Nuclear Instruments and Methods in Physics Research 31 (1984) 219-222 North-Holland, Amsterdam

219

CHARGE TRANSFER MECHANISMS IN THE THERMOLUMINESCENCE OF MgO

Luis DELGADO

Division Fusi& TermonucIear, Junta Energiu Nuclear, Madmd 3, Spain

The opt&l and thermal stability of the optical absorption spectrum and the~o~u~n~nce (TL) of as-cleaved MgO samples subjected to ionizing radiation at room temperature has been studied. UV irradiation leads to V-center formation when tight is absorbed in the Fe’+ absorption bands. Under subsequent illumination in the V-band, Fe3+ ions are again formed. X or y irradiation increase the Fe3+ band (4.3 ev) to a saturation value and produce the V band. The same three glow peaks at - 93,175 and 220°C are observed in X, y or UV irradiated samples. Two maxima at - 4.3 and 5.1 eV are found in the TL excitation spectrum. It is suggested

that, as expected, cation vacancies charge compensate for the Fe3+ . rons. Charge and excitation transfer between both hole and

electron traps and the impurity activators Fe, Cr, Mn and Ni determine the observed TL emission.

1. Introduction

A great deal of work on the optical (absorption and luminescence) and magnetic properties of radiation in- duced defects in MgO has been carried out [1,2]. Under ionizing radiation, no stable lattice defects are formed and only V-type center formation (holes trapped at oxygen ions adjacent to ever-present cation vacancies) and impurity valence changes have been detected by using electron spin resonance measurements (ESR), luminescence (emission and excitation) and optical ab- sorption measurements [l-3]. On heating, the radiation induced trapped charges are thermally released from their traps, and thermoluminescent electron-hole re- combinations mainly at the impurities, give rise to the observed glow peaks. The identification of the trapping and recombination centers is still a matter of con- troversy [l-12].

This work presents a study on charge transfer and thermoluminescent processes involving V-type centers and impurity ions in commercial grade MgO subjected to ionizing radiation at room temperature as a first step to a characterization of radiation damage in MgO. An assignment of the glow peaks to the thermal release of both electrons and holes and their recombination at the activator impurity ions Fe, Cr, Mn and Ni is made.

2. Experimental

MgO specimens of about 10 X 10 X 1 mm3 were cleaved from a 4 N single crystal block (W.C. Spicer Ltd). Preliminary chemical analysis of these samples show lo-20 ppm Fe. As cleaved samples were irradia- ted at room temperature with: y-rays from a 6oCo source (1 X lo6 R h-l), X-rays through an ahuninium filter 1 mm thick with a Siemens tungsten anode tube

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

operated at 40 kV, 20 mA (2 X lo3 R min-‘) and a Bausch and Lomb (BL) 120 W deuterium lamp for the ultra-violet (UV) irradiations.

The experimental rig used for the TL emission spec- tra measurements has been described elsewhere 1131. A BL scanning mon~hromator operated at a speed of - 20 nm s-r was used to obtain the wavelength spec- trum of the emitted light while the sample was heated at 10°C min-*. Depending on the monochromator slits, the resolution varies between 3 and 19 nm. A Cary 17 spectrophotometer was used for the optical absorption measurements. A pulsed annealing method was used to obtain the thermal stability of the main Fe3” (4.3 eV) and V (2.3 eV) absorption bands. After heating up to each annealing temperature, the absorption spectra were all recorded at room temperature. The optical bleaching experiments were performed with either a deuterium or a tungsten lamp and a BL mon~hromator (high inten- sity model), or appropriate Coming filters.

3. Results and discussion

As-cleaved samples show two absorption bands at 4.3 and 5.7 eV due to Fe 3+ charge transfer transitions

[141. The effect of X irradiation on the optical absorption

spectrum is presented in fig. 1. As shown the 4.3 eV band (Fe3+) increases to a saturation value while the well known 2.3 eV absorption band due to different ~nfigurations of intrinsic (V-) and impurity related (VA,. V,, Vou, * - * ) cation vacancies which have trapped a hole, is formed. The evolution with dose of these two main bands at 4.3 and 2.3 eV is presented in the inset of fig. 1. As many as 1.2 x 10” cmm3 V-type centers could be roughly estimated for a dose of 1.2 X 10’ R by using as average values of the different V-type centers, an

I. CRYSTALLINE OXIDES

L. Delgado / Charge transfer mechanisms

Wowlength Inm I 0

3- LZx105R

I b 3 2 1

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Fig. 1. Variation induced in the optical absorption spectrum of as-cleaved MgO crystals by X-irradiation. The mset shows the evolution with dose of the 4.3 and 2.3 eV absorption bands.

