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Optical characterization of some irradiation-induced centers in diamond
Diamond and Related Materials 10 Ž2001. 18᎐26
Optical characterization of some irradiation-induced centers in diamond K. Iakoubovskii a,U , G.J. Adriaenssens a , N.N. Dogadkinb, A.A. Shiryaev b a
Laboratorium ¨ oor Halfgeleiderfysica, Katholieke Uni¨ ersiteit Leu¨ en, Celestijnenlaan 200 D, B-3001 Leu¨ en, Belgium b Institute of Geochemistry, Kosygin st. 19, B-334 Moscow, Russia Accepted 18 August 2000
Abstract Photoluminescence ŽPL., PL excitation and optical absorption techniques were applied to the 3H, H3 and Si-related centers in natural and synthetic diamond. Thresholds at 4.6 and 3.85 eV are observed in the excitation spectra of the H3 and 3H luminescence, respectively. The 4.6-eV threshold could only be observed if single substitutional nitrogen ŽP1. centers were present in the sample. It is ascribed to the ionization of the P1q centers followed by an energy transfer from the P1 to the H3 centers. The energy transfer from other defects can also account for the temperature-independent PL, which was observed from the 3H, H3, H4 and TR12⬘ centers when they were excited at energies below the zero-phonon line. Si-related centers are detected in natural Ia diamond after neutron irradiation and annealing. Details of the vibronic structure of the Si-V center in diamond grown by chemical vapor deposition are reported. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond defects; Luminescence; Impurities; Micro-Raman spectroscopy
1. Introduction Many defects in diamond have ground and excited states located within the forbidden gap. Optical transitions between those states may produce zero-phonon lines ŽZPLs. with associated vibronic structures, which can be detected in absorption andror luminescence. These lines often have a width of a few meV or less and are mostly present in the visible part of the spectrum, for which sensitive optical detectors are available. Consequently, their intensity and position can be measured with a high accuracy and used for identification and characterization of a defect. However, inter-
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Corresponding author. Fax: q32-1632-7987. E-mail address: [email protected] ŽK. Iakoubovskii..
pretation of the obtained spectra is often not straightforward and it is much facilitated if the same defect center is studied by several techniques in different types of samples. This was done in the present work, where photoluminescence ŽPL., PL excitation ŽPLE. and optical absorption were applied to the 3H, H3 and silicon-vacancy ŽSi-V. centers in natural and synthetic diamonds. 1.1. The H3 center The defect responsible for the H3 center consists of two next-nearest-neighbor substitutional nitrogen atoms separated by a vacancy w1x. It is produced if a nearest-neighbor nitrogen pair Žthe A center. captures a vacancy. The H3 center has C 2v symmetry with a ²110: symmetry axis. Two charge states are known,
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 0 . 0 0 3 6 1 - 7
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neutral ŽwN-V-Nx 0 . and negative ŽwN-V-Nxy. . Optical transitions between the ground state and the first excited state of the wN-V-Nx 0 produce a system, labeled H3, of ZPL at 2.463 eV and associated vibronic band w1x. The excitation spectrum of the H3 emission contains several non-luminescent lines around 3.4 eV Žthe so-called H13 system., which are attributed w2x to the transitions from the ground state to the higher-lying excited states at the wN-V-Nx0 . An absorption and PL system H2, with a ZPL at 1.257 eV, is associated w3x with wN-V-Nxy. Illumination with light of energy above 2.1 eV converts wN-V-Nxy to wN-V-Nx0 . Therefore, it was suggested w3x that the ground state of wN-V-Nxy is located 2.1 eV below the conduction band edge Žat EC y 2.1 eV.. The position in the forbidden gap of the ground state of the wN-V-Nx 0 is not identified yet. Here we report on an additional transition observed at 4.6 eV in the excitation spectra of the H3 luminescence. This transition is seen only if the single substitutional nitrogen ŽP1. center is present in the sample, and it is interpreted in terms of an energy transfer from the P1 to the H3 center. Moreover, it is shown that the H3 and some other centers ŽH4, 3H and TR12⬘. can be excited by light with energy less than ZPL and the efficiency of such excitation may be weakly temperature dependent. A mechanism for this sub-threshold PL is proposed.
1.2. The 3H center
The defect responsible for the 3H center is believed to be a primary irradiation product. Similarly to the H3, it has C 2v symmetry with a ²110: axis and a ZPL at 2.463 eV w4x. However, contrary to the H3, the 3H center has a very weak vibronic structure with sharp local mode peaks at 169.3, 182.4, 186.7 and 217.8 meV w5x. Presence of a local mode is characteristic for interstitial-related centers w6x. Moreover, these single local mode peaks split into triplets in diamond containing comparable amounts of 12 C and 13 C isotopes, indicating that only two carbon atoms are involved in the vibrations. Therefore, it was suggested that the 3H is a split interstitial w5x. This center shows complex annealing behavior: most of its absorption disappears already at 400⬚C, but complete annealing occurs only in the temperature range 900᎐1000⬚C w7x. In this paper the PLE spectrum from the 3H center is reported. Peaks at 2.63, 2.68, 3.02 eV and a threshold at 3.85 eV are detected. The first three peaks are attributed to the excited states of the 3H center. An additional weak PL feature at 2.523 eV is assigned to the 3H spectrum. Remarkably, this line has a preferen-
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tial direction of polarization perpendicular to the one of the main transitions, and its intensity is almost temperature independent. Similar behavior is observed for some other centers in naturally-irradiated Ia diamond. 1.3. The green band II Many diamonds show a featureless broadband green emission. Recent PLE results w8,9x show that it can consist of two bands. The green band I is ascribed to radiative transitions at the amorphous carbon inclusions present in diamond grown by chemical vapor deposition ŽCVD.. Band II is attributed to the donor᎐acceptor pair recombination, where the acceptor is boron and the donor has an optical ionization threshold to the valence band at 3.25 eV; the donor’s origin being unknown. This band may be observed in as-grown CVD and high-pressure᎐high-temperature ŽHPHT. synthetic diamonds, boron- or nitrogen-doped w8᎐10x. Here we report the observation that the green band II is produced in IIb Žboron-rich. natural diamond by neutron irradiation and annealing at 800⬚C and suggest that the donor responsible for this band may be a structural defect. 1.4. The Si-V center This center is observed in any CVD or HPHT diamond if silicon was present in the growth reactor, but it has not yet been detected in natural diamond. Despite a large number of reports, only fragmentary details of its structure are known: it probably contains one Si atom and a vacancy w11,12x, has a ground state at EC y 2.05 eV w8,9,13x, and a ²110: C 2 symmetry axis w14x. Calculation suggests that it consists of a Si atom at the center of a divacancy w15x, but this structure has not yet been confirmed by experiment. The Si-V center has a ZPL at 1.682 eV with a very weak vibronic structure and it is often accompanied w16x by a broader and weaker line at 1.679 eV whose origin is unknown. The details of the Si-V PL spectrum are thoroughly characterized w12,17x, however, only two reports are known on Si-V absorption w11,13x. Meanwhile, two important diamond centers have ZPLs close to that of the Si-V, the neutral vacancy ŽGR1, ZPL at 1.673 eV. and the ²100: split interstitial ŽR2, lines at 1.685 and 1.86 eV. w6x. Moreover, three other Si-related centers at 1.679, 1.691 and 1.711 eV were detected in absorption from Si-doped HPHT diamond w18x. Clearly, absorption study of these centers is required to reveal their possible interrelation. Consequently, we also examined absorption and PL of the Si-related centers. It is suggested that one of the
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1.679 eV lines is a stress-induced vibronic transition at the Si-V center. In addition, three Si-related centers are detected in neutron irradiated natural Ia diamond annealed at 800⬚C.
2. Experimental CVD diamond films were grown on Si substrates in an ASTeX PDS-17 microwave plasma reactor. Typical deposition parameters were: microwave power 5 kW, gas flow rate 300 cm3rmin, substrate temperature 800⬚C, feed gas mixture 99:1 H 2 :CH 4 . The substrate was removed by etching in HF:HNO3 mixture. Freestanding polycrystalline films composed of randomly oriented grains have been studied. ²100: oriented natural IIb and synthetic Ib crystals were purchased from Drukker International and De Beers, respectively. The IIb diamond used for this paper contained 0.2 ppm of uncompensated boron acceptors, as deduced from IR transmittance spectra. Several groups of natural Ia diamonds from Australia ŽArgyle pipe., Yakutia, Guinea Žlower mantle, below 600 km from the surface . and Venezuela Žplacers. have been made available by the Institute of Geochemistry, Moscow. Some natural and CVD diamonds were irradiated by an integrated dose of 2 = 10 18 neutronsrcm 2 at nominally room temperature at the Research Reactor of Moscow Engineering Physics Institute. IR absorption was measured at 300 K with a commercial Fourier-transform spectrometer. All other measurements were performed in the temperature range 80᎐300 K. Raman and most PL spectra were recorded with a confocal micro-Raman spectrometer using one of the lines Ž2.708, 2.602, 2.540, 2.497, 2.472 or 2.410 eV. from an Arq laser. PL was excited and detected normal to the sample’s surface. The spectrum of Fig. 5 was measured with a home-built spectrometer under polarized laser excitation at 2.708 eV. PL was excited normal to the surface and detected in the perpendicular direction. In PLE experiments the intensity of PL at fixed wavelength, selected by a monochromator, was measured as a function of excitation photon energy. Excitation was provided by a 250-W halogen lamp with a double monochromator in the range 2.45᎐3.3 eV or 150-W Xe lamp with a single monochromator in the range 3.3᎐6 eV. The whole PL spectrum was then measured at different PLE energies in order to check for the presence of underlying PL bands. In polarization measurements PL signals I < < and I H were detected at polarizations parallel and perpendicular to that of the exciting laser beam. The value Ps ŽI < < y I H .rŽI < < q I H . was used as a measure of the degree of polarization. Polarization response of the detection
Fig. 1. PL and PLE spectra from the H3 center measured at 80 K in HPHT Ib and natural Ia diamonds. Photoconductivity ŽPhC. spectrum w19x, measured on the same Ib sample at 300 K, is also shown for comparison. PLE was measured for 2.4 eV PL, PL was excited at 2.708 eV.
system was measured independently and taken into account.
