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Photoinduced absorption lines related to nickel impurity in annealed synthetic diamonds
$B[AMOND RELATED MATERIALS
ELSEVIER
Diamond and Related Materials 4 (1995) 177-185
Photoinduced
absorption
lines related to nickel impurity in annealed synthetic diamonds
A.P. Yelisseyev a, V.A. Nadolinny
b
a lnstitute qf Mineralogy and Petrography, b Institute of Inorganic Chemistry, 630090 Novosibirsk, Russia Received 15 June 1994; accepted in final form 12 October 1994
Abstract The absorption lines at 539.9, 546.6 and 552.9 nm induced by visible or UV light illumination in synthetic diamonds annealed at T&2000 K and 5.5 GPa have been studied. The lines are associated with two different nitrogen-nickel defects, while the photoinduction effect is a result of internal changes in the electronic configuration of the defects, reversion to the initial state being possible only upon annealing at 650 K. Analysis of generation spectra allows us to relate the defects to nitrogen-nickel paramagnetic centres such as NE1 or NE2 (possibly NE3) with optical analogues such as vibronic S3, S2 systems in the case of the 546.6 nm line and to a similar defect but containing only one nitrogen atom for the 552.9 nm line. An increase in annealing temperature results in further impurity aggregation, an increase in the 546.6 nm line intensity and a decrease in the others. Photoexcitation also stimulates ionization of donor nitrogen and of NEl-NE3 defects, when some of the generated electrons are located in purely nickel defects such as NE6 or in nickel-nitrogen defects such as NE7, operating as shallow traps responsible for the thermostimulated luminescence peaks at T< 180 K. Keywords: Synthetic
diamonds;
Defects; Nickel impurities
1. Introduction The investigation of synthetic diamonds is interesting in connection with recent successes in obtaining large crystals under laboratory conditions and the possibility of using them in various devices. Nitrogen and nickel are known to be the main impurities in synthetic diamonds grown in the Fe-Ni-C system at high temperatures and pressures [ 11. Tens of nitrogen defects have been studied in detail during the last three decades, the structures being determined for many of them [2,3]. At the same time only vibronic systems with zero-phonon lines (ZPLs) at 484, 882 nm in cathodoluminescenceabsorption and the 658 nm absorption line in optics as well as a single line with g= 2.031 [4] and NIRIM 1, NIRIM 2 [ 5,6] systems in electron spin resonance (ESR) can be unambiguously related to nickel defects. In the case of 484, 882 nm systems the conclusion is a result of ZPL splitting due to nickel isotopes [7]. A set of additional absorption lines appears in diamonds with relatively high contents of both nickel and nitrogen at 1970 K annealing. These were related to nickel-nitrogen defects created as a result of impurity diffusion, principally nitrogen impurities [8,9]. The latter become Elsevier Science S.A. SSDI 0925-9635(94)00240-l
mobile already at 1800 K [ 8,9]. In Ref. [lo] some of these lines associated with broad bands at 330 and 440 nm were related to S2, S3 luminescence systems, which have been well studied in natural diamonds [ 21. Their analogues in ESR were revealed and the structures of the main three, denoted NEl, NE2 and NE3, determined [9]. The defects were found to contain nickel ions located in the divacancy position or in the position of double semivacancy according to the Masters approach developed for Ge and Si. This is a fragment which is common to all of them and is surrounded by nitrogen atoms whose number is n=2 for NE1 and II= 3 for NE2, NE3; n is also expected to take other values and indeed centres with n=O,l were described in Ref. [lo]. Taking into account that nickel ions can be in various charge states, including non-paramagnetic ones such as Ni” and Ni2+ , we used a preliminary photoexcitation in order to transform defects into the Ni+ paramagnetic state [ 111. Earlier such an approach was proved to be fruitful when studying a single vacancy in diamond, which undergoes a photoinduced transition from the V” state, responsible for the GRl system in absorptionluminescence, to the V- state with the ND1 system [Z].
178
A. P. Y&weyev.
V. A. Nudolinn~ i Diamond and R&ted
The same process takes place in the case of nitrogenvacancy defects such as N2V2, for which a transition from the H3 to the H2 vibronic system takes place upon illumination [ 121. Preliminary photoexcitation made it possible to reveal three other paramagnetic defects related to nickel-containing ones: NE5, which is analogous to NE1 with n=2 [lo], but where, in contrast with the latter, the nitrogen atoms are separated by vacancies from the nickel ion, thus forming an N-V( VNi-V))V-N chain [lo]; as well as NE6 and NE7 centres with n=O and 1 respectively. NE6 and NE7 correlate with thermostimulated luminescence (TSL) peaks at 130 and 160 K and are found to be shallow traps which have captured an electron. The present paper is devoted to a quest for photoinduced systems in optical absorption spectra which could be the optical analogues of NE5 in ESR [ 111. Investigation of the kinetics, generation spectra and optical and thermal stabilities showed that the photoinduced lines at 539.9, 546.6 and 552.9 nm correspond to two different nickel-nitrogen defects which are supposed to contain two or three nitrogen atoms in the case of the 546.6 nm line and only one nitrogen atom for the 552.9 (539.9) nm lines, while the photoinduction effect is a result of internal changes in the electron configuration of the defects.
2. Experimental
details
To investigate the photoinduction phenomena, a setup based on an MDR-2 diffraction monochromator with Xe and glow lamps as light sources was used. The generation spectra for the photoinduced lines were obtained point by point. In every case at first the absorption spectrum at 80 K was recorded, then the sample was illuminated directly inside a liquid nitrogen cryostat at the same temperature using light of a certain wavelength and the spectrum was recorded a second time for the intensity of absorption lines. After that the diamond was heated to 700 K, at which temperature the photoinduced lines are found to be removed, and again cooled to 80 K to obtain the next point. The intensity value for a particular absorption line was plotted vs. wavelength. When the thermal stability of photoinduced lines was studied, the sample was excited at 80 K and then heated to a certain temperature, after which the line intensity was measured at 80 K. In a similar way the spectral dependence of the efficiency of light sum storage in various TSL peaks was obtained. In all cases the kinetics of absorption or of TSL signal growth was preliminarily studied in order to provide operation under linear dependence of the measured parameter on excitation duration. The ESR spectra were recorded using an El09 “Varian” spectrometer operating in the X band and
Mutrriuls
4 i 1995) I77
IX5
80-400 K temperature range, while photoexcitation was produced by a mercury DRSh-500 lamp directly inside an ESR spectrometer cavity. The IR absorption spectra were obtained on Perkin-Elmer 325 spectrophotometer with a microdevice. Synthetic diamonds up to 7 mm in size, mainly of octahedral habit, were grown in the Design and Technological Institute of Monocrystals in Novosibirsk in a polyanvil high pressure apparatus of the splitsphere type [ 131. The as-grown yellow diamonds, obtained in the FeeNi-C system using the thermal gradient technique at 1620-1670 K and 5.5 GPa, were annealed at T > 2000 K in the same apparatus for 4 h. After annealing, the yellow colour diminished and the diamonds became slightly greenish. The nickel concentration as centres with g = 2.031 was about lO”~- 1018 cm-j in as-grown diamonds, while the donor nitrogen content was estimated to be 10’9~1020 cmm3 according to ESR and IR absorption data. After annealing at T>2000 K and 5.5 GPa, the signal from Ni+ with g= 2.031 disappears from the ESR spectrum and the concentration of donor nitrogen (or C centres [l-3]) decreases by approximately two orders. Simultaneously, signals from paramagnetic NEl, NE2 and NE3 centres [lo] appear, but the total nickel concentration in these forms is several times lower than in as-grown diamonds. This is additional proof that various forms of nickel defects are present in diamonds, including non-paramagnetic ones.
3. Results and discussion 3.1. Optical
absorption
spectra
The yellow colour of as-grown synthetic diamonds is mainly a result of C centre absorption [l-3]. The transmission spectrum for as-grown diamond 2 mm thick is shown in Fig. 1 (curve a). The transmission edge is situated at 480 nm. Annealing at T> 2000 K is accompanied by a weakening of the yellow colour, which is associated with absorption of donor nitrogen, and the transmission edge shifts to 350 nm. The greenish colour is a result of absorption in a vibronic system with ZPL at 793 nm. The maximum of the system is located at 700 nm and its short wave wing enters the visual range, weakening the red light transmission in the crystal. Another reason for the yellow colour is the broad band at 440 nm which is well defined on the background owing to absorption of the remaining C centres (Fig. 1). This band corresponds to the CI band in luminescence excitation-absorption spectra of defects responsible for the intensive green luminescence of these crystals in S2 and S3 vibronic bands. The other broad band (p) typical for these systems is usually situated at 330 nm but is hardly observed here because of C centres absorption.
