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New infrared absorption centres in electron irradiated and annealed type Ia diamonds
Diamond and Related Materials 8 (1999) 1576–1580 www.elsevier.com/locate/diamond
New infrared absorption centres in electron irradiated and annealed type Ia diamonds I. Kiflawi a, *, G. Davies b, D. Fisher c, H. Kanda d a J.J. Thomson Physical Laboratory, The University of Reading, Reading, RG6 6AF UK b Department of Physics, King’s College London, Strand, London WC2R 2LS, UK c DeBeers DTC Research Centre, Belmont Road, Maidenhead SL6 6JW, UK d NIRIM, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Accepted 30 November 1998
Abstract New absorption centres having zero phonon lines at 2678 (332 meV ), 2913.6 (361 meV ), 4392.9 (544 meV ) and 4442.2 cm−1 (550 meV ) have been produced in heavily electron-irradiated and annealed type Ia diamonds. Their temperature dependence and annealing behaviour have been investigated along with the effect of the radiation dose and isotopic shifts caused by the substitution of 15N and 13C for 14N and 12C, respectively. The results are consistent with the centres being caused by electronic transitions. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Infrared; Type Ia diamond; Annealing
1. Introduction A group of infrared (IR) optical absorption centres referred to in the literature as H1a, b and c, are the product of irradiating and annealing of type I diamonds [1,2]. Nitrogen-containing type I diamonds can be divided into different sub-categories according to the state of aggregation of the nitrogen: type Ib contain dispersed single substitutional nitrogen atoms (C-centres), type IaA contain nitrogen pairs in neighbouring substitutional sites (A-centres), while type IaB contain centres consisting of four substitutional nitrogen atoms surrounding a vacancy (B-centres). The H1a absorption line involves the creation of a local vibrational mode at 1450 cm−1 due to interstitial nitrogen [3], H1b is an electronic transition at 4941 cm−1 (612 meV ) seen in type IaA diamonds and is thought to be related to A-centres [2]. H1c is also an electronic transition at 5171 cm−1 (641 meV ) seen in type IaB and is thought to be related to B-centres [2]. Four new IR absorption centres, which we propose naming as H1d, H1e, H1f and H1g (in line with the H1a, b and c centres mentioned above), have been produced in heavily irradiated and annealed type Ia * Corresponding author. Fax: +44-1189752030. E-mail address: [email protected] (I. Kiflawi)
diamonds. Their annealing behaviour has been investigated along with the effect of irradiation dose and the isotopic shifts caused by the substitution of 15N and 13C for 14N and 12C, respectively. A possible relationship with other known impurity centres in diamond will be discussed.
2. Experimental Natural and synthetic type Ia diamonds have been irradiated with various doses of 2 MeV electrons ranging from 1018 to 1019 electrons cm−2; apart from the measurements of Section 3.6 where the effect of changing the irradiation dose was studied, all the results reported in this work were obtained with diamonds irradiated with a dose of 1019 electrons cm−2. Four type IaA diamonds and four type IaB as well as two 13C synthetic diamonds were investigated. The type IaB diamonds used in this work were not absolutely pure type IaB, they contained 5–10% nitrogen in A-centres. The as grown 13C synthetic diamonds were initially type Ib; to convert them to type Ia, with A and B centres, they were annealed before irradiation for 4 h at 2650°C, using the set-up described by Evans and Qi [4]. IR absorption spectra were measured at various temperatures between 80 and 300 K.
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peaks. The H1d (2668.2 cm−1) and H1g (4442.4 cm−1) centres can be produced only in type IaA diamonds, the H1e (2918.2 cm−1) centre can be produced only in type IaB diamonds while the H1f (4396 cm−1) centre can be produced in both types IaA and IaB. As shown in Section 3.2 the H1d and the H1g are formed in a different temperature range than the H1e and H1f centres. A decomposition, using Lorentzian curves, of the spectrum on the high energy side of the H1e ZPL shows that it contains three broad components centred at 3081 (very weak), 3237.4 and 3437 cm−1. The intensity and the sharpness of the band at 3237 cm−1 increase with the reduction of the temperature, it is very likely that the band at the 3437 cm−1 is the side band of the H1e centre. Fig. 1 also shows a minor peak (shoulder) at 2850.8 cm−1. No detailed study of this minor peak was conducted. It is to be noted that the phonon side bands of H1d, H1e and H1f centres are very similar suggesting some common structure to these centres. The sharp peak at 3107 cm−1 in Fig. 1 is due to hydrogen[5]. In addition to the above centres a broad band with a maximum between 3094–3110±3 cm−1 was observed in the heavily irradiated and annealed type IaA and type IaB diamonds. The position of the maximum of this band is specimen dependent. We will refer to it henceforth as the broad-band. The broad-band appears immediately after irradiation and anneals out at 1000°C. Fig. 2 shows the spectrum of this band.
