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Defect formation in electron-irradiated synthetic diamond annealed in the temperature range 820–1120 K
March 1998
Materials Letters 34 Ž1998. 143–147
Defect formation in electron-irradiated synthetic diamond annealed in the temperature range 820–1120 K E.M. Shishonok a
a,)
, V.B. Shipilo a , G.P. Popelnuk a , I.I. Azarko b, A.A. Melnikov b, A.R. Filipp b
Institute of Solid State and Semiconductor Physics, Academy of Sciences of Belarus, P. BroÕka Str. 17, 220072 Minsk, Belarus b Belarusian State UniÕersity, F. Skaryna AÕe. 4, 220050 Minsk, Belarus Received 20 February 1997; revised 7 May 1997; accepted 20 May 1997
Abstract The phenomenon of the extreme increase of the concentration of dispersed paramagnetic nitrogen in electron-irradiated synthetic diamond specimens annealed in the temperature range 820–1120 K was revealed. It is established that the order of the reaction responsible for the decrease of paramagnetic defects on annealing at temperatures Tann ) 1020 K or times no more than one hour is equal to 2, with the activation energy of the process being Ea s 1.07 eV. q 1998 Elsevier Science B.V. PACS: 71.55.-i; 61.72.Ji; 78.60.Hk; 76.30.-v Keywords: Synthetic diamond; Nitrogen impurity; Paramagnetic
1. Introduction Synthetic diamonds ŽSD. are still not adequately explored, unlike the naturally occurring ones, and this is due primarily to its nonequilibrium defect structure. It is known that a nitrogen impurity plays a leading part in determining the physical properties of diamond. The possibilities of transmutation of various nitrogen-containing defects in a diamond lattice, including those at high pressures and temperatures, have been investigated in w1,2x. The present work reports mainly experimental findings concerned with the possibility of modifying a defect SD structure by electron irradiation and heat treatment at lower tem-
)
Corresponding author. Fax: q375-172-324644.
peratures ŽT - 1270 K.. A choice of the methods of modifying the diamond structure in the indicated temperature range is essentially based upon our investigations of cubic boron nitride w3,4x.
2. Experimental We have investigated SD powders with a grain size no more than 160 m m and some single SD crystals synthesized at the Institute of Solid-State and Semiconductor Physics ŽMinsk.. The powders were irradiated by 4 MeV electrons in vacuum and in a carbon-containing atmosphere Žfor example CO 2 . at doses ranging from 1 = 10 17 cmy2 to 3 = 10y8 cmy2 . The following heat treatment was accomplished in vacuum at Tann s 670–1270 K. EPR spec-
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 1 4 9 - 3
144
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
tra were recorded by use of a 3 cm range spectrometer, ‘Varian E 112’ model, and cathodoluminescence ŽCL. was excited by 15 keV electrons at T s 80 K.
