Conference-ISSS-7-Growth Rate and Electrochemical Properties of Boron-Doped Diamond Films Prepared by Hot-Filament Chemical Vapor Deposition Methods

Boron-doped diamond (BDD) films have good electrochemical performance with a wide potential window and chemical stability in aqueous solutions, compared with other electrode materials made of Pt, glassy carbon, and so forth. BDD electrodes have been investigated for various industrial applications, such as ozone-dissolved water and effluent water treatment. In this study, to achieve a high synthesis rate of BDD films, trimethyl borate was additionally introduced to a hot-filament chemical vapor deposition (HF-CVD) system as a reactant gas. It was found that the growth rate and quality of diamond prepared using the HF-CVD system depended on the effect of CH4 concentration on hydrogen, distance from filament to substrate, and supply B/C ratios. BDD films with a high growth rate in the range from 2 to 4 μm/h have been obtained at a filament-to-substrate distance of 5 mm, a CH4 concentration of 4%, and B/C ratios of 0.3-2.0%. Cyclic voltammograms of a Pt and BDD electrodes in 0.2 M KNO3 have been investigated in terms of the effect of supply B/C ratios of 0.3, 0.5, and 2.0%. It was found that BDD electrodes had a wide potential window and a low background current compared with conventional Pt electrodes. The BDD films prepared and characterized in this study are efficient as electrodes for environmental applications. [DOI: 10.1380/ejssnt.2016.53]


INTRODUCTION
Diamond films, because of their mechanical hardness, high thermal conductivity, and excellent optical properties, are commercially important in a wide range of applications [1,2].Boron-doped diamond (BDD) films prepared by chemical vapor deposition (CVD) have a number of advantages over conventional materials such as platinum or glassy carbon.BDD electrodes have a wide electrochemical potential window and a very low background current in aqueous electrolyte, in addition to chemical and physical stability.Therefore, BDD electrodes have been investigated for application to wastewater treatment.
Hot-filament (HF) and microwave (MW) methods are widely used in major CVD diamond-coating techniques.The deposition area of a MW reactor is limited by the uniformity of plasma density.The maximum deposition area in state-of-the-art MW systems is limited to about 150 mm diameter.On the other hand, the flexibility of the HF-CVD method [3] is shown by commercial systems that can uniformly coat a silicon wafer of 300 mm diameter with diamond.HF-CVD systems are one of the most practical and economical methods of CVD diamond.One of the main disadvantages of HF-CVD systems is the low growth rate, typically about 1 µm/h.Because of the high production cost for BDD electrodes using these systems, its industrial-scale application has not been widespread.
In the HF-CVD system, a mixture of H 2 and CH 4 gases is activated by a filament through thermal heating.The activation of gases is the first step in the diamond thinfilm growth.The growth of diamond films is dependent on many parameters [4][5][6][7][8][9][10], such as filament temperature, substrate temperature, substrate material, and reactant gas mixture.
Although the HF-CVD systems have been used for diamond coatings, the elementary chemical processes in the gas phase as well as the surface reactions are not completely understood.Recently, the deposition rate has been improved by using graphite etching with only hydrogen as the reactor gas [11][12][13].For the high growth rate of diamond, it is necessary to know the methyl radical and the temperature distribution around the filament [14][15][16][17].The spatial profile of methyl radical concentration in a hot-filament reactor using cavity ring-down spectroscopy (CRDS) has been reported [18][19][20].In these results, methyl radicals decayed rapidly as a function of distance from the filament.The optimization of the distance between the filament and the substrate [18][19][20] was a contributing factor in the growth rate of diamond.For the purpose of reducing the production cost of BDD electrodes and coating the BDD films on a large area, the HF-CVD system was designed.A high growth rate of 3-5 µm/h and large-area (300 mm in diameter) deposition were achieved for nondoped diamond films.
In this study, to achieve the high growth rate of BDD films, trimethyl borate was additionally introduced to the HF-CVD system as a reactant gas.A series of BDD films were deposited on Si wafer substrates by HF-CVD.The film thickness was measured by observing a section of the films by scanning electron microscopy (SEM).Diamond films were characterized by Raman scattering spectroscopy and X-ray diffraction (XRD).The BDD electrodes were characterized by cyclic voltammetry (CV).In this work, the growth rate and quality of BDD films prepared by HF-CVD were investigated in terms of their  effect on process parameters.