Fig. 1. IR spectra showing the four new centres and their side bands. The spectra are shown after subtracting the intrinsic diamond absorption. The specimen temperature was 80 K. For clarity the spectra of the H1d and the H1g centres are shifted vertically by 10 cm−1.
To obtain the spectra due to the new centres the intrinsic diamond phonon absorption was subtracted using the spectrum of a nitrogen-free type II diamond. Absorption spectra in the ultraviolet ( UV )-visible range were also measured at a temperature of 80 K. To study the effect of isochronal annealing two specimens of each type were annealed between 400 and 1800°C, at intervals of 100–150oC, for 4 h at each temperature. The annealing below 1500°C was carried out either in vacuum or in argon at atmospheric pressure. Annealing above 1500°C was carried out under a stabilising pressure of 8 GPa.
3.2. The effect of isochronal annealing Fig. 3 shows the isochronal annealing behaviour of the broad-band, the H1d and H1e centres. The isochronal annealing behaviour of H1g and H1f are respectively similar to the H1d and H1e. Fig. 3 also shows that the isochronal annealing behaviour of the broad-band compliments that of the H1d and H1g centres. In parallel to the above measurements we monitored the UV-visible spectra of the examined specimens.
3. Results 3.1. The results of the IR spectroscopy Fig. 1 shows the spectra of the four new centres and Table 1 gives a summary of the positions of the observed Table 1 The spectral positions of the new centres, their side bands and isotopic shift Centre Specimen type H1d H1e H1f H1g
12C 13C IaBd 12C 13C IaA and IaBd 12C 13C IaA 12C 13C IaA
Position ZPL (300 K ) Position ZPL (80 K ) FWHM (80 K ) Isotopic shift (cm−1)±0.5 (cm−1)±0.5 (cm−1 ) (cm−1)
Position side band (cm−1)± 5
Phonon energy. (meV )± 0.6
2678.7 2670.2 2917.0 2896.7 4400.0 4392.4 4437.0 4446.7
3184 3160 3437 b 4915
63.9 63.9 64.9
2668.2 2659.0 2912.8 a 4396.0 a 4442.4 4452.0
60 −9.2 77 −20.3 13 −7.6 25 +9.6
a Not measured. b Too weak to detect. c The side band, if it exists, is obscured by the presence of the very strong H1b centre at 4941 cm−1. d These are not pure IaB diamonds, they have 5–10% of the nitrogen in A-centres.
c c
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Fig. 2. The spectrum of the broad-band taken at room temperature. The spectrum is shown after subtracting the intrinsic diamond absorption.
(220±20 atm ppm) and in a third case of 14±2% (110±16 atm ppm). The change, if any, after subsequent annealing at 1450°C was within experimental error. To calculate the change in the concentration of the A-centres a conversion factor of 16.5 atm ppm per cm−1 of IR absorption at 1282 cm−1 was used [6 ]. The changes in the concentration of B-centres in two type IaB diamonds were also examined, there were no observable changes. One should take into consideration that the conversion factor of the nitrogen content in B-centres is 80 atm ppm cm [7], so it will be more difficult to detect small changes. However, if the changes were of the same order of magnitude as those observed in the case of the type IaA diamonds, they could have been detected. This reduction in the concentration of A-centres, observed for the first time due to the high irradiation dose, gives direct evidence of the involvement of A-centres with the formation at 900°C of very intense H1b and H3 centres in these diamonds and maybe also the H1d and H1g centres. 3.3. The effect of changing the C and N isotope As seen in Table 1, changing the carbon isotopes introduced a very small shift (ca 1 meV ) in the positions of these centres, giving the first indication that they are due to electronic transitions. There was no shift in the position of the zero-phonon line ( ZPL) of the H1d and the H1f centres when 14N was replaced by 15N. It was not possible to form an adequate concentration of B-centres in the 15N doped synthetic diamonds to measure the effect of changing the nitrogen isotope on the H1e centre.
