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Change of luminescence character of Ib diamonds with HPHT treatment
Diamond and Related Materials 10 Ž2001. 1665᎐1669
Change of luminescence character of Ib diamonds with HPHT treatment H. Kandaa,U , X. Jiab a
National Institute for Research in Inorganic Materials (NIRIM), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b National Key Laboratory of Superhard Materials, Jilin Uni¨ ersity, Changchun 130023, PR China
Abstract Type Ib high pressure synthetic diamonds have been examined using photo- and cathodo-luminescence topography and spectroscopy before and after heat treatment. The type Ib crystals were grown from Fe instead of Ni and Co in order to avoid influence of Ni and Co luminescence centers. The crystal exhibited the 389-nm band and a broad band with a maximum around 550 nm as well as additional weak peaks at 485 and 535 nm. With annealing at 1800⬚C and 6 GPa, the peaks were annealed out and the N3, H3 and 575-nm bands appeared instead. The H3 and 575-nm bands were stronger in 1134 sectors than in 1114, whereas the N3 band was weaker in the 1134 sectors. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cathodoluminescence; HPHT diamond; Nitrogen impurity; Defect
1. Introduction Luminescence spectroscopy is a useful technique to provide information on defects of diamond with high sensitivity. Luminescence spectra are readily recorded, and many hundreds of luminescence bands have been documented w1x, but it is generally difficult to know structures, concentrations or behavior of defects from the spectra because of lack of knowledge on correlation between spectra and defects. This study aims at accumulation of the knowledge. In this study, luminescence properties of diamond grown from Fe have been investigated, because it was expected that Fe impurity is incorporated into diamond to become optically active centers in analogy with Ni and Co w2,3x. However, no new luminescence peak was found, but well known nitrogen related peaks such as
U
Corresponding author. Tel.: q81-298-51-4005; fax: q81-298-514005. E-mail address: [email protected] ŽH. Kanda..
the 389-nm, N3, H3 and 575-nm bands were observed. In this study, therefore, behaviors of the nitrogen related peaks have been investigated in relation to thermal stability, concentrations of nitrogen and strain.
2. Experimental Diamond crystals were grown by the temperature gradient method. Details of the growth method have been presented elsewhere w4x. Solvent᎐catalyst for the growth was pure iron. Pressure and temperature were 6 GPa and 1500⬚C, and 2᎐3 mm crystals were grown for 20᎐40 h. The grown crystals were facetted by 1114 surfaces predominantly, accompanying small 1134 , 1104 and 1004 surfaces. The crystals were mechanically polished on a rotating iron disc named a scaife to 0.5᎐1 mm thick slab with large parallel 1104 surfaces so that internal sector morphology was readily observed. The 1114 sectors were yellow in color, while the other sectors were colorless. The concentrations of nitrogen
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 3 9 2 - 2
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
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in 1114 and 1134 sectors were 50 and 5 ppm, respectively, according to microscopic FTIR measurements. The other sectors were too small to be measured by the FTIR apparatus. The slabs were heat treated in graphite powder filled in a graphite capsule at 6 GPa and 1800᎐2000⬚C for 5 h using a belt-type high pressure apparatus, which was also used for the diamond growth. The samples recovered from the apparatus had been broken into several pieces, and their surface appeared frosted because of slight etching. According to decomposition of FTIR spectra, 30% of nitrogen has transformed to the A form in 1114 sectors, but for 1134 sectors absorption intensity was too weak to determine the degree of transformation. Cathodoluminescence spectra and images were taken using a Topcon SX-40A scanning electron microscope fitted with a spectrometer, a photo-multiplier detector and a CCD. Photoluminescence ŽPL. spectra were taken excited with a 325-nm He᎐Cd laser using a Renishaw Raman imaging microscope system. Samples were cooled down to 110 K for both the CL and PL measurements.
