Solid State Communications, Vol. 50, No. 10, pp. 943-946, 1984. Printed in Great Britain.

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

THERMOLUMINESCENCE IN ROOM TEMPERATURE IRRADIATED MgO : Ni

L. Delgado

Div. de Fusi6n Termonuclear, Junta de Energia Nuclear, Madrid 3, Spain

(Received 7 March 1984 by S. Amelinckx)

MgO : Ni single crystals submitted to ionizing irradiation at room tempera- ture produce absorption bands at 4.3 eV and 2.63 eV previously ascribed t o Fe 3+ and Ni 3+ centers, respectively. A composite weak TL peak is observed in correlation with the annealing stages of the 4.3 and 2.63 eV absorption bands. The results are discussed in terms of charge transfer processes between impurities.

IT IS WELL KNOWN that submitting "pure" and impurity doped MgO samples to ionizing radiation primarily produces impurity valence changes and V-type center formation (i.e. holes trapped at oxygen ions adjacent to ever present cation vacancies) [1-3]. Optical absorption and luminescence measurements yield useful information on the charge trapping and recombination center structure in irradiated samples. However in spite of a great deal of work, the impurity role in the thermo- luminescence (TL) of MgO is still a matter of con- troversy [2-11 ]. Most of the work has been concerned with Cr and Fe ions. The line structure in the red TL emission at ~ 1.7 eV (730 nm) has been unambiguously assigned to Cr a÷ ions [2, 3]. The interaction during annealing of the radiation induced Cr 2÷ and V-type centers would yield the Cr 3÷ characteristic emission [2, 4, 10]. The red underlying emission band has been tentatively ascribed to manganese or Fe 2+ ions [4, 5]. Little work has been done with regard to other impurities such as Ni [6, 7] in which the number of charge compensating cation vacancies available to trap holes under irradiation can be severely diminished since no charge compensation for Ni 2÷ is necessary. In an attempt to gain better understanding of the impurity role in the TL mechanisms, this work presents a study of optical absorption and TL in room temperature irradiated MgO : Ni single crystals.

The MgO : Ni samples used were cleaved from a single crystal ingot provided by Dr Chen and grown at Oak Ridge National Laboratory [ 12]. Spectrographic analysis showed 4700 ppm Ni, 10 ppm Fe. In spite of the high Ni concentration it is mostly dissolved as Ni 2÷ in the matrix [ 13]. As-cleaved samples were X-irradiated at room temperature with a siemens tungsten anode tube operated at 40 kV, 20 mA (2 x 103R min-1). The exper- imental set-ups used for the optical absorption, TL emission and photoluminescence (PL) emission spectra have been fully described elsewhere [ 14]. Due to weak

TL intensity the TL spectral distribution had to be obtained with adequate Corning filters. A pulse anneal- ing method was used to obtain the thermal stability of the optical absorption spectrum.

The absorption spectrum of as-grown MgO : Ni samples is given in Fig. 1. It shows four complex absorp- tion groups in the 1.07, 1.80, 2.67 and 3.07 eV (1160, 690, 465 and 404 nm) regions ascribed to substitutional octahedrally coordinated Ni 2÷ ions. At room tempera- ture the Ni 2÷ absorption groups are poorly resolved. Unlike in normal "pure" MgO samples [ 1-3, 14], the Fe 3÷ absorption band at 4.3 eV (288 nm) was not present in the unirradiated Ni doped samples. After X-irradiation the most important change in the optical absorption spectrum is the creation of the well known Fe 3÷ electron transfer band at 4.3 eV (288 nm) [18] which increases with dose. In addition an absorption band at ~ 2.63 eV (471 nm) is also increased by irradia- tion as shown in the difference optical absorption spectra presented in the inset of Fig. 1. Although coinciding with the vibronic sidebands of Ni 2÷ in un- irradiated samples, the radiation induced absorption band at ~ 2.63 eV has been tentatively ascribed to Ni 3÷ ions [19], since EPR spectra showed evidence for the presence of Ni 3÷ ions after 3' irradiation [3, 19]. Unlike in nominally pure MgO samples no V-type centers absorbing at 2.3 eV could be optically detected after irradiation [ 1-3, 14].

Photostimulation in the main Ni 2÷ absorption band at 3.07 eV (404 nm) yields a dominant green emission band at 2.53 eV (490 nm) together with a weaker one at 2.21 eV (560 nm) [see Fig. 2(a)]. The emission band at 2.53 eV has been previously observed in TL [6], X-ray luminescence [7] and PL [16, 17] emission spectra of MgO : Ni crystals. It has been success- fully identified as the IT2g -+ 3A2g transition in single Ni 2÷ ions [3].

