Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/15—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components having potential barriers, specially adapted for light emission
  • H01L27/153—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components having potential barriers, specially adapted for light emission in a repetitive configuration, e.g. LED bars
  • H01L27/156—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components having potential barriers, specially adapted for light emission in a repetitive configuration, e.g. LED bars two-dimensional arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
  • H01L33/10—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector
  • H01L33/105—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector with a resonant cavity structure
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L33/36—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes
  • H01L33/40—Materials therefor
  • H01L33/405—Reflective materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L33/48—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor body packages
  • H01L33/50—Wavelength conversion elements

Definitions

• This invention relates to light emitting devices, such as displays and lamps, and more particularly relates to such devices incorporating phosphors.
• the most widely known display device which employs a phosphor display screen is the color cathode ray tube employed in televisions and computer monitors.
• the phosphors used are cathodoluminescent, that is, excitable by cathode rays from the electron gun in the neck of the tube. Conversion of the cathode rays to visible light is relatively energy intensive, with operating voltages of 20 to 30 kV being typical. Moreover, the conversion must take place in a vacuum, which is maintained by the sealed glass envelope of the tube. Attempts have been made to flatten the conventional cathode ray tube, in order to broaden the range of applications. However, the most successful implementation of flat display devices so far has been the liquid crystal display (LCD). Due in part to its lower energy consumption and lack of the need for a vacuum environment, the LCD is widely used in portable computers, and other special purpose display applications, such as watches, calculators and instrument panels.
• LCD liquid crystal display
• Lamps incorporating phosphors are also known.
• conventional fluorescent lamps have a coating of a UV-excitable phosphor on the inside surface of the lamp's glass envelope.
• Hg in the lamp fill emits UV radiation, which excites visible light emissions from the phosphor coating.
• CTRs like CRTs, such lamps require a glass envelope to maintain a suitable environment (Hg vapor) for UV emission.
• Light emitting devices are known in which the visible (blue) light from an LED is enhanced by a fluorescent screen coupled to the LED (Japanese Patent Abstract of Application 07176794) or by a fluorescent dye impregnated into a resin encapsulating the LED (Japanese Patent Abstract of Application 5-152609).
• a display device described in Japanese Patent Abstract of Application 62- 189770 includes an infra-red emitting LED and a fluorescent layer for converting the infra ⁇ red radiation to visible light. Such long-to-short wavelength energy conversions are not very efficient.
• a visible light emitting device comprises a phosphor screen of one or more UV-excitable, visible light-emitting phosphors, and a source of UV radiation for exciting visible light emission from the phosphor screen, characterized in that the UV source consists of at least one GaN-based light emitting diode (LED) or laser.
• LED GaN-based light emitting diode
• the LED is a multilayer epitaxial structure on a single crystal substrate, the structure comprising a first GaN contact layer of a first conductivity type on the substrate, a first In x ALGa j . x . y N cladding layer of the first conductivity type on the contact layer, an active region of Al y Ga ⁇ y N on the first cladding layer, an In ⁇ AlyGa ! N cladding layer of a second conductivity type on the active region, a second GaN contact layer of the second conductivity type on the second cladding layer, and a top metal contact layer on top of the second GaN contact layer.
• the first conductivity type is n and the second conductivity type is p
• the first contact layer is n+ GaN
• the first cladding layer is n In ⁇ Al Ga ⁇ . ⁇ N
• the second cladding layer is p Ir ⁇ AlyGa ⁇ . y N
• the second contact layer is p+ GaN.
• the active region may be a single quantum well structure or a multi quantum well structure.
• the substrate may be of a material which is transmissive to visible light, such as sapphire, silicon carbide or zinc oxide, and the phosphor screen is located on or adjacent to the substrate.
• the top metal contact layer is transmissive to visible light, and the phosphor screen is located on or adjacent to the top metal contact layer.
• the LED includes a resonant cavity (RC), and is defined by at least one distributed Bragg reflection (DBR) region located between one of the cladding layers and its contact layer.