Fig. 3. The results of the isochronal annealing of the H1d and H1e centres and the broad-band. The isochronal annealing behaviour of the H1g and the H1f are respectively similar to the H1d and H1e.
Immediately after irradiation we observed the usual vacancy centres GR1 (neutral vacancy) and ND1 (negatively charged vacancy), as well as some of the nitrogenvacancy centres such as the 1.945 eV (single nitrogen+vacancy), H3 (A-centre+vacancy) and H4 (B-centre+vacancy), which are usually formed after annealing when a lower irradiation dose is used. After annealing we observed the formation of the 2.068 eV (unknown) and the TH5 (believed to be due to di-vacancies [1,8]) centres. Only the GR1, ND1, 2.068 eV and the TH5 centres annealed out in a complimentary fashion with the formation of the H1d and the H1g centres, while the other nitrogen vacancies centres were formed, or their intensity increased, in the same temperature range as the H1d and H1g centres. A comparison of the concentration the A centres of three type IaA specimens before and after annealing at 900°C shows a reduction in two cases of 25±2%
3.4. The temperature dependence Reducing the temperature from 300 to 80 K reduced the full-width at half maximum (FWHM ) of the ZPL of the H1d, H1e and H1g centres by 2 meV, and of the H1f centre by 3 meV. These values are substantially larger than the sharpening of local vibrational modes which are only fractions of a meV (for example, the local mode at 1450 cm−1 shows a reduction in the FWHM of 0.17 meV ). The exceptionally broad ZPLs of the H1d and H1e centres [~60 cm−1 (7 meV )] can be due to their proximity to the edge of the diamond intrinsic two phonons absorption at 2664 cm−1. The temperature dependence of the intensity of the new lines was found to be satisfactorily described by the formula: I=exp(−S); S=S(0) (1+2n); where S(0) is the Huang Rhys factor and n is the Bose–Einstein population term, given by n= 1/[exp(Bv/kT )−1].
I. Kiflawi et al. / Diamond and Related Materials 8 (1999) 1576–1580
3.5. The effect of irradiation dose None of the new centres were observed after an irradiation dose of up to 1018 electrons cm−2. They were observed at low intensity for an irradiation dose of 4×1018 electrons cm−2. Two type IaA diamonds with the same concentration of nitrogen (~270 atm ppm) were irradiated with doses of 1019 and 4×1018 electrons cm−2, the integrated intensities of the ZPL of the H1d centre were respectively 400 and 40 cm−2. 3.6. Correlation with the nitrogen concentration A correlation has been found between the integrated intensity of the ZPL of the H1d centre and the absorption due to A centres (Fig. 4a). This reached saturation at a nitrogen concentration of ca 700 atm ppm. A correlation was also found between the intensity of the ZPL of the H1e centre and the nitrogen concentration in
Fig. 4. (a) The correlation between the nitrogen concentration in A centres and the integrated intensity of the ZPL of the H1d centre, showing saturation at high N concentration. Results from various locations of three specimens represented by different symbols. (b) The correlation between the nitrogen concentration in B centres and the integrated intensity of the ZPL of the H1e centre. Results from various locations of the same specimen.
