898 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 40. NO. 5. MAY I993

Memory Effect in ZnS:Mn AC Thin-Film Electroluminescent Devices with Low

Mn Concentration Ian P. McClean and Clive B. Thomas

Abstract-Electroluminescent (EL) memory is exhibited in ZnS : Mn ac thin-film EL (ACTFEL) devices with Mn concen- trations between 0.2 and 0.7 wt % (f0.2%). Maximum hyster- esis width is observed for Mn concentrations of 0.3 wt % (+0.2%), compared to 1 wt % for previous devices exhibiting memory. The phenomenon is seen in Zn-rich, but not in S-rich ZnS : Mn films. Low-field electrical characterization has pre- viously shown that the presence of shallow donor sites in Zn- rich ZnS at 0.11 eV below the conduction band. Electron ejec- tion of donors near the interfaces is believed to sustain elec- troluminescence at voltages below threshold. The trap is thought to be created by an excess of S vacancies (donor sites) over other trapping or recombination centers.

I. INTRODUCTION USEFUL facility for electroluminescent (EL) flat- A panel displays is intrinsic memory associated with the

light-emitting pixel. The first ac thin-film electrolumines- cent (ACTFEL) devices found to possess memory under conventional drive conditions were fabricated by Yamau- chi et al. [ l ] . These films had an active layer of ZnS : Mn with a high atomic percentage of Mn (above 1 wt %) sandwiched between two insulating layers. EL flat-panel displays were subsequently fabricated with these devices and were demonstrated at the SID conference of 1976 [2], [3]. Intensive research by the IBM Corporation [4], [5] led to the conclusion that the phenomenon was a result of deep hole traps, created by excess Mn incorporation over the substitutional allowance in Zn [6]. Holes were created by hot-electron impact ionization of the lattice. These were captured by the deep hole traps creating a positive space charge. The internal field was consequently en- hanced, allowing hot electron formation at voltages below threshold [7], [8]. Electron beam irradiation and ultra- violet light illumination were found to turn these devices ON from an OFF-state when biased just below threshold

Unfortunately, coupled with additional analysis data from other groups [lo]-[ 121 problems were identified that

Manuscript received October 28, 1992; revised December 29, 1992. The work of I . P. McClean was supported by the Science and Engineering Re- search Council. The review of this paper was arranged by Associate Editor J . J . Coleman.

The authors are with the Department of Electrical and Electronic Engi- neering, University of Bradford, Bradford, West Yorkshire, BD7 IDP, United Kingdom.

141, [91.

IEEE Log Number 9207585.

were not solved. Memory was found typically in films with a concentration of >0.5 wt % (although it had been observed very weakly at 0.2 wt % [lo]) which is above the concentration providing optimum efficiency previ- ously reported to be at values between 0.2 wt % [12] and 0.5 wt % [13]. The necessity of Mn doping meant that other dopants could not be substituted to provide a colour memory display [ 141. Finally, stable operating lifetimes were short. Memory was observed to completely disap- pear within 100 to 300 h of operation [4], [5] and thus were not suitable for display applications.

A method of obtaining memory in more optimally doped devices was illustrated by Zhu et al. [15], [16] by driving devices with fast rise time pulses. The pulse train resulted in electrons, driven to the cathode interface being unable to relax to states near the interface before the re- verse pulse was applied (the relaxation time of trapped electrons was estimated to be lop4). An electron source had thus been formed near to the interface which would allow electron ejection at lower voltages than threshold, thus proving hysteresis.

In this study we have used a shallow donor center to give a memory effect in ZnS : Mn ACTFEL devices with low Mn concentrations. Although the formation of the do- nor centers is not induced by the Mn doping concentra- tion, the concentration of uncompensated donor sites is. The paper describes the preliminary observations of mem- ory characterization in the ACTFEL devices.