A. P. Yelisseyev, V. A. Nadolinny J Diamond and Related Materials 4 (1995) 177-185
179
A
;I;;: e d
I
540
350
Fig. 1. Transmission spectra of synthetic 2000 K (b) and after additional excitation other for convenience.
400
450
500 h (nm)
550
I
550 h (nm)
600
diamonds at 80 K for as-grown diamond of yellow colour (a), for the same sample after annealing at by 366 (c, d) and 546 nm (e) Hg lines. Spectra b and c as well as d and e are shifted with respect to each
A rich fine structure has been recorded after annealing in these spectra in the 330-370 nm region and at A>465 nm. The full list of lines observed at 80 K is given in Table 1. In yellow as-grown crystals only a few vibronic systems are observed (columns 1 and 2). The systems with ZPLs at 494, 658 and 793 nm were related in Refs. [8,9] to purely nickel defects, while the other one with ZPL at 732 nm and two vibration replicas due to the ho = 52 f 1 meV phonon were associated with nickelnitrogen defects. In columns 3 and 4 the positions of lines recorded in annealed synthetic diamonds are given in comparison with lines in annealed synthetic diamonds grown by H. Kanda at NIRIM (Japan) [ 891 and natural diamonds with green luminescence from Yakutia deposits [ 14,151. It can be seen that most of the lines are found in annealed diamonds from both NIRIM and the Design and Technological Institute of Monocrystals, Novosibirsk, excluding maybe some at long waves. Most of the lines were associated with nickel-nitrogen defects in Refs. [ 8,9]. Column 5 gives the results of decomposition of the fine structure in the 360-500 nm range into three groups, each of which is due to a certain defect [lo]. One of these defects is responsible for the S3 vibronic system with ZPL at 496.7 nm in luminescence at 80 K. The other ZPLs, which are situated at 532.2 (the A line according to Refs. [14,15]), 489.1 (B), 477.8 (C), 472.5 (D) and 470.5 nm (E), were thought to be related to a single vibronic system such as S2 [8,9]. The use of the luminescence excitation technique showed them to
be due to at least two different defects [lo]. A certain confusion could result from a previous paper [lo] where we associated the “S2” notation with the 489.1 nm line (B), while most authors relate it to the A line at 532.2 nm [ 1,16,17]. To avoid further misunderstanding, we shall use “S2” to indicate both defects and give the ZPL position when speaking about one of these defects. These defects appear to be very similar because they both contain the nickel ion in a divacancy position surrounded by three nitrogen atoms, but located in a different way in the diamond lattice [IS]. Thus in column 5 of Table 1 (a) denotes the lines which are associated with the system with ZPL at 489.1 nm in luminescence and NE2 in ESR, (b) 496.7 nm (S3 system), NE1 and (c) 523.2 nm, NE3 [lo]. Detailed analysis of the fine structure of the B band in the luminescence excitation spectrum for the 489.1 nm ZPL shows that in contrast with previous papers [ 14,151 there is only a ZPL at 366.7 nm (3.380 eV) accompanied by several replicas due to a single phonon with ho= 32 + 2 meV. Since this value is lower than 68 meV, which corresponds to the minimum energy for vibration in the diamond lattice at various points in the first Brillouin zone [ 1,3,17], the mentioned maxima in the spectra can be related to a quasi-local vibration. The low energy value is likely to indicate participation of a heavy ion such as nickel [ 171. Long-term luminescence decay as well as large values, up to 5-10, of the HuanggRhys factor, which characterizes the electron-phonon coupling for a particular centre, are typical for such cases [ 171. Besides S2, S3 systems, other examples are systems
180
A. P. Yelisseyev,
Table 1 The fine structure (at 80 K) Synthetic As-grown E, eV
diamonds
in absorption
spectra
V. A. Nadolinny j Diamond urzd Rebted
of as-grown
and annealed
synthetic
dtamonds
1.563 1.692 1.746 1.797
793.6 132.4 711.0 689.9
1.884
658.2
Annealed E, eV
m compartson
with natural
i. nm 996.0 987.2 913.9 8X3.2 860.2 793.6
1.565
at 2000 K
1.245 1.256 1.357 1.404 1.441 1.563
4 ( 1995) 177- 1X5
Synthetic diamonds from NIRIM (annealed at 2000 K [8,9] E. eV
from DTIMC
i, nm
Muteriuls
diamonds
from Yakutia
deposits
Natural diamonds [ 14.151 E. eV
1.693 1.744 I .794 I X47 1.884
1.938 2.200 2.242” 2.268 2.296 2.351” 2.374 2.385 2.394 2.405 2.429” 2.458 2.419 2.515
563.5 552.9b 546.6’ 539.9b 521.3 522.2 520.2 518.0 515.7 510.4 502.2 500.2
2.24 2.267 2.296 2.351 2.383 2.392 2.404 2.421 2.465 2.476
493 2.523
2.582” 2.598 2.623” 2.627” 2.652 2.658
478.4’ 477.2cd” 472.8’ 471.9d 461.6’ 466.5*
3.341 3.380” 3.412” 3.442” 3.474 3.502 3.560 3.572 3.607 3.689
371.1d 366.1d 363.4d 360.1d 356.9d 354.0* 348.2* 347.1’ 343.7’ 336.1’
a These lines can sometimes be found also in the (111) sectors: b The photoinduced lines in absorption: ’ ZPL in absorption/luminescence excitation d ZPL in absorption/luminescence excitation ’ ZPL in absorption/luminescence excitation
specific points
spectra spectra spectra
of as-grown
2.574 2.589 2.596 2.618 2.648
2.655
J
3.380 3.412 3.443 3.414
synthetic
diamonds
(near seed, large inclusions
and in junctions
between
related to vibronic system with ZPL at 2.370 eV (NE3 [IO]): related to vibronic system with ZPL at 2.535 eV in luminescence (NE2): related to vjibronic system with ZPL at 2.496 eV (S3 vibronic system. NE1 [lo]).
with ZPLs at 578.08 nm with ht~=30 meV vibration and at 455.7 nm (34 meV) in brown natural diamonds, which differ in their large content of heavy elements such as Fe, Ni, Mn, etc. [16]. Another example is the system with 484 nm ZPL and lzto= 20 and 35 meV
vibrations in cathodoluminescence and absorption which was proved to be of nickel nature [ 181. It should be noted that as-grown synthetic diamonds can be quite uneven in colour and luminescence. Along with intensively yellow octahedral sectors. there are
A. P. Yelisseyev, V. A. Nadolinny / Diamond and Related Materials 4 (1995) 177-185
181
6r
250
300
350
400
450 h
Fig. 2. Normalized
generation
spectra
500
for 546.6 (1) and 552.9 nm (2) absorption
regions where the colour is much lighter and bands due to pairs of nitrogen atoms (A centres), with the main are observed in IR asorption. one at 1280 cm-’ [l-3], The same zones also show a more intensive green luminescence in S2, S3 systems. This occurs at junctions of adjacent octahedral sectors as well as in regions near a seed or large metal inclusions and shows an opportunity of creating complex defects including several impurity atoms directly during the growth process at relatively low temperatures such as 1700 K. The most intensive lines which are typical for annealed diamonds can be observed in spectra obtained at these points. These lines are marked (a) in column 3 of Table 1. The three lines marked (b) in column 4 appear only after preliminary illumination of annealed diamonds. The transmission spectra obtained before and after excitation at 80 K are shown in Fig. 1, while the three photoinduced lines at 539.9, 546.6 and 552.9 nm are shown in detail in the inset, The last two of these are ZPLs, while the line at 539.9 nm is a little weaker and wider and corresponds to a vibration replica from the 552.9 nm line with ho = 54 meV energy. Its small value, which is lower than 68 meV, can be an argument in favour of possible participation of a heavy ion such as nickel in defect formation. The intensity of the photoinduced lines is comparable with that recorded for other ordinary lines of nickel-nitrogen nature which are present in this spectral range (Fig. 1) [S,9] and reaches several cm ‘. In photoluminescence spectra these lines look like sharp holes on the background of S2 and S3 vibronic systems and can be very deep, down to complete luminescence quenching because of self-absorption and
550
600
650
(nm) lines in annealed
synthetic
diamonds
at 80 K.
large optical path lengths for luminescence inside a sample in some cases. 3.2. Conditions
of photoinduced
emission
line generation
In Fig. 2 the normalized generation spectra for the photoinduced lines at 546.6 and 552.9 nm, obtained after excitation from an Xe 1 kW lamp through the MDR-2 monochromator, are given. The intensity dependence on excitation duration was measured preliminarily. The kinetics of the photoinduced line intensities at 430 nm excitation are shown in Fig. 3. Both lines are generated
t (min) Fig. 3. Kinetics of absorption lines under 440 nm excitation
growth of 546.6 (1) and 552.9 nm (2) at 80 K.