• DBR distributed Bragg reflection
• a second DBR region may be located between the other cladding layer and its contact layer.
• a reflective metal layer is used, such as the top metal contact layer. The distance between the two reflective surfaces defines the cavity width. Typically, this width is approximately ⁇ /2n, and the active region is located in the antinode of the cavity.
• the light emitting device is a display device in which the phosphor screen comprises an array of phosphor elements, and the array is photo pumped by a row of LEDs which scans the array with UV light in accordance with a display signal.
• the scanning may be achieved either by moving the row of LEDs or by using optical scanning means, such as a rotating prism.
• a display device is comprised of a matrix array of individually addressable LED-phosphor devices.
• a lamp is comprised of a matrix array LED-phosphor devices which are addressed enough.
• Fig. 1 is a cross section of one embodiment of an LED-phosphor structure of the invention, in which the phosphor is located on top of the LED;
• Fig. 2 is a cross section of another embodiment of an LED-phosphor structure of the invention, in which the phosphor is located on the substrate under the LED;
• Fig. 3 is a cross section of yet another embodiment similar to that of Fig. 1 , except that the LED includes a resonant cavity;
• Fig. 4 is a cross section of yet another embodiment similar to that of Fig. 2, except that the LED includes a resonant cavity structure;
• Fig. 5 is a schematic illustration of one embodiment of a color display device in which a row of LEDs is scanned across a screen of color phosphor elements;
• Fig. 6 is a schematic illustration of another embodiment of a color display device similar to that of Fig. 5, except that the row of LEDs is stationary, and the screen is optically scanned;
• Fig. 7 is a schematic illustration of yet another embodiment of a color display device of a matrix of individually addressable LED-phosphor picture elements;
• Figs. 8(a) through 8(g) are cross-sections illustrating stages in the fabrication of a row of picture elements of the device of Fig. 7;
• Fig. 9 is a schematic circuit diagram of a portion of the matrix display of Fig.
• Fig. 10 is a cross section of a portion of the LED structure of Fig. 4, showing the structures of the bottom and top DBR layers;
• Fig. 11 is a schematic diagram of yet another embodiment of a color projection system of the invention, using a single UV LED and x and y scanning optics to scan a phosphor screen;
• Fig. 12 is a schematic diagram of a variation of the color display device of Fig. 5, employing three rows of LEDs.
• GaN- based (InAlGaN) light emitting diodes LEDs
• LEDs GaN- based light emitting diodes
• This UV LED-phosphor device requires a simple GaN-based LED, which typically has a broad spectrum with a peak around 363 nm and emission which tails into the visible range. This tail is not very useful for photo pumping visible range phosphors. Therefor, microcavities, sometimes referred to herein as resonant cavities, are introduced into the LEDs to narrow the width of the emission band and raise the peak emission.
• Fig. 1 shows in cross section a simple structure for a UV LED/phosphor device 10.
• a single crystal substrate 12 of for example, sapphire, silicon carbide or zinc oxide
• an epitaxial buffer/contact layer 14 of n+ GaN On this buffer layer is the LED structure including the following epitaxial layers in sequence: lower cladding layer 16 of n AlGaN, active region 18 of i GaN, and upper cladding layer 20 of p AlGaN.
• a p+ GaN contact layer 22 On top of this LED structure is a p+ GaN contact layer 22, semi-transparent metal contact layer 24, of for example a Au/Ni alloy, and voltage electrode 26, with phosphor layer 28, of a UV-excitable phosphor, on contact layer 24.
• Metallization layers 30 and 34 are provided on the surface of buffer/ contact layer 14 on either side of the LED structure.
• Layer 30 provides grounding via grounding electrode 32, while layer 34 serves as an addressing electrode.
• the contact layer 22 or the metallization layer 24 preferably has an anti-reflective coating on its upper surface.
• Such a coating should have low reflectivity and low absorption in the UV (eg. , below 450 nm), and high reflectivity and low absorption in the visible wavelength range (eg. , 450-650), to prevent light generated in the phosphor layer from backscattering into the device structure.
