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The creation of the 3107 cm−1 hydrogen absorption peak in synthetic diamond single crystals
D IAMOND AND RELATED MATERIALS ELSEVIER
Diamond and Related Materials 5 (i996) 1516-1518
The creation of the 3107 cm-1 hydrogen absorption peak in synthetic diamond single crystals I. Kiflawi a,,, D. Fisher b, H. K a n d a °, G. Sittas a a
j.j. Thomson Physical Laboratory, The University of Reading, Reading RG6 6AF, UK u D.T.C. Research Centre, Belmont Road, Maidenhead,, Berkshire SL6 6JW, UK ° NIRIM Namki t-I, Tsukuba-shi, Ibaraki 305, Japan
Received 24 May 1996; accepted 24 June 1996
Abstract The 3107 cm -1 hydrogen related tocal mode was produced in HPHT grown diamonds after annealing at temperatures above 2100°C. A correlation was found between the intensity of the peak and the concentration of nitrogen at different locations of the same specimen. The peak position did not shift in ZSN doped samples. Keywords: Hydrogen absorption peak; Synthetic diamond; HPHT
1. Introduction The 3107 cm -1 IR absorption peak is attributed to a C - H stretch vibration and has been observed in natural type Ia diamonds [ 1-3]. Woods and Collins [3 ] investigated the spectra of 50 type Ia diamonds and found the 3107 cm -1 peak in every one of them; they found no such absorption in type II diamonds. Type II diamonds are defined as those in which the nitrogen content is below that detectable by infrared absorption spectroscopy ( ~ 1 atomic ppm). Woods and Collins also observed that the 3107 cm-1 peak has a satellite peak at 3097.8 cm -1. The position of this satellite peak is consistent with 13C replacing 12C in the carbon-hydrogen vibration, and its intensity relative to the intensity of the 3107 cm-1 peak was found to be compatible with the 1.1% natural abundance of 13C. They inferred that the 3107 cm -~ centre does not involve nitrogen. A peak at 1405 cm -~ is also found in samples containing the 3107 cm- ~ absorption and this is attributed to the C - H wagging vibration. This paper reports the first observation of the 3107 cm -1 absorption in synthetic diamonds grown by the high pressure-high temperature (HPHT) method. The centre is produced by high temperature annealing of such synthetic diamonds. The production of this absorp* Correspondingauthor. 0925-9635/96/$15.00© 1996ElsevierScienceS.A. All rights reserved PII S0925-9635 (96) 00568-7
tion in synthetic diamonds enabled us to study the effect of changing the nitrogen isotope on the position of the peak.
2. Experimental Specimens were annealed at temperatures up to 2650°C under a stabilising pressure of 9 GPa, using the experimental set-up described previously by Evans and Qi [4]. High temperature treatment can cause inclusioncontaining diamonds to break during the treatment. Therefore all the specimens examined were free from any visible inclusions. This was achieved by cutting and polishing the treated specimens to exclude the inclusions. The natural diamonds examined were homogeneously clear, gem quality diamonds with no visible defects. Infrared absorption spectra were acquired using a Nicolet Magna-IR 750 FTIR spectrometer with a microscope attachment. This arrangement allowed spectra to be obtained from 60 x 60 ~m regions of the samples.