11. EXPERIMENTAL

ZnS : Mn triple-layer devices were grown on n+ (100) Si substrates at 150°C. Si being the preferred substrates used in this laboratory which is concerned with the for- mation of optoelectronic integrated circuits. 1 -pm-thick layers of ZnS : Mn were sandwiched between two 0.3-pm- thick layers of Y203. The ZnS : Mn layers were grown by co-evaporating ZnS and Mn from separate Knudsen cells in an MBE system with a base pressure of lo-'' mbarr. Two types of ZnS source were used (a S-rich powder from DLR Chemicals Ltd. and Zn-rich crystals from Sasoon Advanced Materials) in order to compare EL emission. Films with five different Mn concentrations were grown using the Zn-rich ZnS source material. The Mn concen-

0018-9383/93$03.00 0 1993 IEEE

MCCLEAN AND THOMAS: MEMORY EFFECT I N ZnS : Mn THIN-FILM DEVICES 899

tration was varied by growing each film with a different Mn Knudsen cell temperature (T,,). S-rich devices were fabricated with two Mn concentrations corresponding to memory emission in the Zn-rich ACTFEL devices. The Y203 layers were grown by magnetron sputtering. Both the S- and Zn-rich films were either left as-grown or an- nealed at 500°C for 1 h in 2 bar of Ar.

Finally, A1 top and bottom contacts were made by ther- mal evaporation. 3-mm-long, 1-mm-wide top contacts were initially grown and a cleave was made (creating two 1.5 x 1 mm contacts) exposing the edge of the device. Emission directly out of the active layer, through the ex- posed edge, was characterized which is in line with the investigations undertaken in this laboratory. Brightness- voltage meausurements were made using a 5-kHz sine wave driving voltage at room temperature; voltages were measured as peak values. EL output was measured on an oscilloscope, via a photomultiplier tube and intensities were determined on a Minolta Luminance meter (model LS 1 10). Lifetime measurements were conducted in a vac- uum chamber using the same photomultiplier tube. Elec- troluminescent spectra were measured on an EG&G Princeton Applied Research Spectrometer system utiliz- ing a Detector Interface model 1461.

111. RESULTS WITH DISCUSSION

B-V characteristics for Zn-rich ZnS : Mn films all show memory behavior. Depending on the maximum voltage applied any number of return paths is possible, as illus- trated in Fig. 1 for a device grown with T,, = 750°C. The maximum intensity of the emission from the phos- phor layer was measured at 82 000 fL, which was 1.5 times larger than an equivalent device grown in a mag- netron sputtering system in the same laboratory [17]. As the Mn concentration was increased the memory margin (AV, = threshold voltage - turn-off voltage) increased up to T,, = 750°C then decreased with a further increase of Mn (Fig. 2). EL emission with memory was observed for films at comparable brightness which had no post-de- position annealing. AV, was smaller for the as-grown de- vices.

Memory is not observed in S-rich films at either Mn concentration of T,, = 725°C or T,, = 750"C, as-grown or annealed, as illustrated in Fig. 3 , with a typical B-V characteristic with T,, = 750°C.

The Mn concentration was measured by electron dis- persive X-ray analysis using an electron microprobe (Jeol JXA 50A) to be between 0.2 and 0.7 wt % ( f 0 . 2 % ma- chine error) for the Mn Knudsen cell variation between 700 and 800°C. The concentration providing maximum memory margin was 0.3 wt %; the corresponding maxi- mum memory concentrations for the Yamauchi-type de- vice was in the order of l wt % [12]. The maximum AV, reported for the earlier devices was 20 V [7], half of that in the present study. It is well documented that in photo- luminescent (PL) studies of ZnS:Mn a red emission emerges at a Mn concentration near 0.5 wt % [18], [19].

~

260

Bias (Volts)

Fig. 1. Brightness-voltage characteristic of ACTFEL exhibiting memory, with T,, = 750°C.

Mn Knudsen Cell Temperature ("C)

Fig. 2. Variation of memory margin with Mn Knudsen cell temperature. Insert depicts conductivity of ZnS : Zn films with varying Zn concentration.

Bias (Volts)

Fig. 3 . Brightness-voltage characteristic for S-rich ACTFEL with T,n,, = 750°C exhibiting no memory.

