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Characterization of defects in monocrystalline CVD diamond films by electron spin resonance
Diamond and Related Materials 12 (2003) 511–515
Characterization of defects in monocrystalline CVD diamond films by electron spin resonance K. Iakoubovskiia,*, A. Stesmansa, K. Suzukib, J. Kuwabarac, A. Sawabec a
Physics Department, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium b Toplas Engineering Co., Ltd., 1-9-9 Nishi-Tsutsujigaoka, Chofu-shi, Tokyo 182-0006, Japan c Department of Electrical Engineering and Electronics, Aoyama-Gakuin University, 6-16-1 Chitosedai, Setagaya-ku, Tokyo 157, Japan
Abstract Electron spin resonance (ESR) measurements on free-standing monocrystalline CVD diamond samples allowed us to deduce the symmetry of the previously reported KUL1 and KUL3 centers as trigonal and monoclinic-I, respectively. Based on the correlation between the intensities of the KUL3 ESR signals and of the 1.68 eV Si-related luminescence peak, and on the analysis of the observed hyperfine interaction structure, the KUL3 center is suggested to involve one Si and one hydrogen atom. Two previously undocumented centers, labeled as KUL16 and KUL17, have been characterized and attributed to two N100M-oriented surface defects, each involving one hydrogen atom. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond film; Defect characterization; Impurities
1. Introduction An important feature of chemical vapor deposition (CVD) of diamond is that the diamond growth mostly occurs using a hydrogen-rich gas mixture in a Sicontaining reactor. As a result, CVD diamond can be distinguished from other types of diamond by its relatively high concentrations of hydrogen and Si atoms. Indeed, electron spin resonance (ESR) applied to CVD diamond revealed a number of defect centers involving one hydrogen atom, such as H1 and H2 w1–3x, KUL2– 4, and KUL9 w4–6x, and two centers involving one Si atom (KUL1 and KUL8 w4–6x). Determination of the microscopic structure of those centers is important. However, this was impeded by the unfortunate morphology of CVD diamond: Most CVD diamond samples studied so far by ESR are polycrystalline films with random orientation of the grains. ESR spectra from such samples (the so-called ‘powder-pattern’ spectra) provide only limited information on the defect symmetry, which is essential for the defect modeling. In an attempt to overcome this problem, in the present paper we report ESR measurements on monocrystalline CVD diamond *Corresponding author. Tel.: q32-16-327-705; fax: q32-16-327987. E-mail address: [email protected] (K. Iakoubovskii).
samples, which reveal the symmetry of the KUL1 and KUL3 centers. Moreover, two previously undocumented centers, labeled as KUL16 and KUL17, are characterized and assigned to two surface defects, each involving one hydrogen atom. 2. Experimental details Monocrystalline CVD films can be easily grown on monocrystalline diamond substrates. However, such samples were not studied here because many ESR centers are present in CVD diamond in small concentrations (-1 ppm) and therefore can be masked by background signals from the substrate. Additionally, removal of a diamond substrate is a painstaking process. Instead, in the present work a monocrystalline diamond film ;5=10=0.03 mm3 in size was epitaxially deposited on a N100M-oriented Ir substrate. The detailed description of the sample preparation is given elsewhere w7x. The sample, hereafter referred to as sample 1, was grown by DC plasma CVD from an H2:CH4 gas mixture at a substrate temperature Ts1000 8C. After growth, the sample was immersed for 3 h in a Sn–Pb eutectic alloy kept at 400 8C to enable Ir etching. The metals were then dissolved first in HNO3 and then in a NaOH water solution kept at 60 8C.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635(02)00380-1
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512
Microscopical examinations revealed that samples 1 and 2 contained a few internal grain boundaries and therefore, strictly speaking, are not monocrystalline. However, mapping the ESR angular dependences of the P1 center revealed that the samples 1 and 2 might be considered as monocrystalline for the purposes of the ESR studies of the present work. ESR spectra were recorded at room temperature using a commercial Qband (;33 GHz) Bruker spectrometer. 3. Experimental results and analysis Several ESR centers, including P1, KUL1, 3, 5 and 6, H1 and H2, were detected in samples 1–3. Among those, the P1 center has previously been thoroughly characterized in monocrystalline synthetic and natural diamonds and assigned to the neutral single substitutional nitrogen center (NS8) w8x, while the characterization of the H1 center in monocrystalline CVD diamond has been reported recently w9x. Therefore, in this work we shall focus on the KUL1, 3, 5 and 6, and H2 centers. 3.1. KUL1 center
Fig. 1. Angular variation of the ESR signal resonance fields for the KUL1 (a) and KUL3 (b) center (300 K, Q band) measured for rotating the sample in a (01-1) plane. Lines present simulations, generated using the spin Hamiltonian parameters of Table 1.
