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ELECTROLUMINESCENT FILM AND METHOD FOR PREPARING SAME Radio frequency sputtered films of zinc sulfide containing about 0.2 - 0.4 mol percent of copper and 0.05 - 0.8 mol percent of manganese are electroluminescent. Cells thereof have high brightness and high power efficiency.
Primary Examiner: Kaplan; G. L. Assistant Examiner: Langel; Wayne A. Attorney, Agent or Firm: 1. A method of preparing an electroluminescent film of zinc sulfide activated with manganese and copper which comprises radio frequency sputtering a film onto a substrate from a target comprising zinc sulfide partly overlaid with copper and manganese metal in an inert atmosphere at from 10 - 15 millitorr pressure at a substrate temperature of about 2. A method according to claim 1 wherein the substrate is glass coated with 3. A method according to claim 1 wherein the substrate temperature is 4. A method according to claim 1 wherein the manganese and copper are present in amounts sufficient to form a zinc sulfide film containing from about 0.05 - 0.8 mol percent of manganese and from about 0.2 - 0.4 mol 6. In a dc electroluminescent cell comprising a substrate, a conductive coating on the substrate, a dc electroluminescent layer on the conductive coating and an electrode on the dc electroluminescent layer, the improvement which comprises employing as the dc electroluminescent layer a 7. In a dc electroluminescent storage cell comprising a substrate, a conductive coating on the substrate, a photoconductive layer on the conductive coating, a dc electroluminescent layer on the photoconductive layer, and a conductive coating on the dc electroluminescent layer, the improvement which comprises employing as the dc electroluminescent layer a film prepared according to the method of claim 1. This invention relates to thin films of zinc sulfide which display dc electroluminescence of improved brightness and power efficiency. More particularly, this invention relates to thin films of zinc sulfide activated with manganese and copper prepared by radio frequency sputtering techniques. The invention hereunder was made in the course of or under a contract with the government of the United States. BACKGROUND OF THE DISCLOSURE Zinc sulfide activated with manganese and copper is known as a dc electroluminescent material and has been investigated in the form of powder layers, thin films and single crystals. The electroluminescent properties of activated zinc sulfide varies with the form of zinc sulfide and the method by which it is prepared. Zinc sulfide powders having a copper rich layer on the surface display high brightness values but they exhibit what is known as a "forming" process whereby a lag in time occurs between application of a current and onset of electroluminescent emission. On first applying a voltage to the cell, a high current flows but no electroluminescent emission is noted. Gradually the current reduces in value and electroluminescence commences and increases in value. This forming process can take from a few seconds to an hour or more depending upon the method of fabrication. This phenomenon is probably due to reaction or diffusion of the copper and zinc sulfide-mangenese composition. Forming is particularly disadvantageous when large area panel displays containing many electroluminescent cells addressable separately are to be fabricated. Single crystals of zinc sulfide have been made which exhibit dc electroluminescence, but they are very difficult to prepare and they cannot be made large enough for use in large panel display devices. Thin films of manganese-and copper-activated zinc sulfide have also been prepared by evaporation techniques. However, both deposition rate and power efficiency of the evaporated films vary according to the evaporation temperature. The deposition rate decreases rapidly in the range from about room temperature to about 260.degree.C., whereas the power efficiency increases rapidly in that range. In order to prepare films of high power efficiency at economic deposition rates, the films are evaporated at low temperatures and then annealed at high temperatures, i.e., about 700.degree.C. The resultant films display good brightness but their power efficiency is still low, on the order of 0.001 to 0.05 percent. Further this method severely limits the substrate which can be employed for the film because the substrate must be able to withstand high annealing temperatures without degradation. For example, quartz may be used, but inexpensive ordinary glass cannot be used. For use in large panel display devices, it would be highly desirable to obtain films of electroluminescent materials which can be made at low temperatures, and which do not exhibit a forming process. SUMMARY OF THE INVENTION It has been discovered that electroluminescent films of zinc sulfide-manganese-copper can be prepared by radio frequency sputtering techniques. These films display high brightness, have improved power efficiency and do not exhibit forming. The films described herein can be prepared at low temperatures and thus low melting, inexpensive substrates can be employed. These improved films can be used in low cost, large area flat panel displays as well as in cathodoluminescent screens and as transparent films. