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Photochemical Modification of Diamond Surfaces John B. Miller*,† and Duncan W. Brown‡ Chemistry Department, Western Michigan University, Kalamazoo, Michigan 49008, and Advanced Technology Materials, Inc., 7 Commerce Drive, Danbury, Connecticut 06810 Received September 6, 1994. In Final Form: July 30, 1996X Photochemical and thermal surface modifications of diamond powders and (100), (110), and (111) oriented single crystals were achieved. Clean, hydrogen-terminated diamond was chlorinated by heating in Cl2(g) or by UV irradiation in Cl2(g). The surface chlorine titer was shown to correlate with the dangling bond density on the single-crystal faces. Aminated diamond was produced by irradiating the chlorinated diamond in NH3(g). Surface nitrogen loading correlated with the dangling bond density. The structural features of the amine-terminated surface led to a decrease in the vibrational frequencies of the surface amine groups.
Introduction The technology of electronic devices based upon the properties of wide band gap semiconductors such as diamond is expanding rapidly.1 Thick, polycrystalline diamond films are technologically relevant due to their use in multichip module and heat sink applications, while thin films and single crystals are candidates for use in many electronic devices. Production and operation of all electronic devices rely heavily on the surface properties of the materials used in fabricating them. The surface chemical properties of the semiconductor play a major role in determining the electrical and mechanical interfaces of the semiconductor to other materials, including metals, other semiconductors, and insulators. Diamond poses especially vexing problems in adhesion and electrical contact formation due to its inherently low chemical reactivity and high charge-carrier activation energy.2,3 In this work, we investigated the chemical modification of the diamond surface by both thermal and photochemical reactions. We examined the reactivity of specific, lowindex faces of diamond single crystals and micrometer size diamond powders. The modification of polycrystalline diamond films is described elsewhere.4 From a practical standpoint, the powders are a logical model to use for studying diamond surface chemistry for several reasons. First, the surface area to volume ratio is relatively large; therefore measurable changes in the mass due to surface reactions are possible using a reasonably small amount of material. Second, diffuse reflectance infrared (DRIFT) spectroscopy provides a rapid technique for obtaining the surface vibrational spectrum of the diamond powders without the confounding effects of mulling or pelleting agents and with minimal contribution from the bulk lattice vibrational modes. However, diamond powders expose many different crystal faces; therefore the DRIFT spectra presented herein are the convolution of the spectra of many individual faces. Controlled chemical modification of the diamond (C*) surface has proven difficult. Although many surfaces are quite reactive, the diamond surface is generally inert to †
Western Michigan University. Advanced Technology Materials, Inc. X Abstract published in Advance ACS Abstracts, October 1, 1996. ‡
(1) Davis, R. F.; Sitar, Z.; Williams, B. E.; Kong, H. S.; Kim, H. J.; Palmour, J. W.; Edmond, J. A.; Ryu, J.; Glass, J. T.; Carter, C. H. Mater. Sci. Eng. 1988, B1, 77. (2) Das, K.; Venkatesan, V.; Miyata, K.; Dreifus, D. L.; Glass, J. T. Thin Solid Films 1992, 212, 19 and references therein. (3) Collins, A. T.; Lightowlers, E. C. Electrical Properties. In The Properties of Diamond; Field, J. E., Ed.; Academic Press: London, 1979. (4) Miller, J. B.; Brown, D. W. Diamond Relat. Mater. 1995, 4, 435.