3. Results 3.1. The H3 center and sub-threshold PL excitation Fig. 1 shows PL and PLE spectra from the H3 center measured at 80 K in HPHT Ib and natural Ia diamonds. The photoconductivity spectrum from Iakoubovskii and Adriaenssens w19x, which was measured on the same Ib sample at 300 K, is also shown for comparison. PLE was detected for the 2.4 eV emission, PL was excited at 2.708 eV. There is an evident deviation from the mirror symmetry between PL and PLE. It is explained w1x by the presence of an optically-inactive vibronic level 2.479 eV above the ground state. A threshold at 5.47 eV corresponds to the band-to-band excitation. A PLE threshold at 4.6 eV is seen in Ib but not in Ia diamond. The same PLE threshold was reported for the broad blue PL band w8,9x, but no such band could be detected in the HPHT samples studied here. Re-examination of those earlier data suggests that the tail of the H3 system, which can extend into the blue region at room temperature, might be responsible for the threshold observed in w8,9x. The spectra of Fig. 1 were reproduced on 10 natural diamonds with concentration of A and B nitrogen centers in the range 1᎐1000 ppm and on five HPHT crystals with concentration of P1 centers in the range 50᎐500 ppm. Fig. 2 presents absorption and PL, excited at 2.41 eV, from the H3 center at 300 and 120 K. At 300 K the H3 absorption extends below the 2.463 eV ŽZPL. energy, at 120 K the vibronic structure significantly sharpens and the absorption below 2.463 eV strongly decreases. This might suggest that light with energy less than 2.463 eV can excite the H3 luminescence and the
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
efficiency of such sub-threshold excitation should strongly decrease as temperature is lowered. PLE measurements at energies lower than ZPL are unreliable, because it is not certain of which PL line the excitation is monitored. Therefore, the H3 luminescence was excited with a 2.41 eV Arq line. The results in Fig. 2 show that the H3 PL can be excited at photon energy below the ZPL. The efficiency of excitation at 2.41 eV was approximately 10 times lower than at 2.54 eV. The decrease in temperature down to 80 K led to the sharpening and shift of ZPL, showing that the H3 centers cooled down along with the lattice, but the total intensity of the H3 system was almost temperature independent. The polarization degree P of the H3 ZPL, measured for several excitation energies in the range 2.410᎐2.708 eV, is shown by the open squares in Fig. 2. While the value of P is close to 0.5 in the range 2.71᎐2.5 eV, it is at least 10 times smaller at 2.41 eV. The value Ps 0.5 is consistent both with the theoretical prediction for a ²110: oscillator and with experimental results for the H3 luminescence w20x. The results on the sub-threshold excitation of the H3 center were reproduced on HPHT and Ia natural diamonds with different amount of A and B nitrogen centers. Only weak sample dependence was observed. Similar temperature-independent sub-threshold PL was also seen from the 3H ŽZPL at 2.463 eV. and H4 ŽZPL at 2.499 eV. centers and from a recently-characterized w7᎐9x TR12⬘ system ŽZPL at 2.65 eV. under 2.41 eV excitation in different samples. Dependence of the sub-threshold PL intensity on the excitation power, measured for these centers at 120 and 300 K, was linear or slightly sublinear depending on the sample. 3.2. The 3H center Fig. 3 presents PL and PLE spectra from the 3H
Fig. 2. Absorption and PL from the H3 center measured at 300 K Žthick lines. and 120 K Žthin lines.. PL was excited at 2.41 eV. Open squares show the polarization degree P of the H3 ZPL luminescence measured as a function of excitation energy.
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Fig. 3. PL and PLE spectra for the 3H center at 80 K. PL was excited at 2.709 eV, PLE was measured for the 2.463 eV emission. Sample is natural IIb diamond irradiated by 2 = 10 18 neutronsrcm 2 and annealed at 800⬚C for 1 h.
center at 80 K. PL was excited at 2.709 eV, PLE was measured for the 2.463 eV emission. The 3H center was produced in natural IIb diamond by neutron irradiation. The sample was annealed at 850⬚C for 1 h in order to remove an interfering TR12 system. This procedure does not anneal out the 3H luminescence system, but sharpens its ZPL w7x. The PLE spectrum shows peaks at 2.63, 2.68 and 3.02 eV, followed by a threshold at 3.85 eV. Details of the 3H vibronic structure observed in absorption and PL at 80 K and 300 K are shown in Fig. 4. Spectra are plotted as a function of a shift from the ZPL; zero corresponds to 2463 meV and 2459 meV for curves recorded at 80 K and 300 K, respectively. Absorption had to be recorded on as-irradiated sample, because it becomes undetectable after annealing at 350⬚C w7x. Structure above 170 meV could not be reliably linked to the 3H system because of the TR12 signals w4x. PL spectra were recorded after annealing Žthe same as for Fig. 3.. Narrow peaks at y169.3, y182.4 Žweak., y186.7 Žweak. and y217.8 meV in PL, and at 169 meV in absorption are local modes w5x. Both absorption and PL spectra show a weak temperature dependence: the decrease in temperature from 300 to 80 K only sharpens the ZPL and local vibronic mode lines and shifts them slightly towards higher energies. Vibronic peaks at 64 and 132 meV are seen in absorption, the 64-meV line is asymmetric and may be deconvoluted to the two Gaussian peaks centered at 60 and 70 meV with a halfwidth of 17 and 34 meV, as shown by dotted lines in Fig. 4. Along with the ZPL, three PL peaks are observed at y140, y70 and 60 meV with a width of approximately 20, 40 and 20 meV, respectively. The peak at 60 meV is unusual because it appears above the ZPL and its intensity is temperature independent. In order to check whether this peak belongs to the 3H system, PL spectra were measured under
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Fig. 4. Absorption Ždashed lines. and PL Žsolid lines. from the 3H center measured at 80 K Žthin lines. and 300 K Žthick lines.. Curves are plotted vs. a shift from ZPL, zero corresponds to 2463 meV for curves recorded at 80 K and to 2459 meV for those measured at 300 K. PL spectra measured at 150 and 200 K Žnot shown. were similar to the 80-K graph. Absorption was measured on as-irradiated IIb diamond. It was annealed at 850⬚C for 1 h prior to PL measurements. Absorption peak at 64 meV is deconvoluted to the two Gaussian lines at 60 and 70 meV.
different excitation energies in the range 2.41᎐5.5 eV, in different samples. The ratio of ZPL and the y70 meV vibronic line to the 60 meV peak was constant in all spectra, even under 2.472 meV excitation, which excited the zero-phonon and the y70 meV lines in the normal mode, but the 60 meV Ž2.523 eV. peak in the sub-threshold regime. Relying on this observation we suggest that the 2.523 eV line belongs to the 3H center. Fig. 5 shows PL spectra from the 3H center measured on the IIb crystal used for Figs. 3 and 4. PL was detected at polarizations parallel and perpendicular to that of 2.708-eV excitation beam. Spectral resolution was lowered to 35 meV in order to increase the signalto-noise ratio and the ZPL width is spectrally integrated. Rotation of the incident polarization in the
Fig. 5. PL spectra from the 3H center measured on the same IIb sample used for Figs. 3 and 4. PL was detected with a 35-meV spectral resolution at polarizations parallel and perpendicular to the linearly-polarized excitation beam of 2.708 eV energy. Rotation of the incident polarization in the ²100: plane resulted in the same shape of spectra recorded at polarizations parallel and perpendicular to the incident one.
Fig. 6. PL spectra measured at 80 K Žthin line. and 300 K Žthick lines. under 2.41 eV excitation in naturally-irradiated Australian diamonds.