3. Results and discussion Cathodoluminescence ŽCL. imaging revealed clear sector dependence for the as-grown crystal as shown in Fig. 1, which is a monochromatic image taken at 520 nm. CL images were the same at wavelengths between 400 and 700 nm, indicating that one luminescence band
Fig. 1. Monochromatic CL image Ž520 nm. of 1104 cross-section of an as-grown crystal grown from Fe. Sector dependence of CL intensity is revealed.
is dominant from the sample. As seen in the figure, CL intensity from 1114 and 1104 sectors is higher than that from 1134 sectors. CL from 1004 was not observed because of smallness or similarity to adjacent 1134 sectors. It may be noticed that CL intensity in 1104 is stronger at the periphery than at the center of the sector. Typical CL spectra from 1114 , 1104 and 1134 sectors are shown in Fig. 2. A broad band with a maximum around 550 nm is seen in 1114 sectors. In addition, small peaks at 389 nm and 484 nm are also present. No additional peak was seen between 230 nm and 700 nm except weak free exciton peaks. 1104 sectors gave a spectrum similar to 1114 sectors as shown in Fig. 2b. The intensity of the band was variable in the sector as
Fig. 2. CL spectra of an as-grown crystal from Ža. 1114, Žb. 1104 and Žc. 1134 sectors.
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
found from the CL image ŽFig. 1.. Behavior of the CL intensity of the 1104 sector was different from that of 1114 , when changing focussing control of electron beam of SEM w5x. When defocused, the CL intensity dramatically increased from 1114 sector, whereas that from 1104 did not change. We do not know the origin of the change of CL intensity with changing focussing, but it is suggested that the luminescence center in 1104 is different from that in 1114 . Spectra from 1134 sectors were different from those of 1114 and 1104 , namely, sharp peaks were observed unambiguously as shown in Fig. 2c, although the broad band with a maximum around 550 nm is also present. Positions of the sharp peaks are at 389, 485 and 535 nm. Weak peaks are seen additionally. The 389-nm band is commonly observed in diamond w1x, but the others have not been documented in detail. Free exciton peaks were seen in all of the sectors, but its intensity was weak in 1114 sectors. This sector dependence is reasonable in terms of nitrogen concentrations. CL spectra changed dramatically with heat treatment. Sector dependence of the luminescence persisted as shown in Fig. 3, in which CL images at two different wavelengths are demonstrated. 1114 sectors are
Fig. 3. Monochromatic CL image of an annealed crystal at Ža. 420 nm and Žb. 520 nm.
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Fig. 4. CL spectra of an annealed crystal from Ža. 1114 and Žb. 1134 sectors.
brighter than 1134 at 420 nm, while darker at 520 nm. Typical CL spectra from 1114 and 1134 sectors are also shown in Fig. 4. All of the CL bands seen in the pre-heated crystals have disappeared and other bands have been produced. These are N3, H3, 575-nm bands and so called Band A, which is a broad band with a maximum around 430 nm. The bright image at 420 nm of Fig. 3a is contributed by the N3 band and Band A, and that at 520 nm seen in 1134 sectors is due to the H3 band. Mapping of photoluminescence spectra ŽFig. 5. confirms that the N3 is stronger in 1114 sectors than in 1134 sectors, whereas H3 and 575-nm bands are stronger in 1134 sectors. The mapping indicates that the ratio of the zero-phonon peak of the H3 band to that of the 575-nm is almost constant throughout the crystal, and therefore, the H3 and the 575-nm centers may be formed in a similar process. The sector dependence of the luminescence bands may suggest there is a correlation between the luminescence bands and nitrogen concentrations, namely, the H3 and 575-nm bands became weaker with higher concentrations of nitrogen, whereas the N3 band became stronger although this does not sensitively depend on the concentrations. Fig. 3 shows striations of luminescence in addition to sector dependence. The striations may be related to plastic deformation formed by stress during the HPHT treatment, because they are parallel to ²110: directions, i.e. most readily formed slip lines. The luminescence is stronger at the periphery of the crystal, where deformation is larger, indicating that the plastic deformation enhances the CL intensity. The striations were seen in CL images at 370, 420, 520 and 575 nm. The contrast at 370 nm is due to a tail of the Band A, and it is confirmed that Band A is striated. The image at 420 nm is formed by contribution of both Band A and the N3 band, but the striation images at 370 nm and 420 nm were different from each other, confirming that the N3 band is also striated. It is unambiguously judged from the image at 520 nm that the H3 band is striated. As for the 575-nm band,
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H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
Fig. 5. Mapping of PL spectra of annealed crystal. The spectra were taken along a line crossing a sector boundary from 1114 to 1134. Sharp spikes are instrumental artifacts produced by cosmic rays.
