Phosphors and Other Emissive Materials

PHOSPHORS AND OTHER EMISSIVE MATERIALS

Nguyen Van An

Institute of Engineering Physics, Hanoi University of Technology

01 Dai Co Viet Str., Hanoi, Vietnam

Abstract

Emissive materials, particularly phosphors, are used in most display technologies, including electroluminescent, cathode ray tube, field emission, plasma, and liquid crystal, as direct light emitters or as illumination sources. The performance of display products depends upon phosphor efficiencies, spectral distribution, long-term stability, and electrical characteristics. Often it is the materials constraints of emissive materials that limit improvements in displays. In particular, for electroluminescent (EL) and light-emitting diode (LED) displays, the requirements for materials improvements include greater stability of ZnS phosphor, a more efficient red, green, and blue (RGB) filterable white phosphor, and a more efficient blue phosphor. The phosphors for plasma displays need a breakthrough in luminous efficiency, greater long-term stability in a gas plasma, and better thick-film, high- resolution phosphor deposition processes. Improvements in cathode ray tubes and field emission displays require phosphors with higher efficiency/brightness, greater long-term stability, more efficient blue color, and low-voltage performance.

Introduction

At the Volga Research and Development Institute in Saratov, powder phosphors are developed for use in vacuum fluorescent displays (VFDs) produced by the neighboring company Reflector. The goals for R&D are to develop low voltage phosphors in the range of 4-20 V for color displays. Volga develops phosphors that are then mass-produced in Stavropol. The Volga laboratories include a lab to develop specialized phosphors, indium-tin-oxide evaporation equipment, and reactive plasma etching equipment. For phosphor development, the emphasis is on low-voltage materials using ZnO for green, Cd:ZnS for red, and ZnS for blue. For 200-300 V applications, ZnS is used for green and blue, and Y-oxide for red (TV analog materials). The screen processing methods used are silk-screening and electrophoretic deposition. The ZnO for green has a brightness of 10-12 lm/W. Using silk- screening techniques, lines of 100-150 mm thick of phosphor have been deposited. Volga's scientists are studying ZnS and ZnO for FEDs. Some work has been done on thick-film (10 mm) Zn sulfides and oxides, but there has been no thin-film materials work.

Reflector in Saratov obtained patents on phosphors and device structures for low- voltage VFDs that it manufactures. The phosphors are mass-produced at Russia's main phosphor production enterprise, Luminophor in Stavropol. Reflector and the Volga Institute developed a red, green, and blue multicolor VFD production process. Production modules include glass substrate cleaning, phosphor screen printing, and carbon coating and screening. Reflector claims several technical advantages for its VFD, including the low voltage structure that provides very high brightness, high lifetime (approx. 100,000 hrs), and multicolor capability.

In Fryazino, Platan develops and produces the powder phosphors used in its products, CRTs and CRT projection systems. The phosphor facility is large and well-equipped. The company's scientists are working on low-voltage phosphors and multicomponent phosphors that emit different colors depending on the beam voltage or current. The company claims it has a 10 V phosphor; a 30 V phosphor was demonstrated. One phosphor screen, a material doped with Eu, showed strong evidence of laser action; that is, it exhibited threshold-like intensity behavior with significant spectral narrowing. Platan has developed a line of CRTs that uses the color-modulated phosphors. The voltage- induced color shift was sufficient to produce five distinct hues, and the colors were used to color-code targets and write symbology in a large (approx. 30") tube. Dr. Saschin at the phosphor lab showed phosphor encapsulated in polyethylene/polypropylene for converting UV radiation to 610-700 nm, used to enhance plant growth and to warm enclosures. Phosphors are mass-produced at factories in Kustova and Dorogomilovsky.

