Technical Field

The invention is concerned with luminescent color display devices _*

Background of the Invention

Display devices such as, e.g., cathode ray tubes typically include a luminescent screen which locally emits optical radiation in response to locally incident electron or electromagnetic radiation. Luminescent screen materials may be chosen with respect to a number of criteria; in many applications such as, e.g., high resolution displays, projection systems, and direct viewing systems under strong ambient lighting conditions there is a particular concern with brightness of emitted radiation. Accordingly, luminescent screen materials are desired which are highly efficient in emitting radiation of a desired wavelength.

Up to a point brightness is directly related to the intensity of stimulating electron or UV radiation; however, eventual "burnout" of luminescent material limits the usable intensity of such stimulating radiation. Thus, in the interest of high brightness, it is desirable to find luminescent materials having a high burnout intensity threshold.

Display devices may include luminescent materials or phosphors in powder form; however, such materials tend to have shortcomings such as, e.g., poor thermal properties, degradation of efficiency with stimulating radiation dose, and limited resolution. On the -other hand, single-crystal slabs or layers of luminescent materials have been found to be capable of withstanding input power densities in excess of 10 10W/m2 without measurable degradation and to have improved resolution as compared to powder phosphor targets.

While luminescence properties of single-crystal phosphors tend to be similar to those of powder phosphors

of the same composition, features tend to differ in detail in a generally unpredictable manner. For example, since a powder phosphor is typically formed by a different process than the single-crystal phosphor, chemical differences such as, e.g., phase differences may be present. Also, while the crystal lattice in powder particles can be expected to be strained and to have numerous defects, the lattice of a single-crystal phosphor typically is relatively free of strain and of defects. Since luminescence is sensitive to details of the crystal field, such lattice differences can result in significant differences in luminescence. The following- printed items are cited as representative of the literature on luminescent materials, their preparation, and their use in display devices. U. S. Patent No. 3,431,066, "Method for Producing

Yttrium Aluminum Oxide Garnet Crystals", issued March 4, 1969 to R. Seitz;

U. S. Patent No. 3,564,322, "Cathode-Ray Tube for Flying-Spot Scanning", issued February 16, 1971 to G. Blasse et al.;

U. S _* Patent o, 3,839,219, "Europium Activated Alkaline Earth Magnesium Aluminum Silicate Luminescent Material", issued October 1, 1974 to J. M. P. J. Verstegen et al. ; U. S. Patent No. 4,003,845, "Luminescent

Material", issued January 18, 1977 to P. F. J. van den Boom et al. ;

U. S. Patent No. 4,024,070, "Method of Manufacturing a Cerium Activated Luminescent Rare-Earth Aluminate", issued May 17, 1977 to R. E. Schull;

U. S. Patent No. 4,093,890, "Terbium-Activated Luminescent Garnet Material and Mercury Vapor Discharge Lamp Containing The Same", issued June 6, 1978 to J. G. Verriet et al. ; U. S. Patent No. 4,180,477, "Luminescent

Materials", issued December 25, 1979 to R. G. L. Barnes;

U. S. Patent No. 4,216,408, "Luminescent Material

and Discharge Lamp and Cathode Ray Tube Containing the Same", issued August 5, 1980 to J. M. P. J. Verstegen et al. ;

U. S. Patent No. 4,298,820, "Luminescent Screen", issued November 3, 1981 to P. F. Bongers et al.;

G. Blasse et al., "Study of Energy Transfer from S „,b3+, „Bi.3+, C„e3+ ,t_o S _. m3+,

Eu , Tb , Dy " , The Journal of

Chemical Physics, Vol. 47, No. 6, September 1967, pp. 1920- 1926;

G. Blasse et al., "Investigation of Some Ce 3+-Activated Phosphors", Journal of Chemical

Physics, Vol. 47, No. 12, December 1967, pp. 5139-5145;

G. Blasse, "Energy Transfer in Oxidiσ Phosphors", Physics Letters, Vol. 28A, December 1968, pp. 444-445;

J. Kvapil et al. , "0~ Centre Formation in YAG

Crystals Doped with Rare Earth Ions", Kristall und Technik,

Vol. 10, No. 2, 1975, pp. 161-165; T. R. Johansen et al., "Lead Free Bismuth

Substituted Garnet Films by L.P.E-", AIP Conference

Proceedings, No. 29, Magnetism and Magnetic Materials-1975,

American Institute of Physics, 1976, pp. 580-582;

J. M. Robertson et al., "Thin Single Crystalline Phosphor Layers Grown by Liquid Phase Epitaxy",

Philips J Res. 35, 1980, pp. 354-371;

J. M. Robertson, "Epitaxially Grown

Monocrystalline Garnet Cathode-Ray Tube Phosphor Screens",

Appl. Phys. Lett. 37, No. 5, 1980, pp. 471-472; V. A. Andriiσhuk, "Photolu inescence of Garnets

Activated by Bismuth Ions", Fiz. Elektron (Lvov), Vol. 20,

1980, pp. 80-81; and

F. Kellendonk et al. , "On the Luminescence of

Bismuth, Cerium, and Chromium and Yttrium Aluminum Borate", J. Chem. Phys. 76, No. 3, 1982, pp. 1194-1201.

