Electroluminescent displays (ELDs) have their origins in scientific discoveries in the first decade of the twentieth century, but they did not become commercially viable products until the1980s. ELDs are particularly useful in applications where full color is not required but where ruggedness, speed, brightness, high contrast, and a wide angle of vision is needed. Color Electroluminescent Display technology has advanced significantly in recent years, especially for microdisplays. The two main firms that have developed and commercialized ELDs are Sharp in Japan and Planar Systems in the United States.
There are two main ways of producing light: incandescence and luminescence. In incandescence, electric current is passed through a conductor (filament) whose resistance to the passage of current produces heat. The greater the heat of the filament, the more light it produces. Luminescence, in contrast, is the name given to “all forms of visible radiant energy due to causes other than temperature.”
There are a number of different types of luminescence, including (among others): electroluminescence, chemiluminescence, cathodoluminescence, triboluminescence, and photoluminescence. Most “glow in the dark” toys take advantage of photoluminescence: light that is produced after exposing a photoluminescent material to intense light. Chemiluminescence is the name given to light that is produced as a result of chemical reactions, such as those that occur in the body of a firefly. Cathodoluminescence is the light given off by a material being bombarded by electrons (as in the phosphors on the faceplate of a cathode ray tube). Electroluminescence is the production of visible light by a substance exposed to an electric field without thermal energy generation.
An electroluminescent (EL) device is similar to a laser in that photons are produced by the return of an excited substance to its ground state, but unlike lasers EL devices require much less energy to operate and do not produce coherent light. EL devices include light emitting diodes, which are discrete devices that produce light when a current is applied to a doped p-n junction of a semiconductor, as well as EL displays (ELDs) which are matrix-addressed devices that can be used to display text, graphics, and other computer images. EL is also used in lamps and backlights.
There are four steps necessary to produce electroluminescence in Electroluminescent Displays:
- Electrons tunnel from electronic states at the insulator/phosphor interface
- Electrons are accelerated to ballistic energies by high fields in the phosphor
- The energetic electrons impact-ionize the luminescent center or create electron-hole pairs that lead to the activation of the luminescent center
- The luminescent center relaxes toward the ground state and emits a photon.
All Electroluminescent Displays have the same basic structure. There are at least six layers to the device. The first layer is a baseplate (usually a rigid insulator like glass), the second is a conductor, the third is an insulator, the fourth is a layer of phosphors, and the fifth is an insulator, and the sixth is another conductor.
Electroluminescent Displays are quite similar to capacitors except for the phosphor layer. You can think of an ELD as a “lossy capacitor” in that it becomes electrically charged and then loses its energy in the form of light. The insulator layers are necessary to prevent arcing between the two conductive layers.
An alternating current (AC) is generally used to drive an ELD because the light generated by the current decays when a constant voltage is applied. There are, however, EL devices that are DC driven (see below).
Electroluminescent Scientific Origins
Electroluminescence was first observed in silicon carbide (SiC) by Captain Henry Joseph Round in 1907. Round reported that a yellow light was produced when a current was passed through a silicon carbide detector. Round was an employee of the Marconi Company and a personal assistant to Guglielmo Marconi. He was an inventor in his own right with 117 patents to his name by the end of his life.
The second reported observation of electroluminescence did not occur until 1923, when O.V. Lossev of the Nijni-Novgorod Radio Laboratory in Russia again reported electroluminescence in silicon carbide crystals.
B. Gudden and R.W. Pohl conducted experiments in Germany in the late 1920s with phosphors made from zinc sulfide doped with copper (ZnS:Cu). Gudden and Pohl were solid state physicists at the Physikalisches Institute. They reported that the application of an electrical field to the phosphors changed the rate of photoluminescent decay.
The next recorded observation of electroluminescence was by Georges Destriau in 1936, who published a report on the emission of light from zinc sulfide (ZnS) powders after applying an electrical current. Destriau worked in the laboratories of Madame Marie Curie in Paris. The Curies had been early pioneers in the field of luminescence because of their research on radium. According to Gooch, Destriau first coined the word “electroluminescence” to refer to the phenomenon he observed.
Gooch also argues that one should keep in mind the differences between the “Lossev effect” and the “Destriau effect:”
The Lossev effect should be distinguished from the Destriau effect. Destriau’s work involved zinc sulphide phosphors, and he observed that those phosphors could emit light when excited by an electric field…[The Lossev effect, in contrast, involves electroluminescence] in p-n junctions.
During World War II, a considerable amount of research was done on phosphors in connection with work on radar displays (which was later to benefit the television industry in the form of better cathode ray tubes). Wartime research also included work on the deposition of transparent conductive films for deicing the windshields of airplanes. That work was later to make possible a whole generation of new electronic devices.