3. Results and discussion At the microwave radiation power of 1 mW the EPR spectra of all the initial SD powder specimens displayed the signal of paramagnetic centers ŽPC. P1 with the central component, identical to the Lorentz line Ž D H f 3 G. w4x, generated by substitutional atoms of the dispersed nitrogen with concentration NP1 s 1.2–1.7 = 10 18 cmy2 w5x. With an increase of the radiation power up to 100 mW, the dominating signal in the spectra was that of paramagnetic centers generated presumably by the exchange-bound pairs of nitrogen atoms with g-factor equal to 2.002 " 0.001 Ž D H s 16–19 G. N s 1.3 = 10 19 spinrg.. The signal’s form was similar to the Lorentz one. After the irradiation of SD in the above dose range, the concentration of paramagnetic defects was NP1 s 1.4–1.8 = 10 18 cmy2 . The A-band, with maximum intensity at 2.35–2.38 eV and width 0.50–0.53 eV, was detected in CL spectra of some single SD crystals and in powder specimens w6x. In the 200–350 nm region of the CL spectrum, the SD specimens investigated did not display any luminescence. After annealing at different temperatures and irradiation we detected the A-band and the ZPL line at 575 nm w7x, depending on the irradiation dose; D s 1 = 10 17 cmy2 at Tann ) 870 K and D s 1 = 10y8 cmy2 at Tann ) 770 K. Immediately after irradiation, D G 3 = 10 18 cmy2 . The ZPL Žzero phonon line. of 884 nm of a nickelcontaining center and the ZPL line of 781 nm initiated by an unknown defect were also detected in the spectra of single SD crystals. It was found that annealing of the single crystals and powder specimens of synthetic diamond in the low-temperature range, Tann - 1270 K, leads to regular changes in the defect structure of the material and, consequently, in the EPR and CL spectra. Thus, in the temperature range of Tann s 1000–1100 K the relative intensity of all luminescence centers was observed to increase, especially of the ZPL center at 575 nm, and to narrow at the same time which was
Fig. 1. Relative intensity of the ZPL at 575 nm in the CL spectra of the SD specimens: Ž1–3. powders irradiated in vacuum with the doses 3=10 17 cmy2 , 7=10 17 cmy2 , 3=10 18 cmy2 ; Ž4. single SD crystals irradiated in vacuum with the dose Ds 5=10 17 cmy2 ; Ž5 and 6. SD powders irradiated in the carbon-containing atmosphere with the doses 3=10 17 cmy2 and 3=10 18 cmy2 versus the annealing temperature.
distinctly recorded in the spectra of separate single crystals of different size ŽFig. 1.. The changes discussed were accompanied by an abrupt decrease of the concentration of all paramagnetic defects, P1, which has been increasing till this moment and attained its maximum at Tann s 973 K Ž NP1 s 3.9–4.3 = 10y8 spinrg.. Kinetic curves of the investigation of the effect of annealing Žwith the temperature steps DT s 100 K. on the width of the signal generated by PC P1 in the EPR spectra of SD specimens irradiated in vacuum are shown in Fig. 2a. The concentration dependencies are similar in nature. It is clearly seen that annealing promotes defect formation in SD, at first it causes an increase of the signal width D H ŽP1 concentration., which attains its maximum and saturation at Tann s 1073 K, and then it decreases to D H s 2.75 G Ž NP1 s 1.3–1.5 = 10 18 spinrg. for more than one hour at Tann s 1073 K when the concentration decreases for an annealing time of 360 min. Because the signal width D H Žconcentration of PC P1. of the indicated centers in the EPR spectra of the SD specimens annealed at Tann s 1073 K Ž tann s 5 min. exceeds that in the spectra of those annealed at Tann s 973 K Ž tann s 6 h., we conducted some additional kinetic studies to determine critical temperatures more precisely and to elucidate the order of the reaction of the observed
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
145
The kinetic curves did not allow unambiguous determination of the order of the reaction leading to an increase of the PC P1 concentration at Tann s 923 K. It was established, however, that the experimental data on decreasing the concentration of the indicated defects in diamond at Tann ) 1023 K can be described in terms of second-order kinetics for annealing times no more than one hour; at larger annealing times the process slows down ŽFig. 3.. The activation energy of the process was calculated by use of the known Eqs. Ž1. and Ž2. below, to be
k ts
1
1 y
Ct
C0
,
Ea
ž /
k s A exp y
Fig. 2. Width Ž D H . of the signal generated by PC P1 in the EPR spectra of SD specimens irradiated in vacuum with the dose 3=10 17 cmy2 and Ža. annealed at Ž1. 673 K, Ž2. 773 K, Ž3. 873 K, Ž4. 973 K, Ž5. 1073 K, Ž6. 1173 K versus annealing time; Žb. annealed for 1–5 min, 2–10 min, 3–30 min, 4–60 min versus the annealing temperature.