II. EXPERIMENTAL METHODS
Three-inch Si wafers were used as substrates.These substrates were scratched using 1-3-µm-particle-size diamond paste.Every substrate was cleaned in an ultrasonic bath for 10 min with ethanol.The HF-CVD system used is shown schematically in Fig. 1.The process parameters of methane concentration, filament-to-substrate distance, and supply B/C ratios are shown in Table I.Tantalum wires of 130 mm length and 0.15 mm diameter were used as filament materials.The grid filament [21] having dimensions of 130 × 100 mm 2 consists of several tantalum wires arranged in parallel.Silicon substrates were set up on a water-cooled molybdenum substrate holder of 100 mm diameter.This substrate holder can be adjusted to regulate the distance from the substrate to the filament.The filament is resistively heated with DC current.In the presence of methane, at elevated temperatures, a tantalum filament is converted into tantalum carbide.Trimethyl boron (TMB) as the boron source in dilute hydrogen was introduced into the HF-CVD reactor for the BDD films.The thermoregulated temperature of TMB was maintained at the range from 0 to 25 the boron source of supply B/C ratios from 0.3 to 2.4% during the film growth.The filament temperature during diamond growth was measured using an optical pyrometer (Chino IR-CAS).The molybdenum substrate holder temperature was measured using a thermocouple inserted into this holder.
The film thickness was measured by observing the cross section of the films by SEM (HITACHI, Miniscope TM3000).The diamond films were characterized by XRD (Rigaku, Smart Lab) and Raman scattering spectroscopy (RENISHAW inVia Reflex, 532 nm laser).
The CV for the BDD/Si electrodes of various B/C ratios was performed in an aqueous solution of 0.2 M KNO 3 .The scan rate was 50 mV/s.A standard three-electrode cell system was set up to evaluate the electrochemical performance of the electrodes using a potentiostat (AUTOhttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

LAB, PGSTAT302
).A platinum wire and Ag/AgCl were used as counter electrode and reference electrode, respectively.All electrochemical experiments were carried out by exposing a constant geometric area of 1 mm 2 for the working electrode.

A. Growth rate of nondoped diamond films
To determine the growth rate and quality of diamond films, the CH 4 concentration in CH 4 -H 2 is the most important process parameter in HF-CVD.The dependence of the normalized diamond growth rate on CH 4 concentration is shown in Fig. 2. The reactor pressure was 5 kPa.The filament temperature was 2700 K.The filament-tosubstrate distance was 6 mm.In the case of CH 4 concentrations of 1, and 4%, Fig. 2 shows SEM images of typical polycrystalline diamond films grown on Si wafer.In the CH 4 concentration range from 1 to 4%, SEM images show typical polycrystalline diamond films grown on the substrate surface.In the case of 5% CH 4 concentration, the surface SEM image of the film shows a cauliflower morphology and nanocrystalline diamond particles that aggregate into roughly spherical structures.The growth rate of nondoped diamond increased with increasing CH 4 FIG. 5. Depth profile of the AlGaN sample etched with CF4 plasma at a gas pressure of 10 mTorr and a processing-time of 100 min.The normalized intensity at the unetched surface calculated using the peak intensity is equal to one.concentration up to 5%.However, it is desirable that the CH 4 concentration is lower than 4% for achieving polycrystalline diamond growth.
The growth rate of diamond films also depends strongly on the filament-to-substrate distance.In the case of 4% CH 4 concentration, Fig. 3 shows the dependence of diamond growth rate on the filament-to-substrate distance.The filament-to-substrate distance was varied from 5 to 20 mm.The substrate holder temperature was changed in the range of 900-1100 K.The growth rate of diamond increased rapidly with decreasing filament-to-substrate distance.For the high growth rate of diamond, it is necessary to know the spatial profile measurement results of methyl radical concentration in a hot-filament reactor.It was reported [14][15][16][17] that the methyl radicals decayed rapidly as a function of distance from the filament.It was recognized that the distance between the filament and the substrate was a contributing factor in the growth rate of diamond.
In the case of 3% CH 4 concentration, the surface SEM image and cross-sectional SEM image of nondoped diamond films are shown in Figs.4(a) and 4(b), respectively.The SEM morphology shows typical polycrystalline diamond films grown on the substrate surface.A cross section of this diamond film shows the growth to be essentially columnar.The X-ray diffraction patterns of the diamond films are shown in Fig. 5.In the case of CH 4 concentrations of 3 and 4%, the polycrystalline diamond structures of these films were identified on the basis of XRD patterns.The strong (111) peak of diamond films can be seen in the XRD pattern, indicating that the diamond films have a (111) preferred orientation.Raman spectra of nondoped diamond films in the case of CH 4 concentrations of 3, 4, and 5% are shown in Fig. 6.The characteristic sharp peak at 1332 cm −1 corresponds to sp 3 -diamond.The broad band at around 1550 cm −1 corresponds to a nondiamond phase such as sp 2 -bonded graphite.The peak intensity of the 1550 cm −1 band was increasing with increasing CH 4 concentration.It is generally accepted that atomic hydrogen etches sp 2 -bonded graphite much faster than sp 3 -bonded diamond.The presence of hydrogen is to prevent the growth of graphite.In the case of 5% CH 4 concentration, the rate of nondiamond growth exceeds its etch rate.

