Patent Application
20050110032
1. Field of the Invention
The present invention relates to light-emitting diodes and semiconductor light-emitting devices.
2. Background Art
In recent years, the development of white light-emitting diodes (LEDs) into practical, working devices has flourished. White LEDs utilize short-wavelength LEDs—blue LEDs or ultraviolet LEDs—and phosphors that by the agency of light emitted from these LEDs fluoresce. Such white LEDs are utilized for example as the light source for liquid-crystal display backlights. Reference is made to
Apart from the configurations illustrated in
White LEDs of a configuration in which a blue LED is covered with a synthetic polymer containing a phosphorescent material, as is the case with the configuration disclosed in Pat. App. Pub. Nos. H07-99345 and H10-93146, suffer from problems such as follows. In mixing phosphorescent material into the synthetic polymer, in general a phosphorescent material in powdered form is employed, but the phosphor powder tends to clump within the synthetic polymer, making it difficult to obtain uniform (yellow light) luminescence. The white light is consequently not uniform, with mottled hues being produced instead.
In the configuration disclosed in Pat. App. Pub. Nos. H11-31845 and H11-46019, meanwhile, a phosphorescent material is spread onto a blue LED. Spreading the phosphorescent material on at a uniform thickness is, however, problematic; moreover, special equipment is required in order to apply the phosphorescent material, which is in powder or paste form, onto the LED. Finally, the method as in Pat. App. Pub. No. H11-46015, in which a phosphorescent film is formed using a sputtering technique, is impracticable in that too much time is required for forming the phosphorescent film to the necessary thickness, making productivity poor.
Taking the problems discussed above into consideration, it occurred to the present inventors that if the phosphorescent material is utilized in solid form its thickness can be readily adjusted to yield uniform luminescence and keep the white light from being blotchy. Nevertheless, in for example the configuration illustrated in
An object of the present invention, which has been brought about taking the foregoing issues into consideration, is to make available light-emitting diodes and semiconductor light-emitting devices that enable an emission spectrum in which mottling is minimized to be achieved.
In order to resolve the problems discussed above, a first light-emitting diode according to the present invention is characterized in being furnished with: (1) a semiconductor laminate composed of nitride semiconductors, and having a first electroconductive semiconductor layer, a second electroconductive semiconductor layer, and an active layer, provided in between the first electroconductive semiconductor layer and the second electroconductive semiconductor layer, for emitting light in a first band of wavelengths; (2) a first optically reflective layer provided on a surface of the laminate opposite the surface of the first electroconductive semiconductor layer that faces onto the active layer; (3) a second optically reflective layer provided on a surface of the laminate opposite the surface of the second electroconductive semiconductor layer that faces onto the active layer; and (4) a phosphor, provided on a side face of the semiconductor laminate, for receiving light of the first wavelength band and emitting light in a second band of wavelengths.
In the above-described first light-emitting diode, the fact that the semiconductor laminate is sandwiched between the first and the second reflective layers means that the band #1 light output from the active layer is reflected at the first and second reflective layers and emitted through the side face of the laminate. Then inasmuch as a phosphor is provided on the side face of the semiconductor laminate, the phosphor is pumped by the band #1 light and emits band #2 light. An emission spectrum that is tuned depending on the band #1 light and the band #2 light is thus produced. Pursuant to a first light-emitting diode as set forth above, band #1 light from the active layer is gathered on the side face of the semiconductor laminate, and because the phosphor is meanwhile provided on the side face of the semiconductor laminate, by arbitrarily adjusting the thickness of the phosphor the intensity of the band #2 light can readily be made to conform with the intensity of the band #1 light. An emission spectrum in which unevenness of hue is minimized can accordingly be achieved.
The light-emitting diode further may be characterized in that the semiconductor laminate is additionally furnished, in between the first electroconductive semiconductor layer and the first optically reflective layer, with a substrate made from a GaN-based compound. Substrates in general are sufficiently thicker than the semiconductor layers, which are typically formed by an epitaxial growth technique. This accordingly means that by providing the first electroconductive semiconductor layer, active layer, and second electroconductive semiconductor layer on a substrate, electric current supplied to the layers will diffuse throughout the substrate and therefore band #1 light generated in the active layer will spread along the plane of the active layer (that is, heading toward the side faces of the semiconductor laminate). Pursuant to this light-emitting diode, band #1 light can consequently be taken out from the side face of the semiconductor laminate more efficiently.