kinetic processes as well as the influence of the annealing time on their activation. Annealing was carried out at temperatures ranging from 923 to 1123 K with temperature steps DT s 25 K in the time range of 1–360 min. Even a 5 min anneal at Tann s 1023 K causes a maximum increase in the concentration of PC P1 in the SD specimens irradiated in vacuum. Moreover, the larger annealing time, the lower the temperatures at which the concentration starts increasing and attains its minimum due to the heat treatment. With the maximum being unchanged at Tann s 1023 K, its low- and high-temperature branches are displaced towards the region of low temperatures ŽFig. 2b..
kT
Ž 1. ,
Ž 2.
where k is the reaction rate constant, C0 is the initial concentration of defects, Ct is the concentration in the specimens annealed for time t at the temperature T and Ea is the activation energy. It is noteworthy that an extreme increase of the paramagnetic nitrogen concentration in the SD specimens was observed in the temperature region where the activation of vacancy motion occurs, but in the CL spectra of the investigated diamonds the luminescence centers initiated by the neutral vacancy ŽGR-1. are not observed. However, it has been suggested that irradiation of SD results in formation of charged vacancies in the latter, unlike natural diamond, and
Fig. 3. 1r C versus time dependence for different annealing temperatures and lnŽ k . versus 10 4 r T corresponding to the activation energy of 1.07 eV Žin frame.: Ž1. 973 K, Ž2. 1023 K, Ž3. 1073 K.
146
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
the activation of the motion of nitrogen atoms is observed in the lattice at Tann ) 1073 K w8,9x. To determine a possible relation of the discovered phenomenon with the presence of vacancies in SD, we irradiated the SD powders in the carbon-containing atmosphere. It was found that the center at 575 nm recorded in the spectra of the specimens irradiated in vacuum, particularly after annealing Žfor D - 3 = 10 18 cmy2 ., was detected immediately after irradiation of the SD specimens in the atmosphere by smaller doses, while for the heat-treated specimens at the same dose at lower annealing temperatures. The relative intensity of the zero-phonon lines ŽZPL. exhibited a several-fold decrease, as compared to the specimens irradiated in vacuum; the halfwidth decreased from 5.5 to 4.3 meV Žat the dose ; 3 = 10 18 cmy2 . thus pointing to a decrease of inhomogeneous distortions in the lattice as compared to the specimens irradiated in vacuum. As it is seen in Fig. 1 the temperature dependences of the relative ZPL intensity at 575 nm in the spectra of SD irradiated in the carbon atmosphere do not display any maxima for doses D - 3 = 10 18 cmy2 . The plots of concentration of paramagnetic center, P1, versus annealing temperature also do not display a maximum at Tann ; 973 K. As for the absolute concentration of PC P1, it is lower than that in the SD specimens annealed after irradiation in vacuum Ž NP1 s 2.4 = 10 18 - 3.9–4.3 = 10 18 spinrg. ŽFig. 4..
Since the SD irradiation in the atmosphere does not change the mechanism of interaction of an accelerated electron with its lattice, it is hardly probable that an additional amount of the 575 nm defects appear in the material in the case of luminescence build-up at less severe conditions of radiative and thermal treatments. Owing to the improvement of structure perfection of synthetic diamond due to a decrease in the concentration of recombination levels in its forbidden band, the above mentioned decrease in the ZPL intensity of the 575 nm centers with their simultaneous narrowing can be caused by a decrease of the vacancy concentration and, as a consequence, of the 575 nm centers in the SD specimens. This is well explained within the framework of the model of forced diffusion w8x when positive carbon ions from the radiation-ionized CO 2 atmosphere penetrates into the near-surface layers of a negatively charged crystal, diffuse in it, and annihilate with vacancies.
4. Conclusion An increase in the concentration of the PC P1 at Tann s 1023 K can be caused by activation of the motion of charged vacancies since the luminescence centers initiated by the neutral vacancy ŽGR-1. have not been detected, as a result of which the impurity nitrogen are recharged. The positive carbon ions, diffused into the SD lattice from the carbon atmosphere, hinder the motion of vacancies and nitrogen recharging thus suppressing an increase of the PC P1 concentration in synthetic diamond. The process of formation of defects at 575 nm being most pronounced at Tann ) 1000 K results in a shift towards the region of activation of motion of the nitrogen impurity.