Summary of the Invention

Visual display devices embodying the invention

comprise a source of stimulating electron or electromagnetic radiation, typically including deflection and modulation means, and a member comprising a substantially single-crystal phosphor material which is a garnet material consisting essentially of a composition substantially as represented by the formula

Y 3_-x-yBixRyAl5.-zGaz0 _i 1 _n 2 where R represents at least one rare earth element, where x_ is in the preferred range from 0.005 to 0.5, where preferably is less than or equal to 2.995, and where _z preferably is less than or equal to 5. In the interest of limiting extraneous luminescence, rare earth elements" Nd, Eu, Tm, and Tb are limited to amounts corresponding to y- values which are less than 0.01 and preferably less than or equal to 0.005.

More narrow preferred limits are as follows: Preferred yttrium aluminum garnets have z_ less than or equal to 0.4 and preferably τ_ less than or equal to 0 _^ 05,. and rare, earth elemeata other than Lu and Gd are preferably limited such that their combined contribution to the combined amount of rare earth elements is represented by a value of which is less than or equal to 0.2. When stimulated by electron or ultraviolet radiation, suitable bismuth-containing garnet materials emit electromagnetic radiation whose spectrum has a peak at a wavelength which is greater than or equal to 425 nanometers. Brief Description of the Drawing

FIG. 1 is a schematic cross-sectional view of a cathode ray display device in accordance with the invention; and

FIG. 2 is a graphic representation of spectra of light output of two luminescent materials. Detailed Description FIG. 1 shows a cathode ray display device comprising electron radiation source 11 with electrical connection pins 12, enclosure 13, garnet substrate 14, and

bismuth-activated luminescent film 15 in accordance with the invention.

FIG. 2 shows light output intensity curve 21 representing luminescence intensity of a prior-art polycrystalline bismuth-activated garnet material, and curve 22 representing luminescence intensity of a bismuth- activated garnet material in accordance with the invention. Curve 22 shows an intensity peak corresponding to a wavelength near 450 nanometers. Display devices embodying the invention comprise a member such as, e.g., a screen comprising a substantially single-crystal material having a composition as described above. (While single-crystal structure is preferred, substantially single-crystal structure may include faults such as, e.g., dislocations.) A member may comprise other phosphor materials which luminesce at different wavelengths as, e.g., in color display devices; however, monochromatic devices are not precluded.

A luminescent material, typically is .in the form of a layer as epitaxially deposited on a crystallographically compatible substrate. Yttrium- aluminum garnet substrates are customary, typically having a [111] crystalline orientation.

Also suitable for purposes of the invention are more specific compositions such as, e.g., those which comprise little or no yttrium and for which y_ is in a preferred range of from 2.5 to 2.995. In the interest of lattice matching to yttrium-aluminum garnet, such materials preferably comprise a significant amount of gallium as, e.g., in a composition such as

^Bi 0.05 ^u 2.95 ^Ga 2 ^Al 3°12 ^; this and similar combinations of substrate and epitaxial layer result in an optical waveguide structure.

Alternately, and essentially in the absence of yttrium and aluminum, and if R is primarily gadolinium, a resulting layer may be conveniently matched to a gadolinium-gallium garnet substrate. ^■

Deposition of the film on the substrate is conveniently effected by liquid phase epitaxy, i.e., by bringing a surface of a substrate in contact with a fluxed melt of garnet constituent oxides while melt temperature is regulated to result in supersaturation of garnet constituents in the vicinity of the liquid-solid interface.

Example. A melt was prepared by heating a mixture of approximately 18.40 grams Y2O3, approximately 33.58 grams AI2O3, approximately 1100.0 grams Bi2θ3, and approximately 41.07 grams B2O3. An yttrium-aluminum garnet substrate was brought in contact with the melt surface while the melt was at a temperature of 1025 degrees C, and a film having a thickness of approximately 5.86 micrometers was thus grown at a rate of approximately 1.43 micrometers per minute. The grown film had a composition as approximately represented by the formula * ₂ .95 ^Bi 0.05 ^Al 5°12*

Internal efficiency was determined by low-power- density excitation and was found to be approximately

1.9 percent.. Power saturation was achieved at a power of 10 watts/ at which point internal efficiency was approximately 0.6 percent.