3. Results and discussion A suite of 32 De Beers grown synthetic diamonds was used for this study. The nitrogen contents were typically 200-300 atomic ppm, although in one case (sample V17)
I. Kiflmvi et al./Diamond and Related Materials 5 (1996) 1516-1518
the nitrogen concentration was unusually high at about 900 ppm. Annealing at temperatures in excess of 2100°C resulted in the formation of a weak absorption line at 3107 cm -~ in 20 of the samples, with absorption coefficients varying from 0.05 to 1.0 cm -~. Due to the weakness of the 3107 cm -~ peak, the associated peak at t405 cm-~ was not detectable. The 3107 cm -~ peak was not observed in the 12 other specimens; the peak was either too small to be detected or it was not produced. The nitrogen concentration in these specimens was less than 150 ppm. The intensity of the 3107 cm - ~ absorption produced by this method was found to increase with the annealing temperature, but it quickly reached a limit beyond which there was no further dependence on the annealing temperature. Production of the 3107 cm -~ absorption is shown most clearly in the case of the high nitrogen sample (V17) as illustrated in Fig. 1 where the spectra initially and after annealing at 2650°C for 5 h are shown. It is also possible to produce the peak in electron irradiated samples at the significantly lower annealing temperature of 1900°C. Ten synthetic diamonds of similar nitrogen content (300-400 ppm) grown at the National Institute for Research in Inorganic Materials (NIRIM) were annealed under similar conditions. The 3107 cm-~ centre was not produced in any of these samples. It is thought that the hydrogen necessary for the formation of the 3107 cm - t centre is introduced into the crystal during growth. This hydrogen is initially trapped in an optically inactive
2.8
1517
state from which it is released during high temperature annealing to form the defect responsible for the 3107 cm -1 absorption. The difference in behaviour between the De Beers and NIRIM grown samples is possibly due to different growth capsule components or due to the higher inclusion content in the NIRIM samples as the inclusions and their associated defects could act as additional traps for hydrogen. In view of the different incidences of the 3107 cm -~ absorption in type I and type II diamonds noted by Woods and Collins [3], an investigation into possible correlation between nitrogen content and the strength of the 3107 cm- ~ peak in the annealed synthetic samples was made. The concentration of nitrogen in its various forms was obtained by measuring the infrared absorption in the one-phonon region [5-7]. It should be noted that the annealing process produced the usual aggregation of the singly substituted nitrogen (C-centres) to form A-centres (nitrogen pairs) and that annealing at temperatures above 2500°C resulted in the formation of B-centres (four nitrogen atoms surrounding a common vacancy) and platelets [8]. No definite correlation was found between the nitrogen concentrations and the strength of the 3107 cm -~ absorption when comparing different samples. However, in one specimen (V17) the 3107 cm -z peak was strong enough to carry out a quantitative analysis of its behaviour with respect to nitrogen concentration at different locations of the sample. A linear dependence was found between the total nitrogen concentration and the strength of the 3107 cm -~ peak as is shown in Fig. 2. tt should be
2.6 &--, 5
2.4
E '~
.~ 4
2.2
Q
. m
-ff
o ~o
E o
2
, D
o
3
bQ
'~ 2 e-
1.8
=
1.6
1
I
1.4 3130
o
3120
3110
3100
3090
3080
wavenumbers (cm -1) Fig. 1. Comparison of the IR spectra in the spectral range of 3130-3180 c m - ~ before (dotted fine) and after annealing at 2650°C for 5 h (solid line), showing the introduction of the 3107 cm -1 peak.
0
200
400
600
800
1000
Nitrogen concentration (atm. ppm) Fig. 2. The correlation of the intensity of the 3107 cm -1 line and the total nitrogen concentration at various locations of specimen V17 (correlation coelficient equals 0.96).
1518
/. Kiflawi et at./Diamond and Related Materials 5 (1996) 1516-1518
pointed out that, although this sample contains different growth sectors, no growth sector dependence of the correlation between the nitrogen concentration and the intensity of the 3107 cm -1 peak was found. In other samples, where quantitative comparisons were more difficult due to the weakness of the 3107 cm -1 peak, results were qualitatively similar. The 3107 cm - t peak was not observed in locations where the nitrogen concentration was low (less than about 100 ppm). A possible explanation for the observed correlation is that the conditions that favour the incorporation of nitrogen during the growth of the diamond might also favour the incorporation of hydrogen. No such correlation exists when the intensities of the 3107 c m - 1 peak are compared with the N concentration in natural diamonds; examining three natural diamonds, we found correlation in only two of them. In general, however, as has been observed in previous work [31, the presence of the 3107 cm -1 line is associated with higher nitrogen content. A very small 3107 cm-1 peak, having an absorption coefficient of 0.02 cm-1, was found in a natural type IIa diamond. The 3107 cm - i peak was also produced in seven specimens, grown by De Beers, containing both 14N and tSN. The heavier nitrogen isotope was introduced by including a few drops of 15N-containing aniline in the growth capsule. The ratio of 14N to 15N in these samples was estimated using the relative intensities of the two lines derived from the H l a centre [91 due to the different isotopes. The 15N content was found to vary between 50 and 66%. No shift was observed in the peak position o f the 3107 cm -1 line when 14N was replaced by lSN confirming the result of the study by Woods and Collins [3] that the centre does not include nitrogen.