References
Fig. 4. PC P1 concentration in the SD specimens irradiated in the carbon-containing atmosphere with the doses: Ž1. 7=10 17 cmy2 , Ž2. 1=10 18 cmy2 , Ž3. 3=10 18 cmy2 .
w1x R.M. Brozel, T. Evans, R.F. Stephenson, Proc. R. Soc. London A 361 Ž704. Ž1978. 109. w2x Ju.A. Kluev, A.M. Naletov, V.P. Nepsha, L.D. Belichenko, V.A. Laptev, M.I. Samoilovich, J. Phys. Chem. 56 Ž3. Ž1982. 524, in Russian. w3x E.M. Shishonok, V.B. Shipilo, I.I. Azarko, N.G. Anichenko, Proc. of the 5th Conf. Prospects for Diamond Application in Technology and Electronics, Moscow, May, 1995 Žin Russian..
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147 w4x E.M. Shishonok, V.B. Shipilo, I.I. Azarko, N.G. Anichenko, Abstr. of the Topical Workshop on 3–5 Nitrides, Japan, September, 1995. w5x W.V. Smith, P.P. Sorokin, L.L. Gelles, G.J. Lasher, Phys. Rev. 115 Ž1959. 1546. w6x S. Lowsom, G. Davies, A. Collins, A. Mainwood, J. Phys. Condens. Matter. 4 Ž1992. 125.
147
w7x L. Nishida, Mater. Sci. Forum ŽSwitzerland. 38–41 Ž1989. 561. w8x G. Popovici, T. Sung, M.A. Prelas, R.J. Wilson, Proc. III Int. Conf. on Application of Diamond Films and Related Materials, USA, Ž1995. 169. w9x A.M. Zaitsev, dissertation, Minsk, 1992 Žin Russian..
Materials Letters 34 Ž1998. 143–147
Defect formation in electron-irradiated synthetic diamond annealed in the temperature range 820–1120 K E.M. Shishonok a
a,)
, V.B. Shipilo a , G.P. Popelnuk a , I.I. Azarko b, A.A. Melnikov b, A.R. Filipp b
Institute of Solid State and Semiconductor Physics, Academy of Sciences of Belarus, P. BroÕka Str. 17, 220072 Minsk, Belarus b Belarusian State UniÕersity, F. Skaryna AÕe. 4, 220050 Minsk, Belarus Received 20 February 1997; revised 7 May 1997; accepted 20 May 1997
Abstract The phenomenon of the extreme increase of the concentration of dispersed paramagnetic nitrogen in electron-irradiated synthetic diamond specimens annealed in the temperature range 820–1120 K was revealed. It is established that the order of the reaction responsible for the decrease of paramagnetic defects on annealing at temperatures Tann ) 1020 K or times no more than one hour is equal to 2, with the activation energy of the process being Ea s 1.07 eV. q 1998 Elsevier Science B.V. PACS: 71.55.-i; 61.72.Ji; 78.60.Hk; 76.30.-v Keywords: Synthetic diamond; Nitrogen impurity; Paramagnetic
1. Introduction Synthetic diamonds ŽSD. are still not adequately explored, unlike the naturally occurring ones, and this is due primarily to its nonequilibrium defect structure. It is known that a nitrogen impurity plays a leading part in determining the physical properties of diamond. The possibilities of transmutation of various nitrogen-containing defects in a diamond lattice, including those at high pressures and temperatures, have been investigated in w1,2x. The present work reports mainly experimental findings concerned with the possibility of modifying a defect SD structure by electron irradiation and heat treatment at lower tem-
)
Corresponding author. Fax: q375-172-324644.
peratures ŽT - 1270 K.. A choice of the methods of modifying the diamond structure in the indicated temperature range is essentially based upon our investigations of cubic boron nitride w3,4x.