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a dc electroluminescent cell. FIG. 2 is a cross-sectional view of a laser addressable storage dc electroluminescent cell. FIG. 3 is a graph of the deposition rates of zinc sulfide, copper and manganese as a function of dc sheath voltage. FIG. 4 is a graph of the deposition rate of zinc sulfide as a function of sample number sputtered from a given target. FIG. 5 is a graph of copper and manganese concentration in sputtered zinc sulfide films as a function of sample number sputtered from a given target. FIG. 6 is a schematic arrangement of a target for cosputtering a three component film of zinc sulfide, manganese and copper. DETAILED DESCRIPTION OF THE INVENTION The films described more particularly hereinafter are useful in fabricating electroluminescent cells. FIG. 1 is a cross-sectional view of a suitable dc electroluminescent cell. The cell comprises a glass substrate 1 having a transparent conductive coating of indium oxide 2 thereon, a sputtered electroluminescent layer of zinc sulfide-mangenese-copper 3 and an evaporated aluminum electrode 4 which is 1/16 inch in diameter. The conductive coating 2 which acts as a first electrode and the electrode 4 which acts as the second electrode are connected to a battery 5, the source of dc power. When the power is turned on and the threshold voltage has been reached, electroluminescence is observed. As is known, the threshold voltage can be decreased, or the light output increased, if the cell is illuminated with UV light when the power is on. Thus when the UV light source 6 is turned on, electroluminescence of the cell is observed at lower operating voltage than when the UV light is removed. The films described herein can also be used in making laser-addressable storage dc electroluminescent cells. FIG. 2 is a cross-sectional view of such a storage cell comprising a glass substrate 10 having a transparent conductive coating of indium oxide 11, a cadmium sulfur selenide photoconductive layer 12, a sputtered electroluminescent layer of zinc sulfide-manganese-copper 13 and a transparent conductive aluminum metal electrode layer 14. The conductive layers 11 and 14 are connected to a battery 15, the source of dc power. When the voltage is applied and a scanning laser beam 16 impinges on a location of the cell, the photoconductor, which is chosen so as to be sensitive to both the laser light and the electroluminescent light, is activated. The voltage across the electroluminescent layer at this location is lowered and the electroluminescent light turns on. The electroluminescent light in turn keeps the photoconductor in its conductive state and the image is stored after the laser light has moved on to another location. At the end of the scan, the voltage to the cell is interrupted and the stored image is erased. Improved electroluminescent films of zinc sulfide-manganese-copper can be prepared by radio frequency (hereinafter r.f.) sputtering from a single multicomponent target onto a substrate in an inert atmosphere at temperatures of from about 260.degree.-500.degree.C. The target of zinc sulfide-manganese-copper is prepared by cold pressing a disk of zinc sulfide powder onto a suitable substrate, such as an aluminum platter. Manganese and copper metal pieces in the form of strips, plates, squares and the like are placed on the zinc sulfide in the required amounts, evenly distributed in a regular array so as to give a uniform composition on the substrate. Methods of calculating the amount of each material required to sputter a film of desired constant composition onto a substrate, based on the deposition rates and thickness of the film are given in copending application of Hanak entitled "Determination of the Composition of Co-Sputtered films and Preparation of Large area Co-Sputtered Films of Desired Compositions," Ser. No. 175,587 filed Aug. 27, 1971, now abandoned. Films suitable for electroluminescent display devices have the composition ZnS:Mn.sub.x Cu.sub.y wherein x can be from about 0.05 - 0.8 mol percent of the zinc sulfide, preferably about 0.6 - 0.8 mol percent, and y can be from about .001 - 0.4 mol percent of the zinc sulfide, preferably from about 0.2 - 0.4 mol percent. The nature of the substrate is not critical and depends on the application for which the electroluminescent film is to be used. For the preparation of electroluminescent cells, plates of glass or fused silica coated with a transparent conductive film, such as a film of indium oxide or tin oxide, can be employed. The substrate is mounted above and parallel to the target. The substrate can be held in a fixed position or can be oscillated during sputtering to improve film uniformity. The substrate is heated within the temperature range of about 260.degree. - 500.degree.C., preferably about 300.degree. - 425.degree.C., during sputtering. The temperature of the substrate affects the rate of deposition of the zinc sulfide and the dopants and thus must be closely controlled to obtain films of constant composition. The voltage applied during sputtering should be kept constant in order to obtain films of constant composition. By