Abstract:
A light emitting diode capable of emitting first light having a first peak wavelength is combined with a first phosphor layer overlying the light emitting diode, the first phosphor layer capable of absorbing the first light and emitting second light having a second peak wavelength and a second phosphor layer overlying the light emitting diode, the second phosphor layer capable of emitting third light having a third peak wavelength.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. Pat. No. 6,696,703, granted Feb. 24, 2004 and incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to light emitting diodes (LEDs) and, more particularly, to a phosphor-converted LED device that utilizes one or more phosphor thin films for converting primary light emitted by the LED into one or more other frequencies of light in order to generate white light.  
           [0004]    2. Description of Related Art  
           [0005]    With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emission of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light. LEDs that produce white light are useful for signaling and/or illumination purposes.  
           [0006]    Currently, state-of-the-art phosphor conversion of a portion of the primary emission of the LED is attained by placing a phosphor in an epoxy that is used to fill the reflector cup, which houses the LED within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED and is subsequently cured.  
           [0007]    The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary light emitted by the LED passes through the epoxy without impinging on the phosphor particles, whereas a portion of the primary light emitted by the LED impinges on the phosphor particles, thereby causing the phosphor particles to emit complimentary light. The combination of the primary blue light and the phosphor-emitted light produces white light. One disadvantage of utilizing the epoxy comprising the phosphor particles is that uniformity in the white light emitted by the LED is difficult, if not impossible, to obtain. This non-uniformity is caused by non-uniformity in the sizes of the phosphor particles mixed into the epoxy slurry. Currently, phosphor powders having uniform phosphor particle sizes generally are not available. When the phosphor powder is mixed into the epoxy slurry, the larger phosphor particles sink faster than the smaller phosphor particles. This non-uniformity in spatial distribution of the phosphor particles exists in the epoxy once it has been cured.  
           [0008]    Therefore, obtaining a uniform distribution of the phosphor particles within the epoxy is very difficult, if not impossible, due to the non-uniformity of the sizes of the phosphor particles. This inability to control the sizes of the phosphor particles and their locations within the epoxy results in difficulties in producing LED lamps that emit white light in a consistent manner. Therefore, the quality of the white light produced by LED lamps may vary from one lamp to another, even for a given model manufactured by a particular manufacturer.  
           [0009]    Attempts have been made to overcome the disadvantages of using phosphor powders mixed with epoxies by placing luminescent organic dye films on a lens that encases the LED. The dye is carefully positioned on the lens at a particular location so that it totally absorbs all of the primary light impinging thereon and converts the primary light to light of a longer wavelength. A fraction of the primary light emitted passes through the lens without impinging on the dye. The primary light that does not impinge on the dye then combines with the longer-wavelength light to produce white light. Since the dye totally absorbs the primary light impinging thereon, any variation in the fraction of the primary light that is summed with the longer-wavelength light is supposed to be eliminated.  
           [0010]    However, this latter approach also has several disadvantages. The placement of the dye on the lens is subject to manufacturing uncertainties, which may result in variations in the white light produced. Also, dyes that are stable over long periods of time generally are not available. As a result, wide spread use of wavelength-converting dyes has not occurred.  
           [0011]    Accordingly, a need exists for a phosphor-converted LED that overcomes these problems and disadvantages.  
         SUMMARY  
         [0012]    In accordance with embodiments of the invention, a light emitting diode capable of emitting first light having a first peak wavelength is combined with a first phosphor layer overlying the light emitting diode, the first phosphor layer capable of absorbing the first light and emitting second light having a second peak wavelength and a second phosphor layer overlying the light emitting diode, the second phosphor layer capable of emitting third light having a third peak wavelength. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a perspective view of the light emitting diode device of the present invention before a phosphor thin film has been deposited thereon.  
         [0014]    [0014]FIG. 2 is a side view of the light emitting diode device of the present invention in accordance with a first embodiment.  
         [0015]    [0015]FIG. 3 is a side view of the light emitting diode device of the present invention in accordance with a second embodiment.  
         [0016]    [0016]FIG. 4 is a side view of the light emitting diode device of the present invention in accordance with a third embodiment.  
         [0017]    [0017]FIG. 5 is a side view of the light emitting diode device of the present invention in accordance with a fourth embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIG. 1 is a perspective view of a light emitting diode (LED)  1  that is suitable for incorporating one or more phosphor thin films therein in accordance with the present invention. However, it should be noted that the LED of the present invention is not limited to any particular LED. Those skilled in the art will understand that a variety of known LEDs are suitable for use with the present invention.  