S0743-7463(94)00716-X CCC: $12.00
most chemical reagents. Many potential modificationreaction pathways are apparently precluded by two major factors: (1) the steric bulk of the diamond surface constrains the attack trajectory for an incoming reagent to a near surface-normal vector and (2) the sp3-hybridized carbons of the diamond bulk and the C-H groups presented by the unreconstructed diamond surface are inherently low in reactivitysthey offer little in the way of useful molecular orbitals for attack by either nucleophiles or electrophiles. There are several notable exceptions to the generally low reactivity of diamond. First, diamond surfaces can undergo a phase transformation to graphite,5 and the graphite is then used as the basis for further surface reactions. This may not be particularly useful in the context of electronic devices relying on properties specific to diamond. Second, diamond oxidizes. The prima facie most obvious oxidative method, heating in the presence of O2, graphitizes the diamond surface prior to any further oxidation.6 Oxidative surface modification may also be accomplished by an etching procedure employing molten alkali nitrates.7 This technique is neither especially safe nor controllable, and it could prove problematic when designing device processing steps to preserve delicate microstructures. A more practical method for surface modification is the direct reaction of diamond via a radical reaction. The radicals that are known to react directly with diamond include hydrogen atoms,8 fluorine atoms,9 and chlorine atoms.10 The diamond surface is unreactive to the molecular species H2, F2, and Cl2.11 Although molecular H2 and Cl2 have been used as reagents, the reaction conditions required are such that the atomic species are produced. Those conditions are either very vigorous and corrosive (for example, Cl2/400-500 °C) or unsuitable for (5) The diamond to graphite phase transformation may be induced by any of the following methods. (a) Simple heating, see: Matsumoto, S.; Sato, Y.; Setaka, N. Carbon 1981, 19, 234. (b) Ion bombardment, see: Cong, Y.; Collins, R. W.; Messier, R.; Vedam, K.; Epps, G. E.; Windischmann, H. J. Vac. Sci. Technol. 1991, A9, 1123. (c) High-energy laser radiation, see: Ageev, V. P.; Chapliev, N. I.; Konov, V. I.; Kuz’michev, A. V.; Pimenov, S. M.; Ral’chenko, V. G. Phys. Res. 1990, 13, 318. (d) High-power laser radiation, see: Rothschild, M.; Arnonr, C.; Ehrlich, D. J. J. Vac. Sci. Technol. 1986, B4, 310. (6) Sappok, R.; Boehm, H. P. Carbon 1968, 6, 573. (7) Tolansky, S.; Miller, R. F.; Punglia, J. Philos. Mag. 1972, 26, 1275. (8) Yamada, T.; Chuang, T. J.; Seki, H.; Mitsuda, Y. Mol. Phys. 1991, 76, 887. (9) Freedman, A.; Stinespring, C. D. Appl. Phys. Lett. 1990, 57, 1194. (10) Sappok, R.; Boehm, H. P. Carbon 1968, 6, 283. (11) Tapraeva, F. M.; Pushkin, A. N.; Epshina, N. I.; Kulakova, I. I.; Rudenko, A. P. Zh. Fiz. Khim. 1986, 60, 1814.
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implementation on a production scale (i.e., fluorine atomic beams). A previously reported thermal amination procedure12 proved irreproducible. More recently, a chlorinecontaining ammonolysis product was observed during high-temperature (425 °C) ammonolysis; however, the identity of that product remains speculative.13 We sought a mild and controllable procedure for modifying the diamond surface that might then open further pathways for chemical modification. For application to diamond-based electronics, the process should be compatible with the current device processing technology and electronic microstructures. Since radical reactions could be a practical method for modifying diamond, we investigated the use of photochemically generated radicals for this purpose. This work describes the result of a comparison of thermally and photochemically induced diamond surface modification. Experimental Section Materials. Diamond single crystals, type IIa, were obtained from Dubbeldee Harris. The crystals were cleaned, ex situ, by a three-step process starting with a 5 min etch in 160 °C chromic acid, following with a 10 min wash in a 60 °C solution of 1:1 H2O2/NH4OH, and finishing with a 1-2 min dip in aqueous HF. Only occasional trace amounts of surface oxygen were observed by Auger electron spectroscopy of the crystals prepared in this fashion. Line shape analysis of the CKLL Auger transition14 indicated that the surface