- Email: [email protected]
Annealing study of the formation of nickel-related paramagnetic defects in diamond
Diamond and Related Materials 11 (2002) 623–626
Annealing study of the formation of nickel-related paramagnetic defects in diamond R.N. Pereiraa,b,*, A.J. Nevesa, W. Gehlhoffb, N.A. Soboleva, L. Rinoa, H. Kandac a
Department of Physics, University of Aveiro, 3810-193 Aveiro, Portugal b Institute of Solid State Physics, TU-Berlin, D-10623, Germany c National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Japan
Abstract We report an annealing study of paramagnetic defects found in synthetic diamond grown from nickel solventycatalysts. Substitutional N0 and Niy defects in diamond show a similar behavior in the course of annealing at temperatures in the range 1550–2000 8C. New paramagnetic centers labeled AB6 and AB7 become detectable upon annealing. The AB5 center is found to appear in as-grown HPHT diamond rich in nitrogen, and its concentration decreases with increasing annealing temperature. The AB1, AB3, AB6 and AB7 defects are produced at early stages of the annealing process and are suppressed by heating diamond to temperatures in excess of 1900 8C. Possible origins of the AB paramagnetic centers are discussed in the light of the recently proposed mechanisms of nitrogen aggregation and nickel–nitrogen complexes formation in HPHT diamond. 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond; Paramagnetic defects; Annealing; Nickel
1. Introduction Depending on the growth conditions, diamond synthesized at high pressures and high temperatures (HPHT) incorporates different types of impurity-related defects. Nitrogen is typically the most abundant impurity in HPHT diamond when nitrogen getters like Ti andyor Zr are not used in its synthesis. As-grown samples incorporate nitrogen mostly at single substitutional sites, either in the neutral charge state (N0s: P1 EPR center w1x) or in the positive charge state (Nq s ) forms w2x. Annealing these crystals at temperatures TaG1500 8C results in the formation of pairs of nearest-neighbor nitrogen atoms (A centers). It was observed that the presence of Ni or Co in HPHT synthetic diamonds enhances the nitrogen aggregation and studies on the aggregation kinetics of N in diamonds containing transition metals have shown significant deviations from second order kinetics w3,4x. Ni assists in the aggregation process by promoting the formation of nitrogen interstitials via carbon interstitials w3,4x. Mechanisms involving *Corresponding author. Fax: q351-234-424965. E-mail address: [email protected] (R.N. Pereira).
vacancies released from nickel-vacancy centers were also proposed w3x. Along with the well-known single substitutional nickel Niy s (W8 EPR center with spin Ss3y2 w5,6x), several other nickel-related paramagnetic centers were found in as-grown and annealed HPHT diamonds w7–12x. Among these defects, the nickel–nitrogen complexes NE1–NE3, NE5 and NE8 (NE centers) have been taken as resulting from nitrogen aggregation to a nickel-related defect at temperatures at which nitrogen is mobile in the diamond lattice w7x. The formation mechanisms of these centres were studied by EPR and infrared absorption spectroscopy measurements on as-grown and annealed HPHT diamonds w13x. Recently, five nickel-related paramagnetic centers (labeled AB1–AB5) have been found in annealed HPHT diamonds (see Table 1) w8,9x. These AB centers are probably formed by a process different from the nitrogen aggregation to nickel (which is assumed for the formation of the NE centers), because no hyperfine splitting due to involved nitrogen could be resolved despite the relatively small width (DBpps0.1– 0.3 mT) of the EPR lines. While the NE and AB1–AB5 centers are formed in nitrogen-rich HPHT diamonds, the NIRIM1 and
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Table 1 Spin Hamiltonian parameters of the AB centers in diamond Center
Symmetry
Spin
g-Values
Zero-field splitting
AB1
Trigonal
Ss1y2
–
AB2
Trigonal
Ss1y2
AB3
Rhombic-I
Ss1y2
AB4
Rhombic-I
Ss1y2
AB5
Trigonal
Ss1
AB6
Triclinic
Ss1y2
AB7
Rhombic-I
Ss1y2