however, it was not possible to confirm that this is striated, because images at 520 nm and 575 nm were the same. We cannot rule out a possibility that the image at 575 nm is predominantly due to a tail of the H3 band. These observations suggest that production of the luminescence bands, the N3, H3 and Band A, is caused by plastic deformation as well as annealing. Some comments on the luminescence bands described above are given in the following. 3.1. 550-nm broad band Crystals grown from Ni or Co exhibit characteristic luminescence bands which are related to Ni or Co, respectively, suggesting that when Fe is used as the growth media, Fe impurity is also incorporated to diamond lattice to produce characteristic luminescence. The 550-nm broad band shown in Fig. 2 had been expected to be related to Fe impurity, but this is not likely, because no characteristic bands were formed with annealing. When Ni or Co impurity is incorporated, characteristic luminescence peaks with high intensity are produced with annealing w3,6x. There is a possibility that the broad band is due to contamination of boron impurity, because boron doped diamond exhibit a similar luminescence band. Further work is required to confirm the possibility.
3.2. The 389-nm band This defect is a radiation damage product, and it has been proposed that an interstitial nitrogen or carbon is involved in the defect w1x. This band has been observed in a non-irradiated sample of as-grown CVD diamond. This may be reasonable if we assume that CVD diamond is usually defective, as the CVD diamond grows under thermodynamically unstable conditions. However, it was found in this study that this center is present even in the as-grown HPHT diamond, indicating that imperfection is formed during growth even in diamond stable conditions. This may not be surprising, because the H3 luminescence band, another radiation damage product, is commonly observed in 1004 growth sectors of commercially available HPHT diamond. The band has been reported to start decreasing its intensity at 1200⬚C, when it is produced by ion implantation w7x, but there is a report that it survives after annealing at 1400⬚C for CVD diamond w8x. The present study supports the latter, because the band is seen in the crystal grown at 1500⬚C. However, it is not so stable as to survive at 1800⬚C. It may be speculated that when the band is formed by ion implantation, additional defects such as vacancy and interstitial atoms are also produced, and that such defects enhance migration of impurities in the crystal, leading to disappearance of the 389-nm band.
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
3.3. The N3 band It is known that there are two sequential stages of aggregation of nitrogen impurity with heat treatment, i.e. type Ib to IaA and IaA to IaB, and that the N3 center is a byproduct formed in the second stage w9x. However, the N3 band was seen in the first stage of aggregation in this study. Low concentrations of the N3 center may be formed at this stage. The luminescence technique is able to detect it in spite of low concentrations, and may be undetectable in absorption. A higher intensity of the N3 band in nitrogen-rich 1114 sectors observed in this study may be reasonable, if we assume higher concentrations of nitrogen produce higher concentrations of the N3 center. An earlier report has proposed that the N3 band is quenched by high concentrations of nitrogen w10x, but the present result is not the case. One hundred parts per million of nitrogen concentration may be too low to quench the luminescence. Luminescence is striated in the 1114 sectors as shown in Fig. 3a, suggesting that plastic deformation also contributes to formation of the N3 center. This may be contradictory with a description that the N3 luminescence observed in plastically deformed natural diamond is diffuse and non-localized in contrast with the striated H3 band w11x. It is expected that the formation process of the N3 center presented in this study is different from that of the natural diamonds. 3.4. The H3 and 575 bands The reverse relationship of sector dependence of CL intensity between the N3 band and the H3 and 575 bands is difficult to interpret. The H3 and 575-nm centers may be hardly formed in the 1114 sectors, or luminescence of the centers may have been quenched in the sectors. 3.5. 485- and 535-nm peaks Similar peaks have been observed in CVD diamonds w8,12x. It is probable that the peaks observed in this study are the same as those observed in CVD diamond.
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It has been reported that the 535-nm peak has been attributed to a nitrogen-vacancy complex w8x, although detailed study on the defect structures has not been done for the peaks. The presence of the peaks in the crystals studied here supports the theory that nitrogen is involved in the defects, because nitrogen is present in the crystals. We expect that the CVD diamond exhibiting the peaks is contaminated by nitrogen.
