IEEE TRANSACTIONS O N ELECTRON DEVICES, VOL. ED-30, NO. 5 , MAY 1983 475

The Mn02 layer imparts some stability to the all thin-fdm structure making it possible to obtain I - V and B-I data up to current densities of 2.5 A/cm2. Efficiencies of 0.005 were ob- tained when 20 ft * L was induced with 50-mA 20-ps, 0.5-per- cent duty pulses. The black MnOz yields good contrast, but unfortunately, after 24-h running, unencapsulated samples deteriorate badly, partly through an increase in threshold volt- age and more seriously through gross loss of areas of MnOz and accompanying A1 cathode. Although these devices are at present of no importance to the display engineer, they may help to identify the conditions under which stable directly- coupled operation may be possible.

CONCLUSIONS Laboratory tests show that OMCVD ZnS : Mn fdms yield elec-

troluminescent properties whlch are comparable with those of films produced by other techniques. In particular, it would be worthwhile to investigate their suitability for capacitatively coupled (ac) film devices. They should also bring nearer the possibility of a directly coupled thin-fdm-only device. Most importantly perhaps, the OMCVD films have potential in an effective, alternative, hybrid-DCEL technology based on com- posite structures. These could have improved contrast if darker powders are used, higher saturation brightness, and no need of an expensive and variable forming process. Several problems remain to be solved before such devices could become competi- tive, perhaps the most important being the achievement of a scaled-up OMCVD process for ZnS :Mn with closer control of film thickness and surface roughness.

ACKNOWLEDGMENT The authors thank J. Evans for taking some of the data and

P. Lloyd and his staff for providing the cadmium stannate, Mn02 ) and aluminum films used in our devices.

REFERENCES A. F. Cattell and A. G . Cullis, “The variation in the luminescent and structural properties of sputter-deposited ZnS:Mn thin films with post deposition annealing,” Thin Solid Films, vol. 92, p. 21 1, 1982. P. J. Wright, B. Cockayne, A. F. Cattell, P . J . Dean, and A. D. Pitt, “Manganese doping of ZnS and ZnSe epitaxial layers grown by organometallic chemical vapour deposition,” J. Cryst. Growth, vol. 59, P. 155. 1982, also G. T. Blackmore, J. Cryst. Growth, vol. 56, 198i, to be published. T. Inowchi. M. Takeda. Y. Kakihara. Y. iKakata. and M. Yoshida, “Stabl; high brightness thin film electroluminescent panels,” in SID Int. Symp. Dig. Tech. Papers, p. 84, 1974. J . Kirton, “Panel electroluminescence the second time around: A successful display in the 1980’s?” in h o c . 1981 European Display Res. Con&, p. 144, 1981. C. J . Alder, A. F. Fray, C. Hilsum, and P . Lloyd, “A composite ZnS thin film powder electroluminescent panel,” Displays, vol. 1, p. 191, 1980. A. Vecht, N. J. Werring, R. Ellis, and P, J. F. Smith, “Direct-cur- rent electroluminescence in zinc sulphide: State of the art,” Proc. IEEE, vol. 61, p. 902, 1913. C. J. Alder, A. F. Cattell, K. F. Dexter, M. Dixon, J. Kirton, and M. S. Skolnick, “An investigation of the electrical and optical properties of dc electroluminescent ZnS : Mn, Cu-powder panels,” IEEE Trans. Electron Devices, vol. ED-28, p. 680, 1981. A. Vecht, N. J . Werring, R. Ellis, and P.J.F. Smith, “Materials con- trol and DC electroluminescence in ZnS:Mln, Cu, C1 powder phos- phors,” J. Phys. D., vol. 2, p. 953, 1969. S. L. McCarthy and J . Lambe, “Thin film electroluminescence in impurity doped Al203,” Appl. Phys. Lett . , vol. 37, p. 554, 1980.