Sample No 2 was prepared by cleaving a large (;200 mm in size) single grain from a 1.5-mm thick defectrich polycrystalline CVD diamond film. The film was grown by microwave plasma CVD on a Si substrate at Ts800 8C. For comparison, a defect-free polycrystalline diamond film (sample 3), grown at similar conditions, has also been studied.
The KUL1 center was detected in sample 1 and its angular map of signal resonance fields for rotation in a (01-1) plane has been obtained (squares in Fig. 1a). The lines in Fig. 1a represent a fit computer generated using the spin Hamiltonian parameters summarized in Table 1. Remarkably, the same values of the spectroscopic splitting factor g and of the spin–spin coupling constant D for the KUL1 center are obtained in the present single-crystal measurements as in previous powder-pattern ESR studies w4–6x. The observation of trigonal symmetry for the KUL1 center is important. This result, combined with the specific value of D, might suggest the assignment of the KUL1 center to a defect containing a Si atom at the center of a divacancy w10x. There is, however, an apparent disagreement between the observed high symmetry and the previously deduced
Table 1 Inferred room-temperature spin Hamiltonian parameters of the observed ESR centers Center
Symmetry
Spin
Matrix
Principal values
KUL1
Trigonal
1
KUL3
Monoclinic-I
1y2
KUL5 KUL6 KUL16
Tetragonal
1y2 1y2 1y2
KUL17
Tetragonal
1y2
g D g A g g g A g A
2.0040(1) y23.86(3) 2.00506(3) y0.10(2) 2.00290(3) 2.00306(3) 2.00318(3) 0.98(2) 2.00256(3) 1.08(2)
2.0035 11.93 2.00426 0 2.00290 2.00306 2.00272 0.72(2)
2.0035 11.93 2.00254 0.26 2.00290 2.00306 2.00272 0.72(2)
The sign of D1 for the KUL1 center is assumed to be negative. Values of D and A are in mT. The angles between the principal axes of the g ¯ and D or g and A matrixes are zero. ¯ ¯ ¯
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513
w4x small number of equivalent carbon sites (two and one), which needs to be resolved to attain adequate modeling of the KUL1 center. 3.2. KUL3 center The description of the KUL3 spectrum has been reported elsewhere w4x. This Ss1y2 ESR center is characterized by an unusually large for diamond anisotropy of the g factor (see Table 1) and by a structure originating from anisotropic hyperfine (hf) interaction with one hydrogen site. The KUL3 center is also detected in sample 1 and its angular map for rotation in a (01-1) plane has been measured (squares in Fig. 1b). The lines in Fig. 1b present a fit, computer generated using the spin Hamiltonian parameters of Table 1. Here again, the same principal values of matrix g and hf ¯ in the matrix A for the KUL3 center were deduced ¯ present single-crystal measurements as in previous powder-pattern ESR studies w4x thus increasing the reliability of the applied fitting procedure. The analysis of our previous measurements, performed over a large series of polycrystalline CVD diamond films, all grown on Si substrates at similar conditions, reveals that the intensity of the KUL3 center is the strongest in samples with the largest surface-tovolume ratio. However, all the equivalent defect orientations appear reflected in the angular map of Fig. 1b, while fewer equivalent orientations should be observed for a typical surface defect. Therefore, the