and their intensities grow at the same rate during the initial stage of the excitation process up to 1 min at a given excitation level. Later the 546.6 nm line intensity continues to grow but at a different rate, while the 552 nm line saturates (Fig. 3). The generation spectra for the 546.65 and 552.9 nm lines were found to differ considerably (Fig. 2). For the first line the spectrum begins at 500 nm and shows maxima at 350 and 440 nm corresponding to the B and c; bands in the luminescence excitation spectra for emission of the 489.1 and 496.7 nm ZPLs [10,14,15]. The hardly pronounced minimum between the bands at 400 nm is likely to show participation of the third defect with emission of the 523.2 nm ZPL, which demonstrates a band at 400 nm in the luminescence excitation spectrum [9]. Thus the generation spectrum 1 in Fig. 2 indicates that transition into a new state which is characterized by absorption at 546.6 nm occurs when NEl, NE2 and possibly NE3 nickel-nitrogen defects are excited. The single line in the absorption of the photoinduced centre indicates that only one certain defect from this group undergoes the transformation. For the 552.9 and 539.9 nm lines the generation spectra are similar and begin already at 600 nm, displaying maxima at 390 and 500 nm (Fig. 2, curve 2). Minima at 340 and 440 nm correspond to maxima in spectrum 1 for the 546.6 nm line as well as to B, x bands in luminescence excitation--absorption of NEl, NE2 defects. Thus spectrum 2 is likely to be related to a defect with an absorption which is continuously growing from 600 nm to shorter waves; variations in intensity are due to competition between this defect and NEl, NE2 defects which absorb part of the exciting light. In analysing a set of defects with suitable characteristics which are present in synthetic diamonds and which could be responsible for the 539.9 and 552.9 nm photoinduced lines, one can focus on a single nitrogen atom in the substitutional position, known as a C centre [l&3]. However, since these lines are related to nickel-nitrogen defects [S,9] and are not found in natural Ib diamonds, they can be associated with a complex defect where in addition to the nickel ion a single nitrogen atom is present as a fragment, possibly separated by carbon atoms from the nickel ion. These assumptions about the structure of the defects responsible for the photoinduced lines agree very well with Ref. [9], where the influence of annealing temperature on the absorption line intensities in synthetic diamonds has been studied. Both the 539.9 and 552.9 nm lines were found to demonstrate a similar behaviour, decreasing in intensity as the annealing temperature was increased from 1970 to 2170 K, in contrast with the 546.6 nm line which was increasing. The changes could be considered as a result of further aggregation of impurities when an increase in annealing temperature stimulates transformation of defects from initial purely nickel or purely nitrogen centres into ones containing
both nickel and nitrogen. On the other hand, nickelnitrogen defects containing only one nitrogen atom (n = 1) are transformed into more complicated ones where 12=2 or 3, to which the above-mentioned NEl, NE2 and NE3 defects are related. It is curious that the simpler defects responsible for the 552.9 nm line can be created with a higher probability directly during growth at relatively low temperatures such as 1700 K (Table l), in contrast with the more complex defects responsible for the 546.6 nm line which were not found in the absorption of as-grown diamonds. 3.3. Remocul
of photoinduced
lines
Attempts to remove the photoinduced lines using illumination by Xe or Hg lamps up to 1 kW in power appeared to be unsuccessful even when selecting the necessary spectral range by broad band filters of coloured glass. The only way to remove the lines is to anneal the crystals. The dependence of the photoinduced line intensity on the temperature to which a sample was heated at a rate of 20 K min-’ is shown in Fig. 4a. The behaviour was found to be quite similar for both the 546.6 and 552.9 nm lines, which show no changes up to 570 K and then are quickly removed by 640 K. Plotting the intensity on an Arrhenius scale, one can evaluate the energy of thermal activation as E=O.SO + 0.10 eV. The fact that only thermal removal takes place shows that the excitation stimulates the photoinduced internal transformation of the electron configuration of the corre-
0.6
5
2 0.4 w 0.2 0
100
200
300
400
500
600
700
800
T (K) Fig. 4. Temperature dependence of absorption of 545.6 and 552.9 nm lines (a) and thermostimulated luminescence curve (b) for annealed synthetic diamonds. In the latter the posItIon of horizontal lines relates to the energy of temperature activation for certain traps, while their length indicates the temperature range where the TSL glow matches with a given Er value. The full lines represent the “initial rise” calculations and the broken lines the corresponding results by the “general” method.
A. P. Yelisseyev, V. A. Nadolinny / Diamond and Related Materials 4 (1995) 177-185
sponding defect. It is curious that the 570-640 K temperature range in which the defects containing a structural fragment such as a double C-C bond and corresponds to a transition into a triplet state [ 191. The alternative mechanism of photoinduced transformation in crystals is a recharging of various defects under photoexcitation as in the case of vacancies [ 1,3] or N2V2 defects responsible for H3, H2 vibronic systems [ 121. 3.4. Recharging luminescence
of defects; thermostimulated
curves
Thermostimulated luminescence may be defined as the emission of a crystal during heating after preliminary excitation at low temperature. The recorded glow peaks are associated with charge carrier release from shallow traps where they are localized at excitation and their radiative recombination on the ionized luminescence centres. On one hand, analysis of TSL curves allows one to reveal the mechanism of the recharging process, the charge transfer between various defects which are considerably remote in the lattice. On the other hand, TSL analysis allows determination of the parameters of the traps and sometimes their structure. Fig. 4b shows the TSL curve for annealed synthetic diamond which was obtained after excitation with the 366 nm Hg line at 80 K and a heating rate of 20 K min-l. The main peaks are situated at 160, 360, 600 and 710 K; a weaker one can be seen as a shoulder on the low temperature side of the 160 K peak. The curve indicates charge carrier release from at least five types of traps. In Table 2 the results of calculations of trap parameters such as depth or energy of thermal activation, Table 2 Parameters
for traps of charge
Position of TSL peak T, K
Halfwidth 6,162 T, K
130
carriers
in annealed
synthetic
183
E,, frequency factor and TSL kinetics order are given. Two methods were used: the “general” method of Antonov-Romanovsky which uses the whole TSL peak [20] and the well-known “initial rise” method where only the initial part of the peak is analysed. In column 5 the temperature range where the glow peak follows certain E,, W, parameters is given. For most of the peaks good agreement between the results of the two methods was found. A somewhat different value of E, = 0.28 eV for the 215-280 K range and a difference in E, obtained using the two methods for the 600 K peak indicate that there are additional traps with ET =0.28 and 0.55 eV. In Fig. 5 the spectral dependence of the efficiency of light sum storage in various TSL peaks is shown. The spectra were found to be similar for the 160, 360 and 600 K peaks. The most effective TSL excitation begins at 500 nm, the spectra being similar to luminescence excitation-absorption spectra for NE1 and NE2 defects [lo]. Two c1 and 0 broad bands are well defined at 440 and 350 nm. For the low temperature peaks at 130 and 160 K TSL excitation is possible also at longer wavelengths (from 600nm), though with much lower efficiency, and donor nitrogen ionization can be supposed. Thus at A< 500 nm excitation the complex nickelnitrogen defects such as NE1 and NE2 as well as donor nitrogen can be ionized. The form of the TSL curves remains invariable at different wavelengths of excitation, showing that the lower excited levels of the c1 group for NEl, NE2 defects are situated close to the edge of the forbidden band just under the bottom of the conduction band or directly inside it. The excitation into the a band is accompanied by defect ionization. To a larger extent
diamonds
Energy of thermal activation E, eV
Method of calculation
0.15
i.r.
Temperature range
Kinetics order
Frequency factor 0, SC’
T, K
80-120
160
22.5/45
0.23 0.22 0.24 0.19
gen. ix. i.r. ix.
130-250 130-150 160&180 18OC230
11
2.5 x 103
360
75/61.5
0.39 0.40 0.37 0.37
gen. i.r. i.r. i.r.
240-350 240-310 270-320 3OOG350
II
4.5 x 102
600
45150
0.63 0.55
gen. i.r.
470-700 470&600
I
3 x 102
710
25/l 7
1.92 1.72
gen. i.r.
6OOG750 6OOG670
I
1.3 x 10’2
gen. is the general at low-temperature
method of Antonov-Romanovski [20]; i.r. is the initial rise method and high-temperature sides of the peak respectively.