DC Electroluminescence in Novel n-p Si/ZnS: Mn Heterostructures

J. M. GALLEGO, H. S. REEHAL, AND C. B. THOMAS

Abstract-The behavior of a novel n+-p Si/ZnS : Mn/ITO light-emitting structure capable of limiting the device current during luminescence is described. In contrast with the very strong dependence of current on voltage observed during luminescence in conventional thin-film ZnS

Manuscript received on October 6, 1982; revised December 31, 1982. This work was supported by S.E.R.C. and C.V.D.

J. M. Gallego was with the Department of Applied Physics, Uni- versity of Bradford, Bradford, West Yorkshire, U.K. He is now with Pilkington Brothers, Ltd., Research and Development Laboratories, Latham, Ormskirk, Lancs, L40 5UF, U.K.

H. S. Reehal and C. B. Thomas are with the Department of Applied Physics, University of Bradford, Bradford, West Yorkshire, BD7 lDP, U.K.

electroluminescent structures, the present strcciure exhibits a pro- nounced “knee” or reduction in the rate of 6;rowth of current with voltage, which coincides with the onset of electroluminescence. Typi- cally, a decay in brightness of <5 percent is observed after twelve-hours continuous operation when operating at -15 f t . L using 0.3-percent duty cycle,.20-~s wide dc pulses.

E INTRODYCTION

LECTROLUMINESCENCE in a simple thin-film struc- ture fabricated from consecutive layers of tin oxide or

indium-tin oxide (ITO), ZnS doped with Mn and aluminum possibly results from impact excitation, either directly or

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416 IEEE TRANSACTIONS O N ELECTRON DEVICES, VOL. ED-30, NO. 5 , MAY 1983

C O L L E C T O R C O N T A C T

II .TO. O R A L I +

- E M I T T E R C O N T A C T

Fig. 1. Schematic cross section of the MISE device structure.

indirectly, of the Mn ions by "hot" electrons. The nature c f the junction, where the barrier height is considerably great1:r than kT, limits the injection of carriers; thus luminescence ,s, necessarily accompanied by avalanching or similar high-fie;.d behavior. In the present paper we report the characteristics, and behavior of ZnS films deposited upon Si, where the elel:. tron affinities are comparable. In particular we explore the behavior of heterojunctions formed between a film of ZnS : M.n deposited upon an n+-p Si diode to form a metal or ITO/''CO.II. ducting insulator"/p-n+ silicon electroluminescent structu:le (MISE). Thus in an electrical sense, it resembles the metal/"i:ll. sulator"/p-n silicon switches (MISS) reported previously .in the literature [2], [3], where, however, the insulating laye:,$ were formed from thin films of SiOz (<45 A) or SiO,N, (<400 A).

PREPARATION Prototype devices were fabricated using the structure shown

in Fig. 1. P regions with an average acceptor concentraticn of cm-3 were diffused to a depth of -10 pm into 11.

type epi-layers deposited onto n+-Si substrates. Then t l t doped Si substrates were diced, chemically cleaned, and heill: treated a t 500°C for -30 minutes in a vacuum of to:r to remove adsorbed moisture. ZnS : Mn films with thickness(:!; between 2000 and 4000 were deposited, by RF sputteriq a suitable solid target, on to the p surfaces of the diodes. Dur. ing the sputtering process the n+-p-Si substrates were maill. tained at a temperature of -2OO0C. Atomic absorptilcn spectroscopy showed an Mn-doping level of -0.7 at. percent. Then either transparent IT0 or A1 films of 1 -mm diameter we1 e: deposited onto the ZnS fdms to provide electrodes and cor:>. plete the structures. By comparison with Kroger and Wegem::. [2] we shall refer to the n', p, and ZnS layers as the emittcr, base, and collector regions, respectively as shown in Fig. I, and ignore the presence of the atmospherically-induced th.n (<20 A) SiOz films existing between the base and co1lecto.r regions.

For comparison with the behavior of the MISE devices, similar films of ZnS : Mn were deposited upon both p+-dopc: ti and n+-doped Si substrates and upon ITO-coated glass su 3-

strates. Again electrodes of either aluminum or I T 0 filnu were deposited upon the ZnS : Mn layers.