carbons were diamond. The diamond powders used were obtained from Kay Industrial Diamond Corporation. Two types of 1-3 µm diamond powders were used. The first was synthetic, type SJK-5, while the second was virgin, natural diamond. The manufacturer’s standard preparation is a sequential cleaning by molten KOH/KNO2 and aqua regia followed by rinsing. However, samples prepared in this fashion were found, by energy dispersive X-ray spectroscopy (EDX), to be contaminated by significant amounts of calcium and phosphorus, presumably from the rinse water. Copious rinsing with deionized water eliminated this contamination. Thereafter, all of the samples obtained were prepared by the manufacturer using exclusively deionized water. The surface area of the natural diamond powder was measured to be 2.80 m2‚g-1, as determined using the Brunauer-Emmett-Teller (BET) method15 with a 3:7 N2/He adsorbate mixture. Argon (zero grade), Cl2 (99.9%), and NH3 (99.99%) were obtained from MG Industries. ND3 (99 atom %-D) was obtained from CIL. All gases were used as received. Thermal Pretreatment of Diamond Powders. A weighed aliquot (ca. 0.5 g typically) of diamond powder was placed into a tared quartz boat (8 cm × 9 mm diameter) capped at the ends by a fine quartz frit. The boat was then placed into a quartz tube attached to a vacuum/gas manifold. The tube was slowly evacuated to less than 1 × 10-4 Torr with a turbomolecular pump. Care was taken to avoid blowing the fine diamond powder or the single crystals. The tube was then slowly heated (15 °C‚min-1) to 950 °C with constant pumping. The tube was maintained at 950 °C for 1 h, then allowed to cool rapidly to room temperature (ca. 21 °C). The tube was backfilled with argon, and the boat and powder were weighed. The surface area of the diamond powders was unaffected by this thermal treatment, as determined by BET analysis. Thermal Chlorination of Diamond. A weighed aliquot (0.5 g typically) of the thermally treated diamond powder or a singlecrystal diamond was placed into a tared quartz boat and then into the quartz tube of the reactor. The tube was evacuated to less than 8 × 10-5 Torr, then rapidly heated to 425 °C under vacuum. One atmosphere of chlorine gas was slowly introduced over several minutes. The slow addition allowed the pressure (12) Makal’skii, V. I.; Loktev, V. In Vliyanie Khim. Fiz.-Khim Vozdeistv. Svoistva Almazov; Bogatyreva, G. P., Ed.; Akad. Nauk Ukr. SSR, Inst. Sverkhtverd Mater.: Kiev, USSR, 1990; pp 48-54. (13) Ando, T.; Yamamoto, K.; Suehara, S.; Kamo, M.; Sato, Y.; Shimosaki, S.; Nishitani-Gamo, M. J. Chin. Chem. Soc. 1995, 42, 285. (14) Lurie, P. G.; Wilson, J. M. Surf. Sci. 1977, 65, 476. (15) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.
Miller and Brown to equilibrate and eliminated blowing the diamond sample. The chlorine was typically left in contact with the hot diamond for about 1 h, after which the tube was allowed to cool rapidly to room temperature. The chlorine was then purged from the tube with argon through an aqueous NaOH scrubber for 5-15 min, and the boat and diamond were weighed. Photochemical Chlorination of Diamond. A weighed aliquot (0.5 g typically) of the thermally treated diamond powder or a previously heated or unheated single-crystal diamond was placed into a tared quartz boat and then into the quartz tube of the reactor. The tube was evacuated to less than 8 × 10-5 Torr and 1 atm of chlorine gas was slowly introduced. The diamond was irradiated with the light from a high-pressure Hg arc lamp (Ace-Hanovia, medium arc, 450 W) at a distance of approximately 20 cm for 1-24 h. After irradiation, the reactor tube surface temperature increased to only 28 °C from the heat of the UV lamp. The chlorine was then purged from the tube with argon through an aqueous NaOH scrubber for 5-15 min, and the boat and diamond were weighed. The surface area of the diamond powder was unaffected by the photochemical chlorination, as determined by BET analysis. Thermal Exposure of Chlorinated Diamond to Ammonia. A weighed aliquot (0.5 g typically) of the chlorinated diamond powder or a diamond single crystal was placed into a tared quartz boat and then into the quartz tube of the reactor. The tube was evacuated to less than 8 × 10-5 Torr, then rapidly heated to 450 °C under vacuum. One atmosphere of ammonia gas was slowly introduced. The ammonia was left in contact with the hot, chlorinated