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A review of neutron radiation damage on corundum crystals
644
Journal of Nuclear Materials 108 & 109 (1982) 644-654 North-Holland Publishing Company
A REVIEW OF NEUTRON RADIATION DAMAGE ON CORUNDUM CRYSTALS J.H. CRAWFORD, Jr Department of Physics and Astronomy, Universiiy of North Carolina at Chapel Hill, Chapel Hiil, NC 27514. USA
The first neutron radiation damage studies were carried out on corundum (Also,) crystals nearly thirty years ago by Levy and Dienes and nearly twenty years ago Compton and Arnold measured the threshold energy for atomic displacement using energetic electrons. However, it was not until about five years ago that a series of optical studies of crystals bombarded with neutrons and with energetic ions laid the basis for identifying radiation-induced lattice defects. Since that time a variety of lattice defects have been identified through their optical absorption and emission characteristics. Some information on the spatial distribution of neutron damage has been obtained through a study of the effect of annealing on the luminescent efficiency which increases as the high local damage concentration in the displacement spike is annealed, thus decreasing concentration quenching of F centre luminescence. In this paper all of the recent developments related to the identification of
lattice defects created by fast-neutron irradiation and the annealing of damage till be reviewed with particular attention to outstanding problems and areas needing addition work.
1. Introduction
Because of its high melting point, chemical stability and excellent optical and dielectric properties, corundum finds many important applications. In addition to serving as a laser host, UV window material and a substrate for VLSI circuits, AlaO, is a potential insulator in a variety of high temperature energy-conversion devices. Hence there is a high interest in its radiation stability and the nature of the radiation damage it sustains upon exposure to high energy particles. Studies of y-ray coloration and fast-neutron damage of Al,O, single crystals goes back some 25 years to the work of Hunt and Schuler [1] who observed that exposure to X-rays introduces optical absorption bands at 230 nm (5.4 eV) and 400 nm (3.1 eV) in such crystals. At almost the same time Levy and Dienes [2] found that a 6.1 eV absorption band resulted from atomic displacements in crystals exposed to fast neutrons in a nuclear reactor and that the absorption could not be produced by ionizing radiation (X-rays and y-rays). Thus quite early it was recognized that ionization-type damage can influence only those imperfections (impurities and defects) already present in the structure but that this type of damage is distinct from structural damage associated with the displacement of lattice ions by elastic collisions involving massive particles. In general, ionization damage saturates at rather low exposures (105-lo6 rad) and the magnitude and type of the spectral changes depend upon the amount and type of impurity present.
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
Fast-particle damage, however, continues to grow to high fluences before saturation begins to set in. This same behaviour is exhibited by other ionic refractory oxides [3] and stands in marked contrast to the response of alkali halides in which X-rays and y-rays are known to produce copious quantities of anion vacancies (F-type centres) and interstitial halogen molecular complexes (V-type centres) even at quite low temperatures [4]. The purpose of this paper is to summarize the rapid recent progress in understanding both the nature of intrinsic lattice defects and their electronic structure in Also, on the one hand and the constitution of fast-neutron radiation damage on the other. The ready availability of high quality single crystals needed for K-device substrate material together with insights gained from radiation effects, studies on alkaline earth oxides have made it possible to identify specific defects responsible for a number of features in the absorption and emission spectra of irradiated Al,O,. Perhaps the most surprising discovery is that in Al,O, the generic, simple lattice defects, e.g. anion and cation vacancies, appear to behave in an analogous way to those in the simple cubic oxides even though the Al,O, structure is much more complex. In this review we have chosen to follow an historical development summarizing first the effects of ionizing radiation, adducing evidence for the various structures involved, and then we examine displacement-type damage and build a case for the identification of the defects responsible for various absorption and emission
J.H. Crawford, Jr /
Neutron
radiation damage on corundum
lation by which the optical and ESR findings could be connected was still missing so it was not possible to state which y-ray-induced absorption band was associated with the defect responsible for the ESR signal. A possible link was provided by infrared spectra associated with OH- impurity in Al,4 crystals. Turner and Crawford [ 121used thick crystals (55 mm) of Union Carbide, UV grade Al,4 and observed an IR absorption band at 3278 cm-’ which was almost completely replaced by a band at 3316 cm-’ when the crystal was exposed to 13’Cs y-rays. A beforeand-after comparison is made in Fig. 1. The thick crystal was necessary because of the low concentration of OH- estimated at - lOi cmu3 1 and the low oscillator strength of its IR absorption. The response of the 3278 cm-’ absorption to radiation is analogous to what is observed for OHin MgO in which the two corresponding absorptions at 3296 cm-’ and 3323 cm-’ have been identified with an OH- situated adjacent to a Mg*+ vacancy V&/centre) and an OH- adjacent to a Mg*+ vacancy which has trapped a hole on the opposite 02- ion [13], respectively. Therefore, it was concluded that these two absorptions arise from analogous centres in Al,O, but, because of the higher negative charge on the Al”’ vacancy, these are designated V& and V&. Considering the analogy between Al 203 and MgO in the IR spectra of Vo,-type centres, similar features in the electronic excitation region of the optical spectrum might also be expected. Again using 55 mm thick specimens because of the small concentration of absorbing centres, the spectrum represented by curve 1 of fig. 2 was obtained upon y irradiation. The dominant features
bands in neutron- and electron-bombarded crystals. Although our main concern is fast-neutron damage, identification of the defects comprising such damage requires evidence from the influence of ionizing radiation and thermochemical treatment as well as displacement-type damage. Therefore, evidence from these other sources will be discussed. However, there are some effects which are characteristic of neutron damage per se because of the spatial ~st~bution of defects around the primary collision site. These will be treated separately.
2. Effectsof ionizingradiation Among all of the absorption bands reported for nominally pure Al,O,, Levy [S] catalogued three as resulting from y irradiation: 5.4 eV, 4.3 eV and 3.1 eV. Gamble et al. [6] found that y irradiation introduced a single ESR resonance line with axial symmetry along the c-axis which they resolved into resonances attributed to (a) a hole trapped on an oxide ion adjacent to a site deficient in positive charge, perhaps a cation vacancy or au A13+ site occupied by a monovalent or divalent cation, and (b) an electron trapped at an anion vacancy. Although the second identification has not proved valid, the first one has. It parallels the ESR and ENDOR characterization of V-type centres in MgO 17-101 as due to O- centres adjacent to Mg2+ vacancies in various stages of compensation. Moreover, Cox [1 l] confirmed these observations by ESR measurements on Al,O, crystals, delibarately doped with Li+ and Mg+ , exposed to y-rays. Unfortunately, the wrre-
r
I
I
I OH
3275 Wavenumber
I
Spectra Before After
---
in Al,
~-Roy
Exposure
0,
3300 (cm-’
645
)
Fig. 1. OH- stretching-mode bands in Ai,O, before and after y-irradiation. (After Turner and Crawford, Ref. 119).
646
J.H. Crawford, Jr / Neutron radiation damage on corundum
6.0 50 0.4
4.0
II
Photon Energy (ev) 3.0 2.5 2.0
I
I
1.75
I
I
I I 500 600 Wovelength (nm)
I 700
I
I5
0.3 2 2 E EO.2 t 0 0’ ._ ‘, 9 2 0.1
OXl-
I/#
I
2!oO
300
I
400
800
Fig. 2. Absorption spectra of y-irradiated Al,O, following 3.0 eV optical bleaching for periods of (1) 0, (2) 5, (3) 15, (4) 35, (5) 75, (6) 155 and (7) 135 min. (After Turner and Crawford, ref.
1131).
are absorption bands at 5.46 eV and 3.02 eV, i.e. the same bands reported by Hunt and Schuler [l], and evidence for the weak band near 4.3 eV mentioned by Levy was also observed. The important feature, however, is the great width of the 3.0-3.1 eV absorption which is similar to that of the V-type absorption near 2.2 eV in MgO [IO]. Thus it seems reasonable to conclude that, analogous to MgO, the broad 3.1 eV absorption is associated with several trapped-hole centres, all involving an O- adjacent to a cation vacancy or some other positive-charge-deficient site as suggested by Gamble et al. [7], but that an important component is the V&n band for which good IR evidence exists. From bleaching studies [12] it was found that the 3.02 eV and 5.45 eV absorption were complementary. The latter could be completely destroyed by irradiating into the 3.02 eV band which was also bleached in the process. The results of this type of optical bleaching is shown by the family of curves in fig. 2. Excitation in the 5.45 eV region produced mutual bleaching of the two bands also, except that approximately one-third of the 3.02 eV band (now shifted to 3.1 ev) remains when then 5.45 eV band is gone. It is therefore concluded that the 5.45 eV band is associated with some type of elec-
tron trapping impurity which is able to accomodate - 2/3 of the electrons required to balance holes trapped at V-type centres. The major contaminant in the crystals used is Cr3+ as detected by its 6.9 eV charge-transfer absorption band [ 141. Moreover, in thermally stimulated luminescence (TSL) studies [ 121 the emission in the strong glow peak at 150°C was due to the Cr*+ ion. Finally, thermal annealing studies reveal that the 3316 cm-’ IR band is removed and the 3278 cm-’ band is restored in the region between 100 and 200°C which is the same annealing region in which removal of - 60% of the 3.02 band occurs and the large Cr2+ TSL glow peak maximum mentioned above appears. Definite evidence for three distinct V-type centres has been obtained by Lee et al. [15,16] from ESR studies. The dominant signal observed for the y-irradiated crystals before further treatment corresponded to a S = i centre with axial symmetry: g,, = 2.018 * 0.002 and (g, = 2.011 f 0.002 with a line width of 45 G. These parameters are somewhat different from those obtained by Gamble et al. [7] in that they exhibit a greater positive g-shift. Upon heating for - 70 h at 1350°C, a treatment which excludes the CH- completely from the crystal, two new resonances appear: (a) an S = i centre with g,, = 2.011 -t- 0.004, G, = 2.013 * 0.004, and a 50 G line width and (b) S = 1 centre with spin resonance parameters almost identical to those reported by Gamble [ 171and Cox [ 111,namely zero-field splitting parameters of D = 3.44 GHz and E = 0.86 GHz. The thermal stabilities of these centres are also quite distinct from one another. Fig. 3 shows the effect of isochronal annealing on the intensities of these three resonances. The solid curves refer to the S= 1 centre and the S = i centre observed after the high temperature anneal. The least stable centre is the S = 1 and,
-Oxidized
Fig. 3. Relative inters&s of hole centres Al 203 crystals during isochronal annealing. (--non-oxidized), S= f, 0 S = 1 (after and Crawford, Ref. [ 171).