The light-emitting diode may be further characterized in that the phosphor is a phosphorescent sheet glued to the side face of the semiconductor laminate. Inasmuch as the thickness of the phosphorescent sheet can be easily adjusted, furnishing the light-emitting diode with the phosphorescent sheet as the phosphor allows an emission spectrum in which mottling is minimized to be readily produced.
A second light-emitting diode according to the present invention is characterized in being furnished with: (5) a semiconductor laminate composed of nitride semiconductors, and having a first electroconductive semiconductor layer, a second electroconductive semiconductor layer, and an active layer, provided in between the first electroconductive semiconductor layer and the second electroconductive semiconductor layer, for emitting light in a first band of wavelengths; (6) a first optically reflective layer provided on a surface of the laminate opposite the surface of the first electroconductive semiconductor layer that faces onto the active layer; and (7) a second optically reflective layer provided on a surface of the laminate opposite the surface of the second electroconductive semiconductor layer that faces onto the active layer; and is characterized in that the semiconductor laminate further includes a substrate made of a GaN-based compound, situated in between the first electroconductive semiconductor layer and the first optically reflective layer, and in that the substrate contains a luminescent source for receiving light of the first wavelength band and emitting light in a second band of wavelengths.
In the above-described second light-emitting diode, inasmuch as the semiconductor laminate includes the substrate containing the luminescent source, the substrate luminescent source is partially pumped by the band #1 light output from the active layer, which generates band #2 light within the substrate. This thus means that rays of light in both bands #1 and #2 are reflected at the first and second reflective layers, mix together with each other, and are emitted from the side face of the semiconductor laminate. An emission spectrum that is tuned depending on the band #1 light and the band #2 light is thus produced. Pursuant to a second light-emitting diode as set forth above, because rays of band #1 light from the active layer and rays of band #2 light generated inside the substrate are gathered on the side face of the semiconductor laminate, the optical intensity of the rays in band #1 and that of the rays in band #2, emitted from the side face of the semiconductor laminate, can readily be made to conform with each other. An emission spectrum in which unevenness of hue is minimized can accordingly be achieved.
The light-emitting diode may be further characterized in being additionally furnished with an optically reflective film provided on a side face among the plural side faces of the semiconductor laminate excepting its light-emitting face. This makes it possible to gather band #1 light (and band #2 light) onto a specified side face (light-emitting face) among the side faces of the semiconductor laminate, thereby restricting the emitting direction of the light and contributing to enhancing the efficiency with which the light is taken out.
In addition, a first semiconductor light-emitting device according to the present invention is characterized in that a plurality of any of the light-emitting diodes described above is arranged stacked in the laminating orientation. The semiconductor light-emitting device enables light of comparatively large luminous energy to be produced, and the configuration enables the device to be scaled down comparatively.
Furthermore, a second semiconductor light-emitting device according to the present invention is a semiconductor light-emitting device equipped with at least one of any of the light-emitting diodes described above, and is characterized in being further furnished with a rectangular light guide having a light-extraction surface; an optically reflective surface, on the side of the guide opposite the light-extraction surface, for reflecting light in the first and second bands of wavelengths; and a side surface that intersects with the orientation in which the light-extraction surface extends and the orientation in which the reflective surface extends. The device is therein characterized in that the light-emitting diode is mounted on the light guide so that the side face of the semiconductor laminate and the side face of the light guide oppose each other. Since light from the side face of the semiconductor laminate in the light-emitting diode is presented to the side face of the light guide, the thickness of the light guide may be the same as the thickness of the semiconductor laminate. Accordingly, the light guide can be made thinner compared with conventional backlights and similar light-emitting devices. And the fact that as the light-emitting diode, the second semiconductor light-emitting device is equipped with any of the light-emitting diodes described above enables light in which unevenness of hue in the emission spectrum is minimized to be emitted from the light-extraction surface.
In accordance with the present invention, by means of a light-emitting diode as well as a semiconductor light-emitting device an emission spectrum in which unevenness of hue is minimized can be achieved.
From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.
In the following a detailed explanation of embodiments of a light-emitting diode as well as a semiconductor light-emitting device according to the present invention will be made while referring to the accompany drawings. It should be understood that in explaining the drawings, identical elements are labeled with identical reference marks, and redundant explanation will be omitted.