g±±s2.0024 ±± w1, 1, 1x gHs2.0920 H w1, 1, 1x g±±s2.0072 ±± w1, 1, 1x gHs2.0672 H w1, 1, 1x g1s2.1105 ±± w1, 0, 0x g2s2.0663 ±± w0, 1, 1x ¯ 1x g3s2.0181 ±± w0, 1, g1s2.0220 ±± w1, 0, 0x g2s2.0094 ±± w0, 1, 1x ¯ 1x g3s2.0084 ±± w0, 1, g±±s2.037 ±± w1, 1, 1x gHs2.022 H w1, 1, 1x g1s2.0185 ±± wy0.79756, 0.42097, 0.43207x g2s2.0244 ±± wy0.00987, 0.70704, y0.70711x g3s2.0742 ±± w0.60316, 0.56822, 0.55975x g1s1.9910 ±± w1, 0, 0x g2s2.0078 ±± w0, 1, 1x ¯ 1x g3s2.0046 ±± w0, 1,
NIRIM2 paramagnetic defects are observed in synthetic diamonds with low nitrogen concentration w10x. Such diamonds are green colored and are typically grown using a nickel solventycatalyst and a nitrogen getter. The proposed models for the NIRIM1 and NIRIM2 Ss 1y2 centers are interstitial nickel in the positive charge state (Niq 1 ) and interstitial nickel associated with a vacancy (Niq 1 –V), respectively w10x. The present article reports an EPR study on synthetic diamonds both nitrogen-rich and with low nitrogen content. The formation of the AB nickel-related paramagnetic centers upon annealing and their relation with other nitrogen- and nickel-containing centers in diamond is discussed. 2. Experimental details Diamond samples used in this work were synthesized at the NIRIM by the temperature gradient method using Ni- or a Ni-alloy as solventycatalyst. The growth temperature was approximately 1500 8C. In order to monitor the formation conditions of the AB nickel-related paramagnetic defects, two of the samples were subjected to an annealing sequence in the temperature range from 1550 to 2000 8C. Heat treatments were carried out using the same apparatus as for the samples growth. These two samples were grown using Ni–2 wt.% Ti (sample A) and Ni–20 wt.% Fe (sample B) alloys as solventy catalysts. Before HPHT treatment, sample A contained less than 10 ppm of substitutional N0 centers as measured by infrared absorption spectroscopy. Sample B contained ;150 ppm of N0s and 50 ppm of A centers. EPR spectra were measured after each annealing step at suitable temperatures in the range from 4.2 to 100 K depending on which paramagnetic center was detected.
– –
–
Ds"31.72 GHz –
–
EPR measurements were carried out on a Bruker ESP 300E spectrometer mounted with X-and Q-band microwave bridges and corresponding cylindrical TE011 microwave resonators. For measuring EPR at low temperatures we used a helium gas-flow cryostat for the X-band and a helium bath cryostat for the Q-band, respectively. Samples were oriented with respect to the external magnetic field either by growth facets or by faces polished parallel to the main crystallographic planes. 3. Results and discussion The different paramagnetic centers, which produce in the Q-band overlapping EPR spectra, were identified through their complete angular dependence upon rotation of the external magnetic field B in a {110} crystallographic plane. These angular dependencies of the line positions were calculated using the spin Hamiltonian HsbBØgØSqSØDØSqSØAØI
(1)
with the fitting parameters given in Table 1 for the AB centers and those given in the corresponding references for the other centers. 3.1. Annealing sequence: sample A Prior to annealing sample A exhibited a green color that changed to brown only after annealing at a temperature as high as 2000 8C w14x. In Table 2 the paramagnetic centers detected in sample A are listed together with the detected changes of their EPR signal intensity induced by sequential annealing at temperatures increasing in the range 1550–2000 8C. In all annealing stages the EPR measurements of sample A show the EPR spectra of the substitutional
R.N. Pereira et al. / Diamond and Related Materials 11 (2002) 623–626 Table 2 Paramagnetic centers in synthetic diamond and behavior of their EPR signal intensities throughout the annealing sequence Sample
Center wRef.x
Annealing temperature (8C) As-grown
1550
1700
1900
2000
A
P1 w1x W8 w5x NIRIM1 w10x NIRIM2 w10x AB7
• • • • –
• • • • –
• • • • •
• • • • ≠
x x x • x
B
P1 w1x W8 w5x AB5 AB1 AB3 AB6 NE1 w7x NE2 w7x NE3 w7x
• • • – – – – – –
x x • – – – – – –
x x x • • • • – –
x x x • – • x • •
x – – – – – x ≠ ≠
–, Center not detected; •, center detected but no change relative to the previous annealing step observed; x, EPR signal intensity decreases with respect to the previous step; ≠, EPR signal intensity increases with respect to the previous step.