KUL3 center is probably not a surface defect. There is one more pertinent observation: Sample 1 was found inhomogeneous with respect to the concentration of Si-related defects, as monitored by the intensity of the 1.68 eV luminescence line. The Si-rich parts did show the KUL3 ESR signals, while none could be detected in the Sipoor regions. This result may be interpreted as the KUL3 center being Si related; in CVD films grown on Si, the KUL3 center is predominantly present in the Sirich regions near the Si-diamond interface and thus its intensity is higher in the samples of a larger area, but not of a larger volume. The involvement of Si in the KUL3 center also logically explains the large observed g anisotropy: The latter is determined by the spin-orbit coupling, which is larger for a Si than for a carbon atom. The experimental data on the KUL3 center are still insufficient to enable reliable modeling. Very speculatively, this center may be associated with a neutral divacancy, where one of the six basal carbon atoms is replaced by Si, and one dangling bond (remote to Si atom) is passivated by a hydrogen atom. This model is consistent with the observed hydrogen hf values for the KUL3 center (A1sy0.10 mT, A2s0, A3s0.26 mT), which are similar to those of the neutral hydrogendivacancy in diamond (i.e. KUL9 center: A1sA2s
Fig. 2. Room-temperature Q-band ESR spectra observed on CVD diamond sample 2 (top curve) and 3 (bottom curve), recorded using a microwave power Pms1 mW and modulation amplitude of 0.02 mT.
y0.04 mT, A3s0.20 mT w5,6x), and with the deduced here monoclinic-I symmetry and the orientation of the KUL3 center (symmetry axis 158 away from N110M axis in a (01-1) plane). 3.3. KUL5, KUL6 and H2 centers The KUL5 spectrum (see top curve in Fig. 2) consists of three equidistant lines attributed to hf interaction with one nitrogen site. It is only observed in nitrogen-rich CVD diamond. On the contrary, the KUL6 center is observed in nitrogen-free CVD films. The signals of those two centers have very similar resonance field positions and therefore are difficult to distinguish in CVD diamond samples with an intermediate nitrogen concentration (;1 ppm). Two lines have been previously attributed to the KUL6 center (see bottom curve in Fig. 2) from measurements on polycrystalline CVD diamond samples w4x. However, the present results suggest that those lines originate from two different centers, which we provisionally label as KUL6 and KUL6a: In the low-strain samples, the KUL6 line is significantly narrower than the KUL6a line, and only the former line is detected in sample 1.
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Fig. 3. Room-temperature Q-band ESR spectrum observed on CVD diamond sample 1. The spectrum was recorded in dispersion mode using a microwave power Pms0.5 mW and modulation amplitude 0.02 mT with the magnetic field along w100x crystal axis (normal to the sample surface).
Fig. 4. variation of the ESR line positions (300 K, Q band) for the KUL16 and KUL17 centers, and for the central line of the P1 center, all measured for rotating the sample in a (01-1) plane. Lines present simulations, generated using the parameters of Table 1. The dashed lines correspond to the defect orientations, which are not observed experimentally.
