Mechanism of Diamond Nucleation on Titanium Substrate under Very Low Pressure

TL;DR

This study investigates the mechanism of diamond nucleation on titanium substrates under very low pressure, revealing high nucleation rates, the importance of supersaturation, and the benefits of low-pressure conditions for rapid, high-density diamond growth.

Contribution

It demonstrates that very low pressure significantly enhances nucleation density and rate, and elucidates the nucleation mechanism involving atomic hydrogen and carbon supersaturation.

Findings

High nucleation densities (10^8-10^{10} cm^{-2}) achieved at 1 torr.

Sufficient supersaturation of carbon species is critical for nucleation.

Very low pressure extends mean free path, promoting rapid nucleation.

Abstract

Nucleation and its mechanism of diamond on titanium substrates under very low pressure was studied using hot-filament chemical vapor deposition. Very high nucleation rates and densities (10^8-10^{10} cm^{-2}) were obtained under 1 torr, which were 1-3 orders of magnitude higher than the counterpart (10^7 cm^{-2}) under conventionally low pressure (tens of torr). The effects of substrate temperature and methane concentration under very low pressure were also investigated, revealing that, overly high substrate temperature leads to a relatively low nucleation density, and that higher CH_4 concentration gives rise to a higher density and a higher rate. The nucleation mechanism is discussed in detail. While a large amount of atomic hydrogen creates nucleating sites, sufficient supersaturation of carbon and/or hydrocarbon species on/near the substrate surface is the key factor for nucleation, in competition against the rapid formation of carbide. Very low pressure leads to long mean free path and other benefiting effects, and hence, is critical for rapid, high-density nucleation. Effects of substrate temperature and CH_4 concentration are also important. This further implies that C_2H_x (x<6) and CH_4 also contribute to nucleation, but CH_{1-3} dominates under very low pressure. The very-low-pressure method seems to be the only candidate to make diamond deposition on titanium films applicable. It also sheds light on how to increase the diamond growth rate.