N0s (P1) and substitutional Niy s (W8) centers. The P1 and W8 EPR line intensities, like the sample color, only changed after annealing at 2000 8C. Beside the P1 and W8 lines, the EPR spectra of sample A prior to annealing exhibited lines related to the NIRIM1 and NIRIM2 centers. For the NIRIM2 EPR signal intensity, no remarkable changes were observed throughout the annealing sequence. The NIRIM1 signal intensity also decreased significantly only after annealing at 2000 8C. This behavior suggests that the processes involved in the decrease of the concentrations of the substitutional nickel and of the NIRIM1 centers may be similar. After annealing of sample A at 1700 8C, lines related to a new paramagnetic center labeled AB7 appear in the EPR spectra. The angular dependence of the EPR line positions can be described by an Ss1y2 system with rhombic-I symmetry w15x. From the g-values obtained it is likely that this defect incorporates nickel w15x. This center exhibits a transitory behavior: its EPR lines intensity increases after the annealing at 1900 8C and a decrease is observed after the annealing at 2000 8C. Thus, the AB7 center is formed at early annealing stages before nitrogen shows significant aggregation. It might be produced through the capture of mobile vacancies andyor interstitials by a nickel-related center. 3.2. Annealing sequence: sample B The as-grown sample B exhibited the yellow color typical of nitrogen-rich diamond. The sample color changed progressively to brown as it was sequentially annealed. In addition to the P1 center the as-grown
625
sample B showed the presence of W8 and AB5 defects. Heating this sample at 1550 8C caused a decrease of the P1 signal, as opposed to sample A where a significant change in the P1 signal intensity was detected only after annealing at 2000 8C. A similar behavior was observed for the W8 signal intensity. A decrease in the P1 concentration is accompanied by a decrease in the W8 signal intensity in both samples A and B, regardless of the differences in the initial nitrogen concentration. This indicates that the main process of the nitrogen aggregation in such type of diamonds should also involve the W8 defect. This result supports the suggestion that the enhancement of the nitrogen aggregation in nickel containing diamonds is due to the generation of carbon interstitials by nickel defects which in turn produce highly mobile nitrogen interstitials w3,4x. The process occurs when a substitutional Ni ion displaces a neighboring C atom, creating a nickel-vacancy defect. The released carbon interstitial can migrate to substitutional N atoms and exchange positions creating a highly mobile N interstitial, which migrates to form A centers. Another proposed mechanism responsible for the increase of the formation rate of nitrogen pairs is based on the release of vacancies from a nickel-vacancy complex. These vacancies would be trapped by nitrogen and in this way would assist in the migration of nitrogen w3,13x. Nadolinny et al. proposed that the nickel-vacancy related NE4 EPR center, formed in the process of creation of carbon interstitials by W8 centers, is also responsible for the releasing of vacancies in a reverse process w13x. Additionally, it was suggested that this center is the basic structure of some of the nickel– nitrogen complexes w7x. In all as-grown and annealed diamond samples we failed to observe any EPR signal from NE4 defects, though we detected the nickel– nitrogen complexes NE1–NE3 and NE5 in a number of these samples. The EPR signal of the AB5 center in sample B decreases throughout the annealing sequence and disappears after heat treatment at 2000 8C. This center was found for the first time in annealed HPHT diamond samples and a nickel–nitrogen pair model was proposed for its structure w8x. EPR spectra measured in the set of diamonds used in this work show that AB5 is always present in as-grown samples synthesized without nitrogen getters. This type of diamond shows a stronger incorporation of both nitrogen and nickel in comparison with samples synthesized using nitrogen getters. A minimum degree of nitrogen and nickel incorporation is needed to produce the AB5 centers. This suggests that, along with nickel, nitrogen is a probable constituent of AB5, supporting the model proposed for this center in Neves et al. w8x. After annealing sample B at 1700 8C we detected the EPR spectra of the AB1, AB3, NE1 defects and from a new paramagnetic center labeled AB6. The Ss1y2