The KUL6 and KUL5 centers were detected in the monocrystalline samples 1 and 2, respectively. Despite the narrow linewidth (;0.02 mT), no angular variation in the line position and shape could be observed. This result might be interpreted in terms of tetrahedral symmetry. However, in that case, relatively intense 13C hf lines would be expected. Yet, those lines could be detected for neither center. It is possible that the symmetry of the KUL5 and KUL6 centers is lower than Td, but, as in the case of the trigonal center P1, the anisotropy may remain unresolved due to the weak spinorbit coupling in carbon. The H2 central line was also observed in sample 1. However, no angular dependence of its position and width could be reliably established. 3.4. KUL16 and KUL17 centers Two rather unusual spectra, labeled as KUL16 and KUL17, are detected in sample 1 (see Fig. 3). Both consist of two narrow lines, which start saturating at rather low microwave power Pm ;1 mW and therefore, are best observed in dispersion mode, at Pm ;0.5 mW. The angular maps of those centers are presented in Fig. 4. Quite unexpectedly, the intensity of the KUL17 doublet strongly varied with the angle, reaching a maximum for the magnetic field direction normal to the sample surface (NN 100) and disappearing at angles as small as 48 away from the w100x axis. Another very unusual feature, observed for the KUL16 and KUL17 centers in Figs. 3 and 4, is the small number (two) of anisotropic lines, of which the angular curves do not cross when the sample is rotated in a (01-1) plane. This observation suggests that both defects only occur with two equivalent sites, both oriented along a high symmetry crystal axis, which should be w100x in the w100xoriented thin film sample 1. Then, the doublet structure, observed for both the KUL16 and KUL17 centers, may be attributed to hf interaction with one hydrogen atom. The spin Hamiltonian parameters of those centers inferred from computer simulations are presented in Table 1. The above observations strongly suggest the KUL16 and KUL17 to be surface related defects. They can hardly originate from surface contamination: During the Ir substrate elimination, the sample was etched for approximately one month in HNO3 and in NaOH solutions. Moreover, the intensity of the KUL16 and KUL17 signals did not change after the following treatments: (1) ultrasonic rinsing in acetone for 1 h; (2) boiling in perchloric, nitric, fluoric or sulfuric acids, or in their mixtures; (3) boiling in an H2SO4:K2Cr2O7 mixture for
K. Iakoubovskii et al. / Diamond and Related Materials 12 (2003) 511–515
1 h. It could be that the former HNO3qNaOH etching either produced or stabilized (via, e.g. surface oxidation) the KUL16 and KUL17 centers. Assuming that the relevant defects are concentrated at both the {100} and {y100} surfaces, the spin density was estimated as 5Ø1012 cmy2 for both centers. 4. Summary ESR measurements on monocrystalline CVD diamond samples have provided important information on the previously reported KUL1, 3, 5 and 6 centers: The symmetries of the KUL1 and KUL3 centers have been determined as trigonal and monoclinic-I, respectively. By correlating the intensity of the KUL3 ESR signals and the 1.68 eV Si-related luminescence peak, and taking into account the previously reported w4x hyperfine structure and g values, the KUL3 center may be assigned to a defect involving one Si and one hydrogen atom. Two previously undocumented centers, labeled as KUL16 and KUL17, have been characterized and are attributed to two N100M-oriented surface defects, each involving one hydrogen atom.
515
Acknowledgments The authors are indebted to M. Nesladek for provision of polycrystalline CVD samples and to the FWOVlaandered for financial support. References w1x X. Zhou, G.D. Watkins, K.M. McNamara Rutledge, R.P. Messmer, S. Chawla, Phys. Rev. B 54 (1996) 7881. w2x D.F. Talbot-Ponsonby, M.E. Newton, J.M. Baker, G.A. Scarsbrook, R.S. Sussmann, A.J. Whitehead, S. Pfenninger, Phys. Rev. B 57 (1998) 2264. w3x D.F. Talbot-Ponsonby, M.E. Newton, J.M. Baker, G.A. Scarsbrook, R.S. Sussmann, A.J. Whitehead, Phys. Rev. B 57 (1998) 2302. w4x K. Iakoubovskii, A. Stesmans, Phys. Status Solidi (a) 186 (2001) 199. w5x K. Iakoubovskii, A. Stesmans, Phys. Status Solidi (a) 193 (2002) 448. w6x K. Iakoubovskii, A. Stesmans, Phys. Rev. B 66 (2002) 195207. w7x K. Ohtsuka, K. Suzuki, A. Sawabe, T. Inuzuka, Jpn. J Appl. Phys. 35 (1996) L1072. w8x J.H.N. Loubser, J.A. van Wyk, Rep. Prog. Phys. 41 (1978) 1201. w9x K. Iakoubovskii, A. Stesmans, K. Suzuki, A. Sawabe, T. Yamada, Phys. Rev. B 66 (2002) 113203. w10x J.P. Goss, R. Jones, S.J. Breuer, P.R. Briddon, S. Oberg, Phys. Rev. Lett. 77 (1996) 3041.