in
y-irradiated oxidized; Lee, Holmberg
J.H. Crawford
Jr / Neuiron radiation damage on corundum
upon decay, it feeds into the S = 4 centres whose concentration is essentially zero at the start of the anneal. The S =s centre reaches its maximum concentration just when the S= 1 centres disappear and it in turn subsides at a higher temperature. This behaviour means that the two centres observed after heat treatment are two different charge states of the same defect. The most reasonable model is one and two holes trapped at a positive-ion vacancy, with the single-hole centre being more stable because of the double negative charge of the centre than the two-hole centre which is expected to have a single negative charge. The bare vacancy or V3centre is evidently created by the thermal destruction of the V& centre by loss of the proton. The stability of the V&n centre is intermediate between that of the S = 1, V- and the S = i, V2- centres as can be seen from fig. 3. Recently TL studies of Cooke et al. [ 181have given indirect evidence of the presence of the two-hole V,, centre. They observe TL glow peaks at 270 and 435 K whose emission spectra are identical and contain Cr3+ R-lines. The upper peak falls where the V&, is expected to lose its hole at a maximum rate peak falls where the Vo, is expected .to lose its hole at a maximum rate and the lower temperature peak is just about at the anticipated position for the loss of a hole from the two-hole, neutral V,, centre. In summary it is clear that O- ions adjacent to sites of positive-charge deficiency absorb in a broad band of photon energies centered near 3.1 eV. There is strong evidence that one such centre is the V&u and that an oxidative heat treatment at 135O’C converts this centre to a ‘room temperature stable two-hole centre, first observed by Gamble [ 171 and confirmed by Cox [ 111, which has been identified as a V- centre, i.e. two holes trapped at an A13+ vacancy. Above - 100°C the Vloses a hole becoming a V2- centre.
3. Optical absorption and emission bands of fast-neutron-bombarded Al 20J Levy [19] has analyzed the absorption spectrum of A1203 crystals irradiated in the nuclear reactor into Gaussian bands which lie at 6.02 eV, 5.34 eV, 4.84 eV, 4.21 eV, 3.74 eV, 2.64 eV and 2.00 eV. Of these the,6.02 eV band, which we refer to as the 6.1 eV and, was by far the most prominent. To get a better insight into the defect structures responsible for this absorption, Mitchell et al. [20] studied the dichroism of these bands produced by bombardment with both fast neutrons and 2 MeV electrons. Such a technique could be quite
647
informative since Al,O, has rhombohedral symmetry with the result that transition moments of the various defects could differ significantly for different orientations of the light propagation and polarization vectors relative to the crystallographic axes. It was found that the 6.1 eV band was optically isotropic. However, the 5.3 eV and 4.8 eV bands were strongly and oppositely polarized, with the 5.3 eV band being scarcely visible for the electric vector parallel to the c-axis. There was evidence of anisotropy in the lower energy bands as well. Another important development was the determination of the threshold energy of bombarding electrons for the creating of the 6.1 eV band by Compton and Arnold [21,22]. They found that an incident electron energy of 0.37 MeV or greater was necessary to produce. this absorption. Mitchell et al. [20] suggested that the relevant defect was an interstitial A13+ which had captured an electron and for such a structure the minimum energy transferred in an elastic collision, the so-called displacement energy Ed, would be 40 eV. On the other hand, if an oxygen vacancy were responsible, the value of Ed would be 90 eV. Because of the optical isotropy and the smaller displacement energy, the interstitial aluminium defect, which is expected to possess a rather high symmetry [20], was favoured as the origin of the 6.1 eV band. In reality, however, the evidence was not particularly strong and, indeed, it was not possible to say whether the centre has trapped an electron or a hole. One the fact that trapped holes are responsible for the 3.0 eV absorption had been established, it was possible by optical bleaching experiments to determine whether the 6.1 eV band arises from a trapped electron. Turner and Crawford [23] made such a determination by utilizing the Long crystals containing - lOI V& defects cm- 3. These were given a very light neutron exposure in the nuclear reactor which introduced between 10” and lOI eV “centres” cm-3. The result was a set of crystals with a comparable concentration of absorption centres for the 3.0 eV and 6.1 eV bands, a situation well suited to studying the relation between the two types of centres. If upon excitation in the 3.0 eV trapped-hole band, the 6.1 band either did not change or increased in amplitude, one could conclude that the latter was due to a defect which had trapped a hole. Gn the other hand, if bleaching with 3.0 eV light caused the 6.1. eV band to decrease in intensity, one has strong evidence that the 6.1 eV band is due to a defect which had trapped electrons. The results of the experiment are shown in fig.4 in which the absorption spectra of a crystal exposed to 4 X 10” n cmm2 after bleaching with
J.H. Crow&or&Jr / Neutron radiation ohwage
on ewundlun
Nautrcn Irrodicitad Al203 Followtng Blecrching at 410 nm.
0
Fig. 5. Nearest-neighbour cation positions around an anion site in Al,O,. 2.0
6.0
Fig. 4. Effect of optical bleaching with 3.0 eV photons on the
absorption spectrum of neutron-bombarded Alz03 (4X lOi5 n en-‘) and y-irradiation: (0) 0, (1) 5, (2) 15, (3) 35, (4) 75 min bleaching at 300 K. (After Tumer and Crawford, Ref. j25J).
3.0 eV light for various periods of time is shown. Obviously, the 6.1 eV band is subs~ti~y reduced, showing that its origin is an electron-trapping centre. It was also possible to obtain mutual bleaching of the two bands by exciting into the 6.1 eV band. Although these studies demonstrate the ch~e~app~ nature of the defect, there is stiIl no indication of its actual structure. The next development, just as in the case of hole centres, features ESR data. ESR investigations by La et al. 1241 have uncovered a 13 line resonance isotropic about the c-axis with a g-factor of 2.0029 in crystals exposed to - lose, cm-* at 77 K. The concentration of paramagnetic centres responsible for this resonance was - lot9 cme3 which is well above the total impurity content. Therefore they conclude that the centre is an intrinsic, structural defect created by the neutron bombardment. A likely candidate is the F+ centre, i.e., one electron trapped at an oxygen vacancy. To support this attribution, they calctdated wave functions for the F+ centre by the variational method using a point-ion model and including s, p and d basis functions. From these wave functions they c.aIculated the hyperfine interaction constants for the interaction of the electron spin with the =Al nuclei in the two non-equivalent pairs of A13+ ions which neighbour the oxygen vacancy. The model for the F+ centre is shown in fig. 5. The theoreti-
cal hyperfine constants were found to have a ratio of A/A, = 0.26, and for such a ratio, provided the resonance half-width AH is large enough to mask the smaller {A,) splitting, theory indicates that the hyperfine lines for the two sets of pairs will interfere in such a way as to give 13 component lines. The observed value of AH = 50 G fits the required magnitude of the half-width and the observed value of A,/A, is 0.27. Hence; even though the calculated magnitudes of A, and A, are a factor of three too large, a discrepancy not unreasonable for a point-ion calculation, the more important value for comparison is their ratio and this is in excellent agreement with experiment as is the 13 line spectrum. Therefore, the evidence that fast-neutron bombardment creates F+ centres in Al203 is quite strong. Unfortunately, they were unable to correlate the ESR spectrum with optical absorption bands in these heavily irradiated crystals. However, it was found that this “F+ resonance” signal could be detected after an exposure of - lO’* n cm-*. Since a large fraction of these centres may contain two electrons, which would render them diamagnetic and undetectable by ESR, the difficulty of observing such a resonance after smaller neutron exposures, i.e. - 10” cm-‘, is easy to understand. Point-ion calculations of the ground and excited state energies by La et al. [24] also provide the basis for identification of optical transitions of the F+ centre. The low symmetry of this centre (C, point group) splits the excited p-state, which would be triply degenerate in an isotropic environment, into three states designated in order of increasing energy as lB, 2A and 2B. Nence three transitions from the 1A ground state are expected:
J.H. Crawfor
Jr / Neutron radiation damage on corundum
1A - IB, lA-2A and IA - 2B with calculated energies of 2.26 eV, 3.39 eV and 5.15 eV respectively. For electric dipole transitions, only u transitions are allowed for A - A and only qr transitions for A - B. Thus these calculations provide guidance for using polarized excitations as a tool for identifying transitions experimentally. Lee and Crawford [25] observed that bleaching reactor-neutron-bombarded crystals with 6.1 eV light substantially decreased the intensity of the 6.1 eV band and increased absorption at 5.4 eV, 4.8 eV, 4.1 eV and 2.7 eV. The difference in absorption due to exposure to 6.1 eV light is shown in fig. 6(a), whereas the difference due to exposure to 4.8 eV light is shown in fig. 6(b) (5.4 eV light also restores the 6.1 eV band). All of these absorption bands have been previously observed by Levy [191 and Mitchell et al. [20]. It was further found that the 5.4 eV band could be well resolved by using polarized light since it absorbs light with E I c but not for E II c polarization (E is the electric vector of the light and c is the direction of the c-axis) in agreement with observations of Mitchell et al. [20]. Since the 6.1 eV band is known to be an electron centre, bleaching by 6.1 eV light is expected to release electrons. Therefore, the absorption bands enhanced by this process must be associated with either centres which have trapped electrons released from the 6.1 eV centre by photo-excitation or the new charge state of the latter. Other evidence [23], namely
6.6
6.0
5.0 4.0 Photon Energy (eV)
3.0
2.0
Fig. 6. (a) Effect of 6.1 eV light and (b) 4.8 eV light irradiation on the absorption spectrum of ELI A1,03 after exposure to 5 X lOI n cm-‘. (After Lee and Crawford, Ref. [30]).
