- Email: [email protected]
Structure and creation conditions of complex nitrogen-nickel defects in synthetic diamonds
1196
Diamond and Related Materials, 3 (1994) 1196-1200
Structure and creation conditions of complex nitrogen-nickel defects in synthetic diamonds V. Nadolinny
and A. Yelisseyev*
Institute of Inorganic Chemistry, Institute of Mineralogy and Petrography, Novosibirsk 630090 (Russian Federation)
(Received September 24, 1993; accepted in final form January 12, 1994)
Abstract The mechanisms of nickel incorporation into the lattice of synthetic diamond, obtained on a seed by the temperature gradient method in the Fe-Ni-C system at 1700 K and 5.5 GPa, as grown or additionally annealed, have been studied using electron spin resonance (ESR) and optical spectroscopy. In the as-grown diamonds, in addition to the interstitial Ni ÷ a new paramagnetic Ni ÷ center with C3v symmetry environment and gll =2.0227, g. =2.0988 (NE4), which correlates with the 882 nm vibronic system, was observed. In diamonds annealed at 2100 K and 5.5 GPa, containing NE1-NE3 centers, three types of photochromic paramagnetic defect (NE5-NE7) were found, which are also due to Ni incorporation into the diamond lattice. They are characterized by gl = 2.0903; g2 = 2.039 and g3 2.0044 and hyperfine splitting (HFS) from two magnetically equivalent nitrogen atoms with Atl= 12.25 G and A± = 9.5 G (NE5); gl = 1.995, g2 = 2.0207 and g3 2.0109 (NE6). For NE7, g> 2 and HFS from one nitrogen is observed while a detailed study of the ESR angular dependence of the ESR spectrum was hampered by superposition from the other systems. The NE6 and NE7 photochromic and thermal properties are typical for shallow electron traps and determined by a Ni2+-to-Ni + charge transfer by ionization of the donor nitrogen, while in the case of NE5 they can be explained in terms of internal transformation processes. =
=
1. Introduction Despite intensive investigations the p r o b l e m of the i n c o r p o r a t i o n of i m p u r i t y a t o m s into the d i a m o n d lattice has r e m a i n e d o p e n in m a n y aspects until now. F r o m several tens of elements detected in d i a m o n d s by analytical m e t h o d s , only the i n c o r p o r a t i o n of c a r b o n n e i g h b o r s in the p e r i o d i c system such as h y d r o g e n , b o r o n a n d n i t r o g e n is established [1, 2]. T h e electron spin resonance (ESR) s p e c t r u m with a g factor of 2.031 due to Ni + [ 3 ] is the only p r o v e n case of the i n c o r p o r a t i o n of a h e a v y element, a l t h o u g h the defect structure has n o t been determined. Besides this initial version of an interstitial nickel ion I-3] the idea of a nickel a t o m in a high s y m m e t r y t e t r a h e d r a l position, when it substitutes for c a r b o n , is also d e v e l o p e d in [4, 5]. I s o y a et al. considered the E S R s p e c t r u m to be associated with the 3d 7 nickel ion state a n d related in some w a y to N i - . It s h o u l d be n o t e d that, in spite of the new d a t a on hyperfine structure ( H F S ) from ~3C for this defect, the i n t e r p r e t a t i o n in refs. 4 a n d 5 was given in the context of an unsatisfactory a n a l o g y with G e a n d Si where the lattice p a r a m e t e r s are m u c h larger t h a n for d i a m o n d . T h e new results o b t a i n e d recently for three c o m p l e x n i t r o g e n - n i c k e l defects, p r o d u c e d in synthetic d i a m o n d s *Author to whom correspondence should be addressed.
0925-9635/94/$7.00 SSD1 0925-9635(94)00182-Q
after P , T t r e a t m e n t [ 6 ] , s t i m u l a t e d a m o r e careful study of b o t h a s - g r o w n a n d a n n e a l e d d i a m o n d s . Expecting nickel to exist n o t only in the N i + charge state in d i a m o n d , b u t also as N i ° a n d N i 2 +, we investigated the p h o t o e x c i t a t i o n effect on nickel defects optically a n d by ESR.
2. Experimental details Synthetic d i a m o n d s g r o w n on a seed by the t e m p e r ature g r a d i e n t m e t h o d in the F e - N i - C system at 1700 K a n d 5.5 G P a , as well as those after a d d i t i o n a l P , T t r e a t m e n t at 2100 K a n d 5.5 G P a for 5 h, have been studied. E S R spectra were r e c o r d e d on a Varian E l 0 9 s p e c t r o m e t e r in the X - b a n d range at 77 a n d 300 K. W h e n investigating the p h o t o i n d u c e d processes, excitation from 0.5 k W m e r c u r y l a m p was carried out directly inside the s p e c t r o m e t e r cavity. The necessary H g lines in the l a m p s p e c t r u m were s e p a r a t e d using a n u m b e r of c o l o r e d glass filters. O p t i c a l a b s o r p t i o n spectra were t a k e n on a P e r k i n - E l m e r 325 I R m a c h i n e a n d a special c o m p u t e r - c o n t r o l l e d set-up on the basis of an M D R - 2 diffraction m o n o c h r o m a t o r for the UV, visible a n d n e a r - I R regions. T h e r m o l u m i n e s c e n c e curves were r e c o r d e d at a heating rate of 40 K m i n - 1 in the range 8O-700 K.