4. Conclusions The results presented provide evidence that hydrogen may be incorporated in synthetic diamond single crystals during H P H T growth. No optical feature associated with the hydrogen in the as-grown specimens has been identified to date. Annealing at temperatures above 2100°C causes this hydrogen to form the hydrogenous defect producing the local mode absorption at 3107 cm -~. Correlation is found between the total nitrogen content and the strength of the peak in a particular sample and this suggests that the same conditions favour the incorporation of both nitrogen and hydrogen in the growing synthetic diamond. The application of this conclusion to the incorporation of hydrogen in natural diamonds is as yet unclear and requires further investigation.
References
[1.] R.M. Chrenko, R.S. McDonald and K.A. Darrow, Nature, 213 (1967) 474. [2] W.A. Runciman and T. Carter, Solid State Commun., 9 (1971) 315. [3] G.S. Woods and A.T. Collins,J. Phys. Chem. Solids, 44 (1983)471. [4] T. Evans and Z. Qi, Proc. R. Soc. Lond., A381 (1982) 159. [51 R.M. Chrenko, R.E. Tuft and H.M. Strong, Nature (London), 270 (1977) 141. [61 S.R.Boyd,I. Kiflawiand G.S.Woods,Phil. Mag. B, 69 (1994) 1149. [7] S.R. Boyd,I. Kiflawiand G.S. Woods,Phil. Mag. B, 72 (1995) 351. [8] T. Evans, Z. Qi and J. Maguire, 3". Phys C, 14, L379. [9] G.S. Woods and A.T. Collins, 3". Phys. C, 15 (1982) L949.
Diamond and Related Materials 5 (i996) 1516-1518
The creation of the 3107 cm-1 hydrogen absorption peak in synthetic diamond single crystals I. Kiflawi a,,, D. Fisher b, H. K a n d a °, G. Sittas a a
j.j. Thomson Physical Laboratory, The University of Reading, Reading RG6 6AF, UK u D.T.C. Research Centre, Belmont Road, Maidenhead,, Berkshire SL6 6JW, UK ° NIRIM Namki t-I, Tsukuba-shi, Ibaraki 305, Japan
Received 24 May 1996; accepted 24 June 1996
Abstract The 3107 cm -1 hydrogen related tocal mode was produced in HPHT grown diamonds after annealing at temperatures above 2100°C. A correlation was found between the intensity of the peak and the concentration of nitrogen at different locations of the same specimen. The peak position did not shift in ZSN doped samples. Keywords: Hydrogen absorption peak; Synthetic diamond; HPHT
1. Introduction The 3107 cm -1 IR absorption peak is attributed to a C - H stretch vibration and has been observed in natural type Ia diamonds [ 1-3]. Woods and Collins [3 ] investigated the spectra of 50 type Ia diamonds and found the 3107 cm -1 peak in every one of them; they found no such absorption in type II diamonds. Type II diamonds are defined as those in which the nitrogen content is below that detectable by infrared absorption spectroscopy ( ~ 1 atomic ppm). Woods and Collins also observed that the 3107 cm-1 peak has a satellite peak at 3097.8 cm -1. The position of this satellite peak is consistent with 13C replacing 12C in the carbon-hydrogen vibration, and its intensity relative to the intensity of the 3107 cm-1 peak was found to be compatible with the 1.1% natural abundance of 13C. They inferred that the 3107 cm -~ centre does not involve nitrogen. A peak at 1405 cm -~ is also found in samples containing the 3107 cm- ~ absorption and this is attributed to the C - H wagging vibration. This paper reports the first observation of the 3107 cm -1 absorption in synthetic diamonds grown by the high pressure-high temperature (HPHT) method. The centre is produced by high temperature annealing of such synthetic diamonds. The production of this absorp* Correspondingauthor. 0925-9635/96/$15.00© 1996ElsevierScienceS.A. All rights reserved PII S0925-9635 (96) 00568-7
tion in synthetic diamonds enabled us to study the effect of changing the nitrogen isotope on the position of the peak.