• Such coatings for example, a 1/4 wave stack (Bragg reflector), are well-known, and not a necessary part of this description.
• UV radiation is emitted from the active layer 18, substantially in one direction, as indicated by the large arrow, then passes through semi-transparent contact layer 24 to land on phosphor layer 28, and excite visible radiation from the phosphor.
• the phosphor emission has a lambertian distribution, as indicated by the smaller arrows.
• Fig. 2 shows a second embodiment 40 of a simple UV LED/phosphor device, in which the phosphor layer is located on the underside of a UV transparent single crystal substrate, of, for example, single crystal sapphire, silicon carbide or zinc oxide.
• the substrate is preferably relatively thin, eg. , on the order of 100 microns, polished and having an anti-reflective coating on its lower surface, of the type formed on the upper surface of the device of Fig. 1.
• an epitaxial buffer/contact layer 44 of n-f GaN Located on the substrate 42 is an epitaxial buffer/contact layer 44 of n-f GaN.
• the LED structure including the following epitaxial layers in sequence: lower cladding layer 46 of n AlGaN, active region 48 of GaN, and upper cladding layer 50 of p AlGaN.
• the UV excitable phosphor layer 64 is located on the underside of the transparent substrate 42.
• the substrate is kept relatively thin, consistent with the needed mechanical strength, for example, around 100 micrometers, to maximize its transparency to UV radiation.
• Buffer/contact layer 44 is grounded by grounding electrode 60 via metallization layer 58, while layer 62 serves as an addressing electrode.
• the active region could be a single or multi- quantum well structure, the multi-quantum well structure being better suited for higher power applications, as is known.
• a resonant cavity (RC)LED/phosphor device is shown in Fig. 3.
• the device 70 includes on a single crystal substrate 72, an n+ GaN contact layer 74, a back Distributed Bragg Reflection (DBR) layer 76.
• DBR Distributed Bragg Reflection
• this back DBR layer is formed a bottom cladding layer 78 of n AlGaN, active layer 80 of GaN, InGaN or AlGaN, a top cladding layer 82 of p AlGaN, a second output DBR layer 84, similar in structure to the first back DBR layer 76, but having a greater UV transmission, according to the relation
• the back DBR typically, the reflectivity of the back DBR would be 90 percent or more, while that of the output DBR would be in the range of 60 to 70 percent.
• the DBR layers must satisfy the condition
• d is the distance between the inner reflective surfaces of the DBR layers 76 and 84, which defines the width of the resonant cavity
• n is the refractive index
• ⁇ is the absorption coefficient of the cavity, respectively, whereby quenching of the resonance is avoided.
• top contact layer 86 of p+ GaN is formed on top of the output DBR layer 84.
• Electrodes 92, 94 and 96 complete the structure.
• the distance d is determined by the equation
• N is an integer (usually 1)
• ⁇ out ( ⁇ ), ⁇ bri ⁇ k ( ⁇ ) are phase changes dunng reflection at the output and back-mirror, respectively
• ⁇ is the resonant wavelength
• d and n are the cavity width and refractive index, respectively.
• phase change is either so small as to be negligible if the first layer of the DBR (the layer in contact with the cladding layer) is a high index layer, or x/2 if the first layer is a low index layer.
• n m , k, n real and imaginary part of the refractive index of the mirror
• n is the refractive index of the cavity.
• the phosphor layer could be deposited on the bottom of a UV transparent substrate, and the positions of the back and output DBR layers would be reversed.
• FIG. 4 Another alternative resonant cavity structure 100 is shown in Fig. 4.
• This structure built onto UV transparent substrate 102, begins with n + contact layer 104 of AlGaN, on top of which is output DBR 106, supporting bottom cladding layer 108 of n- InAlGaN; next are the active layer 1 10 and top cladding layer 1 12, of p InAlGaN. On layer 1 12 is formed top DBR layer 1 14, and top contact layer 1 16 of p+ AlGaN. Completing the structure is top metallic contact 1 18, and electrodes 124 and 126.
• the individual sub-layers of the top and bottom DBR layers 106 and 1 14 of the device of Fig. 4 are shown in Fig. 10.