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B-centres ( Fig. 4b). No saturation was observed in this case. This can be due to the fact that for the same concentration of nitrogen the concentration of B-centres (four nitrogen atoms plus a vacancy) is only half the concentration of A-centres (two nitrogen atoms). 4. Discussion Four new centres (H1d, e, f, g) have been produced in electron irradiated and annealed type Ia diamonds. At this stage we can only say, based on the carbon and nitrogen isotopic data, that all these new lines are due to electronic transitions. If the lines are due to local modes of vibration, then since the centres are made of C, N and irradiation damage (vacancies and interstitials) that is, the chemistry is carbon and nitrogen, we would expect any isotopic shifts to be related to the mass of these elements. No shift was observed for nitrogen isotope change so only C atoms would be vibrating, in which case we should observe reductions in the vibrational frequencies by (12/13)1/2 for the 13C isotope contribution. The observed shifts are far less than this value and in one case, the H1g centre, the shift is in the ‘‘wrong’’ direction. We conclude, therefore, that the new centres are caused by electronic transitions. The sharpening of the ZPL(s) is consistent with this conclusion. The H1d and H1g centres are similar to the H1b centre in that they are only produced in type IaA diamonds and exhibit similar annealing behaviour [2]. However, the H1b centre is observed after irradiation with a lower dose (≤9×1017 electrons cm−2) [2]. Similarly, for type IaB diamonds the H1e centre requires a higher irradiation dose than the H1c centre, although in this case the H1e centre does not show the same annealing behaviour, requiring higher temperature for its formation. The need for a higher irradiation dose in the case of the new centres points to the possibility that more than one vacancy is needed, in conjunction with the presence of A or B centres, to form them. The observed formation and the complementary annealing behaviour of the di-vacancy centre, TH5, might point to the possibility that di-vacancies are involved in the formation of the H1d and H1g centres. It is to be noted that the TH5 is formed when the GR1 (single vacancy) centre anneals out at 700–800°C, and it anneals out at 1000°C [5], that is, at the same temperature that the intensity of the H1d and H1g centres reaches a maximum. The role of the broad-band is not clear. It is clear that further experiments such as studying the effect of uniaxial stress splitting of the ZPLs and ESR measurements might provide information useful in the characterisation of these centres. Acknowledgement The authors would like to thank Dr Jenifer Lomer for useful discussions.
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References [1] C.D. Clark, R.W. Ditchburn, H.B Dyer, Proc. R. Soc. A 237 (1956) 75. [2] A.T. Collins, G. Davies, G.S. Woods, J. Phys. C 19 (1986) 3933. [3] I. Kiflawi, A. Mainwood, H. Kanda, D. Fisher, Phys. Rev. B 54 (1996) 16719.
[4] T. Evans, Z. Qi, Proc. R. Soc. A 381 (1978) 159. [5] R.M Chrenko, R.S McDonald, K.A. Darrow, Nature 213 (1967) 474. [6 ] S.R. Boyd, I. Kiflawi, G.S. Woods, Phil. Mag. B 69 (1994) 1149. [7] S.R. Boyd, I. Kiflawi, G.S. Woods, Phil. Mag. B 72 (1995) 351. [8] M.A. Lea-Wilson, J.N. Lomer, Phil. Mag. B 72 (1995) 81.
New infrared absorption centres in electron irradiated and annealed type Ia diamonds I. Kiflawi a, *, G. Davies b, D. Fisher c, H. Kanda d a J.J. Thomson Physical Laboratory, The University of Reading, Reading, RG6 6AF UK b Department of Physics, King’s College London, Strand, London WC2R 2LS, UK c DeBeers DTC Research Centre, Belmont Road, Maidenhead SL6 6JW, UK d NIRIM, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Accepted 30 November 1998
Abstract New absorption centres having zero phonon lines at 2678 (332 meV ), 2913.6 (361 meV ), 4392.9 (544 meV ) and 4442.2 cm−1 (550 meV ) have been produced in heavily electron-irradiated and annealed type Ia diamonds. Their temperature dependence and annealing behaviour have been investigated along with the effect of the radiation dose and isotopic shifts caused by the substitution of 15N and 13C for 14N and 12C, respectively. The results are consistent with the centres being caused by electronic transitions. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Infrared; Type Ia diamond; Annealing
1. Introduction A group of infrared (IR) optical absorption centres referred to in the literature as H1a, b and c, are the product of irradiating and annealing of type I diamonds [1,2]. Nitrogen-containing type I diamonds can be divided into different sub-categories according to the state of aggregation of the nitrogen: type Ib contain dispersed single substitutional nitrogen atoms (C-centres), type IaA contain nitrogen pairs in neighbouring substitutional sites (A-centres), while type IaB contain centres consisting of four substitutional nitrogen atoms surrounding a vacancy (B-centres). The H1a absorption line involves the creation of a local vibrational mode at 1450 cm−1 due to interstitial nitrogen [3], H1b is an electronic transition at 4941 cm−1 (612 meV ) seen in type IaA diamonds and is thought to be related to A-centres [2]. H1c is also an electronic transition at 5171 cm−1 (641 meV ) seen in type IaB and is thought to be related to B-centres [2]. Four new IR absorption centres, which we propose naming as H1d, H1e, H1f and H1g (in line with the H1a, b and c centres mentioned above), have been produced in heavily irradiated and annealed type Ia * Corresponding author. Fax: +44-1189752030. E-mail address: [email protected] (I. Kiflawi)
diamonds. Their annealing behaviour has been investigated along with the effect of irradiation dose and the isotopic shifts caused by the substitution of 15N and 13C for 14N and 12C, respectively. A possible relationship with other known impurity centres in diamond will be discussed.