Characterization of defects in monocrystalline CVD diamond films by electron spin resonance K. Iakoubovskiia,*, A. Stesmansa, K. Suzukib, J. Kuwabarac, A. Sawabec a
Physics Department, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium b Toplas Engineering Co., Ltd., 1-9-9 Nishi-Tsutsujigaoka, Chofu-shi, Tokyo 182-0006, Japan c Department of Electrical Engineering and Electronics, Aoyama-Gakuin University, 6-16-1 Chitosedai, Setagaya-ku, Tokyo 157, Japan
Abstract Electron spin resonance (ESR) measurements on free-standing monocrystalline CVD diamond samples allowed us to deduce the symmetry of the previously reported KUL1 and KUL3 centers as trigonal and monoclinic-I, respectively. Based on the correlation between the intensities of the KUL3 ESR signals and of the 1.68 eV Si-related luminescence peak, and on the analysis of the observed hyperfine interaction structure, the KUL3 center is suggested to involve one Si and one hydrogen atom. Two previously undocumented centers, labeled as KUL16 and KUL17, have been characterized and attributed to two N100M-oriented surface defects, each involving one hydrogen atom. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond film; Defect characterization; Impurities
1. Introduction An important feature of chemical vapor deposition (CVD) of diamond is that the diamond growth mostly occurs using a hydrogen-rich gas mixture in a Sicontaining reactor. As a result, CVD diamond can be distinguished from other types of diamond by its relatively high concentrations of hydrogen and Si atoms. Indeed, electron spin resonance (ESR) applied to CVD diamond revealed a number of defect centers involving one hydrogen atom, such as H1 and H2 w1–3x, KUL2– 4, and KUL9 w4–6x, and two centers involving one Si atom (KUL1 and KUL8 w4–6x). Determination of the microscopic structure of those centers is important. However, this was impeded by the unfortunate morphology of CVD diamond: Most CVD diamond samples studied so far by ESR are polycrystalline films with random orientation of the grains. ESR spectra from such samples (the so-called ‘powder-pattern’ spectra) provide only limited information on the defect symmetry, which is essential for the defect modeling. In an attempt to overcome this problem, in the present paper we report ESR measurements on monocrystalline CVD diamond *Corresponding author. Tel.: q32-16-327-705; fax: q32-16-327987. E-mail address: [email protected] (K. Iakoubovskii).
samples, which reveal the symmetry of the KUL1 and KUL3 centers. Moreover, two previously undocumented centers, labeled as KUL16 and KUL17, are characterized and assigned to two surface defects, each involving one hydrogen atom. 2. Experimental details Monocrystalline CVD films can be easily grown on monocrystalline diamond substrates. However, such samples were not studied here because many ESR centers are present in CVD diamond in small concentrations (-1 ppm) and therefore can be masked by background signals from the substrate. Additionally, removal of a diamond substrate is a painstaking process. Instead, in the present work a monocrystalline diamond film ;5=10=0.03 mm3 in size was epitaxially deposited on a N100M-oriented Ir substrate. The detailed description of the sample preparation is given elsewhere w7x. The sample, hereafter referred to as sample 1, was grown by DC plasma CVD from an H2:CH4 gas mixture at a substrate temperature Ts1000 8C. After growth, the sample was immersed for 3 h in a Sn–Pb eutectic alloy kept at 400 8C to enable Ir etching. The metals were then dissolved first in HNO3 and then in a NaOH water solution kept at 60 8C.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-9635(02)00380-1
K. Iakoubovskii et al. / Diamond and Related Materials 12 (2003) 511–515
512
Microscopical examinations revealed that samples 1 and 2 contained a few internal grain boundaries and therefore, strictly speaking, are not monocrystalline. However, mapping the ESR angular dependences of the P1 center revealed that the samples 1 and 2 might be considered as monocrystalline for the purposes of the ESR studies of the present work. ESR spectra were recorded at room temperature using a commercial Qband (;33 GHz) Bruker spectrometer. 3. Experimental results and analysis Several ESR centers, including P1, KUL1, 3, 5 and 6, H1 and H2, were detected in samples 1–3. Among those, the P1 center has previously been thoroughly characterized in monocrystalline synthetic and natural diamonds and assigned to the neutral single substitutional nitrogen center (NS8) w8x, while the characterization of the H1 center in monocrystalline CVD diamond has been reported recently w9x. Therefore, in this work we shall focus on the KUL1, 3, 5 and 6, and H2 centers. 3.1. KUL1 center
Fig. 1. Angular variation of the ESR signal resonance fields for the KUL1 (a) and KUL3 (b) center (300 K, Q band) measured for rotating the sample in a (01-1) plane. Lines present simulations, generated using the spin Hamiltonian parameters of Table 1.