[20];
6, and 6, are the half-widths
for TSL peaks, measured
184
A. P. Yelisse_yev, V. A. Nudolinn~ ! Dimmond and R&ted
Matrricrls 4 i 1995) 177-185
The NE5 system is described
600 h (nm)
Fig. 5. Normalized spectral dependence of light sum storage efficiency in thermoluminescence peaks at 160 (l), 350 (2) and 600 K (3) under 80 K excitation.
it is related to excitation into the l3 band, which corresponds to the other group of excited states situated higher in the energy level diagram. It is curious that as in the case of the negatively structure is well charged vacancy V- , the vibronic defined in absorption despite the superposition of energy states of the local centre and states of the conduction band. A different situation is realized for NV defects responsible for the N3 vibronic system, for which the lower excited state is 0.5 eV under the edge of the conduction band [21]. In this case no free electrons are generated at 415 nm excitation and some N,V defects are ionized only as a result of channel transition of an electron to adjacent traps. The latter are likely to be distorted by the luminescence centre and have parameters which are different from those for remote traps. The TSL curves are of different forms for these two cases. All the peaks shown in Fig. 4b and Table 2 are easily bleached under i, > 600 nm illumination. 3.5. Photoinduced
systems in ESR
Preliminary excitation also results in changes in ESR spectra. Thus 366 nm Hg excitation allowed us to reveal three new systems in annealed synthetic diamonds, denoted NE5, NE6 and NE7, which were described in detail in Ref. [ 111. Here only brief information is given in order to compare the behaviour of paramagnetic and optically active centres under illumination and heating.
by the spin hamiltonian
where S = f and g-2.0903 (gi 11(011)), g,=2.039, g,= 2.044 (.Y(g,,(Oli))=8 “). It demonstrates a hyperfine structure (HFS) resulting from two magnetically equivalent nitrogen atoms with Af = Ai= 12.25 G (Af and Ai ~i(lli) and .4:=.4$=9.5G. The defect was shown to be a nickel ion in a divacancy position surrounded by two nitrogen atoms as for the NE1 centre, but here, in contrast with the latter, the nitrogen atoms did not reach the first coordination sphere of nickel and occurred separated from it by vacancies, thus forming an N-V( VNiiV))V-N chain [ 111. The NE5 spectrum is induced after i. ~400 nm excitation and removed by 540-720 nm illumination. Heating to 1100 K does not affect the NE5 intensity. If a signal has been removed by illumination, it can be restored by heating to 570 K. The energy of thermal activation for this process was estimated to be 0.2 eV. The behaviour of the defect with illumination and heating can be understood by assumping an internal transformation inside a defect for which a potential energy function U(r) in a one-coordinate scheme has two minima of different depths separated by an energy barrier. An electron, which initially is in a shallower minimum with more gentle inclination of the walls, at i ~400 nm excitation reaches a slope of a deeper minimum and appears at its bottom after relaxation. It can also get there by heating to overcome the 0.2 eV energy barrier. The reverse transition is likely to need much larger effort and temperature but can occur at 540-720 nm excitation. Detailed investigation of the NE5 system is in progress, but it is clear that it cannot correspond to any of the photoinduced absorption lines. The other two photoinduced centres NE6 and NE7 become paramagnetic after illumination by blue or UV light. They both have S = f. The NE6 system demonstrates no HFS and has g, = 1.995 (gl iI{1 lo)), g2 = 2.207 (gz~~(llO)), g,=2.0109 (gJ/(OOl)). NE7 has a g factor which is considerably larger than 2 and an HFS from one nitrogen atom; the value of the HFS splitting varies from 6 to 10 G depending on the orientation [ 111. Illumination at i > 540 nm results in complete removal of these signals; the same is observed upon heating to 180 K. This behaviour is typical for shallow traps responsible also for the 130 and 160 K TSL peaks, the ESR signal being related to states when they are filled. When the charge transfer between various defects is considered, one of the most important problems is to determine the sign of the carriers. In this context it is necessary to pay attention to the fact that at 366 nm Hg excitation a decrease in intensity of the donor nitrogen signal is observed along with the appearance of NE6 and NE7 spectra in ESR. This occurs only under low temperature (e.g. 80 K) excitation and varies from sample
A. P. Yelisseyev, V. A. Nadolinny / Diumond and Related Materials 4 (1995) 177-185
to sample, reaching 12% in some crystals. No changes were observed under room temperature excitation even at 0.015% sensitivity level. After excitation at 80 K the nitrogen intensity in ESR can be restored by Y > 600 nm illumination, while the NE6 and NE7 systems disappear completely. The same result can also be achieved by heating to 300 K. All these facts characterize NE6 and NE7 centres as shallow traps of electrons generated by donor nitrogen ionization. Since the same TSL peaks appear after ionization of NE1 and NE2 centres (Fig. 5) one can suppose that Ni+ -+NiZf transformation takes place in these defects, accompanied by generation of free electrons. The other TSL peak at higher temperature (600 K) in synthetic diamonds was also found to be caused by nickel impurity [ 22,231.
185
(5) The shallow nickel-containing traps are supposed to cause the n-type conductivity of synthetic diamonds. This can seem particularly curious taking into account the situation with n-type conductivity in diamond: no reliable element, defect or method is known to create it to date. The purely nitrogen defects are known to cause only deep levels in the forbidden band which weakly affect the conductivity of diamonds.
Acknowledgements The authors are grateful to Professor N. Sobolev for interest in the work and to Dr. V. Vins and B. Feigelson for lending them as-grown and annealed synthetic diamonds for investigation.
4. Conclusions (1) The photoinduced absorption lines in annealed synthetic diamonds are due to two different systems with ZPLs at 546.6 and 552.9 nm, while the 539.9 nm line is a phonon replica of the latter with ho = 54 meV. Both defects are similar in generation kinetics, temperature and optical stability. (2) Analysis of generation spectra and the effect of annealing on the intensity of photoinduced lines leads one to suppose that NE1 or NE2 defects are responsible for the 546.6 nm line, while a similar defect but containing only one nitrogen is responsible for the 552.9 nm line. A nickel ion in a divacancy position, which is a main structural fragment of both defects, is surrounded by two or three nitrogen atoms in the case of the 546.6 nm line and by only one nitrogen for the 552.9 nm line. An increase in annealing temperature results in further impurity aggregation in the diamond lattice and changes the ratio between different defects in favour of those which contain more nitrogen atoms, such as NEl, NE2 (546.6 nm line). (3) Along with the internal transformation, a recharging process in complex nickel-nitrogen defects such as NEl, NE2, NE3 and in donor nitrogen takes place at 80 K under photoexcitation. Excitation results in their ionization, with the free electrons generated being captured in the traps. Some of the latter, which are responsible also for TSL peaks at 130 and 160 K, become paramagnetic under excitation and correspond to purely nickel defects such as NE6 or to nickel-nitrogen ones such as NE7. (4) The obtained data demonstrate a variety of nickelnitrogen defects in synthetic diamonds. Defects of this type, which are responsible for S2, S3 and 793 nm vibronic systems, strongly affect the luminescence and colour of annealed crystals. In connection with the prospects of stimulated emission generation in diamonds in some of these centres [17], the absorption lines in the visual region are very important. They can destroy the continuity of wavelength tunning, particularly in the multipass regime.