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centers AB1 and AB3 have trigonal and rhombic-I symmetry, respectively, and, like the AB5 center, were first observed in annealed HPHT diamonds w9x. The new Ss1y2 AB6 w15x center which is measured after the annealing at 1700 8C has triclinic symmetry with no indication of any hyperfine structure in the EPR spectra. Like the other AB paramagnetic centers, the measured g-values give a strong indication that it is a nickelcontaining defect. Although AB1 and AB3 where produced in sample B by annealing at 1700 8C, these centers were observed also in as-grown samples synthesized with 100% Ni solventycatalyst, which demonstrated a stronger incorporation of substitutional nickel than in sample B. Contrary to the AB1 and AB3 defects, the AB6 center has never been detected in as-grown diamond samples studied by us. It is quite likely that the AB1, AB3 and AB6 centers are products of the nickelrelated generation of carbon interstitials that are supposed to enhance the nitrogen aggregation. These centers are transient defects that anneal out at higher temperatures, being transformed into other more stable defects, possibly the NE nickel–nitrogen complexes. At annealing temperatures TaG1700 8C we observed the appearance of the NE1, NE2 and NE3 EPR spectra. According to Nadolinny and co-workers w7,13x, these centers are produced by aggregation of mobile nitrogen atoms to a nickel impurity during the annealing of nickel-containing diamond. Their suggestion is supported by the above mentioned detection of the NE1 EPR signal after annealing at 1700 8C. Additionally, we observed that this signal decreases progressively as we anneal sample B at increasing temperatures, while the NE2 and NE3 EPR lines arise. 4. Conclusions The formation and annealing conditions of paramagnetic defects in HPHT synthetic diamond were studied. Substitutional N0 and Niy defects show a similar behavior when diamond samples are subjected to high temperature heat treatments. AB5 centers occur in as-grown HPHT diamond rich in nitrogen and its concentration
can be decreased by heat treatments. New AB7 and AB6 paramagnetic centers are produced upon annealing diamond samples with high and low Nos concentrations, respectively. AB1, AB3 and AB6 centers exhibit a transitory behavior upon annealing HPHT diamond crystals. These are possibly products of the nickel-related generation of carbon interstitials that are supposed to enhance the nitrogen aggregation. Acknowledgments ¸ ˜ Integradas The work was supported in part by ‘Accoes ˜ Luso-Alemas’, Project No. A-13y99. R.N.P. acknowl¸ ˜ para a edges the financial support from ‘Fundacao ˆ Ciencia e a Tecnologia’ under contract PRAXIS XXIy BDy18405y98. References w1x W.V. Smith, P.P. Sorokin, I.L. Gelles, G.J. Lasher, Phys. Rev. 115 (1959) 1546. w2x S.C. Lawson, D. Fisher, D.C. Hunt, M.E. Newton, J. Phys.: Condens. Matter 10 (1998) 6171. w3x I. Kiflawi, H. Kanda, A. Mainwood, Diamond Relat. Mater. 7 (1998) 327. w4x D. Fisher, S.C. Lawson, Diamond Relat. Mater. 7 (1998) 299. w5x M.I. Samoilovich, G.N. Bezrukov, V.P. Butuzov, JETP Lett. 14 (1971) 379. w6x J. Isoya, H. Kanda, J.R. Norris, J. Tang, M.K. Bowman, Phys. Rev. B 41 (1990) 3905. w7x V.A. Nadolinny, A.P. Yelisseyev, J.M. Baker, et al., J. Phys.: Condens. Matter 11 (1999) 7357. w8x A.J. Neves, R. Pereira, N.A. Sobolev, et al., Diamond Relat. Mater. 9 (2000) 1057. w9x A.J. Neves, R. Pereira, N.A. Sobolev, et al., Physica B 273y 274 (1999) 651. w10x J. Isoya, H. Kanda, Y. Uchida, Phys. Rev. B 42 (1990) 9843. w11x C.J. Noble, Th. Pawlik, J.-M. Spaeth, J. Phys.: Condens. Matter 10 (1998) 11781. w12x Th. Pawlik, C. Noble, J.-M. Spaeth, J. Phys.: Condens. Matter 10 (1998) 9833. w13x V.A. Nadolinny, A.P. Yelisseyev, J.M. Baker, et al., Diamond Relat. Mater. 9 (2000) 883. w14x S.C. Lawson, H. Kanda, J. Appl. Phys. 73 (1993) 3967. w15x R.N. Pereira, W. Gehlhoff, A.J. Neves, N.A. Sobolov, H. Kanda, to be published.