A broad small glow peak at 110°C is obtained in

943

944 THERMOLUMINESCENCE IN ROOM TEMPERATURE IRRADIATED MgO : Ni

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Fig. 1. Optical absorption spectra of as-cleaved MgO: Ni crystals before irradiation (0) and irradiated to a dose of: 104R (1), and 1.2 x lO s R (2). The inset shows the different optical absorption spectra between a sample irradiated to 104R (1) and 1.2 x 10SR (2) and an as-cleaved sample.

the TL spectrum of X-irradiated samples. In correlation with this glow peak the radiation induced Fe 3÷ absorp- tion band is thermally annealed as shown in Fig. 2(b). By using adequate Coming f'dters the spectral distri- bution of the glow peak could be resolved, two differ- ent glow curves peaking at 110°C (red) and 80°C (green) being observed. No blue TL could be detected through a pure blue triter CS 5-58 (maximum transmission at

3.02 eV (410 nm)). The intensity of the red emission is " 100 times higher than that of the green one. In correlation with the green glow peak the absorption

band at 2.63 eV shows an annealing stage. This supports the identification of the green TL emission as being due to the Ni 2÷ transition observed in PL. Such an assign- ment was also made by Sathyamoorthy and Luthra [6] who also observed the red (1.7 eV) and green (2.53 eV) emissions in less heavily Ni doped MgO samples (0.01%) although the relative intensity of the green and red emissions was changed, since in our samples (0.4% Ni) the green emission is clearly diminished. This may be due to a concentration quenching effect. TL in the red region of the spectrum has been widely observed in pure

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Fig. 2. (a) Typical photoluminescence spectra of as-cleaved MgO: Ni crystals under excitation at 3.07 eV. (b) Left: Growth curves of the 4.3 eV and 2.63 eV absorption bands in samples irradiated up to a dose 1.2 x 10 s R. Right: Total glow curve and pulsed annealing curves of the 4.3 and 2.63 eV bands in samples irradiated to a dose of 1.2 x l0 s R_Also shown glow curves obtained under identical irradiation conditions and detected through the Corning filters (1) red CS 2-64 transmitting less than 0.01% at energies >i 1.94 eV and (2) green CS 4-74 with maximum transmission at ~ 2.53 eV and transmitting less than 0.1% at 1.7 eV.

and impurity doped MgO samples. As stated above, in most of the cases this red emission is composed of the line structure of Cr 3÷ ions with a broad underlying band ascribed to manganese or Fe 2÷ ions [3, 4]. PL measure- ments in MgO : Ni samples at 78 K show the Cr a+ line structure, due to trace amounts of Cr 3÷ in the crystals [16,17].

The formation of the Fe 3+ and Ni 3÷ bands during irradiation and their annealing in correlation with the observed red and green glow peaks respectively, suggest the interaction of these hole trapped centers with some

radiation induced electron centers during annealing. The results on the red TL could be consistent with the suggested Fe3+-Cr 2÷ interaction during annealing [20, 21 ]. Nevertheless one should note the large temperature width of the red glow peak, while the Cr 3÷ emission lines when observed occur in a narrow temperature range [21 ]. This suggests that most of the red emission here observed is the broad red band ascribed to Fe 2÷ or manganese ions [4]. In fact, the Fe 2÷ formation during annealing could yield the broad red emission. In addition, the green emission ascribed to Ni 2÷ ions is

946 THERMOLUMINESCENCE IN ROOM TEMPERATURE IRRADIATED MgO:Ni Vol. 50, No. 10

correlated with the annealing stage of the 2.63 eV absorption band, tentatively assigned to Ni a+ ions. Obviously the presence in the lattice of some electron centers unstable at ~ 80°C is also consistent with this result. According to previous results, Fe 1+ and Cr 2+ could be the electron centers, since evidence for electron release peaks from Fe 1+ as well as electron trapping at Cr a+ ions in several irradiated MgO samples has been reported [3, 9]. On the other hand, most of the Ni is dissolved in the matrix as substitutional Ni 2+ ions. This causes a large decrease in the number of charge compen- sating cation vacancies available to trap holes under irradiation, probably related with the observed inhibition of the V-center formation

Acknowledgement - The author is indebted to Dr E.R. Hodgson for his critical reading of the manuscript.

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