²100: plane and acquisition of spectra at polarizations parallel and perpendicular to it resulted in the same spectra. From Fig. 5 it is seen that the non-phonon and y70 meV lines are partially polarized parallel to the exciting light, but that the 60 meV peak is mostly polarized perpendicular to it. The observation of a PL line at energy above the ZPL, which has a preferential polarization direction perpendicular to the ZPL and which has a weak temperature dependency, is unusual. However, the same behavior was observed for three more centers with ZPLs at 2.106, 1.886 and 1.768 eV. PL spectra from these centers recorded at 80 and 300 K under 2.41 eV excitation are plotted in Fig. 6. A temperature-independent line above the ZPL is observed in all these centers, and polarization PL measurements Žnot shown. reveal that this line is polarized perpendicular to the ZPL. These three centers are not commonly observed. They were found in approximately 25 of 50 natural diamonds from Argyle pipe. Absorption and PL measurements reveal that those diamonds are of mixed IaArIaB type and that they contain neutral vacancy ŽGR1. centers. The latter fact suggests that those diamonds were naturally irradiated at temperatures below 600⬚C such that the 2.106, 1.886 and 1.768 eV centers may be primary irradiation products. Annealing at 700⬚C for 2 h results in complete removal of the GR1 and of those three centers in agreement with the above hypothesis. These Argyle pipe diamonds were rather inhomogeneous and it was possible to ascertain whether the spectral features belong to the same center by scanning the crystal in the micro-Raman setup. 3.3. The green band II Prior to irradiation, the IIb diamond used for Figs. 4 and 5 showed only weak blue band emission with
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Fig. 7. Absorption Žcurves 1 and 2. and PL Žcurve 3. measured at 80 K on natural Ia Ž1 and 3. diamond from the lower mantle and on CVD Ž2. films. PL spectrum from CVD sample was similar to that presented by curve 3. Samples were irradiated by approximately 10 18 neutronsrcm 2 and annealed at 800⬚C for 2 h.
undetectable green band. After irradiation by 2 = 10 18 neutronsrcm 2 and annealing at 800⬚C the green band II emerged with enough strength to suppress the blue band. PL and PLE spectra were identical to those presented in our previous work w8,9x and therefore are not shown. 3.4. Si-related centers Fig. 7 presents absorption Žcurves 1 and 2. and PL spectra Žcurve 3. measured at 80 K on natural Ia Ž1 and 3. and on CVD Ž2. diamonds. Both samples were irradiated by 2 = 10 18 neutronsrcm 2 and annealed at 800⬚C for 2 h. Lines at 1.682, 1.691, 1.712, 1.744, 1.765 and 1.806 eV, similar to those reported in w18x, are seen in absorption, but only the 1.682 eV peak is PL-active. The 1.691 eV line is unobservable in CVD film. All these lines together produce a broad absorption band in the red part of the spectrum, which gives annealed crystals a greenish hue. Annealing at 1200⬚C for 2 h removes all those signals except for the 1.682 eV ŽSi-V. line, in agreement with results of Kiflawi et al. w18x. In the samples used in w18x the 1.682, 1.691 and 1.711 eV lines were sharper than in ours, and these lines were interpreted as ZPLs. The above Si-related lines were observed in six of 12 natural diamonds from Yakutia and Guinea. Details of the vibronic structure of the 1.682 eV center in CVD diamond recorded in absorption and PL at 80 K are presented in Fig. 8. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces of the same sample, which was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals w13,21x. Preliminary PL and absorption measurements performed on each individual piece confirmed similarity of their optical properties. PL was measured on a single piece and therefore, the effect of self-absorption should be negligible. In Fig. 8 a weak PL and absorption structure above
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the Si-V ZPL could be resolved after subtraction of underlying broadband signals. Peaks y42, 30, 85, 126, 156 and 220 meV away from the ZPL are seen in the absorption spectrum. The y42 meV Ž1.64 eV. line was first mentioned in w11x as unrelated to the Si-V feature. Here we confirm this result: as it was previously reported w21x, in a series of CVD diamond films grown in the same run, an increase in amorphous carbon related defects leads to the decrease in the Si-V absorption. However, in the same series the amplitude of the 1.64 eV peak was constant. Moreover, UV illumination may lead to the strong Žup to 10 times. increase in the Si-V absorption w13,21x, but it has a minor effect on the 1.64 eV line. All this may indicate that the defect responsible for the 1.64 eV line has a ground state well below that of the Si-V. The PL spectrum of Fig. 8 shows a series of signals at y154, y126, y85, y65, y42, 85, 126, 156, 200 and 220 meV. Remarkable is that the intensity ratio of the high-energy to the low-energy lines is similar at 80 K Žthis work. and 10 K w22x. It was suggested w22x that the origin of the high-energy peaks is an anti-Stokes PL from the Si-V center. This could be one more example of temperature-independent anti-Stokes PL, but we should note that the 126, 156 and 220 meV peaks were also observed in PL from CVD films with undetectable Si-V absorption but with a series of strong lines attributed to the Ni-V center w8,9,23x. Part of that Ni-V PL spectrum, recorded at the same spectral resolution Ž10 meV. is shown by curve R in Fig. 8. Moreover, a peak at 200 meV, which was also reported in w22x, might well be a vibronic feature of the 1.945 center. Therefore, despite the y126 and y156 meV PL peaks
Fig. 8. Absorption and PL spectra from the Si-V center, measured at 80 K in a CVD diamond film. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces after it was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals. PL was measured under 2.54 eV excitation on a single piece. The y-axis roughly corresponds to absorption coefficient in cmy1 for absorption curve. Curve R presents a fragment of PL spectrum from a CVD film, where the Si-V center was relatively weak, but the ‘Ss 1’ center attributed to a Ni-V center w8,9,23x was exceptionally strong. Underlying broad background is subtracted from all curves.
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K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
the spectra measured before and after annealing are shown in Fig. 10. It is seen that the annealing resulted in the decrease in broad underlying signals at y19 meV ŽPL. and 12 meV Žabsorption..
4. Proposed interpretation of experimental results 4.1. The H3 center and sub-threshold PL excitation
Fig. 9. PL excited at 2.54 eV at 80 K from CVD diamond. The width of diamond Raman peak is 2, 3, 6 cmy1 for films 1, 2, 3, respectively, that indicates an increase in stress from film 1 to 3.
being attributed to the Si-V system w17x, the assignment of the 126, 156, 200 and 220 meV peaks to the same system seems not certain. Measurements on a large Ž) 50. series of CVD films reveal differences in PL structure around the Si-V ZPL. An example is given in Fig. 9, where PL spectra recorded at 80 K under 2.54 eV excitation are shown. The three films used for the figure were grown at similar conditions, but they have a different width of the diamond Raman peak, 2, 3 and 6 cmy1 for samples 1, 2 and 3, respectively. It is seen that a broad underlying band increases for films with a wider Raman peak, i.e. with a larger stress w24x. Similar conclusions can be drawn from the results of Sharda et al. w25,26x. Film 2 was further annealed at 1400⬚C for 2 h in a vacuum 10y8 torr. Absorption, PL and the difference between
Fig. 10. PL Žcurves 1᎐3. and absorption Žcurves 1⬘᎐3⬘. measured at 80 K in the same CVD diamond sample before Ž1, 1⬘. and after Ž2, 2⬘. annealing at 1400⬚C for 2 h in 10y8 torr. Curve 3 Ž3⬘. is a difference between the curves 1 Ž1⬘. and 2 Ž2⬘.. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces after it was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals. PL was measured under 2.54 eV excitation on a single piece. Underlying broad background is subtracted from all curves.
The shape of the 4.6-eV threshold in Fig. 1 is typical for an optical transition between a defect-induced level and the conduction or valence band. Such transition can excite a ZPL if it ionizes an electron from the center to the conduction band and the generated electron is then rapidly captured by an excited state of the same center. Then the PLE threshold should be present in any diamond with the H3 centers. On the contrary, it is observed only in diamonds containing single substitutional nitrogen. Those crystals also show the photocurrent threshold whose shape and position are remarkably similar to those of the 4.6-eV PLE feature suggesting their common origin. That photocurrent threshold is shown to be due to ionization of holes from the positively-charged P1 center ŽP1q. to the valence band w19x. Relying on the above observations we interpret the PLE results of Fig. 1 as indication that the H3 center can be excited via the ionization of the P1q center. The mechanism of such excitation is not clear. In order to return to equilibrium, the ionized P1q Žor P10 . center should re-trap a hole. It may be possible that the energy released from the P10 ª P1q relaxation excites the H3 centers. Remarkable is the observation of the temperatureindependent sub-threshold excitation of the H3 center. Fig. 2 reveals that while the PL excited at energies above ZPL preserves the polarization of incident light, polarization is almost lost for the sub-threshold excitation. This may indicate that the sub-threshold PL is generated via an energy transfer from some defect center, different from H3, and the polarization is lost during that process. Probably the center responsible for excitation of the sub-threshold PL is the one, which dominates absorption in the sample, i.e. P1 in Ib and A nitrogen in IaA diamond. However, one may also consider the following mechanism: The 2.41 eV light could ionize the H2 center ŽwN-V-Nxy with a proposed ground state at EC y 2.1 eV w3x. to the conduction band. It is possible that the ionization of the wN-V-Nxy converts it to the excited wN-V-Nx 0 , and polarization is lost during this process. With respect to this proposal it is worth noting that electron spin resonance measurements reveal w27x that the 5A 2 excited state of the neutral vacancy ŽV 0 . in diamond is produced via ionization of Vy rather than via internal excitation at V 0 . The above discussion may also be valid for the
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
observed sub-threshold excitation of PL from the H4, 3H and TR12⬘ centers. Unlike for the H3, no differently charged states have been detected for these centers yet. This, however, is by no means proof of their non-existence.