ELECTRO-OPTICAL CHARACTERISTICS OF THE HETEROSTRUCTURES

Shown in Fig. 2, for reference, are the current-density/ele ;- tric-field characteristics of ZnS : Mn films with different thick. nesses. The films were deposited, by RF sputtering the san.e

10'' 1 o4 l o 5 lo6 10'

Field I Vcm-' I

Fig. 2. Current-density/electric-field characteristics for glass/ITO/ ZnS: Mn/M devices having ZnS: Mn film thicknesses of 500 A (v), 2000 A (A), 2700 A (m), 3700 A (n), and 5500 A ( 0 ) .

target and using the same conditions which were described previously, upon ITO-coated glass. Then the ZnS layers were coated with A1 films to fabricate sandwich structures. For these glass/ITO/ZnS : Mn/Al structures, the onset of electro- luminescence occurred at fields of -9 X lo5 V cm-'. How- ever, meaningful luminescence, Le., brightness >10 ft * L using dc pulses of 2 0 ; ~ s width with a 0.3-percent duty cycle, re- quired electric fields of -1.2 X IO6 V cm-' . As shown in Fig. 2, avalanching-type of behavior occurs at this value of the electric field. (Typically these devices have a life of ap- proximately 2 h when the initial brightness is -12 ft e L; the lifetime is defined as the time to reach half brightness. Oc- casionally devices exhibited lifetimes of 12 h).

Shown in Fig. 3 are the current-voltage characteristics of ITO/ZnS : Mn/n'-Si, ITO/ZnS : Mn/p+-Si, and A.l/ZnS : Mn/p-n+- Si structures. Also shown for comparison is the behavior of a glass/ITO/ZnS : Mn/M device. The ZnS : Mn film thicknesses for all these structures were in the range -1800-3000 8. When the IT0 contacts of the ITO/ZnS : Mn/n+-Si structures were biased positively, then negligible currents flowed even at electric fields as high as 1.8 X lo6 V cm-'. The curve, for these particular structures shown in Fig. 3 refers to the reverse bias condition, Le., the n+-Si biased positively. The dependence of current upon voltage shows a low-field region labelled AB where the current ( I ) is related to the voltage (VI as

I - exP (VPO) (1)

and a high field region, where the current increases consider- ably faster than the rate of increase of the applied voltage, i.e., region BC. This latter region, which occurs at field strengths > 1.2 X lo6 V cm-' (a figure obtained by assuming that all the voltage drops across the ZnS:Mn film) suggests an avalanching type of process and has a qualitative appear-

GALLEGO et al.: DC ELECTROLUMINESCENCE IN NOVEL n-p Si/ZnS:Mn HETEROSTRUCTURES 417

1

1 0- & L

10 20 30 40

V O L T S

Fig, 3. Current-voltage characteristics of various device structures: (a) Al/ZnS: Mn (-0.3 pm)/p-n+-Si, (A) ITO/ZnS : Mn (0.18 pm)/P+- Si, (e) ITO/ZnS: Mn (0.18 pm)/n+-Si, and (0) ITO/ZnS: Mn (0.20 m ) l i U .

ance similar to that shown for the ITO/ZnS:Mn/Al structures of Fig. 2. However, at lower field strengths the current density (J) and electric field ( t ) of these latter ITO/ZnS :Mn/Al de- vices are related according to the simple space charge limited expression

J-- L t2

( 2 )

where L is the thickness of the ZnS layer. This difference s u g gests that at low fields the conduction mechanism in ZnS films is controlled strongly by the nature of the injecting electrodes. Electroluminescence occurred from these ITO/ZnS : Mn/n+-Si structures but was extremely weak and unstable, as observed by eye. The light was not emitted from a uniform region but occurred in various regions of the surface. Also, the emissive regions moved as the applied voltage was varied.