In the 1950s, GTE Sylvania fired various coatings, including EL phosphors onto heavy steel plates to create ceramic EL lamps. During this period, most research focused on powder EL phosphors to get bright lamps requiring minimal power and with a potentially long lifetime. Research funding was cut back when it was determined that product lifetimes were too short (approximately 500 hours).
The first thin-film EL structures were fabricated in the late 1950s by Vlasenko and Popkov. These two scientists observed that luminance increased markedly in EL devices when they used a thin film of Zinc Sulfide doped with Manganese (ZnS:Mn). Luminance was much higher in thin film EL (TFEL) devices than in those using powdered substances. Such devices however were still too unreliable for commercial use.
Russ and Kennedy introduced the idea of depositing insulating layers under and above the phosphor layer on a TFEL device. The implications for reliability of TFEL devices was not appreciated at the time, however.
Soxman and Ketchpel conducted research between1964 and 1970 that demonstrated the possibility of matrix addressing a TFEL display with high luminance, but again unreliability of the devices remained a problem.
In the mid-1960s, there was a revival of EL research in the United States focused on display applications. Sigmatron Corporation first demonstrated a thin-film EL (TFEL) dot-matrix display in 1965. Unfortunately, Sigmatron was unable to successfully commercialize these displays and it folded in 1973.
In 1968, Aron Vecht first demonstrated a direct current (DC) powered EL panel using powdered phosphors. Research on powdered phosphor DC-EL devices continued, especially for use in watch dials, nightlights and backlights, but most subsequent research on ELDs focused on thin-film AC driven devices. An early example was the work of Peter Brody and his associates at Westinghouse Research Laboratories on EL and AM-EL devices between 1968 and 1974.
In 1974, Toshio Inoguchi and his colleagues at Sharp Corporation introduced an alternating current (AC) TFEL approach to ELDs at the annual meeting of the Society for Information Display (SID). The Sharp device used zinc sulfide doped with manganese (ZnS:Mn) as the phosphor layer and yttrium oxide (Y2O3) for the sandwiching insulators. This was the first high-brightness long-lifetime ELD ever made. Sharp introduced a monochrome ELD television in 1978. The paper Inoguchi published on his group’s research helped to reinvigorate EL research in the rest of the world, including at Tektronix, a U.S. electronics firm based in Portland, Oregon.
Tektronix’ research on EL began in 1976. The management at Tektronix were familiar with the work reported by Inoguchi’s team. They decided to start a new program on Electroluminescent Displays at Tektronix Applied Research Laboratories. The work begun there was continued when the Tektronix researcher left to create a spinoff firm called Planar Systems. Several other large U.S. companies also were conducting research on ELDs in the 1970s, including: IBM, GTE, Westinghouse, Aerojet General, and Rockwell. All these companies realized that ELDs had potential advantages over existing LCD technology in the following areas:
- Viewing angle.
The most important problem that had to be solved before mass production of Electroluminescent Displays could begin was increasing the reliability of the EL thin film stack. Since the devices operated at very high field levels — about 1.5 MV/cm — there was a high probability that they would break down, especially if there was insufficient uniformity in the stack. Sharp, Tektronix, and Lohja Corporation in Finland were able to solve this problem between 1976 and 1983 using slightly different approaches.
The second major problem was to get access to high-voltage drivers for the displays. Sharp ended up developing their own; Tom Engibous developed drivers for EL displays at Texas Instruments by modifying the design his group had done for plasma displays. Planar used the TI drivers in its products until it could find additional suppliers.
The introduction to the market in 1985 of Grid and Data General laptops with EL displays from Sharp and Planar respectively helped to build the foundations for the nascent laptop computer industry at a time when LCDs did not have sufficient brightness or contrast to be used in commercial products. Both Planar and Sharp monochrome ELDs used a phosphor layer made from zinc sulfide doped with manganese (ZnS:Mn). These displays gave off an amber (orange-yellow) color that was bright but also pleasing to the eye.
The History of Sharp’s Electroluminescent Operations
The head of research at Sharp, Sanai Mita, was convinced that Electroluminescent Displays could be used eventually to make flat TVs. Mito was formerly a professor at Osaka Municipal University. He and his team mounted a major effort in the mid 1970s to develop TFELs.
The key research at Sharp was done by Toshio Inoguchi and his colleagues. The successful demonstration of a working TFEL display in September 1978 at the Consumer Electronics Show in Chicago was the “finest hour” of Inoguchi’s group. This display was only a few inches in diagonal, but it was also only 3 cm thick.
Sharp began mass production of Electroluminescent Displays in 1983. One of its earliest displays was used in the U.S. Space Shuttle’s obital navigation system in that same year. Another early application of a Sharp ELD was in a Grid laptop computer. This display provided resolution equivalent to a quarter VGA (320×240).