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centered at 60 meV Ž17 meV width. and at 70 meV Ž35 meV width.. The first peak is seen at the same energy in PL and it may be a ZPL due to the G ª E2 transition. The second has a mirror image at y70 meV in PL and is possibly a vibronic feature of the G᎐E1 transition.
4.2. The 3H center and sub-threshold PL emission 4.3. Green band II The above discussion of the H3 PLE spectrum suggests that the 3.85-eV threshold in the PLE spectrum of the 3H center ŽFig. 3. currently cannot be assigned to any defect center. The peaks at 2.63, 2.68 and 3.02 eV may be ascribed to the excited states of the 3H center. The 2.63-eV peak was also observed in absorption ŽFig. 4.. An interesting observation is the 2.523 eV line at the 3H center, whose emission is polarized perpendicular to the ZPL, and whose intensity is temperature independent in the range 80᎐300 K. The easiest explanation would be that the intensity correlation of the 2.523 and 2.463 eV lines is fortuitous and the 2.523 eV feature originates from a center relative to 3H. However, a very similar temperature-independent feature at above ZPL energy was also observed in at least six other centers: those with ZPL at 2.106, 1.886, 1.768 eV ŽFig. 6. and 1.950 eV w28x; TR12 and TR12⬘ w7᎐9,28x. Remarkably, all those centers are irradiation induced. The above observation suggests that some common phenomenon takes place in all those centers, a model for which is proposed below on the basis of the results from the 3H center. We suggest that the 2.523 eV line belongs to the 3H center and originates from radiative recombination between the level Žwhich we call E2. at 60 meV above the first excited state ŽE1. and the ground state ŽG.; the E2 level corresponds to an excited state of the 3H center, which has orientation of electric dipole perpendicular to the one of ZPL. This can be imagined as the 2.463 eV ŽZPL. and the 2.532 eV transitions having Zand XY-dipoles, respectively. Different orientations of those dipoles can explain the weak temperature dependence of the E2 ª G transition: although the energy distance between the E2 and E1 states is within a phonon quantum, the E2 ª E1 transition may be forbidden by symmetry and almost no thermalization then occurs between E2 and E1. It is worth noting that the existence of excited states with perpendicularly-oriented electric dipoles was also suggested for the positively charged interstitial Ni center in diamond w29x. However, the splitting between those lines was 2.7 meV and the ratio of their intensities was temperature activated. It is possible that the asymmetric 64-meV peak in the absorption spectrum of Fig. 4 is a sum of two lines
The change in the Fermi level position can have a drastic effect on the intensity of optical signals w13,21x. Before irradiation, the Fermi level in the IIb diamond should be at the boron acceptor center Ž E V q 0.37 eV w4x.. After irradiation and annealing of the IIb diamond the green band II could be detected under sub-bandgap excitation. Such excitation can induce electronic transitions from the valence band to the donor level. Holes generated in the valence band may be captured by the boron acceptors. Subsequent transition of an electron from an occupied donor to an unoccupied acceptor then produces the green band II w8᎐10x. In this model a shift of the Fermi level down from the E V q 0.37 position would increase the green band II PL, because the hole does not need to be captured by the acceptor any more, and an upward shift would decrease the green band because some donor levels become occupied. Resistivity and IR measurements show that irradiation and annealing of the IIb sample leads to the compensation of boron acceptors by irradiation-induced donors and consequently the Fermi level is moved up, but the strong increase in the green band is observed. These changes can hardly be explained by a Fermi level shift. An alternative explanation is that irradiation and annealing produce the donors responsible for the green band II. They are produced in a rather pure sample, and one may expect them to be structural defects. One more observation is in agreement with this suggestion: Exactly the same absorption threshold at 3.25 eV, which is responsible for the excitation of the green band II w8,9x was observed in our study Žunpublished. of a series of approximately 50 small Ž- 1 mm. natural diamonds from Venezuela. Microscopic examination revealed that all these diamonds had abundant extended structural defects. 4.4. Si-related centers Results shown in Figs. 7᎐10 demonstrate that identification of the vibronic structure of the Si-V center is not straightforward. Data presented in Fig. 10 may suggest that a peak at 1.679 eV, which often lies under the Si-V spectrum is not an independent center, but a
26
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
stress-induced feature at the Si-V center. The 30-meV absorption peak has a position of 1.712 eV, the same as for a center shown in Fig. 7. The intensity of the former was unchanged by annealing at 1400⬚C, but annealing at 1200⬚C completely removed the latter. Thus, we assign the 1.712 eV peak in Figs. 8 and 10 to a vibronic feature at the Si-V center. Remarkable is that this peak has no exact counterpart in PL and probably corresponds to the y42 meV PL line. The intensity ratio of the 30-meV to the non-phonon line is 0.05, which is approximately five times smaller than reported in w11x. However, the sample measured in w11x was electron irradiated and annealed at 800⬚C. This treatment is favorable for production of a 1.712 eV absorption line w18x, and these two centers could interfere. Indeed, our absorption measurements on CVD diamond, which was irradiated and annealed at 800⬚C Žcurve 2 of Fig. 7., show that after such treatment the 1.712 and 1.682 eV peaks can even have comparable intensities. 5. Summary and conclusions Combined application of PL, PLE and optical absorption spectroscopies to natural and synthetic diamond has provided new information on the 3H, H3 and Si-related centers. Si-related centers were also observed in natural Ia diamond after neutron irradiation and annealing. Excited states at 2.63, 2.68 and 3.02 eV above the ground state are detected for the 3H center. The importance of energy transfer processes between different defect centers in excitation of internal ZPL transitions is revealed. In particular, the 4.6-eV threshold in the PLE spectrum of the H3 center is assigned to ionization of holes from the P1q center followed by an energy transfer from the P1 to the H3 centers. Such processes might also be responsible for the observed temperature-independent sub-threshold PL from the H3, H4, 3H and TR12⬘ centers. Results of polarization-resolved PL measurements suggest that several centers in diamond, in particular the 3H, TR12, TR12⬘ and previously undocumented irradiation-related centers with ZPLs at 2.106, 1.886 and 1.768 eV, have excited states with mutually perpendicular orientation of their dipole moments. Acknowledgements Provision of CVD diamond films by M. Nesladek, IMO, LUC, Belgium is gratefully acknowledged. The authors are indebted to both referees for helpful comments, to W. Deweerd for high-temperature annealing
and to the FWO Žgrant G.0014.96. for financial support.
References
w1x G. Davies, M.H. Nazare, M.F. Hammer, Proc. R. Soc. A 351 Ž1976. 245. w2x A.T. Collins, J. Phys. C 16 Ž1983. 6691. w3x Y. Mita, H. Kanehara, Y. Adachi, Y. Nishida, M. Okada, M. Kobayashi, J. Phys.: Condens. Matter 2 Ž1990. 8567. w4x J. Walker, Rep. Prog. Phys. 42 Ž1979. 1605. w5x J.W. Steeds, T.J. Davis, S.J. Charles, J.M. Hayes, J.E. Butler, Diamond Relat. Mater. 8 Ž1999. 1847. w6x G. Davies, Physica B 273r274 Ž1999. 15. w7x K. Iakoubovskii, G.J. Adriaenssens, Phys. Stat. Solidi Ža., 181 Ž2000. in press. w8x K. Iakoubovskii, G.J. Adriaenssens, Phys. Rev. B 61 Ž2000. 10174. w9x K. Iakoubovskii, G.J. Adriaenssens, Diamond Relat. Mater. 9 Ž2000. 1017. w10x K. Watanabe, S.C. Lawson, J. Isoya, H. Kanda, Y. Sato, Diamond Relat. Mater. 6 Ž1997. 99. w11x A.T. Collins, L. Allers, C.J.H. Wort, G.A. Scarsbrook, Diamond Relat. Mater. 3 Ž1992. 932. w12x C.D. Clark, H. Kanda, I. Kiflawi, G. Sittas, Phys. Rev. B 51 Ž1995. 16681. w13x K. Iakoubovskii, G.J. Adriaenssens, M. Nesladek, J. Phys.: Condens. Matter 12 Ž2000. 189. w14x S.W. Brown, S.C. Rand, J. Appl. Phys. 78 Ž1995. 4069. w15x J.P. Goss, R. Jones, S.J. Breuer, P.R. Briddon, S. Oberg, Phys. Rev. Lett. 77 Ž1996. 3041. w16x L. Bergman, R.J. Nemanich, J. Appl. Phys. 78 Ž1995. 6709. w17x A.A. Gorokhovsky, A.V. Turukhin, R.R. Alfano, W. Phillips, Appl. Phys. Lett. 66 Ž1995. 43. w18x I. Kiflawi, G. Sittas, H. Kanda, D. Fisher, Diamond Relat. Mater. 6 Ž1997. 146. w19x K. Iakoubovskii, G.J. Adriaenssens, J. Phys.: Condens. Matter 12 Ž2000. L77. w20x C.D. Clark, G.W. Maycraft, E.W.J. Mitchell, J. Appl. Phys. 33 Ž1962. 378. w21x K. Iakoubovskii, G.J. Adriaenssens, Diamond Relat. Mater. 9 Ž2000. 1349. w22x T. Feng, B.D. Schwartz, J. Appl. Phys. 73 Ž1993. 1415. w23x K. Iakoubovskii, A. Stesmans, B. Nouwen, G.J. Adriaenssens, submitted. w24x K. Iakoubovskii, A. Stesmans, G.J. Adriaenssens, R. Provoost, Silverans, V. Raiko, Phys. Stat. Solidi Ža. 174 Ž1999. 137. w25x T. Sharda, A. Vaidya, D.S. Misra et al., J. Appl. Phys. 83 Ž1998. 1120. w26x T. Sharda, A. Vaidya, D.S. Misra et al., Diamond Relat. Mater. 7 Ž1998. 250. w27x J.A. van Wyk, O.D. Tucker, M.E. Newton, J.M. Baker, G.S. Woods, P. Spear, Phys. Rev. B 52 Ž1995. 12657. w28x J.W. Steeds, S.J. Charles, T.J. Davis, I. Griffin, Diamond Relat. Mater. 9 Ž2000. 397. w29x A.T. Collins, J. Phys.: Condens. Matter 1 Ž1989. 439.
