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Formation of color centers in anion-doped crystals
J. Phys. Chem
Solrdr Vol.
51.
0022-3697.90
ho 7.pp.737-715. 19%
53.00 + 0.00
C 1990 Pcrgamon Press plc
Printed in Gnat Bntain.
FORMATION OF COLOR CENTERS ANION-DOPED CRYSTALS
IN
MASAMITSU HIRAI Department of Applied Physics, Faculty of Engineering, Tohoku University, Sendai, 980 Japan Abstract-Point defects such as F, XT, H centers and so on are produced in nominally pure alkali halide crystals under irradiation by ionizing radiation. These defects are the products of purely electronic processes within the ions of the matrix. On the other hand, a small concentration of halogen or chalcogen anions as impurities in the crystal, acts as a catalyst or a depressor for the formation of defects. The functions of anions for defect formation, mainly in alkali halide crystals, are described in this paper. Keywords: Anion impurities, point defects, defect formation, alkali halide crystals, radiation damage. chalcogen anions, color centers.
1. INTRODUCTION This paper introduces the formation of color centers in crystals doped with anions such as halogen or chalcogen ions as impurities. These anions can be introduced into alkali-, silver- or copper-halides, and exhibit characteristic absorption bands in ultraviolet (UV) or visible spectral regions depending on the impurities. The illumination of the crystal with light absorbed into the absorption bands leads to the formation of F centers or other photochemical reactions. The mechanism of F center formation connected with such impurities differs from that in the intrinsic process discussed in previous papers. The absorption spectra due to these anions, the formation mechanism of the color centers, and the structure of the products formed are introduced in Sections 2-7: H- ions, OH- ions, F- ions, Cl- ions, Br- ions, and I- ions, respectively. Section 8 deals with optical properties of chalcogen ions, and these are further described in Sections 9-11. Section 9, 02- ions, Section 10, .Y?- ions, and Section 11, Se*-, Te’- and Po2- ions, respectively.
2. H- IONS The most classical and famous anions in alkali halide crystals are H- ions, from which F centers can be formed by the illumination of UV light. The Hion occupies an anion site substitutionally. This center is called the U center, and exhibits the U band at 228 nm in KBr [l], for instance. The illumination of light into the U band results in the formation of F centers above about 200 K as shown in Fig. 1, in which the temperature dependence of the quantum yield of the U-F conversion is shown for hydrogen and deuterium. The yield increases at high temperatures. The conversion process may be understood as follows. An electron of the H- ion becomes delocalized on the neighboring metal ions after
excitation due to light illumination, leaving a hydrogen atom at the original site. If the temperature of the crystal is high enough, the hydrogen atom moves out from its site before the return of the electron. After the hydrogen atom moves away the electron is captured by the anion vacancy to form an F center [2,3]; but if the temperature is low, the U center is converted into the U, center, which is an interstitial H- ion, or to the U2 center which is an interstitial hydrogen atom, HP [4]. The optical properties of these centers have been reviewed by Fowler in 1968 [S].’ Since then, several investigations have been reported. KC1 doped with H- or D- ions exhibits a new absorption band at 630nm under UV-light illumination. The F center in this crystal acts as a trap for hydrogen atoms [6]. Hydrogen HO,and deuterium Di atoms are produced in KCI containing not only the U center but also Na or Ag impurity ions by UV-light irradiation [7]. The optical bleaching of interstitial atomic hydrogen (HP or UZ center) produces U and H(Cl]-) centers in KC]. Here, the H center is not a hydrogen atom, but Xi- ions occupy ing three adjoining anion sites in one of the (110) directions of the alkali halide crystals. Here X stands for a halogen atom. To form the H center, a Cl0 atom is ejected and forms a molecular Cl; crowdion in only (111) directions. It is noteworthy that this direction is perpendicular to the (111) optical transition moment of the U2 charge transfer transition [8]. A new defect, which is a molecular ion ‘H; , is created in LiF with high LiH content under irradiation by @‘Cogamma-rays [9]. Defect pairs of F and U centers [Fn(H-) centers] are created by laser-induced aggregation of F centers in NaCl, KC1 and RbCl doped with high concentrations of substitutional H- ions (U centers) [lo]. UV-light irradiation results in the destruction of the crystal lattice in NaCl doped with hydrogen [ 111. The role of hydrogen or deuterium atoms in color center formation in alkaline earth halides and MgO
MASAMITSUHIRAI
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-200
a
+200
+OJ
+600
%
TEMPERATURE
Fig. 1. Temperature dependence of the quantum yield for the U-F conversion in KBr containing hydrogen or deutexium ions as U centers [2]. has been reported by several investigators. X-Ray irradiation of SrF,, BaF, and CaFr doped with hydrogen at 77 K produces hydrogen atoms in fluorine sites (Hy centers) and trapped electron centers [ 121.The X-ray irradiation of CaF,, SrF, and BaFr incorporating hydrogen and deuterium produces interstitial and substitutional hydrogen and deuterium atoms. The dependence of the hyperfme constants of nuclei in several shells on the lattice constants of alkaline earth fluorides has been discussed [13, 141. The impurity hydrogen acts also as the governing factor in the formation of the V- center in MgO under exposure to gamma- and electronirradiation. V- centers are produced by the displacement of hydrogen from the V,,_ sites by ionizing radiation [ 151.
3. OH- IONS U center formation in neutron-irradiated LiF : OH has been confirmed by ESR and IR absorption measurements (161. X-Ray irradiation at room temperature of OH- in LiF dissociates OH-, and produces U, centers. Annealing at 35O’C restores the initial state of the crystal. In such annealing, the direct conversions of U to OH-, and of Ho to OH-, and also the conversion of Ho to U, are suggested [ 171. Morato and Luty performed a systematic investigation of the UV-light-induced photodissociation of substitutional OH- defects in KC1 [18]. H; centers are produced by the reaction of mobile Cl, crowdions with H- defects. The H; defect is composed of the same electronic and ionic ingredients as the LJ, center. The formation of hydrogen atoms occupying interstitial tetrahedral sites was confirmed by Raman scattering measurement on alkali halides containing OH- under UV-light excitation. The heavier halogen ion impurities are effective in trapping the interstitial hydrogen next to them. The heavier halogen ion and interstitial hydrogen form a singly-perturbed II, center of C,, symmetry and a doubly-perturbed Uz center of C,, symmetry [19]. The formation of U2 (HP) centers was also observed in BaClF and SrClF crystals (201.
IR absorption measurements suggested the formation of a hydrogen bond between the OH- ion and the radiation-induced defect in NaF under gammaray irradiation (2 I]. TO stabilize the laser-active F; center, special attention has been focused on the defect-stabilized F; center in alkali halide crystals. Such laser-active F; centers can be achieved with OH- doping and heavy radiation damage [22-241. A U.S.S.R. group [24] found fast and slow steps in the formation process of O*- : F2+centers. The former step involves diffusion of thermally unstable F: centers, and the latter one involves the diffusion of single O*- : V: dipoles. O*- ions are produced from OH- ions by radiation such as X-rays.
4.
F-IONS
X-Ray irradiation of a mixed KCI-KF crystal at liquid-nitrogen temperature (LNT) produces molecular heterohalogen FCI- centers oriented along the (111) directions of the crystal [25]. The FCI- center is a hole-type center like the X; centers in nominally pure crystals. The electron structure (251 and the fundamental internal vibration [26] of this center have been reported. (11 I)-oriented FCI-, FBr- and FI- centers occupying one negative ion vacancy are also created in KCI-KF, KCl-KF-KBr and KCI-KF-Kl under X- or gamma-ray irradiation at 77 K [27] and exhibit absorption bands at 300, 294 and 275 nm, respectiveiy. FCI-, FBr- and FIcenters become unstable above about 120, 170 and 270 K, but the actual temperatures depend on the host crystals involving these centers. No investigation of the role of F- ions on F center formation has been found.