© 1994
Elsevier Science S.A. All rights reserved
V. Nadolinny, A. Yelisseyev / Complex N Ni defects in synthetic diamonds
3. Results and discussion
According to ESR data the as-grown diamonds are characterized by single substitutional nitrogen and paramagnetic nickel contents up to 1019 c m - 3 and 10 ~8 cm 3 respectively. At 77 K, weak spectra from the N E 1 - N E 3 centers [-6] as well as a new spectrum with S = i without any HFS, denoted NE4, were observed (Fig. 1 ). Analysis of the angular dependence of the ESR spectra (Fig. 2) showed the NE4 center to be of C3~ symmetry and described by the spin hamiltonian I2I=oa[H4S with parameters gll = 2.0227 and g± = 2.0988, where gll II(111). The select symmetry axis of the third order for the NE4 defect indicates substitution of an impurity atom for carbon and distortion of the lattice parameters along one of the (111) directions. As follows from an analysis of the g factor anisotropy, the center is due to a d ion with a more than half filled d shell. This state may be the 3d 9 electronic configuration of Ni+. Analysis of the possible positions of a large Ni + ion, with 1.15 ~. covalent radius, in the diamond lattice allows us to conclude that a vacancy occupies one of the carbon sites which are the nearest to the nickel. In this case the nickel ion shifts towards the vacancy and arranges a structure Ni +
Hu [001] NE4
i
I
I
N
I 3194
I 3234
I 3274
I 3314
1197
analogous to the double semivacancy which has been calculated by Masters [7] and proven to exist in Ge and Si. In luminescence spectra of the crystals under investigation the 882 nm vibronic system is observed together with the $2, $3 and "523.2nm" systems. Employing isotopic effects, Nazar6 et al. [8] proved the nickel nature of the 882 nm system and showed it to be due to the 3d 9 state of an Ni + ion with C3v local symmetry. The above-mentioned NE4 defect, parallel with nickel centers studied in refs. 4 and 5, may explain the structure of the defects responsible for the 882 nm system. After P,T treatment at 2 1 0 0 K and 5 . 5 G P a the spectrum from Ni with g=2.031 disappears in the ESR spectra of synthetic diamonds while the concentration of single substitutional nitrogen as well as of NE4 defects decreases by two orders. The treatment is accompanied by an increase in the concentration of N E I - N E 3 nickelcontaining centers for which the total content reaches (3 5) x 101~ cm -3. In order to understand in which form the remainder of the nickel ions are present in diamonds, and expecting the existence of Ni ° and Ni 2+ states in analogy with other crystals, we employed photoexcitation to produce charge transfer of the nickel to the Ni + paramagnetic state. With excitation by ). ~<360 nm light at room temperature an ESR spectrum with a g factor exceeding 2 and H F S from two magnetically equivalent nitrogen atoms appears (Fig. 3). This spectrum, denoted NE5, is superimposed on the NE1 and NE2 spectra [6]. Therefore the analysis of the NE5 spectrum parameters was carried out for ESR spectra recorded in Htl <111), (110) and (100) orientations. The NE5 spectrum is described by the spin hamiltonian
I 3354
i
where S= l, g]=2.0903 (gl4l {011)), g2=2.039, g3 =
~,G Fig. 1. The ESR spectrum of as-grown synthetic diamond at 77 K.
HII [100] 90 _[1101 70 --
~
k/k
~
/x*--
\
[111] ~ - ~ "
d
( 30-
~"4 "X"X,"
10 -[1001 3154
\
If--
t
~1
\
,' "~
NE2NE1 '
I '[ ~ N E 5 1
I I I
/// //
~-
)~
3194
~ ~ ~x-"
3234
9=2.0025 I
3274
~,G Fig. 2. Angular dependence of the ESR spectrum for the NE4 center.
3220
Iq
3280
I
I
3340
3400
~,G Fig. 3. The ESR spectrum of annealed synthetic diamond after ,i~ 360 nm excitation at 300 K.