The ITO/ZnS :Mn/p+-Si structure has characteristics similar to those observed from ZnS :Mn films deposited upon n+-Si substrates. Current flow, as demonstrated by the appropriate curve of Fig. 3, occurs when the p+-Si is biased negatively and has a significantly larger magnitude than in the ITO/ZnS : Mn/n+- Si case. Again, at relatively low fields, Le., region AB of Fig. 3, the current ( I ) and voltage ( V ) are related by the description of (1). At higher fields p 1 . 2 X lo6 V cm-l), avalanching or a similar behavior may be occurring, and also at these fields luminescence occurs. Again, the emission, although brighter than from the previous structure, is weak and patchy and also varies spatially as the voltage is changed. Apart from the weak and almost totally absent emission, these structures demon- strate the worst feature of the simple glass/ITO/ZnS :Mn/Al structure-namely a high probability of current runaway at the onset and during emission (a typical curve for this latter structure has been included in Fig. 3 for comparison).

The characteristics of the heterostructure of Al/ZnS : Mn/p-n+- Si are shown also in Fig. 3 when the n+ emitter (see Fig. 1) is biased negatively (forward bias condition), No appreciable current flow occurred with the reverse bias. These MISE de-

012

- 1

- 0

- -1

- r 2

VOLTAGE,volts

Fig. 4. Current- (v) and brightness-(e) voltage characteristics of an ITO/ZnS:Mn/p-n+-Si structure utilizing a -10.4-pm thick film of ZnS : Mn.

vices, unlike any of the previous structures, show no presence of an avalanching type of characteristic in the device as a whole. In the relatively lowfield region, Le., below the “knee,” the current-voltage characteristics are described again by (1). However, above the knee th.e behavior of Fig. 3 indicates a “negative deviation” from the exponential be- havior of (1). Fig. 4 illustrates typical current-voltage-bright- ness characteristics for ITO/ZnS : Mn/p-n+-Si heterostruc- tures. The electrical characteristics are similar to those of the aluminum electroded MISE, but the “kneeing” of the current-voltage characteristics corresponds to the onset of electroluminescence. For the measurements illustrated in Fig. 4 the same driving conditions described previously have been used, Le., a dc pulsewidth of 20 ps and a duty cycle of 0.3 percent, Apart from, very importantly, limiting the cur- rent during luminescence, these heterostructure devices typically show a decay in brightness of <S percent after 12-h continuous operation when operating at -15 ft * L. A similarly small reduction was observed when these MISE de- vices were driven with dc pulses of 4-ms duration at a duty cycle of 0.4 percent. This should be compared with the ITO/ZnS:Mn/Al structures which had a half-life, at best, of 5 12 h at -12 ft . L. Hence the inbuilt prevention of current runaway or current limiting appears t o dramatically increase the lifetime of electroluminescent devices.

DISCUSSION The characteristics described in this paper indicate that

only the ITO/ZnS :Mn/p-n+-Si heterostructure can be thought of as a potentially useful device. The internal prevention of current runaway obviously enhances the lifetime of electro- luminescence. We shall consider a possible qualitative descrip- tion of the operating mode of these MISE devices. Fig. 5(a) illustrates a probable equilibrium band structure of the hetero- junction devices since the electron affinitilas of ZnS and Si are comparable [4], [SI. Fig. 5(b) shows the probable energy- level diagram when the device is forward biased. These devices are not dissimilar to those described by Kroger and Wegener [2], where the insulating region was provided by thin films of various dielectrics. Even at low fields, as pointed out by Kroger and Wegener, these dielectric layers were relatively

418 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-30, NO. 5, MAY 1983

i

ZnS:Mn p . S i $.Si

-

Fig. 5. Schematic energy level diagrams of the MISE structure und1:r (a) equilibrium conditions and (b) forward bias. The effects of interface states have been ignored.