Annealing study of the formation of nickel-related paramagnetic defects in diamond R.N. Pereiraa,b,*, A.J. Nevesa, W. Gehlhoffb, N.A. Soboleva, L. Rinoa, H. Kandac a
Department of Physics, University of Aveiro, 3810-193 Aveiro, Portugal b Institute of Solid State Physics, TU-Berlin, D-10623, Germany c National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Japan
Abstract We report an annealing study of paramagnetic defects found in synthetic diamond grown from nickel solventycatalysts. Substitutional N0 and Niy defects in diamond show a similar behavior in the course of annealing at temperatures in the range 1550–2000 8C. New paramagnetic centers labeled AB6 and AB7 become detectable upon annealing. The AB5 center is found to appear in as-grown HPHT diamond rich in nitrogen, and its concentration decreases with increasing annealing temperature. The AB1, AB3, AB6 and AB7 defects are produced at early stages of the annealing process and are suppressed by heating diamond to temperatures in excess of 1900 8C. Possible origins of the AB paramagnetic centers are discussed in the light of the recently proposed mechanisms of nitrogen aggregation and nickel–nitrogen complexes formation in HPHT diamond. 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond; Paramagnetic defects; Annealing; Nickel
1. Introduction Depending on the growth conditions, diamond synthesized at high pressures and high temperatures (HPHT) incorporates different types of impurity-related defects. Nitrogen is typically the most abundant impurity in HPHT diamond when nitrogen getters like Ti andyor Zr are not used in its synthesis. As-grown samples incorporate nitrogen mostly at single substitutional sites, either in the neutral charge state (N0s: P1 EPR center w1x) or in the positive charge state (Nq s ) forms w2x. Annealing these crystals at temperatures TaG1500 8C results in the formation of pairs of nearest-neighbor nitrogen atoms (A centers). It was observed that the presence of Ni or Co in HPHT synthetic diamonds enhances the nitrogen aggregation and studies on the aggregation kinetics of N in diamonds containing transition metals have shown significant deviations from second order kinetics w3,4x. Ni assists in the aggregation process by promoting the formation of nitrogen interstitials via carbon interstitials w3,4x. Mechanisms involving *Corresponding author. Fax: q351-234-424965. E-mail address: [email protected] (R.N. Pereira).
vacancies released from nickel-vacancy centers were also proposed w3x. Along with the well-known single substitutional nickel Niy s (W8 EPR center with spin Ss3y2 w5,6x), several other nickel-related paramagnetic centers were found in as-grown and annealed HPHT diamonds w7–12x. Among these defects, the nickel–nitrogen complexes NE1–NE3, NE5 and NE8 (NE centers) have been taken as resulting from nitrogen aggregation to a nickel-related defect at temperatures at which nitrogen is mobile in the diamond lattice w7x. The formation mechanisms of these centres were studied by EPR and infrared absorption spectroscopy measurements on as-grown and annealed HPHT diamonds w13x. Recently, five nickel-related paramagnetic centers (labeled AB1–AB5) have been found in annealed HPHT diamonds (see Table 1) w8,9x. These AB centers are probably formed by a process different from the nitrogen aggregation to nickel (which is assumed for the formation of the NE centers), because no hyperfine splitting due to involved nitrogen could be resolved despite the relatively small width (DBpps0.1– 0.3 mT) of the EPR lines. While the NE and AB1–AB5 centers are formed in nitrogen-rich HPHT diamonds, the NIRIM1 and
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R.N. Pereira et al. / Diamond and Related Materials 11 (2002) 623–626
624
Table 1 Spin Hamiltonian parameters of the AB centers in diamond Center
Symmetry
Spin
g-Values
Zero-field splitting
AB1
Trigonal
Ss1y2
–
AB2
Trigonal
Ss1y2
AB3
Rhombic-I
Ss1y2
AB4
Rhombic-I
Ss1y2
AB5
Trigonal
Ss1
AB6
Triclinic
Ss1y2
AB7
Rhombic-I
Ss1y2
