Formation of optical centers in CVD diamond by electron and neutron irradiation

Diamond and Related Materials, 1 (1992) 470 477

470

Elsevier Science Publishers B.V., Amsterdam

Formation of optical centers in CVD diamond by electron and neutron irradiation Y. Yokota, H. Kotsuka, T. Sogi, J. S. Ma, and A. Hiraki Department of Electrical Engineering, Faculty of Engineering, Osaka University, Osaka 565 (Japan)

H. Kawarada School of Science and Engineering, Waseda University, Shinjuku, Tokyo 169 (Japan)

K. Matsuda and M. Hatada Japan Atomic Energy Research Institute, Neyagawa, Osaka 572 (Japan)

Abstract An investigation of the formation of optical centers in chemical vapor deposited diamond (CVD diamond) by electron and neutron irradiation has been carried out. Cathodoluminescence was mainly used for observation of the optical centers. Several optical centers have been observed after the irradiation and subsequent annealing at 900 °C, such as the 5RL center and the centers with zerophonon lines at 3.19 eV, 2.16 eV and 1.94 eV. Because these centers are considered to have relations to nitrogen atoms, vacancies or interstitials, it is expected that their luminescence intensities depend on the amount of nitrogen doping and radiation dose. However, decrease in the luminescence intensity with the amount of nitrogen doping has been observed. Boron-doped specimens have also been irradiated and annealed, but the formation of defects related to boron has not been observed.

1. Introduction The effects of electron and neutron irradiation on the luminescence spectra of natural and high-pressure synthesized diamonds have been reported in many publications. There are, however, many optical centers that need further work to determine their atomic structure. Diamond can be formed easily and reproducibly by chemical vapor deposition (CVD) methods. These methods have a great advantage in controlling imlSuntaes in diamond, compared with natural or high-pressure processes. Using the microwave-assisted plasma CVD method [,1], the diamond that corresponds with type IIa natural diamond in the point of CL spectrum can be obtained easily [-2]. In the present paper, an "undoped" CVD diamond means this kind of CVD diamond. Doping impurities into diamond in CVD process is easily done by mixing a gas involving a desired element with a reaction gas [-2, 3]. The amount of impurity in diamond is therefore controllable. Thus, systematical studies of the formation of defects by irradiation, concerning impurity, can be carried out more easily by using chemical-vapor deposited diamonds (CVD diamonds). Previously, an investigation of the effect of irradiation on the cathodoluminescence (CL) of nitrogen-doped CVD diamond has been carried out [4]. The main results are as follows. Electron or neutron irradiation

0925-9635/92/$5.00 © Elsevier Science Publishers B.V. All rights reserved

alone induces the 5RL center and the 3.19eV center, and destroys the band A. Subsequent annealing to the irradiation forms the 2.16 eV centers and the 1.94 eV centers. The irradiation-induced changes of the CL spectrum of CVD diamonds are similar to those of highpressure synthesized diamonds. Luminescent areas are characteristic of each center or crystal growth sector: the 5RL center, {100} sectors; the 3.19 eV center, both {111} and {100} sectors; the 2.16 eV center, {111} sectors. The typical centers and their origins reported in irradiated bulk diamond are as follows: (a) the 5RL center (the main zero-phonon line at 4.58 eV, 271 nm), attributed to intrinsic defects such as interstitial carbon [-4-6]; (b) the 3.19 eV center (zero-phonon line observed at 3.19 eV, 389 nm), attributed to interstitial nitrogen or a nitrogen-interstitial carbon complex [,6-10]; (c) the 2.16eV center (zero-phonon line observed at 2.16eV, 575 nm), attributed to a nitrogen-vacancy complex [,11, 12]. These centers have been also observed in ionimplanted diamond [-6, 8, 11]. There are few reports on irradiated type lib diamond, but the 2BD center (two zero-phonon lines at 4.777 eV and 4.698 eV, related to boron) has been reported [-5]. In this study, investigations on the formation of optical centers in CVD diamond by electron and neutron irradiation have been carried out with varying parameters. Cathodoluminescence was mainly used for observation of the optical centers.