In the Department of Physical and Colloidal Chemistry at L'viv State University, Ukraine, Professors M.S. Pidzyrailo and M.M. Soltys discussed R&D on cathodoluminescent screens based on three kinds of phosphor powders (425, 450, and 540 nm). The following materials (1-4 micron diameter powders) with small crystal grains are typically used: ZnS:Ag, Y3Al5O12:Ce, and Y2SiO5:Ce. L'viv's collaboration with Erotron has resulted in highly informative displays with 60 micron lines. The screen operates in a two-color mode (red and green); the threshold of the color switching is 5-10 kV. Dr. V.D. Bondar in the Laboratory of Physical Electronics performs R&D on the luminescent screens. The screen is composed of two luminescent films possessing different colors (e.g., red Y2O2S:Eu). The color of luminescence depends on the intensity of the electron beam that defines its penetration depth.

The Kyyiv Scientific Research Institute for Microelectronics Technique and Materials in Kyyiv, Ukraine, has facilities for producing EL devices. The material used is ZnS:Mn (yellow color), which in the past was purchased from Zelenograd (Russia). Presently, in Ukraine the source of the pure materials is located in Odessa. The lifetime for the display with this phosphor is 2,000 hrs; the goal is to reach 10,000-15,000 hrs using pure materials. EL indicators operate in the temperature range of -50 degrees centigrade to 80 degrees centigrade. The basic technical parameters of the EL indicators are: integral luminance of >40 cd/m2, contrast of better than 2:1 with external illuminance of 30,000 lx and better than 10:1 with 500 lx, a viewing angle >120 degrees, and operating voltages of 5 V and 250 V.

In the Department of Optoelectronics, Institute of Semiconductors at the Ukraine Academy of Sciences, thin-film EL structures are based on materials such as ZnS, ZnSe, and ZnS:Mn with colors of yellow orange, red, green, and blue. Recently, a new type of EL indicator based on the integration of the thin-film luminescent structure and active layers of ferroelectric ceramics has been developed. Two- colored matrix screen on glass and ceramic substrates are manufactured.

The Moscow State Institute of Electronics and Mathematics has done considerable work in EL displays at brightness levels greater than 50,000 lx, and claims to have an EL comparable to a CRT in a 15 cm x 15 cm format. The color was reported as yellow green, and the lifetime was given as greater than 10,000 hrs. The institute has operated the device over a variety of temperature ranges.

In display technology, the Elorma Scientific Industrial Corporation has investigated a DC and AC EL display. The role of Elorma in making the display was to modify the interface to enhance electron mobility of the phosphors with use of a siloxane film. The company's interests are in using the siloxane material developed at Elorma as a sublayer for photodioxides and possible substrate coating for displays. The proposed advantage of this organic is that it can protect the phosphors from alkalis from soda-lime glass substrates and thus enhance reliability and stability. This EL display consisted of ZnO - ZnS p-n junction with an organic polymer dielectric. The display was a 4" x 5" matrix with a yellow green color and an integral brightness of 180-250 cd/cm2, which was visible in sunlight. The luminescent material was a modification of existing phosphor in which ZnS particles were oxidized, covered with patches of Cu islands, and coated with a siloxane film. Other phosphors were also used, such as rare earth oxides for blue. The siloxane used in the display was patented; the patent was to be released in February 1994. Possible future applications of this organic material were proposed, such as a substrate in an LCD and as a photodioxide screen in a LED with narrowband emission.

LED MATERIALS

Sapphire Research and Production Amalgamation in Moscow demonstrated the first SiC blue light-emitting diode, and has continued to do considerable research and development in SiC light emitters. An experimental blue LED operating at 480 nm with a 2 candela maximum output and a green LED at 520 nm with 3- 4 candela output were demonstrated. Dr. Sushkov described an ultraviolet SiC LED operating at 410 nm. He suggested that such an LED could be used to stimulate emission from a phosphor. The efficiency of the UV LED is very low, but the effect was observable.

Reference

1. R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416-419.

2. Y.Y. Chen, J.G. Duh, B.S. Chiou, C.G. Peng, Thin Solid Films 392 (2001) 50-55.

3. K. Manzoor et al, Appl. Phys. Lett. 84 (2004) 284-286.

4. A.A. Khosravi et al, Appl. Phys. Lett. 67 (1995) 2702-2704.