5. Cl- IONS The formation of the molecular heterohalogen BrCI- centers aligned along (1 IO) directions in KCI doped with a small amount of KBr has been confirmed by optical and EPR studies after X- or gamma-ray irradiation and proper thermal treatment [28,29]. Strong and much weaker optical absorption bands are observed at 382 and 760nm with sigmapolarized transitions. Similar molecular heterohalogen FCI- centers are also observed in single KF crystals doped with PbCl, and KCI. The difference in the optical and ESR parameters betweeen FCl- in the (111) directions in KC1 doped with KF and FCI- in the (110) direction in KF doped with KC1 have been discussed qualitatively [30]. Irradiation with 6.5-7.5 eV photons creates anion Frenkel defects (F-H center pairs and a-I center pairs) in KBr doped with KC1 at 4.2 K [31]. It is suggested that the self-localization of opticallyformed excitons creates such defects. X-Ray irradiation of the crystal at 4.2 K also showed the decay of
Formation of color centers in anion-doped crystals
self-trapped excitons with the creation of F-H and z-1 center pairs on regular lattice sites [32]. The formation of C!; centers in K20-BrO,-KC! and NarO-B,Or-NaCI glass is an interesting result. The centers in the glass are more stable than those in alkali halide crystals [33]. 6. Br- IONS Absorption bands due to Br- ions in KC! appear on the low energy side of the fundamental absorption bands in the UV spectra! region [34]. These absorption bands originate from the creation of excitons at Br- ions. Extensive investigation on the behavior of the bands has been reported by Tomiki [35]. The molecular heterohalogen BrCl- center introduced before is thermally converted to another center, which consists of BrCI- and a positive ion vacancy in a nearest-neighbor position to both nuclei of the molecule ion [36]. The new center [BrCI-(v,)] exhibits a strong absorption band at 368 nm to the sigma-polarized transition and also a weak band at 910nm. The internuclear axis of the BrCl- ion in this center lies in a (100) plane and is inclined at an angle of 12.5” to the (110) direction. The new centers are converted into Cl; (v, ) centers by 405 nm light excitation at 77 K [37]. Further complex centers such as the BrCli- center in KC1 containing Na and Br [38], or the BrCI- center in an anion site with nearest-neighbor Sr’+ and an associated positive ion vacancy, have been confirmed in KCI containing Br- and Sr*+ or Ba?+ ions [39] under X-ray irradiation. Ionizing irradiation and suitable thermal and optical treatment can create BrCI- centers occupying a single negative ion site next to a substitutional Li+ ion in KC! strongly doped with Li+ and Br- ions [40]. The internuclear axis of the BrCI- molecule ion lies in a (100) plane and makes an angle of 25” to one of the (100) directions. ( 1IO) oriented H,, (Na+) and H, (Na+)type BrCIi- centers in KC1 doped with both Na+ and Br- ions have also been reported [41]. Still and Pooley were the first to observe the effect of heavy ions on F-center formation in KCI-KBr and KCI-RbCI [42]. Introduction of heavy ions such as Br- or Rb+ reduces the formation yield of the F centers. The reduction of the yield was expected in such mixed crystals because of the disruption of replacement collision sequences. On the other hand, the formation of H centers in KCI containing CIBrwas the first report to confirm color-center formation through self-trapped excitons (STE) [43]. KC1 containing Tl’ was irradiated by 2 MeV electrons at LNT to form Cl;, CIBr- (due to a small amount of substitutional Br- impurity), TlO, T12+, F and z centers. ESR absorption confirmed the existence of ClBr-. Then, by releasing electrons from Tl”, and by trapping them at CIBr-, the formation of H centers was confirmed by ESR. It was also confirmed that Br- ions in KC! act effectively on the F-center
739
formation (441. KC! crystals containing various concentrations of substitutional Br- ions were excited by photons between 7.0 and 8.2eV. The F-center formation yield became a maximum in the spectral region where the Br- absorption band peaks, but not in the fundamental exciton absorption region of KC!. A two-photon absorption process of N, laser light also creates F and H centers in KC! containing Br- or I- ions [45]. The absorption and emission spectra due to F centers and the absorption spectra due to H centers thus produced in the crystal containing Br- or I- ions are all similar to those in undoped crystals. This result suggests the formation of the regular type of F and H centers even if Br- or I- ions are selectively excited. However, the molecular heterohalogen CIBr- seems to play an important role in color center formation. Similar results in KC1 containing Br- or I- ions are also reported by U.S.S.R. groups [46]. They observed the creation of a-1 center pairs as well as F-H center pairs. A thermoluminescence study discusses the formation and thermal stability of defects created in KC1 : Br under gamma irradiation [47]. 7. I- IONS Mahr [48] was the first to report the absorption and emission characteristics of I- ions in KC!. The existence of the molecular heterohalogen ICl- has been confirmed by ESR absorption [49]. UV-light irradiation produces the heterohalogen self-trapped excitons, IC12-*, in KC1 [50] and RbC! [51] containing I ions. An absorption band due to relaxed IC12-* excitons occurs at 1.65 eV at 80 K after irradiation of KC1 : I by a pulsed electron beam [52]. The characteristics of the lowest relaxed triplet state of these excitons have been investigated by observing the temperature dependence of the luminescence from this state [53,54]. The luminescence study on KC1 : I and RbCl : I under UV-light irradiation suggests coexistence of two different types of relaxed exciton states. The first is an IC12-* state of the two-center type, i.e. ICI*-* occupying two anion vacancy sites. The second is a one-center type, i.e. IC12-* occupying three anion vacancy sites, where the central iodide ion traps an electron-hole pair [55]. IBr2-* exciton has been identified in NaBr [56]. Such heterohalogen excitons are produced by a weakly temperature dependent process of conversion of intrinsic relaxed excitons localized near impurities. IOHmolecule ions could also be produced in KC1 : I under X-, gamma- or UV-irradiation at 80 K [57]. Introduction of I- ions as the heavy ions into KBr reduces the production rate of F centers [58]. I- ions in KBr seem to disrupt the replacement collision sequences for the F-H formation as do Br- ions in KC! [42]. A new type of H center consisting of Br, I’is expected to be produced in this mixed crystal. A similar heterohalogen H center (C1313-) occupying
MASAMITSUHIRAI
740
+
t
-I-
-
+
-
-
+
Cl-
+
+
ICI-
-l
-
cl-
+
-
+
+
-
+
-
-8
-
6
Fig. 2. A schematic model of the heterohalogen
H center.
three anion sites in one of the (110) directions is also observed in KCI : I under X-ray irradiation at LHeT as shown in Fig. 2 [59]. The ICI- ion in the center occupies the central anion site. The formation of F and H centers has been investigated by exciting selectively I- ions in KC1 and KBr under irradiation by an ArF-excimer laser (601. It is interesting to note that the absorption spectra due to F and H centers thus produced are almost the same as those in pure crystals. The creation of regular F and H centers in pairs from I--localized excitons was concluded. The H centers in this case are not associated with I- ions. A selective photoexcitation of localized excitons perturbed by I- ions in KC1 : I creates effectively F-H center pairs at 4.2 K [61]. As a formation mechanism for F-H center pairs from such heterohalogen excitons, the translational shift of the near-impuritylocalized homohalide excitons has been discussed. For a clear understanding of the F-center formation mechanism in crystals doped with heavy halogen ions, further systematic investigations are desired. Another type of color center formation in doped KC1 with I- ions is the formation of V-type centers [62]. The center exhibits an absorption band at 247 nm after X-ray irradiation of the crystal. The
5 4 3 2 PHOTON ENERGY CeV)
Fig. 3. Absorption spectra of KCI containing @- ions at 78 K. Solid curve: before UV-light illumination; broken curve: after UV-light illumination [68].
center consists of I,Clof the host lattice.
lying in the (1 IO) directions
8. OPTICAL PROPERTIES
OF CHALCOGEN
2.58 3.93 5.08
2.80 3.68 4.92
3.37 4.46 5.35
2.98 3.58 4.49
2.94 3.48 4.29
3.05 4.12 4.89
3.05 3.32
3.14 3.34
2.87 3.12
2.81 3.06
2.87 3.07
4.19 4.46
4.26 4.51
4.06 4.36
3.79 4.08
3.91 4.15
2.85 4.40 5.85
2.82 4.10 5.61
S?- + F-
3.15 3.85 4.70
3.21 3.86 5.70
Se?- + F-
3.14 3.41
Te?- + F-
IONS
8.1. Optical absorption spectra Chalcogen ions can be introduced into alkali halide crystals, and form anion-vacancy pairs to maintain the electric neutrality of the crystals. The pairs exhibit two or three absorption bands in the UV and visible spectral regions. Illumination of the crystal with light absorbed by the band in the UV region leads to the formation of F centers in alkali halide crystals, and other photochemical reactions in other crystals. The solid curve in Fig. 3 illustrates the typical absorption spectrum due to O’- + anion vacancy pairs (O*- + F+) occupying two nearest anion sites in one of the (110) directions in KC1 at 78 K [63]. F+ stands for the anion vacancy. Hennl [64] has summarized the data on the position of the absorption maxima in K- and Rb-halide crystals as listed in Table 1. Chalcogen-vacancy centers (Ch2- + F+) have two, three or four absorption bands in the
Table I. The peak position of the absorption bands at 78 K (in electron volts) 1641 RbI RbBr KBr KI RbCl KC1 O?- + F-
1
2.88 3.45
2.67 3.24
4.16 4.78
3.86 4.45
Formation of color centers in anion-doped crystals
741
factor u is a function of the center and of the host-lattice anion, and it decreases from iodide to chloride, as listed in Table 2. Gol’denberg et al. [65] attributed two absorption bands at 228 nm (5.43 eV) and 290 nm (4.27 eV) to (O*- + F+) in NaCl from their intensity, thermal stability and reaction to light. A band at 265 nm (4.68 eV) is due to a complex of these dipoles. 8.2. Luminescence 2.5
3.0
3.5
4.0
PHOTON ENERGY (eV)
Fig. 4. Double-logarithmic plot of the anionxation distance vs the photon energy of the violet absorption band of the chalcogen-vacancy centers at 78 K. Straight lines connect points of the same pair of lattice anion and center. (a): 02-, (0): S*-, (+): Se’-, ( x ): Te*--F+ centers [64]. Table 2. Constant a of the Mollwo-Ivev relation 1641 02- + F-
S2- + FSe2-+FTti-+F-
Chlorides 5.03 5.25 5.19 -
Bromides 5.48 5.38 -
Iodides 5.92 5.75 5.58
near-UV spectral region. The maxima of the lowest energy absorption band follow the modified Mollow-Ivey relation,
E Mix= &.08 2 0.10,
(1)
with a factor u characteristic of each (Ch*- + F+). In Fig. 4 the photon energies of the lowest energy absorption band are plotted as a function of the anion-cation distance in a double-logarithmic scale as well as those for S*- in NaCl and NaBr 1641.The
bank
In Table 3 [64] are listed the peak positions and full-widths at half-maximum (FWHM) of the luminescence bands due to (Ch*- + F+) in K-halide crystals at 78 K (O*- at 20 K). (O*- + F+) exhibits only two luminescence bands. (S*- + F+) exhibits two bands around 3 and 2 eV. Three bands in the visible region due to (Se*- + F+) and (Te*- + F+) consist of two bands due to spin-orbit coupling and an independent band. In the near-IR, three or four luminescence bands between 0.4 and 0.9 eV with FWHM of 0.09 eV or smaller are observed for all (S’- + F+), (Se*- + F+) and (Te*- + F+). Hennl discusses the characteristics of the luminescence as follows. By excitation in the lowest energy absorption band, (S*- + F+), (Se*- + F+) and (Te*- + F+) return to the ground state with radiation, but (O*- + F+) without radiation. Excitation in the second absorption band results in the radiative transition in (O*- + F+), but the non-radiative transition in all other cases. Three IR luminescence bands of (S*- + F+), (Se*- + F+) and (Te?- + F+) are excited by illumination in any of the absorption bands. Hennl assumes a luminescence cascade for the IR bands as a first
Table 3. The peak position (E) and FWHM (H) of emission bands in electron volts at 78 K (02- at 20 K) [64] KC1
KBr
E
H
E
O*- + F-
2.67 1.18
0.33 0.25
2.36 0.89
S2- + F-
3.48 2.07 0.899 0.585 0.435
0.21 0.205 0.058 0.058 0.036
Se’- + F-
2.82 2.15 2.04 0.895 0.579 0.433
0.254 0.39 0.24 0.063 0.047 0.035
Te2- + F-
KI
H
E
H
2.99 2.08 0.902 0.577 0.432
0.26 0.27 0.065 0.062 0.049
3.02 2.09 1.475 0.713 0.407
0.30 0.21 0.351 0.149
2.83 2.20 1.93 0.896 0.575 0.43 1
0.18 0.15 0.21 0.062 0.056 0.043
2.88 2.22 2.02 0.873 0.573 0.430
0.19 0.26 0.303 0.088 0.063 0.043
3.16 2.07 1.75 0.892 0.785 0.569 0.429
0.18 0.208 0.317 0.076 0.091 0.056
MASAMITWHIRAI
742
possibility. His second interpretation for the IR bands is the formation of a weak homopolar bond of Ch- with one of the six neighboring alkali ions. The Ch- can be formed by ionization of Ch2- by light illumination. According to his discussion, depending on the relative position of the nearby anion vacancy (F+) and the weak homopolar bond, three different potential energy curves are formed in the excited state to result in three bands.