Fig. 4 illustrates the intensity of the PL emission of ZnS:Mn films on Si grown by the authors at 670 nm; clearly the red emission emerges at a higher Mn Knudsen cell temperature than 750 "C thus providing some confir- mation of the measured Mn concentrations. Additionally, it is reported that a Mn concentration of >0.2 wt % is required for memory in the Yamauchi-type device [12] (although higher values have been stated in some reviews [4], [20], [21]) at which concentration the memory mar- gin for 0.2 wt % was much less than l % of the threshold voltage. In the present study at the same Mn concentra- tion the memory margin is over 7 % of the threshold volt-

900 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 5, MAY 1993

1 ' 1 A

'~ 0.0 O.' 680

7 /

710 740 770 800

Mn Knudsen Cell ("0 Fig. 4. Photoluminescent emission peak at 670 nm for ZnS:Mn films grown with various Mn Knudsen cell temperatures. Insert depicts spectrum for ZnS : Mn films with low and high Mn concentrations.

age and from Fig. 2 it seems likely that at much lower concentrations memory will still be manifested.

Several differences in the characteristics of devices in this study to the previous memory devices have been ob- served. An attempt to induce electroluminescent emission was made by illuminating a device, biased just below threshold, with ultraviolet (UV) light. The devices were biased at various voltages below threshold, in the OFF- state, and UV light was irradiated onto the open ZnS : Mn active layer surface at the device edge. Under no bias con- dition or any Mn concentration was any change in emis- sion observed. EL spectrum measurements taken on ACTFEL devices with the five Mn concentrations can be seen in Fig. 5. Coupled with the normal Mn yellow emis- sion at 580 nm is a red emission observed in devices with a Mn Knudsen cell temperature of T,, = 750°C and above. A similar result was observed by Marello et al. E121 and was attributed to an increase in recombination centers at the higher Mn concentrations. There is a dif- ference of 30 nm between the peak red emission in Mar- ello's devices, centered on 670 nm, and that observed in the present study, which is centered on 700 nm. By re- ducing the applied voltage of a device, while in its mem- ory state just below threshold, to zero and reapplying it after a delay time the devices of Yamauchi type would still be on after several seconds delay due to deep hole traps releasing charge slowly [5]. In the present study the 50% decay time was in the order of 0 .4 s (Fig. 6), much lower than the Yamauchi device.

PL spectrum taken at room temperature of thin films of Zn-rich ZnS, grown by the MBE reactor used in the pres- ent study, are dominated by a strong blue, self-activated (SA) emission centered on 460 nm [22]. The emission has been attributed to a S-to-Zn vacancy electron transition (few other recombination centers were observed to exist) as similarly observed by Riehl [23]. Addition of Zn, to the films, resulted in a reduction in the SA emission in-

I I

Wavelength (nm) Fig. 5 . Electroluminescent spectrum for ACTFEL's which exhibit mem-

ory, with Mn Knudsen cell temperature as shown.

Tme (secoads)

Fig. 6 . Band diagram of insulatoriactive layer interface illustrating shal- low trap contribution to emission.

tensity, due to the reduction of Zn vacancies. A similar reduction of the SA emission was observed when Mn was co-sublimated with the ZnS material. Two emission peaks in the ZnS : Mn films were observed at lower and only one at higher Mn concentrations, which provided direct con- firmation of the positioning of Mn atoms in Zn sites (in- sert of Fig. 4).

In order to determine if the usual autocompensation ef- fect of ZnS [24] was manifesting itself as a concomitant reduction of S and Zn vacancies as the Zn content in- creased, current-voltage measurements were taken on ZnS:Zn films grown onto p- epilayers (on p+ Si sub- strates) with varying concentrations of Zn [25]. An in- crease in conductivity was observed with increasing Zn content, which indicated that S vacancy donor centers were dominating over Zn vacancy sites. At a particular Zn concentration, an ohmic contact was formed between the Si and ZnS : Zn, confirming the presence of donor sites below the conduction band. Together with the PL data and the knowledge that S vacancies act as donor sites [23], these results provided confirmation that S vacancies were present in the film, and that by the addition of Zn the concentration of uncompensated donor sites could be con- trolled. Temperature measurements on the ohmic films found that the donor level existed only 0.1 1 eV below the conduction band. A very similar trapping level had been seen previously in ZnS by Hoogenstraaten and was deter- mined to be an inherent defect but its origin was never surmised [26].

