Khan, M H

Maheshwari, R C

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Indian J. Phys. 49, 300-304 (1976)Spectral distribution and brightness wave forms o f the electroluminescent ZnS: Cu, Sn luminophorR . C. Ma iie sh w a r i a n d  M. H . K h a nDepartm f vt of Physics, University of Oorakhpur Gorakhpur 273001,(Kemived 14 November 1974, revised 27 January 1975)Tho (*.ffoct of voJiage and frequency on the spectral distribution of ZiiS : Cu, Sn j)hosphf)r has bt^en investigated. The electrolumines- cvat emission spectrum consists of a single band in the region 4000- (iOOOA. Tlic peak of the electroluminescent spectral band shifts towards sliorier wave length side by increasing the frequency of tho exciting field, but it remains unchanged with the increase or decrease of the applied voltage. The brightness wave forms of this phosphor has also been studied at different voltages and frequencies of the ap­plied field. One primary and (jno secondary peak has been observed in (^ ac!h half cycle of the applied sinusoidal voltage. The results have been discussed in the light of the existing theories.1. I n t r o d u c t io nIt is now well known that with the addition of impurity in the controlled amouni, the ZnS bt^comes photolurninescent and under specific conditions of preparatiop. it also acquires the property of electroluminescence. Now a days, eloctrolu- jninosconee may bo produced in any desired region of the electromagnetic spec­trum by a proper choice of a base lattic(\ donor-acceptor pair and tho energy absorbing interphaso. The light- omission inside the ZnS lattice very much depends upon tho relative concentration of the activator and the co-activator ions, the* firing temperature and the preparation technique etc.Apart from tho investigations like voltage, frequency and temperature de­pendence on the intensity of the emitted light, the study of spectral distribution and the brightness wave forms is also important for understanding the mechanism of (d(H4iroluminescenco. The present communication reports for the first time effect of voltage and frequency of the apjdied a.c. field on the spectral distribution and the brightness wave forms of a now ZnS : Cu, Sn luminophor. *2. E x p e r im e n t a lElectroluminescent sample was prepared by firing the raw mixture, contain­ing copper as activator and tin as coactivator, at a temperature of 950°C for 90 minutes in a tubulai’ furnace under the continuous flow of pure nitrogen gas. Details of sample preparation have already been described by Maheshwari &300Khan (1974), in earlier publication, A permanent of electroluminev«icont cell was used. One electrode of this cell was a plane polished A1 sheet and the other was a transparent conducting glass plate, having resistance lOOQ/cm®. The sample in fine powdered form was mixed in araldite and was put pressed hoi ween the two electrodes. A. C. voltages in the audio frequency range were developed using an audio-oscillator (range 60 Hz to 200 KHz) coupled with a wide band ampli­fier (range 0 to 750V (rms)) and were applied to the electroluminescent cell.The wave forms of the light output and the applied field were traced simul- tanoouly on the screen of a cathode ray oscillograph in stationary positions. The study of the electroluminescent spectral distribution was made by using a prism monochromator, stabilized d.c. power supply, a multiflex galvanometei and a RCAIP21 photomultiplier tube. The photomultiplier tube was calibrated with the help of tungusten filament lamp of known colour temperature.Spectral distribution of ZnS : Cw, 8n luminophor 3013. R e su l t s  a n d  D isc u ssio nThe spectral distribution of electroluminescent brightness at 500 volts (rms) and at different frequencies of the applied sinusoidal field is shown in figure 1 . From this figure we observe that (i) the light emission extends from 4000 to fiOOOA, (ii) the phosphor shows only one distinct maxima at each frequency, (Hi) by increasing the excitation frequency, the peak of the light omission shifts towards shorter wave length side.The spectral band does not shift with the increase or decrease of the voltage of the exciting field as shown in figure 2 .A complete explanation of the frequency dependence of electroluminescent spectral emission can be given with the help of the Schon (1942) Klason’s (1946) model. When an alternating electric field is applied, the luminescent centres are ionized and the freed electrons move to the conduction band and are held there on the positive side of