V. Nadolinny, A. Yelisseyev / Complex N-Ni defects in synthetic diamonds
1198
2.0044 (~ (g3, (01i))=8°), Air1= Atl2= 12.25 G (All1 and All2 II (11[)) and A± 1 = A±2=9.5 G. Electronic spin S = ½ and g factor values typical for a d ion with a more than half-filled d shell, together with the absence of the HFS, allow us to relate the NE5 centers to the Ni + 3d 9 state. The g factors for NE5, as in the case of NE1-NE4 centers, correspond to the 3z 2 - r 2 ground state of Ni ÷ in the crystal field of a distorted octahedron (gll=2; g ± > 2 ) I-9]. As one of the possible NE5 structures, in analogy with the NE1 center where two magnetically equivalent nitrogen atoms are also present, a four-vacancy formation can be assumed. Nitrogen atoms could be located at its ends and a Ni ÷ ion in the center. The N - N i - N direction is at 8 ° to the (011) second-order axis which corresponds to the g3 direction for this center. Certainly it is difficult to expect that such a four-vacancy structure really exists. The NE5 structure is more likely to be similar to that of the NE1 center where nitrogen atoms did not reach the Ni first coordination sphere during the diffusion process but stopped in the second coordination sphere, thus forming an N - C - V - N i - V - C - N chain. The study of photoexcitation by the other wavelengths showed that the NE5 intensity decreases to complete disappearance when the wavelength changes from 560 to 650 nm. In Fig. 4 the kinetics of the intensity changes for the X line in the NE5 spectrum (Fig. 3) in various conditions are given. In darkness at room temperature the ESR spectrum demonstrates no changes after cutoff of the preliminary 2 ~<360 nm excitation which gives a strong NE5 response or after complete quenching at 2>~560nm illumination (Fig. 4, time intervals 3 and 5 respectively). Weakening of the exciting light using neutral density filters produces no changes in the NE5 behaviour at either 360 or 560 nm but makes the curves slope more gentle and the transition time larger (time intervals 2 and 4). 360nm
The temperature influence on the NE5 spectrum was quite unexpected. After complete quenching by 560 nm illumination at room temperature, heating the sample to 570 K in darkness completely restored the NE5 signal. Analysis of the temperature dependence gives the thermal activation energy of this process to be about 0.2 eV. Such a behavior means that the level from which the electron is excited by UV illumination or by heating is situated higher than the NE5 paramagnetic state. Because of the possibility of recombination processes with charge transfer between the centers, we carried out a special investigation of synthetic diamonds using a thermoluminescence technique which allows shallow traps with levels situated close to the conduction or valence bands to be revealed. They are able to capture free carriers which are generated in the crystal by UV excitation and release them on heating. A thermoluminescence curve demonstrating three main peaks at 160, 370 and 600 K, as well as an additional peak at 130 K, is shown in Fig. 5 while the calculated traps parameters are given in Table 1. If excited at 77 K, two new photochromic centers appear in the ESR spectra of annealed synthetic diamonds. The single substitutional nitrogen also demonstrates photochromic properties but in quite a different way from in the case described in ref. 11, in which charge transfer between the H2 and H3 center in synthetic diamond was investigated. One of the photochromic centers (NE6) is characterized by an electron state with S = ½, has no HFS and is described by a spin hamiltonian with the following parameters: gl = 1.995 (gl II(110)), g2 = 2.0207 (g2 II( 1i0)) and g3 = 2.0109 (g3 II(001)). The other, denoted NE7, has a g factor considerably larger than 2 and HFS from one nitrogen atom; the value of the H F splitting varies from 6 to 10 G depending on orientation. We failed to record the angular dependence of this spectrum because of
2,.2.560n m
p;~6 -'4
3 e-
_-2
0.1
0.2 t, h
0.3
0.4
Fig. 4. Kinetics of X-line intensity changes in the NE5 spectrum at room temperature during 3 6 5 n m and 2 > 5 6 0 n m excitation (time intervals 2 and 4 respectively) while during the time intervals 1, 3 and 5 the sample was in darkness. The upper curve demonstrates the temporal regime of the excitation.
100
300
500
700
~K
Fig. 5. Thermoluminescence curve for an annealed synthetic diamond after 360 n m excitation at 80 K.