Optical characterization of some irradiation-induced centers in diamond K. Iakoubovskii a,U , G.J. Adriaenssens a , N.N. Dogadkinb, A.A. Shiryaev b a
Laboratorium ¨ oor Halfgeleiderfysica, Katholieke Uni¨ ersiteit Leu¨ en, Celestijnenlaan 200 D, B-3001 Leu¨ en, Belgium b Institute of Geochemistry, Kosygin st. 19, B-334 Moscow, Russia Accepted 18 August 2000
Abstract Photoluminescence ŽPL., PL excitation and optical absorption techniques were applied to the 3H, H3 and Si-related centers in natural and synthetic diamond. Thresholds at 4.6 and 3.85 eV are observed in the excitation spectra of the H3 and 3H luminescence, respectively. The 4.6-eV threshold could only be observed if single substitutional nitrogen ŽP1. centers were present in the sample. It is ascribed to the ionization of the P1q centers followed by an energy transfer from the P1 to the H3 centers. The energy transfer from other defects can also account for the temperature-independent PL, which was observed from the 3H, H3, H4 and TR12⬘ centers when they were excited at energies below the zero-phonon line. Si-related centers are detected in natural Ia diamond after neutron irradiation and annealing. Details of the vibronic structure of the Si-V center in diamond grown by chemical vapor deposition are reported. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond defects; Luminescence; Impurities; Micro-Raman spectroscopy
1. Introduction Many defects in diamond have ground and excited states located within the forbidden gap. Optical transitions between those states may produce zero-phonon lines ŽZPLs. with associated vibronic structures, which can be detected in absorption andror luminescence. These lines often have a width of a few meV or less and are mostly present in the visible part of the spectrum, for which sensitive optical detectors are available. Consequently, their intensity and position can be measured with a high accuracy and used for identification and characterization of a defect. However, inter-
U
Corresponding author. Fax: q32-1632-7987. E-mail address: [email protected] ŽK. Iakoubovskii..
pretation of the obtained spectra is often not straightforward and it is much facilitated if the same defect center is studied by several techniques in different types of samples. This was done in the present work, where photoluminescence ŽPL., PL excitation ŽPLE. and optical absorption were applied to the 3H, H3 and silicon-vacancy ŽSi-V. centers in natural and synthetic diamonds. 1.1. The H3 center The defect responsible for the H3 center consists of two next-nearest-neighbor substitutional nitrogen atoms separated by a vacancy w1x. It is produced if a nearest-neighbor nitrogen pair Žthe A center. captures a vacancy. The H3 center has C 2v symmetry with a ²110: symmetry axis. Two charge states are known,
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 0 . 0 0 3 6 1 - 7
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
neutral ŽwN-V-Nx 0 . and negative ŽwN-V-Nxy. . Optical transitions between the ground state and the first excited state of the wN-V-Nx 0 produce a system, labeled H3, of ZPL at 2.463 eV and associated vibronic band w1x. The excitation spectrum of the H3 emission contains several non-luminescent lines around 3.4 eV Žthe so-called H13 system., which are attributed w2x to the transitions from the ground state to the higher-lying excited states at the wN-V-Nx0 . An absorption and PL system H2, with a ZPL at 1.257 eV, is associated w3x with wN-V-Nxy. Illumination with light of energy above 2.1 eV converts wN-V-Nxy to wN-V-Nx0 . Therefore, it was suggested w3x that the ground state of wN-V-Nxy is located 2.1 eV below the conduction band edge Žat EC y 2.1 eV.. The position in the forbidden gap of the ground state of the wN-V-Nx 0 is not identified yet. Here we report on an additional transition observed at 4.6 eV in the excitation spectra of the H3 luminescence. This transition is seen only if the single substitutional nitrogen ŽP1. center is present in the sample, and it is interpreted in terms of an energy transfer from the P1 to the H3 center. Moreover, it is shown that the H3 and some other centers ŽH4, 3H and TR12⬘. can be excited by light with energy less than ZPL and the efficiency of such excitation may be weakly temperature dependent. A mechanism for this sub-threshold PL is proposed.
1.2. The 3H center
The defect responsible for the 3H center is believed to be a primary irradiation product. Similarly to the H3, it has C 2v symmetry with a ²110: axis and a ZPL at 2.463 eV w4x. However, contrary to the H3, the 3H center has a very weak vibronic structure with sharp local mode peaks at 169.3, 182.4, 186.7 and 217.8 meV w5x. Presence of a local mode is characteristic for interstitial-related centers w6x. Moreover, these single local mode peaks split into triplets in diamond containing comparable amounts of 12 C and 13 C isotopes, indicating that only two carbon atoms are involved in the vibrations. Therefore, it was suggested that the 3H is a split interstitial w5x. This center shows complex annealing behavior: most of its absorption disappears already at 400⬚C, but complete annealing occurs only in the temperature range 900᎐1000⬚C w7x. In this paper the PLE spectrum from the 3H center is reported. Peaks at 2.63, 2.68, 3.02 eV and a threshold at 3.85 eV are detected. The first three peaks are attributed to the excited states of the 3H center. An additional weak PL feature at 2.523 eV is assigned to the 3H spectrum. Remarkably, this line has a preferen-
19
tial direction of polarization perpendicular to the one of the main transitions, and its intensity is almost temperature independent. Similar behavior is observed for some other centers in naturally-irradiated Ia diamond. 1.3. The green band II Many diamonds show a featureless broadband green emission. Recent PLE results w8,9x show that it can consist of two bands. The green band I is ascribed to radiative transitions at the amorphous carbon inclusions present in diamond grown by chemical vapor deposition ŽCVD.. Band II is attributed to the donor᎐acceptor pair recombination, where the acceptor is boron and the donor has an optical ionization threshold to the valence band at 3.25 eV; the donor’s origin being unknown. This band may be observed in as-grown CVD and high-pressure᎐high-temperature ŽHPHT. synthetic diamonds, boron- or nitrogen-doped w8᎐10x. Here we report the observation that the green band II is produced in IIb Žboron-rich. natural diamond by neutron irradiation and annealing at 800⬚C and suggest that the donor responsible for this band may be a structural defect. 1.4. The Si-V center This center is observed in any CVD or HPHT diamond if silicon was present in the growth reactor, but it has not yet been detected in natural diamond. Despite a large number of reports, only fragmentary details of its structure are known: it probably contains one Si atom and a vacancy w11,12x, has a ground state at EC y 2.05 eV w8,9,13x, and a ²110: C 2 symmetry axis w14x. Calculation suggests that it consists of a Si atom at the center of a divacancy w15x, but this structure has not yet been confirmed by experiment. The Si-V center has a ZPL at 1.682 eV with a very weak vibronic structure and it is often accompanied w16x by a broader and weaker line at 1.679 eV whose origin is unknown. The details of the Si-V PL spectrum are thoroughly characterized w12,17x, however, only two reports are known on Si-V absorption w11,13x. Meanwhile, two important diamond centers have ZPLs close to that of the Si-V, the neutral vacancy ŽGR1, ZPL at 1.673 eV. and the ²100: split interstitial ŽR2, lines at 1.685 and 1.86 eV. w6x. Moreover, three other Si-related centers at 1.679, 1.691 and 1.711 eV were detected in absorption from Si-doped HPHT diamond w18x. Clearly, absorption study of these centers is required to reveal their possible interrelation. Consequently, we also examined absorption and PL of the Si-related centers. It is suggested that one of the
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K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
1.679 eV lines is a stress-induced vibronic transition at the Si-V center. In addition, three Si-related centers are detected in neutron irradiated natural Ia diamond annealed at 800⬚C.