649
the decrease of the 6.1 eV band and the growth of the 4.8 eV band by release of holes, either optically or thermally, from V-type centres and the restoration of these bands to their initial intensities by y-irradiation, indicates that the 4.8 eV band arises from the new charge state of the 6.1 eV centre. Excitation in the region of the 4.8 eV absorption produced emission of 3.8 eV light. Since this absorption band was not far from the 5.15 eV 1A - 2B transition predicted by La et al. [24] and since the quantum yield of the emission was greater for those relative orientations of dipoles and E of the polarized exciting light conductive to B excitations, it was concluded that the 4.8 eV absorption could be attributed to the 1A - 2B transition. Evans and Staplebroek [26] showed that this assignment was incorrect since their measurement of the excitation spectrum of the 3.8 eV emission revealed that it is produced with 4.8 eV, 5.4 eV and, probably 6.3 eV photons (fig.7). They assigned the 4.8 eV excitation to the lA- 1B r transition, the 5.4 eV to the IA-2A o transition and the 6.3 eV excitation to the 1A - 2B TL transition. The polarization behaviour accorded with this assignment since E II e produced a larger emission than E Ic for the 4.8 eV II transition and only E Ic at 5.4 eV produced emission. It was further concluded that the 3.8 eV emission common to these excitations is due
Fig. 7. Polarized optical absorption spectra of an Al,Op crystal exposed to 3.9X lOi MeV n cmm2 followed by a 20 min 300°C anneal. Emission or excitation spectra from other crystals exposed to 1.7X lOI n cmm2. (a) E-L c; (b) El/c. (After Evans and Stapelbroek, Ref. [31]).
J.H.
650
Crawford,
Jr / h’eutron radiation damage on corundum
to the 1B - 1A transition. This close correspondence (except in actual energy values) of theoretical predict-
I
shows
a linear correspondence
between
electron state in the F+ centre and procedure an excited F centre. Decay to the original ground state would restore the Ff centre with emission of a 3.8 eV photon. However, if there is a small but finite probability that the hole on the adjacent O- ion could migrate away to a trapped electron before recombination can occur, a stable F centre would result [27]. Recent cluster calculations by Choi [28] indicate that process (b) is indeed feasible.
4. Sub&active
and growth-cdoured
0
the
increase of the intensity of one of these bands and the decrease of the other. The slope of this line together with the room temperature half widths of the 4.8 eV and 6.1 eV bands can be used to estimate the ratio of their oscillator strengths: f,.Jj& = 0.55. The main puzzle in the bleaching studies is why excitation at 4.8 eV and 5.4 eV restores the 6.1 eV band intensity (fig. 7(b)). Two possibilities exist: (a) There is an underlying absorption at this photon energy associated with an occupied electron trap. (b) 4.8 eV and 5.4 eV photon absorption leads to charge-transfer transitions involving the cation vacancy, which adds an electron from a near-neighbour 02- ion to the F+ centre. This latter process would in effect occupy the vacant
I
“93
ions of the point-ion model and experimental observations give confidence to the conclusion that the F+ centre is indeed a product of neutron (and other particle) bombardment and that its optical transitions are the 4.8 eV, 5.4 eV and, quite likely, the 6.3 eV bands. Moreover, the close relationship between centres responsible for the 4.8 eV and the 6.1 eV bands established by bleaching studies [25] are a strong indication that the 6.1 eV band is an optical transition of the F centre, i.e. two electrons on an anion vacancy. Step-wise bleaching
I
6
s Photon
3
4 Energy
(,v
)
Fig. 8. Optical absorption spectra of Al,O,: (1) as received; (2) 1800°C heat treatment in Al metal vapour; (3) 2000°C heat treatment in Al metal vapour. (After Lee and Crawford, Ref.
1331).
“subtractive colouration” rather. than “additive colouration” is used. The half width of this band is -0.6 eV which corresponds to that of the F band created by kither electrons or neutron bombardment. Excitation in the 6.1 eV band results in an -emission at 3.0 eV (301 which is shown in fig. 9. The same emission has been observed at 77 K in neutronbombarded crystals though it is considerably weaker. The 3.0 eV luminescende can also be excited in certain as-received crystals coloured during growth. Draeger and Summers [31] have also noted this and have explored adsorption and emission of both F+ and F centres in such growth-coloured crystals. The subtrac-
tive colouration of Al,O, mentioned above indicates that colouration during growth is a likely process provided the atmosphere over the melt is sufficiently reducing, i.e. the oxygen partial pressure is less than the
AI,OJ 2
Further confirmation of the identification of the 6.1 eV band as a transition of the F centre is provided by subtraction colouration studies of Lee and Crawford [29,30]. Heating Al,O, crystals in the presence of Al metal vapour to temperatures near 2OOO’C for two hours results in the introduction of a substantial 6.1 eV band (fig. 8). It was found that it made little difference whether Al vapour was present or not, provided the atmosphere was strongly reducing. Hence the process must involve extraction of oxygen under conditions that provide electrons to compensate the positive charge on the anion vacancies left behind. For this reason the term
--.7----1-r I--
1
Fig. 9. Excitation and emission spectra of F centres in subtractively coloured Al *O, (After Lee and Crawford. Ref. [34]).
J. H. Crawford
Jr / Neutron radiation damage on corundum
decomposition partial pressure of Al,O, at the growth temperature. Whereas the lifetime of the 3.8 eV emission of the F+ centre is quite short (C 7 ns) as expected for an allowed transition [26], the decay lifetime of the 3.0 eV emission is quite long [30]. Fig. 10 compares the firstorder decay of this emission for a subtractively coloured crystal, a growth-coloured as received crystal and a neutron-bombarded crystal. The emission from the subtractively coloured crystal is the more intense ( lo2 more than for the neutron-bombarded crystal). All three crystals exhibit identical decay constants which correspond to a lifetime of the emitting state of 36 -C4 ms. This lifetime together with the large Stokes shift (6.1 eV compared with 3.0 eV) is similar to the emission behaviour observed for F centres in CaO and suggests a forbidden transition. The electronic structure of the two-electron F centre is helium-like, i.e. the (1s)~ configuration corresponds to the term ‘Sc. Upon excitation, one electron is promoted into either a 2s or 2p orbital, leading to possible singlet and triplet states (‘S, ‘S, ‘P and ‘P). In MgO and CaO, emission results from pumping into the ‘S - ’ P F centre transition (the F band)
3 eV
Emission
6.51
[32,33]. The ‘P state is filled by radiationless decay from the higher energy ‘P state and ihe forbidden transition sp-’ S follows with a long decay time. The situation in AlaOs is analogous, which suggests that the 3.0 eV emission arises ‘from similar transitions. However, it is noted that recent studies of the behaviour of the 3.0 eV decay below 77 K by Brewer and Summers [34] indicates that the situation is probably more complex. At room temperature the 3.0 eV emission occurs with a long phosphorescent tail [30]. This was first reported by Lehmann and Gunthard [35] who analyzed the emission into a first-order, temperature-independent component with a 36 ms lifetime and a second-order, temperature dependent component which they attributed to a metastable state of the impurity responsible for the emission. Lee and Crawford [30] have found the phosphorescent tail to be specimen-dependent (present in some and not in others). As shown in fig. 11 the growth-coloured crystal shows only the 36 ms decay, whereas the subtractively coloured crystal shows the extended phosphorescence. Hence it appears that an electron-trapping impurity which discharges in the order of seconds is present in the latter but not in the former. Although Lee and Crawford [29] found the 4.8 eV and 5.4 eV bands to be too weak to be detected in their subtractively coloured crystals, the presence of F+ centres is revealed by the 3.8 eV emission which can be excited by photons with these energies. To demonstrate that the phosphorescent tail of the 3.0 eV F centre emission is indeed due to temporary storage of electrons originating at F centres via the process F*“- F+ +e,
3eV Emission
I
0
I
40
I
80
I
120
Timelm
I
160
200
set)
Fig. 10. Decay of 3.0 eV emission at 77 K from subtractively colored Al,O, (SLC), from as-received Adolf Meller Al 20, (ARA) and from neutron-bombarded Al,O,. (After Lee and Crawford, Ref. (341).
L
Time (m set)
Fig. 11. Decay of 3.0 eV emission at 300 K from subtractively coloured Al,O, (SCL) and as-received Adolf Meller Al,O, (ARA). (After Lee and Crawford, Ref. [34]).
652
J.H. Crawford
Jr / Neutron radiation damage on corundum Conduction
Time
t cm
CA
(Second) t on
(6.leV
F- center Light)
A’z03
Band
F+ center
Fig. 13. Optical transitions (excitation and emission) associated with the F+ and F centres in Al,O, crystals.
Fig. 12. Effect of 6.1 eV light on (a) 3.0 eV emission (b) 3.8 eV emission with constant 4.8 eV excitation in a subtractively coloured crystal at 300 K. (After Lee and Crawford, Ref. [34]).
the F+ emission was monitored [30] while pumping periodically at 6.1 eV and continuously at 4.8 eV. The results are shown in fig. 12. The upper panel (fig. 12(a) shows the intensity of the F centre emission as the 6.1 eV exciting light is turned on and off. The 3.0 eV emission intensity increases during the first few seconds while the electron traps are being filled to a saturation value. The rise time of this square pulse is approximately the decay time of the traps. Saturation of the 3.0 eV emission intensity occurs as soon as the electron trap occupation reaches a steady state. In this condition nearly every electron released from an F centre recombines with an F+ centre. When’ the exciting light is turned off the 3.0 eV emission rapidly decays initially until the phosphorescent tail due to the discharge of traps takes over. The lower panel (fig. 12(b)) shows the intensity of the 3.8 eV F+ emission through the cycle. The 4.8 eV exciting light is held constant and the small (- 10%) variation of emission intensity is due to the increase and decrease of F+ centre concentration associated with the loading and unloading of the electron traps responsible for the phosphorescent decay. This measurement provides a direct indication of the relation between the 6.1 eV and 4.8 eV exciting transitions and the 3.0 eV and 3.8 eV emissions, strengthening their identification with F centres and F+ centres, respectively. The various transitions considered above are summarized in fig. 13. Draeger and Summers [31] have observed that the photoconductivity peak at 6.1 eV can be observed at 10 IL Therefore, the ‘P state of the F centre must he very close to the bottom of the conduction band as indicated. The electron-trapping impurity level must lie -0.5-0.6 eV below the conduction band
to account for the phosphorescent tail and also for a thermoluminescent peak slightly below room temperature [30,36]. There are no doubt electron traps to account for the F” centres which produce the steady emission of fig. 12(b). Up to this point in the review we have been concerned with establishing the identity of simple point defects in their various charge states, defects which might be expected to result from various processes, i.e. ionization and displacement type damage as’,well as thermochemical treatment either during or subsequent to crystal growth. We now turn to those studies which focus more or less specifically upon fast-neutron damage. That one might expect to find such specific effects, of course, is associated with the rather special spatial distribution of fast-neutron damage, namely a displacement cascade occupying a rather restricted region of dimension - 100 A around the site of primary collision. There are two such studies. The first of these concerns concentration quenching of the 3.0 eV F centre luminescence in neutron-bombarded Al,O,. It was noted above (see fig. 10) that the 3.0 eV emission intensity at 77 K of a reactor-irradiated Al,O, crystal was two orders of magnitude smaller than a subtractively coloured one even though its F centre concentration was at least as large. To explore this matter, Jeffries et al. [37] have studied the influence of thermal annealing neutronbombarded crystals over the range from 300K and 850K. As the annealing proceeded there was an enormous increase in the emission efficiency as indicated by the ratio of relatively light output at 3.0 eV to the 6.1 eV absorption coefficient. This ratio along with the absorption coefficient measured during the course of the annealing is plotted against anneal temperature in fig. 14. The sharp increase in annealing efficiency during the anneal indicates that the fast-neutron damage is highly
J.H. Crawford ?looc
,
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400
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Fig. 14. (a) Relative luminescence intensity per F centre measured at room temperature for the 3.0 eV emission in neutronirradiated sapphire, following 15 min isochronal annealing at successively higher temperatures. (b) Absorption coefficient of the 6.1 eV F band in the samples after each isochronal annealing stage (after Jeffries et al., Ref. [37]).
