New chemical pathway for large-area deposition of doped diamond films by linear antenna microwave plasma chemical vapor deposition

Highlights

• Boron-doped diamond grown only from a trimethyl borate and hydrogen gas mixture, with no further hydrocarbon required.

• Control over the doping level and morphology by changing the B/C and B/O ratios achieved by adding carbon dioxide.

• High growth rates, high doping levels, and low resistivity values were reached.

• Trimethyl borate is a low-cost and safe liquid source for large-area MWCVD growth of doped diamond films.

Abstract

Today, boron-doped diamond (BDD) is among the key materials for electrochemical sensing and advanced oxidation of micropollutants. Therefore, a cost-effective fabrication of large-area BDD electrodes is of high interest. We present an implementation of a liquid-phase boron precursor trimethyl borate for large-area deposition of boron-doped diamond films by linear antenna microwave plasma CVD. Trimethyl borate vapors were used not only as a source of boron for doping but also as the only source of carbon and oxygen, while completely saturating the requirements for the growth of high-quality boron-doped diamond films. However, to allow for control over the doping level through maintaining the B/C and B/O ratios, carbon dioxide was employed as an additional source of carbon and oxygen. The film morphology was controllable from microcrystalline to ultra-nanocrystalline by changing the concentrations of trimethyl borate. Using this unique precursor system, we were able to grow diamond films with a doping level in range from 8 × 10¹⁷ cm⁻³ to 2 × 10²² cm⁻³ and resistivity as low as 1.16 × 10⁻² Ω·cm. Low activation energies were calculated from the Arrhenius plot and growth rates as high as up to 170 nm/h for the low pressure microwave plasma CVD were reached. The investigation of the plasma emission spectrum revealed a chemical composition similar to that of hydrogen-rich plasmas with methane, and the proposed chemical reactions indicate that the diamond growth takes place via the hydroxyl radical. The results demonstrated that trimethyl borate is a suitable source of carbon and boron for the large-area growth of highly boron-doped diamond via low pressure microwave plasma CVD methods.

Introduction

The well-known unique properties of boron-doped diamond make it an ideal candidate for many applications, of which electrochemical detection and degradation using advanced oxidation processes (AOP) are among the most common [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. In particular, the latter one requires large-area boron-doped diamond (BDD) films with low electrical resistivity. Moreover, several micrometers thick BDD film is a must to avoid failure of the electrochemical electrode as a result of penetration of the treated solution into the substrate while causing substrate degradation and consequent BDD delamination. Since any industrial application requires a cost-effective fabrication of BDD electrodes, low-cost large-area methods for producing several micrometers thick layers of highly boron-doped diamond are highly needed for the widespread of BDD electrodes [13]. Commonly, the larger BDD thickness is welcomed as a means to i) minimizing the unwanted presence of voids in the diamond layer (which is highly probable for low thickness), and ii) enhancing chemical resistance by using large diamond crystals (whose size is almost proportional to film thickness) and finally, iii) decreasing of the number of the sp² hybridized carbons preferably formed at the grain boundaries (which area is smaller for the morphology consisting of large diamond crystals). Although the large-area hot-filament CVD method is already available commercially, so far only two large-area microwave plasma CVD (MWCVD) techniques have been explored, namely the distributed antenna array [14] and linear antenna (LA) [15]. The method most intensively studied is that of the linear antenna system, which was introduced in 2006 and later explored for the growth of intrinsic or boron-doped diamond films [16], [17].

Since the first published study, many successful attempts have been already made to produce high-quality BDD films by LA-MWCVD. A comprehensive study of gas precursor composition was published by Taylor et al. [18] who investigated the influence of the gas mixture on the boron doping level in the diamond film. The authors claim that the addition of a small amount of CO₂ is necessary for the growth of the diamond layer. Unfortunately, it was reported that the addition of oxygen negatively influences the incorporation of boron [19] and consequently the electrical properties. Therefore, the flow of the boron source precursor (diborane) corresponding to the B/C ratio in the hundreds of thousands of ppm was necessary for achieving high boron doping levels, which is extremely high as compared to that of a rotational ellipsoid cavity or multimode clamshell cavity microwave CVD systems. However, using high diborane concentrations makes the CVD process more expensive, and additionally, a highly explosive mixture is easily formed in moist air at room temperature which can ignite spontaneously. Not only the factor of large area but also that of low temperature synthesis (below 300 °C) is another advantageous characteristic of the LA-MWCVD method [20], [21]. Unfortunately, under these conditions, the growth rate becomes very low (<70 nm/h) for highly boron-doped diamond films. In summary, the requirements on hundreds of thousands of B/C ratios and slow growth rate make the LA-MWCVD process cost ineffective when several micrometers thick diamond layers are needed, e.g., for the purpose of fabricating large-area BDD electrodes for electrochemical applications.

To overcome the above-mentioned bottlenecks, alternative carbon sources can be investigated, including solid or liquid ones (e.g., graphite, acetone, methanol, ethanol) [22], [23]. These have some advantages over standard gaseous precursors, such as the much lower costs, easier availability, safe handling, and a wide range of compositions. Technological progress has been made in the CVD synthesis of intrinsic or boron-doped diamond films from liquid precursors. The pioneering study on boron doping from a liquid precursor should be assigned to the work of Okano et al. who successfully have grown boron-doped diamond using dissolved non-toxic boron trioxide (B₂O₃) powder in the methanol/acetone mixture in hot-filament CVD system [24]. Although this type of doping attracted the attention of some researchers, the inhomogeneous distribution of boron within the diamond film forced researchers to find alternative precursors. Herein, trimethyl borate (B(OCH₃)₃, TMBT) was first used as a liquid precursor for boron doping in the microwave plasma CVD [25] and HF CVD processes [26], [27], [28], [29]. In these growth experiments, TMBT was always used diluted in another liquid hydrocarbon as a source of dopant to the maximum of 40,000 ppm of B/C in the methane/hydrogen gas atmosphere.

For undiluted TMBT, we can simply achieve very high B/C ratios in the order of hundreds of thousands (up to 333,333 ppm). Such a high ratio is needed for the highly boron doped diamond growth in LA-MWCVD to become successful [18]. Moreover, TMBT alone also contains enough carbon and oxygen which evokes the idea to grow high-quality BDD films directly under hydrogen-rich gas conditions and solve the above-mentioned technological obstacles related to LA-MWCVD.

Herein, we present the first successful implementation of a liquid source precursor (trimethyl borate) in the growth of a large area of highly boron-doped diamond films at low pressure by linear antenna microwave CVD. In the first part of the experiments, evaporated TMBT is used as the only source of boron, carbon, and oxygen in a hydrogen-rich gas mixture. In the following experiments, various amounts of CO₂ were also added to the gas mixture to allow for better control over the structural and electrical properties of the grown BDD films.