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J. Phys. Chem. B 2006, 110, 23888-23897
Raman Imaging and Kelvin Probe Microscopy for the Examination of the Heterogeneity of Doping in Polycrystalline Boron-Doped Diamond Electrodes Sabine Szunerits,*,† Michel Mermoux,*,† Alexandre Crisci,† Bernadette Marcus,† Pierre Bouvier,† Didier Delabouglise,† Jean-Pierre Petit,† Sebastian Janel,‡,§ Rabah Boukherroub,‡,§ and Lilin Tay| Laboratoire d’Electrochimie et de Physicochimie des Mate´ riaux et des Interfaces, CNRS-INPG-UJF, 1130 rue de la piscine, BP 75, 38402 St. Martin d’He` res Cedex, France, Institut de Recherche Interdisciplinaire (IRI), FRE 2963, France, Institut d’Electronique, de Microe´ lectronique et de Nanotechnologie (IEMN), CNRS-8520, Cite´ Scientifique, AVenue Poincare´ - BP 60069, 59652 VilleneuVe d’Ascq, France, and National Research Council, IMS, 1200 Montreal Road, Ottawa, Ontario K1A 0R6 Canada ReceiVed: July 13, 2006; In Final Form: September 8, 2006
The issue of the heterogeneity of boron doping in microcrystalline diamond films was addressed by four different methods: micro-Raman spectroscopy and Raman imaging, Kelvin probe force microscopy, conducting atomic force microscopy, and scanning electrochemical microscopy. The samples were commercially available films from Windsor Scientific, with an average boron concentration of about 5 × 1020 cm-3. In agreement with previous works, all of the methods showed that the boron uptake was nonuniform across the surface of the electrode. Two different types of regions were evidenced, with metallic or semiconducting properties that were characterized with different types of Raman spectra. The line shape of these spectra was strongly dependent on the excitation wavelength. Local variations in electroactivity were evidenced by the SECM curves, which are related to the electronic properties of the individual grains, which, in turn, are governed by the boron content of the individual crystallites. In this study, two different micro-Raman imaging techniques were used that reveal the grain structure of the films: the images constructed from the diamond line intensity perfectly reproduced the optical image obtained by illuminating the sample in reflection. The method also allows detection of the presence of nondiamond carbon, especially in the metallic parts of the samples. Other spectral features (intensity of the boron-related broad lines, as well as the frequency and width of the diamond line) were used to construct images. In every case, the grain structure of the film was revealed, as well as twinning within individual crystallites. All approaches revealed that no enhanced doping or boron depletion occurred at the grain boundaries.
1. Introduction Boron-doped diamond (BDD) thin films have gained remarkable interest because of their specific physical properties. BDD electrodes show excellent mechanical properties, extreme chemical stability, good biocompatibility, good electrical conductivity, low background current densities, and a large potential window in aqueous electrolytes (about -1.35 to 2.3 V vs NHE).1,2 As a result, this new electrode material has found use in electroanalysis,3 wastewater purification,4 and biomedical applications.5-8 Polycrystalline BDD is generally produced by a variety of chemical vapor deposition (CVD) techniques. The boron dopant is added to the source gas (0.3-1% CH4 in H2), often in the form of diborane (B2H6), at concentrations varying between 1 and 10000 ppm. Depending on the growth conditions used, the properties of the diamond interface, such as the boron concentration, the morphology and size of the individual grains, * To whom correspondence should be addressed. E-mail:
[email protected] (S.S.),
[email protected] (M.M.). Phone: +33-4 76.82.65.52 (S.S.), +33-4 76.82.33.12 (M.M.). Fax: 33-4 76.82.66.30 (S.S.), 33-4 76.82.66.30 (M.M.). † Laboratoire d’Electrochimie et de Physicochimie des Mate ´ riaux et des Interfaces. ‡ Institut de Recherche Interdisciplinaire (IRI). § Institut d’Electronique, de Microe ´ lectronique et de Nanotechnologie (IEMN). | National Research Council.