2. Experimental Specimens were annealed at temperatures up to 2650°C under a stabilising pressure of 9 GPa, using the experimental set-up described previously by Evans and Qi [4]. High temperature treatment can cause inclusioncontaining diamonds to break during the treatment. Therefore all the specimens examined were free from any visible inclusions. This was achieved by cutting and polishing the treated specimens to exclude the inclusions. The natural diamonds examined were homogeneously clear, gem quality diamonds with no visible defects. Infrared absorption spectra were acquired using a Nicolet Magna-IR 750 FTIR spectrometer with a microscope attachment. This arrangement allowed spectra to be obtained from 60 x 60 ~m regions of the samples.
3. Results and discussion A suite of 32 De Beers grown synthetic diamonds was used for this study. The nitrogen contents were typically 200-300 atomic ppm, although in one case (sample V17)
I. Kiflmvi et al./Diamond and Related Materials 5 (1996) 1516-1518
the nitrogen concentration was unusually high at about 900 ppm. Annealing at temperatures in excess of 2100°C resulted in the formation of a weak absorption line at 3107 cm -~ in 20 of the samples, with absorption coefficients varying from 0.05 to 1.0 cm -~. Due to the weakness of the 3107 cm -~ peak, the associated peak at t405 cm-~ was not detectable. The 3107 cm -~ peak was not observed in the 12 other specimens; the peak was either too small to be detected or it was not produced. The nitrogen concentration in these specimens was less than 150 ppm. The intensity of the 3107 cm - ~ absorption produced by this method was found to increase with the annealing temperature, but it quickly reached a limit beyond which there was no further dependence on the annealing temperature. Production of the 3107 cm -~ absorption is shown most clearly in the case of the high nitrogen sample (V17) as illustrated in Fig. 1 where the spectra initially and after annealing at 2650°C for 5 h are shown. It is also possible to produce the peak in electron irradiated samples at the significantly lower annealing temperature of 1900°C. Ten synthetic diamonds of similar nitrogen content (300-400 ppm) grown at the National Institute for Research in Inorganic Materials (NIRIM) were annealed under similar conditions. The 3107 cm-~ centre was not produced in any of these samples. It is thought that the hydrogen necessary for the formation of the 3107 cm - t centre is introduced into the crystal during growth. This hydrogen is initially trapped in an optically inactive
2.8
1517
state from which it is released during high temperature annealing to form the defect responsible for the 3107 cm -1 absorption. The difference in behaviour between the De Beers and NIRIM grown samples is possibly due to different growth capsule components or due to the higher inclusion content in the NIRIM samples as the inclusions and their associated defects could act as additional traps for hydrogen. In view of the different incidences of the 3107 cm -~ absorption in type I and type II diamonds noted by Woods and Collins [3], an investigation into possible correlation between nitrogen content and the strength of the 3107 cm- ~ peak in the annealed synthetic samples was made. The concentration of nitrogen in its various forms was obtained by measuring the infrared absorption in the one-phonon region [5-7]. It should be noted that the annealing process produced the usual aggregation of the singly substituted nitrogen (C-centres) to form A-centres (nitrogen pairs) and that annealing at temperatures above 2500°C resulted in the formation of B-centres (four nitrogen atoms surrounding a common vacancy) and platelets [8]. No definite correlation was found between the nitrogen concentrations and the strength of the 3107 cm -~ absorption when comparing different samples. However, in one specimen (V17) the 3107 cm -z peak was strong enough to carry out a quantitative analysis of its behaviour with respect to nitrogen concentration at different locations of the sample. A linear dependence was found between the total nitrogen concentration and the strength of the 3107 cm -~ peak as is shown in Fig. 2. tt should be
2.6 &--, 5
2.4
E '~
.~ 4
2.2
Q
. m
-ff
o ~o
E o
2
, D
o
3
bQ
'~ 2 e-
1.8
=
1.6
1
I
1.4 3130
o
3120
3110
3100
3090
3080
wavenumbers (cm -1) Fig. 1. Comparison of the IR spectra in the spectral range of 3130-3180 c m - ~ before (dotted fine) and after annealing at 2650°C for 5 h (solid line), showing the introduction of the 3107 cm -1 peak.