References Cl1 A.T. Collins, in J.E. Field (ed.), The Properfies of Natural and Synthetic Diamonds, Academic, London, 1992, p. 35. Cl1 E.V. Sobolev, in V. Kuznetsov (ed.), Problems of Petrology in the Earth Crust and Upper Mantle, Nauka, Novosibirsk, 1978, p. 245 (in Russian). c31 G. Davies, Chem. Phys. Carbon, 13 (1977) 1. G. Bezrukov and V. Butuzov, Pisma ZhETF, [41 M. Samoilovich, 14 (1971) 325 (in Russian). [51 J. Isoya, H. Kanda and J.R. Norris, Phys. Rev. B, 41 (1990) 3905. [cl J. Isoya, H. Kanda and Y. Uchida, Phys. Rec. B, 42 (1990) 9843. [71 G. Davies, A.J. Neves and M.H. Nazare, Europhys. Left., 9 (1) (1989) 47. /. Phys. D. App[. Phys., 18 C81 A.T. Collins and M. Stanley, (1985) 2537. 191 S. Lawson and H. Kanda, J. Appi. Phys., 73 ( 1993) 3967. and A. Yelisseyev, Diamond Relut. Mater., 3 Cl01 V. Nadolinny (1993) 17. and A. Yelisseyev, Diamond Relat. Mater., 3 [Ill V. Nadolinny (1994) 1196. Y. Mita, Y. Nishida and K. Suito, J. Phys. C: Condens. Mutter, [=I 21 (1990) 8507. Yu. I. Malinovski, Yu. N. Borsdov, A.F. [I31 Yu. N. Palyanov, Khokhryakov, A.I. Chepurov, A.A. Godovikov and N.V. Sobolev, Dokl. Akad. Nauk SSSR, 315 (1990) 1221 (in Russian). of Inorganic Chemistry, 1141 V.E. Ilyin, Ph.D. Thesis, Institute Novosibirsk, 1970, p. 18. 1151 V.E. Ilyin, E.V. Sobolev and O.P. Yuryeva, SOL,. Phys. ~ Solid State, 12 (1971) 21. Cl61 J.E. Field, in J.E. Field (ed.), The Properties of Naturai and Synfheric Diamonds, Academic, London, 1992, p. 687. in R.P. Rao (ed.), Luminescence Phenomena: Cl71 M.E. Pereira, Phenomena, Materials and Devices, Nova, New York, 1992, p. 137. Cl81 A.T. Collins and P.M. Spear, J. Phys. C: Solid State Phys., 16 (1983) 963. Ph.D. Thesis, Institute of Inorganic Chemistry, 1191 V.A. Nadolinny, Novosibirsk, 1978, p. 19. V.V. Antonov-Romanovsky, Kinetics of Luminescence in WI Crystals, Nauka, Moscow, 1966 (in Russian). 1211 E.V. Sobolev and A.P. Yelisseyev, J. Struct. Chem., 17 (1976) 933 (in Russian). J.E. Lowther, T.L. Nam and R.J. Keddy, 1221 L. Paslovsky, J. Luminesc., 52 (1993) 167. v31 T.L. Nam, R.J. Keddy and J.E. Lowther, J. Hard Mater., 2 (1991) 163.
ELSEVIER
Diamond and Related Materials 4 (1995) 177-185
Photoinduced
absorption
lines related to nickel impurity in annealed synthetic diamonds
A.P. Yelisseyev a, V.A. Nadolinny
b
a lnstitute qf Mineralogy and Petrography, b Institute of Inorganic Chemistry, 630090 Novosibirsk, Russia Received 15 June 1994; accepted in final form 12 October 1994
Abstract The absorption lines at 539.9, 546.6 and 552.9 nm induced by visible or UV light illumination in synthetic diamonds annealed at T&2000 K and 5.5 GPa have been studied. The lines are associated with two different nitrogen-nickel defects, while the photoinduction effect is a result of internal changes in the electronic configuration of the defects, reversion to the initial state being possible only upon annealing at 650 K. Analysis of generation spectra allows us to relate the defects to nitrogen-nickel paramagnetic centres such as NE1 or NE2 (possibly NE3) with optical analogues such as vibronic S3, S2 systems in the case of the 546.6 nm line and to a similar defect but containing only one nitrogen atom for the 552.9 nm line. An increase in annealing temperature results in further impurity aggregation, an increase in the 546.6 nm line intensity and a decrease in the others. Photoexcitation also stimulates ionization of donor nitrogen and of NEl-NE3 defects, when some of the generated electrons are located in purely nickel defects such as NE6 or in nickel-nitrogen defects such as NE7, operating as shallow traps responsible for the thermostimulated luminescence peaks at T< 180 K. Keywords: Synthetic
diamonds;
Defects; Nickel impurities
1. Introduction The investigation of synthetic diamonds is interesting in connection with recent successes in obtaining large crystals under laboratory conditions and the possibility of using them in various devices. Nitrogen and nickel are known to be the main impurities in synthetic diamonds grown in the Fe-Ni-C system at high temperatures and pressures [ 11. Tens of nitrogen defects have been studied in detail during the last three decades, the structures being determined for many of them [2,3]. At the same time only vibronic systems with zero-phonon lines (ZPLs) at 484, 882 nm in cathodoluminescenceabsorption and the 658 nm absorption line in optics as well as a single line with g= 2.031 [4] and NIRIM 1, NIRIM 2 [ 5,6] systems in electron spin resonance (ESR) can be unambiguously related to nickel defects. In the case of 484, 882 nm systems the conclusion is a result of ZPL splitting due to nickel isotopes [7]. A set of additional absorption lines appears in diamonds with relatively high contents of both nickel and nitrogen at 1970 K annealing. These were related to nickel-nitrogen defects created as a result of impurity diffusion, principally nitrogen impurities [8,9]. The latter become Elsevier Science S.A. SSDI 0925-9635(94)00240-l
mobile already at 1800 K [ 8,9]. In Ref. [lo] some of these lines associated with broad bands at 330 and 440 nm were related to S2, S3 luminescence systems, which have been well studied in natural diamonds [ 21. Their analogues in ESR were revealed and the structures of the main three, denoted NEl, NE2 and NE3, determined [9]. The defects were found to contain nickel ions located in the divacancy position or in the position of double semivacancy according to the Masters approach developed for Ge and Si. This is a fragment which is common to all of them and is surrounded by nitrogen atoms whose number is n=2 for NE1 and II= 3 for NE2, NE3; n is also expected to take other values and indeed centres with n=O,l were described in Ref. [lo]. Taking into account that nickel ions can be in various charge states, including non-paramagnetic ones such as Ni” and Ni2+ , we used a preliminary photoexcitation in order to transform defects into the Ni+ paramagnetic state [ 111. Earlier such an approach was proved to be fruitful when studying a single vacancy in diamond, which undergoes a photoinduced transition from the V” state, responsible for the GRl system in absorptionluminescence, to the V- state with the ND1 system [Z].
178
A. P. Y&weyev.
V. A. Nudolinn~ i Diamond and R&ted
The same process takes place in the case of nitrogenvacancy defects such as N2V2, for which a transition from the H3 to the H2 vibronic system takes place upon illumination [ 121. Preliminary photoexcitation made it possible to reveal three other paramagnetic defects related to nickel-containing ones: NE5, which is analogous to NE1 with n=2 [lo], but where, in contrast with the latter, the nitrogen atoms are separated by vacancies from the nickel ion, thus forming an N-V( VNi-V))V-N chain [lo]; as well as NE6 and NE7 centres with n=O and 1 respectively. NE6 and NE7 correlate with thermostimulated luminescence (TSL) peaks at 130 and 160 K and are found to be shallow traps which have captured an electron. The present paper is devoted to a quest for photoinduced systems in optical absorption spectra which could be the optical analogues of NE5 in ESR [ 111. Investigation of the kinetics, generation spectra and optical and thermal stabilities showed that the photoinduced lines at 539.9, 546.6 and 552.9 nm correspond to two different nickel-nitrogen defects which are supposed to contain two or three nitrogen atoms in the case of the 546.6 nm line and only one nitrogen atom for the 552.9 (539.9) nm lines, while the photoinduction effect is a result of internal changes in the electron configuration of the defects.
2. Experimental
details
To investigate the photoinduction phenomena, a setup based on an MDR-2 diffraction monochromator with Xe and glow lamps as light sources was used. The generation spectra for the photoinduced lines were obtained point by point. In every case at first the absorption spectrum at 80 K was recorded, then the sample was illuminated directly inside a liquid nitrogen cryostat at the same temperature using light of a certain wavelength and the spectrum was recorded a second time for the intensity of absorption lines. After that the diamond was heated to 700 K, at which temperature the photoinduced lines are found to be removed, and again cooled to 80 K to obtain the next point. The intensity value for a particular absorption line was plotted vs. wavelength. When the thermal stability of photoinduced lines was studied, the sample was excited at 80 K and then heated to a certain temperature, after which the line intensity was measured at 80 K. In a similar way the spectral dependence of the efficiency of light sum storage in various TSL peaks was obtained. In all cases the kinetics of absorption or of TSL signal growth was preliminarily studied in order to provide operation under linear dependence of the measured parameter on excitation duration. The ESR spectra were recorded using an El09 “Varian” spectrometer operating in the X band and
Mutrriuls
4 i 1995) I77
IX5
80-400 K temperature range, while photoexcitation was produced by a mercury DRSh-500 lamp directly inside an ESR spectrometer cavity. The IR absorption spectra were obtained on Perkin-Elmer 325 spectrophotometer with a microdevice. Synthetic diamonds up to 7 mm in size, mainly of octahedral habit, were grown in the Design and Technological Institute of Monocrystals in Novosibirsk in a polyanvil high pressure apparatus of the splitsphere type [ 131. The as-grown yellow diamonds, obtained in the FeeNi-C system using the thermal gradient technique at 1620-1670 K and 5.5 GPa, were annealed at T > 2000 K in the same apparatus for 4 h. After annealing, the yellow colour diminished and the diamonds became slightly greenish. The nickel concentration as centres with g = 2.031 was about lO”~- 1018 cm-j in as-grown diamonds, while the donor nitrogen content was estimated to be 10’9~1020 cmm3 according to ESR and IR absorption data. After annealing at T>2000 K and 5.5 GPa, the signal from Ni+ with g= 2.031 disappears from the ESR spectrum and the concentration of donor nitrogen (or C centres [l-3]) decreases by approximately two orders. Simultaneously, signals from paramagnetic NEl, NE2 and NE3 centres [lo] appear, but the total nickel concentration in these forms is several times lower than in as-grown diamonds. This is additional proof that various forms of nickel defects are present in diamonds, including non-paramagnetic ones.