Sample No 2 was prepared by cleaving a large (;200 mm in size) single grain from a 1.5-mm thick defectrich polycrystalline CVD diamond film. The film was grown by microwave plasma CVD on a Si substrate at Ts800 8C. For comparison, a defect-free polycrystalline diamond film (sample 3), grown at similar conditions, has also been studied.
The KUL1 center was detected in sample 1 and its angular map of signal resonance fields for rotation in a (01-1) plane has been obtained (squares in Fig. 1a). The lines in Fig. 1a represent a fit computer generated using the spin Hamiltonian parameters summarized in Table 1. Remarkably, the same values of the spectroscopic splitting factor g and of the spin–spin coupling constant D for the KUL1 center are obtained in the present single-crystal measurements as in previous powder-pattern ESR studies w4–6x. The observation of trigonal symmetry for the KUL1 center is important. This result, combined with the specific value of D, might suggest the assignment of the KUL1 center to a defect containing a Si atom at the center of a divacancy w10x. There is, however, an apparent disagreement between the observed high symmetry and the previously deduced
Table 1 Inferred room-temperature spin Hamiltonian parameters of the observed ESR centers Center
Symmetry
Spin
Matrix
Principal values
KUL1
Trigonal
1
KUL3
Monoclinic-I
1y2
KUL5 KUL6 KUL16
Tetragonal
1y2 1y2 1y2
KUL17
Tetragonal
1y2
g D g A g g g A g A
2.0040(1) y23.86(3) 2.00506(3) y0.10(2) 2.00290(3) 2.00306(3) 2.00318(3) 0.98(2) 2.00256(3) 1.08(2)
2.0035 11.93 2.00426 0 2.00290 2.00306 2.00272 0.72(2)
2.0035 11.93 2.00254 0.26 2.00290 2.00306 2.00272 0.72(2)
The sign of D1 for the KUL1 center is assumed to be negative. Values of D and A are in mT. The angles between the principal axes of the g ¯ and D or g and A matrixes are zero. ¯ ¯ ¯
K. Iakoubovskii et al. / Diamond and Related Materials 12 (2003) 511–515
513
w4x small number of equivalent carbon sites (two and one), which needs to be resolved to attain adequate modeling of the KUL1 center. 3.2. KUL3 center The description of the KUL3 spectrum has been reported elsewhere w4x. This Ss1y2 ESR center is characterized by an unusually large for diamond anisotropy of the g factor (see Table 1) and by a structure originating from anisotropic hyperfine (hf) interaction with one hydrogen site. The KUL3 center is also detected in sample 1 and its angular map for rotation in a (01-1) plane has been measured (squares in Fig. 1b). The lines in Fig. 1b present a fit, computer generated using the spin Hamiltonian parameters of Table 1. Here again, the same principal values of matrix g and hf ¯ in the matrix A for the KUL3 center were deduced ¯ present single-crystal measurements as in previous powder-pattern ESR studies w4x thus increasing the reliability of the applied fitting procedure. The analysis of our previous measurements, performed over a large series of polycrystalline CVD diamond films, all grown on Si substrates at similar conditions, reveals that the intensity of the KUL3 center is the strongest in samples with the largest surface-tovolume ratio. However, all the equivalent defect orientations appear reflected in the angular map of Fig. 1b, while fewer equivalent orientations should be observed for a typical surface defect. Therefore, the