Article pubs.acs.org/crystal
Large Area Single-Crystal Diamond Synthesis by 915 MHz Microwave Plasma-Assisted Chemical Vapor Deposition Qi Liang,*,† Chih-shiue Yan,† Joseph Lai,†,‡ Yu-fei Meng,†,‡ Szczesny Krasnicki,† Haiyun Shu,†,‡ Ho-kwang Mao,†,‡ and Russell J. Hemley*,† †
Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, United States Center for High Pressure Science and Technology Advanced Research, 1690 Cailun Rd, Pudong, Shanghai 201203, P.R. China
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ABSTRACT: A 75 kW, 915 MHz microwave plasma-assisted chemical vapor deposition system was adapted and utilized to scale up production of high-quality single-crystal diamonds at high growth rates. A 300 mm diameter plasma discharge was achieved with uniform temperature distributions of ±250 °C on up to 300 single-crystal diamond substrates. Diamond single crystals were synthesized from H2/CH4/N2 gas mixtures at pressures between 90 and 180 Torr, with recorded growth rates from 10 to 30 μm/h. The source of N2 was from vacuum chamber leakage, and it greatly affected synthesis chemistry. Optical emission spectroscopy was used to probe the localized plasma chemistry and plasma uniformity at different gas pressures. Production rates of up to 100 g/day of single-crystal diamonds were demonstrated, with 25% of the material categorized as colorless. Crystals up to 3.5 mm in thickness could be produced during a single deposition run. The quality of the crystals produced was assessed by photoluminescence and UV−visible absorption spectroscopies.
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INTRODUCTION A single-crystal diamond produced by microwave plasmaenhanced chemical vapor deposition (MPCVD) has been one of the centerpieces of CVD diamond science and technology. Advances have been achieved in high growth rates (up to 165 μm/h)1,2 and in the production of large (up to 10 carats)3 and high-quality diamonds.4 In a typical laboratory environment, MPCVD reactors use up to 6 kW of microwave power at 2.45 GHz to generate energetic plasma at high gas pressures from 150 to 300 Torr. To control the substrate growth temperature, aggressive sample cooling on the backside is required.5 Although small volume diamond production using such reactors is possible, the synthesis area is restrained by the plasma discharging area that is normally no larger than 25 mm in diameter.6 It was shown that either decreasing the growth pressure or increasing the incident microwave power can increase the deposition area.1,6 However, currently available tubular 2.45 GHz MPCVD systems are not designed to handle the excessive heat generated under those conditions that can lead to a catastrophic system failure. A MPCVD system operated at 915 MHz has long been considered a cost-effective tool for scaling up high-quality CVD diamond production.7 Recently, efforts have been made in producing up to 70 single-crystal diamond crystals in a batch by a bell jar type MPCVD system. An average growth rate of 14− 21 μm/h was reported for those samples with considerably large amounts of addition of nitrogen, although little information about the characteristics of crystals produced was provided.8 It is worth noting that a large amount of nitrogen added to the gas chemistry introduces nitrogen-related defect centers9 that affect a diamond’s physical properties. Here, we report growth studies in a 915 MHz CVD reactor operated at a © XXXX American Chemical Society
microwave power of up to 70 kW and with a deposition area up to 300 mm in diameter. Plasma chemistry diagnosis was conducted using optical emission spectroscopy (OES). Optical properties of the single-crystal CVD diamond produced were studied using photoluminescence (PL) and UV−visible spectroscopy at room temperature
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EXPERIMENTAL DETAILS
A clamshell type microwave plasma system equipped with a 360 mm diameter water-cooling stage and a 915 MHz, 75 kW maximum, switch-mode microwave generator was used for diamond deposition. A hydrogen generator with an external palladium purifier was able to generate 99.99999% pure hydrogen for the system. A methane purification system was used to decrease the nitrogen content in the research grade methane line to