4. Summary We investigated luminescence properties of diamonds grown from Fe before and after HPHT treatment at 6 GPa and 1800᎐2000⬚C. Any characteristic luminescence band that was expected to be related with Fe impurity was not found. The nitrogen related 389-nm band seen in nitrogen poor growth sector of the as-grown crystal annealed out at 1800⬚C. The N3, H3 and the 575-nm bands formed with the HPHT treatment appeared to be enhanced by plastic deformation. The latter two were weak in nitrogen rich 1114 sectors, whereas the former was stronger. References w1x A.M. Zaitsev, in: M.A. Prelas, G. Popovici, L.K. Bigelow ŽEds.., Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, New York, 1997, pp. 227᎐376. w2x A.T. Collins, P.M. Spear, J. Phys. D 15 Ž1982. L183. w3x S.C. Lawson, H. Kanda, K. Watanabe, I. Kiflawi, Y. Sato, A.T. Collins, J. Appl. Phys. 79 Ž1996. 4348. w4x H. Kanda, T. Ohsawa, O. Fukunaga, I. Sunagawa, J. Crystal Growth 94 Ž1989. 115᎐124. w5x S.C. Lawson, H. Kanda, H. Kiyota, T. Tsutsumi, H. Kawarada, J. Appl. Phys. 77 Ž1995. 1729. w6x V.A. Nadolinny, A.P. Yelisseyev, Diam. Relat. Mater. 3 Ž1993. 17. w7x A.A. Gippius, Diam. Relat. Mater. 2 Ž1993. 640. w8x V.S. Vavilov, A.A. Gippius, A.M. Zaitsev, B.V. Deryagin, B.V. Spitsyn, A.E. Aleksenko, Sov. Phys. Semicond. 14 Ž1980. 1078. w9x T. Evans, in: J.E. Field ŽEd.., The Properties of Natural and Synthetic diamond, Academic Press, London, 1992, pp. 259᎐290. w10x G. Davies, M.D. Crossfield, J. Phys. C 7 Ž1973. L104. w11x W.J.P. Van Enckevort, E.P. Visser, Phil. Mag. B 62 Ž1990. 597. w12x Y.L. Khong, A.T. Collins, L. Allers, Diam. Relat. Mater. 3 Ž1994. 1023.
Change of luminescence character of Ib diamonds with HPHT treatment H. Kandaa,U , X. Jiab a
National Institute for Research in Inorganic Materials (NIRIM), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b National Key Laboratory of Superhard Materials, Jilin Uni¨ ersity, Changchun 130023, PR China
Abstract Type Ib high pressure synthetic diamonds have been examined using photo- and cathodo-luminescence topography and spectroscopy before and after heat treatment. The type Ib crystals were grown from Fe instead of Ni and Co in order to avoid influence of Ni and Co luminescence centers. The crystal exhibited the 389-nm band and a broad band with a maximum around 550 nm as well as additional weak peaks at 485 and 535 nm. With annealing at 1800⬚C and 6 GPa, the peaks were annealed out and the N3, H3 and 575-nm bands appeared instead. The H3 and 575-nm bands were stronger in 1134 sectors than in 1114, whereas the N3 band was weaker in the 1134 sectors. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cathodoluminescence; HPHT diamond; Nitrogen impurity; Defect
1. Introduction Luminescence spectroscopy is a useful technique to provide information on defects of diamond with high sensitivity. Luminescence spectra are readily recorded, and many hundreds of luminescence bands have been documented w1x, but it is generally difficult to know structures, concentrations or behavior of defects from the spectra because of lack of knowledge on correlation between spectra and defects. This study aims at accumulation of the knowledge. In this study, luminescence properties of diamond grown from Fe have been investigated, because it was expected that Fe impurity is incorporated into diamond to become optically active centers in analogy with Ni and Co w2,3x. However, no new luminescence peak was found, but well known nitrogen related peaks such as
U
Corresponding author. Tel.: q81-298-51-4005; fax: q81-298-514005. E-mail address: [email protected] ŽH. Kanda..
the 389-nm, N3, H3 and 575-nm bands were observed. In this study, therefore, behaviors of the nitrogen related peaks have been investigated in relation to thermal stability, concentrations of nitrogen and strain.
