X-ray induced luminescence in BaS phosphors

Mat. R e s . B u l l . , Vol. 21, p p . 249-254, 1986. Printed in the USA. 0025-5408/86 $3.00 + .00 C o p y ~ g h t ( c ) 1986 Pergamon P r e s s Ltd.

X-RAY INDUCED LUMINESCENCE IN BaS PHOSPHORS R.P. Rao* Materials Science Centre Indian Institute of Technology Kharagpur - 721 302

(Received

January

17, 1985; Refereed)

ABSTRACT: X-ray induced luminescence measurements like radioluminescence, afterglow decay and thermoluminescence (TL) of BaS phosphors prepared with different concentrations of Cu or/and Bi have been studied. In general, overlapping TL glow peaks are observed in temperature range 30 ° to 350°C. The TLoutput increases with the increase of impurity concentration upto a certain percentage and then decreases with further increase. The decay (afterglow and TL) follows an exponential law. From these results, it is concluded that the luminescence m a y b e associated with the native defects which have a close relationship with the impurities. MATERIALS INDEX: BaS p h o s p h o r s Introduction Lnninescence and electronic properties of sulphides of II-VI group like ZnS and Cd$ have been widely studied (1-3) and are being used in several optoelectronic devices. However systematic investigations have not been made in detail on alkaline earth sulphides which also belong to the same group. In order to understand the role of impurities and their relationship with the native defects, a comprehensive program (4,5) has been undertaken to prepare and study the luminescence and electrical properties of BaS. In the past, most ,of the workers have concentrated on studies of UV excited luminescence; meagre information is available on similar studies by X- and y-ray excitations (6-9) of this system of phosphors. In this paper, some results on X-ray indu.=ed radioluminescence (RL), afterglow decay (AD), thermoluminescence (TL) and TL spectra of BaS phosphors doped with Cu and/or Bi are presented and discussed. Experimental Several samples of BaS phosphors have been prepared by reducing pure BaSO4 with spectroscopic grade pure carbon in an ago, ron atmosphere 950 o for two hours with different quantities of Bi (0.005 - 0.5% by wt.) and/or Cu (0.001 0.2% by wt.) in the presence of NaCI flux (i0). The X-ray diff$ctograms of of these samples exhibited the presence of BaS only and not BaSO/, or BaO which are otherwise the possible residues in the reduction process, ll~e X-ray analysis also indicated that these material are polycrystalline with f.c.c. structure. The details of materials preparation and characterisation were given in the earlier paper (ii). The powders of 75 ~ average particle size, packed in a brass well of i0 cm diameter and 1 mm depth were used to record the RL, AD, T L a n d TL spectra. i

Presently at C.E.C.R.I., Karaikudi - 623 006 249

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The emission from the samples while irradiating with X-rays (30 kV i0 mA) at different temperatures, was focused on to the entrance slit (50 f~ml of a 0.5 M Jarrell Ash monochromator with the help of a quartz lens (12). The intensity of emission was corrected accordi!~g to the spectral response of photo multiplier tube and grating. Afterglow decay was recorded after X-irradiation at 30uC (room temperature). After the afterglow decay for a particular period (say 15 minutes), TL glow curves were recorded by heating the samples in vacutnn (10 -3 to~r) in the temperature range 300 to 350°C. The details of experimental setup used to record TL and TL spectra are described elsewhere (13). In case of optical bleaching, the X-irradiated samples were exposed to monochromator light from a 60 Watt tungsten lamp at 30°C. The samples after X-irradiation were stored in dark for different durations of time and recorded the TL decay (fading). Results Radioluminescence (RL) spectra of several smr~les doped with Cu and/or Bi were studied at different temperatures (30 ° - 300°C). In all the samples, three emission bands were observed in three wavelength regions viz., 350 to 375 am, 425 to 450 nm and 500 to 550nm. RL spectra of BaS:Bi (0.21% by wt.), one typical concentration is shown in Fig.l. It could be seen from the figure that all the three bands shift towards longer wavelengths with the increase of excitation temperature. Similar behaviour was also observed in case of BaS:Cu and BaS:Cu, Bi phosphors. The emission from the samples were recorded for 15 minutes, though a weak glow persists. Fig.2 depicts the results on AD of BaS:Bi. In order to avoid overlapping, the decay curves are shown with an equal shift (by arrows). To find the nature of decay, arrhenius plot between the intensity and time for one typical case is also shown in Fig.2(B). The decay process is observed to occur in two stages, independent of impurity concentration; where as in case of double doped (BaS: ¢ a - R.T. Cu, Bi) samples, the decay process could be ,oc / ~ ,,-,ooc resolved into three stages by peeling off ~,a //.~ e.,,-3ooc / \\ ~- 2oo c procedure (14) as shown in Fig.3. The data =~ on afterglow~ decay of BaS:Cu was presented ~ 6c / \ \ ~ in Ref.6. The arrehenius plot between 'I'
20

Undoped BaS exhibited practically no TL while BaS doped with Cu &/or'Bj yielded complex TL patterns with overlapping glow maxima, as shown in Fig.3. These glow curves occur essentially in three temperature regions viz., 90 ° to 130 ° , 180 ° to 220 ° and 280 ° to 310°C. It could be seen from Fig.4 that TL increases with increase of concentration of impurities upto certain value (0.23% by wt. in case of Bi and 0.12% by wt. in case of Cu (6)) and then decreases with further increase of Cu and/or Bi. The trapping parameters (E and