KUL3 center is probably not a surface defect. There is one more pertinent observation: Sample 1 was found inhomogeneous with respect to the concentration of Si-related defects, as monitored by the intensity of the 1.68 eV luminescence line. The Si-rich parts did show the KUL3 ESR signals, while none could be detected in the Sipoor regions. This result may be interpreted as the KUL3 center being Si related; in CVD films grown on Si, the KUL3 center is predominantly present in the Sirich regions near the Si-diamond interface and thus its intensity is higher in the samples of a larger area, but not of a larger volume. The involvement of Si in the KUL3 center also logically explains the large observed g anisotropy: The latter is determined by the spin-orbit coupling, which is larger for a Si than for a carbon atom. The experimental data on the KUL3 center are still insufficient to enable reliable modeling. Very speculatively, this center may be associated with a neutral divacancy, where one of the six basal carbon atoms is replaced by Si, and one dangling bond (remote to Si atom) is passivated by a hydrogen atom. This model is consistent with the observed hydrogen hf values for the KUL3 center (A1sy0.10 mT, A2s0, A3s0.26 mT), which are similar to those of the neutral hydrogendivacancy in diamond (i.e. KUL9 center: A1sA2s
Fig. 2. Room-temperature Q-band ESR spectra observed on CVD diamond sample 2 (top curve) and 3 (bottom curve), recorded using a microwave power Pms1 mW and modulation amplitude of 0.02 mT.
y0.04 mT, A3s0.20 mT w5,6x), and with the deduced here monoclinic-I symmetry and the orientation of the KUL3 center (symmetry axis 158 away from N110M axis in a (01-1) plane). 3.3. KUL5, KUL6 and H2 centers The KUL5 spectrum (see top curve in Fig. 2) consists of three equidistant lines attributed to hf interaction with one nitrogen site. It is only observed in nitrogen-rich CVD diamond. On the contrary, the KUL6 center is observed in nitrogen-free CVD films. The signals of those two centers have very similar resonance field positions and therefore are difficult to distinguish in CVD diamond samples with an intermediate nitrogen concentration (;1 ppm). Two lines have been previously attributed to the KUL6 center (see bottom curve in Fig. 2) from measurements on polycrystalline CVD diamond samples w4x. However, the present results suggest that those lines originate from two different centers, which we provisionally label as KUL6 and KUL6a: In the low-strain samples, the KUL6 line is significantly narrower than the KUL6a line, and only the former line is detected in sample 1.
514
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Fig. 3. Room-temperature Q-band ESR spectrum observed on CVD diamond sample 1. The spectrum was recorded in dispersion mode using a microwave power Pms0.5 mW and modulation amplitude 0.02 mT with the magnetic field along w100x crystal axis (normal to the sample surface).
Fig. 4. variation of the ESR line positions (300 K, Q band) for the KUL16 and KUL17 centers, and for the central line of the P1 center, all measured for rotating the sample in a (01-1) plane. Lines present simulations, generated using the parameters of Table 1. The dashed lines correspond to the defect orientations, which are not observed experimentally.