2. Experimental Diamond crystals were grown by the temperature gradient method. Details of the growth method have been presented elsewhere w4x. Solvent᎐catalyst for the growth was pure iron. Pressure and temperature were 6 GPa and 1500⬚C, and 2᎐3 mm crystals were grown for 20᎐40 h. The grown crystals were facetted by 1114 surfaces predominantly, accompanying small 1134 , 1104 and 1004 surfaces. The crystals were mechanically polished on a rotating iron disc named a scaife to 0.5᎐1 mm thick slab with large parallel 1104 surfaces so that internal sector morphology was readily observed. The 1114 sectors were yellow in color, while the other sectors were colorless. The concentrations of nitrogen
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 3 9 2 - 2
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
1666
in 1114 and 1134 sectors were 50 and 5 ppm, respectively, according to microscopic FTIR measurements. The other sectors were too small to be measured by the FTIR apparatus. The slabs were heat treated in graphite powder filled in a graphite capsule at 6 GPa and 1800᎐2000⬚C for 5 h using a belt-type high pressure apparatus, which was also used for the diamond growth. The samples recovered from the apparatus had been broken into several pieces, and their surface appeared frosted because of slight etching. According to decomposition of FTIR spectra, 30% of nitrogen has transformed to the A form in 1114 sectors, but for 1134 sectors absorption intensity was too weak to determine the degree of transformation. Cathodoluminescence spectra and images were taken using a Topcon SX-40A scanning electron microscope fitted with a spectrometer, a photo-multiplier detector and a CCD. Photoluminescence ŽPL. spectra were taken excited with a 325-nm He᎐Cd laser using a Renishaw Raman imaging microscope system. Samples were cooled down to 110 K for both the CL and PL measurements.
3. Results and discussion Cathodoluminescence ŽCL. imaging revealed clear sector dependence for the as-grown crystal as shown in Fig. 1, which is a monochromatic image taken at 520 nm. CL images were the same at wavelengths between 400 and 700 nm, indicating that one luminescence band
Fig. 1. Monochromatic CL image Ž520 nm. of 1104 cross-section of an as-grown crystal grown from Fe. Sector dependence of CL intensity is revealed.
is dominant from the sample. As seen in the figure, CL intensity from 1114 and 1104 sectors is higher than that from 1134 sectors. CL from 1004 was not observed because of smallness or similarity to adjacent 1134 sectors. It may be noticed that CL intensity in 1104 is stronger at the periphery than at the center of the sector. Typical CL spectra from 1114 , 1104 and 1134 sectors are shown in Fig. 2. A broad band with a maximum around 550 nm is seen in 1114 sectors. In addition, small peaks at 389 nm and 484 nm are also present. No additional peak was seen between 230 nm and 700 nm except weak free exciton peaks. 1104 sectors gave a spectrum similar to 1114 sectors as shown in Fig. 2b. The intensity of the band was variable in the sector as
Fig. 2. CL spectra of an as-grown crystal from Ža. 1114, Žb. 1104 and Žc. 1134 sectors.
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
found from the CL image ŽFig. 1.. Behavior of the CL intensity of the 1104 sector was different from that of 1114 , when changing focussing control of electron beam of SEM w5x. When defocused, the CL intensity dramatically increased from 1114 sector, whereas that from 1104 did not change. We do not know the origin of the change of CL intensity with changing focussing, but it is suggested that the luminescence center in 1104 is different from that in 1114 . Spectra from 1134 sectors were different from those of 1114 and 1104 , namely, sharp peaks were observed unambiguously as shown in Fig. 2c, although the broad band with a maximum around 550 nm is also present. Positions of the sharp peaks are at 389, 485 and 535 nm. Weak peaks are seen additionally. The 389-nm band is commonly observed in diamond w1x, but the others have not been documented in detail. Free exciton peaks were seen in all of the sectors, but its intensity was weak in 1114 sectors. This sector dependence is reasonable in terms of nitrogen concentrations. CL spectra changed dramatically with heat treatment. Sector dependence of the luminescence persisted as shown in Fig. 3, in which CL images at two different wavelengths are demonstrated. 1114 sectors are
Fig. 3. Monochromatic CL image of an annealed crystal at Ža. 420 nm and Žb. 520 nm.