2 B
10
$ 5 $ Y 8
9. O*- IONS
/ 0
I
I
0
NUMBER OF ABSORBED
9.1. KC1 Sensitization for the coloration of alkali halide crystals doped with oxygen was first discovered by Hilsch and Pohl[66]. Korth [67] was the first to report the following process, (02- + F+)+(O-
+ F),
(2)
in KCI. (Ch- + F) stands for the Ch- + F center pair occupying two nearest anion sites in one of the (I 10) directions in the alkali halide crystals. Since then, the Gijttingen group have worked extensively on this subject. Figure 3 presents a typical absorption change in 02-doped KC1 after annealing (solid curve) and after irradiation (broken curve) with UV-light at room temperature (RT) [68]. Three bands at 2.92, 4.25 and 5.85 eV due to (O*- + F+) are observed in this case. Irradiation with UV-light decreases the concentration of O*- centers, while that of the F centers increases correspondingly, as shown in Fig. 5. From these results, Fischer and Gummer [68] suggested the photochemical reaction given by eqn (2) as the only reaction occurring in this case. The reaction depends on the photon energy by which the crystal is excited. Only irradiation into the high energy bands yields the decomposition of (02- + F+) with a quantum yield of 0.36 in KC1 above _ 220 K. No reaction occurs by irradiation into the lowest energy band [68]. By UV-light irradiation of a crystal doped with Ch2-, a new absorption band appears in the spectral region of the conventional F band. However, this band has a lower energy peak position and wider FWHM than those of the conventional F band as listed in Table 4 [69]. Hennl [69] attributed the new absorption band to the optical absorption of an F center which is perturbed by a Ch- anion in the nearest-neighbor position.
._
I
2 .lO’V
1
PHOTONS
Fig. 5. Decrease of 02- ion concentration (broken curve) and complementary increase of F center concentration (solid curve) in KC1 at 250 K as a function of absorbed photon number [68]. The peak position and FWHM of the new band due to (O- + F) in KC1 could be interpreted by a calculation on the variation of the energy parameters for F centers in the field of the associated impurity oxygen ion. This calculation suggested the validity of the formation of (O- + F) centers in the photochemical coloration of crystals containing oxygen impurities [70]. Staibe and Scott [71] reported F center formation from (02- + F’) in KCI under UV-light irradiation between 154 and 210 K. They studied further the back reaction, F-+a + e,
(3)
e + O--*02-,
(4)
to find the activation
energy for r center migration.
9.2. KBr and KI Hennl [69] investigated the effect of UV-light irradiation at liquid helium temperature (LHeT) on KBr and KI doped with 02-, S?-, Se?- or Te*- impurities, and found new absorption bands in the spectral region of the conventional F band in each crystal as introduced before. and as listed in Table 4. The new band was named as the F,(Ch) band in analogy to the F, band. Irradiation into the F,(Ch) absorption band at LHeT destroys the F,(Ch) centers. F,(Ch) centers are unstable at LNT, and are converted back into (Ch2- + F+).
Table 4. The peak position of the F band (E), its FWHM (H) and deviation of peak energy from the normal F band peak (AE) in KBr and KI crystals [69] KI
KBr L(O) F”(S) F, (Se) Fu (Te)
E(eV) 2.064 1.987
H(eV) 0.285 0.158
P(%)
AE(eV)
-7
0.077 -
2.032 2.015
0.440 0.415
8 8
0.032 1.049
E(eV) 1.875
H(eV) 0.155
P(%)
1.839 1.828 1.797
0.253 0.245 -
18 8 4
-
AE(eV) 0.036 0.047 0.078
Formation of color centers in anion-doped crystals
9.3. NaCl formation efficiency in NaCl containing (O*- + F+) is much higher than that in pure NaCl under X-ray irradiation [72]. Didyk and Tsal [72] proposed the following process for the enhancement of F-center formation efficiency, F-center
(O*- + F+)+O-
+ e + F+ -+O- + F center,
(5)
0; + e + F+ -+O, + F center,
(6)
and kT
20- -
where, hv and kT are an X-ray quantum and thermal energy, respectively. However, no discussion on the variation due to the O*- centers was given. Light irradiation into the 228 nm (5.43 eV) band in NaCl at 633 K destroys the 228 nm band and yields F centers [65]. Gol’denberg et al. proposed the idea of using the crystal as an image recording material. According to their report, the image in the crystal was preserved for an indefinite time at RT and could be erased in 10 min by heating the crystal to 773 K. They [73] also suggest the recording of 3D holograms in NaCl with dipole oxygen centers.
743
into the _ 5.5 eV band. This band grows even at LNT, but with a formation rate of half of that at RT. An interesting result is the wide FWHM (0.49 eV) of the new band which grows as the crystal is irradiated at LNT. The FWHM becomes equal to that of the conventional F band when the crystal temperature is raised to RT. By cooling the crystal to LNT again, the FWHM remains narrow as the conventional one does. This wide FWHM is attributed to the absorption band due to the F centers perturbed by nearby S- ions at LNT, as introduced by other investigators. At RT, the thermal energy decomposes (S- + F) into individual S- and F centers and results in the narrow FWHM of the band. NaCl does not show such a wide FWHM for the F band even at LNT, suggesting that the complex center (S- + F) does not form. 10.2. KC1 and KBr Fischer and Grundig [77] were the first to observe the absorption bands due to S2- ions at 3.15, 3.85 and 4.7eV, and discussed F-center formation by a photochemical reaction as follows, U-F
+ (1/2)H,,
(9)
and (1/2)H2 + S2- + F+-rSH-
10. S*- IONS
10.1. NaCl and NaBr Baba and coworkers [74,75] have extensively studied F-center formation from (S*- + F+) in NaCl. NaCl doped with S*- shows two absorption bands at 4.15 and _ 6 eV when the crystal is quenched from 873 K to RT. Irradiation of the crystal with UV-light absorbed into the z 6eV band at RT results in F-center formation. The formation rate as a function of the exciting photon energy at RT becomes a maximum near 5.3 eV. They suggest the photochemical reaction, S?-z
S2-*_&-+e,
(7)
and e + F++F,
(8)
where S*-* stands for the excited S2- ion. No formation of F centers is observed at LNT. Irradiation into the 4.15 eV band at RT and LNT does not produce any F centers either. The 4.1 eV emission intensity due to (St- + F+) in NaCl starts to decrease above _ 150 K, while the F-center formation efficiency increases correspondingly in this temperature range. They assume two adiabatic potential curves for the excited (S2- + F+) center, the one leading to the 4.10eV emission and the other leading to F-center formation. NaBr [76] also exhibits two absorption bands at 4.15 and around 5.5 eV. A new band around the conventional F band is formed by irradiation only
+ F.