3. Results and discussion 3.1. Optical
absorption
spectra
The yellow colour of as-grown synthetic diamonds is mainly a result of C centre absorption [l-3]. The transmission spectrum for as-grown diamond 2 mm thick is shown in Fig. 1 (curve a). The transmission edge is situated at 480 nm. Annealing at T> 2000 K is accompanied by a weakening of the yellow colour, which is associated with absorption of donor nitrogen, and the transmission edge shifts to 350 nm. The greenish colour is a result of absorption in a vibronic system with ZPL at 793 nm. The maximum of the system is located at 700 nm and its short wave wing enters the visual range, weakening the red light transmission in the crystal. Another reason for the yellow colour is the broad band at 440 nm which is well defined on the background owing to absorption of the remaining C centres (Fig. 1). This band corresponds to the CI band in luminescence excitation-absorption spectra of defects responsible for the intensive green luminescence of these crystals in S2 and S3 vibronic bands. The other broad band (p) typical for these systems is usually situated at 330 nm but is hardly observed here because of C centres absorption.
A. P. Yelisseyev, V. A. Nadolinny J Diamond and Related Materials 4 (1995) 177-185
179
A
;I;;: e d
I
540
350
Fig. 1. Transmission spectra of synthetic 2000 K (b) and after additional excitation other for convenience.
400
450
500 h (nm)
550
I
550 h (nm)
600
diamonds at 80 K for as-grown diamond of yellow colour (a), for the same sample after annealing at by 366 (c, d) and 546 nm (e) Hg lines. Spectra b and c as well as d and e are shifted with respect to each
A rich fine structure has been recorded after annealing in these spectra in the 330-370 nm region and at A>465 nm. The full list of lines observed at 80 K is given in Table 1. In yellow as-grown crystals only a few vibronic systems are observed (columns 1 and 2). The systems with ZPLs at 494, 658 and 793 nm were related in Refs. [8,9] to purely nickel defects, while the other one with ZPL at 732 nm and two vibration replicas due to the ho = 52 f 1 meV phonon were associated with nickelnitrogen defects. In columns 3 and 4 the positions of lines recorded in annealed synthetic diamonds are given in comparison with lines in annealed synthetic diamonds grown by H. Kanda at NIRIM (Japan) [ 891 and natural diamonds with green luminescence from Yakutia deposits [ 14,151. It can be seen that most of the lines are found in annealed diamonds from both NIRIM and the Design and Technological Institute of Monocrystals, Novosibirsk, excluding maybe some at long waves. Most of the lines were associated with nickel-nitrogen defects in Refs. [ 8,9]. Column 5 gives the results of decomposition of the fine structure in the 360-500 nm range into three groups, each of which is due to a certain defect [lo]. One of these defects is responsible for the S3 vibronic system with ZPL at 496.7 nm in luminescence at 80 K. The other ZPLs, which are situated at 532.2 (the A line according to Refs. [14,15]), 489.1 (B), 477.8 (C), 472.5 (D) and 470.5 nm (E), were thought to be related to a single vibronic system such as S2 [8,9]. The use of the luminescence excitation technique showed them to
be due to at least two different defects [lo]. A certain confusion could result from a previous paper [lo] where we associated the “S2” notation with the 489.1 nm line (B), while most authors relate it to the A line at 532.2 nm [ 1,16,17]. To avoid further misunderstanding, we shall use “S2” to indicate both defects and give the ZPL position when speaking about one of these defects. These defects appear to be very similar because they both contain the nickel ion in a divacancy position surrounded by three nitrogen atoms, but located in a different way in the diamond lattice [IS]. Thus in column 5 of Table 1 (a) denotes the lines which are associated with the system with ZPL at 489.1 nm in luminescence and NE2 in ESR, (b) 496.7 nm (S3 system), NE1 and (c) 523.2 nm, NE3 [lo]. Detailed analysis of the fine structure of the B band in the luminescence excitation spectrum for the 489.1 nm ZPL shows that in contrast with previous papers [ 14,151 there is only a ZPL at 366.7 nm (3.380 eV) accompanied by several replicas due to a single phonon with ho= 32 + 2 meV. Since this value is lower than 68 meV, which corresponds to the minimum energy for vibration in the diamond lattice at various points in the first Brillouin zone [ 1,3,17], the mentioned maxima in the spectra can be related to a quasi-local vibration. The low energy value is likely to indicate participation of a heavy ion such as nickel [ 171. Long-term luminescence decay as well as large values, up to 5-10, of the HuanggRhys factor, which characterizes the electron-phonon coupling for a particular centre, are typical for such cases [ 171. Besides S2, S3 systems, other examples are systems
180
A. P. Yelisseyev,
Table 1 The fine structure (at 80 K) Synthetic As-grown E, eV
diamonds
in absorption
spectra
V. A. Nadolinny j Diamond urzd Rebted
of as-grown
and annealed
synthetic
dtamonds
1.563 1.692 1.746 1.797
793.6 132.4 711.0 689.9
1.884
658.2
Annealed E, eV
m compartson
with natural
i. nm 996.0 987.2 913.9 8X3.2 860.2 793.6
1.565
at 2000 K
1.245 1.256 1.357 1.404 1.441 1.563
4 ( 1995) 177- 1X5
Synthetic diamonds from NIRIM (annealed at 2000 K [8,9] E. eV
from DTIMC
i, nm
Muteriuls
diamonds
from Yakutia
deposits
Natural diamonds [ 14.151 E. eV
1.693 1.744 I .794 I X47 1.884
1.938 2.200 2.242” 2.268 2.296 2.351” 2.374 2.385 2.394 2.405 2.429” 2.458 2.419 2.515
563.5 552.9b 546.6’ 539.9b 521.3 522.2 520.2 518.0 515.7 510.4 502.2 500.2
2.24 2.267 2.296 2.351 2.383 2.392 2.404 2.421 2.465 2.476
493 2.523
2.582” 2.598 2.623” 2.627” 2.652 2.658
478.4’ 477.2cd” 472.8’ 471.9d 461.6’ 466.5*
3.341 3.380” 3.412” 3.442” 3.474 3.502 3.560 3.572 3.607 3.689
371.1d 366.1d 363.4d 360.1d 356.9d 354.0* 348.2* 347.1’ 343.7’ 336.1’
a These lines can sometimes be found also in the (111) sectors: b The photoinduced lines in absorption: ’ ZPL in absorption/luminescence excitation d ZPL in absorption/luminescence excitation ’ ZPL in absorption/luminescence excitation
specific points
spectra spectra spectra
of as-grown
2.574 2.589 2.596 2.618 2.648
2.655
J
3.380 3.412 3.443 3.414
synthetic
diamonds
(near seed, large inclusions
and in junctions
between
related to vibronic system with ZPL at 2.370 eV (NE3 [IO]): related to vibronic system with ZPL at 2.535 eV in luminescence (NE2): related to vjibronic system with ZPL at 2.496 eV (S3 vibronic system. NE1 [lo]).
with ZPLs at 578.08 nm with ht~=30 meV vibration and at 455.7 nm (34 meV) in brown natural diamonds, which differ in their large content of heavy elements such as Fe, Ni, Mn, etc. [16]. Another example is the system with 484 nm ZPL and lzto= 20 and 35 meV
vibrations in cathodoluminescence and absorption which was proved to be of nickel nature [ 181. It should be noted that as-grown synthetic diamonds can be quite uneven in colour and luminescence. Along with intensively yellow octahedral sectors. there are
A. P. Yelisseyev, V. A. Nadolinny / Diamond and Related Materials 4 (1995) 177-185
181
6r
250
300
350
400
450 h
Fig. 2. Normalized
generation
spectra
500
for 546.6 (1) and 552.9 nm (2) absorption
regions where the colour is much lighter and bands due to pairs of nitrogen atoms (A centres), with the main are observed in IR asorption. one at 1280 cm-’ [l-3], The same zones also show a more intensive green luminescence in S2, S3 systems. This occurs at junctions of adjacent octahedral sectors as well as in regions near a seed or large metal inclusions and shows an opportunity of creating complex defects including several impurity atoms directly during the growth process at relatively low temperatures such as 1700 K. The most intensive lines which are typical for annealed diamonds can be observed in spectra obtained at these points. These lines are marked (a) in column 3 of Table 1. The three lines marked (b) in column 4 appear only after preliminary illumination of annealed diamonds. The transmission spectra obtained before and after excitation at 80 K are shown in Fig. 1, while the three photoinduced lines at 539.9, 546.6 and 552.9 nm are shown in detail in the inset, The last two of these are ZPLs, while the line at 539.9 nm is a little weaker and wider and corresponds to a vibration replica from the 552.9 nm line with ho = 54 meV energy. Its small value, which is lower than 68 meV, can be an argument in favour of possible participation of a heavy ion such as nickel in defect formation. The intensity of the photoinduced lines is comparable with that recorded for other ordinary lines of nickel-nitrogen nature which are present in this spectral range (Fig. 1) [S,9] and reaches several cm ‘. In photoluminescence spectra these lines look like sharp holes on the background of S2 and S3 vibronic systems and can be very deep, down to complete luminescence quenching because of self-absorption and
550
600
650
(nm) lines in annealed
synthetic
diamonds
at 80 K.