1667
Fig. 4. CL spectra of an annealed crystal from Ža. 1114 and Žb. 1134 sectors.
brighter than 1134 at 420 nm, while darker at 520 nm. Typical CL spectra from 1114 and 1134 sectors are also shown in Fig. 4. All of the CL bands seen in the pre-heated crystals have disappeared and other bands have been produced. These are N3, H3, 575-nm bands and so called Band A, which is a broad band with a maximum around 430 nm. The bright image at 420 nm of Fig. 3a is contributed by the N3 band and Band A, and that at 520 nm seen in 1134 sectors is due to the H3 band. Mapping of photoluminescence spectra ŽFig. 5. confirms that the N3 is stronger in 1114 sectors than in 1134 sectors, whereas H3 and 575-nm bands are stronger in 1134 sectors. The mapping indicates that the ratio of the zero-phonon peak of the H3 band to that of the 575-nm is almost constant throughout the crystal, and therefore, the H3 and the 575-nm centers may be formed in a similar process. The sector dependence of the luminescence bands may suggest there is a correlation between the luminescence bands and nitrogen concentrations, namely, the H3 and 575-nm bands became weaker with higher concentrations of nitrogen, whereas the N3 band became stronger although this does not sensitively depend on the concentrations. Fig. 3 shows striations of luminescence in addition to sector dependence. The striations may be related to plastic deformation formed by stress during the HPHT treatment, because they are parallel to ²110: directions, i.e. most readily formed slip lines. The luminescence is stronger at the periphery of the crystal, where deformation is larger, indicating that the plastic deformation enhances the CL intensity. The striations were seen in CL images at 370, 420, 520 and 575 nm. The contrast at 370 nm is due to a tail of the Band A, and it is confirmed that Band A is striated. The image at 420 nm is formed by contribution of both Band A and the N3 band, but the striation images at 370 nm and 420 nm were different from each other, confirming that the N3 band is also striated. It is unambiguously judged from the image at 520 nm that the H3 band is striated. As for the 575-nm band,
1668
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
Fig. 5. Mapping of PL spectra of annealed crystal. The spectra were taken along a line crossing a sector boundary from 1114 to 1134. Sharp spikes are instrumental artifacts produced by cosmic rays.
however, it was not possible to confirm that this is striated, because images at 520 nm and 575 nm were the same. We cannot rule out a possibility that the image at 575 nm is predominantly due to a tail of the H3 band. These observations suggest that production of the luminescence bands, the N3, H3 and Band A, is caused by plastic deformation as well as annealing. Some comments on the luminescence bands described above are given in the following. 3.1. 550-nm broad band Crystals grown from Ni or Co exhibit characteristic luminescence bands which are related to Ni or Co, respectively, suggesting that when Fe is used as the growth media, Fe impurity is also incorporated to diamond lattice to produce characteristic luminescence. The 550-nm broad band shown in Fig. 2 had been expected to be related to Fe impurity, but this is not likely, because no characteristic bands were formed with annealing. When Ni or Co impurity is incorporated, characteristic luminescence peaks with high intensity are produced with annealing w3,6x. There is a possibility that the broad band is due to contamination of boron impurity, because boron doped diamond exhibit a similar luminescence band. Further work is required to confirm the possibility.
3.2. The 389-nm band This defect is a radiation damage product, and it has been proposed that an interstitial nitrogen or carbon is involved in the defect w1x. This band has been observed in a non-irradiated sample of as-grown CVD diamond. This may be reasonable if we assume that CVD diamond is usually defective, as the CVD diamond grows under thermodynamically unstable conditions. However, it was found in this study that this center is present even in the as-grown HPHT diamond, indicating that imperfection is formed during growth even in diamond stable conditions. This may not be surprising, because the H3 luminescence band, another radiation damage product, is commonly observed in 1004 growth sectors of commercially available HPHT diamond. The band has been reported to start decreasing its intensity at 1200⬚C, when it is produced by ion implantation w7x, but there is a report that it survives after annealing at 1400⬚C for CVD diamond w8x. The present study supports the latter, because the band is seen in the crystal grown at 1500⬚C. However, it is not so stable as to survive at 1800⬚C. It may be speculated that when the band is formed by ion implantation, additional defects such as vacancy and interstitial atoms are also produced, and that such defects enhance migration of impurities in the crystal, leading to disappearance of the 389-nm band.