2. Experimental We have investigated SD powders with a grain size no more than 160 m m and some single SD crystals synthesized at the Institute of Solid-State and Semiconductor Physics ŽMinsk.. The powders were irradiated by 4 MeV electrons in vacuum and in a carbon-containing atmosphere Žfor example CO 2 . at doses ranging from 1 = 10 17 cmy2 to 3 = 10y8 cmy2 . The following heat treatment was accomplished in vacuum at Tann s 670–1270 K. EPR spec-
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 1 4 9 - 3
144
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
tra were recorded by use of a 3 cm range spectrometer, ‘Varian E 112’ model, and cathodoluminescence ŽCL. was excited by 15 keV electrons at T s 80 K.
3. Results and discussion At the microwave radiation power of 1 mW the EPR spectra of all the initial SD powder specimens displayed the signal of paramagnetic centers ŽPC. P1 with the central component, identical to the Lorentz line Ž D H f 3 G. w4x, generated by substitutional atoms of the dispersed nitrogen with concentration NP1 s 1.2–1.7 = 10 18 cmy2 w5x. With an increase of the radiation power up to 100 mW, the dominating signal in the spectra was that of paramagnetic centers generated presumably by the exchange-bound pairs of nitrogen atoms with g-factor equal to 2.002 " 0.001 Ž D H s 16–19 G. N s 1.3 = 10 19 spinrg.. The signal’s form was similar to the Lorentz one. After the irradiation of SD in the above dose range, the concentration of paramagnetic defects was NP1 s 1.4–1.8 = 10 18 cmy2 . The A-band, with maximum intensity at 2.35–2.38 eV and width 0.50–0.53 eV, was detected in CL spectra of some single SD crystals and in powder specimens w6x. In the 200–350 nm region of the CL spectrum, the SD specimens investigated did not display any luminescence. After annealing at different temperatures and irradiation we detected the A-band and the ZPL line at 575 nm w7x, depending on the irradiation dose; D s 1 = 10 17 cmy2 at Tann ) 870 K and D s 1 = 10y8 cmy2 at Tann ) 770 K. Immediately after irradiation, D G 3 = 10 18 cmy2 . The ZPL Žzero phonon line. of 884 nm of a nickelcontaining center and the ZPL line of 781 nm initiated by an unknown defect were also detected in the spectra of single SD crystals. It was found that annealing of the single crystals and powder specimens of synthetic diamond in the low-temperature range, Tann - 1270 K, leads to regular changes in the defect structure of the material and, consequently, in the EPR and CL spectra. Thus, in the temperature range of Tann s 1000–1100 K the relative intensity of all luminescence centers was observed to increase, especially of the ZPL center at 575 nm, and to narrow at the same time which was
Fig. 1. Relative intensity of the ZPL at 575 nm in the CL spectra of the SD specimens: Ž1–3. powders irradiated in vacuum with the doses 3=10 17 cmy2 , 7=10 17 cmy2 , 3=10 18 cmy2 ; Ž4. single SD crystals irradiated in vacuum with the dose Ds 5=10 17 cmy2 ; Ž5 and 6. SD powders irradiated in the carbon-containing atmosphere with the doses 3=10 17 cmy2 and 3=10 18 cmy2 versus the annealing temperature.