         [0019]    For purposes of describing the typical components of the LED  1 , the LED  1  has been shown without any phosphor thin films disposed thereon. The LED I may comprise, for example, a light emitting structure  2 , which comprises two n-GaN layers  3  and  4 , an SQW or MQW GaInN layer  5 , a p-AlGaN layer  6  and a p-GaN layer  7 . The light emitting structure  2  also comprises an n-electrode bond pad  8 , an n-electrode  3 , a p-electrode bond pad  11  and a p-electrode  12 . The n-electrode  3  is comprised of GaN and the p-electrode  12  is either transmissive or reflective, as discussed below in more detail. The electrode bond pads  8  and  11 , when connected to a power supply (not shown), provide the biasing current for causing the LED  1  to emit light.  
         [0020]    The light emitting structure  2  is disposed on a substrate  13 , which preferably is sapphire, although other materials may be used for the substrate  13 . It should be noted that the materials used for creating the light emitting diode I are not limited to the materials discussed above with reference to FIG. 1. Those skilled in the art will understand that the light emitting diode  1  can be comprised of various types of materials. As stated above, the light emitting diode  1  is not limited to any particular type of light emitting diode, with the exception that the light emitting diode device utilized in accordance with the present invention emits a primary light that is either blue or ultraviolet, as discussed below in detail. Those skilled in the art will understand that various light emitting diodes are known that are suitable for this purpose.  
         [0021]    The light emitting structure  2  that generates the blue or UV primary emission preferably is grown epitaxially on either a sapphire (i.e., Al 2 O 3 ) or silicon carbide (i.e., SiC) substrate. Since both of these substrate materials are transparent to the primary radiation emission, the LED  1  can be mounted on a header (not shown) or in a reflector cup (not shown) in either a “normal” mounting configuration or a “flipchip” mounting configuration, which are known to those skilled in the art. As discussed below in detail, in some cases the substrate material may be transmissive, whereas in other cases it may be opaque. Those skilled in the art will understand that many substrate materials are suitable for these purposes.  
         [0022]    [0022]FIG. 2 is a side view of the LED device of the present invention in accordance with a first embodiment. The LED device  20  comprises a substrate  13  and an LED structure  2 , as shown in FIG. 1. In accordance with this embodiment of the present invention, a single phosphor thin film  21  is disposed on the surface of the light emitting structure  2  opposite the surface of the light emitting structure  2  that is in contact with the substrate  13 . The phosphor thin film  21  may be deposited on the light emitting structure  2  by a plurality of known methods. Phosphor thin films have been used primarily in the thin film electroluminescent display industry. Several known techniques may be used for depositing the phosphor thin film such as, for example, electron beam evaporation, thermal evaporation, rf-sputtering, chemical vapor deposition and atomic layer epitaxy.  
         [0023]    The method that is utilized for depositing the thin film may depend on the desired characteristics of the thin film. For example, if the thin film is to absorb all of the primary radiation emitted by the LED, one particular deposition technique may be used, whereas if the thin film is to allow a percentage of the primary radiation to pass through it, a different technique may be used. Those skilled in the art will understand which type of technique is to be utilized in order to obtain a thin film having the desired characteristics.  
         [0024]    Preferably, the method for depositing the thin film  21  shown in FIG. 2 is rf-sputtering. In accordance with this method, the phosphor powder is pressed into a target of a diameter slightly exceeding the LED wafer diameter such that a homogenous distribution of the phosphor film is ensured. As will be understood by those skilled in the art, the sputter gas characteristics can vary, but preferably the sputter gas mainly comprises Argon, but also comprises approximately 1% to approximately 3% of oxygen. The pressure and RF power input are matched to provide the thin film  21  with optimum thickness and homogeneity. The distance between the target and the substrate preferably is approximately 5 to 10 centimeters. The phosphor powder preferably is a Cerium-doped Yttrium-Aluminum-Garnet (Y 3 Al 5 O 12 :Ce 3+ ), also denoted as YAG:Ce. Those skilled in the art will understand that the present invention is not limited to using any particular type of phosphor for the thin film. Those skilled in the art will understand that other types of phosphors exist that are suitable for this purpose.  
         [0025]    Preferably, the phosphor thin film is deposited after the light emitting structure  2  has been grown on the substrate  13 . However, this may not always be possible, due to the fact that the deposition conditions might be incompatible with the requirements for maintaining the integrity of the light emitting structure  2 . Therefore, in some cases, it may be necessary to deposit the thin film before the light emitting structure is grown.  
         [0026]    The homogeneity of the composition and the thickness of the thin film can be further improved by rotating the LED wafer on a particular trajectory, such as eccentric circles, or on more complicated trajectories, which are sometimes referred to as “planetary motion.” This technique of rotating the wafer to improve the homogeneity of a material is known in the art. Since the manner in which phosphor thin films having a desired homogeneity can be created and deposited is known, no further discussion of the manner in which this is accomplished will be provided herein.  