H. Kanda, X. Jia r Diamond and Related Materials 10 (2001) 1665᎐1669
3.3. The N3 band It is known that there are two sequential stages of aggregation of nitrogen impurity with heat treatment, i.e. type Ib to IaA and IaA to IaB, and that the N3 center is a byproduct formed in the second stage w9x. However, the N3 band was seen in the first stage of aggregation in this study. Low concentrations of the N3 center may be formed at this stage. The luminescence technique is able to detect it in spite of low concentrations, and may be undetectable in absorption. A higher intensity of the N3 band in nitrogen-rich 1114 sectors observed in this study may be reasonable, if we assume higher concentrations of nitrogen produce higher concentrations of the N3 center. An earlier report has proposed that the N3 band is quenched by high concentrations of nitrogen w10x, but the present result is not the case. One hundred parts per million of nitrogen concentration may be too low to quench the luminescence. Luminescence is striated in the 1114 sectors as shown in Fig. 3a, suggesting that plastic deformation also contributes to formation of the N3 center. This may be contradictory with a description that the N3 luminescence observed in plastically deformed natural diamond is diffuse and non-localized in contrast with the striated H3 band w11x. It is expected that the formation process of the N3 center presented in this study is different from that of the natural diamonds. 3.4. The H3 and 575 bands The reverse relationship of sector dependence of CL intensity between the N3 band and the H3 and 575 bands is difficult to interpret. The H3 and 575-nm centers may be hardly formed in the 1114 sectors, or luminescence of the centers may have been quenched in the sectors. 3.5. 485- and 535-nm peaks Similar peaks have been observed in CVD diamonds w8,12x. It is probable that the peaks observed in this study are the same as those observed in CVD diamond.
1669
It has been reported that the 535-nm peak has been attributed to a nitrogen-vacancy complex w8x, although detailed study on the defect structures has not been done for the peaks. The presence of the peaks in the crystals studied here supports the theory that nitrogen is involved in the defects, because nitrogen is present in the crystals. We expect that the CVD diamond exhibiting the peaks is contaminated by nitrogen.
4. Summary We investigated luminescence properties of diamonds grown from Fe before and after HPHT treatment at 6 GPa and 1800᎐2000⬚C. Any characteristic luminescence band that was expected to be related with Fe impurity was not found. The nitrogen related 389-nm band seen in nitrogen poor growth sector of the as-grown crystal annealed out at 1800⬚C. The N3, H3 and the 575-nm bands formed with the HPHT treatment appeared to be enhanced by plastic deformation. The latter two were weak in nitrogen rich 1114 sectors, whereas the former was stronger. References w1x A.M. Zaitsev, in: M.A. Prelas, G. Popovici, L.K. Bigelow ŽEds.., Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, New York, 1997, pp. 227᎐376. w2x A.T. Collins, P.M. Spear, J. Phys. D 15 Ž1982. L183. w3x S.C. Lawson, H. Kanda, K. Watanabe, I. Kiflawi, Y. Sato, A.T. Collins, J. Appl. Phys. 79 Ž1996. 4348. w4x H. Kanda, T. Ohsawa, O. Fukunaga, I. Sunagawa, J. Crystal Growth 94 Ž1989. 115᎐124. w5x S.C. Lawson, H. Kanda, H. Kiyota, T. Tsutsumi, H. Kawarada, J. Appl. Phys. 77 Ž1995. 1729. w6x V.A. Nadolinny, A.P. Yelisseyev, Diam. Relat. Mater. 3 Ž1993. 17. w7x A.A. Gippius, Diam. Relat. Mater. 2 Ž1993. 640. w8x V.S. Vavilov, A.A. Gippius, A.M. Zaitsev, B.V. Deryagin, B.V. Spitsyn, A.E. Aleksenko, Sov. Phys. Semicond. 14 Ž1980. 1078. w9x T. Evans, in: J.E. Field ŽEd.., The Properties of Natural and Synthetic diamond, Academic Press, London, 1992, pp. 259᎐290. w10x G. Davies, M.D. Crossfield, J. Phys. C 7 Ž1973. L104. w11x W.J.P. Van Enckevort, E.P. Visser, Phil. Mag. B 62 Ž1990. 597. w12x Y.L. Khong, A.T. Collins, L. Allers, Diam. Relat. Mater. 3 Ž1994. 1023.