Field responsive photoluminescent display devices
United States Patent 1191 Kazan et al.  
 [ FIELD RESPONSIVE II-IO&#39;IOLUMINESCENT DISPLAY DEVICES [75] Inventors: Benjamin Kazan; Thomas O.  
 Sedgwick, both of Westchester, NY.  
 1451 Apr. 1, 1975 [73] Assignee: International Business Machines Corporation, Armonk, NY. [57] ABSTRACT [22] Filed: Dec. 26, 1972 211 App]. No: 318,136  
 improved image storage and display devices of the type which includes a layer of phosphor material which is stimulated to emit light through photolumi 52 5 (1 H 315 11 315 12 3 3 5 AB nescence. The intensity of emitted light is varied by 313/92 250 3 5 applying an electric field to the phosphor layer. The 511 Int. Cl. 1101, 31/48 field may be applied to the P p layer y sand- [58] Field of Search .1 315/11, 12; 313/65 AB, wishing the layer between a layer of insulating mate- 313/68 A, 92 PH; 250/365 rial and a layer of electrically conductive material and subjecting the insulator to an electron beam. To alter 5 References Cited the field strength at selected locations at writing elec- UNITED STATES PATENTS tron beam is focussed on the insulator at these se 7 S I q lected locations. in the alternative, the phosphor layer z g l g2 may be covered on one surface with insulation materi- 4/196; Turrllcer e d X als forming a charge trap and the phosphor and insula- 3&#39;437&#39;752 4mm; Ford g 5 X tion layers sandwiched between sets of orthognal elec- 3:594:607 7/l97l Frankland 313/68 A x trically conductive strips Selective field alteration is 3.051.362 3/1972 Takita 313/68 A X accomplished by X-Y addressing an orthognal pair of 3,663v8l3 5/1972 Shaw 250/365 X conductive strips. 3.666346 5/l972 Trimhlc ,1 250/365 X FOREIGN PATENTS OR APPLICATIONS 8 Claims, 3 Drawing Figures 2.05l.099 3/l972 France VIEWING DIRECTION 3 X A l- 1 7w Li 4 30 x m m i I iii m 4 I Z O 0 Y (Zn 0) 1 2 1 4 U V, TRANSMITTING GLASS SUBSTRATE *JENTEQHFB 1 i975 4.875.457  
  14 B L, HIGH VOLTAGE FIG j T SUPPLY 12 EXCITATION I f,v| |BL Q u HT INSULATING FILM (Si0 03 H PHOSPHOR LAYER (zn0)- 4 2 METAL BACK PLATE WRITING sun 26 l 2kV VIEWING DIREETION i 23 L X 3 f ///////////////////////////7 X L FL/////////////////////////// Z L Q g (5i 0 2 2 f f 32 [Zn 0) 1 2 3 4 U. V. TRANSMITTING H r I I GLASS SUBSTRATE 3 /s d HcE 14 FIELD RESPONSIVE PHOTOLUMINESCENT DISPLAY DEVICES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is in the field of image storage and display devices and particularly pertains to those devices which use phosphor material excited to emit light through the phenomenon of photoluminescence.  
 2. Description of the Prior Art Various image storage and display devices using phosphor materials are known. In one type of device, there is provided a cathode ray tube, including a target comprised of a layer of phosphor material excited to emit light by flooding the target with an electron beam produced by a flooding electron gun. To control the intensity of luminescence at selected areas of the phosphor material, the cathode ray tube includes a storage grid upon which is stored a charge image corresponding to the light image to be emitted by the target. This stored charge image controls the electron flow between the flooding electron gun and the target to thereby modulate the intensity of the target luminescence in accordance with the stored charge image. Such storage and display devices are explained in greater detail in US. Pat. No. 3,002,124 to Schneeberger, issued Sept. 26, 1961, US. Pat. No. 3,567,984 to Allard, issued Mar. 2, 1971 and US. Pat. No. 3,243,644 to Roe, issued Mar. 29, 1966.  
  These image storage and display devices suffer the disadvantage that continuous luminescence of the target requires a continuous flooding of the phosphor layer with electrons and thus the continued energization of the flood beam. If the flood beam is cut off, the light output drops instantly. Not only does this cause an excessive use of electrical energy, but requires a relatively complicated structure.  