and leads to concentration quenching due to the overlap of the excited state wave function of one centre with an adjacent unexcited F centre. This situation allows energy transfer and radiationless recombination to occur. It was estimated that the local concentration of F centres in displacement cascade is - 2 X 10” crnm3 which is ample for considerable concentration quenching to occur. During anneal the F centre density in each cascade decreases as oxygen atoms or ions recombine with vacancies, thus decreasing wave function overlap and increasing luminescent efficiency. It is interesting to note that F+ luminescence (3.8 eV) was not substantially affected by the anneal which is to be expected for two reasons: the wave function of the F+ centre is expected to be more compact and their concentration was considerably smaller than the F centres. The second experiment concerns the optical behaviour of a composite defect of the type expected to be produced only in high damage concentration. Welch et al. [38] studied the dichroic absorption of a band at 3.5 eV and the polarization of the associated luminescence in oriented Al,4 crystals which were bombarded with lo’* fast neutrons crne2 and 3 X lOI 6 MeV protons cm -*. The polarization behaviour of the absorption produced by neutron bombardment is shown in fig. 15. non-uniform
I 300
I 400 WAVELENGTH
I SO0
I 600
(nm)
Fig. 15. Absorption spectra at 4 K of an a-Also, crystal neutron-irradiated to a dose of lOI n cm-*. The light propagates along the [ lOTO]axis and curves A and B refer to the absorption spectrum for E I][0001] and for E 1[0001] respeu tively. Curve C is the background absorption of the crystal before irradiation. (After Welch et al., Ref. [38]).
Analysis of the 3.5 eV band and the 3.3 eV emission indicates that the electronic transition involves a linear defect inclined 40° to the c-axis. A defect which fits this orientation quite well is the oxygen-vacancy pair, i.e. a d&F centre which is the model they postulate for this centre. Although aaimilar spectrum can be produced by heavy proton (3 X lo’* cm-*) bombardment, the spectrum obtained after fast-neutron bombardment is much better defined, particularly with respect to a zero-phonon line at 3.38 eV associated with this centre. In view of the short range of protons (- 60 MI for a 3 MeV proton in Al,O,) the average damage density is much greater than in the case of reactor neutrons. However, because of the high local defect concentration in the displacement cascades, the probability of anion-vacancy pair formation is greater in the neutron case. Finally it is worth noting that interstitial defects have been detected in fast-neutron-bombarded Al,O, crystals thorugh ESR measurement. Cos and Herve [39] have observed two types of resonances both involving Al*+ pairs which have trapped an electron. One of these centres is created by reactor irradiation at 77 K and is stable for a few days at room temperature. Its axis is parallel to the crystal c-axis. The second resonance is associated with a “tilted” pair inclined 10’ to the c-axis. This defect pair is created by reactor bombardment at 300 K and is stable indefinitely. A third “axial” pair has been detected after 77 K irradia-
654
J.H. Crawford Jr / Neutron radiation damage on corundum
tion but it is unstable at room temperature [40]. These pairs evidently form as A13+ interstitials, take positions along the c-axis and interact with one one-site neighbour more strongly than another. However, in view of their rapid saturation at a rather low concentration (1016-1017 cmF3 range) it is suspected that they may be impurityrelated rather than intrinsic in character. To summarize our present state of knowledge of neutron radiation damage in Al,O,, we can say that the main point defect observable is the anion (oxide) vacancy in two charge states, i.e. as the F+ centre and the F centre. Due to the high concentration of these in displacement cascades, the F centre luminescence at 3.0 eV is largely quenched. Also a substantial anion divacancy centre is produced which exhibits a zero-phonon line at 3.38 eV and optically anisotropic “mirror image” absorption and emission bands at 3.5 eV and 3.3 eV respectively. There is some ESR evidence of A13+ interstitial-type defects in the form of Al pairs. Additional unidentified optical absorption bands at 4.2 eV, 2.6 eV and 2.0 eV have yet to be accounted for. And of course there are some riddles: why does neutron irradiation not produce evidence for A13+ vacancies, e.g. V-type bands? and most important, what form does interstitial oxygen assume. Much work needs to be done on these last two questions to produce a balanced picture of the nature of fast-neutron damage in corundum single crystals.
Acknowledgements
The author wishes to thank the US Department of Energy for support during the preparation of this review and work reported therein conducted at the University of North Carolina.
References [t] R.A. Hunt and R.H. Schuler, Phys. Rev. 89, (1953) 664. [2]’ P.W. Levy and G.J. Dienes, Rep. Bristol Conf. on Defects in Crystalline Solids (Physical Sot., London, 1955) p. 256. [3] For a review see B. Henderson and A.E. Hughes, Point Defects in Solids (Plenum, New York, 1972). Vol. 1, Chap. 7. [4] See S. Sonder and W.A. Sibley, Point Defects in Solids (Plenum, New York, 1972) Vol. 1, Chap. 4. [5] P.W. Levy, Discuss. Faraday Sot. 31, (1961) 118. [a] F.T. Gamble, R.H. Bartram, C.G. Young, O.R. Gilliam, P.W. Levy, Phys. Rev. 134 (1964) A589. [7] R.G. Duvamey and A.K. Garrison, Solid State Commun. 12 (1973) 1235.
[S] W.P. Unruh, Y. Chen and M.M. Abraham, Phys. Rev. Lett. 30 (1973) 607. [9] L.E. Halliburton, L.A. Kappers, D.L. Cowan, F. Dravnieks and J.E. Wertz, Phys. Rev. Lett. 30 (1973) 607. [lo] For a recent summary see Y. Chen and M.M. Abraham, Phys. Rev. Bll (1975) 881. [ 111 R.T. Cox, Solid State Commun. 9 (1971) 1989. (121 T.J. Turner and J.H. Crawford, Solid State Commun. 17 (1975) 167. [ 131 P.W. Kirklin, P. Auzins and J.E. Wertz, J. Phys. Chem. Solids 26 (1965) 1067. [14] H.H. Tippins, Phys. Rev. Bl (1970) 126. [ 151 K.H. Lee, G.E. Holmberg and J.H. Crawford, Solid State Commun. 20 (1976) 183. [16] K.H. Lee, G.E. Holmberg and J.H. Crawford, Phys. Status Solidi (a) 39 (1977) 669. [ 17) F.T. Gamble, Ph.D. Dissertation, University of Connecticut, 1963 (unpublished). Quoted by Cox (Ref. [9]). [ 18) D.W. Cooke, H.E. Roberts and C. Alexander, J. Appl. Phys. 49 (1978) 3451. [19] P.W. Levy, Phys:Rev. 123 (1961) 1226. [20] E.W.J. Mitchell, J.D. Rigden and P.D. Townsend, Phil. Msg. 5 (1960) 1013. [21] G.W. Arnold and W.D. Compton, Phys. Rev. Lett. 4 (1960) 66. [22] W.D. Compton and G.W. Arnold, J. Appl. Phys. 31 (1960) 622. [23] T.J. Turner and J.H. Crawford, Phyys. Rev. B13 (1976) 1735. [24] S.Y. La, R.H. Bartram and R.T. Cox, J. Phys. Chem. Solids 34 (1973) 1079. [25) K.H. Lee and J.H. Crawford, Phys. Rev. B15 (1977) 4065. [26] B.D. Evans and M. Stapelbroek, Phys. Rev. 18 (1978) 7089. [27] J.H. Crawford, Bull. Am. Phys. Sot. (1981). (281 S.-I. Choi, unpublished. [29] K.H. Lee and J.H. Crawford, Appl. Phys. Lett. 33 (1978) 273. [30] K.H. Lee and J.H. Crawford, Phys. Rev. B19 (1979) 3217. [31] B.G. Draeger and G.P. Summers, Phys. Rev. B19 (1979) 1172. (321 B. Henderson, S.E. Stokowski and T.C. Ensign, Phys. Rev. 183 (1969) 826. [33] B. Henderson and DC. O’Connell, Semicond. Insulators 3 (1978) 299. [34] J.B. Brewer, B.T. Jeffries, G.P. Summers, Phys. Rev. B22 (1980) 4900. [35] H.W. Lehman and Hs. H. Gunthard, J. Phys. Chem. Solids 25 (1964) 941. [36] B.T. Jeffries, J.D. Brewer and G.P. Summers, Bull. Am. Phys. Sot. 26 (1981) 268. [37] B. Jeffries, G.P. Summers and J.H. Crawford, J. Appl. Phys. 51 (1980) 3984. [38] L.S. Welch, A.E. Hughes and G.P. Pells, J. Phys. Cl3 (1980) 1805. [39] R.T. Cox and A. Herve, C.R. Acad. Sci., Paris 261 (1965) 5080. [40] R.T. Cox, Phys. Lett. 21 (1966) 503.