I( Nadolinny, A. Yelisseyev / Complex N Ni defects in synthetic diamonds
1199
TABLE 1. Trap parameters for annealed synthetic diamond (calculated from the thermoluminescence curves in Fig. 5t; 61 and 6z are the halfwidths of the thermoluminescence peak from the low and from the high temperatures respectively Thermoluminescence peak maximum T
Half-widths of thermoluminescence peak
{KI
61/~2
130 160 360 600
22.5/45 75/67.5 45/'50
Thermal activation energy (general method [ 10]) E(eV}
Frequency factor o~ (s 1)
Kinetics order
0.23 _+ 0.02 0.39 _+ 0.03 0.63 ± 0.05
2.5 × 103 4.5 x 10z 3.0 × 102
II I! I
strong superposition with the other systems. Both NE6 and NE7 appear at 2~< 360 nm excitation, while they are removed reversibly with ~l>~560 nm illumination. They also revert to a non-paramagnetic state on warming to 300 K. The facts that S = ½, that there is strong anisotropy of the g factors with values which are much larger than 2 and typical for ions with a more than half-filled d shell, that HFS is absent and that the diamonds were grown in an Ni-containing melt allow us to relate these centers also to nickel defects with the Ni + state of a transition metal. A detailed study of the temperature and spectral dependences for efficiency of light sum storage demonstrates full analogy between the 130 and 160 K peaks in thermoluminescence curves and the NE6 and NE7 centers in ESR. Thus the same defects can be supposed to take part in luminescence and ESR, both centers becoming paramagnetic after capturing an electron and reverting to a non-paramagnetic state on warming or with illumination. Single substitutional nitrogen acts as a donor of electrons, which take part in a charge transfer process, its behaviour being opposite to that of the NE6 and NE7 centers on excitation and heating. Changes in the donor nitrogen response in ESR with photoexcitation vary from sample to sample depending on the concentrations of the NE6 and NE7 centers and reach 12% in some crystals. At room temperature no changes in ESR were noticed in the same samples with the same excitation although the sensitivity level was about 0.05%. Thus the photochromic properties of donor nitrogen, as well as of the NE6 and NE7 centers, differ considerably from effects associated with the NE5 defect. The complexity of the data obtained allows us to conclude that the latter are due to internal transitions and transformations inside the NE5 center. The state into which this center changes with 2>~ 560 nm excitation is also expected to be paramagnetic while the corresponding ESR response is likely to be masked by the other spectra or maybe will be observed at lower temperatures.
Thermal activation energy (initial rise method [ 10]) E(eV} 0.15 0.22 0.37 0.53
× 0.02 +_ 0.02 +_ 0.03 +_ 0.05
4. Conclusions The present investigation allows us to identify four new nickel-containing defects in synthetic diamonds, three of them demonstrating photochromic effects. The NE5 photochromic properties are likely to be determined by internal transformations. Two other photochromic centers are traps which capture an electron generated by ionization of single substitutional donor at low temperatures. Analysis of the g factors and HFS from the nitrogen atoms allows us to conclude that the main structural fragment of the N E 1 - N E 5 centers is a nickel-vacancy complex with the structure of a double semivacancy. The latter arranges the octahedral environment around the nickel ion with various numbers of nitrogen atoms, located in the first and in the second coordination spheres of the transition metal ion. As follows from the optical study, in certain zones of a sample the creation of complex nitrogen-nickel defects is possible also directly during the growth process. During P,T treatment the initial nitrogen and nickel defects transform into more complicated defects over all the sample volume, mainly as a result of the diffusion of nitrogen atoms which are smaller than ions of the transition metal. The variety of complex nitrogen-nickel defects is determined by the number of nitrogen atoms, which can range from zero to three, and variations in their location as well as in the charge state of the nickel.
Acknowledgments The authors are grateful to Dr V. Vins for loaning them synthetic diamonds for investigation and to the J. Soros Foundation for financial support.
References 1 G. Bokii, G. Bezrukov, Yu. Kluev, A. Naletov and V. Nepsha, Natural and Synthetic Diamonds, Nauka, Moscow, 1986, p. 1 (in Russiant.
1200
V. Nadolinny, A. Yelisseyev / Complex N-Ni defects in synthetic diamonds
2 E. Sobolev, in N. Sobolev (ed.), Problems of Petrology of the Earth's Crust and Upper Mantle, Nauka, Novosibirsk, 1978, p. 245 (in Russian). 3 M. Samoilovich, G. Bezrukov, and V. Butuzov, Pis'ma v Zh. Eksp. Teor. Fiz., 14 (1971) 325. 4 J. Isoya, H. Kanda and J. R. Norris, Phys. Rev. B,41 (1990) 3905. 5 J. Isoya, H. Kanda, and Y. Uchida, Phys. Rev. B,42 (1990) 9843. 6 A. Yelisseyev and V. Nadolinny, Dokl. Ross. Akad. Nauk, SSSR, 3266 (1992) 524.
7 B. Y. Masters, Solid State Commun., 9 (1971) 283. 8 M. H. Nazar6, A. J. Neves and G. Davies, Phys. Rev. B,43 (1991) 14196. 9 A. Abraham and B. Bleany, ESR of transition ions, Clarendon, Oxford, 1970, p. 1. 10 V. V. Antonov-Romanovski, Kinetics of Crystal Luminescence, Nauka, Moscow, 1966, p. 1 (in Russian). 11 Y. Mira, Y. Nishida and K. Suito, J. Phys: Condens. Matter, 21 (1990) 8507.