diamond overnight (14-16 h), after which the tube was allowed to cool rapidly to room temperature (ca. 21 °C). The ammonia was then purged from the tube with argon (UHP) through a water scrubber for 3-5 min, and the boat and diamond were weighed. A water-soluble white residue was observed on the cold portions of the tube walls when the diamond powder was used. This substance was determined to be NH4Cl on the basis of the standard chloride precipitation and NH3(g) evolution tests. First, a milky precipitate formed when 0.1 M AgNO3 was added to an aqueous solution of the residue; this precipitate darkened overnight. Second, moistened pH test paper turned blue when held above a solution of the residue to which had been added 5 M NaOH, indicating a gaseous base. Photochemical Amination of Chlorinated Diamond. An aliquot (0.5 g typically) of the chlorinated diamond powder or a diamond single crystal was placed into a quartz boat and then into the quartz tube of the reactor. The tube was evacuated to less than 8 × 10-5 Torr, and 1 atm of ammonia gas was slowly introduced. The diamond was irradiated with the Hg arc lamp at a distance of approximately 20 cm for 3-24 h. After irradiation, the reactor tube surface temperature increased from 21 to 28 °C from the heat of the UV lamp. The ammonia was then purged from the tube with argon through a water scrubber for 3-5 min. Diffuse Reflectance Fourier Transform Infrared Spectroscopy. Diffuse reflectance spectra were obtained using a Perkin-Elmer 1600 FT-IR equipped with a Spectra-Tech diffuse reflectance attachment. All of the spectra were recorded as percent reflectance, R, which is the ratio of the spectral intensity of the sample, S, divided by the intensity of a background sample, B; thus %R ) 100(S/B). The background spectra were generally those of the powdered diamond samples that were used as the starting material for a specific treatment. The resulting spectra highlighted the differences between the starting material and product; descending peaks resulted from the functional groups that were more prevalent on the product, while ascending peaks were due to the groups that are more common on the starting powder, i.e., those that were lost during the procedure. When an absolute spectrum was required, dry, powdered KBr was employed as a neutral reflector. To avoid disturbing the fine powders with flowing gas, the sample chamber was not purged. Due to minor variations in the light path length, the P- and R-branches of the gas-phase CO2 asymmetric stretch occasionally appeared as ascending or descending peaks at 2360 and 2338 cm-1 and were unrelated to any surface processes or properties. These bands are indicated by asterisks in the displayed spectra. X-ray Photoelectron Spectroscopy (XPS). The X-ray photoelectron spectroscopy was performed by Surface Science
Photochemical Modification of Diamond Surfaces
Figure 1. Change in mass of as-received diamond powders with heating in vacuo. Laboratories. Excitation was by near-normal incidence of monochromatized Al KR photons in a 600 µm diameter spot. Due to the high resistivity of the diamond samples used, a low-energy electron flood gun was employed to minimize sample charging. To separate the surface and bulk contributions to the spectra, certain samples were also examined using a glancing-angle incident X-ray beam. Data in the C(1s) region were the result of coadding 5 scans, while the N(1s) and Cl(2p) regions were the coaddition of 30 scans. Due to the lower counting rate, more scans were coadded for the glancing-angle measurements: 8 scans in the C(1s) region and 80 scans in the N(1s) and Cl(2p) regions. For quantitative analysis, the electron counting rate was used; the integrated peak area for a given element was normalized to that of C(1s) on the same sample. The energy scale was internally calibrated by referencing the binding energy of the major C(1s) peak to the 285.0 eV literature value for bulk diamond.16 The X-ray photoelectron spectra were fit assuming a 1:1 Gaussian/Lorentzian line shape17 and a linear background using a nonlinear least-squares method. Choosing the number of component peaks to employ was an iterative process. First, a minimum number of component peaks were assumed to make up each spectrum. For example, the N(1s) spectrum will have at least one component, while the Cl(2p) spectrum must have at least two components. A fit was attempted