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Radiation-activated development of defective structure in type Ia diamond
Diamond and Related Materials 8 (1999) 897–902
Radiation-activated development of defective structure in type Ia diamond T.D. Ositinskaya *, V.N. Tkach V.Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev, Ukraine Received 27 July 1998; accepted 7 November 1998
Abstract A type Ia diamond crystal having nitrogen impurity in the form A, B1 and B2 centers was irradiated with 3.5 MeV electrons at successively increasing doses of 5×1016, 2×1017, 4×1017 and 2×1018 e cm−2 and examined before and after each dose using a series of methods, including positron annihilation, EPR and optical spectroscopy in IR and visible regions. After irradiation with the highest dose, a development of the visible structure of the crystal was revealed. A detailed description of the phenomenon as well as EPR and optic studies for different irradiation doses are given. The first results of CL studies of the as-developed structure are presented. These results include CL-topograms for different regions of the crystal as well as their appropriate CL-spectra. Possible known models of defects responsible for main lines in CL-spectra are considered. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; Diamond; EPR-spectroscopy; IR-spectroscopy; Radiation induced defects
1. Introduction We have previously reported [1] some findings in studies of the charge state of vacancies in natural type Ia diamond. To create vacancy defects, a crystal was irradiated with 3.5 MeV electrons. The irradiation dose was increased successively, and before and after each dose the specimen was examined by a series of methods including positron annihilation (ACAR), EPR and optical spectroscopy in IR- and visible regions. We have revealed the effect of a development of the crystal defective structure, which as far as we know has not been observed before. The present paper deals with the detailed description of the phenomenon, data of EPR and optical measurements depending on the irradiation dose, as well as the first results of cathodoluminescence (CL) studies of the as-developed structure.
2. Experimental The specimen under study was a large, natural-diamond crystal that had been ground almost parallel to * Corresponding author. Fax: +7 380 44435 3291; e-mail: [email protected]
the (111) plane into a hexagonal plate of about 10 mm size and 2.1 mm thickness. The plate was colorless and of clear water transparency. Before irradiation the initial EPR, IR and ACAR spectra from the specimen were recorded. Then the specimen was irradiated with 3.5 MeV electrons at the following doses: 5×1016, 2×1017, 4×1017 and 2×1018 e cm−2. After each irradiation dose, the abovementioned as well as optical spectra in the visible region were taken. The diamond crystal was irradiated using a linear accelerator (at the Institute of Physics of the National Academy of Sciences of Ukraine). The irradiation was performed by a pulsing electron beam having an average current density of 5.3 mA cm−2, pulsation being 4 ms and the duty cycle 250 pulses s−1. With the first three doses, the irradiation temperature was 60 °C, while the irradiation with the latest dose occurred at cooling with liquid nitrogen (80–90 K ). A detailed description of the positron annihilation technique and results are given in Ref. [1]. 22Na isotope with activity of 3.7×108 Bq was the source of positrons. ACAR spectra were recorded at room temperature. EPR and IR spectra were recorded at room temperature using a SE/X-2547 radiospectrometer produced by the ‘‘Radiopan’’ of Poland and a German-produced UR-20 spectrophotometer. Optical spectra in the visible
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 42 8 - 2
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T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
region (400–800 nm) were recorded on a spectral calculation universal complex. Transmission or absorption spectra were measured in a wholly white light of a tungsten incandescent lamp at 77 K using a nitrogen cryostat. CL-studies were performed on a device developed on the basis of a ‘‘Camebax’’ scanning electron microscope [2] equipped with a monochromator (200–800 nm optical range). This device allowed us to record CL spectra for regions of 1–7 mm in diameter on the surface of the specimen being studied, to define the integral (or in a given spectral range) intensity of emission along the line of electron probe scanning and observe visually a twodimensional pattern of distribution of integral intensity of the object luminescence, i.e. a CL-topogram. In the mode of recording CL spectra, the device resolution was about 0.5 nm between spectral lines. The accelerating voltage was between 10 and 30 kV; the electron beam current was from 3×10−6 to 10−9 A; and the measurement temperature was 80 K.