oscillator strengthf- 0.2 and a halfwidth W1,2 - 1.9 eV 10” cmm3 in u&radiated samples to - 5.4 x 10” cmv3

[1,3] in Smakula’s formula. With Wi,2 - 0.56 eV and for a dose of 1.2 X 10’ R. In addition, a broad absorp-

f- 0.1 [17,18] the Fe3+ concentration as determined by tion region between - 2.9 and 4 eV is observed to grow

the 4.3 eV absorption band is enhanced from - 4.7 X with dose. A qualitatively similar evolution with dose of

Wavelength fnml

isu 2m

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Fig. 2. Variation induced by illumination (30 min) with UV light in the optical absorption spectrum of as cleaved MgO (curve 1). Curve 2 shows the difference optical absorption spectrum between a sample illuminated for 15 min with a tungsten lamp through a green Coming filter CS 4-64 and a sample illuminated (30 min) with UV light.

L.. Delgado / Charge transfer mechanisms 221

the optical absorption spectrum was found in y-irradia- ted samples up to a dose of 5 x lo6 R.

Fig. 2 presents the effect of optical bleaching (UV and visible) on the absorption spectrum of as cleaved MgO samples. Under UV irradiation the NIR and VIS regions (- 1 to 4 eV) increase, with detectable V-band formation at 2.3 eV, while the UV region (- 4 to 6 eV) decreases showing two minima at 4.3 and 5.7 eV (Fe3+ bands). If this sample is then illuminated with visible light from a tungsten lamp through a green Coming filter CS4-64 the reverse effect occurs and the UV region increases with maxima at 4.3 and 5.7 eV while the VIS-NIR region decreases and the V-band previ- ously formed disappears. Therefore, it seems that a reversible hole transfer process between Fe’+ ions and cation vacancies can be optically induced. V- centers could then be formed by trapping at the cation vacan- cies the holes optically released from Fe3+ ions. This supports previous suggestions about the charge com- pensation of Fe3+ . tons by cation vacancies [2] and on the formation of so-called V,, centers by UV irradiation detected with ESR measurements [9-111.

A quantitative estimation of these V,, centers in- volved in this charge transfer process is difficult be- cause, as far as we know, the optical parameter (transi- tion energy, f and Wi,2) of the V, centers are not well determined. However a rough estimation of the 4.3 and 2.3 eV absorbing centers involved, yields that by far more Fe3+ ions than 2.3 eV (V-type centers) are implied in these processes. This could indicate that other trapped

hole centers, probably absorbing in the broad visible region between - 2.9 and 3.8 eV (see fig. 2) are also involved in this optically induced hole transfer process. On the other hand analysis of ESR spectra of Fe3+ shows octahedral symmetry which implies that most of the cation vacancies that charge compensate Fe3+ ions are not adjacent to Fe3+.

Three glow peaks at - 93, 175 and 22O’C are ob- served in the TL spectrum of X or y irradiated samples (fig. 3a). In this figure the pulsed thermal annealing curves of the 4.3 and 2.3 eV are also showed for comparison purposes.

The TL emission spectrum is presented in fig. 3b. It consists of four emission bands at - 2.95 (420 nm), 2.53 (490 nm), 2.08 (590 nm) and 1.7 eV (730 nm). The red 1.7 eV light intensity is - 100 times higher than the other three emission bands. At the temperature of the main glow peak (93’C) the 1.7 eV emission band shows the well known characteristic structure of Cr3+ super- posed on a broad emission band [5]. From 25’C until - lOO“C, the V-band decreases by thermal release of holes. This may give the necessary holes to recover the Cr3+ ions, previously reduced to Cr*+ by the radiation, yielding the characteristic Cr3+ light emission observed [l]. An annealing step in the Fe3+ (4.3 eV) absorption band is also observed from 60°C up to - 110°C. It is

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Fig. 3(a) Glow curve and pulsed annealing curves of the 4.3 and 2.3 eV absorption bands corresponding to a sample X- irradiated to a dose of 1.2 X lo5 R. (b) Emission spectra of a

sample irradiated to 1.2X10’ R. The initial and final sample temperatures while the light is scanned from 350 to 750 nm are indicated. (c) TL excitation spectrum of UV irradiated samples.