Recent capacitance-voltage measurements on ZnS : Zn and ZnS : Mn films have shown that similar electrical be-

MCCLEAN AND THOMAS: MEMORY EFFECT I N ZnS : Mn THIN-FILM DEVICES

havior is manifested by both [27]. From this it was con- cluded that films of ZnS : Mn may also possess an uncom- pensated donor site below the conduction band, of which the concentration would depend on the Mn doping. This is further indicated by the similar rise and fall of conduc- tivity in ZnS : Zn films with an increase in Zn concentra- tion and the rise and fall of memory margin in ZnS : Mn ACTFEL devices, as indicated by the two graphs of Fig. 2. It was this knowledge of the ZnS grown in the MBE reactor and the measurements of the memory effect which led the authors to believe that the mechanism producing memory in the ACTFEL devices of the present study was different to that of the Yamauchi device.

A memory mechanism which utilizes the uncompen- sated donor sites to exist in the ZnS material is proposed which is similar to the process described by Zhu er al. [15] and is as follows. Initially, by considering the energy diagram in equilibrium (Fig. 7(a)), it is believed that the donor sites are empty, due to the Fermi level being effec- tively pinned at lower energies by the high density of in- terface states [28]. In the diagram only the donor sites near the interface are shown as these are the only ones believed to take part in the memory process. It is believed that an equal density of donor sites exists across the active layer, consequently no energy band bending due to empty positively charged donor sites occurs. When the driving voltage is raised to threshold, electrons are swept across the active layer to the anode interface. Some electrons from the anode interface are captured by anode interface states and others at donor sites near this interface. On re- versal of the field electrons are emitted from both sites (Fig. 7(b)). On reduction of the driving voltage to below threshold electrons can still be emitted from the filled, and captured by empty, anode donor sites sustaining emission and providing memory. This process is very similar to that found by Zhu, who utilized a higher energy interface state to realize an electron source at energies higher than the Fermi level [ 151, [ 161.

The concomitant reduction of AV, with increase in a red EL emission may be accounted for as a reduction of uncompensated donors by an increase in recombination centers in the bulk ZnS. Finally, illumination of the ZnS : Mn layer by ultraviolet light would produce the same effect in these devices as with a conventional non-memory device; no enhancement to emission. The number of elec- trons being induced into the conduction band to be swept across the active layer is dependent only on the number of electron-hole pairs produced, and not on a field en- hancement as in the case of Yamauchi's device.

Ageing measurements have been carried out on Zn-rich devices with the optimum memory margin. Results are extremely promising, compared to those obtained by the Yamauchi-type device. Fig. 8 illustrates a typical thresh- old voltage and AV,,, variation over an operating period of 1000 h. The devices were driven with a 5-kHz signal and in constant luminance mode, corresponding to the mem- ory threshold brightness at zero hours (approximately of the maximum brightness). After a 20-h burn-in period AV,

~

90 I

Shallow Donor Centres

(b) Fig 7 (a) Unbiased and (b) biased energy band diagrams for the memory

device, indicating the contribution from donor site\

Time (hours)

Fig. 8. Ageing measurements for memory ACTFEL with optimum Mn concentration.

is fairly stable, with a slight downward shift near the end of the period. This variation is similar to that of threshold and is comparable to the stability of MOCVD-grown de- vices [20]. The maximum luminance did not vary until 600-h operation when it began to slowly reduce. The rea- son for such stable operation is not yet fully understood, but these results are very encouraging, as devices used were not optimized in any way. With a better understand- ing of the memory process further advances in stabilizing operation time may be possible.

IV. CONCLUSIONS

In summary, we have grown ZnS:Mn ACTFEL de- vices with Mn concentrations between 0.2 and 0.7 wt % (+0.2%) which exhibit memory. The maximum memory margin observed is at lower concentrations than has pre- viously been reported. This occurred at a concentration of 0.3 wt % ( f 0 . 2 % ) Mn, compared to the typically re- ported optimum concentration of 1 wt % or above. The memory margin at this concentration is twice that of any previously reported device. By varying the Mn concentra- tion and post-deposition annealing times the memory

902 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 40, NO. 5, MAY 1993

margin can be altered. Operational lifetimes are as stable as has previously been encountered for memory emission.