         [0027]    During operation, the light emitting structure  2  generates primary blue  30  radiation. The primary emission impinges on the thin film  21 . A portion of the primary emission passes through the thin film  21  without exciting the dopants in the thin film. A portion of the primary emission impinging on the thin film  21  is absorbed by the dopants contained in the thin film and is converted into yellow light. This yellow light is emitted from the thin film  21  and combines with the portion of the unconverted primary radiation to form white light. The dopants are incorporated on an atomic scale into the phosphor thin film  21 . When these dopants are excited by primary radiation, the dopants emit yellow light.  
         [0028]    The total amount of dopants in the thin film is determined by their concentration and by the thickness of the thin film. The spatial distribution of the dopants in the thin film can be controlled with great precision. The techniques used for this purpose are common to thin film deposition processes used in the industry and are known to those skilled in the art. Those skilled in the art will understand the manner in which the amount of light-emitting dopants in the thin film and the spatial distribution of the dopants can be precisely controlled. By precisely controlling these characteristics of the thin film, the fraction of the primary radiation that will pass through the thin film without conversion is predictable and can be controlled. Therefore, the characteristics of the white light produced by the light emitting diode device  20  can be ensured. Thus, manufacturing uncertainties can be eliminated and LED devices having high quality and consistency can be obtained.  
         [0029]    It should be noted that the primary light may comprise light having more than one wavelength. Similarly, the light emitted in response to excitation by the primary light may comprise light of more than one wavelength. For example, the blue light emitted by thin film  21  may correspond to a plurality of wavelengths making up a spectral band. Wavelengths this spectral band may then combine with the unconverted primary light to produce white light. Therefore, although individual wavelengths are discussed herein for purposes of explaining the concepts of the present invention, it will be understood that the excitation being discussed herein may result in a plurality of wavelengths, or a spectral band, being emitted. Wavelengths of the spectral bands may then combine to produce white light. Therefore, the term “spectral band” is intended to denote a band of at least one wavelength and of potentially many wavelengths, and the term “wavelength” is intended to denote the wavelength of the peak intensity of a spectral band.  
         [0030]    [0030]FIG. 3 is a side view of the light emitting diode device  30  of the present invention in accordance with an alternative embodiment. In accordance with this embodiment, a reflective electrode bond pad  25  is disposed on a surface of the light emitting structure  2  and the phosphor thin film  21  is disposed on a surface of the substrate  13  opposite the surface of the substrate  13  that is in contact with the light emitting structure  2 . As stated above, the preferred substrate material is transparent. Therefore, the primary radiation emitted from the light emitting structure  2  that impinges on the reflective electrode bond pad  25  is reflected through the light emitting structure  2  and through the substrate  13  into the phosphor thin film  21 . Once the primary radiation impinges on the phosphor thin film  21 , the results are essentially identical to those discussed above with reference to the operations of the LED device  20  of FIG. 2. The phosphor thin film shown in FIG. 3 may be identical to the phosphor thin film shown in FIG. 2 and may also be created using the above-discussed rf-sputtering technique.  
         [0031]    Since, in accordance with this embodiment, the phosphor thin film  21  is deposited directly onto the substrate  13 , the phosphor thin film  21  may be deposited either before or after the light emitting structure  2  has been created and either before or after the reflective electrode bond pad  25  has been created.  
         [0032]    [0032]FIG. 4 is a side view of the light emitting diode device  40  of the present invention in accordance with an alternative embodiment in which the LED device  40  comprises at least two phosphor thin films  31  and 33 . The thin films  31  and  33  are deposited on the side of the substrate  13  opposite the side of the substrate  13  on which the light emitting structure  2  is to be disposed. In accordance with this embodiment, the phosphor thin films  31  and  33  preferably are deposited using atomic layer epitaxy. Preferably the thin film  31  disposed on the substrate  13  produces red emissions in response to the blue or ultraviolet primary emissions impinging on the light-emitting dopants contained in the thin film  31 . The phosphor thin film  33  disposed on phosphor thin film  31  emits green light in response to the blue or ultraviolet primary emissions impinging on the dopants contained in the thin film  33 . [ 00311  The red light emitted by thin film  31  is not absorbed by thin film  33 . However, the green light emitted by thin film  33  is partly absorbed by thin film  31 , and is converted into red light. Both of the thin film layers allow a portion of the primary blue light emitted by the light emitting structure  2  to pass through the thin films. This unconverted primary blue light combines with the red and green light emitted by thin film layers  31  and  33 , respectively, to produce white light.  
