Choudhury, K

Khare, R P

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Indian J . Phya. 50, 1032-1038 (1076)Electroluminescence of BaO and SrO phosphors activated by manganeseR  P  K h AHE a n d  V  K .  CUOUDHUEY  Deparlmant of Unwe/mfy of Santjar, Sngar {31 P )(Received 30 March 1976)A-.C. oJoctrolumiiiowoonoc (EL) iu BaO . Mu and SrO : Mn powder phosphors if3 invoHtigated. Tt is observwi that the mean EL bright- neap is an moreasing funetion of tho applitni ,ainupoidal voltage and vai-ies exponentially with the latter Tt seems that during EL of these pliosphoi'H, a Mott-Rehottky type*? of >)arrier p’^ ovides tho field eTihanoemeut Th(^  frequem^y dependence of EL brightness is found to be nonlinear in ]iatnre. The brightness waves of the nowder cell consist of two major peaks peJ’ nyele of the applierl voltage' Tt has been inferred that in the present system of phosphors, eleetrolumi- minoseenee takes plae<^  by impatit ionization mechanism1. IntboductionIt is diffioult to excite eleetroluminese(‘nce, in such crystalline phosphors, Avliore the j)erc(mtag(» of ionic ehai’aoter of bond is very high Tho reasons for this may Tu^ that, (i) there is a short mean frec! path for electrons and holes, diu\ to their scatti r^lng by phonons, (ii) thoi'e is a relativtdy wide forbiddini gap between valence and eonduetion bands and (iii) theiiv is a low concentration of free charge carriers so that the regions whi^ ’e the field is concentrated are not formed (Georgobiani 1964:). Alkaline earth oxide phosphors come under the above mentioned group having ionic character of their bonds approx. 70-80%. Tn order to products EL fi'om such compounds, it bec-omos necessary to prepare thin and uniform layers, because, with a rckluction in the layer thickness, the diiVelopmont of cascade breakdown beiiomeS more difficult and hence the pro­portion of electrical energy which goes into ionization of radiation centres is inoroased while tho proportion which goes to heat the crystalline lattice is decreased. The present investigation deals with the studie.s of EL in BaO ; Mn and SrO ; Mn by preparing thin and uniform layers of tho powder phosphors.2. Exfbbimental Details 2.1. Preparation of PhosphorsBaO : Mn and SrO : Mn olootroliirainophors were prepared by solid phase reaction through firing the mixture of the starting materials (pure BaCOg or SrCOa+CaFg—2 % -f-activator compound) in air at 1 2 0 0 ”C. The samples were1032lirod for 4 hour,s and thou slowly cooled to r»)oiu tompevatiuo in abouL 10 lu's In general, imicli higher concontvatiou of tlie activator atoni.s is e^qaivod to produce more brightness as rexiortod by many invostigatovs (ZaJiii 1950, MiitoSisi & Nudoliiian 1959). Tlio results presoutod in this papt«’ a.iv IJioso oblauu-d with 5% concentration of Mn/Ba or Mn/Sr. Uniform thin layers of pJiospliovs M'ece deposited on niica sheets using a transiiarentr adhesive substajuu^  araldite Tl\c use of mica sheet excludes the possibility of carrier injection (Suhlalli 1959) at the electrode contacts.2 2 Expariw.e.ntal Set-upIn the ijreSeut study a iion-syinmetvioal EL cidl was used A ];>h,ospho\' layer deposited on thin mica sheet was sand witched betw^ ium tlie elecl»odc;s an<l theii it was excited by applying a sinusoidal voltage. A L.E oseillator in eon- jLiiietion with a wide band amplifier was used for this xmrx)oso The outimt of the amplifier was fed to tlie two electrodes of the EL coll The moan EL brightneSiS was measured using IP 21 photomultiplier detector coupled with a Rubicon galvanometer (sensitivity 0-0013 ^A/min) TJie voltage doiiendenco of brightness, keejiiug other jiai'anuders constant, w'as studied at different Ire- (liioncjes (30 ejis to 1000 ejis) at room temxinraturo (23“C) Similarly frequency deijondojice at different fixed voltages Avas also studied.Eor studying the hrigh.tness waves, a douhli5 beam Tektronix Oscilloscoxie (tyiie 540) was used in place of the galvanometer The voltage devoloi>pd acr oss the resistance of the anode circuit of the photomultiplier Avas ainjilifiod and apjAliml to one iiair of the Y-jilates of the CRO and the exciting voltage Ava>s put to the othei' i>ajr ol Y-jilatcs Brightness Avaves along with tlie AvaviAS of th(> exciting voltage A^ e^re photographed for analysis.Electrotuminescence of BaO and 8r0 phosphors 10333. R e s u l t sThe powder phosifiiora i>reparod with Mn as activator wore found to bo gray in colour. The darkness in colour increases with increase in Mn-content. This indicates the presence of some other phase in addition to BaO/SrO contain­ing the activator in solid solutions Similar results have been reported by Fisher (1962,1963) in ZnS : Cu phosphors. Goui* & Ranade (1970) and Masoorkar & Ranade (1973) have also confirmed those results in the case of alkaline earth sulphide olectroluminoph ors3 .1 . Voltage and freguency dej^mdence of mean Eh hnyhlnean(a) BaO : Mn. The curves representing the moan EL brightness as a func­tion of voltage at fixed frequencies are sliowii in figure. 