Journal of Nuclear Materials 108 & 109 (1982) 644-654 North-Holland Publishing Company
A REVIEW OF NEUTRON RADIATION DAMAGE ON CORUNDUM CRYSTALS J.H. CRAWFORD, Jr Department of Physics and Astronomy, Universiiy of North Carolina at Chapel Hill, Chapel Hiil, NC 27514. USA
The first neutron radiation damage studies were carried out on corundum (Also,) crystals nearly thirty years ago by Levy and Dienes and nearly twenty years ago Compton and Arnold measured the threshold energy for atomic displacement using energetic electrons. However, it was not until about five years ago that a series of optical studies of crystals bombarded with neutrons and with energetic ions laid the basis for identifying radiation-induced lattice defects. Since that time a variety of lattice defects have been identified through their optical absorption and emission characteristics. Some information on the spatial distribution of neutron damage has been obtained through a study of the effect of annealing on the luminescent efficiency which increases as the high local damage concentration in the displacement spike is annealed, thus decreasing concentration quenching of F centre luminescence. In this paper all of the recent developments related to the identification of
lattice defects created by fast-neutron irradiation and the annealing of damage till be reviewed with particular attention to outstanding problems and areas needing addition work.
1. Introduction
Because of its high melting point, chemical stability and excellent optical and dielectric properties, corundum finds many important applications. In addition to serving as a laser host, UV window material and a substrate for VLSI circuits, AlaO, is a potential insulator in a variety of high temperature energy-conversion devices. Hence there is a high interest in its radiation stability and the nature of the radiation damage it sustains upon exposure to high energy particles. Studies of y-ray coloration and fast-neutron damage of Al,O, single crystals goes back some 25 years to the work of Hunt and Schuler [1] who observed that exposure to X-rays introduces optical absorption bands at 230 nm (5.4 eV) and 400 nm (3.1 eV) in such crystals. At almost the same time Levy and Dienes [2] found that a 6.1 eV absorption band resulted from atomic displacements in crystals exposed to fast neutrons in a nuclear reactor and that the absorption could not be produced by ionizing radiation (X-rays and y-rays). Thus quite early it was recognized that ionization-type damage can influence only those imperfections (impurities and defects) already present in the structure but that this type of damage is distinct from structural damage associated with the displacement of lattice ions by elastic collisions involving massive particles. In general, ionization damage saturates at rather low exposures (105-lo6 rad) and the magnitude and type of the spectral changes depend upon the amount and type of impurity present.
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
Fast-particle damage, however, continues to grow to high fluences before saturation begins to set in. This same behaviour is exhibited by other ionic refractory oxides [3] and stands in marked contrast to the response of alkali halides in which X-rays and y-rays are known to produce copious quantities of anion vacancies (F-type centres) and interstitial halogen molecular complexes (V-type centres) even at quite low temperatures [4]. The purpose of this paper is to summarize the rapid recent progress in understanding both the nature of intrinsic lattice defects and their electronic structure in Also, on the one hand and the constitution of fast-neutron radiation damage on the other. The ready availability of high quality single crystals needed for K-device substrate material together with insights gained from radiation effects, studies on alkaline earth oxides have made it possible to identify specific defects responsible for a number of features in the absorption and emission spectra of irradiated Al,O,. Perhaps the most surprising discovery is that in Al,O, the generic, simple lattice defects, e.g. anion and cation vacancies, appear to behave in an analogous way to those in the simple cubic oxides even though the Al,O, structure is much more complex. In this review we have chosen to follow an historical development summarizing first the effects of ionizing radiation, adducing evidence for the various structures involved, and then we examine displacement-type damage and build a case for the identification of the defects responsible for various absorption and emission
J.H. Crawford, Jr /
Neutron
radiation damage on corundum
lation by which the optical and ESR findings could be connected was still missing so it was not possible to state which y-ray-induced absorption band was associated with the defect responsible for the ESR signal. A possible link was provided by infrared spectra associated with OH- impurity in Al,4 crystals. Turner and Crawford [ 121used thick crystals (55 mm) of Union Carbide, UV grade Al,4 and observed an IR absorption band at 3278 cm-’ which was almost completely replaced by a band at 3316 cm-’ when the crystal was exposed to 13’Cs y-rays. A beforeand-after comparison is made in Fig. 1. The thick crystal was necessary because of the low concentration of OH- estimated at - lOi cmu3 1 and the low oscillator strength of its IR absorption. The response of the 3278 cm-’ absorption to radiation is analogous to what is observed for OHin MgO in which the two corresponding absorptions at 3296 cm-’ and 3323 cm-’ have been identified with an OH- situated adjacent to a Mg*+ vacancy V&/centre) and an OH- adjacent to a Mg*+ vacancy which has trapped a hole on the opposite 02- ion [13], respectively. Therefore, it was concluded that these two absorptions arise from analogous centres in Al,O, but, because of the higher negative charge on the Al”’ vacancy, these are designated V& and V&. Considering the analogy between Al 203 and MgO in the IR spectra of Vo,-type centres, similar features in the electronic excitation region of the optical spectrum might also be expected. Again using 55 mm thick specimens because of the small concentration of absorbing centres, the spectrum represented by curve 1 of fig. 2 was obtained upon y irradiation. The dominant features
bands in neutron- and electron-bombarded crystals. Although our main concern is fast-neutron damage, identification of the defects comprising such damage requires evidence from the influence of ionizing radiation and thermochemical treatment as well as displacement-type damage. Therefore, evidence from these other sources will be discussed. However, there are some effects which are characteristic of neutron damage per se because of the spatial ~st~bution of defects around the primary collision site. These will be treated separately.
2. Effectsof ionizingradiation Among all of the absorption bands reported for nominally pure Al,O,, Levy [S] catalogued three as resulting from y irradiation: 5.4 eV, 4.3 eV and 3.1 eV. Gamble et al. [6] found that y irradiation introduced a single ESR resonance line with axial symmetry along the c-axis which they resolved into resonances attributed to (a) a hole trapped on an oxide ion adjacent to a site deficient in positive charge, perhaps a cation vacancy or au A13+ site occupied by a monovalent or divalent cation, and (b) an electron trapped at an anion vacancy. Although the second identification has not proved valid, the first one has. It parallels the ESR and ENDOR characterization of V-type centres in MgO 17-101 as due to O- centres adjacent to Mg2+ vacancies in various stages of compensation. Moreover, Cox [1 l] confirmed these observations by ESR measurements on Al,O, crystals, delibarately doped with Li+ and Mg+ , exposed to y-rays. Unfortunately, the wrre-
r
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3275 Wavenumber
I
Spectra Before After
---
in Al,
~-Roy
Exposure
0,
3300 (cm-’
645
)
Fig. 1. OH- stretching-mode bands in Ai,O, before and after y-irradiation. (After Turner and Crawford, Ref. 119).
646
J.H. Crawford, Jr / Neutron radiation damage on corundum
6.0 50 0.4
4.0
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Photon Energy (ev) 3.0 2.5 2.0
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I I 500 600 Wovelength (nm)
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0.3 2 2 E EO.2 t 0 0’ ._ ‘, 9 2 0.1
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300
I
400
800
Fig. 2. Absorption spectra of y-irradiated Al,O, following 3.0 eV optical bleaching for periods of (1) 0, (2) 5, (3) 15, (4) 35, (5) 75, (6) 155 and (7) 135 min. (After Turner and Crawford, ref.
1131).
are absorption bands at 5.46 eV and 3.02 eV, i.e. the same bands reported by Hunt and Schuler [l], and evidence for the weak band near 4.3 eV mentioned by Levy was also observed. The important feature, however, is the great width of the 3.0-3.1 eV absorption which is similar to that of the V-type absorption near 2.2 eV in MgO [IO]. Thus it seems reasonable to conclude that, analogous to MgO, the broad 3.1 eV absorption is associated with several trapped-hole centres, all involving an O- adjacent to a cation vacancy or some other positive-charge-deficient site as suggested by Gamble et al. [7], but that an important component is the V&n band for which good IR evidence exists. From bleaching studies [12] it was found that the 3.02 eV and 5.45 eV absorption were complementary. The latter could be completely destroyed by irradiating into the 3.02 eV band which was also bleached in the process. The results of this type of optical bleaching is shown by the family of curves in fig. 2. Excitation in the 5.45 eV region produced mutual bleaching of the two bands also, except that approximately one-third of the 3.02 eV band (now shifted to 3.1 ev) remains when then 5.45 eV band is gone. It is therefore concluded that the 5.45 eV band is associated with some type of elec-
tron trapping impurity which is able to accomodate - 2/3 of the electrons required to balance holes trapped at V-type centres. The major contaminant in the crystals used is Cr3+ as detected by its 6.9 eV charge-transfer absorption band [ 141. Moreover, in thermally stimulated luminescence (TSL) studies [ 121 the emission in the strong glow peak at 150°C was due to the Cr*+ ion. Finally, thermal annealing studies reveal that the 3316 cm-’ IR band is removed and the 3278 cm-’ band is restored in the region between 100 and 200°C which is the same annealing region in which removal of - 60% of the 3.02 band occurs and the large Cr2+ TSL glow peak maximum mentioned above appears. Definite evidence for three distinct V-type centres has been obtained by Lee et al. [15,16] from ESR studies. The dominant signal observed for the y-irradiated crystals before further treatment corresponded to a S = i centre with axial symmetry: g,, = 2.018 * 0.002 and (g, = 2.011 f 0.002 with a line width of 45 G. These parameters are somewhat different from those obtained by Gamble et al. [7] in that they exhibit a greater positive g-shift. Upon heating for - 70 h at 1350°C, a treatment which excludes the CH- completely from the crystal, two new resonances appear: (a) an S = i centre with g,, = 2.011 -t- 0.004, G, = 2.013 * 0.004, and a 50 G line width and (b) S = 1 centre with spin resonance parameters almost identical to those reported by Gamble [ 171and Cox [ 111,namely zero-field splitting parameters of D = 3.44 GHz and E = 0.86 GHz. The thermal stabilities of these centres are also quite distinct from one another. Fig. 3 shows the effect of isochronal annealing on the intensities of these three resonances. The solid curves refer to the S= 1 centre and the S = i centre observed after the high temperature anneal. The least stable centre is the S = 1 and,
-Oxidized
Fig. 3. Relative inters&s of hole centres Al 203 crystals during isochronal annealing. (--non-oxidized), S= f, 0 S = 1 (after and Crawford, Ref. [ 171).