Diamond and Related Materials, 3 (1994) 1196-1200
Structure and creation conditions of complex nitrogen-nickel defects in synthetic diamonds V. Nadolinny
and A. Yelisseyev*
Institute of Inorganic Chemistry, Institute of Mineralogy and Petrography, Novosibirsk 630090 (Russian Federation)
(Received September 24, 1993; accepted in final form January 12, 1994)
Abstract The mechanisms of nickel incorporation into the lattice of synthetic diamond, obtained on a seed by the temperature gradient method in the Fe-Ni-C system at 1700 K and 5.5 GPa, as grown or additionally annealed, have been studied using electron spin resonance (ESR) and optical spectroscopy. In the as-grown diamonds, in addition to the interstitial Ni ÷ a new paramagnetic Ni ÷ center with C3v symmetry environment and gll =2.0227, g. =2.0988 (NE4), which correlates with the 882 nm vibronic system, was observed. In diamonds annealed at 2100 K and 5.5 GPa, containing NE1-NE3 centers, three types of photochromic paramagnetic defect (NE5-NE7) were found, which are also due to Ni incorporation into the diamond lattice. They are characterized by gl = 2.0903; g2 = 2.039 and g3 2.0044 and hyperfine splitting (HFS) from two magnetically equivalent nitrogen atoms with Atl= 12.25 G and A± = 9.5 G (NE5); gl = 1.995, g2 = 2.0207 and g3 2.0109 (NE6). For NE7, g> 2 and HFS from one nitrogen is observed while a detailed study of the ESR angular dependence of the ESR spectrum was hampered by superposition from the other systems. The NE6 and NE7 photochromic and thermal properties are typical for shallow electron traps and determined by a Ni2+-to-Ni + charge transfer by ionization of the donor nitrogen, while in the case of NE5 they can be explained in terms of internal transformation processes. =
=
1. Introduction Despite intensive investigations the p r o b l e m of the i n c o r p o r a t i o n of i m p u r i t y a t o m s into the d i a m o n d lattice has r e m a i n e d o p e n in m a n y aspects until now. F r o m several tens of elements detected in d i a m o n d s by analytical m e t h o d s , only the i n c o r p o r a t i o n of c a r b o n n e i g h b o r s in the p e r i o d i c system such as h y d r o g e n , b o r o n a n d n i t r o g e n is established [1, 2]. T h e electron spin resonance (ESR) s p e c t r u m with a g factor of 2.031 due to Ni + [ 3 ] is the only p r o v e n case of the i n c o r p o r a t i o n of a h e a v y element, a l t h o u g h the defect structure has n o t been determined. Besides this initial version of an interstitial nickel ion I-3] the idea of a nickel a t o m in a high s y m m e t r y t e t r a h e d r a l position, when it substitutes for c a r b o n , is also d e v e l o p e d in [4, 5]. I s o y a et al. considered the E S R s p e c t r u m to be associated with the 3d 7 nickel ion state a n d related in some w a y to N i - . It s h o u l d be n o t e d that, in spite of the new d a t a on hyperfine structure ( H F S ) from ~3C for this defect, the i n t e r p r e t a t i o n in refs. 4 a n d 5 was given in the context of an unsatisfactory a n a l o g y with G e a n d Si where the lattice p a r a m e t e r s are m u c h larger t h a n for d i a m o n d . T h e new results o b t a i n e d recently for three c o m p l e x n i t r o g e n - n i c k e l defects, p r o d u c e d in synthetic d i a m o n d s *Author to whom correspondence should be addressed.
0925-9635/94/$7.00 SSD1 0925-9635(94)00182-Q
after P , T t r e a t m e n t [ 6 ] , s t i m u l a t e d a m o r e careful study of b o t h a s - g r o w n a n d a n n e a l e d d i a m o n d s . Expecting nickel to exist n o t only in the N i + charge state in d i a m o n d , b u t also as N i ° a n d N i 2 +, we investigated the p h o t o e x c i t a t i o n effect on nickel defects optically a n d by ESR.
2. Experimental details Synthetic d i a m o n d s g r o w n on a seed by the t e m p e r ature g r a d i e n t m e t h o d in the F e - N i - C system at 1700 K a n d 5.5 G P a , as well as those after a d d i t i o n a l P , T t r e a t m e n t at 2100 K a n d 5.5 G P a for 5 h, have been studied. E S R spectra were r e c o r d e d on a Varian E l 0 9 s p e c t r o m e t e r in the X - b a n d range at 77 a n d 300 K. W h e n investigating the p h o t o i n d u c e d processes, excitation from 0.5 k W m e r c u r y l a m p was carried out directly inside the s p e c t r o m e t e r cavity. The necessary H g lines in the l a m p s p e c t r u m were s e p a r a t e d using a n u m b e r of c o l o r e d glass filters. O p t i c a l a b s o r p t i o n spectra were t a k e n on a P e r k i n - E l m e r 325 I R m a c h i n e a n d a special c o m p u t e r - c o n t r o l l e d set-up on the basis of an M D R - 2 diffraction m o n o c h r o m a t o r for the UV, visible a n d n e a r - I R regions. T h e r m o l u m i n e s c e n c e curves were r e c o r d e d at a heating rate of 40 K m i n - 1 in the range 8O-700 K.
