of Fig. 6.…”

“…1-6 Furthermore, additions of O-containing atoms to the gas source can improve the diamond film quality, resulting in a lower nondiamond ͑or sp 2 -bonded͒ carbon content, and allowing diamond growth to be extended to lower substrate temperatures while retaining high quality. [7][8][9][10][11][12][13][14][15][16][17] It has been first suggested 3,5,18 and later demonstrated 19 that most of the diamond phase generated by a plasma of CO and H 2 with CO/H 2 ϳ0.05 results from the conversion of CO to hydrocarbon species with the initial step being CO excitation by energetic plasma electrons. In contrast to CH 4 /H 2 mixtures, from which high quality diamond films can be obtained only if CH 4 /H 2 Ͻ0.02, 20 CO/H 2 mixtures yield high quality films over a very wide range 2 -at least two orders of magnitude in the CO/H 2 dilution ratio, from 0.04 to ϳ5, as shown below.…”

Measurements of C2 and CH concentrations and temperatures in a dc arc jet using cavity ring-down spectroscopy

“…Density functional theory ͑DFT͒ calculations, 34 motivated by the experimental observation of deposition of diamond films by C 2 produced from C 60 fragmentation in hydrogen poor plasmas, 47,48 showed that growth by C 2 at a bare ͑i.e., not H terminated͒, clean diamond ͑110͒ surface is also energetically favored, and presents only very low energy barriers to the formation of C 2n chains and, ultimately, new layers. Although there have been reports of increasing C 2 concentrations in low power microwave plasmas that are correlated with degraded diamond film quality, 49,50 Gruen et al observed a positive correlation between C 2 emission and the film growth rate, and OES studies by our group 6 and by others 8 demonstrated that growth of high quality diamond films can proceed hand in hand with strong C 2 dϪa emission in more highly activated gas plasma environments.…”

Deposition of diamond films at low pressure in a planar large-area microwave surface wave plasma source

“…20 can degrade the diamond quality because it makes the etching process of non-sp 3 bonding by the hydrogen atoms insufficient. The C 2 radical illustrated in Ref.…”