3. Results and discussion According to the initial IR spectrum, the specimen contained a considerable amount of nitrogen impurity in the form of A, B1, and B2 centers, which allows the diamond type to be defined Ref. [3] as a Ia A/B ( Table 1). EPR measurements showed that the specimen also contained nitrogen impurity in the form of paramagnetic centers: individual nitrogen atoms substituting for carbon and a complicated paramagnetic center P2 (defined as N3 in optics). The concentration of each paramagnetic center was ≤1017 cm−3. The specimen in the initial state was colorless and transparent. When irradiated with 3.5 MeV electrons, it changed color from sky-blue at a dose of 5×1016 up to blue-green at 4×1017 e/cm2 and remained wholly transparent. After irradiation with a dose of 2×1018 e cm−2 and an abrupt drop in the temperature of irradiation, the specimen became opaque and a structure developed that was visible by transillumination even to the unaided eye. A magnified photo of the crystal (×6) is given in Fig. 1. A dark background is attributed to the dark blue-green color of the diamond crystal. This phenomenon has not been observed in
Fig. 1. Photography of the type Ia diamond with as-developed structure (×6).
irradiation with 1.5–2 MeV electrons even at higher dose [4], and also was not reported in Ref. [5], where a type Ia diamond crystal was irradiated with 3.5 MeV electrons at an integral dose of 1017–1018 e cm−2. To trace dynamics of changes caused in the specimen by an intensive irradiation, let us consider dose-dependencies of EPR and optical measurements. Positron annihilation data and optical spectra in the visible region for different irradiation doses [1] have shown that, along with an increase in intensity of bands GR1 (741 nm) and ND1 (394 nm) associated, as is verified by direct positron annihilation, with an increase in concentrations of neutral and negatively charged vacancies, an intensive growth is observed of structureless absorption with the maximum at about 620 nm and a weak peak at about 667 nm in the GR1 band. New bands in spectra have not been found, but characteristic lines are broadened which can be a testimony to strain broadening of vacancy defects by their local surroundings. IR spectra from our specimen are characterized by a strong absorption in the one-phonon region due to a high concentration of the nitrogen impurity. The assessment of the absorption rate in peaks of bands defined by centers A, B1 and B2 is difficult due to a considerable thickness of the specimen (transmission <10%). Therefore, absorption at 1100 and 1010 cm−1 was mea-
Table 1 Concentration of nitrogen impurity in different forms for type Ia diamond versus the irradiation dose No. spectrum
Dose, e cm−2
N , cm−3 A
N , cm−3 B1
N , cm−3 B2
nmax , cm−1 B2
1 2 3 4 5
0 (initial ) 5×1016 2×1017 4×1017 2×1018
1.21×1020 1.46×1020 1.40×1020 1.13×1020 1.13×1020
8.75×1019 7.71×1019 8.64×1019 9.24×1019 1.12×1020
4.43×1019 4.35×1019 4.69×1019 4.31×1019 5.02×1019
1375 1365 1365 1367 1357
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