  It is also known that phosphor materials can be made to emit light through photoluminescence and that the intensity of the emitted light can be controlled by a direct application of voltage to the material. For example, it is known that phosphor materials such as ZnO and ZnS can be made to emit light when subjected to ultraviolet radiation and that the intensity of this emitted light can be controlled by directly applying an electric potential to the phosphor material. For a discussion of this phenomenon the reader is directed to the article by P. F. Daniels et al, Control of Luminescence by Charge Extraction,&#34; Physical Review, Vol. 111, Sept. 1, 1958, pages 1240-1244 and the article by L. W. Hershinger et al, Very Low Voltage DC Electroquenchable,&#34; Abstract of Electrochemical Society Meeting, May 3-7, 1959.  
  Although these articles suggest a technique for varying the intensity of luminescence in phosphor materials excited by photoluminescence, there is no suggestion of providing an image display device exhibiting high resolution nor a device which has the capability of both image storage and display. With respect to image storage, the technique described in the aforementioned articles requires a sustained applied potential with associated current flow, in order to retain the intensity of the emitted light at one of two levels. As soon as the applied potential and current is removed, the intensity of luminescence changes.  
 SUMMARY OF THE INVENTION The present invention is directed to an improved image storage and display device using phosphor materials excited to emit light by photoluminescence. In accordance with the teaching of the invention, the display device is provided with an image storage and display feature that does not require the direct application of a potential to the phosphor material. We have discovered that the intensity of the light emitted by the phosphor material can be changed by subjecting the material to an electric field without the direct application of a potential to the material. The intensity remains at this new level so long as the field exists.  
  In a basic embodiment of the invention, a phosphor material, such as ZnO or ZnS, is coated with an insulator on one surface thereof and a conductive backplate on the opposite surface to produce an electric field when the insulator is bombarded with electrons. When the phosphor layer is subjected to radiation, it emits lights. For a ZnO or ZnS phosphor layer, luminescence results in response to ultraviolet excitation. The intensity of luminescence is less when the phosphor layer is subjected to an electric field. The luminescence remains at a lower intensity level as long as the field remains. While the field is initiated by bombarding the insulator with an electron beam, the field is retained over a relatively long period of time in the absence of the electron beam by virtue of the charge storage capability of the insulating material. Thus, the phosphor materials continue to luminesce at the lower intensity level after the electron beam has been extinguished.  
  In a second embodiment of the invention, the basic embodiment is modified to provide a new type of display storage tube of the type including a target comprised of a layer of phosphor material. The tube includes a writing gun producing an information modulated writing electron beam which alters the electric field at selected points on the phosphor material in accordance with the writing beam modulation.  
  In the third embodiment, the basic embodiment is modified to provide an X-Y addressed storage and display panel.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the basic embodiment of the invention;  
  FIG. 2 illustrates a modification of the embodiment of FIG. 1 to provide a high resolution image storage and display tube; and  
  FIG. 3 illustrates a third embodiment of the invention which includes the X-Y addressing of selected locations on the phosphor material.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a first embodiment of the present invention. This embodiment will be used to explain the basic operation of the inventive device. The device of FIG. I is comprised of a display panel including a phosphor layer 2, for example a ZnO or ZnS layer. One surface of the phosphor layer 2 is covered with an insulating film 4, which, by way of example only, maybe SiO,. The opposite surface of the phosphor layer is covered by a metallic backplate 6, illustrated as being opaque. This conductive backplate 6 may, however, be transparent or translucent. The phosphor material 2 is caused to luminesce through the phenomenon of pho toluminescence. When the phosphor material 2 is ZnO or ZnS, luminescence results when the material is subjected to ultraviolet radiation from source 14. For ease in understanding the present invention, the invention wiil be explained assuming the phosphor layer to be ZnO and the excitation ultraviolet radiation. It is to be understood, however, that the present invention is not limited to Zn) phosphor material nor to ultraviolet ex citation.  
  The level of luminescence is controlled by flooding the insulating layer 4 with a defocused electron beam produced by a suitable electron gun shown, by way of example, in FIG. I, as a corona generator 8, the cathode of which is connected to the negative terminal ofa high voltage power source 12. By negatively charging the insulator, using the corona generator in air, it has been found that the brightness of the ZnO layer 2 can be decreased to less than one-fourth its initial value. The level of luminescence then begins to increase over a period of several minutes to a new equilibrium level which is about half the initial brightness. Although the reason for this change in brightness level is not completely understood, it is believed that the reduction in light output is caused by upward bending of the bands of the ZnO at the ZnO-SiO interface which interferes with the photoluminescence process.  