2. Experimental Natural and synthetic type Ia diamonds have been irradiated with various doses of 2 MeV electrons ranging from 1018 to 1019 electrons cm−2; apart from the measurements of Section 3.6 where the effect of changing the irradiation dose was studied, all the results reported in this work were obtained with diamonds irradiated with a dose of 1019 electrons cm−2. Four type IaA diamonds and four type IaB as well as two 13C synthetic diamonds were investigated. The type IaB diamonds used in this work were not absolutely pure type IaB, they contained 5–10% nitrogen in A-centres. The as grown 13C synthetic diamonds were initially type Ib; to convert them to type Ia, with A and B centres, they were annealed before irradiation for 4 h at 2650°C, using the set-up described by Evans and Qi [4]. IR absorption spectra were measured at various temperatures between 80 and 300 K.
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peaks. The H1d (2668.2 cm−1) and H1g (4442.4 cm−1) centres can be produced only in type IaA diamonds, the H1e (2918.2 cm−1) centre can be produced only in type IaB diamonds while the H1f (4396 cm−1) centre can be produced in both types IaA and IaB. As shown in Section 3.2 the H1d and the H1g are formed in a different temperature range than the H1e and H1f centres. A decomposition, using Lorentzian curves, of the spectrum on the high energy side of the H1e ZPL shows that it contains three broad components centred at 3081 (very weak), 3237.4 and 3437 cm−1. The intensity and the sharpness of the band at 3237 cm−1 increase with the reduction of the temperature, it is very likely that the band at the 3437 cm−1 is the side band of the H1e centre. Fig. 1 also shows a minor peak (shoulder) at 2850.8 cm−1. No detailed study of this minor peak was conducted. It is to be noted that the phonon side bands of H1d, H1e and H1f centres are very similar suggesting some common structure to these centres. The sharp peak at 3107 cm−1 in Fig. 1 is due to hydrogen[5]. In addition to the above centres a broad band with a maximum between 3094–3110±3 cm−1 was observed in the heavily irradiated and annealed type IaA and type IaB diamonds. The position of the maximum of this band is specimen dependent. We will refer to it henceforth as the broad-band. The broad-band appears immediately after irradiation and anneals out at 1000°C. Fig. 2 shows the spectrum of this band.
Fig. 1. IR spectra showing the four new centres and their side bands. The spectra are shown after subtracting the intrinsic diamond absorption. The specimen temperature was 80 K. For clarity the spectra of the H1d and the H1g centres are shifted vertically by 10 cm−1.
To obtain the spectra due to the new centres the intrinsic diamond phonon absorption was subtracted using the spectrum of a nitrogen-free type II diamond. Absorption spectra in the ultraviolet ( UV )-visible range were also measured at a temperature of 80 K. To study the effect of isochronal annealing two specimens of each type were annealed between 400 and 1800°C, at intervals of 100–150oC, for 4 h at each temperature. The annealing below 1500°C was carried out either in vacuum or in argon at atmospheric pressure. Annealing above 1500°C was carried out under a stabilising pressure of 8 GPa.