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sured [6 ]. Nitrogen concentrations in A and B1 centers were calculated as shown in Ref. [6 ], while the concentration in B2 centers was assessed by the relation [7]: N =(3.5×1018)a , cm−3 B2 B2 where a is the absorption coefficient in the peak of B2 the 1370 cm−1 band. Calculation results from IR spectra are given in Table 1. The data of Table 1 show that in irradiation of a type Ia diamond crystal at an increasing dose, the nitrogen concentration in N , N and N centers does not A B1 B2 change essentially, and only at the highest dose is some decrease in N and an increase in N and N observed. A B1 B2 The last column of Table 1 shows variations in the wavenumber of the absorption peak of the B2 band, nmax. The wavenumber decreases from 1375 cm−1 in the B2 initial state to 1357 cm−1 at the highest dose, which corresponds to an increase in platelet size from about ˚ [7]. But the observed change in nmax is 200 to 1500 A B2 not accompanied by any changes in intensity, halfwidth and symmetry of peak B2 [4]. Unirradiated diamond crystals with B2 defects show up as a wide structureless band in the 540–620 nm range [8]. Therefore, it can be suggested that the increase in absorption observed in spectra of the visible region at about 620 nm is caused by both the platelets and a superposition of the GR1 band. A peak at 667 nm possibly defines an interstitional carbon related center [9]. Fig. 2 illustrates variations of EPR spectrum versus the irradiation dose (the spectrum taken at the irradiation dose of 2×1018 e cm−2 is not given as it is similar to the one taken at 4×1017 e cm−2). In all cases spectra were recorded under the same conditions with the 111 crystal axis being oriented parallel to the direction of the external magnetic field H . 0 An analysis of the spectra shows that intensity of EPR lines from atomic nitrogen impurity decreases with each irradiation dose, and after four irradiations it was decreased by more that on an order of magnitude (in 20 times), while the intensity of lines from the P2 center was almost unchanged at all the irradiation stages (the same was observed for the N3 center at 415 nm in the visible region spectra). After irradiation at a dose of 2×1017 e cm−2, in place of the abruptly decreasing central line of a nitrogen triplet (at g#2.0025) a new single EPR line appeared, whose intensity increased with the irradiation dose. This line had not been identified up until now, but it is possible that it indicates some kind of center, which is most likely due to structural defects. The results of direct CL studies of the as-developed structure are given in Figs. 3 and 4 in the form of CL-topograms for various crystal regions and their respective CL spectra.
Fig. 2. EPR spectra from a type Ia diamond: (a) before irradiation (initial ) and after irradiation with doses: (b) 5×1016; (c) 2×1017; and (d ) 4×1017 e/cm2.
Three characteristic zones of CL-emission have been revealed: Zone 1 with an emission mainly in the region of about 600 nm (orange-red luminescence) contains formations in the form of protruding and located one over the other hexagon-planes. Their protruding parts are seen in Fig. 3a,b, and different angles between the sides of an individual hexagon are seen in Fig. 3c. Zone 2 is adjacent to zone 1 sweeping it. Under a cathode beam, zone 2 develops a bluish color and a luminescence in the green-blue region of the spectrum. The zone contains elongated planar formations of almost rectangular form ( Fig. 3a,b). The mutual arrangement of planes is seen in Fig. 3b. Zone 3, representing the main part of the crystal, is transparent to the naked eye, with luminescence in the blue region of the spectrum at about 400 nm ( Fig. 3d ). CL spectra from zones 1 and 2 (Fig. 4a–c) exhibit main lines: at 575, 490.7, 484.5 and 389 mn, while zone 3 is
900
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
(a)
(b)
(c)
(d)
Fig. 3. CL topograms for zones in type Ia diamond: (a,b) zone 1; (c) zone 2; (d ) zone 3. Scale is: 1 cm–100 mm.