Y. Yokota et al.

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Formation qfoptical centers in CVD diamond

2. Experimental The diamonds were synthesized using a microwaveplasma CVD method [1]. Diamond films and particles were deposited on floating-zone-made silicon (111) substrates. The substrates were put on a quartz block in a quartz reaction chamber. The reaction gases were 15% carbon monoxide and 85% hydrogen in volume. For doping, nitrogen (N2) gas or diborane (BzH6) gas was mixed with the reaction gases. The amount of doping is determined using the atomic ratio of nitrogen and carbon (N/C) or boron and carbon (B/C) in the reaction gases. The substrate temperatures were 920-970 ~C and the pressure was 4.5 x 103 Pa during deposition. Thicknesses of the films were 4 91am and the diameters of the particles were 8 18 ~tm. Electron irradiations were carried out using a linear accelerator (20 MeV), a Van de Graaff accelerator (1.5 MeV, 1.2 MeV) and a transformer rectifier accelerator (800 keV, 500 keV). The specimens were mounted on a water-cooled holder during the electron irradiation. The dose of electron irradiation was 3.6 x 10 ~¢, 7.2 x 10 t9 e/cm 2. Neutron irradiations were carried out using a nuclear reactor, and the dose was 2.2 x 10 ~6 n/cm 2. Annealings after irradiations were carried out at 900 C for l h in a vacuum of <10 3pa. Then the specimens were cooled down rapidly to room temperature. For CL experiments, a scanning electron microscope (JSM-840 type) incorporated with a parabolic mirror and a monochromator (CT-25C type) was used. A diffraction grating (600 line/ram, blaze peak at 300 nm) and a photomultiplier (R-376 type, suitable for 160 850 nm) were used. The accelerating voltage was 5.0 kV (the exception was Fig. 1, 20 kV), where the penetration depth was 300 400 nm. The current and the scanning area of the electron probe were 3.0 x 10 8 A (the exception was Fig. l , 3 . 0 x l 0 7A) a n d 4 . 1 x l 0 - 4 m m 2,respectively. The CL spectra were obtained at 89 K. The slit width was 1.3 nm in wavelength. The CL images were obtained by synchronizing the signal from the photomultiplier with the position of the scanning electron probe. The shapes of the spectra have not been corrected for the wavelength-dependent response of the optical system. To distinguish a graphite phase from diamond, X-ray photoelectron spectroscopy was used.

3. Results and discussions

3.1. Nitrogen-doped and undoped specimens In this section, dependences of irradiation-induced centers in CVD diamond on the amount of nitrogen doping, the doses and the irradiation energies of electrons are illustrated. Then the effects of neutron irradiation to

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Fig. 1. CL spectra of the specimen shown in Fig. 12, spectrum C, obtained by the excitation electron probe with higher energy (20 keV) and larger current (3 ×I0 ~ A) than that in Fig. 12. (al Whole range spectrum. (b) bound and free excitons and the 5RL center.

undoped CVD diamond are shown. Finally, discussions on the formations of each optical center are carried out individually.

3.1.1. Dependences on the amount of nitrogen doping Figures2(a), (b) and (c) show the CL spectra of polycrystalline diamond films doped with various amount of nitrogen. All spectra in Fig. 2 are obtained at the same condition of the CL measurements. Asgrown nitrogen-doped CVD diamonds (Fig. 2(a)) show the band A emission with a peak around 2.92eV (424 nm). After 20 MeV electron irradiation to these specimens, the 3.19 eV emission and the 5RL emission rise (Fig. 2(b)). After annealing subsequent to the irradiation, the 2.16 eV emission rises and the 3.19 eV center and the 5RL center have no obvious change (Fig. 2(c)). The intensity of all these emission centers decreases as the amount of nitrogen increases (Fig. 3). The intensities in Figs 3 5, are calculated by deducting background levels. Here, if a graphite phase exists, it impedes luminescence. Graphitization of diamond films by irradiation and/or annealing, however, does not occur in these conditions. XPS is a very sensitive technique to investigate whether a graphite phase exists or not [13, 14].

Y. Yokota et al. / Formation of optical centers in CVD diamond

472

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Fig. 2. CL spectra of polycrystalline CVD diamond films doped with various amounts of nitrogen: (a) as grown, (b) electron irradiated (20 MeV, 1.8 x 1019 e/cm2), (c) annealed at 900 °C.

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Fig. 3. Dependences of the CL intensities of the optical centers on the amount of nitrogen doping. Results are obtained at 2.92 eV (424 nm, band A) in Fig. 2(a); and at 4.58 eV (271 nm, 5RL), 3.19 eV (389 nm) and 2.16 eV (575 nm) in Fig. 2(c).