The KUL6 and KUL5 centers were detected in the monocrystalline samples 1 and 2, respectively. Despite the narrow linewidth (;0.02 mT), no angular variation in the line position and shape could be observed. This result might be interpreted in terms of tetrahedral symmetry. However, in that case, relatively intense 13C hf lines would be expected. Yet, those lines could be detected for neither center. It is possible that the symmetry of the KUL5 and KUL6 centers is lower than Td, but, as in the case of the trigonal center P1, the anisotropy may remain unresolved due to the weak spinorbit coupling in carbon. The H2 central line was also observed in sample 1. However, no angular dependence of its position and width could be reliably established. 3.4. KUL16 and KUL17 centers Two rather unusual spectra, labeled as KUL16 and KUL17, are detected in sample 1 (see Fig. 3). Both consist of two narrow lines, which start saturating at rather low microwave power Pm ;1 mW and therefore, are best observed in dispersion mode, at Pm ;0.5 mW. The angular maps of those centers are presented in Fig. 4. Quite unexpectedly, the intensity of the KUL17 doublet strongly varied with the angle, reaching a maximum for the magnetic field direction normal to the sample surface (NN 100) and disappearing at angles as small as 48 away from the w100x axis. Another very unusual feature, observed for the KUL16 and KUL17 centers in Figs. 3 and 4, is the small number (two) of anisotropic lines, of which the angular curves do not cross when the sample is rotated in a (01-1) plane. This observation suggests that both defects only occur with two equivalent sites, both oriented along a high symmetry crystal axis, which should be w100x in the w100xoriented thin film sample 1. Then, the doublet structure, observed for both the KUL16 and KUL17 centers, may be attributed to hf interaction with one hydrogen atom. The spin Hamiltonian parameters of those centers inferred from computer simulations are presented in Table 1. The above observations strongly suggest the KUL16 and KUL17 to be surface related defects. They can hardly originate from surface contamination: During the Ir substrate elimination, the sample was etched for approximately one month in HNO3 and in NaOH solutions. Moreover, the intensity of the KUL16 and KUL17 signals did not change after the following treatments: (1) ultrasonic rinsing in acetone for 1 h; (2) boiling in perchloric, nitric, fluoric or sulfuric acids, or in their mixtures; (3) boiling in an H2SO4:K2Cr2O7 mixture for
K. Iakoubovskii et al. / Diamond and Related Materials 12 (2003) 511–515
1 h. It could be that the former HNO3qNaOH etching either produced or stabilized (via, e.g. surface oxidation) the KUL16 and KUL17 centers. Assuming that the relevant defects are concentrated at both the {100} and {y100} surfaces, the spin density was estimated as 5Ø1012 cmy2 for both centers. 4. Summary ESR measurements on monocrystalline CVD diamond samples have provided important information on the previously reported KUL1, 3, 5 and 6 centers: The symmetries of the KUL1 and KUL3 centers have been determined as trigonal and monoclinic-I, respectively. By correlating the intensity of the KUL3 ESR signals and the 1.68 eV Si-related luminescence peak, and taking into account the previously reported w4x hyperfine structure and g values, the KUL3 center may be assigned to a defect involving one Si and one hydrogen atom. Two previously undocumented centers, labeled as KUL16 and KUL17, have been characterized and are attributed to two N100M-oriented surface defects, each involving one hydrogen atom.
515
Acknowledgments The authors are indebted to M. Nesladek for provision of polycrystalline CVD samples and to the FWOVlaandered for financial support. References w1x X. Zhou, G.D. Watkins, K.M. McNamara Rutledge, R.P. Messmer, S. Chawla, Phys. Rev. B 54 (1996) 7881. w2x D.F. Talbot-Ponsonby, M.E. Newton, J.M. Baker, G.A. Scarsbrook, R.S. Sussmann, A.J. Whitehead, S. Pfenninger, Phys. Rev. B 57 (1998) 2264. w3x D.F. Talbot-Ponsonby, M.E. Newton, J.M. Baker, G.A. Scarsbrook, R.S. Sussmann, A.J. Whitehead, Phys. Rev. B 57 (1998) 2302. w4x K. Iakoubovskii, A. Stesmans, Phys. Status Solidi (a) 186 (2001) 199. w5x K. Iakoubovskii, A. Stesmans, Phys. Status Solidi (a) 193 (2002) 448. w6x K. Iakoubovskii, A. Stesmans, Phys. Rev. B 66 (2002) 195207. w7x K. Ohtsuka, K. Suzuki, A. Sawabe, T. Inuzuka, Jpn. J Appl. Phys. 35 (1996) L1072. w8x J.H.N. Loubser, J.A. van Wyk, Rep. Prog. Phys. 41 (1978) 1201. w9x K. Iakoubovskii, A. Stesmans, K. Suzuki, A. Sawabe, T. Yamada, Phys. Rev. B 66 (2002) 113203. w10x J.P. Goss, R. Jones, S.J. Breuer, P.R. Briddon, S. Oberg, Phys. Rev. Lett. 77 (1996) 3041.