(10)
Slightly different peak positions from those reported by Hennl are observed at 3.60 and 4.80 eV for KCI, and at 3.55 and 4.70 eV for KBr containing S2- impurity [78]. A luminescence band around 3.7 eV in KCI or around 3.3 eV in KBr is observed by irradiation into the high energy absorption band in each crystal at LNT. No growth of the (S- + F) absorption is observed in either KC1 or KBr under irradiation into the high energy bands at LNT. Irradiation at RT, however, produced (S- + F) as in other crystals. Mito and Hirai [79] investigated the temperature dependence of the conversion efficiency of (S?- + F+) to F centers. No F centers were produced by irradiation with UV-light near LNT, but three absorption bands at _ 1.9, _ 2.1 and - 2.3 eV (X bands) grew by irradiation between 220 and 260 K. The conventional F band was formed at temperatures between 273 and 330 K. As shown by the curves from 2 to 6 in Fig. 6, the X bands could be converted to the F band by raising the crystal temperature from 220 to 305 K with almost complete conversion from X centers to F centers. They suggest the following explanation. At a relatively low temperature such as 220 K, an electron from S2- is transferred to the anion vacancy (F+) to form an X center consisting of an F center perturbed by an S- ion, i.e. (S- + F). Above + 260 K, the S- ion migrates away from the F center to leave an isolated F center. The destructive energy for (S- + F), possibly for the migration of S, was estimated to be 550meV. The F center is not mobile at temperatures up to around 370 K.
744
MASAMITSU HIRAI /-
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Fig. 6. Thermal conversion of X centers to F centers by raising the temperature of KC1 : S’- from 220 to 305 K [79].
In their sample, two luminescence bands were observed at 3.76 and 2.57 eV. These bands occurred at different positions from those reported by Henne (3.48 and 2.07 eV) [64]. Since their S2- ions [79] were introduced into KC1 by adding Na,S, there is a possibility that the Na+ ion perturbed the (S2- + F+) centers sufficiently to shift the luminescence peaks. The intensity of the 3.76 eV band starts to decrease above _ 200 K as the 4.1 eV luminescence band in NaCI. However, no correlation between the temperature dependence of the luminescence intensity and the F center formation efficiency was observed. This result suggests different excited states for the initial state for the luminescence and for F center formation. Irradiation into the low energy band of KBr under an electric field at LNT aligned the dipoles of (S’- + F+) along the electric field, but no conversion to F centers was observed [SO]. 10.3. KI S’- centers in KI exhibit absorption bands at 3.37, 4.46 and 5.35 eV [81]. The 3.37 eV band occurs due to isolated (S2- + F+) dipoles, whereas the 4.46 and 5.35 eV bands are partly due to dipole aggregates. Irradiation into the 3.37 eV band at temperatures between RT and LNT results in no change in the absorption spectra. However, irradiation into the 4.46 and 5.35 eV bands at 240 K produced F centers, and lowered the height of the 3.37 eV band. No reaction was observed in the temperature range between _ 25 K and LNT. The formation of F centers in this case suggests the photochemical reaction.
(S2-+ F+) hv
S- + F.
(11)
The photochemical reaction at LHeT in the high energy absorption bands produces (S- + F). Alignment of the (S2- + F-) dipole under an electric field and irradiation into the lowest absorption band at 3.37eV were also observed in KI.
A few investigations of the optical absorption spectra and F-center formation in alkali halide crystals doped with Se2-, Te2- and Po2- ions have been reported. Fischer [82] observed absorption bands due to (Se2- + F-) in KC1 and KBr to occur in the near-UV spectral region as listed in Table 1. Absorption bands in KI, RbCl, RbBr and RbI were studied extensively by Hennl [64]. These results are also listed in Table 1. In general (Se2- + F-) exhibits four bands, except in KCI. (Se2- + F+) in KCI, KBr and KI exhibits three emission bands in the visible region and an additional three bands in the near-IR region, as listed in Table 3. Irradiation of KBr and KI doped with Se2- by the intense line of a high pressure mercury lamp at LHeT led to the formation of an absorption band near the conventional F band. However, the peak position and FWHM of the new band differed slightly from those of the conventional F band as well as the band in KBr and KI doped with 02-, S2- or Te2-. Hennl [69] suggested that F centers perturbed by a nearby Ch- ion, i.e. (Ch- + F), are the reason for the different peak position and FWHM. (Te’- + F+) has been investigated only for KI and RbI [64]. The absorption bands in these crystals consist of four bands as listed in Table 1. In Table 3 are listed the peak positions of the luminescence bands due to (Te2- + F+) in KI. Seven bands were found (641. (Te2- + F+) is also converted to (Te- + F), similar to (Se2- + F+). No report has been found on (Po2- + F-) in any crystals.
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104, 599 (1956). 5. Fowler W. B., Physics of Color Centers. Academic, New York (1968). 6. Dumke V. and de Souza M., 1974 International Conference on Color Centers in Ionic Crystals (abstracts) (1974). 7. Badalyan A. G. and Baranov P. G., Fir. tverd. Tela 20, 3727 (1978). 8. Reuter G.,. Lohse F. and Spaeth J. M., Phys. Status Solidi (b) 96, 243 (1979). 9. Grefenshtein S. R. and Chepumoi V. A., Opt. Spekrrosk. 63, 202 (1987). 10. Pan D. S. and Liity F., Phys. Rev. BlS, 1868 (1978). 11. Sukhorebry S. P., Tsal N. A. and Pavlyk B. V., Uir. Fiz. Zh. 32, 239 (1987). 12. Bessent R. G., Hayes W.. Hodby J. W. and Smith R. H. S.. Proc. R. Sot. A309. 69 (1969). 13. Bottcher’ R., Frieser A., Milsch ‘B., Volkel G. and Wartewig S., Phys Status Solidi 38, k97 (1970). 14. Weid J. P. von der and Ribeiro S. C., 1974 Infernational Conference on Color Centers in Ionic Crystals
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Formation of color centers in anion-doped crystals 15. Chen Y., Abraham M. M., Templeton L. C. and Unruh W. P.. Phvs. Rev. Bll. 881 (1975). 16. Kamikawa T., Phys. Sratus‘Solidi (b) 18, 639 (1975).
17. Akhvlediani 2. G., Berg K. J. and Berg G.. Crysl. Lutt. Defects 8, 167 (1980). 18. Morato S. P. and Luty F., Phys. Rev. B22,4980 (1980). 19. Goovaerts E., Zhou Z. F. and Schoemaker D., Radial. Eff. 72, 81 (1983). 20. Yuste M., Lefrant S., Spaeth J. M. and Tatter1 L., J. Phys. c 8, 3491 (1975). 21. Vorozheikina L. F., Albutashvili N. V. and Tatishvili K. D., Opt. Spekrrosk. 60, 535 (1986). 22. Mollenauer L. F., Opt. Lert. 6, 342 (1981). 23. Arkhangelskaya V. A., Guseva E. V., Zinger G. M. and Korolevk N. E., Opt. Spekrrosk. 61, 542 (1986). 24. Bryukvina L. I., Khulugurov V. M. and Parfianovich I. A.. ODI. Soekfrosk. 63. 207 (1987). 25. Wilkins’J. W. and Gabriel J. d., Phys. Rev. 132, 1950 (1963).
26. Susnian S., Phys. Srarus Solidi 37, 561 (1970). 27. Schoemaker D., Phys. Rev. 149, 693 (1966). 28. Delbecq C. J., Schoemaker D. and Yuster P. H., Phys. Rev. 83, 473 (1971). 29. Hausmann A. and Pomplun H., Solid St. Commun. 11, 867 (1972).
30. Delbecq C. J., Hutchinson E., Yuster P. H. and Marshall S. A., J. them. Phys. 79, 1408 (1983). 31. Taiirov M. M., F&. roerd. Tela.25, 450 (1983). 32. Taiirov M. M.. Akilbekov A. T. and Fvodorova 0. A.. Eesri NSV Tead. Akad. Fuus Inst. Uurim. 54, 102 (1983).
33. Maekawa T., Satoh S. and Yokokawa T., YogyoKyokai-Shi (Japanese) 93, 20 (1985). 34. Mahr H., Phys. Rev. 122, 1464 (1961). 35. Tomiki T., J. phys. Sot. Japan 23, 1280 (1967). 36. Delbecq C. J., Schoemaker D. and Yuster P. H., Phys. Ren. B7, 3933 (1973).
37. Delbecq C. J., Schoemaker D. and Yuster P. H., Phys. Rep. B9, 1913 (1974). 38. Schoemaker D. and Lagendijk A., Phys. Rev. BlS, 5927 (1977). 39. Puymbroeck W. van, Schrijvers N., Bouwen A. and Schoemaker D., Phys. Srarus Solidi (b) 112, 725 (1982). 40. Schoemaker D. and Shirkey C. T., Phys. Rev. B6, 1562 (1972). 41. Schoemaker D., 1974 International Conference on Color Centers in Ionic Crystals (abstract) (1974). 42. Still P. B. and Pooley D., Phys. Status Solidi 32, k147 (1969). 43. Keller F. J. and Pattern F. W., Solid St. Commun. 7, 1603 (1969).
44. Maki M., Nagasawa N. and Hirai M., SolidSr. Commun. 17, 1409 (1975).
45. Itoh M., Kan’no K. and Nakai Y., Solid St. Commun. 26, 929 (1978). 46. Vasil’chenko E. A. and Taiirov M., Esri NSV Tead. Akad. Fuus. Inst. Uurim. 53, 172 (1982).
47. Veeresham P., Narasimha Reddy K., Subba Rao U. V. and Hari Babu V., Crysr. Res. Technol. 18, 1585 (1983). 48. Mahr H., Phys. Rev. 12S, 1510 (1962).