not clear that this Fe3+ annealing step is due to thermal release of holes from Fe3+ ions, since, then, one could expect only a thermal annealing step for the radiation induced Fe3+ band. However, a second smaller Fe3+ band annealing stage is detected at - 17O’C and the Fe3+ band does not recover its preirradiated value till h&her temperatures (4OO’C). An alternative mechanism tar the two observed Fe3+ annealing stages would be electron trapping at Fe3+ ions of electrons released from different electron traps. The consequent Fe*+ for- mation might yield the red broad underlying band [7]. A possible electron trapped center could be Cr*+. In fact, electron release from Cr*+ might yield the characteristic Cr3+ light emission. Luminescence bands at - 2.53 eV have been observed in radioluminescence (RL), photo- luminescence (PL) and TL measurements of MgO : Ni crystals [1,8,19]. This emission band has been identified as the ’ T2s -+ 3A *s transition in substitutional Ni*+ ions [1,19]. Similarly, an orange emission band at - 2.08 eV

I. CRYSTALLINE OXIDES

222 L. Delgado / Charge transfer mechanisms

has been detected in the fluorescence spectra (RL and TL) of MgO: Mn samples. It has been assigned to Mn2+ ions [1,7,8].

The origin of the blue emission at - 2.95 eV is more uncertain and several tentative assignments have been made. It has been assigned to the reduction of Cr3+ to Cr2+ by thermally released electrons from Fe’+ [4]. Also, radiative recombinations of thermahy released holes with Fe2+ ions, leading to highly excited states of Fe3+ have been suggested as the blue TL emission source [1,2]. These highly excited states could only be reached by X or y irradiation, but not by UV excitation [ll]. Finally, radiative recombinations in the debris, implying vacancy clusters or even F centers have been proposed as the origin of the strong blue TL observed in deformed samples [6].

The same three glow peaks (- 83, 175, 220°C) are observed in samples excited with UV light. The emitted light intensity is about 100 times smaller than in X or y irradiated samples. Due to the weak TL output, only the composed red emission band at - 1.7 eV could be resolved in the wavelength emission spectrum. However, a very weak blue glow curve, with the same glow peaks as the red emission could be detected through a Coming blue filter CS 5-58 transmitting less than 0.01% at 750 nm. This may indicate that UV irradiation can be energetically efficient enough to reach the assumed highly impurity excited states responsible for the blue TL emission.

In fig. 3c the TL excitation spectrum is presented. It shows two maxima at - 4.3 eV (Fe3”) and 5.1 eV. All the glow peaks are excited at these two photon energies, always keeping the same relative intensity which seems to indicate that the same type of trapped carriers are involved in the three glow peaks. From photoconductiv- ity measurements Peria [20] found an electron release peak at - 5.05 eV ascribed to Fe2+ ions, which under irradiation at this energy, release free electrons, some of which can be trapped by other Fe’+ ions. On the other hand a third Fe3+ absorption band has been reported at - 4.8 eV. Therefore there are overlapping absorption bands due to Fe3+, Fe2+ and probably other impurity ions acting as electron and hole traps, so that excitation at either 5.1 or 4.3 eV seems to lead to the same charge

trapping centers structure as in X or y irradiated sam- ples. It is then suggested that not only holes (as usually assumed by the red TL emission) but also electrons are involved in the three glow peaks. In fact a strong electron release peak from Fe’+ has been previously detected at - 110°C [4]. On the other hand, the blue-green TL emission at - 2.53 eV is due to a Ni2+ transition. However, the evidence from ESR and RL measurements [1,21] is that Ni2+ is converted to Ni3+ by X irradiation. Thermal release of trapped electrons seems, then, to be necessary to yield the observed 2.53 eV TL emission of Ni2+ ions.

In summary, in addition to the well known optically detected hole centers (V, Fe3+) usually related to the large red TL emission, some electron centers appear to be necessary to explain the obtained results on the Fe3’ annealing steps, glow curves and TL emission and exci- tation spectra. According to previous results [l] Fe’+ and Cr2+ could be the electron centers. Charge and excitation transfer between these hole and electron trapped centers and the activator impurity ions Cr, Fe, Mn and Ni ions determine the observed TL emission. The reversible optically excited hole transfer between V-type and Fe3+ centers may raise some questions on the lattice position of the charge compensating cation vacancies for the Fe3+ ions and on the possible interre- lations of both trapped hole centers under X or y irradiation. Up to now no references in the literature to V- centers due to the presence of Fe3+ have been found. The V- centers have been related to the presence of OH- ions [l].

Very helpful discussions with Miss S. Clement and Dr E.R. Hodgson are gratefully acknowledged.

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