B. Growth rate of boron-doped diamond films
A series of BDD films were deposited on Si wafer substrates using the HF-CVD system.Growth rate and electrical resistivity relative to supply B/C ratio and CH 4 concentration are given in Table II.Electrical conductivity was measured by two-point probe methods.Because the film thicknesses of these samples were different in the range from 10 to 24 µm, it is necessary to consider the effects of contact resistance and the thickness variation from sample to sample.In the case of a constant CH 4 concentration of 3%, it was observed that the growth rates of BDD films decrease with increasing supply B/C ratio.This phenomenon was observed by several authors [22][23][24][25][26] and indicates that incorporated boron atoms inhibit the growth of diamond films.In the case of a constant CH 4 concentration of 4%, high growth rates in the range of 2.9-4.7 µm/h were realized by optimizing film formation conditions such as the filament-to-substrate distance.
Figure 7 shows SEM images of BDD films deposited at various B/C ratios.For the B/C ratios in the range of 0.4-1.2%, the BDD films mainly contain pyramid-shaped grains related to (111) facets.As the B/C ratio increased, it was found that the grain size of diamonds became small.In the case of the B/C ratio of 2.4%, the crystal size of diamonds became small and crystal boundaries became http://www.sssj.org/ejssnt( vague.Figures 8 and 9 show the Raman spectra of the BDD films for CH 4 concentrations of 3 and 4%, respectively.New peaks appear at 500 and 1200 cm −1 for the borondoped diamond.The Raman spectra of the BDD films exhibit a characteristic diamond peak at 1332 cm −1 and a band at 1200 cm −1 .The band at 1200 cm −1 became steeper and wider with increasing B/C ratio, and a reduction in the SP 3 -diamond peak is considered to be related to these effects.The 1200 cm −1 band was attributed to the disorder induced in the actual boron incorporation in the lattice [23].A 500 cm −1 band was also observed, which can be attributed to the vibration modes of a pair of boron atoms [25].Fig. 10 shows the X-ray patterns of the BDD films.In the case of B/C ratios of 0.4, 0.8, and 2.4%, the crystal structure of the BDD films was identified by XRD.The strong (111) peak of BDD films can be seen in the XRD pattern, indicating that the BDD films have a (111) preferred orientation.It is found that the position of the (111) peaks downshifts with increasing boron doping quantity, which indicates an expansion of the lattice parameter induced by boron incorporation in the impurity bond of boron [26][27][28][29][30]. Figure 11 shows cyclic voltammograms of a platinum and BDD electrodes [31][32][33][34] with B/C ratios 0.3, 0.5, and 2.0% in 0.2 M KNO 3 .It was found that the potential window of BDD electrodes is clearly the widest in comparison with that of the conventional Pt material.It was observed that the potential window of BDD electrodes decreased with increasing boron doping level.This phenomenon of high boron doping level is related to the grain boundaries containing sp 2 -bonded graphite impurities.

IV. CONCLUSIONS
The growth rate and quality of BDD films prepared by HF-CVD have been investigated in terms of the effects of methane concentration, filament-to-substrate distance, and supply B/C ratios.It was found that the growth rate and quality of the BDD films depended on the CH 4 concentration, filament-to-substrate distance, and supply B/C ratios.BDD films with a high growth rate in the range of 2-4 µm/h were obtained at a filament-tosubstrate distance of 5 mm, a CH 4 concentration of 4%, and supply B/C ratios of 0.3-2.0%.Cyclic voltammo-grams of a platinum and BDD electrodes in 0.2 M KNO 3 were investigated in terms of the effect of supply B/C ratios of 0.3, 0.5, and 2.0%.The potential window of BDD electrodes was clearly the widest in comparison with that of the conventional Pt material.It was observed that the potential window of BDD electrodes decreased with increasing boron doping level.The BDD films prepared and characterized in this study are efficient as electrodes for environmental applications.
FIG. 4. SEM images of nondoped polycrystalline diamond films grown on Si wafer.

TABLE I .
Process parameters.
. 2. Dependence of nondoped diamond growth rate on CH4 concentration at the filament temperature of 2700 K.
• C with FIGFIG.3. Relationship between nondoped diamond growth rate and filament-to-substrate distance.

TABLE II .
J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Growth rate and electrical resistivity of BDD films relative to supply B/C ratio and CH4 concentration.