© 1994
Elsevier Science S.A. All rights reserved
V. Nadolinny, A. Yelisseyev / Complex N Ni defects in synthetic diamonds
3. Results and discussion
According to ESR data the as-grown diamonds are characterized by single substitutional nitrogen and paramagnetic nickel contents up to 1019 c m - 3 and 10 ~8 cm 3 respectively. At 77 K, weak spectra from the N E 1 - N E 3 centers [-6] as well as a new spectrum with S = i without any HFS, denoted NE4, were observed (Fig. 1 ). Analysis of the angular dependence of the ESR spectra (Fig. 2) showed the NE4 center to be of C3~ symmetry and described by the spin hamiltonian I2I=oa[H4S with parameters gll = 2.0227 and g± = 2.0988, where gll II(111). The select symmetry axis of the third order for the NE4 defect indicates substitution of an impurity atom for carbon and distortion of the lattice parameters along one of the (111) directions. As follows from an analysis of the g factor anisotropy, the center is due to a d ion with a more than half filled d shell. This state may be the 3d 9 electronic configuration of Ni+. Analysis of the possible positions of a large Ni + ion, with 1.15 ~. covalent radius, in the diamond lattice allows us to conclude that a vacancy occupies one of the carbon sites which are the nearest to the nickel. In this case the nickel ion shifts towards the vacancy and arranges a structure Ni +
Hu [001] NE4
i
I
I
N
I 3194
I 3234
I 3274
I 3314
1197
analogous to the double semivacancy which has been calculated by Masters [7] and proven to exist in Ge and Si. In luminescence spectra of the crystals under investigation the 882 nm vibronic system is observed together with the $2, $3 and "523.2nm" systems. Employing isotopic effects, Nazar6 et al. [8] proved the nickel nature of the 882 nm system and showed it to be due to the 3d 9 state of an Ni + ion with C3v local symmetry. The above-mentioned NE4 defect, parallel with nickel centers studied in refs. 4 and 5, may explain the structure of the defects responsible for the 882 nm system. After P,T treatment at 2 1 0 0 K and 5 . 5 G P a the spectrum from Ni with g=2.031 disappears in the ESR spectra of synthetic diamonds while the concentration of single substitutional nitrogen as well as of NE4 defects decreases by two orders. The treatment is accompanied by an increase in the concentration of N E I - N E 3 nickelcontaining centers for which the total content reaches (3 5) x 101~ cm -3. In order to understand in which form the remainder of the nickel ions are present in diamonds, and expecting the existence of Ni ° and Ni 2+ states in analogy with other crystals, we employed photoexcitation to produce charge transfer of the nickel to the Ni + paramagnetic state. With excitation by ). ~<360 nm light at room temperature an ESR spectrum with a g factor exceeding 2 and H F S from two magnetically equivalent nitrogen atoms appears (Fig. 3). This spectrum, denoted NE5, is superimposed on the NE1 and NE2 spectra [6]. Therefore the analysis of the NE5 spectrum parameters was carried out for ESR spectra recorded in Htl <111), (110) and (100) orientations. The NE5 spectrum is described by the spin hamiltonian
I 3354
i
where S= l, g]=2.0903 (gl4l {011)), g2=2.039, g3 =
~,G Fig. 1. The ESR spectrum of as-grown synthetic diamond at 77 K.
HII [100] 90 _[1101 70 --
~
k/k
~
/x*--
\
[111] ~ - ~ "
d
( 30-
~"4 "X"X,"
10 -[1001 3154
\
If--
t
~1
\
,' "~
NE2NE1 '
I '[ ~ N E 5 1
I I I
/// //
~-
)~
3194
~ ~ ~x-"
3234
9=2.0025 I
3274
~,G Fig. 2. Angular dependence of the ESR spectrum for the NE4 center.
3220
Iq
3280
I
I
3340
3400
~,G Fig. 3. The ESR spectrum of annealed synthetic diamond after ,i~ 360 nm excitation at 300 K.