  By making use of the above field effect control of photoluminescence, a new type of display storage tube can be built as illustrated in FIG. 2. This tube comprises a conventionally shaped tube including a transparent or translucent glass faceplate 22 at the front thereof. Directly behind the faceplate 22 is placed a target or screen 7 consisting of a transparent conductive layer [8, phosphor layer 2 and insulating layer 4. Common elements in FIGS. 1, 2 and 3 will be designated by common numerical designators. Situated behind target 7 is a collector mesh 15. Photoexcitation of the phosphor layer is produced by the radiation source 14, which is shown, by way of example in FIG. 2, as an ultraviolet source. The tube wall, the collector mesh 15, as well as the insulating layer 4, are made from materials transparent to ultraviolet radiation, so that the phosphor layer 2 may be excited into luminescence. The tube also includes an electron flooding gun 28 and a writing electron gun 26, the function and operation of which will be described in detail hereinbelow.  
  The operation of the display tube will now be described. To place the tube in its initial or erased state, the flooding gun 28 is turned on while the potential of conductive layer 18 is pulsed to IOO with respect to ground by closing switch 17 for a short period of time thereby connecting the conductive layer to source 19. With switch 17 closed, capacitor 23 charges to l00 volts. When switch I7 is opened, capacitor 17 begins to discharge through resistor 25. The values of the capacitor 23 and resistor 15 are selected so that the dis charge time of the capacitor is several tenths of a second.  
  The energy of flooding beam 29 is controlled, by setting the cathode potential of electron gun 28 in such a way that during a substantial portion of the discharge interval the ratio of the number of secondary electrons emitted by insulating layer 4 to the number of primary electrons of beam 29 is less than unity. In the embodiment illustrated insulating layer 4 is SiO and the cath- Ode of gun 28 is set at -l 00V. In this erased condition,  
 excitation of ZnO layer 2 by ultraviolet source 14 re sults in a low level of light output.  
  To accomplish a writing operation, flooding beam 29 is turned off and insulating layer 4 is scanned by a writing beam 27 produced by writing electron gun 26. The position of writing beam 27 is controlled by electrical input signals in a conventional manner; for example, the position of the writing beam can be controlled in accordance with a suitable input signal to deflecting plates 24. In the preferred embodiment, the cathode of writing gun 26 is set at 2k\/.  
  The setting of the potential of the cathode of the writing gun is such that the energy of writing beam 27 contains sufficient energy to render the ratio of the number of secondary electrons freed from the insulating layer to the number of primary electrons of the writing beam greater than unity. The result is that where insulating layer 4 is struck by the focused writing beam the potential rises close to ground potential as grounded collector mesh 15 substantially absorbs all electrons emitted secondarily. The reduction of the electric field over the phosphor layer at areas of the insulating layer where the writing beam has struck, will result in an increase in the efficiency of luminescence at corresponding areas of phosphor layer 2 producing a bright trace on a relatively dark background. Assuming no charge leakage through insulating layer 4, the stored image may be viewed for many minutes after writing beam 27 is cut off.  
  To maintain an image for very long periods of time, the flooding beam 29 is switched on while the conductive plate 18 is connected to ground potential. The action of the flooding beam will be to maintain the areas on the insulating layer 4 at either one of two stable potential states; lOO volts, corresponding to the erase state, at those areas not scanned by beam 27, and substantially ground at those locations which have been scanned by the writing beam 27.  
  More specifically, those areas of the target which have not been written upon, may tend to leak charge through phosphor layer 2, and the transparent conductive layer 18 to ground. Thus, the potential at the points where charge leakage has occurred may rise to some potential above l0() volts and the luminescence at these points begin to increase. As the flood beam strikes these areas, the charge thereon returns to -l 00 volts, assuming the potential at those points has not risen sufficiently due to leakage such that the energy of the flooding beam 29 would be sufficient to cause secondary emission with a factor greater than unity. How ever, at the locations which have been scanned by the writing beam 17, the potential is sufficiently more positive than that of the flooding beam 29 such that the energy of the beam with respect to those areas is sufficient to cause secondary emission with a factor greater than unity. Therefore, the number of secondary electrons at these locations is greater than the number of primary electrons causing these locations to continue to luminesce brightly.  
  As indicated above, to erase an image, switch 17 is closed momentarily causing the potential of the transparent conductive layer 18 to drop to J00 volts with respect to ground and slowly rise to ground potential while the flooding beam 29 is on. As a result, the energy of the flood beam is insufficient, during a substantial portion of the erase time, to cause secondary emission with a factor greater than unity, and the entire insulating layer assumes a potential l() volts.  