3.2. The effect of isochronal annealing Fig. 3 shows the isochronal annealing behaviour of the broad-band, the H1d and H1e centres. The isochronal annealing behaviour of H1g and H1f are respectively similar to the H1d and H1e. Fig. 3 also shows that the isochronal annealing behaviour of the broad-band compliments that of the H1d and H1g centres. In parallel to the above measurements we monitored the UV-visible spectra of the examined specimens.
3. Results 3.1. The results of the IR spectroscopy Fig. 1 shows the spectra of the four new centres and Table 1 gives a summary of the positions of the observed Table 1 The spectral positions of the new centres, their side bands and isotopic shift Centre Specimen type H1d H1e H1f H1g
12C 13C IaBd 12C 13C IaA and IaBd 12C 13C IaA 12C 13C IaA
Position ZPL (300 K ) Position ZPL (80 K ) FWHM (80 K ) Isotopic shift (cm−1)±0.5 (cm−1)±0.5 (cm−1 ) (cm−1)
Position side band (cm−1)± 5
Phonon energy. (meV )± 0.6
2678.7 2670.2 2917.0 2896.7 4400.0 4392.4 4437.0 4446.7
3184 3160 3437 b 4915
63.9 63.9 64.9
2668.2 2659.0 2912.8 a 4396.0 a 4442.4 4452.0
60 −9.2 77 −20.3 13 −7.6 25 +9.6
a Not measured. b Too weak to detect. c The side band, if it exists, is obscured by the presence of the very strong H1b centre at 4941 cm−1. d These are not pure IaB diamonds, they have 5–10% of the nitrogen in A-centres.
c c
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Fig. 2. The spectrum of the broad-band taken at room temperature. The spectrum is shown after subtracting the intrinsic diamond absorption.
(220±20 atm ppm) and in a third case of 14±2% (110±16 atm ppm). The change, if any, after subsequent annealing at 1450°C was within experimental error. To calculate the change in the concentration of the A-centres a conversion factor of 16.5 atm ppm per cm−1 of IR absorption at 1282 cm−1 was used [6 ]. The changes in the concentration of B-centres in two type IaB diamonds were also examined, there were no observable changes. One should take into consideration that the conversion factor of the nitrogen content in B-centres is 80 atm ppm cm [7], so it will be more difficult to detect small changes. However, if the changes were of the same order of magnitude as those observed in the case of the type IaA diamonds, they could have been detected. This reduction in the concentration of A-centres, observed for the first time due to the high irradiation dose, gives direct evidence of the involvement of A-centres with the formation at 900°C of very intense H1b and H3 centres in these diamonds and maybe also the H1d and H1g centres. 3.3. The effect of changing the C and N isotope As seen in Table 1, changing the carbon isotopes introduced a very small shift (ca 1 meV ) in the positions of these centres, giving the first indication that they are due to electronic transitions. There was no shift in the position of the zero-phonon line ( ZPL) of the H1d and the H1f centres when 14N was replaced by 15N. It was not possible to form an adequate concentration of B-centres in the 15N doped synthetic diamonds to measure the effect of changing the nitrogen isotope on the H1e centre.
Fig. 3. The results of the isochronal annealing of the H1d and H1e centres and the broad-band. The isochronal annealing behaviour of the H1g and the H1f are respectively similar to the H1d and H1e.
Immediately after irradiation we observed the usual vacancy centres GR1 (neutral vacancy) and ND1 (negatively charged vacancy), as well as some of the nitrogenvacancy centres such as the 1.945 eV (single nitrogen+vacancy), H3 (A-centre+vacancy) and H4 (B-centre+vacancy), which are usually formed after annealing when a lower irradiation dose is used. After annealing we observed the formation of the 2.068 eV (unknown) and the TH5 (believed to be due to di-vacancies [1,8]) centres. Only the GR1, ND1, 2.068 eV and the TH5 centres annealed out in a complimentary fashion with the formation of the H1d and the H1g centres, while the other nitrogen vacancies centres were formed, or their intensity increased, in the same temperature range as the H1d and H1g centres. A comparison of the concentration the A centres of three type IaA specimens before and after annealing at 900°C shows a reduction in two cases of 25±2%
3.4. The temperature dependence Reducing the temperature from 300 to 80 K reduced the full-width at half maximum (FWHM ) of the ZPL of the H1d, H1e and H1g centres by 2 meV, and of the H1f centre by 3 meV. These values are substantially larger than the sharpening of local vibrational modes which are only fractions of a meV (for example, the local mode at 1450 cm−1 shows a reduction in the FWHM of 0.17 meV ). The exceptionally broad ZPLs of the H1d and H1e centres [~60 cm−1 (7 meV )] can be due to their proximity to the edge of the diamond intrinsic two phonons absorption at 2664 cm−1. The temperature dependence of the intensity of the new lines was found to be satisfactorily described by the formula: I=exp(−S); S=S(0) (1+2n); where S(0) is the Huang Rhys factor and n is the Bose–Einstein population term, given by n= 1/[exp(Bv/kT )−1].