Fig. 5. Dependences of the CL intensities of the 2.16 eV center on the energies of electron irradiation. Results are obtained at 2.16eV (575 nm) in Fig. 7. N(8)

09 18 a6 22 Dose (x10~gelcm2 )

Fig. 4. Dependences of the CL intensities of the optical centers on the dose of 20 MeV electron irradiation. Results are obtained at 4.58 eV (271 nm, 5RL), 3.19 eV (389 nm) and 2.16 eV (575 nm). The specimens are polycrystalline C V D diamond films doped with nitrogen (N/C =

2%).

Figure6 shows the carbon ls core photoemission spectrum of the specimen showing the CL spectrum D in Fig. 2(c). The peak position, the linewidth (FWHM) and the half linewidth (HWHM) are 285.5 eV, 1.98 eV and 0.89 eV, respectively. These are in agreement with those of natural diamond measured by Y. Iijima [13].

Cls

294 292 290 288 286 284 282 280 278 Binding Energy (eV)

Fig. 6. XPS spectrum (carbon ls core) of the specimen showing the CL spectrum D in Fig. 2(c).

The good symmetry of the peak indicates that the specimen consists of only one component of carbon phase. Therefore, the absence of a graphite phase in the surface of the specimen (the depth of XPS signal is about 2 nm) has been confirmed. Similar XPS spectra are shown by the other specimens. Because the 20 MeV incident electrons have too much energy to be stopped around a special depth of diamond films of several micrometers in thickness, they should have a uniform

Y. Yokota et al.

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Formation o[optical ~enters in C VD diamond

depth profile. This indicates that the XPS results represent the information much deeper than the surface. Therefore, at the informative depth of CL (300 400 nm at Sky), at least, there should be no graphitization. There is other evidence that the specimens are charged up during SEM observation. Due to the reasons mentioned above, the effects of the graphite phase on CL measurements can be negligible.

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3.1.2. Dependences on the dose o f electron irradiation

Figure 4 shows the CL intensities of zero-phonon lines of nitrogen-doped (N/C = 2%) CVD diamond films after annealing subsequent to the electron irradiation with various doses. Here, the incident energy is kept constant at 20 MeV. Obviously, the 5RL center, the 3.19 eV center and the 2.16 eV center are more intense in higher doses of irradiation.

Fig. 8. CL spectrum of a polycrystallineCVD diamond film irradiated by 800 keV electrons with a dose of 2.25 × 10~"ecru 2 and annealed at 900 C. The dose is higher than that in the case of Fig. 7. The specimen is doped with nitrogen (N:C=4%L to the 2.16 eV line (I3.t9/12.16) in the Fig. 8 is less than that of the specimens irradiated by 20 MeV electrons (Fig. 2(c)). The band A emission remains after 800 keV irradiation but disappears after 20 MeV irradiation.

3.1.3. Dependences on the energy q[electron irradiation

The CL spectra and the CL intensities of nitrogendoped ( N / C = 4 % ) CVD diamond films after annealing subsequent to electron irradiations of various energies are shown in Figs 7 and 5, respectively. The dose of all the specimens is the same, 3.6 x 1016 e/cm 2. The 2.16 eV center is more intense with higher energies of irradiation. In this case, the 3.19 eV emission and the 5RE emission are very weak. Figure 8 shows the CL spectrum of a nitrogen-doped ( N / C = 2 % ) CVD diamond film after annealing subsequent to 800keV electron irradiation. The dose (2.25x10 TM e/cm 2) is higher than that in Fig. 7. The intensity of the zero-phonon line at 2.16eV of this specimen is comparable to that of the specimen irradiated by 20 MeV electrons with the dose of 2 3 x l 0 ~9 e/cm 2. However, a relative intensity of the 3.19 eV line

3.1.4. Undoped specimens irradiated by neutron

The features of CL spectra by neutron irradiation are qualitatively similar to those by electron irradiation, as shown in a previous study [4]. Also, the irradiationinduced changes of CL spectra of nitrogen-doped CVD diamonds are similar to those of undoped diamonds. Figure 9 shows the CL spectra of an undoped specimen. The band A with a peak around 2.92eV and the free exciton recombination radiations [15 17] with the main peak at 5.27 eV (235 nm) are observed in the spectrum of the as-grown specimen (spectrum A). This spectrum is almost the same as that shown in natural type IIa