distinctly recorded in the spectra of separate single crystals of different size ŽFig. 1.. The changes discussed were accompanied by an abrupt decrease of the concentration of all paramagnetic defects, P1, which has been increasing till this moment and attained its maximum at Tann s 973 K Ž NP1 s 3.9–4.3 = 10y8 spinrg.. Kinetic curves of the investigation of the effect of annealing Žwith the temperature steps DT s 100 K. on the width of the signal generated by PC P1 in the EPR spectra of SD specimens irradiated in vacuum are shown in Fig. 2a. The concentration dependencies are similar in nature. It is clearly seen that annealing promotes defect formation in SD, at first it causes an increase of the signal width D H ŽP1 concentration., which attains its maximum and saturation at Tann s 1073 K, and then it decreases to D H s 2.75 G Ž NP1 s 1.3–1.5 = 10 18 spinrg. for more than one hour at Tann s 1073 K when the concentration decreases for an annealing time of 360 min. Because the signal width D H Žconcentration of PC P1. of the indicated centers in the EPR spectra of the SD specimens annealed at Tann s 1073 K Ž tann s 5 min. exceeds that in the spectra of those annealed at Tann s 973 K Ž tann s 6 h., we conducted some additional kinetic studies to determine critical temperatures more precisely and to elucidate the order of the reaction of the observed
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
145
The kinetic curves did not allow unambiguous determination of the order of the reaction leading to an increase of the PC P1 concentration at Tann s 923 K. It was established, however, that the experimental data on decreasing the concentration of the indicated defects in diamond at Tann ) 1023 K can be described in terms of second-order kinetics for annealing times no more than one hour; at larger annealing times the process slows down ŽFig. 3.. The activation energy of the process was calculated by use of the known Eqs. Ž1. and Ž2. below, to be
k ts
1
1 y
Ct
C0
,
Ea
ž /
k s A exp y
Fig. 2. Width Ž D H . of the signal generated by PC P1 in the EPR spectra of SD specimens irradiated in vacuum with the dose 3=10 17 cmy2 and Ža. annealed at Ž1. 673 K, Ž2. 773 K, Ž3. 873 K, Ž4. 973 K, Ž5. 1073 K, Ž6. 1173 K versus annealing time; Žb. annealed for 1–5 min, 2–10 min, 3–30 min, 4–60 min versus the annealing temperature.
kinetic processes as well as the influence of the annealing time on their activation. Annealing was carried out at temperatures ranging from 923 to 1123 K with temperature steps DT s 25 K in the time range of 1–360 min. Even a 5 min anneal at Tann s 1023 K causes a maximum increase in the concentration of PC P1 in the SD specimens irradiated in vacuum. Moreover, the larger annealing time, the lower the temperatures at which the concentration starts increasing and attains its minimum due to the heat treatment. With the maximum being unchanged at Tann s 1023 K, its low- and high-temperature branches are displaced towards the region of low temperatures ŽFig. 2b..
kT
Ž 1. ,
Ž 2.
where k is the reaction rate constant, C0 is the initial concentration of defects, Ct is the concentration in the specimens annealed for time t at the temperature T and Ea is the activation energy. It is noteworthy that an extreme increase of the paramagnetic nitrogen concentration in the SD specimens was observed in the temperature region where the activation of vacancy motion occurs, but in the CL spectra of the investigated diamonds the luminescence centers initiated by the neutral vacancy ŽGR-1. are not observed. However, it has been suggested that irradiation of SD results in formation of charged vacancies in the latter, unlike natural diamond, and
Fig. 3. 1r C versus time dependence for different annealing temperatures and lnŽ k . versus 10 4 r T corresponding to the activation energy of 1.07 eV Žin frame.: Ž1. 973 K, Ž2. 1023 K, Ž3. 1073 K.
146
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147
the activation of the motion of nitrogen atoms is observed in the lattice at Tann ) 1073 K w8,9x. To determine a possible relation of the discovered phenomenon with the presence of vacancies in SD, we irradiated the SD powders in the carbon-containing atmosphere. It was found that the center at 575 nm recorded in the spectra of the specimens irradiated in vacuum, particularly after annealing Žfor D - 3 = 10 18 cmy2 ., was detected immediately after irradiation of the SD specimens in the atmosphere by smaller doses, while for the heat-treated specimens at the same dose at lower annealing temperatures. The relative intensity of the zero-phonon lines ŽZPL. exhibited a several-fold decrease, as compared to the specimens irradiated in vacuum; the halfwidth decreased from 5.5 to 4.3 meV Žat the dose ; 3 = 10 18 cmy2 . thus pointing to a decrease of inhomogeneous distortions in the lattice as compared to the specimens irradiated in vacuum. As it is seen in Fig. 1 the temperature dependences of the relative ZPL intensity at 575 nm in the spectra of SD irradiated in the carbon atmosphere do not display any maxima for doses D - 3 = 10 18 cmy2 . The plots of concentration of paramagnetic center, P1, versus annealing temperature also do not display a maximum at Tann ; 973 K. As for the absolute concentration of PC P1, it is lower than that in the SD specimens annealed after irradiation in vacuum Ž NP1 s 2.4 = 10 18 - 3.9–4.3 = 10 18 spinrg. ŽFig. 4..