g±±s2.0024 ±± w1, 1, 1x gHs2.0920 H w1, 1, 1x g±±s2.0072 ±± w1, 1, 1x gHs2.0672 H w1, 1, 1x g1s2.1105 ±± w1, 0, 0x g2s2.0663 ±± w0, 1, 1x ¯ 1x g3s2.0181 ±± w0, 1, g1s2.0220 ±± w1, 0, 0x g2s2.0094 ±± w0, 1, 1x ¯ 1x g3s2.0084 ±± w0, 1, g±±s2.037 ±± w1, 1, 1x gHs2.022 H w1, 1, 1x g1s2.0185 ±± wy0.79756, 0.42097, 0.43207x g2s2.0244 ±± wy0.00987, 0.70704, y0.70711x g3s2.0742 ±± w0.60316, 0.56822, 0.55975x g1s1.9910 ±± w1, 0, 0x g2s2.0078 ±± w0, 1, 1x ¯ 1x g3s2.0046 ±± w0, 1,
NIRIM2 paramagnetic defects are observed in synthetic diamonds with low nitrogen concentration w10x. Such diamonds are green colored and are typically grown using a nickel solventycatalyst and a nitrogen getter. The proposed models for the NIRIM1 and NIRIM2 Ss 1y2 centers are interstitial nickel in the positive charge state (Niq 1 ) and interstitial nickel associated with a vacancy (Niq 1 –V), respectively w10x. The present article reports an EPR study on synthetic diamonds both nitrogen-rich and with low nitrogen content. The formation of the AB nickel-related paramagnetic centers upon annealing and their relation with other nitrogen- and nickel-containing centers in diamond is discussed. 2. Experimental details Diamond samples used in this work were synthesized at the NIRIM by the temperature gradient method using Ni- or a Ni-alloy as solventycatalyst. The growth temperature was approximately 1500 8C. In order to monitor the formation conditions of the AB nickel-related paramagnetic defects, two of the samples were subjected to an annealing sequence in the temperature range from 1550 to 2000 8C. Heat treatments were carried out using the same apparatus as for the samples growth. These two samples were grown using Ni–2 wt.% Ti (sample A) and Ni–20 wt.% Fe (sample B) alloys as solventy catalysts. Before HPHT treatment, sample A contained less than 10 ppm of substitutional N0 centers as measured by infrared absorption spectroscopy. Sample B contained ;150 ppm of N0s and 50 ppm of A centers. EPR spectra were measured after each annealing step at suitable temperatures in the range from 4.2 to 100 K depending on which paramagnetic center was detected.
– –
–
Ds"31.72 GHz –
–
EPR measurements were carried out on a Bruker ESP 300E spectrometer mounted with X-and Q-band microwave bridges and corresponding cylindrical TE011 microwave resonators. For measuring EPR at low temperatures we used a helium gas-flow cryostat for the X-band and a helium bath cryostat for the Q-band, respectively. Samples were oriented with respect to the external magnetic field either by growth facets or by faces polished parallel to the main crystallographic planes. 3. Results and discussion The different paramagnetic centers, which produce in the Q-band overlapping EPR spectra, were identified through their complete angular dependence upon rotation of the external magnetic field B in a {110} crystallographic plane. These angular dependencies of the line positions were calculated using the spin Hamiltonian HsbBØgØSqSØDØSqSØAØI
(1)
with the fitting parameters given in Table 1 for the AB centers and those given in the corresponding references for the other centers. 3.1. Annealing sequence: sample A Prior to annealing sample A exhibited a green color that changed to brown only after annealing at a temperature as high as 2000 8C w14x. In Table 2 the paramagnetic centers detected in sample A are listed together with the detected changes of their EPR signal intensity induced by sequential annealing at temperatures increasing in the range 1550–2000 8C. In all annealing stages the EPR measurements of sample A show the EPR spectra of the substitutional
R.N. Pereira et al. / Diamond and Related Materials 11 (2002) 623–626 Table 2 Paramagnetic centers in synthetic diamond and behavior of their EPR signal intensities throughout the annealing sequence Sample
Center wRef.x
Annealing temperature (8C) As-grown
1550
1700
1900
2000
A
P1 w1x W8 w5x NIRIM1 w10x NIRIM2 w10x AB7
• • • • –
• • • • –
• • • • •
• • • • ≠
x x x • x
B
P1 w1x W8 w5x AB5 AB1 AB3 AB6 NE1 w7x NE2 w7x NE3 w7x
• • • – – – – – –
x x • – – – – – –
x x x • • • • – –
x x x • – • x • •
x – – – – – x ≠ ≠
–, Center not detected; •, center detected but no change relative to the previous annealing step observed; x, EPR signal intensity decreases with respect to the previous step; ≠, EPR signal intensity increases with respect to the previous step.