and the ratio between nondiamond and diamond carbon, can be varied in a wide range. Although the above properties make BDD electrodes an attractive interface, many factors influence their electrochemical behavior. The most influential are (a) the dopant concentration and resulting dopant density, (b) the surface termination, (c) nondiamond carbon impurities (e.g., sp2 inclusions), (d) crystallographic orientation, and (e) fraction of grain boundaries. The dopant concentration probably has the greatest influence on the electrical and electrochemical properties of BDD electrodes. Schematically, for doping concentrations below ∼1019 cm-3, the apparent thermal activation energy equals the ionization energy Ea,opt ) 0.36 eV found in optical experiments. Because of the high bonding energy of the boron acceptors, only a small fraction (∼0.2%) of them are ionized at room temperature. Current conduction is a result of compensationdependent band conduction. In the intermediate boron concentration range [1019-(2 × 1020) cm-3], current conduction is dominated by nearest-neighbor hopping, and the hopping-related activation energy depends on the boron concentration. At still higher boron concentrations, “metallic” conduction is observed, with room-temperature conductivities on the order of 102 Ω-1 cm-1.10 In this concentration range, the carrier concentration is comparable to the solid-state boron concentration. This phe-
10.1021/jp064429+ CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
Doping in Polycrystalline Boron-Doped Diamond Electrodes nomenon is related to the decrease in the activation energy with increasing boron concentration. Boron atoms can also accumulate at grain boundaries. Grain boundary effects were reported by Holt et al. using C-AFM and SECM on very rough surfaces.9 However, the coating at the tip apex can quickly wear at such surfaces, resulting in artifacts particularly at steps and edges where the side of the metal-coated tip makes contact with the surface. The surface termination is a second important parameter for BDD electrodes. Because the deposition of diamond films is undertaken in a reductive medium (hydrogen plasma), the asdeposited diamond surface is hydrogen-terminated. However, this surface can be easily oxidized by a number of methods. The presence of oxygen on the diamond surface has a significant influence on the chemical reactivity,11,12 electrical conductivity,1,2,13,14 and electron affinity.15 Various conditions for diamond oxidation are reported in the literature, including thermal,16,17 plasma,18,19 photochemical,20 electrochemical,21-23 and singlet oxygen24 techniques. Thermal oxidation of hydrogen-terminated diamond under various conditions showed a final surface composition dominated by ether (CsOsC) and carbonyl (Cd O) groups.17 Although hydrogen termination has been found to promote the surface conductivity of intrinsic diamond,25 for heavily doped films, the conversion from hydrogen to oxygen surface termination does not significantly change the surface conductivity.26 The third factor is the fraction of sp2-bonded carbon often present within the grain boundaries or defect sites, which can show activity differing from the surrounding BDD material.27 All of these factors add to the heterogeneity of the surface of polycrystalline BDD. An additional complication is that boron uptake is known to depend on the crystallographic surface.28,29 Boron is more readily included into (111) facets than (100) sectors. Consequently, different grains can show different electrical and electrochemical behaviors ranging from semiconducting to metallic characteristics because of different boron uptakes. Different approaches have been used by various research groups to image the spatial distribution of the boron concentration and the variation in electroactivity at the surface of highly doped polycrystalline diamond. Honda et al. used electrogenerated chemiluminescence (ECL) generated by the reaction of Ru(bpy)32+ and tripropylamine to show the location and size of individual active regions.30 The ECL intensities for (100)oriented growth sectors were much lower than those for other growth sectors and remained at about 50% of those for (111) sectors. Raman imaging was used by the same group and showed that, by mapping the intensities of the one-phonon diamond line, the regions of maximum line intensity correspond to (100)-oriented microcrystallites. Confocal micro-Raman imaging was also used by our group to investigate the structural and, to some extent, chemical inhomogeneities that exist in a high-quality, free-standing, polished, optically transparent borondoped CVD diamond disk.31 Steeds et al. used transmission electron microcopy (TEM) as well as photoluminescence and micro-SIMS to investigate the nature of the inhomogeneities on boron-doped CVD diamond films.32-34 TEM showed that grains in thick polycrystalline CVD diamond form clusters with a common growth direction. The individual grains within a cluster are related by various twinning operations, which are characteristic of columnar CVD growth and do not depend on boron doping. Micro-SIMS clearly indicated the nonuniform introduction of boron into diamond samples, but it was unable to provide quantitative information.32 Bard and Swain used