in
y-irradiated oxidized; Lee, Holmberg
J.H. Crawford
Jr / Neuiron radiation damage on corundum
upon decay, it feeds into the S = 4 centres whose concentration is essentially zero at the start of the anneal. The S =s centre reaches its maximum concentration just when the S= 1 centres disappear and it in turn subsides at a higher temperature. This behaviour means that the two centres observed after heat treatment are two different charge states of the same defect. The most reasonable model is one and two holes trapped at a positive-ion vacancy, with the single-hole centre being more stable because of the double negative charge of the centre than the two-hole centre which is expected to have a single negative charge. The bare vacancy or V3centre is evidently created by the thermal destruction of the V& centre by loss of the proton. The stability of the V&n centre is intermediate between that of the S = 1, V- and the S = i, V2- centres as can be seen from fig. 3. Recently TL studies of Cooke et al. [ 181have given indirect evidence of the presence of the two-hole V,, centre. They observe TL glow peaks at 270 and 435 K whose emission spectra are identical and contain Cr3+ R-lines. The upper peak falls where the V&, is expected to lose its hole at a maximum rate peak falls where the Vo, is expected .to lose its hole at a maximum rate and the lower temperature peak is just about at the anticipated position for the loss of a hole from the two-hole, neutral V,, centre. In summary it is clear that O- ions adjacent to sites of positive-charge deficiency absorb in a broad band of photon energies centered near 3.1 eV. There is strong evidence that one such centre is the V&u and that an oxidative heat treatment at 135O’C converts this centre to a ‘room temperature stable two-hole centre, first observed by Gamble [ 171 and confirmed by Cox [ 111, which has been identified as a V- centre, i.e. two holes trapped at an A13+ vacancy. Above - 100°C the Vloses a hole becoming a V2- centre.
3. Optical absorption and emission bands of fast-neutron-bombarded Al 20J Levy [19] has analyzed the absorption spectrum of A1203 crystals irradiated in the nuclear reactor into Gaussian bands which lie at 6.02 eV, 5.34 eV, 4.84 eV, 4.21 eV, 3.74 eV, 2.64 eV and 2.00 eV. Of these the,6.02 eV band, which we refer to as the 6.1 eV and, was by far the most prominent. To get a better insight into the defect structures responsible for this absorption, Mitchell et al. [20] studied the dichroism of these bands produced by bombardment with both fast neutrons and 2 MeV electrons. Such a technique could be quite
647
informative since Al,O, has rhombohedral symmetry with the result that transition moments of the various defects could differ significantly for different orientations of the light propagation and polarization vectors relative to the crystallographic axes. It was found that the 6.1 eV band was optically isotropic. However, the 5.3 eV and 4.8 eV bands were strongly and oppositely polarized, with the 5.3 eV band being scarcely visible for the electric vector parallel to the c-axis. There was evidence of anisotropy in the lower energy bands as well. Another important development was the determination of the threshold energy of bombarding electrons for the creating of the 6.1 eV band by Compton and Arnold [21,22]. They found that an incident electron energy of 0.37 MeV or greater was necessary to produce. this absorption. Mitchell et al. [20] suggested that the relevant defect was an interstitial A13+ which had captured an electron and for such a structure the minimum energy transferred in an elastic collision, the so-called displacement energy Ed, would be 40 eV. On the other hand, if an oxygen vacancy were responsible, the value of Ed would be 90 eV. Because of the optical isotropy and the smaller displacement energy, the interstitial aluminium defect, which is expected to possess a rather high symmetry [20], was favoured as the origin of the 6.1 eV band. In reality, however, the evidence was not particularly strong and, indeed, it was not possible to say whether the centre has trapped an electron or a hole. One the fact that trapped holes are responsible for the 3.0 eV absorption had been established, it was possible by optical bleaching experiments to determine whether the 6.1 eV band arises from a trapped electron. Turner and Crawford [23] made such a determination by utilizing the Long crystals containing - lOI V& defects cm- 3. These were given a very light neutron exposure in the nuclear reactor which introduced between 10” and lOI eV “centres” cm-3. The result was a set of crystals with a comparable concentration of absorption centres for the 3.0 eV and 6.1 eV bands, a situation well suited to studying the relation between the two types of centres. If upon excitation in the 3.0 eV trapped-hole band, the 6.1 band either did not change or increased in amplitude, one could conclude that the latter was due to a defect which had trapped a hole. Gn the other hand, if bleaching with 3.0 eV light caused the 6.1. eV band to decrease in intensity, one has strong evidence that the 6.1 eV band is due to a defect which had trapped electrons. The results of the experiment are shown in fig.4 in which the absorption spectra of a crystal exposed to 4 X 10” n cmm2 after bleaching with
J.H. Crow&or&Jr / Neutron radiation ohwage
on ewundlun
Nautrcn Irrodicitad Al203 Followtng Blecrching at 410 nm.
0
Fig. 5. Nearest-neighbour cation positions around an anion site in Al,O,. 2.0
6.0
Fig. 4. Effect of optical bleaching with 3.0 eV photons on the
absorption spectrum of neutron-bombarded Alz03 (4X lOi5 n en-‘) and y-irradiation: (0) 0, (1) 5, (2) 15, (3) 35, (4) 75 min bleaching at 300 K. (After Tumer and Crawford, Ref. j25J).
3.0 eV light for various periods of time is shown. Obviously, the 6.1 eV band is subs~ti~y reduced, showing that its origin is an electron-trapping centre. It was also possible to obtain mutual bleaching of the two bands by exciting into the 6.1 eV band. Although these studies demonstrate the ch~e~app~ nature of the defect, there is stiIl no indication of its actual structure. The next development, just as in the case of hole centres, features ESR data. ESR investigations by La et al. 1241 have uncovered a 13 line resonance isotropic about the c-axis with a g-factor of 2.0029 in crystals exposed to - lose, cm-* at 77 K. The concentration of paramagnetic centres responsible for this resonance was - lot9 cme3 which is well above the total impurity content. Therefore they conclude that the centre is an intrinsic, structural defect created by the neutron bombardment. A likely candidate is the F+ centre, i.e., one electron trapped at an oxygen vacancy. To support this attribution, they calctdated wave functions for the F+ centre by the variational method using a point-ion model and including s, p and d basis functions. From these wave functions they c.aIculated the hyperfine interaction constants for the interaction of the electron spin with the =Al nuclei in the two non-equivalent pairs of A13+ ions which neighbour the oxygen vacancy. The model for the F+ centre is shown in fig. 5. The theoreti-
cal hyperfine constants were found to have a ratio of A/A, = 0.26, and for such a ratio, provided the resonance half-width AH is large enough to mask the smaller {A,) splitting, theory indicates that the hyperfine lines for the two sets of pairs will interfere in such a way as to give 13 component lines. The observed value of AH = 50 G fits the required magnitude of the half-width and the observed value of A,/A, is 0.27. Hence; even though the calculated magnitudes of A, and A, are a factor of three too large, a discrepancy not unreasonable for a point-ion calculation, the more important value for comparison is their ratio and this is in excellent agreement with experiment as is the 13 line spectrum. Therefore, the evidence that fast-neutron bombardment creates F+ centres in Al203 is quite strong. Unfortunately, they were unable to correlate the ESR spectrum with optical absorption bands in these heavily irradiated crystals. However, it was found that this “F+ resonance” signal could be detected after an exposure of - lO’* n cm-*. Since a large fraction of these centres may contain two electrons, which would render them diamagnetic and undetectable by ESR, the difficulty of observing such a resonance after smaller neutron exposures, i.e. - 10” cm-‘, is easy to understand. Point-ion calculations of the ground and excited state energies by La et al. [24] also provide the basis for identification of optical transitions of the F+ centre. The low symmetry of this centre (C, point group) splits the excited p-state, which would be triply degenerate in an isotropic environment, into three states designated in order of increasing energy as lB, 2A and 2B. Nence three transitions from the 1A ground state are expected:
J.H. Crawfor
Jr / Neutron radiation damage on corundum
1A - IB, lA-2A and IA - 2B with calculated energies of 2.26 eV, 3.39 eV and 5.15 eV respectively. For electric dipole transitions, only u transitions are allowed for A - A and only qr transitions for A - B. Thus these calculations provide guidance for using polarized excitations as a tool for identifying transitions experimentally. Lee and Crawford [25] observed that bleaching reactor-neutron-bombarded crystals with 6.1 eV light substantially decreased the intensity of the 6.1 eV band and increased absorption at 5.4 eV, 4.8 eV, 4.1 eV and 2.7 eV. The difference in absorption due to exposure to 6.1 eV light is shown in fig. 6(a), whereas the difference due to exposure to 4.8 eV light is shown in fig. 6(b) (5.4 eV light also restores the 6.1 eV band). All of these absorption bands have been previously observed by Levy [191 and Mitchell et al. [20]. It was further found that the 5.4 eV band could be well resolved by using polarized light since it absorbs light with E I c but not for E II c polarization (E is the electric vector of the light and c is the direction of the c-axis) in agreement with observations of Mitchell et al. [20]. Since the 6.1 eV band is known to be an electron centre, bleaching by 6.1 eV light is expected to release electrons. Therefore, the absorption bands enhanced by this process must be associated with either centres which have trapped electrons released from the 6.1 eV centre by photo-excitation or the new charge state of the latter. Other evidence [23], namely
6.6
6.0
5.0 4.0 Photon Energy (eV)
3.0
2.0
Fig. 6. (a) Effect of 6.1 eV light and (b) 4.8 eV light irradiation on the absorption spectrum of ELI A1,03 after exposure to 5 X lOI n cm-‘. (After Lee and Crawford, Ref. [30]).