N0s (P1) and substitutional Niy s (W8) centers. The P1 and W8 EPR line intensities, like the sample color, only changed after annealing at 2000 8C. Beside the P1 and W8 lines, the EPR spectra of sample A prior to annealing exhibited lines related to the NIRIM1 and NIRIM2 centers. For the NIRIM2 EPR signal intensity, no remarkable changes were observed throughout the annealing sequence. The NIRIM1 signal intensity also decreased significantly only after annealing at 2000 8C. This behavior suggests that the processes involved in the decrease of the concentrations of the substitutional nickel and of the NIRIM1 centers may be similar. After annealing of sample A at 1700 8C, lines related to a new paramagnetic center labeled AB7 appear in the EPR spectra. The angular dependence of the EPR line positions can be described by an Ss1y2 system with rhombic-I symmetry w15x. From the g-values obtained it is likely that this defect incorporates nickel w15x. This center exhibits a transitory behavior: its EPR lines intensity increases after the annealing at 1900 8C and a decrease is observed after the annealing at 2000 8C. Thus, the AB7 center is formed at early annealing stages before nitrogen shows significant aggregation. It might be produced through the capture of mobile vacancies andyor interstitials by a nickel-related center. 3.2. Annealing sequence: sample B The as-grown sample B exhibited the yellow color typical of nitrogen-rich diamond. The sample color changed progressively to brown as it was sequentially annealed. In addition to the P1 center the as-grown
625
sample B showed the presence of W8 and AB5 defects. Heating this sample at 1550 8C caused a decrease of the P1 signal, as opposed to sample A where a significant change in the P1 signal intensity was detected only after annealing at 2000 8C. A similar behavior was observed for the W8 signal intensity. A decrease in the P1 concentration is accompanied by a decrease in the W8 signal intensity in both samples A and B, regardless of the differences in the initial nitrogen concentration. This indicates that the main process of the nitrogen aggregation in such type of diamonds should also involve the W8 defect. This result supports the suggestion that the enhancement of the nitrogen aggregation in nickel containing diamonds is due to the generation of carbon interstitials by nickel defects which in turn produce highly mobile nitrogen interstitials w3,4x. The process occurs when a substitutional Ni ion displaces a neighboring C atom, creating a nickel-vacancy defect. The released carbon interstitial can migrate to substitutional N atoms and exchange positions creating a highly mobile N interstitial, which migrates to form A centers. Another proposed mechanism responsible for the increase of the formation rate of nitrogen pairs is based on the release of vacancies from a nickel-vacancy complex. These vacancies would be trapped by nitrogen and in this way would assist in the migration of nitrogen w3,13x. Nadolinny et al. proposed that the nickel-vacancy related NE4 EPR center, formed in the process of creation of carbon interstitials by W8 centers, is also responsible for the releasing of vacancies in a reverse process w13x. Additionally, it was suggested that this center is the basic structure of some of the nickel– nitrogen complexes w7x. In all as-grown and annealed diamond samples we failed to observe any EPR signal from NE4 defects, though we detected the nickel– nitrogen complexes NE1–NE3 and NE5 in a number of these samples. The EPR signal of the AB5 center in sample B decreases throughout the annealing sequence and disappears after heat treatment at 2000 8C. This center was found for the first time in annealed HPHT diamond samples and a nickel–nitrogen pair model was proposed for its structure w8x. EPR spectra measured in the set of diamonds used in this work show that AB5 is always present in as-grown samples synthesized without nitrogen getters. This type of diamond shows a stronger incorporation of both nitrogen and nickel in comparison with samples synthesized using nitrogen getters. A minimum degree of nitrogen and nickel incorporation is needed to produce the AB5 centers. This suggests that, along with nickel, nitrogen is a probable constituent of AB5, supporting the model proposed for this center in Neves et al. w8x. After annealing sample B at 1700 8C we detected the EPR spectra of the AB1, AB3, NE1 defects and from a new paramagnetic center labeled AB6. The Ss1y2