A donor site has previously been observed in low-field measurements to exist at 0.11 below the conduction band [ 2 2 ] . Memory has been proposed to be a result of electron capture and subsequent emission on field reversal from these donor near the interface. Emission of the donors (which are believed to be empty before the drive voltage has been taken over threshold, due to Fermi level pinning by the interface states) may take place at voltages below threshold thus providing the memory effect. As-grown films have comparable brightness to annealed ones as well as exhibiting the memory effect. This is a very useful fea- ture for optoelectronic integrated circuits where the ACT- FEL devices are grown directly onto substrates which al-

[I71 W. M. Cranton, l . P. McClean, D. M. Spink, R. Stevens, and C. B. Thomas, Semicond. Sci. Techno/. (submitted 1992).

[18] 0. Goede, and D. D. Thong, Phys. Sratus Solidi (b) , vol. 124, p. 343, 1984.

[I91 J . Benoit, P. Benalloul, A. Geffroy, N. Balbo, C. Barthou, J . P. Denis, and B. Blanzat, Phys. Srarus Solidi ( a ) , vol. 83, p. 709, and 1984.

[20] K. Hirabayashi and H. Kozawaguchi, Japan. J . Appl. Phys., vol. 25, p. L379, 1986.

[21] K. Taniguchi, K. Tanaka, T. Ogura, Y. Kakihara, S. Nakajima, and T . , Inoguchi, in Proc. 4rh Display Res. Conf: (Society for Informa- tion Display, Los Angeles, CA, 1984), p. 89.

[22] I. P. McClean, and C. B. Thomas, Semicond. Sci. Technol., vol. 7, p. 1394, 1992.

1231 N. Riehl, J. Lumin., vol. 24/25, p. 335, 1981. [24] A. G. Fischer, “Electroluminescence in 11-VI compounds,” U.S.

Air Force Cambridge Res. Lab., Office of Aerospace Research, Tech. Rep., p. 541, 1964.

[25] I. P. McClean and C. B. Thomas, J. Appl. Phys., vol. 72, p. 4749, 1992.

readv Dosses on-board drive circuitrv. [26] W. Hoogenstraaten, Philips Res. Reps., vol. 13, p. 357, 1958.

it for display applications.

ACKNOWLEDGMENT The authors wish to thank Ultra Silicon Technology and

colleagues whose help is greatly appreciated; W. M. Cranton, Dr. A. J . Simons, R . Stevens, B. Mickle- thwaite, and Dr. D. M. Spink.

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(Budapest, Hungary, Akad, Kaido 1968), p. 1177. [7] 0. Sahni, P. M. Alt, D. B. Dove, W. E. Howard, and D. J. McClure,

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639, 1982. [9] 0 . Sahni, W. E. Howard, and P. M. Alt, IEEE Trans. Electron De-

vices, vol. ED-28, p. 459, 1981. [ lo] V Marrello and A. Onton, J. Electrochem. Soc., vol. 127, p. 2220,

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Bradford to begin Ph.D ZnS for electroluminesc towards the ac electrolL

Mr. McClean is an ai gineers in England.

Ian P. McClean was born in England. He re- ceived the B. Eng. (honors) degree in electrical engineering (extended and enhanced) in 1989 and the M. Eng. degree in electrical engineering (ex- tended and enhanced), both from the University of Bradford, United Kingdom.

He was a design engineer for Marconi Instru- ments in England while working towards the M. Eng. degree. He has also worked for GEC Plessey Telecommunications in Auckland, New Zealand. In January 1990 he returned to the University of

8 . research into molecular beam epitaxial growth of :ent applications. His work has recently been aimed iminescent device field for display applications. ssociate member of the Institution of Electrical En-

Clive B. Thomas was born in Wales, United Kingdom. He received the B.Sc. and M.Sc. degrees from the University of Cardiff, Cardiff, Wales, UK, and the Ph.D. degree from the University of Bath, Bath, UK. His Ph.D. work was concerned with thin film growth and analysis.

Subsequently, he worked at R.S.R.E., Malvern, UK, in several project fields, including that of chalcogenides glass analysis. Presently, he is a Reader in Physical Electronics in the Department of Electrical and Elec- tronic Engineering at the University of Bradford, Bredford, UK, and has held a consultancy at AT&T Bell Laboratories, Holmdel, NJ, since 1989. He is the author or co-author of over seventy papers and several patents, principally on the physics of thin films and associated devices. His partic- ular interest since the 1970’s has been thin film dc and ac electrolumines- cence.