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BaS PHOSPHORS

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FIG.2 Afterglow decay of BaS:Bi phosphors s) are calculated from the glow peaks using Chen's formula (15). The values of E obtained are in the range 0.4 to 0.42 eV, 0.6 to 0.8 eV and 0.9 to 1.3 eV corresponding to peaks at ii0 °, 200 ° and 290°C respectively. It is observed that low temperature peak follows first order of kinetics and second order for peaks above 150°C (13). The 520 nm light bleaches the TL peaks faster than the other wavelengths in the visible spectrL~n. Figs.5 and 6 show the optical bleaching of TL glow curves of BaS:Cu and BaS:Bi phosphors respectively. From the figures, a decrease in TL output is obser'° ~ , r e d with the increase of exposure time. ~o But, the peaks could not be bleached completely even after longer exposures. ~o In the bleaching process, the low temperature peak has been removed completely _ ~ and the high temperature peak has been reduced gradually with the time of exposure, as shown in the inset of this figure. The TL glow curves of BaS:Bi \ stored in dark for different durations of time after irradiation are shown in Fig.7 as a typical example. The low temperature peak has been destroyed com, \, pletely after 3 hours storage. The , ~.... ,. . . . :, ,, trend of fall in the intensity of one of the high temperature peaks (200°C) FIG.3 Afterglow decay of BaS:Cu,Bi is shown in the figure (Fig.7) which phosphors indicates a two stage process, each one being exponential in nature. In TL spectra the emission indicates overlapping bands (13) in the wavelength region 350 to 600 nm which could also be divided into three regions as in RL spectra. A slight shift is observed in the band maxima towards the longer wavelengths with an increase in temperature (12).

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FIG. 5 Optical bleaching of TL glow curves of BaS:Bi phosphors Discussion

The results of RL, AD and TL are to be understood in the light of similar information and models available on radiation produced traps in well studied materials, like alkali halides. Similarities in the properties of several alkali halides and alkaline earth sulphides were reported to exist (16). BaS, in general, shows less ionic character than the alkali halide, CsBr. Though ionicity of these sulphides is low compared with the alkali halides, most of the defect controlled properties, such as luminescence, are explained by treating them as ionic models, analogous to alkali halides (17).

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The luminescence (RL, TL) is observed in BaS phosphors only after d o p i n g w i t h i m p u r i t i e s l i k e Cu and Bi. The glow c u r v e s obtaine~t a r e n e a r l y i n the same region of temperature independent of activating impurities and exhibit e m i s s i o n i n the same s p e c t r a l r e g i o n i n d i c a t e t h a t the l u m i n e s c e n c e i n a c t i v a t e d

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BaS phosphors is in some manner connected with the native defects of the material and the associated traps bear a close relationship with the impurities. To explain the results of the present investigation the role of defects like [Vs]' , [Vs- GUBa] and [BIBa j as posslble tPappzng sztes for electrons and [CUB]- [VBa] ~and [V~-BiBa]- for holes have been proposed. All the possible complexes with impurlty-native defects in BaS doped with Cu and Bi were explained in %he previous publications (18,19). The trap depth values obtained from AD and TL glow peaks indicate that the involved electron and hole traps are located near each other. The high temperature glow peaks attributed that they are associated with the distribution of trapped electron and hole centres. The RL and TL spectra observed in the same spectral regions also prove that the distribution of acceptor like levels are available in the system during recombination. The increase in TLyield with the increase of concentrations of dopents is due to increase in ntmlber of traps. The decrease with further increase of concentration is because of quenching of trapping centres

(19). Results on dielectric constant (20), electrical conductivity (21,22) of these phosphors gave ample support for the above proposed defects. It is concluded that the electron and hole traps are located near each other and are related to the unassociated traps in BaS phosphors.

References i.D.Curie,

Luminescence in Crystals 9 Methuen~ London (1963).

2. S.Shinoya, J. Lt~inescencel,2, 3. R.H.Bube,

Electronics

in

17 (1970).

Solids , Academic

Press,

New

York

(1981).

4. R.P. Rao, Ph.D thesis, Indian Inst. Technology, Kharagpur (1980). 5. A.K.Reddy,

Ph.D. thesis, Indian Inst. Technology, Kharagpur (1985).

6. R.P.Rao, D.R.Rao and H.D.Banerjee, Mat.Res.Bull. 13, 491 (1978). 7. R.P.Rao, Radiat. Effects 77~ 159 (1983). 8. R.P.Rao and D.R.Rao, Health Phys. 45, i001 (1983). 9.

R.P.Rao

J.Gasiot

and

J.P.Fillard,

J.Lt~inescence

31/32, 213

(1984).

i0. R.P.Rao and D.R.Rao, Bull. Mat. Sci. 5, 29 (1983). 11. R.P.Rao, J.Mat.Sci.Lett. 2, 106 (1983). 12. R.P.Rao and D.R.Rao, J.Mat.Sci.Lett. i, 435 (1983). 13. idem, Chem. and Phys. Materials 9, 501 (1983). 14. R.H.Bube, Phys. Rev. 8_0, 655 (1950). 15. R.Chen, J. Appl. Phys. 40, 570 (1969). 16. V.P.Rombakh, Optics and Spectrosc. i0, 360 (1961). 17. M.Aven and J.S.Prener (Eds.), Physics and Chemistry of II - VI

Compounds,

North-Hollond Publishers, Amsterdam (1967). 18. R.P.Rao, J. Luminescence 33~ 87 (1985). 19. P.pringhshin Fluroscence New York (1949).

and

Phosphorescence

, Wiley

(Inter

Science)

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20. R.P.Rao and D.R.Rao, Physica Scripta 26, 467 (1982). 21. idem, J. Mat. Sci. lett. i, 17 (1982). 22. idem, Physica Scripta 25, 592 (1982).

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