the phosphor till the expiry of the field. In the mean time some electrons from the valence band go to the vacant low lying centres. At low values of the applied field frequency (the time period being large) all the vacant low lying centres are filled by the electrons from the valence band, there- foie, the recombination of electrons of the conduction band takes place at higher centres only. This gives rise to emission at low energy i.e., towards higher wave length side. At higher frequencies of the applied voltage (the time period being small) some of the low lying centres will remain vacant therefore, the recombina- nation of excited electrons takes place at low lying centres. This gives rise to emission in the higher energy region i.e., towards shorter wave length side. Thus there is a shift of spectral band towards shorter wave length side with the increase of the frequency of the applied field.302 R. C. Maheshwari and M. H, KhanWML LlNQTH tN m4iKift. 1. Spootral clistribiiiion of eloctroluminoHCont brightnoss of ZnS ; Cu, Sn phosphor ai difforont froquonoios (at 600V)tVAVf LCNGTN m  l» «  —F ig . 2. S p e c tr a l d is tr ib u tio n  o f  e le c tr o lu m in e s o o n t  b r ig h tn e s s  o f  Z n S  : C u  S n  phosphor a t  d iffe r e n t  v o lta g e s  (a t  IO K H 25 fr e q u e n c ie s ) .The experimental results have been found to be explained weU with the Schon-Klasen model of luminescence, thus we conclude that the luminescence emission is due to electron transitions from the conduction band into the discrete localized levels (luminescent centres) as discussed earlier.The variations of the integrated light output and the applied sinusoidal volt­age vs time at (a) 600 Hz : 400V, (b) IKHz; 600V, (c) 2KHz; 400V; 600V, (d) 5KHz: 400V, (e) 8KHz :300 V; 400V and (f) 10 KHz :400V are shown in figure 3. From the figure we observe that the primary peaks of the brightness waves lag in phase with the applied sinusoidal voltage (ii) there exist one primary and one secondary peak in each half cycle of the applied voltage (Hi) at a fixed frequency, with the increase of the applied voltage the secondary peaks become better pro­nounced.Spectral distrilmtum of ZnS : Qu, Sn luminophor 303H)Kig. 3. Electroluminesoent birghtnees wave-forme of ZnS : Cu, Sn, luminophor at(a) 600Hz, 400V (b) IKHz, 500V; (o) (i) 2KHz, 400V, (ii) 2KHz, 600V; (d) 6KHz, 400V; (e)(i) 8KHz, 300V, (ii) SlOIz 400V & (f) lOKHZ, 400V.From earlier investigations of Destriau (1956) and Ourie (1963) it is well known that the sinusoidal electric field Ea in the dielectric around the phosphor304 R. C. Mahesliwari and M. H. KhangraiuH lagK jii pliaao with the hold on the phosphor. The phase lag between tilt*- two is giv(3ii bytail (j) Kf)io_ 2_  Kpf "vvliere K ropreHeiits diclectrie coiiMtaut of tJic phosphor and p its specific resistance.Recently it has hcen observed by Ivey (1963), Singh & Mohan (1969) that the (icid inside t he dielectric lags in phase also with the applied sinusoidal voltage V in a complex manner. In our case since the amount of phosphor is less tlian the mass of the diclta-tric;, so Ea will be in phase with Ep and thus the same phase diifermice ^ \\ill t^xist btdween the brightness wave and the applied sinusoidal field.The appearance of the secondary peaks may be duo to delayed transition of electrons from the deeper trap levels to the fundamental Cu level. Since more cixcitation energy is required for these transitions to occur, therefore, at high voltages the secondary peaks start appearing.The [H’omineneo of the primary peaks than the secondary peaks at loM^ ap- voltage suggests that in this phosphor deeper trap levels aie less in number.A ck no w ledg m entThajiks are due to the Council of Scientific and Industrial Research, N gav Delhi for financial assistaixee to one of us (MHK).R efeeeno esCurio D. Liindnoscence in Crystal/}, John Wiley & Sons, iric. New York, p 262.Dostriau G. 1956 III Eng. 51, 197.Ivoy H. F. 1903 Electroluminescence and related effects. Academic Press, Inc. Now York, Klasons H. A. 1946 Nature 158, 306.Maheshwari K. C. & Khan M. H. 1974 Ind. J Eure A2?pl. Phys. 12, 32. fcJohon M. 1942 Z. Phys. 119, 463.Singh V. 13. & Mohan Jl. 1969 Z. Agnew. Physik. 27, 360.

Spectral Distribution and Brightness Wave Forms of the Electroluminescent Zns: Cu, Sn Luminophor

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Spectral Distribution and Brightness Wave Forms of the Electroluminescent Zns: Cu, Sn Luminophor

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