V. Nadolinny, A. Yelisseyev / Complex N-Ni defects in synthetic diamonds
1198
2.0044 (~ (g3, (01i))=8°), Air1= Atl2= 12.25 G (All1 and All2 II (11[)) and A± 1 = A±2=9.5 G. Electronic spin S = ½ and g factor values typical for a d ion with a more than half-filled d shell, together with the absence of the HFS, allow us to relate the NE5 centers to the Ni + 3d 9 state. The g factors for NE5, as in the case of NE1-NE4 centers, correspond to the 3z 2 - r 2 ground state of Ni ÷ in the crystal field of a distorted octahedron (gll=2; g ± > 2 ) I-9]. As one of the possible NE5 structures, in analogy with the NE1 center where two magnetically equivalent nitrogen atoms are also present, a four-vacancy formation can be assumed. Nitrogen atoms could be located at its ends and a Ni ÷ ion in the center. The N - N i - N direction is at 8 ° to the (011) second-order axis which corresponds to the g3 direction for this center. Certainly it is difficult to expect that such a four-vacancy structure really exists. The NE5 structure is more likely to be similar to that of the NE1 center where nitrogen atoms did not reach the Ni first coordination sphere during the diffusion process but stopped in the second coordination sphere, thus forming an N - C - V - N i - V - C - N chain. The study of photoexcitation by the other wavelengths showed that the NE5 intensity decreases to complete disappearance when the wavelength changes from 560 to 650 nm. In Fig. 4 the kinetics of the intensity changes for the X line in the NE5 spectrum (Fig. 3) in various conditions are given. In darkness at room temperature the ESR spectrum demonstrates no changes after cutoff of the preliminary 2 ~<360 nm excitation which gives a strong NE5 response or after complete quenching at 2>~560nm illumination (Fig. 4, time intervals 3 and 5 respectively). Weakening of the exciting light using neutral density filters produces no changes in the NE5 behaviour at either 360 or 560 nm but makes the curves slope more gentle and the transition time larger (time intervals 2 and 4). 360nm
The temperature influence on the NE5 spectrum was quite unexpected. After complete quenching by 560 nm illumination at room temperature, heating the sample to 570 K in darkness completely restored the NE5 signal. Analysis of the temperature dependence gives the thermal activation energy of this process to be about 0.2 eV. Such a behavior means that the level from which the electron is excited by UV illumination or by heating is situated higher than the NE5 paramagnetic state. Because of the possibility of recombination processes with charge transfer between the centers, we carried out a special investigation of synthetic diamonds using a thermoluminescence technique which allows shallow traps with levels situated close to the conduction or valence bands to be revealed. They are able to capture free carriers which are generated in the crystal by UV excitation and release them on heating. A thermoluminescence curve demonstrating three main peaks at 160, 370 and 600 K, as well as an additional peak at 130 K, is shown in Fig. 5 while the calculated traps parameters are given in Table 1. If excited at 77 K, two new photochromic centers appear in the ESR spectra of annealed synthetic diamonds. The single substitutional nitrogen also demonstrates photochromic properties but in quite a different way from in the case described in ref. 11, in which charge transfer between the H2 and H3 center in synthetic diamond was investigated. One of the photochromic centers (NE6) is characterized by an electron state with S = ½, has no HFS and is described by a spin hamiltonian with the following parameters: gl = 1.995 (gl II(110)), g2 = 2.0207 (g2 II( 1i0)) and g3 = 2.0109 (g3 II(001)). The other, denoted NE7, has a g factor considerably larger than 2 and HFS from one nitrogen atom; the value of the H F splitting varies from 6 to 10 G depending on orientation. We failed to record the angular dependence of this spectrum because of
2,.2.560n m
p;~6 -'4
3 e-
_-2
0.1
0.2 t, h
0.3
0.4
Fig. 4. Kinetics of X-line intensity changes in the NE5 spectrum at room temperature during 3 6 5 n m and 2 > 5 6 0 n m excitation (time intervals 2 and 4 respectively) while during the time intervals 1, 3 and 5 the sample was in darkness. The upper curve demonstrates the temporal regime of the excitation.
100
300
500
700
~K
Fig. 5. Thermoluminescence curve for an annealed synthetic diamond after 360 n m excitation at 80 K.