characterized by a set of lines at 511.5, 490.7, 484.5, 439, 430 and 415 nm. The 389 nm system taken at a longer recording time is shown separately ( Fig. 4b). In CL spectra from zones 1 and 2, the system at 575 nm is the most intensive. Papers reviewing centers of defects in diamond [8–10] indicate that this system usually manifests itself after either an irradiation and annealing above 500 °C or activation of an electron beam as in our case. It is stated in Ref. [10] that the center at 575 nm incorporates a single atom of nitrogen and a vacancy. However, the authors of Ref. [11] have concluded that the 575 nm system represents a vacancy bonded with the nearest single nitrogen atom in a tetrahedral interstice in the 100 direction. As a result of irradiation, the 389 nm center manifests itself in all types diamonds, but there is no unambiguous interpretation of its model. For example, according to Ref. [10] this center contains interstitial carbon and a single atom of nitrogen, and the following possible variants of the model are given in Ref. [9]:
(1) an interstitial atom of nitrogen; (2) a substituting atom of nitrogen bonded with interstitial atoms of carbon; (3) a substituting atom of nitrogen shifted along the
111 axis to form a carbon–nitrogen elongated pseudomolecule. It was clearly stated in Ref. [12] that the 490.7 nm system is associated with plastically deformed regions of a diamond crystal and is often observed inside crystal growth bands. Though it remains transparent, the main part of the crystal — zone 3 — has the highest quantity of defect centers. The center 511.5 nm is assigned to intrinsic interstitials; at 439 nm to a vacancy-nitrogen defect; at 430 nm to neutral vacancies; and the center at 415 nm is a well-known N3 center, i.e. three substituting atoms of nitrogen in the (111) plane bonded with a common vacancy [8–10,13]. It follows from the above that the main lines (excluding the 490.7 nm system) in CL spectra from all zones
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
901
Fig. 4. CL spectra from zones in type Ia diamond: (a) zone 1; (b) zone 1 (the system 389 nm taken at a longer recording time); (c) zone 2; (d ) zone 3.
are attributable to the interstitial carbon, nitrogen in a substituting and interstitial positions, vacancies and/or combinations of these defects. It has been shown [14] that directly during the irradiation of type IIa diamond with 300 keV electrons at 16 and 87 K, there is a formation and growth of clusters of defects due to the mobility of interstitial carbon. In our case, during an intensive irradiation of the diamond crystal having a sufficiently high concentration of nitrogen impurity with higher-energy electrons at a higher temperature, interstitial-impurity complexes, probably, form. Evidently, CL spectra of zones exhibit this fact in main lines. Considering planar formations (Fig. 3) as a final result of irradiations with electrons, some additional irradiation with positrons and excitation with analytical light, one may assume that these formations have grown to visible sizes. Mobile interstitial carbon that is bound with nitrogen is likely to be a basic building material for these planar defects. It is interesting to note that patterns of zones 1 and 2 in CL topograms are very similar to the models of planar defects described by Sobolev [15]. However, the
size of hexagons we revealed exceeds that of even ‘‘giant’’ platelets. The occurrence of a line at 490.7 nm supports the assumption that hexagons originated as growth defects of a crystal. Based on only first results of studies of the developed structure, one cannot unambiguously determine its origin and give a comprehensive answer to the question about the structure of planar defects. These problems call for further detailed studies and we invite the interested investigators to joint studies of the as-developed structure of this diamond specimen. We believe that the effect of structure development in a type Ia diamond crystal in intensive radiative influences opens new possibilities for experimental studies of both the formation and structure of defects in a mixed-type diamond crystal and the role of intrinsic and impurity point defects in their formation.
Acknowledgements The authors thank Dr G.A. Podzyarey and Dr V.G. Malogolovets for EPR- and IR-spectra.
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References [1] N.V. Novikov, T.D. Ositinskaya, V.S. Mikhalenkov, Diam. Relat. Mater. 7 (1998) 756–760. [2] V.N. Tkach, A.S. Vishnevskii, J. Superhard Mater. 3 (1992) 10. [3] G.S. Woods, G.C. Purser, A.S.S. Mtimkulu, A.T. Collins, J. Phys. Chem. Solids 51 (1990) 1191. [4] G. Davies (Ed.), Properties and Growth of Diamond, INSPEC, London, 1994, p. 94. [5] S. Dannefaer, P. Mascher, D. Kerr, Diam. Relat. Mater. 1 (1992) 407. [6 ] G. Davies, Ind. Diam. Rev. 12 (1980) 466. [7] E.V. Sobolev, V.I. Lisoivan, S.E. Lenskaya, Russ. J. Struct. Chem. 9 (1968) 1029. [8] G.B. Bokii, G.N. Bezrukov, Yu A. Klyuev, A.M. Naletov, V.I.