Since the SD irradiation in the atmosphere does not change the mechanism of interaction of an accelerated electron with its lattice, it is hardly probable that an additional amount of the 575 nm defects appear in the material in the case of luminescence build-up at less severe conditions of radiative and thermal treatments. Owing to the improvement of structure perfection of synthetic diamond due to a decrease in the concentration of recombination levels in its forbidden band, the above mentioned decrease in the ZPL intensity of the 575 nm centers with their simultaneous narrowing can be caused by a decrease of the vacancy concentration and, as a consequence, of the 575 nm centers in the SD specimens. This is well explained within the framework of the model of forced diffusion w8x when positive carbon ions from the radiation-ionized CO 2 atmosphere penetrates into the near-surface layers of a negatively charged crystal, diffuse in it, and annihilate with vacancies.
4. Conclusion An increase in the concentration of the PC P1 at Tann s 1023 K can be caused by activation of the motion of charged vacancies since the luminescence centers initiated by the neutral vacancy ŽGR-1. have not been detected, as a result of which the impurity nitrogen are recharged. The positive carbon ions, diffused into the SD lattice from the carbon atmosphere, hinder the motion of vacancies and nitrogen recharging thus suppressing an increase of the PC P1 concentration in synthetic diamond. The process of formation of defects at 575 nm being most pronounced at Tann ) 1000 K results in a shift towards the region of activation of motion of the nitrogen impurity.
References
Fig. 4. PC P1 concentration in the SD specimens irradiated in the carbon-containing atmosphere with the doses: Ž1. 7=10 17 cmy2 , Ž2. 1=10 18 cmy2 , Ž3. 3=10 18 cmy2 .
w1x R.M. Brozel, T. Evans, R.F. Stephenson, Proc. R. Soc. London A 361 Ž704. Ž1978. 109. w2x Ju.A. Kluev, A.M. Naletov, V.P. Nepsha, L.D. Belichenko, V.A. Laptev, M.I. Samoilovich, J. Phys. Chem. 56 Ž3. Ž1982. 524, in Russian. w3x E.M. Shishonok, V.B. Shipilo, I.I. Azarko, N.G. Anichenko, Proc. of the 5th Conf. Prospects for Diamond Application in Technology and Electronics, Moscow, May, 1995 Žin Russian..
E.M. Shishonok et al.r Materials Letters 34 (1998) 143–147 w4x E.M. Shishonok, V.B. Shipilo, I.I. Azarko, N.G. Anichenko, Abstr. of the Topical Workshop on 3–5 Nitrides, Japan, September, 1995. w5x W.V. Smith, P.P. Sorokin, L.L. Gelles, G.J. Lasher, Phys. Rev. 115 Ž1959. 1546. w6x S. Lowsom, G. Davies, A. Collins, A. Mainwood, J. Phys. Condens. Matter. 4 Ž1992. 125.
147
w7x L. Nishida, Mater. Sci. Forum ŽSwitzerland. 38–41 Ž1989. 561. w8x G. Popovici, T. Sung, M.A. Prelas, R.J. Wilson, Proc. III Int. Conf. on Application of Diamond Films and Related Materials, USA, Ž1995. 169. w9x A.M. Zaitsev, dissertation, Minsk, 1992 Žin Russian..