J. Phys. Chem. B, Vol. 110, No. 47, 2006 23889 scanning electrochemical microscopy (SECM) as well as conductive atomic force microscopy to study hydrogenterminated boron-doped diamond electrodes with different doping levels.9 SECM has also been employed in our group to determine the local doping level of boron-doped polycrystalline electrodes.35 Recently, Macpherson et al. used SECM in conjunction with cathodoluminescence and conducting atomic force microscopy (C-AFM) to study the impact of graindependent boron uptake on the electrochemical and electrical properties of polished, polycrystalline, and oxidized BDD electrodes.36 Whereas C-AFM showed no evidence for enhanced grain boundary conductivity, different characteristic conductivity domains were observed corresponding to different crystallites in the diamond electrode. Using Ru(NH3)63+ as a redox mediator, SECM detected local currents of the magnitude expected for metal-like behavior in some regions, whereas slower electron transfer was apparent in others. In this article, we address the issue of the heterogeneity of boron doping in microcrystallite diamond films by examining the local optical and electrical characteristics using micro-Raman spectroscopy, Raman imaging, Kelvin probe microscopy (KFM), and conducting AFM (C-AFM). In particular, we used two different micro-Raman imaging techniques to investigate the chemical and structural inhomogeneities that exist in the films, at micrometer resolution. KFM, which is based on the noncontact atomic force microscopy mode, offers the possibility to obtain the sample’s surface work function with a lateral resolution in the nanometer range, enabling the study of the grain boundaries in significant detail. SECM was used in an effort to estimate the doping level of the sample. The polycrystalline diamond films studied were polished freestanding films, with a reported average boron concentration of 5 × 1020 boron atoms cm-3. These films are commercially available from Windsor Scientific (Slough Berkshire, U.K.) and are similar to those that were examined in ref 36. 2. Experimental Section Materials. Potassium chloride, hydrochloric acid, and iridium chloride (IrCl6-3) were purchased from Aldrich and used without further purification. Pt wire (10 µm in diameter) was obtained from Goodfellow. Boron-Doped Diamond Films. Polycrystalline boron-doped free-standing diamond films were purchased from Windsor Scientific (Slough Berkshire, U.K.). Briefly, highly boron-doped polycrystalline CVD diamond layers were grown to a thickness greater than 500 µm by adding diborane to the methane and hydrogen source gases supplied to a microwave CVD reactor. The samples were polished on both the nucleation and growth sides to a mirror finish. The final BDD electrodes had a thickness of about 500 µm. The electrodes were supplied as 5 × 5 mm square plates. The resultant samples had a bulk electrical resistance of about 0.75 Ω m. The average solid-state boron concentration of this material was reported by the manufacturer to be about 5 × 1020 cm-3 as determined by secondary ion mass spectroscopy (SIMS). Prior to use, samples were cleaned in 3:1 (v/v) concentrated H2SO4/H2O2 (30%) for 15 min, rinsed copiously with Milli-Q water, and then slightly polished using diamond paste (0.3 µm) to obtain a smooth particle-free surface. Experiments were carried out on different samples with similar results. Raman Spectroscopy. Micro-Raman scattering measurements were performed using two different spectrometers. The first was a Jobin-Yvon T64000 triple monochromator spectrometer that allowed measurements in both the visible and UV
23890 J. Phys. Chem. B, Vol. 110, No. 47, 2006 spectral ranges. For this purpose, it was equipped with a UVenhanced liquid-nitrogen-cooled CCD detector, a microscope, two different confocal optics, and interchangeable gratings (2400, 1800, 600 grooves/mm). A long-working-distance 50× (numerical aperture or NA ) 0.5) objective and a UV-dedicated 40× (NA ) 0.5) objective were used to focus the laser on the sample surface and collect the scattered light. The second spectrometer was an InVia Renishaw spectrometer that allowed measurements in the visible and near-infrared spectral ranges. It was equipped with an air-cooled CCD detector and a microscope. Spectra were acquired using a 50× (NA ) 0.95) objective. In the present work, samples were first examined with conventional micro-Raman spectroscopy at different excitation wavelengths. The three excitation wavelengths used were the 514-nm line of a krypton-argon laser, the 785-nm line of a laser diode, and the 244-nm line of a frequency-doubled argon laser (Coherent 90C FreD series). Although the powers of the three lasers were adjusted over a wide range during the measurements, no dramatic changes in the shape of the spectra or in the frequency and width of the diamond line were observed, indicating a high thermal conductivity of the samples. Using the acquisition conditions described above, the spectral resolution of the two different systems was