0
200
400
600
800
1000
Nitrogen concentration (atm. ppm) Fig. 2. The correlation of the intensity of the 3107 cm -1 line and the total nitrogen concentration at various locations of specimen V17 (correlation coelficient equals 0.96).
1518
/. Kiflawi et at./Diamond and Related Materials 5 (1996) 1516-1518
pointed out that, although this sample contains different growth sectors, no growth sector dependence of the correlation between the nitrogen concentration and the intensity of the 3107 cm -1 peak was found. In other samples, where quantitative comparisons were more difficult due to the weakness of the 3107 cm -1 peak, results were qualitatively similar. The 3107 cm - t peak was not observed in locations where the nitrogen concentration was low (less than about 100 ppm). A possible explanation for the observed correlation is that the conditions that favour the incorporation of nitrogen during the growth of the diamond might also favour the incorporation of hydrogen. No such correlation exists when the intensities of the 3107 c m - 1 peak are compared with the N concentration in natural diamonds; examining three natural diamonds, we found correlation in only two of them. In general, however, as has been observed in previous work [31, the presence of the 3107 cm -1 line is associated with higher nitrogen content. A very small 3107 cm-1 peak, having an absorption coefficient of 0.02 cm-1, was found in a natural type IIa diamond. The 3107 cm - i peak was also produced in seven specimens, grown by De Beers, containing both 14N and tSN. The heavier nitrogen isotope was introduced by including a few drops of 15N-containing aniline in the growth capsule. The ratio of 14N to 15N in these samples was estimated using the relative intensities of the two lines derived from the H l a centre [91 due to the different isotopes. The 15N content was found to vary between 50 and 66%. No shift was observed in the peak position o f the 3107 cm -1 line when 14N was replaced by lSN confirming the result of the study by Woods and Collins [3] that the centre does not include nitrogen.
4. Conclusions The results presented provide evidence that hydrogen may be incorporated in synthetic diamond single crystals during H P H T growth. No optical feature associated with the hydrogen in the as-grown specimens has been identified to date. Annealing at temperatures above 2100°C causes this hydrogen to form the hydrogenous defect producing the local mode absorption at 3107 cm -~. Correlation is found between the total nitrogen content and the strength of the peak in a particular sample and this suggests that the same conditions favour the incorporation of both nitrogen and hydrogen in the growing synthetic diamond. The application of this conclusion to the incorporation of hydrogen in natural diamonds is as yet unclear and requires further investigation.
References
[1.] R.M. Chrenko, R.S. McDonald and K.A. Darrow, Nature, 213 (1967) 474. [2] W.A. Runciman and T. Carter, Solid State Commun., 9 (1971) 315. [3] G.S. Woods and A.T. Collins,J. Phys. Chem. Solids, 44 (1983)471. [4] T. Evans and Z. Qi, Proc. R. Soc. Lond., A381 (1982) 159. [51 R.M. Chrenko, R.E. Tuft and H.M. Strong, Nature (London), 270 (1977) 141. [61 S.R.Boyd,I. Kiflawiand G.S.Woods,Phil. Mag. B, 69 (1994) 1149. [7] S.R. Boyd,I. Kiflawiand G.S. Woods,Phil. Mag. B, 72 (1995) 351. [8] T. Evans, Z. Qi and J. Maguire, 3". Phys C, 14, L379. [9] G.S. Woods and A.T. Collins, 3". Phys. C, 15 (1982) L949.