745
49. Goldberg L. S. and Meistrich M. L., Phys. Rer. 172, 877 (1968). 50. Fujita M., Nakamura K. and Nakai Y.. J. phys. Sot. Japan 49, 1937 (1980). 51. Arimoto 0.. Sasaki K.. Kan’no K. and Nakai Y., J. phys. Sot, Japan 54, 3188 (1985). 52. Malyshev A. A. and Yakovlev V. Yu.. Fiz. rcerd. Tela 24, 2296 (1982). 53. Nakatani H., Arimoto 0.. Kan’no K. and Nakai Y., J. Spectrosc. Sot. Japan 33, 240 (1984). 54. Kan’no K.. Nakatani H.. Mudai T.. Arimoto 0. and Nakai Y., i. phys. Sot. Japan 55, 2443 (1986). 55. Arimoto 0.. Sasaki K., Nakatani H., Kan’no K. and Nakai Y.. J. Luminesc. 31-32. 333 (1984). 56. Lisitsyn V. M., Malyshev A. A. and Yakovlev V. Yu., Fiz. rverd. Tela 25, 3356 (1983). 57. Schoemaker D. and Waldner F., Heb. phys. Acta 44,559
(1971). 58. Hirai M., Solid St. Commun. 10, 493 (1972). 59. Katsuyama T., Wakita S. and Hirai M., J. phys. Sot. Japan 36, 1058 (1974). 60. Ahmoto 0.. Kan’no K., Nakamura K. and Nakai Y., J. phvs. Sot. Jawn 53. 70 (1984). 61. Vasil’chenko E., Salbmatov V., Tajirov XI. and Osmonaliev K., Eesri NSV Tead. Akad. Fuur. Inst. Uurim. 58, 67 (1986). 62. Hattori S., Phil. Mug. B37, 703 (1978). 63. Gummer G., Z. Phys. 215, 256 (1968). 64. Hennl P., Phys. Status Solidi (a) 46, 147 (1978). 65. Gol’denberg A. B.. Golubtsov V. V. and Lukashuk S. B., Opt. Spekrrosk. (U.S.S.R.) 44, 942 (1978). 66. Hilsch R. and Pohl R. W., Gorringer Nachr. marh.-phys. Kl., N.F. 1, 115 (1934). 67. Korth K.. Gotringer Nachr math.-phys. Kl., .L’.F. 1, 221 (1935). Fischer F. and Gummer G., Z. Phys. 189, 97 (1966). Hennl P., Phys. Srarus Solidi (b) 84. K151 (19i7). 70. Dmitrieva E. I., Kristofel N. N. and Salomatov V. N., Opt. Spektrosk. 60, 427 (1986). 71. Staibe T. F. and Scott A. B., Phys. Status. Solidi (b) 67, K107 (1975). 72. Didyk R. I. and Tsal N. A., Fizz. roerd. Tela (U.S.S.R.)
2
14, 1840 (1972). 73. Golubtsov V. V., Gol’denberg A. B., Lukashuk S. B. and MandelV. E., Opt. Spekrrosk.(CI.S.S.R.)47, 146(1979). 74. Baba M., Ikeda T. and Yoshida S., Jap. J. appl. Phys. 14, 1273 (1975). 75. Baba M., Ikeda T. and Yoshida S., Jap. J. appl. Phys. 15, 231 (1976). 76. Baba M., Kimura N. and Ikeda T., Jap. J. appl. Phys. 22, 1837 (1983). 77. Fischer F. and Grundig H., Z. Phys. 184, 299 (1965). 78. Baba M. and Ikeda T., Jap. J. appl. Phys. 24, 646 (1985). 79. Mito K. and Hirai M., J. phys. Sot. Japan 56, 1874 (1987). 80. Prakash J., Phys. Status Solidi (b) 117, 161 (1983). 81. Prakash J., Jap. J. appl. Phys. 17, 1067 (1978). 82. Fischer F., Z. Phys. 187. 262 (1965).
Solrdr Vol.
51.
0022-3697.90
ho 7.pp.737-715. 19%
53.00 + 0.00
C 1990 Pcrgamon Press plc
Printed in Gnat Bntain.
FORMATION OF COLOR CENTERS ANION-DOPED CRYSTALS
IN
MASAMITSU HIRAI Department of Applied Physics, Faculty of Engineering, Tohoku University, Sendai, 980 Japan Abstract-Point defects such as F, XT, H centers and so on are produced in nominally pure alkali halide crystals under irradiation by ionizing radiation. These defects are the products of purely electronic processes within the ions of the matrix. On the other hand, a small concentration of halogen or chalcogen anions as impurities in the crystal, acts as a catalyst or a depressor for the formation of defects. The functions of anions for defect formation, mainly in alkali halide crystals, are described in this paper. Keywords: Anion impurities, point defects, defect formation, alkali halide crystals, radiation damage. chalcogen anions, color centers.
1. INTRODUCTION This paper introduces the formation of color centers in crystals doped with anions such as halogen or chalcogen ions as impurities. These anions can be introduced into alkali-, silver- or copper-halides, and exhibit characteristic absorption bands in ultraviolet (UV) or visible spectral regions depending on the impurities. The illumination of the crystal with light absorbed into the absorption bands leads to the formation of F centers or other photochemical reactions. The mechanism of F center formation connected with such impurities differs from that in the intrinsic process discussed in previous papers. The absorption spectra due to these anions, the formation mechanism of the color centers, and the structure of the products formed are introduced in Sections 2-7: H- ions, OH- ions, F- ions, Cl- ions, Br- ions, and I- ions, respectively. Section 8 deals with optical properties of chalcogen ions, and these are further described in Sections 9-11. Section 9, 02- ions, Section 10, .Y?- ions, and Section 11, Se*-, Te’- and Po2- ions, respectively.
2. H- IONS The most classical and famous anions in alkali halide crystals are H- ions, from which F centers can be formed by the illumination of UV light. The Hion occupies an anion site substitutionally. This center is called the U center, and exhibits the U band at 228 nm in KBr [l], for instance. The illumination of light into the U band results in the formation of F centers above about 200 K as shown in Fig. 1, in which the temperature dependence of the quantum yield of the U-F conversion is shown for hydrogen and deuterium. The yield increases at high temperatures. The conversion process may be understood as follows. An electron of the H- ion becomes delocalized on the neighboring metal ions after
excitation due to light illumination, leaving a hydrogen atom at the original site. If the temperature of the crystal is high enough, the hydrogen atom moves out from its site before the return of the electron. After the hydrogen atom moves away the electron is captured by the anion vacancy to form an F center [2,3]; but if the temperature is low, the U center is converted into the U, center, which is an interstitial H- ion, or to the U2 center which is an interstitial hydrogen atom, HP [4]. The optical properties of these centers have been reviewed by Fowler in 1968 [S].’ Since then, several investigations have been reported. KC1 doped with H- or D- ions exhibits a new absorption band at 630nm under UV-light illumination. The F center in this crystal acts as a trap for hydrogen atoms [6]. Hydrogen HO,and deuterium Di atoms are produced in KCI containing not only the U center but also Na or Ag impurity ions by UV-light irradiation [7]. The optical bleaching of interstitial atomic hydrogen (HP or UZ center) produces U and H(Cl]-) centers in KC]. Here, the H center is not a hydrogen atom, but Xi- ions occupy ing three adjoining anion sites in one of the (110) directions of the alkali halide crystals. Here X stands for a halogen atom. To form the H center, a Cl0 atom is ejected and forms a molecular Cl; crowdion in only (111) directions. It is noteworthy that this direction is perpendicular to the (111) optical transition moment of the U2 charge transfer transition [8]. A new defect, which is a molecular ion ‘H; , is created in LiF with high LiH content under irradiation by @‘Cogamma-rays [9]. Defect pairs of F and U centers [Fn(H-) centers] are created by laser-induced aggregation of F centers in NaCl, KC1 and RbCl doped with high concentrations of substitutional H- ions (U centers) [lo]. UV-light irradiation results in the destruction of the crystal lattice in NaCl doped with hydrogen [ 111. The role of hydrogen or deuterium atoms in color center formation in alkaline earth halides and MgO
MASAMITSUHIRAI
738
-200
a
+200
+OJ
+600
%
TEMPERATURE
Fig. 1. Temperature dependence of the quantum yield for the U-F conversion in KBr containing hydrogen or deutexium ions as U centers [2]. has been reported by several investigators. X-Ray irradiation of SrF,, BaF, and CaFr doped with hydrogen at 77 K produces hydrogen atoms in fluorine sites (Hy centers) and trapped electron centers [ 121.The X-ray irradiation of CaF,, SrF, and BaFr incorporating hydrogen and deuterium produces interstitial and substitutional hydrogen and deuterium atoms. The dependence of the hyperfme constants of nuclei in several shells on the lattice constants of alkaline earth fluorides has been discussed [13, 141. The impurity hydrogen acts also as the governing factor in the formation of the V- center in MgO under exposure to gamma- and electronirradiation. V- centers are produced by the displacement of hydrogen from the V,,_ sites by ionizing radiation [ 151.
