US3445724A - Solid state electroluminescent device - Google Patents

Description

United States Patent 3,445,724 SOLID STATE ELECTROLUMINESCENT DEVICE John J. Lambe, Birmingham, and William C. Vassell, Dearborn, Mich., assignors to Ford Motor Company, Dearborn, Mich-., a corporation of Delaware Filed Aug. 31, 1966, Ser. No. 576,372 Int. Cl. H0511 33/10; H01j 63/04 US. Cl. 315-246 ABSTRACT OF THE DISCLOSURE A solid state electroluminescent device is formed of a semiconducting crystal of cadmium fluoride and an ionically conductive solid conductor of beta alumina. The two materials are in intimate contact with one another at an interfacial area and an electric potential is established between the two materials.

This invention relates to the art of electroluminescence and more particularly to a process and device for the generation of light by the passage of an electrical current across the interface between a semiconducting crystal and an ionically conductive solid conductor.

The invention has been elucidated by three figures of drawings. Each of these rfigures of drawing are strictly schematic and are designed to explain the theory of operation of the invention.

FIGURE 1 represents schematically an electroluminescent device in which a single semiconducting crystal is placed in contact with a single ionically conductive solid conductor.

FIGURE 2 is similar to FIGURE 1 except that in this case the single semiconducting crystal is sandwiched between a pair of ionically conductive solid contuctors.

FIGURE 3 is similar to FIGURE 1 and has been modified to show the probable mechanism by which this invention operates.

Any suitable semiconductor crystal may be employed in the structures shown in the drawing, but the preferred substance is cadmium fluoride. The cadmium fluoride is doped with either Samarium, gadolinium, terbium, europium or mixtures of these elements. A preferred structure is cadmium fluoride doped with 0.1 mole percent europium. It will be noted that the doping elements are adjacent lanthanides having atomic numbers from 62 to 65.

The preferred substance to be employed as the ionically conductive solid conductor is beta alumina. Beta alumina or sodium beta alumina is a material conventionally represented by the formula Na O-1lAl O and may be thought of as a series of layers of A1 0 held apart by a column of linear Al-O bond chains with sodium ions occupying sites between the aforementioned layers and column. It is widely used in the manufacturing of refractory bricks for lining furnaces subject to corrosion from a basic melt and/ or slag.

According to our present understanding of the electroluminescence process, the light is produced by the action of a strong electric field upon a thin surface layer of the electroluminescent, semiconducting material. The origin of this field is, presumably, the charged ions in the electrolyte which cannot enter the semiconductor and collect at the interface between the electrolyte and the semiconductor. The externally applied electric field drives 7 Claims 3,445,724 Patented May 20, 1969 "ice the ions against the interface. A schematic picture of this situation is shown in FIGURE 3. If the ionic conductor is a liquid, there is no problem of getting the mobile negative ions into intimate contact with the semiconductor surface and electroluminescence is observed. However, if the ionic conductor is a solid, there is a real possibility that the surfaces of the semiconductor and ionic conductor will be too rough to allow the ions to get close enough to the semiconductor to generate electroluminescence.

A practical device using a solid ionic conductor was constructed as follows. A 1 cm. diameter x 0.3 cm. thick disc of cadmium fluoride doped with 0.1 mole percent europium was prepared with one face polished on a felt pad impregnated with one micron diamond particles. The electrode contacting the opposite face was made of an indium-tantalum-mercury amalgam with a drop of conducting epoxy to connect the electrical lead to the electrode. The ionic conductor was made from a similar sized disc of ,B-alumina whose room temperature resistivity was approximately 300 ohm cm. One face of this slab was also polished on a felt pad impregnated with one micron diamond particles. The conducting electrode attached to the 3-alumina was simply the mechanical contact between a metal plate and the surface opposite the polished surface. The polished surfaces on the fi-alumina and on the cadmium fluoride were brought together and held with a small force and a voltage of approximately volts was applied. Light was emitted from the interface. The light came from many spots with a variety of sizes. These were presumably regions of very good mechanical contact. Wetting the surfaces with a very concentrated sodium hydroxide solution before bringing them into contact served to make an excellent contact and light was observed to come uniformly from the entire interface.

The collection of interfaces between the metal wires, the semiconductor and the ionic conductor are easily polarized by the application of a DC electric field. When polarized, the intensity of light emission falls off. For this reason the best source of voltage is an AC voltage which supplies a backwards voltage and depolarizes the interfaces every cycle. In fact, the best results have been obtained by using a backwards voltage approximately three times the value of the forward voltage. Thus the best results were obtained with a square wave voltage source which applied approximately 100 volts for about 50 milliseconds in the forward (light emitting) direction and approximately 300 volts for 50 milliseconds in the backwards direction.

We claim as our invention:

1. A solid state electroluminescent device comprising a semiconducting crystal, an ionically conductive solid conductor of beta alumina in intimate contact with the semiconducting crystal and means for establishing an electrical potential difference between the semiconducting crystal and the ionically conductive solid conductor.

2. The structure recited in claim 1 in which the semiconducting crystal is cadmium fluoride.

3. The structure recited in claim 2 in which the cadmium fluoride semiconducting crystal is doped with samarium, gadolinium, terbium, europium or mixtures of these elements.

4. The structure recited in claim 1 in which the contact between the semiconducting crystal and the ionically conducting solid conductor of beta alumina is enhanced by the addition of a solution of an electrolyte.

3 4 5. The structure recited in claim 4 in which the elec- References Cited trolyte is a strong solution of sodium hydroxide. UNITED STATES PATENTS 6. The process of generating light comprising passing an electrical current across an interface between a semi- :12: conducting crystal of cadmium fluoride and an ionically 5 3271198 9/1966 fg 'gg 'f X conducting solid conductor of beta alumina, the semicon- 3:350:597 10/1967 Cole et a1. 313 1O8 ducting crystal and ionically conducting solid conductor being in intimate contact at an interface to :permit the JAMES W LAWRENCE, P i E i passage of current.

7. The process recited in claim 6 in which the applica- 10 PALMER DEMEO Amstant Exammer' tion of a voltage in the forward direction is followed by CL the application of a high voltage in the backward direction. 313 10

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