based on the minimum assumption. If the fit showed a systematic deviation from the observed spectrum, additional peaks were added. The best fit was considered to be the one with the lowest statistical χ2 ) (fobs - ffit)2/fobs, starting from two or more sets of initial conditions. Auger Electron Spectroscopy. Auger spectroscopy was performed in a stainless steel ultrahigh vacuum (UHV) chamber pumped by a 220 L‚s-1 ion pump and a 100 L‚s-1 cryopump and equipped with a Perkin-Elmer Φ 15-110A Auger Analyzer having an integral electron gun. The incident beam was 2 keV with an emission current of 1.2 mA and was modulated to 2 eV peakto-peak. Sample currents were measured to be ca. 0.04-0.10 µA; charging was occasionally observed, but rarely prevented repeatable results. Spectra were obtained in the first derivative mode, dN(E)/dE, using a lock-in amplifier and an X-Y chart recorder. Energy Dispersive X-ray Spectroscopy. EDX spectra were obtained using an ISI scanning electron microscope (SEM) equipped with a KEVEX energy dispersive X-ray spectrometer. The primary electron beam energy was 10 keV, and the beam intersected the sample at an angle of 25°. The SEM magnification setting was typically 100×. Count rates were adjusted to be at or below 3000 counts‚s-1. Regardless of the count rate, the signal was integrated until about 2.5 × 105 total counts had been recorded. Powdered samples were lightly pressed into a thin uniform layer on a circle of mounting adhesive placed on an aluminum stud. The areas that were examined by EDX were (16) Morar, J. F.; Himpsel, F. J.; Hollinger, G.; Jordan, J. L.; Hughes, G.; McFeely, F. R. Phys. Rev. B 1986, 33, 1340. (17) Sherwood, P. M. A. Data Analysis in X-ray Photoelectron Spectroscopy. In Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1983.
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Figure 2. Thermal desorption from as-received diamond powders as determined by total pressure as a function of temperature. (For comparison, see ref 20.) covered with a thick layer of diamond powder, as observed in the SEM images. No peaks from the stud (Al, O) or adhesive (Si, O) were observed.
Results Preparation of Hydrogen-Terminated Diamond Powders. To rationally apply any of the methods for the surface modification of diamond, it was essential to obtain a consistent starting material. Unfortunately, commercially available diamond powders had a highly oxidized surface, which did not reflect the type of surface found on CVD grown diamond films18 or on natural or synthetic diamond single crystals;19 these are generally hydrogen terminated. Vacuum annealing of the diamond powders was found to be an effective means of generating a hydrogen-terminated surface which may be similar to that of both the films and single crystals. Material was lost from the as-received diamond powders upon heating. Figure 1 shows the dependence of the mass changes on the annealing temperature. An average decrease of 0.32% in the mass of the powder sample was observed after heating to 950 °C in vacuo (to form the H-terminated surface) with no further mass changes measured upon extended annealing at that temperature. The mass loss during the heating of the as-received diamond powders apparently originated from volatilization of several surface species, presumably produced by the manufacturer’s highly oxidizing preparation. Thermal desorption data, plotted in Figure 2, shows the total pressure as a function of temperature during slow heating (15 °C‚min-1). The shape of this desorption curve and its peak desorption rate near 650 °C were consistent with the previous accounts of measured desorptions by Matsumoto et al.20 Examination of the DRIFT spectra taken of the diamond powders at various stages during the heat treatment, shown in Figure 3, revealed a correlation between the lost mass and a decrease in at least four absorption bands between 1200 and 1800 cm-1. Recall that the spectra shown in Figure 3 are difference spectra, using the asreceived diamond powder as the background, so ascending peaks are due to groups that were more common on the starting powder, i.e., those that were lost upon heating. (18) Mansour, A.; Indlekofer, G.; Oelhafen, P. Appl. Surf. Sci. 1991, 48/49, 312. (19) Derry, T. E.; Madiba, C. C. P.; Sellschop, J. P. F. Nucl. Instrum. Methods Phys. Res. 1983, 218, 559. (20) The desorbing species were reported to include H2O (