large optical path lengths for luminescence inside a sample in some cases. 3.2. Conditions
of photoinduced
emission
line generation
In Fig. 2 the normalized generation spectra for the photoinduced lines at 546.6 and 552.9 nm, obtained after excitation from an Xe 1 kW lamp through the MDR-2 monochromator, are given. The intensity dependence on excitation duration was measured preliminarily. The kinetics of the photoinduced line intensities at 430 nm excitation are shown in Fig. 3. Both lines are generated
t (min) Fig. 3. Kinetics of absorption lines under 440 nm excitation
growth of 546.6 (1) and 552.9 nm (2) at 80 K.
and their intensities grow at the same rate during the initial stage of the excitation process up to 1 min at a given excitation level. Later the 546.6 nm line intensity continues to grow but at a different rate, while the 552 nm line saturates (Fig. 3). The generation spectra for the 546.65 and 552.9 nm lines were found to differ considerably (Fig. 2). For the first line the spectrum begins at 500 nm and shows maxima at 350 and 440 nm corresponding to the B and c; bands in the luminescence excitation spectra for emission of the 489.1 and 496.7 nm ZPLs [10,14,15]. The hardly pronounced minimum between the bands at 400 nm is likely to show participation of the third defect with emission of the 523.2 nm ZPL, which demonstrates a band at 400 nm in the luminescence excitation spectrum [9]. Thus the generation spectrum 1 in Fig. 2 indicates that transition into a new state which is characterized by absorption at 546.6 nm occurs when NEl, NE2 and possibly NE3 nickel-nitrogen defects are excited. The single line in the absorption of the photoinduced centre indicates that only one certain defect from this group undergoes the transformation. For the 552.9 and 539.9 nm lines the generation spectra are similar and begin already at 600 nm, displaying maxima at 390 and 500 nm (Fig. 2, curve 2). Minima at 340 and 440 nm correspond to maxima in spectrum 1 for the 546.6 nm line as well as to B, x bands in luminescence excitation--absorption of NEl, NE2 defects. Thus spectrum 2 is likely to be related to a defect with an absorption which is continuously growing from 600 nm to shorter waves; variations in intensity are due to competition between this defect and NEl, NE2 defects which absorb part of the exciting light. In analysing a set of defects with suitable characteristics which are present in synthetic diamonds and which could be responsible for the 539.9 and 552.9 nm photoinduced lines, one can focus on a single nitrogen atom in the substitutional position, known as a C centre [l&3]. However, since these lines are related to nickel-nitrogen defects [S,9] and are not found in natural Ib diamonds, they can be associated with a complex defect where in addition to the nickel ion a single nitrogen atom is present as a fragment, possibly separated by carbon atoms from the nickel ion. These assumptions about the structure of the defects responsible for the photoinduced lines agree very well with Ref. [9], where the influence of annealing temperature on the absorption line intensities in synthetic diamonds has been studied. Both the 539.9 and 552.9 nm lines were found to demonstrate a similar behaviour, decreasing in intensity as the annealing temperature was increased from 1970 to 2170 K, in contrast with the 546.6 nm line which was increasing. The changes could be considered as a result of further aggregation of impurities when an increase in annealing temperature stimulates transformation of defects from initial purely nickel or purely nitrogen centres into ones containing
both nickel and nitrogen. On the other hand, nickelnitrogen defects containing only one nitrogen atom (n = 1) are transformed into more complicated ones where 12=2 or 3, to which the above-mentioned NEl, NE2 and NE3 defects are related. It is curious that the simpler defects responsible for the 552.9 nm line can be created with a higher probability directly during growth at relatively low temperatures such as 1700 K (Table l), in contrast with the more complex defects responsible for the 546.6 nm line which were not found in the absorption of as-grown diamonds. 3.3. Remocul
of photoinduced
lines
Attempts to remove the photoinduced lines using illumination by Xe or Hg lamps up to 1 kW in power appeared to be unsuccessful even when selecting the necessary spectral range by broad band filters of coloured glass. The only way to remove the lines is to anneal the crystals. The dependence of the photoinduced line intensity on the temperature to which a sample was heated at a rate of 20 K min-’ is shown in Fig. 4a. The behaviour was found to be quite similar for both the 546.6 and 552.9 nm lines, which show no changes up to 570 K and then are quickly removed by 640 K. Plotting the intensity on an Arrhenius scale, one can evaluate the energy of thermal activation as E=O.SO + 0.10 eV. The fact that only thermal removal takes place shows that the excitation stimulates the photoinduced internal transformation of the electron configuration of the corre-
0.6
5
2 0.4 w 0.2 0
100
200
300
400
500
600
700
800
T (K) Fig. 4. Temperature dependence of absorption of 545.6 and 552.9 nm lines (a) and thermostimulated luminescence curve (b) for annealed synthetic diamonds. In the latter the posItIon of horizontal lines relates to the energy of temperature activation for certain traps, while their length indicates the temperature range where the TSL glow matches with a given Er value. The full lines represent the “initial rise” calculations and the broken lines the corresponding results by the “general” method.
A. P. Yelisseyev, V. A. Nadolinny / Diamond and Related Materials 4 (1995) 177-185
sponding defect. It is curious that the 570-640 K temperature range in which the defects containing a structural fragment such as a double C-C bond and corresponds to a transition into a triplet state [ 191. The alternative mechanism of photoinduced transformation in crystals is a recharging of various defects under photoexcitation as in the case of vacancies [ 1,3] or N2V2 defects responsible for H3, H2 vibronic systems [ 121. 3.4. Recharging luminescence
of defects; thermostimulated
curves
Thermostimulated luminescence may be defined as the emission of a crystal during heating after preliminary excitation at low temperature. The recorded glow peaks are associated with charge carrier release from shallow traps where they are localized at excitation and their radiative recombination on the ionized luminescence centres. On one hand, analysis of TSL curves allows one to reveal the mechanism of the recharging process, the charge transfer between various defects which are considerably remote in the lattice. On the other hand, TSL analysis allows determination of the parameters of the traps and sometimes their structure. Fig. 4b shows the TSL curve for annealed synthetic diamond which was obtained after excitation with the 366 nm Hg line at 80 K and a heating rate of 20 K min-l. The main peaks are situated at 160, 360, 600 and 710 K; a weaker one can be seen as a shoulder on the low temperature side of the 160 K peak. The curve indicates charge carrier release from at least five types of traps. In Table 2 the results of calculations of trap parameters such as depth or energy of thermal activation, Table 2 Parameters
for traps of charge
Position of TSL peak T, K
Halfwidth 6,162 T, K
130
carriers
in annealed
synthetic
183
E,, frequency factor and TSL kinetics order are given. Two methods were used: the “general” method of Antonov-Romanovsky which uses the whole TSL peak [20] and the well-known “initial rise” method where only the initial part of the peak is analysed. In column 5 the temperature range where the glow peak follows certain E,, W, parameters is given. For most of the peaks good agreement between the results of the two methods was found. A somewhat different value of E, = 0.28 eV for the 215-280 K range and a difference in E, obtained using the two methods for the 600 K peak indicate that there are additional traps with ET =0.28 and 0.55 eV. In Fig. 5 the spectral dependence of the efficiency of light sum storage in various TSL peaks is shown. The spectra were found to be similar for the 160, 360 and 600 K peaks. The most effective TSL excitation begins at 500 nm, the spectra being similar to luminescence excitation-absorption spectra for NE1 and NE2 defects [lo]. Two c1 and 0 broad bands are well defined at 440 and 350 nm. For the low temperature peaks at 130 and 160 K TSL excitation is possible also at longer wavelengths (from 600nm), though with much lower efficiency, and donor nitrogen ionization can be supposed. Thus at A< 500 nm excitation the complex nickelnitrogen defects such as NE1 and NE2 as well as donor nitrogen can be ionized. The form of the TSL curves remains invariable at different wavelengths of excitation, showing that the lower excited levels of the c1 group for NEl, NE2 defects are situated close to the edge of the forbidden band just under the bottom of the conduction band or directly inside it. The excitation into the a band is accompanied by defect ionization. To a larger extent
diamonds
Energy of thermal activation E, eV
Method of calculation
0.15
i.r.