• Such DBR layers are composed of sub-layers of alternating high and low refractive index.
• the reflectivity of the DBR layers is determined by the number of sub-layers and the difference in refractive index between the high and low index sub-layers.
• Suitable high index materials for the top back reflective DBR layer include Si 3 N 4 , MgO, TiO 2 , MgF 2 , HfO 2 , Ta 2 O 5 and ZnS; while suitable low index materials include SiO 2 , Al 2 O 3 , CaF 2 and HfF 4 .
• Layer 1 14 is composed of 12 sub-layer of SiO 2 (r.i.
• the thicknesses of the high and low index layers of the back DBR are 455 and 609A, respectively, while the thicknesses of the high and low index layers of the output DBR are 352 and 413 A, respectively.
• the back DBR 1 14 may be eliminated and instead the reflective lower surface of the top metal contact layer 1 16 is relied upon to define the back surface of the resonant cavity.
• the reflectivity of such a metallic mirror is independent of the angle of incidence of the reflected radiation, and provides good electrical contact to adjacent layers.
• Aluminum is a particulary good UV reflective material.
• Another advantage of this arrangement is that the structure can be tuned to the maximum output by regrowth of the p+ contact layer 1 18 to change the length d of the resonant cavity.
• the phosphor layer can be deposited on a separate substrate, and the LED can be positioned in close proximity, where it will photo-pump the phosphor layer.
• This decoupling of the LED and the phosphor layer makes possible a multicolor display device, wherein an array of different color phosphor pixels on a substrate such as a display window can be scanned by a moving LED or array of LEDs, with the intensity of the LED outputs controlled by a display signal, eg. , a video signal.
• a display signal eg. , a video signal.
• a principal advantage of such an arrangement is that a multicolor display may be obtained without the necessity of fabricating different LED structures for each of the desired colors of emission.
• the LED array can be an array of identical devices, which can be formed simultaneously on a single substrate.
• phosphor display screen 210 is composed of a repetitive pattern of R, G, B triplets
• the UV exciting radiation is supplied from a horizontal row 218 of UV emitting LEDs or lasers, identified as R, G, B, to indicate the particular phosphor stripes which they excite, ie, LED 220 excites stripe 212, LED 222 excites stripe 214, LED 224 excites stripe 216, and so on.
• the LED row only excites a portion of each stripe, shown as the horizontal row 226 on the display screen 210. This row defines a single row of pixels of the display.
• the LED row is scanned vertically along the screen, as indicated by the arrows, in synchronism with the display signal.
• a display input signal 230 controls the intensity of the UV output of the individual LEDs as it scans the display screen, thereby to create a full color display.
• each row of LEDs is addressed with the same signal information, but with a time delay.
• row 218c is addressed first, then row 218b, and finally row 218a.
• pixel R is illuminated by the three LEDs in column Cl , G by the LEDs in C2, etc., so that each pixel is illuminated three times in succession with the same display information during each row addressing period, thereby increasing the brightness of the display three times without sacrificing resolution of the display image.
• FIG. 6 Another embodiment of a color display device 300 of the invention is shown in Fig. 6.
• This device is similar to that shown in Fig. 5, in having a phosphor screen 310 composed of triplets (312,314,316) of vertical phosphor stripes, and a row 318 of LEDs (320,322,324) for exciting the screen with UV radiation in accordance with a display input signal 330.
• a single UV emitting LED or laser 610 is used. UV light from LED 610 is focussed by lens 612 into a beam and directed to prism 614, which spins about its axis A to achieve scanning of the beam in the x direction across phosphor screen 618. However, prior to landing on screen 618, the beam is reflected from mirror 616, which spins about its axis A' to achieve scanning of the beam in the y direction on the screen 618.
• Another way of achieving a color display device using the invention is to directly deposit layers of different color phosphors onto the output side, for example, the top, semi- transparent electrode surfaces of an array of UV emitting LEDs.
• Such a color display device 400 of the invention is shown in Fig. 7.