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Y. Yokota et al. / Formation of optical centers in CVD diamond

diamond [2, 16]. After neutron irradiation with a dose of 2.2 x 1016 n/cm 2 (spectrum B), the 5RL emission and the 3.19 eV emission rise and band A is destroyed. After annealing at 900 °C (spectrum C), the 2.16eV center appears. Considering it is said that the 3.19 eV center and the 2.16eV center involve nitrogen [6-12], the above results suggest that nitrogen is contained even in "undoped" CVD diamond which shows a similar CL spectrum to that of natural type IIa diamond. The origin of the nitrogen is not yet understood. The free exciton emission can also be observed in undoped CVD diamond after irradiation. The distinction between "undoped" CVD diamond and nitrogen-doped CVD diamond is seen in the free exciton emission and the 5RL emission. In undoped CVD diamond, these emissions are more intense. These results are consistent with the proposal that the edge emission and the 5RL center are intrinsic [5 7], as well as there being an absorption edge due to isolated substitutional nitrogen in the UV region [18]. Figures 10(a) and (b) show a secondary electron (SE) image and the corresponding CL image, respectively, of the undoped as-grown CVD diamond with CL spectrum shown as Fig. 9(A). The SE image (Fig. 10(a)) shows that the diamond particle is composed of four {111} growth sectors and three {100} sectors. The corresponding monochromatic CL image (Fig. 10(b)) taken with a 2.92 eV signal (band A) shows that the {100} sectors are luminescent, but that the {111} sectors are not. This indicates that the distribution of defects and/or impurities forming band A is in the {100} sector [2, 19]. The CL images after neutron irradiation and subsequent annealing with CL spectrum shown in Fig. 9(C) are shown in Figs 10(c), (d), (e) and (f). The particles in

Fig. 10 are the same. Figures 10(c), (d) and (e) represent luminescent areas of the 5RL center, the 3.19 eV center and the 2.16 eV center, respectively. The 5RL center is in {100} sectors. Since the 5RL center is intrinsic [5-7], it is suggested that {100} sectors are better in quality than {111 } sectors. The 3.19 eV center is in {111 } sectors rather than in {100} sectors. The 2.16eV center is in {111 } sectors. The distortion of the images is caused by the equipment for CL measurement. These results are similar to electron-irradiated CVD diamond and neutron-irradiated CVD diamond doped with nitrogen [4], except for the 3.19 eV center. The 3.19 eV center of nitrogen-doped specimens irradiated with neutrons is also in {100} sectors, as reported in the previous study [4]. The above results show that the broad band A peak disappears after the irradiation and subsequent annealing. CL imaging is very useful for identifying whether the band A is destroyed by the irradiation or not. For example, in CL spectra (Fig. 9) it looks as if the band A remained after irradiation - - i.e. the band around 2.9 eV was due to the band A. The luminescent area of the band around 2.9 eV (Fig. 10(f)), is clearly the same as that of the 3.19 eV line, {111 }, and different from that of the band A, {100} (Fig. 10(b)). Therefore, it is clear that the side band is associated with the 3.19 eV center, and that band A has been practically destroyed. Based upon the above results, a summary of the optical centers of the 5RL, the 3.19 eV and the 2.16 eV has been made as follows. 3.1.5.

The 5RL center

The 5RL center is formed by irradiation alone and can also be observed after annealing at 900 °C. The

Fig. I0. Secondary electron (SE) image (a) and the corresponding CL images (b)-(f) of undoped CVD diamond particles showing similar spectra to those in Fig. 9. The CL images are obtained by signals of photons with (b) 2.92eV (424 nm, band A), (c) 4.33eV (286rim, 5RL), (d) 3.19eV (389nm), (e) 2.16eV (575nm) and (f) 2.92eV (424nm). (a),(b) As grown; (c)-(f) neutron irradiated and annealed at 900°C. Bright areas are luminescent.

