Abstract:
A visible light emitting device includes: three types of LED elements stacked one on another; and first and second optical filters. Each of the LED elements has a light emitting layer configured to emit light of one of three primary colors. Each of the first and second optical filters is disposed between two adjacent ones of the LED elements, and each of the first and second optical filters is operable to reflect or absorb a shorter wavelength light of the lights emitted from two adjacent LED elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-105976, filed on Apr. 13, 2007; the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a light emitting device. 
         [0004]    2. Background Art 
         [0005]    There are conventionally known light emitting devices with a stacked structure of three types of LED elements each having a light emitting layer for any of the three primary colors. 
         [0006]    JP-A 10-319877(Kokai) (1998) discloses a compact light emitting device having a plurality of wavelengths and high brightness, made by combining a semiconductor light emitting element with wavelength conversion materials such as phosphors in various configurations. In this patent document, light emitting elements having different emission wavelengths are stacked into a compact multi-wavelength light source to serve as a light source for an image display device. In this light source, a red light emitting element is stacked via a connection means on a blue light emitting element, and a green light emitting element is further stacked thereon via a connection means. When a current is supplied to such stacked light emitting elements, blue light from the blue light emitting element can be extracted upward without being shaded by the other light emitting elements. Red light from the red light emitting element passes through the green light emitting element and can be extracted upward. Green light from the green light emitting element can be extracted upward without being shaded by the other light emitting elements. Thus a compact light source having high brightness can be realized by stacking light emitting elements for different colors in this manner. In this device, no filter is disposed between the elements. 
         [0007]    JP-A 8-213657(Kokai) (1996) discloses a light emitting device in which light emitting layers for the three primary colors of blue, green, and red are bonded and stacked together by annealing to allow multicolor light emission. This document has no description on an interlayer filter. In such a structure, light with a shorter wavelength excites a light emitting element with a longer wavelength, failing to emit light of a desired color. JP-A 11-233827(Kokai) (1999) discloses a light emitting device in which light emitting layers for the three primary colors are stacked by epitaxial growth. In this light emitting device, each light emitting layer has a light confinement layer. Hence, apparently, light can be extracted only in the horizontal direction with respect to the layered structure. Furthermore, with regard to conventional white illuminations based on LED devices, it is pointed out that phosphors suitable to red light have yet to be found, resulting in poor color rendition. 
       SUMMARY OF THE INVENTION 
       [0008]    According to an aspect of the invention, there is provided a visible light emitting device including: three types of LED elements stacked one on another, each of the LED elements having a light emitting layer configured to emit light of one of three primary colors; and first and second optical filters, each of the first and second optical filters disposed between two adjacent ones of the LED elements, and each of the first and second optical filters being operable to reflect or absorb a shorter wavelength light of the lights emitted from two adjacent LED elements. 
         [0009]    According to an aspect of the invention, there is provided a visible light emitting device including a stacked body including: a first light emitting element which emits a light having a spectrum peak at a first wavelength range; a second light emitting element which emits a light having a spectrum peak at a second wavelength range which is shorter than the first wavelength range; and an optical filter provided between the first and second light emitting elements, at least one of absorbance and reflectance of the optical filter being higher at the second wavelength range than at the first wavelength range. 
         [0010]    According to another aspect of the invention, there is provided a visible light emitting device comprising a stacked body including: a first light emitting element which emits a light having a spectrum peak at a first wavelength range; a third light emitting element which emits a light having a spectrum peak at a third wavelength range which is different from the first wavelength range; a second light emitting element provided between the first and third light emitting elements, the second light emitting element emitting a light having a spectrum peak at a second wavelength range which is different from the first and third wavelength ranges; a first optical filter provided between the first and second light emitting elements, at least one of absorbance and reflectance of the first optical filter being higher at the second wavelength range than at the first wavelength range; and a second optical filter provided between the second and third light emitting elements, at least one of absorbance and reflectance of the second optical filter being higher at the third wavelength range than at the second wavelength range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A ,  1 B and  1 C are cross-sectional views of a red LED element, a green LED element and a blue LED element according to the embodiment. 
           [0012]      FIG. 2  is a schematic cross-sectional view of a stacked body in which the blue LED element, the green LED element, and the red LED element are stacked. 
           [0013]      FIGS. 3A and 3B  are a schematic cross-sectional view and a schematic plan view of a visible light emitting device comprising the stacked body of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The invention will now be described with reference to an embodiment. 
         [0015]    The embodiment is described with reference to  FIGS. 1 to 3 . 
         [0016]      FIG. 1A  is a cross-sectional view of a red LED element emitting red light,  FIG. 1B  is a cross-sectional view of a green LED element emitting green light,  FIG. 1C  is a cross-sectional view of a blue LED element emitting blue light,  FIG. 2  is a schematic cross-sectional view of a stacked body in which the blue LED element, the green LED element, and the red LED element are stacked,  FIG. 3A  is a schematic cross-sectional view of a visible LED device comprising the stacked body of  FIG. 2 , and  FIG. 3B  is a schematic plan view of the visible LED device of  FIG. 3A . 