[9]
[10] [11] [12] [13]
[14] [15]
Nepsha, Natural and Synthetic Diamonds, Nauka Press, Moskow, 1986 (in Russian). A.M. Zaitsev, Optical properties of diamond, chapter XIII, in: Lehrgebiet Baumelemente der Elektrotechnik, Fernuniversita¨t GSH, Hagen, 1995. A.T. Collins, Diam. Relat. Mater. 1 (1992) 457. A.M. Zaitsev, A.G. Uliyashin, H. Ali Noor, Soviet J. Superhard Mater. 1 (1991) 18. A.T. Collins, G.S. Woods, Phil. Mag. B. 45 (1982) 385. A.M. Zaitsev, Luminescence of ion-implanted diamond and cubic boron nitride, Dissertation for Doctor’s degree, University of Minsk, 1992 (in Russian). J. Koike, T.E. Mitchell, D.M. Parkin, Appl. Phys. Lett. 59 (1991) 2515. E.V. Sobolev, Harder than Diamond, Nauka Press, Novosibirsk, 1989 (in Russian).
Radiation-activated development of defective structure in type Ia diamond T.D. Ositinskaya *, V.N. Tkach V.Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev, Ukraine Received 27 July 1998; accepted 7 November 1998
Abstract A type Ia diamond crystal having nitrogen impurity in the form A, B1 and B2 centers was irradiated with 3.5 MeV electrons at successively increasing doses of 5×1016, 2×1017, 4×1017 and 2×1018 e cm−2 and examined before and after each dose using a series of methods, including positron annihilation, EPR and optical spectroscopy in IR and visible regions. After irradiation with the highest dose, a development of the visible structure of the crystal was revealed. A detailed description of the phenomenon as well as EPR and optic studies for different irradiation doses are given. The first results of CL studies of the as-developed structure are presented. These results include CL-topograms for different regions of the crystal as well as their appropriate CL-spectra. Possible known models of defects responsible for main lines in CL-spectra are considered. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; Diamond; EPR-spectroscopy; IR-spectroscopy; Radiation induced defects
1. Introduction We have previously reported [1] some findings in studies of the charge state of vacancies in natural type Ia diamond. To create vacancy defects, a crystal was irradiated with 3.5 MeV electrons. The irradiation dose was increased successively, and before and after each dose the specimen was examined by a series of methods including positron annihilation (ACAR), EPR and optical spectroscopy in IR- and visible regions. We have revealed the effect of a development of the crystal defective structure, which as far as we know has not been observed before. The present paper deals with the detailed description of the phenomenon, data of EPR and optical measurements depending on the irradiation dose, as well as the first results of cathodoluminescence (CL) studies of the as-developed structure.
2. Experimental The specimen under study was a large, natural-diamond crystal that had been ground almost parallel to * Corresponding author. Fax: +7 380 44435 3291; e-mail: [email protected]
the (111) plane into a hexagonal plate of about 10 mm size and 2.1 mm thickness. The plate was colorless and of clear water transparency. Before irradiation the initial EPR, IR and ACAR spectra from the specimen were recorded. Then the specimen was irradiated with 3.5 MeV electrons at the following doses: 5×1016, 2×1017, 4×1017 and 2×1018 e cm−2. After each irradiation dose, the abovementioned as well as optical spectra in the visible region were taken. The diamond crystal was irradiated using a linear accelerator (at the Institute of Physics of the National Academy of Sciences of Ukraine). The irradiation was performed by a pulsing electron beam having an average current density of 5.3 mA cm−2, pulsation being 4 ms and the duty cycle 250 pulses s−1. With the first three doses, the irradiation temperature was 60 °C, while the irradiation with the latest dose occurred at cooling with liquid nitrogen (80–90 K ). A detailed description of the positron annihilation technique and results are given in Ref. [1]. 22Na isotope with activity of 3.7×108 Bq was the source of positrons. ACAR spectra were recorded at room temperature. EPR and IR spectra were recorded at room temperature using a SE/X-2547 radiospectrometer produced by the ‘‘Radiopan’’ of Poland and a German-produced UR-20 spectrophotometer. Optical spectra in the visible
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 42 8 - 2
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T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
region (400–800 nm) were recorded on a spectral calculation universal complex. Transmission or absorption spectra were measured in a wholly white light of a tungsten incandescent lamp at 77 K using a nitrogen cryostat. CL-studies were performed on a device developed on the basis of a ‘‘Camebax’’ scanning electron microscope [2] equipped with a monochromator (200–800 nm optical range). This device allowed us to record CL spectra for regions of 1–7 mm in diameter on the surface of the specimen being studied, to define the integral (or in a given spectral range) intensity of emission along the line of electron probe scanning and observe visually a twodimensional pattern of distribution of integral intensity of the object luminescence, i.e. a CL-topogram. In the mode of recording CL spectra, the device resolution was about 0.5 nm between spectral lines. The accelerating voltage was between 10 and 30 kV; the electron beam current was from 3×10−6 to 10−9 A; and the measurement temperature was 80 K.