Y. Yokota et al. ; Formation (~Joptical centers in CVD diamond

intensity of the 5RL center is high in undoped and boron-doped (see subsection 3.2) specimens, and decreases with an increase in nitrogen concentration. This tendency is likely because the 5RE center is an intrinsic defect --- i.e. interstitial carbon [5 7] produced by irradiation. So, a high dose of electron irradiation enhances the 5RL emission. The 5RL emission can be observed only at low temperature. This may be explained by the fact that since the interstitial carbon is the same atom as the surrounding atoms, strain around the interstitial carbon is easily dispersed at room temperature. In the crystals composed of {1001 sectors and ',111', sectors, the 5RL center tends to be in 11001 sectors. This is because the I100', sectors have less impurities and defects than '~1 I 1~, sectors. 3.1.6. The 3.19 eV center The 3.19 eV center is formed by irradiation alone, and can be observed also after annealing at 900 C . It has been reported that the origin is an interstitial nitrogen or a complex of nitrogen and interstitial carbon [6 10] produced by irradiation. If the 3.19eV center involves a nitrogen atom as reported, it is reasonable to expect that doping with a high amount of nitrogen would enhance the intensity of the 3.19 eV emission. However, the experimental results in this study suggest the opposite to such an expectation. Since only certain rates of nitrogen atoms are changed to interstitials by certain doses of irradiation, substitutional nitrogen atoms increase with the amount of nitrogen doping. The substitutional nitrogen atoms are isolated in CVD diamonds. They form levels at 1.7 eV below the conduction band, and play as nonradiating centers or cause an absorption from blue to UV region. This may be the most probable explanation of the results if the 3.19 eV center involves a nitrogen atom. Further work is necessary. The 3.19 eV center is in both {111'~ sectors and ll00} sectors. In the specimens with a very low nitrogen concentration, it tends to be in 11111 sectors rather than l l00l sectors. 3.1.7. The 2.16 eV center The 2.16 eV center is formed by annealing subsequent to irradiation. By annealing at 900 C , vacancies induced by irradiation are movable, and are therefore able to make complexes with nitrogen atoms, forming optical centers such as the 2.16 eV center and the 1.94 eV center. Since the annealing temperature (900 °C) is too low to move nitrogen atoms in diamond, highly complicated defects involving more than two nitrogen atoms, such as the center called H3 or N3 [5], cannot be formed. The 2.16eV emission decreases with the amount of nitrogen and increases with the doses of the irradiation, the same as the other centers mentioned above. The dependence on nitrogen doping can be explained as the

475

act of nonradiating center due to substitutional nitrogen, though further work is necessary to declare it. The 2.16 eV emission increases with the doses of the irradiation by a factor more than one. The 2.16 eV center is formed by the association of vacancy and substitutional nitrogen, not interstitial nitrogen. The association of vacancy and interstitial atom means extinction of vacancy and the creation of substitutional atoms. Therefore the formation rate of the nitrogen vacancy complexes is not simply dependent on the doses of irradiation. In the truncated octahedron crystals composed of ll001 sectors and llll', sectors, the 2.16eV center tends to be in '~1111 sectors. This indicates that the distribution coefficient of nitrogen is larger in the ',11 I', sectors than in the ll00', sectors. Some specimens, however, show luminescent areas of the 2.16 eV center at 1100 ~,sectors (Fig. I 1). This happens on the specimens deposited at relatively high substrate temperature. Their surfaces of *,111~, sectors are very rough. Therefore, it is supposed that there are too many defects in these ~111', sectors to emit luminescence out. 3.2. Boron-doped specimens Figure 12 shows the CL spectra of polycrystalline CVD diamond films doped with boron. As-grown specimens (spectrum A) show the band A with peaks around 2.92 eV (424 nm) and around 2.47 eV (502 nm) [2]. The 2.92eV band is the same band as that observed in undoped CVD diamonds and type II natural diamonds [2]. The 2.47 eV band is due to boron doping [2]. In UV region, the recombination radiations of the excitons bound by neutral acceptors and free excitons are observed. The intensities of the 2.47 eV band and the bound exciton lines depend on the amount of boron doping [2, 17]. The band around 4.8 eV (260 nm) has not been identified yet. Its peak has similar energy to

Fig. II. SE image and the corresponding CE image of the 2.16eV center. I100', sectors are luminescent. The specimen, doped with nitrogen (N:C - 2%), is deposited at a relatively high substrate temperature (960 C), followed by 20 MeV electron irradiation and annealing at 900 C.

Y. Yokota et al. / Formation of optical centers in CVD diamond

476

1.6 1.8

Photon Energy (eV) 2.2 3.0 4.0 5.

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800

700

600 500 400 Wavelength (nm)

I

300

200

Fig. 12. CL spectra of a polycrystalline CVD diamond film doped with boron (B/C= 1000 ppm): (A) as grown; (B) 20 MeV electron irradiated with a dose of 1.8 x 1019 e/cmZ; (C) annealed.