“conductive.” Hence the application of an appropriate bias would result in the creation of an expanding depletion layer in the p-Si, due to charge leakage, and not inversion as in a conventional MOS structure. Thus by analogy and since th.8 characteristics of Fig. 3 reveal that the ZnS:Mn films are C O I -

ductive at high fields, the application of a positive bias t o th: collector probably induces a depletion layer of width Wl:, in the base of the MISE structures. Hence when a voltage VL#, is applied across the emitter-collector of a MISE, t h s may b: divided up as potential drops across the ZnS : Mn, the deple- tion layer of width WD and the forward-biased p-n+-Si junc- tion. Hence VA may be expressed simply as

where

VI is the voltage across the insulator VD is the voltage across the depletion layer VEB is the emitter-base voltage.

The current-voltage characteristics of Figs. 3 and 4 for MIS]; devices suggest the possibility of two different mechanism; controlling the current in the regions below and above tho “knee.” Also, at the voltage corresponding to the knee, ligh: emission occurs. In the other devices investigated there ma:!’ be different mechanisms responsible for the low-field regions, as evidenced by (1) and (2). However in the simple devices tht: experimental evidence suggests very strongly that the creatiolr~ of “hot” electrons, possibly by avalanching, is necessary fo: luminescence. Since the onset of luminescence in MISE de:. vices probably occurs at comparable fields in the ZnS : Mn, i.e., -8 X lo5 V cm-’ , it is not unlikely that “hot” electron crea.. tion occurs; probably also avalanching which is, however, con. trolled internally. Thus for voltages corresponding to field;; of 68 X lo5 V cm-’ , we shall assume that VI >> V D , VEB Hence ( 3 ) becomes simply V VI.

At fields exceeding -8 X lo5 V cm-’ , as the ZnS:Mn laye:. starts to become more conductive, it is likely that the appliec. voltage is shared between the ZnS:Mn layer and other ele’ ments of the device. We shall assume that VI becomes equa. t o some approximately constant voltage (=V,) correspondini;

to that necessary for the creation of “hot” electrons within the ZnS layer. Furthermore, we shall suppose that the emitter- base voltage is relatively small and can be ignored. Thus at high voltages, i.e., above the knee in the current-voltage char- acteristic, (4) becomes

V- - V , = V o . (4)

Now assuming a uniformly doped base region

where N is the acceptor concentration and E is the dielectric constant of Si5. Hence (3) can be re-expressed as

For a relatively lightly doped base of a MISS device (i.e., N 5 lo1’ ~ m - ~ ) “punchthrough” to the emitter junction depletion zone could occur when WD equals the width of the base region measured from the semiconductor surface to the edge of the emitter junction-depletion zone. When punchthrough occurs in MISS structures, the devices switch to a low-impedance state, where voltage is dropped predominantly across the in- sulator [2], [3]. For a MISS device with a heavily-doped base (N 2 5.1016 ~ m - ~ ) , avalanching occurs in the narrow deple- tion region formed at the insulator-base interface; this also leads to a switching [3].

Thus by comparison with MISS devices, the width and doping of the p-layer of MISE structures, as expressed by ( 9 , may well determine the maximum value of V, - V,. The depletion region under the ZnS:Mn in the p-layer may also be acting as a nonlinear resistance element. Experimen- tally we have observed that the present MISE structures utilizing -4000-8 thick ZnS : Mn layers break down elec- trically at applied voltages of -80 V. However, we cannot say whether switching of the whole device to a lower im- pedance state, with concomitant increase of the current injected into ZnS, has occurred. This and other questions (such as equipping the base region of Fig. 1 with an injecting contact to realize three terminal structures) form part of the on-going research program.

Finally, for devices with very heavily doped base regions, V D in (4) should be negligible at high fields, and thus V, VI. We have experimentally observed evidence for this in MISE structures fabricated upon substrates with p+ base re- gions. These devices exhibit electrical behavior not unlike that of the ITO/ZnS : Mn/p+-Si structure shown in Fig, 3; no evidence for a “knee” in the current-voltage characteristics is seen up to breakdown. However, light emission comparable to the ITO/ZnS: Mn/Al structures is obtained, unlike the emis- sion from ITO/ZnS : Mn/p+-Si devices.