2. Experimental CVD diamond films were grown on Si substrates in an ASTeX PDS-17 microwave plasma reactor. Typical deposition parameters were: microwave power 5 kW, gas flow rate 300 cm3rmin, substrate temperature 800⬚C, feed gas mixture 99:1 H 2 :CH 4 . The substrate was removed by etching in HF:HNO3 mixture. Freestanding polycrystalline films composed of randomly oriented grains have been studied. ²100: oriented natural IIb and synthetic Ib crystals were purchased from Drukker International and De Beers, respectively. The IIb diamond used for this paper contained 0.2 ppm of uncompensated boron acceptors, as deduced from IR transmittance spectra. Several groups of natural Ia diamonds from Australia ŽArgyle pipe., Yakutia, Guinea Žlower mantle, below 600 km from the surface . and Venezuela Žplacers. have been made available by the Institute of Geochemistry, Moscow. Some natural and CVD diamonds were irradiated by an integrated dose of 2 = 10 18 neutronsrcm 2 at nominally room temperature at the Research Reactor of Moscow Engineering Physics Institute. IR absorption was measured at 300 K with a commercial Fourier-transform spectrometer. All other measurements were performed in the temperature range 80᎐300 K. Raman and most PL spectra were recorded with a confocal micro-Raman spectrometer using one of the lines Ž2.708, 2.602, 2.540, 2.497, 2.472 or 2.410 eV. from an Arq laser. PL was excited and detected normal to the sample’s surface. The spectrum of Fig. 5 was measured with a home-built spectrometer under polarized laser excitation at 2.708 eV. PL was excited normal to the surface and detected in the perpendicular direction. In PLE experiments the intensity of PL at fixed wavelength, selected by a monochromator, was measured as a function of excitation photon energy. Excitation was provided by a 250-W halogen lamp with a double monochromator in the range 2.45᎐3.3 eV or 150-W Xe lamp with a single monochromator in the range 3.3᎐6 eV. The whole PL spectrum was then measured at different PLE energies in order to check for the presence of underlying PL bands. In polarization measurements PL signals I < < and I H were detected at polarizations parallel and perpendicular to that of the exciting laser beam. The value Ps ŽI < < y I H .rŽI < < q I H . was used as a measure of the degree of polarization. Polarization response of the detection
Fig. 1. PL and PLE spectra from the H3 center measured at 80 K in HPHT Ib and natural Ia diamonds. Photoconductivity ŽPhC. spectrum w19x, measured on the same Ib sample at 300 K, is also shown for comparison. PLE was measured for 2.4 eV PL, PL was excited at 2.708 eV.
system was measured independently and taken into account.
3. Results 3.1. The H3 center and sub-threshold PL excitation Fig. 1 shows PL and PLE spectra from the H3 center measured at 80 K in HPHT Ib and natural Ia diamonds. The photoconductivity spectrum from Iakoubovskii and Adriaenssens w19x, which was measured on the same Ib sample at 300 K, is also shown for comparison. PLE was detected for the 2.4 eV emission, PL was excited at 2.708 eV. There is an evident deviation from the mirror symmetry between PL and PLE. It is explained w1x by the presence of an optically-inactive vibronic level 2.479 eV above the ground state. A threshold at 5.47 eV corresponds to the band-to-band excitation. A PLE threshold at 4.6 eV is seen in Ib but not in Ia diamond. The same PLE threshold was reported for the broad blue PL band w8,9x, but no such band could be detected in the HPHT samples studied here. Re-examination of those earlier data suggests that the tail of the H3 system, which can extend into the blue region at room temperature, might be responsible for the threshold observed in w8,9x. The spectra of Fig. 1 were reproduced on 10 natural diamonds with concentration of A and B nitrogen centers in the range 1᎐1000 ppm and on five HPHT crystals with concentration of P1 centers in the range 50᎐500 ppm. Fig. 2 presents absorption and PL, excited at 2.41 eV, from the H3 center at 300 and 120 K. At 300 K the H3 absorption extends below the 2.463 eV ŽZPL. energy, at 120 K the vibronic structure significantly sharpens and the absorption below 2.463 eV strongly decreases. This might suggest that light with energy less than 2.463 eV can excite the H3 luminescence and the
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
efficiency of such sub-threshold excitation should strongly decrease as temperature is lowered. PLE measurements at energies lower than ZPL are unreliable, because it is not certain of which PL line the excitation is monitored. Therefore, the H3 luminescence was excited with a 2.41 eV Arq line. The results in Fig. 2 show that the H3 PL can be excited at photon energy below the ZPL. The efficiency of excitation at 2.41 eV was approximately 10 times lower than at 2.54 eV. The decrease in temperature down to 80 K led to the sharpening and shift of ZPL, showing that the H3 centers cooled down along with the lattice, but the total intensity of the H3 system was almost temperature independent. The polarization degree P of the H3 ZPL, measured for several excitation energies in the range 2.410᎐2.708 eV, is shown by the open squares in Fig. 2. While the value of P is close to 0.5 in the range 2.71᎐2.5 eV, it is at least 10 times smaller at 2.41 eV. The value Ps 0.5 is consistent both with the theoretical prediction for a ²110: oscillator and with experimental results for the H3 luminescence w20x. The results on the sub-threshold excitation of the H3 center were reproduced on HPHT and Ia natural diamonds with different amount of A and B nitrogen centers. Only weak sample dependence was observed. Similar temperature-independent sub-threshold PL was also seen from the 3H ŽZPL at 2.463 eV. and H4 ŽZPL at 2.499 eV. centers and from a recently-characterized w7᎐9x TR12⬘ system ŽZPL at 2.65 eV. under 2.41 eV excitation in different samples. Dependence of the sub-threshold PL intensity on the excitation power, measured for these centers at 120 and 300 K, was linear or slightly sublinear depending on the sample. 3.2. The 3H center Fig. 3 presents PL and PLE spectra from the 3H
Fig. 2. Absorption and PL from the H3 center measured at 300 K Žthick lines. and 120 K Žthin lines.. PL was excited at 2.41 eV. Open squares show the polarization degree P of the H3 ZPL luminescence measured as a function of excitation energy.
21
Fig. 3. PL and PLE spectra for the 3H center at 80 K. PL was excited at 2.709 eV, PLE was measured for the 2.463 eV emission. Sample is natural IIb diamond irradiated by 2 = 10 18 neutronsrcm 2 and annealed at 800⬚C for 1 h.
center at 80 K. PL was excited at 2.709 eV, PLE was measured for the 2.463 eV emission. The 3H center was produced in natural IIb diamond by neutron irradiation. The sample was annealed at 850⬚C for 1 h in order to remove an interfering TR12 system. This procedure does not anneal out the 3H luminescence system, but sharpens its ZPL w7x. The PLE spectrum shows peaks at 2.63, 2.68 and 3.02 eV, followed by a threshold at 3.85 eV. Details of the 3H vibronic structure observed in absorption and PL at 80 K and 300 K are shown in Fig. 4. Spectra are plotted as a function of a shift from the ZPL; zero corresponds to 2463 meV and 2459 meV for curves recorded at 80 K and 300 K, respectively. Absorption had to be recorded on as-irradiated sample, because it becomes undetectable after annealing at 350⬚C w7x. Structure above 170 meV could not be reliably linked to the 3H system because of the TR12 signals w4x. PL spectra were recorded after annealing Žthe same as for Fig. 3.. Narrow peaks at y169.3, y182.4 Žweak., y186.7 Žweak. and y217.8 meV in PL, and at 169 meV in absorption are local modes w5x. Both absorption and PL spectra show a weak temperature dependence: the decrease in temperature from 300 to 80 K only sharpens the ZPL and local vibronic mode lines and shifts them slightly towards higher energies. Vibronic peaks at 64 and 132 meV are seen in absorption, the 64-meV line is asymmetric and may be deconvoluted to the two Gaussian peaks centered at 60 and 70 meV with a halfwidth of 17 and 34 meV, as shown by dotted lines in Fig. 4. Along with the ZPL, three PL peaks are observed at y140, y70 and 60 meV with a width of approximately 20, 40 and 20 meV, respectively. The peak at 60 meV is unusual because it appears above the ZPL and its intensity is temperature independent. In order to check whether this peak belongs to the 3H system, PL spectra were measured under
22
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
Fig. 4. Absorption Ždashed lines. and PL Žsolid lines. from the 3H center measured at 80 K Žthin lines. and 300 K Žthick lines.. Curves are plotted vs. a shift from ZPL, zero corresponds to 2463 meV for curves recorded at 80 K and to 2459 meV for those measured at 300 K. PL spectra measured at 150 and 200 K Žnot shown. were similar to the 80-K graph. Absorption was measured on as-irradiated IIb diamond. It was annealed at 850⬚C for 1 h prior to PL measurements. Absorption peak at 64 meV is deconvoluted to the two Gaussian lines at 60 and 70 meV.