1 It is clear fiom t,l\ese curves that tlie incroaso in brightness is sharper at high voltages tlian at ]oav voltages The shape of the curves shows that the linear relationship does not61034 R. P. Khare and V. K. dhoudharyhold well between the brightneas (JB) and applied voltage (F). Therefore an exponential or power* law relation may exist between B and V.F ig . 1. Voltage dependence of EL brightness of 13aO : Mn(a) at low froquenoioB,(b) at high frequencies.Tn genera], the plots of logaritliin of 5  as an mvtvrso function of F^  (figure 2) ai‘e straight linos. Thus, it can be concluded that tlui exponential law holds well.. ‘ *•' *• \  \T ? ^\  \ \  \■k\ \  \\  \  \.r'l *\'  \ \ ^ \\  X ' N  \\  \  \ ^  w’'T  v*. \ \ O ' ^\•> \  ‘ V \\\  \  \  \\  \  \PIG. 2  0  \  \  V \ F ia . 2  bLOW f r e q u e n c i e s  \ \  \ HIGH F R CQ U E N Cie S»  3J 3« JS 3S 97 90 9» 40 4 lOOO//^ 3a 93 34 33 30 37 Sa 30 40 41Fig. 2 . Voltage dependonco of EL brightness (log B  Vs V“* plots) of BaO : Mn(a) at low frequoncioB,(b) at high frequencies.Tho reproaentativo curves illustrating tlio frequency (lO dependence of brightness (logi? as a function of logp) are given in figure 3 Tho curves are observed to be non-lineai' iir nature, that is, at fixed voltages, brightness first decreases with frequency (from 30 cps to 100 cps) and then it starts increasing more rapidly than linearly, with increase in frequency. No saturation occursElectroluminescence of BaO and BrO phosphors 1035Figi 3. Frequency dependence of EL brightness at fixed voltages of BaO ; MnFig. 4. Voltage dependence of EL brightness of SrO : Mh(a) at low frequencies,(b) at high Irequenoies,1036 R. P. Khare and V. K. Choudharyupto about 1000 ops. Tt is quito possible that the saturation might occur at liigho^ ' frequencies (>• 1 0 0 0  ops), which was not tried in the present study.{h)SrO : Mn. Tht» voltage dependence of B at various frequencies in SrO : Mn phosphofs i,s shoAvn in figure 4. Since log B Vs. curves ore alllinear (figure 5 ), it can be concluded that the exponential law holds hero too. The Fig. 6  depicts tlie ivdationship between frequency and brightness at constant voltages. Hero also the curves a,»‘e all non-linear and therefore no ompi.^ ’ical ’‘elatioiiship between B and i> was found to hold well. Tn SrO : Mn phospl o^.rs the saturation in brightness is observed after -v.'GOO cx>.s. at higher voltages3 2, Brightness WavesIn all the ca.sc^ s, two emission peaks witliin one cycle of the applied sinusoidal voltage arc^  obi e^rvcil (Figs 7 & 8 ) At fixed frequencies, the amplitude of botli poak.s jjicroa.ses with an in(;roase of voltage for BaO ; Mn (Figs, 7a & b) and SrO ' Mn (Figs^  8 a &; b) phosphors4 Discussion4 1. Fo/^ «f/e Dependence.It is found practically in both types of p}\osphors (BaO ‘ Mu and SrO : Mn) that the mean EL brightness is an increasing function of the voltages Qualita­tively, all the xihosphora have similar type of dcTiendcnce. It can bo understood on this basis that initially the jinmber of paitides in which the oleotrclumincscenee takes place is small, but on iiuneasing the voltage, more and more active regions arii exposed to voltage gradients above the threshold level The plots betwt‘en log B and V  ^ fronted in tl\o figures 2  and 5 show almosi. linear relationship for diffevoni rrcquencies. Thus the following relationship bi'i.weon the moan EL Iwightness {B) and voltage (F) seems to hold well,B -B o^^xp(-(7 /F l) ... (1 )(i?