1983 was also the year that Shinji Morozumi at Seiko announced that his group was able to build a TFT LCD television. That announcement took Sharp by surprise and they redirected their efforts toward catching up with Seiko in LCDs. By 1987, Sharp was able to market their own TFT LCD television. They were able to capitalize on their lead in mass production of STN LCDs for calculators to quickly develop production technologies for high-volume TFT manufacturing. After 1987, TFT LCD production was far more important to Sharp’s corporate strategy than EL production. Nevertheless, the firm remained active in both research and production of ELDs, providing strong competition to Planar and Lohja. Sharp continues to market EL displays for niche markets.
A Brief History of the Finlux Display Division of Lohja Oy
In 1975, a research group headed by Dr. Tuomo Suntola recognized that thin film electroluminescence would be an ideal flat panel display technology provided that luminance stability and reliability problems could be overcome. To solve these problems a new thin-film deposition method called atomic layer epitaxy (ALE) was developed (see Figure 6). The basic idea was to build thin films layer by layer using surface-controlled chemical exchange reactions. The result is a dense, pinhole film with very good step coverage properties. This research activity started in a small company called Intrumentarium that was acquired in 1977 by Lohja Oy, a Finnish conglomerate which was primarily a manufacturer of construction material. Lohja was the second largest Finnish electronics company after Nokia, and the new Electroluminescent Display technology was considered a good fit for its strategy of diversification into electronics.
Excellent Electroluminescent Display results based on its proprietary ALE technology were for the first time presented at the annual meeting of the Society for Information Display (SID) in 1980, where they received a lot of attention. In 1983, three large information boards were delivered to the Helsinki Vantaa airport. Each of these was comprised of more than 700 character modules. They proved that ALE technology could meet reliability requirements necessary for commercial use. That technology was licensed to Sintra Alcatel in France in 1983. However, the driver costs of the ELD character modules were too high to make them commercially viable, and as a result Finlux began development of a 9-inch 512×256 matrix display for computer and industrial applications. A large manufacturing plant was constructed in a new science park set up in Espoo close to Helsinki. Core manufacturing technologies, including ALE deposition equipment, were developed in-house, which delayed the start of mass production until 1986. Half-page ELD matrix displays with resolutions of 640×200, 650×350 and 640×400 were subsequently manufactured at this plant.
The investments and development costs for Electroluminescent Displays were essentially funded internally by Lohja Oy because little public or customer-paid funding was available. This situation changed when color ELD development was started in 1988 as part of an EU-supported international consortium. The first color EL display based on an innovative device structure was brought to market in 1993.
Lohja Corporation was never able to make the Finlux Display Division profitable because of a lack of experience in managing microelectronics businesses. The Finnish economy benefited from rapid economic growth from the late 1970s until the late 1980s. But when the Soviet Union broke apart in 1991, the Finnish economy suffered because of its dependence upon the Soviet Union as a customer for exports. In 1991, the Finlux Display Division was sold to Planar Systems and was renamed Planar International.
The two ELD operations were of approximately the same size at the time of the merger. The merger permitted savings in marketing costs and materials purchases. Planar Systems succeeded in making Planar International profitable in just a few years by using more experienced management, but without changing manufacturing technology and with only minor changes in staffing. The ALE manufacturing technology still forms the basis for the production of high volume ELDs at both Planar Systems and Planar International. Much of the color development results that were achieved in Finland were also of direct benefit to the work on color ELDs at Planar Systems in the United States, and in particular the AMEL microdisplays discussed above.
In addition, in 1996, Planar Systems began to market a new generation of monochrome Electroluminescent Displays called ICEBrite displays. The ICEBrites combined ALE grown phosphors and insulators with high contrast layers developed by Eric Dickey in the late 1980s.
Electroluminescent Diplays: Conclusions
Electroluminescent displays (ELDs) have a venerable history starting with the experiments of Captain Henry J. Round in 1907, O.V. Lossev in the Soviet Union, and Georges Destriau in France. Electroluminescence was mostly a scientific curiosity until the invention of thin film deposition techniques and the discovery that a sandwich of conductors, insulators and phosphors could result in a very efficent and long-lasting form of emissive display. ELDs were very important in the early days of the laptop computer industry and remained important in niche markets for military, medical and industrial equipment where high brightness, speed, contrast, and ruggedness are necessary. The rise of the color TFT LCD display forced the ELD producers to engage in research on color ELDs with the result that there are now multicolor ELDs on the market and full-color AMELs in development for microdisplays. The Electroluminescent Display industry is currently limited to two major players: Planar and Sharp. Planar acquired its only European competitor, the Finlux Display Division of Lohja Oy, in 1990. Sharp remains committed to competing in ELDs but its main focus is on liquid crystal displays. Most of the important research on ELDs remains within the corporate laboratories of Planar and Sharp, but several publicly funded research laboratories and consortia have also made important contributions to Electroluminescent Display technology.