• a two dimensional matrix 402 of individually addressable R,G,B picture elements (402,404,406, etc) are driven in the conventional line-at-a-time manner, using row and column drivers 410 and 412, respectively, which receive display information from input signal source 414.
• the picture elements each consist of a UV emitting LED having an active region of InGaN, and the LEDs are covered with a layer of a UV excitable, visible light emitting phosphor.
• substrate 500 supports a row of LEDs 502, 504, 506, etc, having identical structures similar to that of the LED of Fig. 1 or the RC LED of Fig. 3.
• Deposited on the LEDs is a layer of a red phosphor/photoresist slurry composition 508, the photoresist component of which becomes insoluble upon being exposed to UV radiation.
• the layer 508 is selectively exposed to UV light by activation of the LEDs corresponding to the red picture elements of the display, rendering the layer surrounding these LEDs insoluble.
• the layer is then developed, ie, the unexposed portions of the layer are removed by treatment with a solvent, leaving red layers 510 on the "red" LEDs.
• This procedure is then repeated for the green picture elements, by coating the array with a green slurry composition 512, selectively activating the LEDs corresponding to the green picture elements to selectively insolubilize the photoresist, developing to remove the still soluble portions and leave green layers 514 on the "green” LEDs. Finally, the procedure is repeated for the "blue” LEDs, using blue slurry coating 516 to leave blue layers 518 on the "blue” LEDs.
• the emission spectrum of the LEDs may extend from the UV into the blue region of the visible spectrum. In this case, since the LEDs have some blue emission in addition to the UV emission, the blue LEDs may be left uncoated, as shown in Fig. 8(g).
• the LEDs may be coated by the so-called dusting technique, in which a UV sensitive photoresist is coated onto the LED array, and then selected portions of the coating are exposed to insolubilize them. This exposure step also results in the photoresist surface becoming tacky. While in this tacky stage, the coating is dusted with a dry powder of phosphor particles, which particles adhere to the tacky surface. The coating is then developed by rinsing away the non-exposed portions.
• dusting technique in which a UV sensitive photoresist is coated onto the LED array, and then selected portions of the coating are exposed to insolubilize them. This exposure step also results in the photoresist surface becoming tacky. While in this tacky stage, the coating is dusted with a dry powder of phosphor particles, which particles adhere to the tacky surface. The coating is then developed by rinsing away the non-exposed portions.
• FIG. 9 A schematic diagram of an exemplary driving scheme is shown in Fig. 9, in which four columns XI , X2, X3, X4, and four rows Yl , Y2, Y3, Y4 of a matrix array are shown, with an array of LEDs interconnected to the rows and columns.
• Typical UV excitable phosphors which may be used in the devices of the invention are: red YO 2 S 2 :Eu green ZnS:Cu, Ag blue BaMgAl j0 0 17 :Eu
• the various picture elements are individually and selectively addressed in order to produce a color display such as a video image.
• the device becomes a lamp, emitting a white light having a color temperature determined by the color coordinates and intensities of the individual R,G,B elements.
• the color temperature can be adjusted by changing the intensity, as well as the composition and mix of the individual color elements.
• Such lamps are of course intended to be included within the scope of the invention. In the alternative, such lamps could emit different colors of light in rapid sequence, by the sequential activation of selected ones of the LEDs.
• activating all red, then all green, then all blue LEDs would provide a source of alternating red, blue and green light, which would be useful as a back light in combination with a light valve such as a liquid crystal display, to form a frame sequential color display system.
• a light valve such as a liquid crystal display
• such systems rely upon the sequential display of the separate red, blue and green components of a color display signal at a frequency such that the observer integrates the separate components into a full color display image.

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• Devices For Indicating Variable Information By Combining Individual Elements (AREA)
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• 1997
  • 1997-05-28 EP EP97920917A patent/EP0856202A2/de not_active Withdrawn
  • 1997-05-28 JP JP10501390A patent/JPH11510968A/ja active Pending
  • 1997-05-28 WO PCT/IB1997/000606 patent/WO1997048138A2/en not_active Application Discontinuation

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