3. Results and discussion According to the initial IR spectrum, the specimen contained a considerable amount of nitrogen impurity in the form of A, B1, and B2 centers, which allows the diamond type to be defined Ref. [3] as a Ia A/B ( Table 1). EPR measurements showed that the specimen also contained nitrogen impurity in the form of paramagnetic centers: individual nitrogen atoms substituting for carbon and a complicated paramagnetic center P2 (defined as N3 in optics). The concentration of each paramagnetic center was ≤1017 cm−3. The specimen in the initial state was colorless and transparent. When irradiated with 3.5 MeV electrons, it changed color from sky-blue at a dose of 5×1016 up to blue-green at 4×1017 e/cm2 and remained wholly transparent. After irradiation with a dose of 2×1018 e cm−2 and an abrupt drop in the temperature of irradiation, the specimen became opaque and a structure developed that was visible by transillumination even to the unaided eye. A magnified photo of the crystal (×6) is given in Fig. 1. A dark background is attributed to the dark blue-green color of the diamond crystal. This phenomenon has not been observed in
Fig. 1. Photography of the type Ia diamond with as-developed structure (×6).
irradiation with 1.5–2 MeV electrons even at higher dose [4], and also was not reported in Ref. [5], where a type Ia diamond crystal was irradiated with 3.5 MeV electrons at an integral dose of 1017–1018 e cm−2. To trace dynamics of changes caused in the specimen by an intensive irradiation, let us consider dose-dependencies of EPR and optical measurements. Positron annihilation data and optical spectra in the visible region for different irradiation doses [1] have shown that, along with an increase in intensity of bands GR1 (741 nm) and ND1 (394 nm) associated, as is verified by direct positron annihilation, with an increase in concentrations of neutral and negatively charged vacancies, an intensive growth is observed of structureless absorption with the maximum at about 620 nm and a weak peak at about 667 nm in the GR1 band. New bands in spectra have not been found, but characteristic lines are broadened which can be a testimony to strain broadening of vacancy defects by their local surroundings. IR spectra from our specimen are characterized by a strong absorption in the one-phonon region due to a high concentration of the nitrogen impurity. The assessment of the absorption rate in peaks of bands defined by centers A, B1 and B2 is difficult due to a considerable thickness of the specimen (transmission <10%). Therefore, absorption at 1100 and 1010 cm−1 was mea-
Table 1 Concentration of nitrogen impurity in different forms for type Ia diamond versus the irradiation dose No. spectrum
Dose, e cm−2
N , cm−3 A
N , cm−3 B1
N , cm−3 B2
nmax , cm−1 B2
1 2 3 4 5
0 (initial ) 5×1016 2×1017 4×1017 2×1018
1.21×1020 1.46×1020 1.40×1020 1.13×1020 1.13×1020
8.75×1019 7.71×1019 8.64×1019 9.24×1019 1.12×1020
4.43×1019 4.35×1019 4.69×1019 4.31×1019 5.02×1019
1375 1365 1365 1367 1357
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
899