different excitation energies in the range 2.41᎐5.5 eV, in different samples. The ratio of ZPL and the y70 meV vibronic line to the 60 meV peak was constant in all spectra, even under 2.472 meV excitation, which excited the zero-phonon and the y70 meV lines in the normal mode, but the 60 meV Ž2.523 eV. peak in the sub-threshold regime. Relying on this observation we suggest that the 2.523 eV line belongs to the 3H center. Fig. 5 shows PL spectra from the 3H center measured on the IIb crystal used for Figs. 3 and 4. PL was detected at polarizations parallel and perpendicular to that of 2.708-eV excitation beam. Spectral resolution was lowered to 35 meV in order to increase the signalto-noise ratio and the ZPL width is spectrally integrated. Rotation of the incident polarization in the
Fig. 5. PL spectra from the 3H center measured on the same IIb sample used for Figs. 3 and 4. PL was detected with a 35-meV spectral resolution at polarizations parallel and perpendicular to the linearly-polarized excitation beam of 2.708 eV energy. Rotation of the incident polarization in the ²100: plane resulted in the same shape of spectra recorded at polarizations parallel and perpendicular to the incident one.
Fig. 6. PL spectra measured at 80 K Žthin line. and 300 K Žthick lines. under 2.41 eV excitation in naturally-irradiated Australian diamonds.
²100: plane and acquisition of spectra at polarizations parallel and perpendicular to it resulted in the same spectra. From Fig. 5 it is seen that the non-phonon and y70 meV lines are partially polarized parallel to the exciting light, but that the 60 meV peak is mostly polarized perpendicular to it. The observation of a PL line at energy above the ZPL, which has a preferential polarization direction perpendicular to the ZPL and which has a weak temperature dependency, is unusual. However, the same behavior was observed for three more centers with ZPLs at 2.106, 1.886 and 1.768 eV. PL spectra from these centers recorded at 80 and 300 K under 2.41 eV excitation are plotted in Fig. 6. A temperature-independent line above the ZPL is observed in all these centers, and polarization PL measurements Žnot shown. reveal that this line is polarized perpendicular to the ZPL. These three centers are not commonly observed. They were found in approximately 25 of 50 natural diamonds from Argyle pipe. Absorption and PL measurements reveal that those diamonds are of mixed IaArIaB type and that they contain neutral vacancy ŽGR1. centers. The latter fact suggests that those diamonds were naturally irradiated at temperatures below 600⬚C such that the 2.106, 1.886 and 1.768 eV centers may be primary irradiation products. Annealing at 700⬚C for 2 h results in complete removal of the GR1 and of those three centers in agreement with the above hypothesis. These Argyle pipe diamonds were rather inhomogeneous and it was possible to ascertain whether the spectral features belong to the same center by scanning the crystal in the micro-Raman setup. 3.3. The green band II Prior to irradiation, the IIb diamond used for Figs. 4 and 5 showed only weak blue band emission with
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
Fig. 7. Absorption Žcurves 1 and 2. and PL Žcurve 3. measured at 80 K on natural Ia Ž1 and 3. diamond from the lower mantle and on CVD Ž2. films. PL spectrum from CVD sample was similar to that presented by curve 3. Samples were irradiated by approximately 10 18 neutronsrcm 2 and annealed at 800⬚C for 2 h.
undetectable green band. After irradiation by 2 = 10 18 neutronsrcm 2 and annealing at 800⬚C the green band II emerged with enough strength to suppress the blue band. PL and PLE spectra were identical to those presented in our previous work w8,9x and therefore are not shown. 3.4. Si-related centers Fig. 7 presents absorption Žcurves 1 and 2. and PL spectra Žcurve 3. measured at 80 K on natural Ia Ž1 and 3. and on CVD Ž2. diamonds. Both samples were irradiated by 2 = 10 18 neutronsrcm 2 and annealed at 800⬚C for 2 h. Lines at 1.682, 1.691, 1.712, 1.744, 1.765 and 1.806 eV, similar to those reported in w18x, are seen in absorption, but only the 1.682 eV peak is PL-active. The 1.691 eV line is unobservable in CVD film. All these lines together produce a broad absorption band in the red part of the spectrum, which gives annealed crystals a greenish hue. Annealing at 1200⬚C for 2 h removes all those signals except for the 1.682 eV ŽSi-V. line, in agreement with results of Kiflawi et al. w18x. In the samples used in w18x the 1.682, 1.691 and 1.711 eV lines were sharper than in ours, and these lines were interpreted as ZPLs. The above Si-related lines were observed in six of 12 natural diamonds from Yakutia and Guinea. Details of the vibronic structure of the 1.682 eV center in CVD diamond recorded in absorption and PL at 80 K are presented in Fig. 8. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces of the same sample, which was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals w13,21x. Preliminary PL and absorption measurements performed on each individual piece confirmed similarity of their optical properties. PL was measured on a single piece and therefore, the effect of self-absorption should be negligible. In Fig. 8 a weak PL and absorption structure above
23
the Si-V ZPL could be resolved after subtraction of underlying broadband signals. Peaks y42, 30, 85, 126, 156 and 220 meV away from the ZPL are seen in the absorption spectrum. The y42 meV Ž1.64 eV. line was first mentioned in w11x as unrelated to the Si-V feature. Here we confirm this result: as it was previously reported w21x, in a series of CVD diamond films grown in the same run, an increase in amorphous carbon related defects leads to the decrease in the Si-V absorption. However, in the same series the amplitude of the 1.64 eV peak was constant. Moreover, UV illumination may lead to the strong Žup to 10 times. increase in the Si-V absorption w13,21x, but it has a minor effect on the 1.64 eV line. All this may indicate that the defect responsible for the 1.64 eV line has a ground state well below that of the Si-V. The PL spectrum of Fig. 8 shows a series of signals at y154, y126, y85, y65, y42, 85, 126, 156, 200 and 220 meV. Remarkable is that the intensity ratio of the high-energy to the low-energy lines is similar at 80 K Žthis work. and 10 K w22x. It was suggested w22x that the origin of the high-energy peaks is an anti-Stokes PL from the Si-V center. This could be one more example of temperature-independent anti-Stokes PL, but we should note that the 126, 156 and 220 meV peaks were also observed in PL from CVD films with undetectable Si-V absorption but with a series of strong lines attributed to the Ni-V center w8,9,23x. Part of that Ni-V PL spectrum, recorded at the same spectral resolution Ž10 meV. is shown by curve R in Fig. 8. Moreover, a peak at 200 meV, which was also reported in w22x, might well be a vibronic feature of the 1.945 center. Therefore, despite the y126 and y156 meV PL peaks
Fig. 8. Absorption and PL spectra from the Si-V center, measured at 80 K in a CVD diamond film. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces after it was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals. PL was measured under 2.54 eV excitation on a single piece. The y-axis roughly corresponds to absorption coefficient in cmy1 for absorption curve. Curve R presents a fragment of PL spectrum from a CVD film, where the Si-V center was relatively weak, but the ‘Ss 1’ center attributed to a Ni-V center w8,9,23x was exceptionally strong. Underlying broad background is subtracted from all curves.
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K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
the spectra measured before and after annealing are shown in Fig. 10. It is seen that the annealing resulted in the decrease in broad underlying signals at y19 meV ŽPL. and 12 meV Žabsorption..
4. Proposed interpretation of experimental results 4.1. The H3 center and sub-threshold PL excitation
Fig. 9. PL excited at 2.54 eV at 80 K from CVD diamond. The width of diamond Raman peak is 2, 3, 6 cmy1 for films 1, 2, 3, respectively, that indicates an increase in stress from film 1 to 3.
being attributed to the Si-V system w17x, the assignment of the 126, 156, 200 and 220 meV peaks to the same system seems not certain. Measurements on a large Ž) 50. series of CVD films reveal differences in PL structure around the Si-V ZPL. An example is given in Fig. 9, where PL spectra recorded at 80 K under 2.54 eV excitation are shown. The three films used for the figure were grown at similar conditions, but they have a different width of the diamond Raman peak, 2, 3 and 6 cmy1 for samples 1, 2 and 3, respectively. It is seen that a broad underlying band increases for films with a wider Raman peak, i.e. with a larger stress w24x. Similar conclusions can be drawn from the results of Sharda et al. w25,26x. Film 2 was further annealed at 1400⬚C for 2 h in a vacuum 10y8 torr. Absorption, PL and the difference between
Fig. 10. PL Žcurves 1᎐3. and absorption Žcurves 1⬘᎐3⬘. measured at 80 K in the same CVD diamond sample before Ž1, 1⬘. and after Ž2, 2⬘. annealing at 1400⬚C for 2 h in 10y8 torr. Curve 3 Ž3⬘. is a difference between the curves 1 Ž1⬘. and 2 Ž2⬘.. Absorption was measured on a stack of three 2 = 2 = 0.1 mm3 pieces after it was illuminated by a 3.7 eV light for 15 min in order to increase the Si-V signals. PL was measured under 2.54 eV excitation on a single piece. Underlying broad background is subtracted from all curves.