voltage is meant the dc sheath voltage which develops at the target during sputtering. The voltage applied will depend on the equipment employed but any wide variation in voltage during sputtering would result in a change in the composition of the sputtered film. FIG. 3 shows the variations in deposition rates in angstroms per minute for zinc sulfide (curve 1), copper (curve 2) and manganese (curve 3) as a function of increasing dc sheath voltage. It is apparent from the curves that large increases in the voltage applied to the system and developed at the target results in large changes in the deposition rates; and that the deposition rate of zinc sulfide increases more rapidly than the deposition rates for the metals. Thus variations in voltage will result in changes in the composition of the sputtered film. During sputtering, an inert gas atmosphere, such as of argon, neon, krypton and the like, of about 10 - 15 millitor pressure is maintained. The above conditions will provide sputtered films of zinc sulfide-manganese-copper of nearly constant composition at a rate of from about 500 to 900 angstroms per minute. However, as sputtering proceeds for lengthy periods, erosion of the target occurs and the rates of deposition of the components change. FIG. 4 shows the change in deposition rate of a zinc sulfide target for a series of 15 samples, taken consecutively over a period of 13 hours of sputtering time. The deposition rate decreases at about 3 percent per sample. FIG. 5 shows the change in deposition rates for manganese (curve 1) and copper (curve 2) activators sputtered from a zinc sulfide-manganese-copper target. Both activator deposition rates increase, but at different rates; about 7 percent of the initial rate for manganese and about 13 percent of the initial rate for copper. However, since these rate changes are fairly regular and predictable and can be determined for each target, they can be compensated for by lowering the substrate temperature at a controlled rate to obtain films of constant composition. Sputtering is continued until a film of the desired thickness is obtained. Films suitable for display devices are from about 0.2 - 3 microns thick, preferably about 0.2 - 2 microns thick. In general, electroluminescent brightness increases with increasing thickness of the film throughout the preferred range, when optimum operating voltage is employed. The invention will be further illustrated by the following examples but it is to be understood that the invention is not meant to be limited to the details described therein. EXAMPLE 1 Zinc sulfide powder was purified by firing in a silica tube in a stream of hydrogen sulfide at 1100.degree.C. for one hour, cooling and washing with hot toluene. The purified zinc sulfide powder was cold pressed at a pressure of 3 to 4 tons/inch onto an aluminum platter to form an 8 inch diameter disk. Nine squares of copper metal 0.1 inch .times. 0.1 inch (target area 0.009 inches.sup.2) were placed on the disk in regular order. A small piece of graphite sheet was placed under each copper square to prevent interaction between the copper and the zinc sulfide. Fifty-four squares of manganese metal 0.1 inch .times. 0.1 inch (target area 0.54 inches.sup.2) were also regularly placed on the disk. The approximate arrangement is shown in FIG. 6 wherein the square marked 1 and like squares represent copper and the square marked 2 and like squares represent manganese. The target prepared as above was placed in position as the cathode in an r.f. sputtering system. A glass plate 3.5 inches .times. 5 inches coated with a transparent film of indium oxide as the substrate, was mounted on a graphite holder, which had an imbedded thermocouple, parallel to and over the target and about 2 inches therefrom. The system was evacuated and an argon pressure of 12 microns maintained during sputtering. The substrate was heated to about 390.degree.C. The deposition rate was about 780 angstroms per minute. When a film about 2.3 microns thick had been sputtered onto the substrate, sputtering was discontinued and the substrate cooled. The resultant film was zinc sulfide containing 0.28 mol percent of copper and 0.71 mol percent of manganese, as determined by x-ray fluorescence spectroscopy. An electroluminescent cell was made from this film by applying silver dot electrodes on the surface of the film. When connected to a battery the cells had a maximum brightness of 440 foot-lamberts at 190 volts and a power efficiency of 0.1 percent. No forming was noted. EXAMPLES 2-3 The procedure of Example 1 was followed except using a target having 12 copper squares and 42 manganese squares and varying the substrate temperature. Results are summarized in the Table below. ______________________________________ Example Substrate Deposition Manganese Copper Temp. Rate Mol% Mol% .degree.C. A/min. ______________________________________ 2 260 1064 0.22 0.16 3 396 535 0.54 0.36 ______________________________________ For U.S. patent law, rules, and procedures see MPEP. Disclaimer. Information presented on this page while believed to be reliable, is provided "as is" with no warranties of its accuracy or timeliness. For legal advice seek help of a licensed professional. |