ACKNOWLEDGMENT The authors acknowledge the assistance of G. Jackson in de-

vice fabrication and A. J. Warren for the data of Fig. 2. Special thanks to Dr. A. Tsui for providing the silicon substrates.

REFERENCES [3] J. G. Simmons and A. El-Badry, “Theory of switching phenomena in metal/semi-insulatorjn-p+ silicon devices,” Solid-State Elec-

8 216 684,1982. [4] A. G. Milnes and D. L. Fencht, Heterojunctions and MetaI-Serni-

of two terminal inversion-controlled switches,” Solid-state Elec- [5] S . M. Sze, Physics of Semiconductor Devices. New York: Wiley, tron., vol. 21, p. 643-654, 1978. 1969.

[ l ] J. M. Gallego, H. S. Reehal, and C. B. Thomas, British patent tron., vol. 20, p. 955-961, 1977.

[ 21 H. Kroger and H. A. Richard Wegener, “Steady-state characteristics conductor Junctions. New York: Academic Press, 1972.

Novel Synthesis Technique for Zinc-Silicate Phosphors

Abstract-A novel but simple synthesis technique for producing improved zinc-silicate phosphors with improved efficiency and per- sistence control as well as tight and uniform particle-size distribution is described. Silicic acid powder with a predetermined range of particle sizes is first coated with a solution containing manganese and arsenic ions and very f i ie zinc-oxide powder (100 A) and then Tied in a wet nitrogen furnace at 1180°+/- 50 C for 15+/-5 minutes. This method eliminates grinding and produces P1- and P39-type phosphors with cathodoluminescence performance superior to commercial samples.

INTRODUCTION

T HE initial interest in cathodoluminescent phosphor re- search was primarily motivated by the requirements of

television displays. With the television technology maturing and its market stablizing, the effort in phosphor research has been disappearing. However, recently, a growing demand for information display has placed different and more stringent requirements on CRT’s in general and on CRT phosphors in particular. This is because the requirements of information displays are quite different from those of television displays as shown in Table I. The information displays generally require more picture .elements and higher resolution and emphasize contrast rather than brightness, but high information content still demands high phosphor luminous efficiency. They also demand longer operating life from CRT’s due to their longer usage (hours per day) and high beam current operation. Since information displays mostly deal with static information, screen flicker due to short phosphor persistence (tolerable in TV’s dynamic image) becomes a very critical

Manuscript received September 17, 1982; revised December 13, 1982. The authors are with the IBM Thomas J. Watson Research Center,

Yorktown Heights, NY 10598.

TABLE I DIFFERENCES IN REQUIREMENTS BETWEEN INFORMATION DISPLAYS

AND TELEVISION DISPLAYS

w INFORMATION DISPLAY w TV(VIDE0) DISPLAY

210’’ PELS . 2 5 x lo1 . HIGH RESOLUTION . LOWER RESOLUTION

CONTRAST IMPORTANT - BRIGHTNESS IMPORTANT . 230 KHR DESIRED 2 T O 3 IKHR . STILL INFORMATION * D Y N A M I C

FLICKER CRITICAL SMEARING IMPORTANT

COLOR AND WHITE DESIRED COLOR NECESSARY

POLARITY CHANGE * N O ISSUE

problem. One can increase the refresh ra.te to reduce flicker; however, this increases the bandwidth required in logic and memory circuits. Furthermore, due to their applications in text processing, there is also an emphasis on white emitting phosphors with adequate persistence and a desire for alternate colors, not necessarily the primary colorsused in the TVtubes. All these point to the fact that the phosphors available to TV CRT’s today are not entirely satisfactory for information displays.

Manganese-doped zinc silicate is an exceptionally useful phosphor in information CRT’s due to its high efficiency, good electron burn resistance, and relatively long persistence. The zinc silicates have been known for many years in mineral form as willemite [ l ] and in commercial products as P1 and P39 [ 2 ] . P39 is zinc silicate doped with manganese and arsenic having extended persistence. In spite of their long history and extensive usage today, little systematic work has been reported that advances the understanding and optimization of these phosphors. As the demand for information displays is growing

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