the 2BD defect in irradiated type IIb natural diamonds [-5]. However, it may be different from the 2BD defect because the specimen is not irradiated. After 20 MeV electron irradiation with a dose of 1.8xl0a9e/cm 2 (spectrum B), the 5RL emission and the 3.19 eV emission rise. After annealing (spectrum C), the 2.16 eV center appears, whereas the 3.19 eV center remains. The bound exciton emission and the 2.47 eV band also exist after irradiation and annealing. The 2.47 eV band is completely different from the 2.92 eV band because the latter is easily destroyed by irradiation but the former is not. This means that origins, or atomic structures of these emissions are different fundamentally. The 2.47 eV band is clearly related to boron, while the 2.92 eV band may be due to some intrinsic strain on the crystal lattice of diamond. This strain may be around dislocations [-2, 20], and may relax if vacancies or interstitials exist, the 2.92 eV band then being destroyed. This speculation may also explain why the 2.92 eV band is enhanced at low temperature [-2]. Figures l(a) and (b) show the CL spectra of the same specimen as shown in Fig. 12 obtained by the excitation electron probe with 20 keV energy and 3 x 1 0 - 7 A current after annealing following irradiation. In this case, fine structures can be seen more clearly. A shoulder at 5.35eV (232nm) and lines at 5.21eV (238nm) and 5.05 eV (245 nm) are attributed to the bound exciton components with zero phonon, one transverse optical (TO) phonon, one T O phonon and one zone center optical phonon, respectively [-15, 17]. A line at 5.26 eV (236 nm) is attributed to the free exciton emission with one T O phonon [15-17]. Lines between 4.58eV (271 nm) and 3.87 eV (320 nm) are attributed to the 5RL center. A shoulder around 4.8 eV (260 rim) has not been identified with the 2BD defect or the 4.8 eV band observed before irradiation.

An investigation of the formation of optical centers in CVD diamond by electron and neutron irradiation has been carried out. The 5RL center, the 3.19 eV center and the 2.16 eV center are formed by irradiation or subsequent annealing. The luminescence intensity of these optical centers decreases with the amount of nitrogen doping, increases with the dose of irradiation, and increases with the energy of electron irradiation. Considering the 3.19 eV center and the 2.16 eV center involving a nitrogen atom, the decrease of these centers with the amount of nitrogen is the opposite of a simple expectation. It may be due to an increase of substitutional nitrogen atoms. The band A around 2.92 eV is destroyed by high energy irradiation and large dose irradiation. The boron-related broad band around 2.47 eV, the free exciton emission and the bound exciton emission remain even after irradiation. The luminescent area - - i.e. the distribution, is characteristic of each center and each growth sector.

Acknowledgments Electron irradiation (20 MeV) was carried out at the Institute of Scientific and Industrial Research in Osaka University, and neutron irradiation was carried out at the Kyoto University Research Reactor. The authors are grateful to Prof. Y. Nishida and Dr Y. Mita for assistance in neutron irradiation, Mr M. Yuasa of Sekisui Chemical Co., Ltd. for assistance in XPS measurements and Dr J. Wei, Mr Y. Mori, Mr H. Yagyu, Mr T. Yara and Mr N. Eimori for their assistance. They also wish to thank Mr K. Nishimura of Osaka Diamond Ind. Co., Ltd. for technical support.

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Y. Yokota et al. ~ Formation ~[optical centers m C VD diamond 10 L. H. Robins, L. P. Cook, E. N. Farabaugh and A. Feldman, Phys. Rer. B. 39 1[989) 13367. II A. M. Zaitsev, A. A. Gippius and V. S. Vavilov, Sot?. Phys. Semicond., 16 (1982) 252. 12 A. T. Collins, M. Stanley and G. S. Woods, d. Phys. D. 2(I (1987) 969. 13 Y. Iijima, Nippon Denshi News, 30 1199/)i 50 {in Japanese). 14 M. Yuasa, O. Arakaki, J. S. Ma, A. Hiraki and H. Kawarada, in R. F. Messier el al. (eds.) Proc. 2rot European Cm!/i on Diamond. Diamond-like and Related Coatings, Nice, September, 1991, Elsevier Sequoia S. A., Lausanne. 1991. Submitted for Publication.

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15 P. J. Dean, E. C. kightowlers and D. R. Wight, Phys. Rer. 140 (1965) A352. 16 A. T. Collins, M. K a m o and Y. Sato, .l. Phys., Condens. Matter, I (1989) 4029. 17 H. Kawarada, Y. Yokota and A. Hiraki, Appl. Phys. Lelt.. 57 (1990) 1889. 18 C. D. Clark, E. W. J. Mitchell and B. J. Parsons, in J. E. Field (ed.), The Properties ~/' Diamond, Academic Press, k(mdon. 1979, Chap. 2, p. 26. 19 Y. Yokota, H. Kawarada, J. S. Ma, K. Nishimura and A. Hiraki, J. Crvst. Growth, 103 (1990) 65.