3. OH- IONS U center formation in neutron-irradiated LiF : OH has been confirmed by ESR and IR absorption measurements (161. X-Ray irradiation at room temperature of OH- in LiF dissociates OH-, and produces U, centers. Annealing at 35O’C restores the initial state of the crystal. In such annealing, the direct conversions of U to OH-, and of Ho to OH-, and also the conversion of Ho to U, are suggested [ 171. Morato and Luty performed a systematic investigation of the UV-light-induced photodissociation of substitutional OH- defects in KC1 [18]. H; centers are produced by the reaction of mobile Cl, crowdions with H- defects. The H; defect is composed of the same electronic and ionic ingredients as the LJ, center. The formation of hydrogen atoms occupying interstitial tetrahedral sites was confirmed by Raman scattering measurement on alkali halides containing OH- under UV-light excitation. The heavier halogen ion impurities are effective in trapping the interstitial hydrogen next to them. The heavier halogen ion and interstitial hydrogen form a singly-perturbed II, center of C,, symmetry and a doubly-perturbed Uz center of C,, symmetry [19]. The formation of U2 (HP) centers was also observed in BaClF and SrClF crystals (201.
IR absorption measurements suggested the formation of a hydrogen bond between the OH- ion and the radiation-induced defect in NaF under gammaray irradiation (2 I]. TO stabilize the laser-active F; center, special attention has been focused on the defect-stabilized F; center in alkali halide crystals. Such laser-active F; centers can be achieved with OH- doping and heavy radiation damage [22-241. A U.S.S.R. group [24] found fast and slow steps in the formation process of O*- : F2+centers. The former step involves diffusion of thermally unstable F: centers, and the latter one involves the diffusion of single O*- : V: dipoles. O*- ions are produced from OH- ions by radiation such as X-rays.
4.
F-IONS
X-Ray irradiation of a mixed KCI-KF crystal at liquid-nitrogen temperature (LNT) produces molecular heterohalogen FCI- centers oriented along the (111) directions of the crystal [25]. The FCI- center is a hole-type center like the X; centers in nominally pure crystals. The electron structure (251 and the fundamental internal vibration [26] of this center have been reported. (11 I)-oriented FCI-, FBr- and FI- centers occupying one negative ion vacancy are also created in KCI-KF, KCl-KF-KBr and KCI-KF-Kl under X- or gamma-ray irradiation at 77 K [27] and exhibit absorption bands at 300, 294 and 275 nm, respectiveiy. FCI-, FBr- and FIcenters become unstable above about 120, 170 and 270 K, but the actual temperatures depend on the host crystals involving these centers. No investigation of the role of F- ions on F center formation has been found.
5. Cl- IONS The formation of the molecular heterohalogen BrCI- centers aligned along (1 IO) directions in KCI doped with a small amount of KBr has been confirmed by optical and EPR studies after X- or gamma-ray irradiation and proper thermal treatment [28,29]. Strong and much weaker optical absorption bands are observed at 382 and 760nm with sigmapolarized transitions. Similar molecular heterohalogen FCI- centers are also observed in single KF crystals doped with PbCl, and KCI. The difference in the optical and ESR parameters betweeen FCl- in the (111) directions in KC1 doped with KF and FCI- in the (110) direction in KF doped with KC1 have been discussed qualitatively [30]. Irradiation with 6.5-7.5 eV photons creates anion Frenkel defects (F-H center pairs and a-I center pairs) in KBr doped with KC1 at 4.2 K [31]. It is suggested that the self-localization of opticallyformed excitons creates such defects. X-Ray irradiation of the crystal at 4.2 K also showed the decay of
Formation of color centers in anion-doped crystals
self-trapped excitons with the creation of F-H and z-1 center pairs on regular lattice sites [32]. The formation of C!; centers in K20-BrO,-KC! and NarO-B,Or-NaCI glass is an interesting result. The centers in the glass are more stable than those in alkali halide crystals [33]. 6. Br- IONS Absorption bands due to Br- ions in KC! appear on the low energy side of the fundamental absorption bands in the UV spectra! region [34]. These absorption bands originate from the creation of excitons at Br- ions. Extensive investigation on the behavior of the bands has been reported by Tomiki [35]. The molecular heterohalogen BrCl- center introduced before is thermally converted to another center, which consists of BrCI- and a positive ion vacancy in a nearest-neighbor position to both nuclei of the molecule ion [36]. The new center [BrCI-(v,)] exhibits a strong absorption band at 368 nm to the sigma-polarized transition and also a weak band at 910nm. The internuclear axis of the BrCl- ion in this center lies in a (100) plane and is inclined at an angle of 12.5” to the (110) direction. The new centers are converted into Cl; (v, ) centers by 405 nm light excitation at 77 K [37]. Further complex centers such as the BrCli- center in KC1 containing Na and Br [38], or the BrCI- center in an anion site with nearest-neighbor Sr’+ and an associated positive ion vacancy, have been confirmed in KCI containing Br- and Sr*+ or Ba?+ ions [39] under X-ray irradiation. Ionizing irradiation and suitable thermal and optical treatment can create BrCI- centers occupying a single negative ion site next to a substitutional Li+ ion in KC! strongly doped with Li+ and Br- ions [40]. The internuclear axis of the BrCI- molecule ion lies in a (100) plane and makes an angle of 25” to one of the (100) directions. ( 1IO) oriented H,, (Na+) and H, (Na+)type BrCIi- centers in KC1 doped with both Na+ and Br- ions have also been reported [41]. Still and Pooley were the first to observe the effect of heavy ions on F-center formation in KCI-KBr and KCI-RbCI [42]. Introduction of heavy ions such as Br- or Rb+ reduces the formation yield of the F centers. The reduction of the yield was expected in such mixed crystals because of the disruption of replacement collision sequences. On the other hand, the formation of H centers in KCI containing CIBrwas the first report to confirm color-center formation through self-trapped excitons (STE) [43]. KC1 containing Tl’ was irradiated by 2 MeV electrons at LNT to form Cl;, CIBr- (due to a small amount of substitutional Br- impurity), TlO, T12+, F and z centers. ESR absorption confirmed the existence of ClBr-. Then, by releasing electrons from Tl”, and by trapping them at CIBr-, the formation of H centers was confirmed by ESR. It was also confirmed that Br- ions in KC! act effectively on the F-center
739
formation (441. KC! crystals containing various concentrations of substitutional Br- ions were excited by photons between 7.0 and 8.2eV. The F-center formation yield became a maximum in the spectral region where the Br- absorption band peaks, but not in the fundamental exciton absorption region of KC!. A two-photon absorption process of N, laser light also creates F and H centers in KC! containing Br- or I- ions [45]. The absorption and emission spectra due to F centers and the absorption spectra due to H centers thus produced in the crystal containing Br- or I- ions are all similar to those in undoped crystals. This result suggests the formation of the regular type of F and H centers even if Br- or I- ions are selectively excited. However, the molecular heterohalogen CIBr- seems to play an important role in color center formation. Similar results in KC1 containing Br- or I- ions are also reported by U.S.S.R. groups [46]. They observed the creation of a-1 center pairs as well as F-H center pairs. A thermoluminescence study discusses the formation and thermal stability of defects created in KC1 : Br under gamma irradiation [47]. 7. I- IONS Mahr [48] was the first to report the absorption and emission characteristics of I- ions in KC!. The existence of the molecular heterohalogen ICl- has been confirmed by ESR absorption [49]. UV-light irradiation produces the heterohalogen self-trapped excitons, IC12-*, in KC1 [50] and RbC! [51] containing I ions. An absorption band due to relaxed IC12-* excitons occurs at 1.65 eV at 80 K after irradiation of KC1 : I by a pulsed electron beam [52]. The characteristics of the lowest relaxed triplet state of these excitons have been investigated by observing the temperature dependence of the luminescence from this state [53,54]. The luminescence study on KC1 : I and RbCl : I under UV-light irradiation suggests coexistence of two different types of relaxed exciton states. The first is an IC12-* state of the two-center type, i.e. ICI*-* occupying two anion vacancy sites. The second is a one-center type, i.e. IC12-* occupying three anion vacancy sites, where the central iodide ion traps an electron-hole pair [55]. IBr2-* exciton has been identified in NaBr [56]. Such heterohalogen excitons are produced by a weakly temperature dependent process of conversion of intrinsic relaxed excitons localized near impurities. IOHmolecule ions could also be produced in KC1 : I under X-, gamma- or UV-irradiation at 80 K [57]. Introduction of I- ions as the heavy ions into KBr reduces the production rate of F centers [58]. I- ions in KBr seem to disrupt the replacement collision sequences for the F-H formation as do Br- ions in KC! [42]. A new type of H center consisting of Br, I’is expected to be produced in this mixed crystal. A similar heterohalogen H center (C1313-) occupying
MASAMITSUHIRAI
740
+
t
-I-
-
+
-
-
+
Cl-
+
+
ICI-
-l
-
cl-
+
-
+
+
-
+
-
-8
-
6
Fig. 2. A schematic model of the heterohalogen
H center.
three anion sites in one of the (110) directions is also observed in KCI : I under X-ray irradiation at LHeT as shown in Fig. 2 [59]. The ICI- ion in the center occupies the central anion site. The formation of F and H centers has been investigated by exciting selectively I- ions in KC1 and KBr under irradiation by an ArF-excimer laser (601. It is interesting to note that the absorption spectra due to F and H centers thus produced are almost the same as those in pure crystals. The creation of regular F and H centers in pairs from I--localized excitons was concluded. The H centers in this case are not associated with I- ions. A selective photoexcitation of localized excitons perturbed by I- ions in KC1 : I creates effectively F-H center pairs at 4.2 K [61]. As a formation mechanism for F-H center pairs from such heterohalogen excitons, the translational shift of the near-impuritylocalized homohalide excitons has been discussed. For a clear understanding of the F-center formation mechanism in crystals doped with heavy halogen ions, further systematic investigations are desired. Another type of color center formation in doped KC1 with I- ions is the formation of V-type centers [62]. The center exhibits an absorption band at 247 nm after X-ray irradiation of the crystal. The
5 4 3 2 PHOTON ENERGY CeV)
Fig. 3. Absorption spectra of KCI containing @- ions at 78 K. Solid curve: before UV-light illumination; broken curve: after UV-light illumination [68].