The shape of the 4.6-eV threshold in Fig. 1 is typical for an optical transition between a defect-induced level and the conduction or valence band. Such transition can excite a ZPL if it ionizes an electron from the center to the conduction band and the generated electron is then rapidly captured by an excited state of the same center. Then the PLE threshold should be present in any diamond with the H3 centers. On the contrary, it is observed only in diamonds containing single substitutional nitrogen. Those crystals also show the photocurrent threshold whose shape and position are remarkably similar to those of the 4.6-eV PLE feature suggesting their common origin. That photocurrent threshold is shown to be due to ionization of holes from the positively-charged P1 center ŽP1q. to the valence band w19x. Relying on the above observations we interpret the PLE results of Fig. 1 as indication that the H3 center can be excited via the ionization of the P1q center. The mechanism of such excitation is not clear. In order to return to equilibrium, the ionized P1q Žor P10 . center should re-trap a hole. It may be possible that the energy released from the P10 ª P1q relaxation excites the H3 centers. Remarkable is the observation of the temperatureindependent sub-threshold excitation of the H3 center. Fig. 2 reveals that while the PL excited at energies above ZPL preserves the polarization of incident light, polarization is almost lost for the sub-threshold excitation. This may indicate that the sub-threshold PL is generated via an energy transfer from some defect center, different from H3, and the polarization is lost during that process. Probably the center responsible for excitation of the sub-threshold PL is the one, which dominates absorption in the sample, i.e. P1 in Ib and A nitrogen in IaA diamond. However, one may also consider the following mechanism: The 2.41 eV light could ionize the H2 center ŽwN-V-Nxy with a proposed ground state at EC y 2.1 eV w3x. to the conduction band. It is possible that the ionization of the wN-V-Nxy converts it to the excited wN-V-Nx 0 , and polarization is lost during this process. With respect to this proposal it is worth noting that electron spin resonance measurements reveal w27x that the 5A 2 excited state of the neutral vacancy ŽV 0 . in diamond is produced via ionization of Vy rather than via internal excitation at V 0 . The above discussion may also be valid for the
K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
observed sub-threshold excitation of PL from the H4, 3H and TR12⬘ centers. Unlike for the H3, no differently charged states have been detected for these centers yet. This, however, is by no means proof of their non-existence.
25
centered at 60 meV Ž17 meV width. and at 70 meV Ž35 meV width.. The first peak is seen at the same energy in PL and it may be a ZPL due to the G ª E2 transition. The second has a mirror image at y70 meV in PL and is possibly a vibronic feature of the G᎐E1 transition.
4.2. The 3H center and sub-threshold PL emission 4.3. Green band II The above discussion of the H3 PLE spectrum suggests that the 3.85-eV threshold in the PLE spectrum of the 3H center ŽFig. 3. currently cannot be assigned to any defect center. The peaks at 2.63, 2.68 and 3.02 eV may be ascribed to the excited states of the 3H center. The 2.63-eV peak was also observed in absorption ŽFig. 4.. An interesting observation is the 2.523 eV line at the 3H center, whose emission is polarized perpendicular to the ZPL, and whose intensity is temperature independent in the range 80᎐300 K. The easiest explanation would be that the intensity correlation of the 2.523 and 2.463 eV lines is fortuitous and the 2.523 eV feature originates from a center relative to 3H. However, a very similar temperature-independent feature at above ZPL energy was also observed in at least six other centers: those with ZPL at 2.106, 1.886, 1.768 eV ŽFig. 6. and 1.950 eV w28x; TR12 and TR12⬘ w7᎐9,28x. Remarkably, all those centers are irradiation induced. The above observation suggests that some common phenomenon takes place in all those centers, a model for which is proposed below on the basis of the results from the 3H center. We suggest that the 2.523 eV line belongs to the 3H center and originates from radiative recombination between the level Žwhich we call E2. at 60 meV above the first excited state ŽE1. and the ground state ŽG.; the E2 level corresponds to an excited state of the 3H center, which has orientation of electric dipole perpendicular to the one of ZPL. This can be imagined as the 2.463 eV ŽZPL. and the 2.532 eV transitions having Zand XY-dipoles, respectively. Different orientations of those dipoles can explain the weak temperature dependence of the E2 ª G transition: although the energy distance between the E2 and E1 states is within a phonon quantum, the E2 ª E1 transition may be forbidden by symmetry and almost no thermalization then occurs between E2 and E1. It is worth noting that the existence of excited states with perpendicularly-oriented electric dipoles was also suggested for the positively charged interstitial Ni center in diamond w29x. However, the splitting between those lines was 2.7 meV and the ratio of their intensities was temperature activated. It is possible that the asymmetric 64-meV peak in the absorption spectrum of Fig. 4 is a sum of two lines
The change in the Fermi level position can have a drastic effect on the intensity of optical signals w13,21x. Before irradiation, the Fermi level in the IIb diamond should be at the boron acceptor center Ž E V q 0.37 eV w4x.. After irradiation and annealing of the IIb diamond the green band II could be detected under sub-bandgap excitation. Such excitation can induce electronic transitions from the valence band to the donor level. Holes generated in the valence band may be captured by the boron acceptors. Subsequent transition of an electron from an occupied donor to an unoccupied acceptor then produces the green band II w8᎐10x. In this model a shift of the Fermi level down from the E V q 0.37 position would increase the green band II PL, because the hole does not need to be captured by the acceptor any more, and an upward shift would decrease the green band because some donor levels become occupied. Resistivity and IR measurements show that irradiation and annealing of the IIb sample leads to the compensation of boron acceptors by irradiation-induced donors and consequently the Fermi level is moved up, but the strong increase in the green band is observed. These changes can hardly be explained by a Fermi level shift. An alternative explanation is that irradiation and annealing produce the donors responsible for the green band II. They are produced in a rather pure sample, and one may expect them to be structural defects. One more observation is in agreement with this suggestion: Exactly the same absorption threshold at 3.25 eV, which is responsible for the excitation of the green band II w8,9x was observed in our study Žunpublished. of a series of approximately 50 small Ž- 1 mm. natural diamonds from Venezuela. Microscopic examination revealed that all these diamonds had abundant extended structural defects. 4.4. Si-related centers Results shown in Figs. 7᎐10 demonstrate that identification of the vibronic structure of the Si-V center is not straightforward. Data presented in Fig. 10 may suggest that a peak at 1.679 eV, which often lies under the Si-V spectrum is not an independent center, but a
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K. Iakoubo¨ skii et al. r Diamond and Related Materials 10 (2001) 18᎐26
stress-induced feature at the Si-V center. The 30-meV absorption peak has a position of 1.712 eV, the same as for a center shown in Fig. 7. The intensity of the former was unchanged by annealing at 1400⬚C, but annealing at 1200⬚C completely removed the latter. Thus, we assign the 1.712 eV peak in Figs. 8 and 10 to a vibronic feature at the Si-V center. Remarkable is that this peak has no exact counterpart in PL and probably corresponds to the y42 meV PL line. The intensity ratio of the 30-meV to the non-phonon line is 0.05, which is approximately five times smaller than reported in w11x. However, the sample measured in w11x was electron irradiated and annealed at 800⬚C. This treatment is favorable for production of a 1.712 eV absorption line w18x, and these two centers could interfere. Indeed, our absorption measurements on CVD diamond, which was irradiated and annealed at 800⬚C Žcurve 2 of Fig. 7., show that after such treatment the 1.712 and 1.682 eV peaks can even have comparable intensities. 5. Summary and conclusions Combined application of PL, PLE and optical absorption spectroscopies to natural and synthetic diamond has provided new information on the 3H, H3 and Si-related centers. Si-related centers were also observed in natural Ia diamond after neutron irradiation and annealing. Excited states at 2.63, 2.68 and 3.02 eV above the ground state are detected for the 3H center. The importance of energy transfer processes between different defect centers in excitation of internal ZPL transitions is revealed. In particular, the 4.6-eV threshold in the PLE spectrum of the H3 center is assigned to ionization of holes from the P1q center followed by an energy transfer from the P1 to the H3 centers. Such processes might also be responsible for the observed temperature-independent sub-threshold PL from the H3, H4, 3H and TR12⬘ centers. Results of polarization-resolved PL measurements suggest that several centers in diamond, in particular the 3H, TR12, TR12⬘ and previously undocumented irradiation-related centers with ZPLs at 2.106, 1.886 and 1.768 eV, have excited states with mutually perpendicular orientation of their dipole moments. Acknowledgements Provision of CVD diamond films by M. Nesladek, IMO, LUC, Belgium is gratefully acknowledged. The authors are indebted to both referees for helpful comments, to W. Deweerd for high-temperature annealing
and to the FWO Žgrant G.0014.96. for financial support.
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