sured [6 ]. Nitrogen concentrations in A and B1 centers were calculated as shown in Ref. [6 ], while the concentration in B2 centers was assessed by the relation [7]: N =(3.5×1018)a , cm−3 B2 B2 where a is the absorption coefficient in the peak of B2 the 1370 cm−1 band. Calculation results from IR spectra are given in Table 1. The data of Table 1 show that in irradiation of a type Ia diamond crystal at an increasing dose, the nitrogen concentration in N , N and N centers does not A B1 B2 change essentially, and only at the highest dose is some decrease in N and an increase in N and N observed. A B1 B2 The last column of Table 1 shows variations in the wavenumber of the absorption peak of the B2 band, nmax. The wavenumber decreases from 1375 cm−1 in the B2 initial state to 1357 cm−1 at the highest dose, which corresponds to an increase in platelet size from about ˚ [7]. But the observed change in nmax is 200 to 1500 A B2 not accompanied by any changes in intensity, halfwidth and symmetry of peak B2 [4]. Unirradiated diamond crystals with B2 defects show up as a wide structureless band in the 540–620 nm range [8]. Therefore, it can be suggested that the increase in absorption observed in spectra of the visible region at about 620 nm is caused by both the platelets and a superposition of the GR1 band. A peak at 667 nm possibly defines an interstitional carbon related center [9]. Fig. 2 illustrates variations of EPR spectrum versus the irradiation dose (the spectrum taken at the irradiation dose of 2×1018 e cm−2 is not given as it is similar to the one taken at 4×1017 e cm−2). In all cases spectra were recorded under the same conditions with the 111 crystal axis being oriented parallel to the direction of the external magnetic field H . 0 An analysis of the spectra shows that intensity of EPR lines from atomic nitrogen impurity decreases with each irradiation dose, and after four irradiations it was decreased by more that on an order of magnitude (in 20 times), while the intensity of lines from the P2 center was almost unchanged at all the irradiation stages (the same was observed for the N3 center at 415 nm in the visible region spectra). After irradiation at a dose of 2×1017 e cm−2, in place of the abruptly decreasing central line of a nitrogen triplet (at g#2.0025) a new single EPR line appeared, whose intensity increased with the irradiation dose. This line had not been identified up until now, but it is possible that it indicates some kind of center, which is most likely due to structural defects. The results of direct CL studies of the as-developed structure are given in Figs. 3 and 4 in the form of CL-topograms for various crystal regions and their respective CL spectra.
Fig. 2. EPR spectra from a type Ia diamond: (a) before irradiation (initial ) and after irradiation with doses: (b) 5×1016; (c) 2×1017; and (d ) 4×1017 e/cm2.
Three characteristic zones of CL-emission have been revealed: Zone 1 with an emission mainly in the region of about 600 nm (orange-red luminescence) contains formations in the form of protruding and located one over the other hexagon-planes. Their protruding parts are seen in Fig. 3a,b, and different angles between the sides of an individual hexagon are seen in Fig. 3c. Zone 2 is adjacent to zone 1 sweeping it. Under a cathode beam, zone 2 develops a bluish color and a luminescence in the green-blue region of the spectrum. The zone contains elongated planar formations of almost rectangular form ( Fig. 3a,b). The mutual arrangement of planes is seen in Fig. 3b. Zone 3, representing the main part of the crystal, is transparent to the naked eye, with luminescence in the blue region of the spectrum at about 400 nm ( Fig. 3d ). CL spectra from zones 1 and 2 (Fig. 4a–c) exhibit main lines: at 575, 490.7, 484.5 and 389 mn, while zone 3 is
900
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
(a)
(b)
(c)
(d)
Fig. 3. CL topograms for zones in type Ia diamond: (a,b) zone 1; (c) zone 2; (d ) zone 3. Scale is: 1 cm–100 mm.