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3.5. The effect of irradiation dose None of the new centres were observed after an irradiation dose of up to 1018 electrons cm−2. They were observed at low intensity for an irradiation dose of 4×1018 electrons cm−2. Two type IaA diamonds with the same concentration of nitrogen (~270 atm ppm) were irradiated with doses of 1019 and 4×1018 electrons cm−2, the integrated intensities of the ZPL of the H1d centre were respectively 400 and 40 cm−2. 3.6. Correlation with the nitrogen concentration A correlation has been found between the integrated intensity of the ZPL of the H1d centre and the absorption due to A centres (Fig. 4a). This reached saturation at a nitrogen concentration of ca 700 atm ppm. A correlation was also found between the intensity of the ZPL of the H1e centre and the nitrogen concentration in
Fig. 4. (a) The correlation between the nitrogen concentration in A centres and the integrated intensity of the ZPL of the H1d centre, showing saturation at high N concentration. Results from various locations of three specimens represented by different symbols. (b) The correlation between the nitrogen concentration in B centres and the integrated intensity of the ZPL of the H1e centre. Results from various locations of the same specimen.
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B-centres ( Fig. 4b). No saturation was observed in this case. This can be due to the fact that for the same concentration of nitrogen the concentration of B-centres (four nitrogen atoms plus a vacancy) is only half the concentration of A-centres (two nitrogen atoms). 4. Discussion Four new centres (H1d, e, f, g) have been produced in electron irradiated and annealed type Ia diamonds. At this stage we can only say, based on the carbon and nitrogen isotopic data, that all these new lines are due to electronic transitions. If the lines are due to local modes of vibration, then since the centres are made of C, N and irradiation damage (vacancies and interstitials) that is, the chemistry is carbon and nitrogen, we would expect any isotopic shifts to be related to the mass of these elements. No shift was observed for nitrogen isotope change so only C atoms would be vibrating, in which case we should observe reductions in the vibrational frequencies by (12/13)1/2 for the 13C isotope contribution. The observed shifts are far less than this value and in one case, the H1g centre, the shift is in the ‘‘wrong’’ direction. We conclude, therefore, that the new centres are caused by electronic transitions. The sharpening of the ZPL(s) is consistent with this conclusion. The H1d and H1g centres are similar to the H1b centre in that they are only produced in type IaA diamonds and exhibit similar annealing behaviour [2]. However, the H1b centre is observed after irradiation with a lower dose (≤9×1017 electrons cm−2) [2]. Similarly, for type IaB diamonds the H1e centre requires a higher irradiation dose than the H1c centre, although in this case the H1e centre does not show the same annealing behaviour, requiring higher temperature for its formation. The need for a higher irradiation dose in the case of the new centres points to the possibility that more than one vacancy is needed, in conjunction with the presence of A or B centres, to form them. The observed formation and the complementary annealing behaviour of the di-vacancy centre, TH5, might point to the possibility that di-vacancies are involved in the formation of the H1d and H1g centres. It is to be noted that the TH5 is formed when the GR1 (single vacancy) centre anneals out at 700–800°C, and it anneals out at 1000°C [5], that is, at the same temperature that the intensity of the H1d and H1g centres reaches a maximum. The role of the broad-band is not clear. It is clear that further experiments such as studying the effect of uniaxial stress splitting of the ZPLs and ESR measurements might provide information useful in the characterisation of these centres. Acknowledgement The authors would like to thank Dr Jenifer Lomer for useful discussions.
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