         [0017]      FIG. 1A  is a cross-sectional schematic view showing the structure of the red LED element constituting the visible LED device. 
         [0018]    In this embodiment, LED elements emitting the three primary colors of red, green, and blue are grown on respective substrates and bonded together with an optical filter interposed between the LEDs. With electrodes formed thereon, the LED elements for the three primary colors are formed into one chip. In this figure, the light emitting direction of the visible LED device is downward. 
         [0019]    The red LED element  100  comprises a p-AlGaAs lower cladding layer  2 , a p-AlGaAs active layer  3 , an n-AlGaAs upper cladding layer  4 , and an n-AlGaAs contact layer  5  formed on a p-GaAs substrate  1 . The semiconductor layers formed on the p-GaAs substrate  1  are sequentially formed by MOCVD (metal organic chemical vapor deposition), for example. The red LED element  100  emits a light having a spectrum peak at a wavelength range of red. 
         [0020]    The green LED element  200  comprises a p-GaP layer  11  and an n-GaP layer  12  formed on a p-GaP substrate  10 . The semiconductor layers formed on the p-GaP substrate  10  are sequentially formed by LPE (liquid phase epitaxy), for example. The green LED element  200  emits a light having a spectrum peak at a wavelength range of green. 
         [0021]    The blue LED element  300  comprises a GaN buffer layer  21 , an n-GaN contact layer  22 , an n-AlGaN lower cladding layer  23 , a p-InGaN active layer  24 , a p-AlGaN upper cladding layer  25 , and a p-GaN contact layer  26  formed on a sapphire substrate  20 . The semiconductor layers formed on the sapphire substrate  20  are sequentially formed by MOCVD, for example. The blue LED element  300  emits a light having a spectrum peak at a wavelength range of blue. 
         [0022]      FIG. 2  shows the situation where these LED elements are stacked. The stacked LED elements constitute one element of the visible LED device of this embodiment. The blue LED element, the green LED element, and the red LED element are stacked with the substrates  20 ,  10 , and  1  facing down to constitute one LED element of the visible LED device of this embodiment. 
         [0023]    Further, an optical filter  31  is provided between the red LED element  100  and the green LED element  200 . At least one of absorbance and reflectance of the optical filter  31  is higher at the wavelength range of green than at the wavelength range of red. The optical filter  31  may be a band-pass filter which allows a light of the wavelength range of red which is included in the light emitted from the red LED element  100  to pass through. Alternatively, the optical filter  31  may be a high-cut filter which reflects or absorbs lights having wavelengths shorter than that of a light of the wavelength range of red which is included in the light emitted from the red LED element  100 . 
         [0024]    An optical filter  32  is also provided between the green LED element  200  and the blue LED element  300 . At least one of absorbance and reflectance of the optical filter  32  is higher at the wavelength range of blue than at the wavelength range of blue. The optical filter  32  may be a band-pass filter which allows a light of the wavelength range of green which is included in the light emitted from the green LED element  200  and a light of the wavelength range of red which is included in the light emitted from the red LED element  100  to pass through. Alternatively, the optical filter  32  may be a high-cut filter which reflects or absorbs lights having wavelengths shorter than that of a light of the wavelength range of green which is included in the light emitted from the green LED element  200 . 
         [0025]    Next, a process for forming this LED element is described. 
         [0026]    As shown in  FIG. 2 , as the optical filter  31 , a dichroic filter which blocks light at green and shorter wavelengths is formed by vapor deposition on the n-GaP layer  12  of the green LED element  200 , for example. Furthermore, as the optical filter  32 , a dichroic filter which blocks light at blue and shorter wavelengths is formed by vapor deposition on the p-GaN layer  26  of the blue LED element  300 , for example. 
         [0027]    A translucent resin  33  such as an epoxy resin adhesive is applied onto the green LED element  200  provided with the dichroic filter  31 , and the substrate  1  constituting the red LED element  100  is bonded onto the green LED element  200 . A translucent resin  34  such as an epoxy resin adhesive is applied onto the blue LED element  300  provided with the dichroic filter  32 , and the substrate  10  constituting the green LED element  200  is bonded onto the blue LED element  300 . That is, in this structure, the green LED element is adjacent to the blue LED element, and the red LED element is adjacent to the green LED element. 
         [0028]    Next, the stacked body including the LED elements and optical filters shown in  FIG. 2  is used to form a visible LED device. 