characterized by a set of lines at 511.5, 490.7, 484.5, 439, 430 and 415 nm. The 389 nm system taken at a longer recording time is shown separately ( Fig. 4b). In CL spectra from zones 1 and 2, the system at 575 nm is the most intensive. Papers reviewing centers of defects in diamond [8–10] indicate that this system usually manifests itself after either an irradiation and annealing above 500 °C or activation of an electron beam as in our case. It is stated in Ref. [10] that the center at 575 nm incorporates a single atom of nitrogen and a vacancy. However, the authors of Ref. [11] have concluded that the 575 nm system represents a vacancy bonded with the nearest single nitrogen atom in a tetrahedral interstice in the 100 direction. As a result of irradiation, the 389 nm center manifests itself in all types diamonds, but there is no unambiguous interpretation of its model. For example, according to Ref. [10] this center contains interstitial carbon and a single atom of nitrogen, and the following possible variants of the model are given in Ref. [9]:
(1) an interstitial atom of nitrogen; (2) a substituting atom of nitrogen bonded with interstitial atoms of carbon; (3) a substituting atom of nitrogen shifted along the
111 axis to form a carbon–nitrogen elongated pseudomolecule. It was clearly stated in Ref. [12] that the 490.7 nm system is associated with plastically deformed regions of a diamond crystal and is often observed inside crystal growth bands. Though it remains transparent, the main part of the crystal — zone 3 — has the highest quantity of defect centers. The center 511.5 nm is assigned to intrinsic interstitials; at 439 nm to a vacancy-nitrogen defect; at 430 nm to neutral vacancies; and the center at 415 nm is a well-known N3 center, i.e. three substituting atoms of nitrogen in the (111) plane bonded with a common vacancy [8–10,13]. It follows from the above that the main lines (excluding the 490.7 nm system) in CL spectra from all zones
T.D. Ositinskaya, V.N. Tkach / Diamond and Related Materials 8 (1999) 897–902
901
Fig. 4. CL spectra from zones in type Ia diamond: (a) zone 1; (b) zone 1 (the system 389 nm taken at a longer recording time); (c) zone 2; (d ) zone 3.
are attributable to the interstitial carbon, nitrogen in a substituting and interstitial positions, vacancies and/or combinations of these defects. It has been shown [14] that directly during the irradiation of type IIa diamond with 300 keV electrons at 16 and 87 K, there is a formation and growth of clusters of defects due to the mobility of interstitial carbon. In our case, during an intensive irradiation of the diamond crystal having a sufficiently high concentration of nitrogen impurity with higher-energy electrons at a higher temperature, interstitial-impurity complexes, probably, form. Evidently, CL spectra of zones exhibit this fact in main lines. Considering planar formations (Fig. 3) as a final result of irradiations with electrons, some additional irradiation with positrons and excitation with analytical light, one may assume that these formations have grown to visible sizes. Mobile interstitial carbon that is bound with nitrogen is likely to be a basic building material for these planar defects. It is interesting to note that patterns of zones 1 and 2 in CL topograms are very similar to the models of planar defects described by Sobolev [15]. However, the
size of hexagons we revealed exceeds that of even ‘‘giant’’ platelets. The occurrence of a line at 490.7 nm supports the assumption that hexagons originated as growth defects of a crystal. Based on only first results of studies of the developed structure, one cannot unambiguously determine its origin and give a comprehensive answer to the question about the structure of planar defects. These problems call for further detailed studies and we invite the interested investigators to joint studies of the as-developed structure of this diamond specimen. We believe that the effect of structure development in a type Ia diamond crystal in intensive radiative influences opens new possibilities for experimental studies of both the formation and structure of defects in a mixed-type diamond crystal and the role of intrinsic and impurity point defects in their formation.
Acknowledgements The authors thank Dr G.A. Podzyarey and Dr V.G. Malogolovets for EPR- and IR-spectra.
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