649
the decrease of the 6.1 eV band and the growth of the 4.8 eV band by release of holes, either optically or thermally, from V-type centres and the restoration of these bands to their initial intensities by y-irradiation, indicates that the 4.8 eV band arises from the new charge state of the 6.1 eV centre. Excitation in the region of the 4.8 eV absorption produced emission of 3.8 eV light. Since this absorption band was not far from the 5.15 eV 1A - 2B transition predicted by La et al. [24] and since the quantum yield of the emission was greater for those relative orientations of dipoles and E of the polarized exciting light conductive to B excitations, it was concluded that the 4.8 eV absorption could be attributed to the 1A - 2B transition. Evans and Staplebroek [26] showed that this assignment was incorrect since their measurement of the excitation spectrum of the 3.8 eV emission revealed that it is produced with 4.8 eV, 5.4 eV and, probably 6.3 eV photons (fig.7). They assigned the 4.8 eV excitation to the lA- 1B r transition, the 5.4 eV to the IA-2A o transition and the 6.3 eV excitation to the 1A - 2B TL transition. The polarization behaviour accorded with this assignment since E II e produced a larger emission than E Ic for the 4.8 eV II transition and only E Ic at 5.4 eV produced emission. It was further concluded that the 3.8 eV emission common to these excitations is due
Fig. 7. Polarized optical absorption spectra of an Al,Op crystal exposed to 3.9X lOi MeV n cmm2 followed by a 20 min 300°C anneal. Emission or excitation spectra from other crystals exposed to 1.7X lOI n cmm2. (a) E-L c; (b) El/c. (After Evans and Stapelbroek, Ref. [31]).
J.H.
650
Crawford,
Jr / h’eutron radiation damage on corundum
to the 1B - 1A transition. This close correspondence (except in actual energy values) of theoretical predict-
I
shows
a linear correspondence
between
electron state in the F+ centre and procedure an excited F centre. Decay to the original ground state would restore the Ff centre with emission of a 3.8 eV photon. However, if there is a small but finite probability that the hole on the adjacent O- ion could migrate away to a trapped electron before recombination can occur, a stable F centre would result [27]. Recent cluster calculations by Choi [28] indicate that process (b) is indeed feasible.
4. Sub&active
and growth-cdoured
0
the
increase of the intensity of one of these bands and the decrease of the other. The slope of this line together with the room temperature half widths of the 4.8 eV and 6.1 eV bands can be used to estimate the ratio of their oscillator strengths: f,.Jj& = 0.55. The main puzzle in the bleaching studies is why excitation at 4.8 eV and 5.4 eV restores the 6.1 eV band intensity (fig. 7(b)). Two possibilities exist: (a) There is an underlying absorption at this photon energy associated with an occupied electron trap. (b) 4.8 eV and 5.4 eV photon absorption leads to charge-transfer transitions involving the cation vacancy, which adds an electron from a near-neighbour 02- ion to the F+ centre. This latter process would in effect occupy the vacant
I
“93
ions of the point-ion model and experimental observations give confidence to the conclusion that the F+ centre is indeed a product of neutron (and other particle) bombardment and that its optical transitions are the 4.8 eV, 5.4 eV and, quite likely, the 6.3 eV bands. Moreover, the close relationship between centres responsible for the 4.8 eV and the 6.1 eV bands established by bleaching studies [25] are a strong indication that the 6.1 eV band is an optical transition of the F centre, i.e. two electrons on an anion vacancy. Step-wise bleaching
I
6
s Photon
3
4 Energy
(,v
)
Fig. 8. Optical absorption spectra of Al,O,: (1) as received; (2) 1800°C heat treatment in Al metal vapour; (3) 2000°C heat treatment in Al metal vapour. (After Lee and Crawford, Ref.
1331).
“subtractive colouration” rather. than “additive colouration” is used. The half width of this band is -0.6 eV which corresponds to that of the F band created by kither electrons or neutron bombardment. Excitation in the 6.1 eV band results in an -emission at 3.0 eV (301 which is shown in fig. 9. The same emission has been observed at 77 K in neutronbombarded crystals though it is considerably weaker. The 3.0 eV luminescende can also be excited in certain as-received crystals coloured during growth. Draeger and Summers [31] have also noted this and have explored adsorption and emission of both F+ and F centres in such growth-coloured crystals. The subtrac-
tive colouration of Al,O, mentioned above indicates that colouration during growth is a likely process provided the atmosphere over the melt is sufficiently reducing, i.e. the oxygen partial pressure is less than the
AI,OJ 2
Further confirmation of the identification of the 6.1 eV band as a transition of the F centre is provided by subtraction colouration studies of Lee and Crawford [29,30]. Heating Al,O, crystals in the presence of Al metal vapour to temperatures near 2OOO’C for two hours results in the introduction of a substantial 6.1 eV band (fig. 8). It was found that it made little difference whether Al vapour was present or not, provided the atmosphere was strongly reducing. Hence the process must involve extraction of oxygen under conditions that provide electrons to compensate the positive charge on the anion vacancies left behind. For this reason the term
--.7----1-r I--
1
Fig. 9. Excitation and emission spectra of F centres in subtractively coloured Al *O, (After Lee and Crawford. Ref. [34]).
J. H. Crawford
Jr / Neutron radiation damage on corundum
decomposition partial pressure of Al,O, at the growth temperature. Whereas the lifetime of the 3.8 eV emission of the F+ centre is quite short (C 7 ns) as expected for an allowed transition [26], the decay lifetime of the 3.0 eV emission is quite long [30]. Fig. 10 compares the firstorder decay of this emission for a subtractively coloured crystal, a growth-coloured as received crystal and a neutron-bombarded crystal. The emission from the subtractively coloured crystal is the more intense ( lo2 more than for the neutron-bombarded crystal). All three crystals exhibit identical decay constants which correspond to a lifetime of the emitting state of 36 -C4 ms. This lifetime together with the large Stokes shift (6.1 eV compared with 3.0 eV) is similar to the emission behaviour observed for F centres in CaO and suggests a forbidden transition. The electronic structure of the two-electron F centre is helium-like, i.e. the (1s)~ configuration corresponds to the term ‘Sc. Upon excitation, one electron is promoted into either a 2s or 2p orbital, leading to possible singlet and triplet states (‘S, ‘S, ‘P and ‘P). In MgO and CaO, emission results from pumping into the ‘S - ’ P F centre transition (the F band)
3 eV
Emission
6.51
[32,33]. The ‘P state is filled by radiationless decay from the higher energy ‘P state and ihe forbidden transition sp-’ S follows with a long decay time. The situation in AlaOs is analogous, which suggests that the 3.0 eV emission arises ‘from similar transitions. However, it is noted that recent studies of the behaviour of the 3.0 eV decay below 77 K by Brewer and Summers [34] indicates that the situation is probably more complex. At room temperature the 3.0 eV emission occurs with a long phosphorescent tail [30]. This was first reported by Lehmann and Gunthard [35] who analyzed the emission into a first-order, temperature-independent component with a 36 ms lifetime and a second-order, temperature dependent component which they attributed to a metastable state of the impurity responsible for the emission. Lee and Crawford [30] have found the phosphorescent tail to be specimen-dependent (present in some and not in others). As shown in fig. 11 the growth-coloured crystal shows only the 36 ms decay, whereas the subtractively coloured crystal shows the extended phosphorescence. Hence it appears that an electron-trapping impurity which discharges in the order of seconds is present in the latter but not in the former. Although Lee and Crawford [29] found the 4.8 eV and 5.4 eV bands to be too weak to be detected in their subtractively coloured crystals, the presence of F+ centres is revealed by the 3.8 eV emission which can be excited by photons with these energies. To demonstrate that the phosphorescent tail of the 3.0 eV F centre emission is indeed due to temporary storage of electrons originating at F centres via the process F*“- F+ +e,
3eV Emission
I
0
I
40
I
80
I
120
Timelm
I
160
200
set)
Fig. 10. Decay of 3.0 eV emission at 77 K from subtractively colored Al,O, (SLC), from as-received Adolf Meller Al 20, (ARA) and from neutron-bombarded Al,O,. (After Lee and Crawford, Ref. (341).
L
Time (m set)
Fig. 11. Decay of 3.0 eV emission at 300 K from subtractively coloured Al,O, (SCL) and as-received Adolf Meller Al,O, (ARA). (After Lee and Crawford, Ref. [34]).
652
J.H. Crawford
Jr / Neutron radiation damage on corundum Conduction
Time
t cm
CA
(Second) t on
(6.leV
F- center Light)
A’z03
Band
F+ center
Fig. 13. Optical transitions (excitation and emission) associated with the F+ and F centres in Al,O, crystals.
Fig. 12. Effect of 6.1 eV light on (a) 3.0 eV emission (b) 3.8 eV emission with constant 4.8 eV excitation in a subtractively coloured crystal at 300 K. (After Lee and Crawford, Ref. [34]).
the F+ emission was monitored [30] while pumping periodically at 6.1 eV and continuously at 4.8 eV. The results are shown in fig. 12. The upper panel (fig. 12(a) shows the intensity of the F centre emission as the 6.1 eV exciting light is turned on and off. The 3.0 eV emission intensity increases during the first few seconds while the electron traps are being filled to a saturation value. The rise time of this square pulse is approximately the decay time of the traps. Saturation of the 3.0 eV emission intensity occurs as soon as the electron trap occupation reaches a steady state. In this condition nearly every electron released from an F centre recombines with an F+ centre. When’ the exciting light is turned off the 3.0 eV emission rapidly decays initially until the phosphorescent tail due to the discharge of traps takes over. The lower panel (fig. 12(b)) shows the intensity of the 3.8 eV F+ emission through the cycle. The 4.8 eV exciting light is held constant and the small (- 10%) variation of emission intensity is due to the increase and decrease of F+ centre concentration associated with the loading and unloading of the electron traps responsible for the phosphorescent decay. This measurement provides a direct indication of the relation between the 6.1 eV and 4.8 eV exciting transitions and the 3.0 eV and 3.8 eV emissions, strengthening their identification with F centres and F+ centres, respectively. The various transitions considered above are summarized in fig. 13. Draeger and Summers [31] have observed that the photoconductivity peak at 6.1 eV can be observed at 10 IL Therefore, the ‘P state of the F centre must he very close to the bottom of the conduction band as indicated. The electron-trapping impurity level must lie -0.5-0.6 eV below the conduction band
to account for the phosphorescent tail and also for a thermoluminescent peak slightly below room temperature [30,36]. There are no doubt electron traps to account for the F” centres which produce the steady emission of fig. 12(b). Up to this point in the review we have been concerned with establishing the identity of simple point defects in their various charge states, defects which might be expected to result from various processes, i.e. ionization and displacement type damage as’,well as thermochemical treatment either during or subsequent to crystal growth. We now turn to those studies which focus more or less specifically upon fast-neutron damage. That one might expect to find such specific effects, of course, is associated with the rather special spatial distribution of fast-neutron damage, namely a displacement cascade occupying a rather restricted region of dimension - 100 A around the site of primary collision. There are two such studies. The first of these concerns concentration quenching of the 3.0 eV F centre luminescence in neutron-bombarded Al,O,. It was noted above (see fig. 10) that the 3.0 eV emission intensity at 77 K of a reactor-irradiated Al,O, crystal was two orders of magnitude smaller than a subtractively coloured one even though its F centre concentration was at least as large. To explore this matter, Jeffries et al. [37] have studied the influence of thermal annealing neutronbombarded crystals over the range from 300K and 850K. As the annealing proceeded there was an enormous increase in the emission efficiency as indicated by the ratio of relatively light output at 3.0 eV to the 6.1 eV absorption coefficient. This ratio along with the absorption coefficient measured during the course of the annealing is plotted against anneal temperature in fig. 14. The sharp increase in annealing efficiency during the anneal indicates that the fast-neutron damage is highly
J.H. Crawford ?looc
,
5
g60-
(a)AI,O,
I
I
I
653
Jr / Neutron radiation damage on corundum I
I 4.0-
u
’
’ AM
I
’
!E60 y. x 40!! r 2o3 c
t 0 1.0-
0 200
100 200
300
400
500
600
TEMPERATURE WI
Fig. 14. (a) Relative luminescence intensity per F centre measured at room temperature for the 3.0 eV emission in neutronirradiated sapphire, following 15 min isochronal annealing at successively higher temperatures. (b) Absorption coefficient of the 6.1 eV F band in the samples after each isochronal annealing stage (after Jeffries et al., Ref. [37]).