  The above-described image storage and display tube 20 can be modified by deleting the flooding gun 28, in which case the writing gun 26 is used for flooding purposes by defocusing the writing beam while its cathode is set at 100 volts rather than 2 kilovolts.  
  Because of the continuous nature of the phosphor layer 2 and insulating layer 4, high resolution stored images can be produced. As an example of the brightness obtainable, a 5 watt black light source can excite a phosphor area of about 5 inches by 5 inches to foot- Lamberts brightness or higher. Since the life of the ZnO layer under ultraviolet excitation is very long, the reliability of the tube is extremely great.  
  A further embodiment of the invention is illustrated in FIG. 3. In this embodiment, addressing of the display screen is provided by a plurality of X-Y conductors rather than through the use of electron beams. More specifically, the display screen is comprised of a layer 2 of phosphor material, one surface of which is covered by an insulating layer 4 of a first insulating material such as SiO Above this first insulating layer 4 is positioned a second insulating layer 30 of a second insulating material such as Si N the relative thicknesses of the two insulating layers being controlled to provide charge trapping in the manner well known in the field of silicon memory technology to permit sustained excitation. Above the Si N insulating layer 30 is positioned a plurality of parallel, semi-transparent conductive strips X,X On the surface of the phosphor layer 2 opposite the surface bearing the SiO layer 4 is positioned a plurality of parallel semi-transparent conductive strips Y,-Y situated orthogonal to the conductive strips X -X The display screen is preferably supported on a glass substrate 32 which is transparent to the phosphor exciting light generated by source 14. Fewer or additional X and Y conductors can be used as desired.  
  In this configuration, a negative charge storage at the SiO Si l l interface will produce a depleted surface on the ZnO, thus lowering luminescence when excited by source 14. By comparison, a positive or zero charge stored at the SiO Si;,N,, interface will result in relatively high luminescence. To alter the charge at the insulating material interface at a local region, coincident pulses of W2 and V/2 are applied to a pair of selected X-Y conductors. Thus, only the region between the selected conductors will experience the full pulse of a voltage sufficient to exceed some threshold voltage V where V/2 V V. This results in a tunnelling of the carriers from the ZnO through the SiO to the Si O -SEN, interface. This charge may be removed by applying voltage pulses of opposite sign to the selected conductors.  
  While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.  
 What is claimed is:  
 1. An image storage and display device comprising: a solid layer of phosphor material having formed on one surface thereofa layer of electrically insulating material and on its opposite surface a layer of conductive material; means for exposing said phosphor material to light to stimulate it to a level of luminescence dependent upon the electric field across said phosphor layer. and  
 means for applying a charge pattern to said insulating layer resulting in an electric field pattern across said phosphor layer corresponding to said charge pattern.  
  2. The device of claim 1 wherein said means for exposing said phosphor material to light includes means for producing light in the ultraviolet region of the spectrum.  
  3. The device of claim 1 wherein said means for applying a charge pattern comprises means for locally charging the insulating layer with electrons.  
  4. The device of claim 3 wherein said means for applying a charge pattern further includes writing means for producing a focussed writing beam of electrons, said writing means including means for controlling the position of said writing beam.  
  5. The device of claim 1 wherein said means for applying a charge pattern comprises an electronic gun means for producing an electron beam impinging said insulating layer, said electron gun means including means for selectively focussing and defocussing said electron beam and means for altering the energy of said electron beam.  
  6. The device of claim 1 further including a second layer of electrically insulating material disposed over said layer of insulating material, said means for applying a charge pattern comprising a first set of conductive strips disposed over said second insulating layer and a second set of conductive strips, orthogonal to said first set forming said layer of conductive material.  
  7. In an image display device including a layer of phosphor material stimulated to luminesce by photoluminescence, the improvement comprising:  
 a continuous layer of insulating material formed over one surface of said phosphor layer and a continuous layer of conductive material formed over the opposite surface of said phosphor layer, light source stimulating said phosphor layer to a level of luminescence dependent upon the electric field across said phosphor layer,  
 a writing electron gun for subjecting selected locations on said insulating layer with a focussed electron beam to thereby change the intensity of the electric field acting on said phosphor layer at said selected locations, and a flooding electron gun means for flooding the insulating layer with electrons to sustain the local electrical fields acting on the phosphor layer. 8. The device of claim 7 wherein said phosphor material is ZnO, said insulating material SiO and said radi ation source a source of ultraviolet radiation.