         [0029]    As shown in  FIG. 3B , by using a photoresist, a corner of the upper surface of the stacked body including the LED elements is dry etched to partly expose the n-GaN contact layer  22  and the p-GaN contact layer  26  of the blue LED element, the p-GaP substrate  10  and the n-GaP layer  12  of the green LED element, and the p-GaAs substrate  1  of the red LED element. Electrodes are formed later on each of the exposed portions and the n-AlGaAs contact layer  5  at the upper surface. 
         [0030]    Next, an insulating film  40  is formed on the surface of the etched LED element. The insulating film  40  is illustratively made of silicon oxide. 
         [0031]    Next, the above-mentioned electrodes are formed. To form the electrodes, the insulating film  40  is trench etched so that trenches reaching the n-GaN contact layer  22 , the p-GaN contact layer  26 , the p-GaP substrate  10 , the n-GaP layer  12 , the p-GaAs substrate  1 , and the n-AlGaAs contact layer  5  are formed on the respective layers. Then the trenches are filled with copper, for example, to form electrodes  41 - 46  in the trenches and on the surface of the insulating film  40 . The electrode  41  is connected to the n-GaN contact layer  22 , the electrode  42  is connected to the p-GaN contact layer  26 , the electrode  43  is connected to the p-GaP substrate  10 , the electrode  44  is connected to the n-GaP layer  12 , the electrode  45  is connected to the p-GaAs substrate  1 , and the electrode  46  is connected to the n-AlGaAs contact layer  5  ( FIG. 3A ). 
         [0032]    Consequently, the basic structure of the visible LED device is formed. 
         [0033]    The visible LED device emits blue light upon application of voltage between the electrodes  41  and  42 , emits green light upon application of voltage between the electrodes  43  and  44 , and emits red light upon application of voltage between the electrodes  45  and  46 . The light emitting direction is the stacked direction of the LED elements  100 ,  200 , and  300 , which is the direction of the arrow in the figure. One of the blue light, the green light, and the red light is emitted by appropriately applying a voltage between the electrodes. A mixed color light including two of the blue light, the green light and the red light can also be emitted. Further, the voltages between the electrodes can be adjusted to mix the red light, green light, and blue light into mixed color light including desired visible light to be emitted. For example, a white light having a desired spectrum can be obtained. When the mixed color light is emitted, the wavelength and the spectrum thereof can be adjusted by appropriately adjusting the current flown through the LED elements  100 ,  200 , and  300 , respectively. 
         [0034]    The dichroic filter can be formed by laminating thin films. The film material can be oxides, fluorides, sulfides, or metals. The dichroic filter has a characteristic of transmitting a particular wavelength band of visible light and reflecting the other wavelength bands. The transmitted and reflected wavelengths can be varied by changing the type of film materials and the manner of stacking. 
         [0035]    In this embodiment, a dichroic filter is used as an optical filter. However, a color filter may be used instead. The dichroic filter is made of a multilayer dielectric film, having the characteristic of transmitting and reflecting particular wavelengths. 
         [0036]    As a major method to form a color filter, pigment having a particle diameter of approximately 0.1 μm is dispersed using a dispersant. The type of pigment can be monoazo-based or triphenylmethane-based, and can be suitably selected to vary the transmitted wavelength. 
         [0037]    The color filter is illustratively based on a resist, having the characteristic of transmitting a particular range of wavelengths. Furthermore, the color filter can be used as an adhesive between the LED elements, thereby eliminating the need to use an extra adhesive. 
         [0038]    While the filter  32  between the blue LED element and the green LED element is a filter blocking light at blue and shorter wavelengths in the above embodiment, a filter blocking only blue light may also be used. Likewise, while the filter  31  between the green LED element and the red LED element is a filter blocking light at green and shorter wavelengths in the above embodiment, a filter blocking only green light may also be used. 
         [0039]    In the above embodiment, the optical filter has the characteristic of blocking the shorter wavelength light of the adjacent LED elements. However, the filter may include the characteristic of reflecting such light. 
         [0040]    In the above embodiment, the filter between the blue LED element and the green LED element is formed on the blue LED element and then bonded to the green LED element. However, this filter may be formed on the green LED element using an adhesive. The same also applies to the filter between the green light emitting layer and the red light emitting layer. 
         [0041]    Furthermore, in the emission of white light and other multicolor light, the three primary colors are not limited to red, green, and blue. In such a case (in the case of three primary colors other than the set of red, green, and blue), the characteristic of the optical filter can be selected so as to block the shorter wavelength light of the lights from the adjacent LED elements. 
         [0042]    Moreover, a filter reflecting red light may be disposed on the n-AlGaAs contact layer  5  of the red LED element  100  to increase the light emission efficiency. 
         [0043]    Thus, in the visible LED device including stacked LED elements according to this embodiment, light with a shorter wavelength is prevented from being incident on the LED element with a longer wavelength, and light of a desired color can be emitted with a desired intensity.