and leads to concentration quenching due to the overlap of the excited state wave function of one centre with an adjacent unexcited F centre. This situation allows energy transfer and radiationless recombination to occur. It was estimated that the local concentration of F centres in displacement cascade is - 2 X 10” crnm3 which is ample for considerable concentration quenching to occur. During anneal the F centre density in each cascade decreases as oxygen atoms or ions recombine with vacancies, thus decreasing wave function overlap and increasing luminescent efficiency. It is interesting to note that F+ luminescence (3.8 eV) was not substantially affected by the anneal which is to be expected for two reasons: the wave function of the F+ centre is expected to be more compact and their concentration was considerably smaller than the F centres. The second experiment concerns the optical behaviour of a composite defect of the type expected to be produced only in high damage concentration. Welch et al. [38] studied the dichroic absorption of a band at 3.5 eV and the polarization of the associated luminescence in oriented Al,4 crystals which were bombarded with lo’* fast neutrons crne2 and 3 X lOI 6 MeV protons cm -*. The polarization behaviour of the absorption produced by neutron bombardment is shown in fig. 15. non-uniform
I 300
I 400 WAVELENGTH
I SO0
I 600
(nm)
Fig. 15. Absorption spectra at 4 K of an a-Also, crystal neutron-irradiated to a dose of lOI n cm-*. The light propagates along the [ lOTO]axis and curves A and B refer to the absorption spectrum for E I][0001] and for E 1[0001] respeu tively. Curve C is the background absorption of the crystal before irradiation. (After Welch et al., Ref. [38]).
Analysis of the 3.5 eV band and the 3.3 eV emission indicates that the electronic transition involves a linear defect inclined 40° to the c-axis. A defect which fits this orientation quite well is the oxygen-vacancy pair, i.e. a d&F centre which is the model they postulate for this centre. Although aaimilar spectrum can be produced by heavy proton (3 X lo’* cm-*) bombardment, the spectrum obtained after fast-neutron bombardment is much better defined, particularly with respect to a zero-phonon line at 3.38 eV associated with this centre. In view of the short range of protons (- 60 MI for a 3 MeV proton in Al,O,) the average damage density is much greater than in the case of reactor neutrons. However, because of the high local defect concentration in the displacement cascades, the probability of anion-vacancy pair formation is greater in the neutron case. Finally it is worth noting that interstitial defects have been detected in fast-neutron-bombarded Al,O, crystals thorugh ESR measurement. Cos and Herve [39] have observed two types of resonances both involving Al*+ pairs which have trapped an electron. One of these centres is created by reactor irradiation at 77 K and is stable for a few days at room temperature. Its axis is parallel to the crystal c-axis. The second resonance is associated with a “tilted” pair inclined 10’ to the c-axis. This defect pair is created by reactor bombardment at 300 K and is stable indefinitely. A third “axial” pair has been detected after 77 K irradia-
654
J.H. Crawford Jr / Neutron radiation damage on corundum
tion but it is unstable at room temperature [40]. These pairs evidently form as A13+ interstitials, take positions along the c-axis and interact with one one-site neighbour more strongly than another. However, in view of their rapid saturation at a rather low concentration (1016-1017 cmF3 range) it is suspected that they may be impurityrelated rather than intrinsic in character. To summarize our present state of knowledge of neutron radiation damage in Al,O,, we can say that the main point defect observable is the anion (oxide) vacancy in two charge states, i.e. as the F+ centre and the F centre. Due to the high concentration of these in displacement cascades, the F centre luminescence at 3.0 eV is largely quenched. Also a substantial anion divacancy centre is produced which exhibits a zero-phonon line at 3.38 eV and optically anisotropic “mirror image” absorption and emission bands at 3.5 eV and 3.3 eV respectively. There is some ESR evidence of A13+ interstitial-type defects in the form of Al pairs. Additional unidentified optical absorption bands at 4.2 eV, 2.6 eV and 2.0 eV have yet to be accounted for. And of course there are some riddles: why does neutron irradiation not produce evidence for A13+ vacancies, e.g. V-type bands? and most important, what form does interstitial oxygen assume. Much work needs to be done on these last two questions to produce a balanced picture of the nature of fast-neutron damage in corundum single crystals.
Acknowledgements
The author wishes to thank the US Department of Energy for support during the preparation of this review and work reported therein conducted at the University of North Carolina.
References [t] R.A. Hunt and R.H. Schuler, Phys. Rev. 89, (1953) 664. [2]’ P.W. Levy and G.J. Dienes, Rep. Bristol Conf. on Defects in Crystalline Solids (Physical Sot., London, 1955) p. 256. [3] For a review see B. Henderson and A.E. Hughes, Point Defects in Solids (Plenum, New York, 1972). Vol. 1, Chap. 7. [4] See S. Sonder and W.A. Sibley, Point Defects in Solids (Plenum, New York, 1972) Vol. 1, Chap. 4. [5] P.W. Levy, Discuss. Faraday Sot. 31, (1961) 118. [a] F.T. Gamble, R.H. Bartram, C.G. Young, O.R. Gilliam, P.W. Levy, Phys. Rev. 134 (1964) A589. [7] R.G. Duvamey and A.K. Garrison, Solid State Commun. 12 (1973) 1235.
[S] W.P. Unruh, Y. Chen and M.M. Abraham, Phys. Rev. Lett. 30 (1973) 607. [9] L.E. Halliburton, L.A. Kappers, D.L. Cowan, F. Dravnieks and J.E. Wertz, Phys. Rev. Lett. 30 (1973) 607. [lo] For a recent summary see Y. Chen and M.M. Abraham, Phys. Rev. Bll (1975) 881. [ 111 R.T. Cox, Solid State Commun. 9 (1971) 1989. (121 T.J. Turner and J.H. Crawford, Solid State Commun. 17 (1975) 167. [ 131 P.W. Kirklin, P. Auzins and J.E. Wertz, J. Phys. Chem. Solids 26 (1965) 1067. [14] H.H. Tippins, Phys. Rev. Bl (1970) 126. [ 151 K.H. Lee, G.E. Holmberg and J.H. Crawford, Solid State Commun. 20 (1976) 183. [16] K.H. Lee, G.E. Holmberg and J.H. Crawford, Phys. Status Solidi (a) 39 (1977) 669. [ 17) F.T. Gamble, Ph.D. Dissertation, University of Connecticut, 1963 (unpublished). Quoted by Cox (Ref. [9]). [ 18) D.W. Cooke, H.E. Roberts and C. Alexander, J. Appl. Phys. 49 (1978) 3451. [19] P.W. Levy, Phys:Rev. 123 (1961) 1226. [20] E.W.J. Mitchell, J.D. Rigden and P.D. Townsend, Phil. Msg. 5 (1960) 1013. [21] G.W. Arnold and W.D. Compton, Phys. Rev. Lett. 4 (1960) 66. [22] W.D. Compton and G.W. Arnold, J. Appl. Phys. 31 (1960) 622. [23] T.J. Turner and J.H. Crawford, Phyys. Rev. B13 (1976) 1735. [24] S.Y. La, R.H. Bartram and R.T. Cox, J. Phys. Chem. Solids 34 (1973) 1079. [25) K.H. Lee and J.H. Crawford, Phys. Rev. B15 (1977) 4065. [26] B.D. Evans and M. Stapelbroek, Phys. Rev. 18 (1978) 7089. [27] J.H. Crawford, Bull. Am. Phys. Sot. (1981). (281 S.-I. Choi, unpublished. [29] K.H. Lee and J.H. Crawford, Appl. Phys. Lett. 33 (1978) 273. [30] K.H. Lee and J.H. Crawford, Phys. Rev. B19 (1979) 3217. [31] B.G. Draeger and G.P. Summers, Phys. Rev. B19 (1979) 1172. (321 B. Henderson, S.E. Stokowski and T.C. Ensign, Phys. Rev. 183 (1969) 826. [33] B. Henderson and DC. O’Connell, Semicond. Insulators 3 (1978) 299. [34] J.B. Brewer, B.T. Jeffries, G.P. Summers, Phys. Rev. B22 (1980) 4900. [35] H.W. Lehman and Hs. H. Gunthard, J. Phys. Chem. Solids 25 (1964) 941. [36] B.T. Jeffries, J.D. Brewer and G.P. Summers, Bull. Am. Phys. Sot. 26 (1981) 268. [37] B. Jeffries, G.P. Summers and J.H. Crawford, J. Appl. Phys. 51 (1980) 3984. [38] L.S. Welch, A.E. Hughes and G.P. Pells, J. Phys. Cl3 (1980) 1805. [39] R.T. Cox and A. Herve, C.R. Acad. Sci., Paris 261 (1965) 5080. [40] R.T. Cox, Phys. Lett. 21 (1966) 503.