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centers AB1 and AB3 have trigonal and rhombic-I symmetry, respectively, and, like the AB5 center, were first observed in annealed HPHT diamonds w9x. The new Ss1y2 AB6 w15x center which is measured after the annealing at 1700 8C has triclinic symmetry with no indication of any hyperfine structure in the EPR spectra. Like the other AB paramagnetic centers, the measured g-values give a strong indication that it is a nickelcontaining defect. Although AB1 and AB3 where produced in sample B by annealing at 1700 8C, these centers were observed also in as-grown samples synthesized with 100% Ni solventycatalyst, which demonstrated a stronger incorporation of substitutional nickel than in sample B. Contrary to the AB1 and AB3 defects, the AB6 center has never been detected in as-grown diamond samples studied by us. It is quite likely that the AB1, AB3 and AB6 centers are products of the nickelrelated generation of carbon interstitials that are supposed to enhance the nitrogen aggregation. These centers are transient defects that anneal out at higher temperatures, being transformed into other more stable defects, possibly the NE nickel–nitrogen complexes. At annealing temperatures TaG1700 8C we observed the appearance of the NE1, NE2 and NE3 EPR spectra. According to Nadolinny and co-workers w7,13x, these centers are produced by aggregation of mobile nitrogen atoms to a nickel impurity during the annealing of nickel-containing diamond. Their suggestion is supported by the above mentioned detection of the NE1 EPR signal after annealing at 1700 8C. Additionally, we observed that this signal decreases progressively as we anneal sample B at increasing temperatures, while the NE2 and NE3 EPR lines arise. 4. Conclusions The formation and annealing conditions of paramagnetic defects in HPHT synthetic diamond were studied. Substitutional N0 and Niy defects show a similar behavior when diamond samples are subjected to high temperature heat treatments. AB5 centers occur in as-grown HPHT diamond rich in nitrogen and its concentration
can be decreased by heat treatments. New AB7 and AB6 paramagnetic centers are produced upon annealing diamond samples with high and low Nos concentrations, respectively. AB1, AB3 and AB6 centers exhibit a transitory behavior upon annealing HPHT diamond crystals. These are possibly products of the nickel-related generation of carbon interstitials that are supposed to enhance the nitrogen aggregation. Acknowledgments ¸ ˜ Integradas The work was supported in part by ‘Accoes ˜ Luso-Alemas’, Project No. A-13y99. R.N.P. acknowl¸ ˜ para a edges the financial support from ‘Fundacao ˆ Ciencia e a Tecnologia’ under contract PRAXIS XXIy BDy18405y98. References w1x W.V. Smith, P.P. Sorokin, I.L. Gelles, G.J. Lasher, Phys. Rev. 115 (1959) 1546. w2x S.C. Lawson, D. Fisher, D.C. Hunt, M.E. Newton, J. Phys.: Condens. Matter 10 (1998) 6171. w3x I. Kiflawi, H. Kanda, A. Mainwood, Diamond Relat. Mater. 7 (1998) 327. w4x D. Fisher, S.C. Lawson, Diamond Relat. Mater. 7 (1998) 299. w5x M.I. Samoilovich, G.N. Bezrukov, V.P. Butuzov, JETP Lett. 14 (1971) 379. w6x J. Isoya, H. Kanda, J.R. Norris, J. Tang, M.K. Bowman, Phys. Rev. B 41 (1990) 3905. w7x V.A. Nadolinny, A.P. Yelisseyev, J.M. Baker, et al., J. Phys.: Condens. Matter 11 (1999) 7357. w8x A.J. Neves, R. Pereira, N.A. Sobolev, et al., Diamond Relat. Mater. 9 (2000) 1057. w9x A.J. Neves, R. Pereira, N.A. Sobolev, et al., Physica B 273y 274 (1999) 651. w10x J. Isoya, H. Kanda, Y. Uchida, Phys. Rev. B 42 (1990) 9843. w11x C.J. Noble, Th. Pawlik, J.-M. Spaeth, J. Phys.: Condens. Matter 10 (1998) 11781. w12x Th. Pawlik, C. Noble, J.-M. Spaeth, J. Phys.: Condens. Matter 10 (1998) 9833. w13x V.A. Nadolinny, A.P. Yelisseyev, J.M. Baker, et al., Diamond Relat. Mater. 9 (2000) 883. w14x S.C. Lawson, H. Kanda, J. Appl. Phys. 73 (1993) 3967. w15x R.N. Pereira, W. Gehlhoff, A.J. Neves, N.A. Sobolov, H. Kanda, to be published.