Temperature range
Kinetics order
Frequency factor 0, SC’
T, K
80-120
160
22.5/45
0.23 0.22 0.24 0.19
gen. ix. i.r. ix.
130-250 130-150 160&180 18OC230
11
2.5 x 103
360
75/61.5
0.39 0.40 0.37 0.37
gen. i.r. i.r. i.r.
240-350 240-310 270-320 3OOG350
II
4.5 x 102
600
45150
0.63 0.55
gen. i.r.
470-700 470&600
I
3 x 102
710
25/l 7
1.92 1.72
gen. i.r.
6OOG750 6OOG670
I
1.3 x 10’2
gen. is the general at low-temperature
method of Antonov-Romanovski [20]; i.r. is the initial rise method and high-temperature sides of the peak respectively.
[20];
6, and 6, are the half-widths
for TSL peaks, measured
184
A. P. Yelisse_yev, V. A. Nudolinn~ ! Dimmond and R&ted
Matrricrls 4 i 1995) 177-185
The NE5 system is described
600 h (nm)
Fig. 5. Normalized spectral dependence of light sum storage efficiency in thermoluminescence peaks at 160 (l), 350 (2) and 600 K (3) under 80 K excitation.
it is related to excitation into the l3 band, which corresponds to the other group of excited states situated higher in the energy level diagram. It is curious that as in the case of the negatively structure is well charged vacancy V- , the vibronic defined in absorption despite the superposition of energy states of the local centre and states of the conduction band. A different situation is realized for NV defects responsible for the N3 vibronic system, for which the lower excited state is 0.5 eV under the edge of the conduction band [21]. In this case no free electrons are generated at 415 nm excitation and some N,V defects are ionized only as a result of channel transition of an electron to adjacent traps. The latter are likely to be distorted by the luminescence centre and have parameters which are different from those for remote traps. The TSL curves are of different forms for these two cases. All the peaks shown in Fig. 4b and Table 2 are easily bleached under i, > 600 nm illumination. 3.5. Photoinduced
systems in ESR
Preliminary excitation also results in changes in ESR spectra. Thus 366 nm Hg excitation allowed us to reveal three new systems in annealed synthetic diamonds, denoted NE5, NE6 and NE7, which were described in detail in Ref. [ 111. Here only brief information is given in order to compare the behaviour of paramagnetic and optically active centres under illumination and heating.
by the spin hamiltonian
where S = f and g-2.0903 (gi 11(011)), g,=2.039, g,= 2.044 (.Y(g,,(Oli))=8 “). It demonstrates a hyperfine structure (HFS) resulting from two magnetically equivalent nitrogen atoms with Af = Ai= 12.25 G (Af and Ai ~i(lli) and .4:=.4$=9.5G. The defect was shown to be a nickel ion in a divacancy position surrounded by two nitrogen atoms as for the NE1 centre, but here, in contrast with the latter, the nitrogen atoms did not reach the first coordination sphere of nickel and occurred separated from it by vacancies, thus forming an N-V( VNiiV))V-N chain [ 111. The NE5 spectrum is induced after i. ~400 nm excitation and removed by 540-720 nm illumination. Heating to 1100 K does not affect the NE5 intensity. If a signal has been removed by illumination, it can be restored by heating to 570 K. The energy of thermal activation for this process was estimated to be 0.2 eV. The behaviour of the defect with illumination and heating can be understood by assumping an internal transformation inside a defect for which a potential energy function U(r) in a one-coordinate scheme has two minima of different depths separated by an energy barrier. An electron, which initially is in a shallower minimum with more gentle inclination of the walls, at i ~400 nm excitation reaches a slope of a deeper minimum and appears at its bottom after relaxation. It can also get there by heating to overcome the 0.2 eV energy barrier. The reverse transition is likely to need much larger effort and temperature but can occur at 540-720 nm excitation. Detailed investigation of the NE5 system is in progress, but it is clear that it cannot correspond to any of the photoinduced absorption lines. The other two photoinduced centres NE6 and NE7 become paramagnetic after illumination by blue or UV light. They both have S = f. The NE6 system demonstrates no HFS and has g, = 1.995 (gl iI{1 lo)), g2 = 2.207 (gz~~(llO)), g,=2.0109 (gJ/(OOl)). NE7 has a g factor which is considerably larger than 2 and an HFS from one nitrogen atom; the value of the HFS splitting varies from 6 to 10 G depending on the orientation [ 111. Illumination at i > 540 nm results in complete removal of these signals; the same is observed upon heating to 180 K. This behaviour is typical for shallow traps responsible also for the 130 and 160 K TSL peaks, the ESR signal being related to states when they are filled. When the charge transfer between various defects is considered, one of the most important problems is to determine the sign of the carriers. In this context it is necessary to pay attention to the fact that at 366 nm Hg excitation a decrease in intensity of the donor nitrogen signal is observed along with the appearance of NE6 and NE7 spectra in ESR. This occurs only under low temperature (e.g. 80 K) excitation and varies from sample
A. P. Yelisseyev, V. A. Nadolinny / Diumond and Related Materials 4 (1995) 177-185
to sample, reaching 12% in some crystals. No changes were observed under room temperature excitation even at 0.015% sensitivity level. After excitation at 80 K the nitrogen intensity in ESR can be restored by Y > 600 nm illumination, while the NE6 and NE7 systems disappear completely. The same result can also be achieved by heating to 300 K. All these facts characterize NE6 and NE7 centres as shallow traps of electrons generated by donor nitrogen ionization. Since the same TSL peaks appear after ionization of NE1 and NE2 centres (Fig. 5) one can suppose that Ni+ -+NiZf transformation takes place in these defects, accompanied by generation of free electrons. The other TSL peak at higher temperature (600 K) in synthetic diamonds was also found to be caused by nickel impurity [ 22,231.
185
(5) The shallow nickel-containing traps are supposed to cause the n-type conductivity of synthetic diamonds. This can seem particularly curious taking into account the situation with n-type conductivity in diamond: no reliable element, defect or method is known to create it to date. The purely nitrogen defects are known to cause only deep levels in the forbidden band which weakly affect the conductivity of diamonds.
Acknowledgements The authors are grateful to Professor N. Sobolev for interest in the work and to Dr. V. Vins and B. Feigelson for lending them as-grown and annealed synthetic diamonds for investigation.
4. Conclusions (1) The photoinduced absorption lines in annealed synthetic diamonds are due to two different systems with ZPLs at 546.6 and 552.9 nm, while the 539.9 nm line is a phonon replica of the latter with ho = 54 meV. Both defects are similar in generation kinetics, temperature and optical stability. (2) Analysis of generation spectra and the effect of annealing on the intensity of photoinduced lines leads one to suppose that NE1 or NE2 defects are responsible for the 546.6 nm line, while a similar defect but containing only one nitrogen is responsible for the 552.9 nm line. A nickel ion in a divacancy position, which is a main structural fragment of both defects, is surrounded by two or three nitrogen atoms in the case of the 546.6 nm line and by only one nitrogen for the 552.9 nm line. An increase in annealing temperature results in further impurity aggregation in the diamond lattice and changes the ratio between different defects in favour of those which contain more nitrogen atoms, such as NEl, NE2 (546.6 nm line). (3) Along with the internal transformation, a recharging process in complex nickel-nitrogen defects such as NEl, NE2, NE3 and in donor nitrogen takes place at 80 K under photoexcitation. Excitation results in their ionization, with the free electrons generated being captured in the traps. Some of the latter, which are responsible also for TSL peaks at 130 and 160 K, become paramagnetic under excitation and correspond to purely nickel defects such as NE6 or to nickel-nitrogen ones such as NE7. (4) The obtained data demonstrate a variety of nickelnitrogen defects in synthetic diamonds. Defects of this type, which are responsible for S2, S3 and 793 nm vibronic systems, strongly affect the luminescence and colour of annealed crystals. In connection with the prospects of stimulated emission generation in diamonds in some of these centres [17], the absorption lines in the visual region are very important. They can destroy the continuity of wavelength tunning, particularly in the multipass regime.
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