center consists of I,Clof the host lattice.
lying in the (1 IO) directions
8. OPTICAL PROPERTIES
OF CHALCOGEN
2.58 3.93 5.08
2.80 3.68 4.92
3.37 4.46 5.35
2.98 3.58 4.49
2.94 3.48 4.29
3.05 4.12 4.89
3.05 3.32
3.14 3.34
2.87 3.12
2.81 3.06
2.87 3.07
4.19 4.46
4.26 4.51
4.06 4.36
3.79 4.08
3.91 4.15
2.85 4.40 5.85
2.82 4.10 5.61
S?- + F-
3.15 3.85 4.70
3.21 3.86 5.70
Se?- + F-
3.14 3.41
Te?- + F-
IONS
8.1. Optical absorption spectra Chalcogen ions can be introduced into alkali halide crystals, and form anion-vacancy pairs to maintain the electric neutrality of the crystals. The pairs exhibit two or three absorption bands in the UV and visible spectral regions. Illumination of the crystal with light absorbed by the band in the UV region leads to the formation of F centers in alkali halide crystals, and other photochemical reactions in other crystals. The solid curve in Fig. 3 illustrates the typical absorption spectrum due to O’- + anion vacancy pairs (O*- + F+) occupying two nearest anion sites in one of the (110) directions in KC1 at 78 K [63]. F+ stands for the anion vacancy. Hennl [64] has summarized the data on the position of the absorption maxima in K- and Rb-halide crystals as listed in Table 1. Chalcogen-vacancy centers (Ch2- + F+) have two, three or four absorption bands in the
Table I. The peak position of the absorption bands at 78 K (in electron volts) 1641 RbI RbBr KBr KI RbCl KC1 O?- + F-
1
2.88 3.45
2.67 3.24
4.16 4.78
3.86 4.45
Formation of color centers in anion-doped crystals
741
factor u is a function of the center and of the host-lattice anion, and it decreases from iodide to chloride, as listed in Table 2. Gol’denberg et al. [65] attributed two absorption bands at 228 nm (5.43 eV) and 290 nm (4.27 eV) to (O*- + F+) in NaCl from their intensity, thermal stability and reaction to light. A band at 265 nm (4.68 eV) is due to a complex of these dipoles. 8.2. Luminescence 2.5
3.0
3.5
4.0
PHOTON ENERGY (eV)
Fig. 4. Double-logarithmic plot of the anionxation distance vs the photon energy of the violet absorption band of the chalcogen-vacancy centers at 78 K. Straight lines connect points of the same pair of lattice anion and center. (a): 02-, (0): S*-, (+): Se’-, ( x ): Te*--F+ centers [64]. Table 2. Constant a of the Mollwo-Ivev relation 1641 02- + F-
S2- + FSe2-+FTti-+F-
Chlorides 5.03 5.25 5.19 -
Bromides 5.48 5.38 -
Iodides 5.92 5.75 5.58
near-UV spectral region. The maxima of the lowest energy absorption band follow the modified Mollow-Ivey relation,
E Mix= &.08 2 0.10,
(1)
with a factor u characteristic of each (Ch*- + F+). In Fig. 4 the photon energies of the lowest energy absorption band are plotted as a function of the anion-cation distance in a double-logarithmic scale as well as those for S*- in NaCl and NaBr 1641.The
bank
In Table 3 [64] are listed the peak positions and full-widths at half-maximum (FWHM) of the luminescence bands due to (Ch*- + F+) in K-halide crystals at 78 K (O*- at 20 K). (O*- + F+) exhibits only two luminescence bands. (S*- + F+) exhibits two bands around 3 and 2 eV. Three bands in the visible region due to (Se*- + F+) and (Te*- + F+) consist of two bands due to spin-orbit coupling and an independent band. In the near-IR, three or four luminescence bands between 0.4 and 0.9 eV with FWHM of 0.09 eV or smaller are observed for all (S’- + F+), (Se*- + F+) and (Te*- + F+). Hennl discusses the characteristics of the luminescence as follows. By excitation in the lowest energy absorption band, (S*- + F+), (Se*- + F+) and (Te*- + F+) return to the ground state with radiation, but (O*- + F+) without radiation. Excitation in the second absorption band results in the radiative transition in (O*- + F+), but the non-radiative transition in all other cases. Three IR luminescence bands of (S*- + F+), (Se*- + F+) and (Te?- + F+) are excited by illumination in any of the absorption bands. Hennl assumes a luminescence cascade for the IR bands as a first
Table 3. The peak position (E) and FWHM (H) of emission bands in electron volts at 78 K (02- at 20 K) [64] KC1
KBr
E
H
E
O*- + F-
2.67 1.18
0.33 0.25
2.36 0.89
S2- + F-
3.48 2.07 0.899 0.585 0.435
0.21 0.205 0.058 0.058 0.036
Se’- + F-
2.82 2.15 2.04 0.895 0.579 0.433
0.254 0.39 0.24 0.063 0.047 0.035
Te2- + F-
KI
H
E
H
2.99 2.08 0.902 0.577 0.432
0.26 0.27 0.065 0.062 0.049
3.02 2.09 1.475 0.713 0.407
0.30 0.21 0.351 0.149
2.83 2.20 1.93 0.896 0.575 0.43 1
0.18 0.15 0.21 0.062 0.056 0.043
2.88 2.22 2.02 0.873 0.573 0.430
0.19 0.26 0.303 0.088 0.063 0.043
3.16 2.07 1.75 0.892 0.785 0.569 0.429
0.18 0.208 0.317 0.076 0.091 0.056
MASAMITWHIRAI
742
possibility. His second interpretation for the IR bands is the formation of a weak homopolar bond of Ch- with one of the six neighboring alkali ions. The Ch- can be formed by ionization of Ch2- by light illumination. According to his discussion, depending on the relative position of the nearby anion vacancy (F+) and the weak homopolar bond, three different potential energy curves are formed in the excited state to result in three bands.
2 B
10
$ 5 $ Y 8
9. O*- IONS
/ 0
I
I
0
NUMBER OF ABSORBED
9.1. KC1 Sensitization for the coloration of alkali halide crystals doped with oxygen was first discovered by Hilsch and Pohl[66]. Korth [67] was the first to report the following process, (02- + F+)+(O-
+ F),
(2)
in KCI. (Ch- + F) stands for the Ch- + F center pair occupying two nearest anion sites in one of the (I 10) directions in the alkali halide crystals. Since then, the Gijttingen group have worked extensively on this subject. Figure 3 presents a typical absorption change in 02-doped KC1 after annealing (solid curve) and after irradiation (broken curve) with UV-light at room temperature (RT) [68]. Three bands at 2.92, 4.25 and 5.85 eV due to (O*- + F+) are observed in this case. Irradiation with UV-light decreases the concentration of O*- centers, while that of the F centers increases correspondingly, as shown in Fig. 5. From these results, Fischer and Gummer [68] suggested the photochemical reaction given by eqn (2) as the only reaction occurring in this case. The reaction depends on the photon energy by which the crystal is excited. Only irradiation into the high energy bands yields the decomposition of (02- + F+) with a quantum yield of 0.36 in KC1 above _ 220 K. No reaction occurs by irradiation into the lowest energy band [68]. By UV-light irradiation of a crystal doped with Ch2-, a new absorption band appears in the spectral region of the conventional F band. However, this band has a lower energy peak position and wider FWHM than those of the conventional F band as listed in Table 4 [69]. Hennl [69] attributed the new absorption band to the optical absorption of an F center which is perturbed by a Ch- anion in the nearest-neighbor position.
._
I
2 .lO’V
1
PHOTONS
Fig. 5. Decrease of 02- ion concentration (broken curve) and complementary increase of F center concentration (solid curve) in KC1 at 250 K as a function of absorbed photon number [68]. The peak position and FWHM of the new band due to (O- + F) in KC1 could be interpreted by a calculation on the variation of the energy parameters for F centers in the field of the associated impurity oxygen ion. This calculation suggested the validity of the formation of (O- + F) centers in the photochemical coloration of crystals containing oxygen impurities [70]. Staibe and Scott [71] reported F center formation from (02- + F’) in KCI under UV-light irradiation between 154 and 210 K. They studied further the back reaction, F-+a + e,
(3)
e + O--*02-,
(4)
to find the activation
energy for r center migration.
9.2. KBr and KI Hennl [69] investigated the effect of UV-light irradiation at liquid helium temperature (LHeT) on KBr and KI doped with 02-, S?-, Se?- or Te*- impurities, and found new absorption bands in the spectral region of the conventional F band in each crystal as introduced before. and as listed in Table 4. The new band was named as the F,(Ch) band in analogy to the F, band. Irradiation into the F,(Ch) absorption band at LHeT destroys the F,(Ch) centers. F,(Ch) centers are unstable at LNT, and are converted back into (Ch2- + F+).