I( Nadolinny, A. Yelisseyev / Complex N Ni defects in synthetic diamonds
1199
TABLE 1. Trap parameters for annealed synthetic diamond (calculated from the thermoluminescence curves in Fig. 5t; 61 and 6z are the halfwidths of the thermoluminescence peak from the low and from the high temperatures respectively Thermoluminescence peak maximum T
Half-widths of thermoluminescence peak
{KI
61/~2
130 160 360 600
22.5/45 75/67.5 45/'50
Thermal activation energy (general method [ 10]) E(eV}
Frequency factor o~ (s 1)
Kinetics order
0.23 _+ 0.02 0.39 _+ 0.03 0.63 ± 0.05
2.5 × 103 4.5 x 10z 3.0 × 102
II I! I
strong superposition with the other systems. Both NE6 and NE7 appear at 2~< 360 nm excitation, while they are removed reversibly with ~l>~560 nm illumination. They also revert to a non-paramagnetic state on warming to 300 K. The facts that S = ½, that there is strong anisotropy of the g factors with values which are much larger than 2 and typical for ions with a more than half-filled d shell, that HFS is absent and that the diamonds were grown in an Ni-containing melt allow us to relate these centers also to nickel defects with the Ni + state of a transition metal. A detailed study of the temperature and spectral dependences for efficiency of light sum storage demonstrates full analogy between the 130 and 160 K peaks in thermoluminescence curves and the NE6 and NE7 centers in ESR. Thus the same defects can be supposed to take part in luminescence and ESR, both centers becoming paramagnetic after capturing an electron and reverting to a non-paramagnetic state on warming or with illumination. Single substitutional nitrogen acts as a donor of electrons, which take part in a charge transfer process, its behaviour being opposite to that of the NE6 and NE7 centers on excitation and heating. Changes in the donor nitrogen response in ESR with photoexcitation vary from sample to sample depending on the concentrations of the NE6 and NE7 centers and reach 12% in some crystals. At room temperature no changes in ESR were noticed in the same samples with the same excitation although the sensitivity level was about 0.05%. Thus the photochromic properties of donor nitrogen, as well as of the NE6 and NE7 centers, differ considerably from effects associated with the NE5 defect. The complexity of the data obtained allows us to conclude that the latter are due to internal transitions and transformations inside the NE5 center. The state into which this center changes with 2>~ 560 nm excitation is also expected to be paramagnetic while the corresponding ESR response is likely to be masked by the other spectra or maybe will be observed at lower temperatures.
Thermal activation energy (initial rise method [ 10]) E(eV} 0.15 0.22 0.37 0.53
× 0.02 +_ 0.02 +_ 0.03 +_ 0.05
4. Conclusions The present investigation allows us to identify four new nickel-containing defects in synthetic diamonds, three of them demonstrating photochromic effects. The NE5 photochromic properties are likely to be determined by internal transformations. Two other photochromic centers are traps which capture an electron generated by ionization of single substitutional donor at low temperatures. Analysis of the g factors and HFS from the nitrogen atoms allows us to conclude that the main structural fragment of the N E 1 - N E 5 centers is a nickel-vacancy complex with the structure of a double semivacancy. The latter arranges the octahedral environment around the nickel ion with various numbers of nitrogen atoms, located in the first and in the second coordination spheres of the transition metal ion. As follows from the optical study, in certain zones of a sample the creation of complex nitrogen-nickel defects is possible also directly during the growth process. During P,T treatment the initial nitrogen and nickel defects transform into more complicated defects over all the sample volume, mainly as a result of the diffusion of nitrogen atoms which are smaller than ions of the transition metal. The variety of complex nitrogen-nickel defects is determined by the number of nitrogen atoms, which can range from zero to three, and variations in their location as well as in the charge state of the nickel.
Acknowledgments The authors are grateful to Dr V. Vins for loaning them synthetic diamonds for investigation and to the J. Soros Foundation for financial support.
References 1 G. Bokii, G. Bezrukov, Yu. Kluev, A. Naletov and V. Nepsha, Natural and Synthetic Diamonds, Nauka, Moscow, 1986, p. 1 (in Russiant.
1200
V. Nadolinny, A. Yelisseyev / Complex N-Ni defects in synthetic diamonds
2 E. Sobolev, in N. Sobolev (ed.), Problems of Petrology of the Earth's Crust and Upper Mantle, Nauka, Novosibirsk, 1978, p. 245 (in Russian). 3 M. Samoilovich, G. Bezrukov, and V. Butuzov, Pis'ma v Zh. Eksp. Teor. Fiz., 14 (1971) 325. 4 J. Isoya, H. Kanda and J. R. Norris, Phys. Rev. B,41 (1990) 3905. 5 J. Isoya, H. Kanda, and Y. Uchida, Phys. Rev. B,42 (1990) 9843. 6 A. Yelisseyev and V. Nadolinny, Dokl. Ross. Akad. Nauk, SSSR, 3266 (1992) 524.
7 B. Y. Masters, Solid State Commun., 9 (1971) 283. 8 M. H. Nazar6, A. J. Neves and G. Davies, Phys. Rev. B,43 (1991) 14196. 9 A. Abraham and B. Bleany, ESR of transition ions, Clarendon, Oxford, 1970, p. 1. 10 V. V. Antonov-Romanovski, Kinetics of Crystal Luminescence, Nauka, Moscow, 1966, p. 1 (in Russian). 11 Y. Mira, Y. Nishida and K. Suito, J. Phys: Condens. Matter, 21 (1990) 8507.