„ and C being tJic constants).Very dose agreement of the experimental curves (figures 2  and 5) with tlio theoretical formula (1 ) indicates the existence of a Mott-Sohottky barrier m tJie pre.sent syatoiu of plxosphors The square root term in the exponential"clepond- onc(^  ol‘ bn'gixtness and voltage can be interpreted on the basis of the acceleration —collision meoJ\anism by taking the probability of impact—ionization to be proxmrtional to ox]}{~G'jE), E being the electric field and G' a constant along with the fact that E itself is propo,rtional to in Mott-Sohottkey barrier.4.2. Frequency DependenceIn the most gontv'al sense, a certain amount of light is expected to be generated at each instant, tb.o luminous condenser is cha»-ged or diac.harged. It is assumed tjio EL brightneoss can be regaT-deKf as being duo to instantaneous concentra-R. P. Khare & V. K. Choudhaby Indian J. Phys. Vol. i50 No, 12. 1976Fig. 7. Oscillograms of BaO : Mn at 30 ops (sweep duration 2 m-sec/em). (a1 at 680 volts; (b) at 720 volts. The curve (1) is the wave form of the applied voltage while the curve (2) is the cwave form of the light emitted from the EL cell.R, P. KnABB & V. K, Chotohaky Indian J. Phys. Vol, 50 No, 12. 1976Fig. 8 . OscillogramH of SrO : Mn at 30 cpa (sweep duration 20 in-sec/cm). (a) at 520 volts; (b) at 560 volts. The curve (1) is the wave form of the applied voltage and the curve (2) is the wave form of the light emitted from the EL cell.Electroluminescence of BaO and SrO phosphors 1 0 3 7Fig. 5. Voltago doponclonco of KL bnglHiioss (log B  Vh plots) of SrO : Mn(a) at low lioqupiicics,(b) at high fi’eqiioncioa.Fig. 6 , Froquenoy dependence of brightneaa at fixed voltagea of SrO ; Mn.1 0 3 8 R, P. Khare and V. K. Choudharyt,ion of light omitting point soiircos. At higli froquonoioB, tho omitting point- soiirooB will 1)0 larger and honoo groator would bo tho omitted light The expori mental obBowation that tho EL brightnoBs increasoB with frequency from 1 0 0  ops onwards (figures .3 and 6 ) conbrms this view. However, the peculiar behaviour of doovoase in B with increase in Irequoncy within the range 30 to 100 cps in both typoR of phosphors is obisorvod. This may be attributed to bo due to the polanza tion effects.Tlio increases m EL bviglitnoss abo\n‘ 100 cps can ho understood on the basis tliat mniitying and refilling of the lumineseent centres tahos place more rapidly witli increase in frequency (Ivey 1957). When tho tune period of the applied AC cycle becomes comjiarable with tlie life time of excited electron, the increas­ing fr(upiencies can not impart a propoi'tioiial increase in average intensity as the ol(Mitron does not hav^ e enough time tf> einorgo out from the trap. Hence saturation can be expected in tho higher frequency raaigc which w^ as oxjicri- mentally observed in the (uise of SrO : Mn (figure 6 )4 3. Bi igMmss WavesLitter & Waymouih (1954) liaA’e sliown tliat ihe light omission from a single gram is conoent’“ated in ca^ riam small spots Tliese spots emit hriglitnoss waves. Aeenrding to Thornton (1956), it ean be assumed that theses spots behave as minia titl e rectifying junctions Some of tho junctions will be excited on the applica­tion of one half (iycle wJiile others on the application of the other half cycle. Tn tJiis way, two peahs arc expected in one complete cycle of applied field, wliich was experimentally ohsei’ved in the present system of phosphors (Figs 7 and 8 ).AcknowledgmentsIt is tho pleasant duty of the authors to thank Prof. J. D. Ranado, M.Sc., rh T)., JVT.Tnst.P Jor valuable suggestions. They are also thankful to Miss Manjula tSlirivast,a\^ a and Miss Anita Has for assistance rendered during the course of tlie i^ xpei imental rvoikReferencesBiUor, F. & Waymouih .1. F. (10.54) P h ys , R ev, 95, 941.Fiahor, A C! (1962) J. Electrodiem  Soc. 109, 1043; (1963) 110, 733.Georgobarn, A. N. (1964) Transactions o f P . N .  Lebedev Plvyaica (English translation)U. S. Cunsultants Bureau, N. Y. Voi, 23, p. 11 Gour, S. I?}". & Bauade, J. D. (1970) : In d ia n  J . P a r t  A p p l. P hya. 8, 294.Ivoy, H. F. (1967) J . EUctruchem. Soc. 104, 740.Masooi-ker, N. & Banade, J. D. (1973) In d ia n  J . P u re  A p p l. P h ya. 11, 6 8 8 .Hatossi, F. & Nudelman, S. (1959) M eth od s o f  E xperim en tal P h ysics , S olid  State Phyaica, Vol. tPart B, p. f  3.Sidalli, G. (19.59) Vacuum , 7-8, 61.Thornton W. A. (1956) P h ys  R ev . 102, 38; (1956) 103, 1586,Zalm P. (1956) P U l .  R ea. R epts, 1 1 , 417.

Electroluminescence of Bao and Sro Phosphors Activated by Manganese

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Electroluminescence of Bao and Sro Phosphors Activated by Manganese

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