         [0033]    The thin film  33  that emits green light preferably is comprised of SrS:Ce and the thin film  31  that emits red light preferably is comprised of SrS:Eu. However, those skilled in the art will understand that the present invention is not limited with respect to the materials utilized for these thin films or with respect to the manner in which these thin films are deposited. The reflective electrode bond pad  25  reflects primary light generated by the light emitting structure  2  toward the thin films  31  and  33  and improves the efficiency of the LED device  40 .  
         [0034]    Alternatively, a thin film  31  totally absorbs all of the primary light and converts the primary light into blue light. Thin film  33  passes some of the blue light and converts some of the blue light into yellow light. The yellow light and the unconverted blue light combine to form white light, in the same manner described above with reference to FIGS. 2 and 3.  
         [0035]    [0035]FIG. 5 is a side view of the light emitting diode device  50  of the present invention in accordance with another embodiment. This embodiment of the present invention also utilizes a plurality of phosphor thin films  37  and  38 . However, in accordance with this embodiment, a dielectric mirror  36  is disposed between thin film  37  and the substrate  13 . The dielectric mirror  36  is fully transparent to the primary emission of the light emitting structure  2  but is highly reflective at the wavelength of the emissions of the phosphor thin films  37  and 38 . The phosphor thin films  37  and  38  may be identical to the phosphor thin films  31  and  33  shown in FIG. 4. Thus, the phosphor thin films  37  and  38  may produce red and green light, respectively, or blue and yellow light, respectively. Preferably, atomic layer epitaxy is used to deposit the thin films  37  and  38  in the manner discussed above with reference to FIG. 4.  
         [0036]    Incorporating the dielectric mirror  36  into the LED device  50  increases the overall efficiency of the LED device  50  by isolating the light emitting structure  2  from luminescence light produced by the phosphor thin films  37  and  38 , which might otherwise be re-absorbed by the light emitting structure  2 . The thin films  37  and  38  of FIG. 5 operate in a manner that is essentially identical to the manner in which the thin films  31  and  33  of FIG. 4 operate to produce white light. The red and green light emitted by thin films  37  and  38 , respectively, combines with the unconverted primary light emitted by the light emitting structure  2  to produce white light.  
         [0037]    The phosphor thin film  37  preferably is deposited onto the dielectric mirror  36  and then the phosphor thin film  38  is deposited onto the phosphor thin film  37 . It should be noted that the present invention is not limited with respect to the order in which the layers of the LED device  50  are incorporated into the LED device  50 . For example, the phosphor thin films can be deposited after the dielectric mirror  36  has been fabricated on the substrate, but before the light emitting structure  2  and the reflective electrode  25  have been fabricated. Alternatively, the phosphor thin films  37  and  38  may be deposited after all of the other layers, including the dielectric mirror  36 , the light emitting structure  2  and the reflective electrode  25 , have been formed.  
         [0038]    It will be understood by those skilled in the art that the present invention has been described with reference to particular embodiments, but that the present invention is not limited to these embodiments. Those skilled in the art will understand that various modifications may be made to the embodiments discussed above, which are within the scope of the present invention. As stated above, the present invention is not limited with respect to the materials used in the LED device, except that the LED device must be capable of generating primary emissions that are blue light or ultraviolet light. Those skilled in the art will also understand that, unless expressly stated herein, the present invention is not limited with respect to the order in which the layers or components of the LED device are formed. It will also be understood by those skilled in the art that the geometric arrangement or configuration of the phosphor thin films is not limited to any particular arrangement.  
         [0039]    For example, rather than using overlapping phosphor thin films in the manner described above with reference to FIGS. 4 and 5, a plurality of phosphor thin film segments, each of which luminesces a different color of light in response to blue or ultraviolet primary radiation impinging thereon, may be deposited on a common surface, For example, three different phosphor thin films segments (not shown) could be deposited in a checker-board configuration on the dielectric minor  36  shown in FIG. 5. In this case, one of the thin film segments would luminesce red light, one would luminesce green light and one would luminesce or transmit blue light, depending on whether the primary light is UV or blue. The segments would be arranged to effect total absorption of the primary radiation emitted from the light emitting structure  2 . The red, green and blue light generated by the respective phosphor thin films would then combine to create white light. Those skilled in the art will understand how various other configurations of thin film layers and segments could be incorporated into an LED device to produce an LED device that generates white light.  
         [0040]    Furthermore, it should be noted that it is not required that white light be produced by the LED device of the present invention. Those skilled in the art will understand the manner in which a phosphor thin film can be produced and utilized in accordance with the principles of the present invention to obtain an LED device that produces other colors of light. For example, those skilled in the art will understand, in view of the description provided herein, how a phosphor thin film may be obtained that produces green light by totally absorbing the blue or UV primary emission.