Table 4. The peak position of the F band (E), its FWHM (H) and deviation of peak energy from the normal F band peak (AE) in KBr and KI crystals [69] KI
KBr L(O) F”(S) F, (Se) Fu (Te)
E(eV) 2.064 1.987
H(eV) 0.285 0.158
P(%)
AE(eV)
-7
0.077 -
2.032 2.015
0.440 0.415
8 8
0.032 1.049
E(eV) 1.875
H(eV) 0.155
P(%)
1.839 1.828 1.797
0.253 0.245 -
18 8 4
-
AE(eV) 0.036 0.047 0.078
Formation of color centers in anion-doped crystals
9.3. NaCl formation efficiency in NaCl containing (O*- + F+) is much higher than that in pure NaCl under X-ray irradiation [72]. Didyk and Tsal [72] proposed the following process for the enhancement of F-center formation efficiency, F-center
(O*- + F+)+O-
+ e + F+ -+O- + F center,
(5)
0; + e + F+ -+O, + F center,
(6)
and kT
20- -
where, hv and kT are an X-ray quantum and thermal energy, respectively. However, no discussion on the variation due to the O*- centers was given. Light irradiation into the 228 nm (5.43 eV) band in NaCl at 633 K destroys the 228 nm band and yields F centers [65]. Gol’denberg et al. proposed the idea of using the crystal as an image recording material. According to their report, the image in the crystal was preserved for an indefinite time at RT and could be erased in 10 min by heating the crystal to 773 K. They [73] also suggest the recording of 3D holograms in NaCl with dipole oxygen centers.
743
into the _ 5.5 eV band. This band grows even at LNT, but with a formation rate of half of that at RT. An interesting result is the wide FWHM (0.49 eV) of the new band which grows as the crystal is irradiated at LNT. The FWHM becomes equal to that of the conventional F band when the crystal temperature is raised to RT. By cooling the crystal to LNT again, the FWHM remains narrow as the conventional one does. This wide FWHM is attributed to the absorption band due to the F centers perturbed by nearby S- ions at LNT, as introduced by other investigators. At RT, the thermal energy decomposes (S- + F) into individual S- and F centers and results in the narrow FWHM of the band. NaCl does not show such a wide FWHM for the F band even at LNT, suggesting that the complex center (S- + F) does not form. 10.2. KC1 and KBr Fischer and Grundig [77] were the first to observe the absorption bands due to S2- ions at 3.15, 3.85 and 4.7eV, and discussed F-center formation by a photochemical reaction as follows, U-F
+ (1/2)H,,
(9)
and (1/2)H2 + S2- + F+-rSH-
10. S*- IONS
10.1. NaCl and NaBr Baba and coworkers [74,75] have extensively studied F-center formation from (S*- + F+) in NaCl. NaCl doped with S*- shows two absorption bands at 4.15 and _ 6 eV when the crystal is quenched from 873 K to RT. Irradiation of the crystal with UV-light absorbed into the z 6eV band at RT results in F-center formation. The formation rate as a function of the exciting photon energy at RT becomes a maximum near 5.3 eV. They suggest the photochemical reaction, S?-z
S2-*_&-+e,
(7)
and e + F++F,
(8)
where S*-* stands for the excited S2- ion. No formation of F centers is observed at LNT. Irradiation into the 4.15 eV band at RT and LNT does not produce any F centers either. The 4.1 eV emission intensity due to (St- + F+) in NaCl starts to decrease above _ 150 K, while the F-center formation efficiency increases correspondingly in this temperature range. They assume two adiabatic potential curves for the excited (S2- + F+) center, the one leading to the 4.10eV emission and the other leading to F-center formation. NaBr [76] also exhibits two absorption bands at 4.15 and around 5.5 eV. A new band around the conventional F band is formed by irradiation only
+ F.
(10)
Slightly different peak positions from those reported by Hennl are observed at 3.60 and 4.80 eV for KCI, and at 3.55 and 4.70 eV for KBr containing S2- impurity [78]. A luminescence band around 3.7 eV in KCI or around 3.3 eV in KBr is observed by irradiation into the high energy absorption band in each crystal at LNT. No growth of the (S- + F) absorption is observed in either KC1 or KBr under irradiation into the high energy bands at LNT. Irradiation at RT, however, produced (S- + F) as in other crystals. Mito and Hirai [79] investigated the temperature dependence of the conversion efficiency of (S?- + F+) to F centers. No F centers were produced by irradiation with UV-light near LNT, but three absorption bands at _ 1.9, _ 2.1 and - 2.3 eV (X bands) grew by irradiation between 220 and 260 K. The conventional F band was formed at temperatures between 273 and 330 K. As shown by the curves from 2 to 6 in Fig. 6, the X bands could be converted to the F band by raising the crystal temperature from 220 to 305 K with almost complete conversion from X centers to F centers. They suggest the following explanation. At a relatively low temperature such as 220 K, an electron from S2- is transferred to the anion vacancy (F+) to form an X center consisting of an F center perturbed by an S- ion, i.e. (S- + F). Above + 260 K, the S- ion migrates away from the F center to leave an isolated F center. The destructive energy for (S- + F), possibly for the migration of S, was estimated to be 550meV. The F center is not mobile at temperatures up to around 370 K.
744
MASAMITSU HIRAI /-
X
0.22 .
2 _ .r:. :-3.“.,.;.....~~~‘ ,,: p
z v, z
0.20
x 2 r 8
0.16 .
0.16
O.lL
.
F ‘. _.’ ;; .::; ...‘;’ .F,,.T’ -. ..” ,. .: .-: 5:” : 1 (..’ .? ,;;. ;,;: ~.&_‘
11. Se*-, Te*- AND PO*- IONS
F
-*
i’--.,
.:.,
,.: ,.:_I. *_a.:. . .. ....1.‘..”
%!_*b -, .‘-Zy:_ ..,,.,;_ .“:(:,~~~/~’ 2, -.. 3 2.! ..,, I.._,_... ...p:y.~. ._. _.,**.. _,” ,._’ ,.u._-” c, .. . . .s_ , _._... ii-2. 22OK :.. :’ ,;._::c.. :j“ *; ,g I ,_. 3: 250K ,z.. / . .. ,.;-i ,y.-.
,&;;;*:<*<.. $3? .-
1.4
;-..
: ;.y_ ,,.. :. ... ‘::: _, ,_ ..:.
1.6
.*.
1.6 2.0 PHOTON
4 : 270 K s: 280K 6 : 305 K
2.2 2.L 2.6 ENERGY (ev)
2.6
3.0
Fig. 6. Thermal conversion of X centers to F centers by raising the temperature of KC1 : S’- from 220 to 305 K [79].
In their sample, two luminescence bands were observed at 3.76 and 2.57 eV. These bands occurred at different positions from those reported by Henne (3.48 and 2.07 eV) [64]. Since their S2- ions [79] were introduced into KC1 by adding Na,S, there is a possibility that the Na+ ion perturbed the (S2- + F+) centers sufficiently to shift the luminescence peaks. The intensity of the 3.76 eV band starts to decrease above _ 200 K as the 4.1 eV luminescence band in NaCI. However, no correlation between the temperature dependence of the luminescence intensity and the F center formation efficiency was observed. This result suggests different excited states for the initial state for the luminescence and for F center formation. Irradiation into the low energy band of KBr under an electric field at LNT aligned the dipoles of (S’- + F+) along the electric field, but no conversion to F centers was observed [SO]. 10.3. KI S’- centers in KI exhibit absorption bands at 3.37, 4.46 and 5.35 eV [81]. The 3.37 eV band occurs due to isolated (S2- + F+) dipoles, whereas the 4.46 and 5.35 eV bands are partly due to dipole aggregates. Irradiation into the 3.37 eV band at temperatures between RT and LNT results in no change in the absorption spectra. However, irradiation into the 4.46 and 5.35 eV bands at 240 K produced F centers, and lowered the height of the 3.37 eV band. No reaction was observed in the temperature range between _ 25 K and LNT. The formation of F centers in this case suggests the photochemical reaction.
(S2-+ F+) hv
S- + F.
(11)
The photochemical reaction at LHeT in the high energy absorption bands produces (S- + F). Alignment of the (S2- + F-) dipole under an electric field and irradiation into the lowest absorption band at 3.37eV were also observed in KI.
A few investigations of the optical absorption spectra and F-center formation in alkali halide crystals doped with Se2-, Te2- and Po2- ions have been reported. Fischer [82] observed absorption bands due to (Se2- + F-) in KC1 and KBr to occur in the near-UV spectral region as listed in Table 1. Absorption bands in KI, RbCl, RbBr and RbI were studied extensively by Hennl [64]. These results are also listed in Table 1. In general (Se2- + F-) exhibits four bands, except in KCI. (Se2- + F+) in KCI, KBr and KI exhibits three emission bands in the visible region and an additional three bands in the near-IR region, as listed in Table 3. Irradiation of KBr and KI doped with Se2- by the intense line of a high pressure mercury lamp at LHeT led to the formation of an absorption band near the conventional F band. However, the peak position and FWHM of the new band differed slightly from those of the conventional F band as well as the band in KBr and KI doped with 02-, S2- or Te2-. Hennl [69] suggested that F centers perturbed by a nearby Ch- ion, i.e. (Ch- + F), are the reason for the different peak position and FWHM. (Te’- + F+) has been investigated only for KI and RbI [64]. The absorption bands in these crystals consist of four bands as listed in Table 1. In Table 3 are listed the peak positions of the luminescence bands due to (Te2- + F+) in KI. Seven bands were found (641. (Te2- + F+) is also converted to (Te- + F), similar to (Se2- + F+). No report has been found on (Po2- + F-) in any crystals.
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