The laminate 6 is composed of a substrate 7, an n-type cladding layer 9 laminated onto the substrate 7 and serving as a first conductive semiconductor layer, an active layer 11 provided on the n-type cladding layer 9, and a p-type cladding layer 13 and a p-type contact layer 15 successively laminated onto the active layer 11 and each serving as a second conductive semiconductor layer.
The substrate 7 is made of a conductive GaN-based compound, and in the present embodiment, the substrate 7 is composed of GaN. The substrate 7 allows light L1 generated in the active layer 11 to be transmitted through it, and has a resistivity of, for example, not more than 0.5 Ωcm, a thickness of, for example, 100 μm to 200 μm. In planar dimension the substrate 7 is, for example, a rectangle 0.5 mm to 30 mm to a side.
The n-type cladding layer 9 is composed of a nitride semiconductor doped with n-type impurities. For example, in the present embodiment, the n-type cladding layer 9 is composed of AlX1Ga1-X1N (0≦X1<1) doped with Si, and is formed on a principal surface 7a of the substrate 7.
Carriers are introduced to the active layer 11 to generate the light L1 in a first band of wavelengths. In accordance with the present embodiment, the first band of wavelengths is that of, for example, blue light (420 nm to 490 nm). The active layer 11 is formed on the n-type cladding layer 9 and has a multiple quantum well structure.
The barrier layers 38a-38c and the well layers 39a and 39b are made of a GaN-based semiconductor represented by a chemical equation such as AlX2InY2Ga1-X2-Y2N (0≦X2<1; 0≦Y2<1; 0≦X2+Y2<1). In accordance with the present embodiment, the compositions of the barrier layers 38a-38c are represented by the above equation with 0<X2<1 and Y2=0, while the compositions of the well layers 39a and 39b are represented by the same equation with 0<X2<1 and 0<Y2<1. In addition, the compositions of the barrier layers 38a-38c and the well layers 39a and 39b are adjusted so that the band gaps of the barrier layers 38a-38c will be larger than those of the well layers 39a and 39b. This configuration traps carriers introduced into the active layer 11 in the well layers 39a and 39b.
With reference to
The p-type contact layer 15 is provided to electrically connect the p-type cladding layer 13 and an anode electrode (described below), and is composed of a nitride semiconductor doped with p-type impurities. For example, in the present embodiment, the p-type contact layer 15 is composed of GaN doped with Mg. The p-type contact layer 15 is formed on the p-type cladding layer 13.
The reflective layer 17 is provided on a side of the p-type contact layer 15 opposite the side on which the active layer 11 is provided. It is preferable that the reflective layer 17 is provided on the entire surface of the p-type contact layer 15. The reflective layer 17 is made of, for example, a metal such as Ag or Al, and reflects the light L1 generated in the active layer 11. Although the anode is integrated with the reflective layer 17 in the present embodiment, the anode can also be provided separately. In addition, the reflective layer 17 and the p-type contact layer 15 can be in ohmic contact.
The reflective layer 19 is provided on a side of the n-type cladding layer 9 opposite the side on which the active layer is provided. It is preferable that the reflective layer 19 is provided on the entire surface of the n-type cladding layer 9. In accordance with the present embodiment, since the semiconductor laminate 6 includes the substrate 7, the reflective layer 19 is provided on the rear surface 7b of the substrate 7. Similar to the reflective layer 17, the reflective layer 19 is also made of, for example, a metal such as Ag or Al, and reflects the light L1 generated in the active layer 11. Although the reflective layer 19 is provided separately from the cathode in the present embodiment, the reflective layer 19 can also be integrated with the cathode. In addition, the reflective layer 19 and the substrate 7 can be in ohmic contact.
The reflective film 25 is provided on a side of the semiconductor laminate 6 excepting side with a light-emitting surface 6a, and the insulating layer 23 is interposed between the semiconductor laminate 6 and the reflective film 25. In accordance with the present embodiment, the optically reflective film 25 and the insulating layer 23 are provided on three of the four side surfaces of the semiconductor laminate 6, and the side surface on which no optically reflective film 25 is provided serves as the light-emitting surface 6a. The reflective film 25 is isolated from the semiconductor laminate 6, the reflective layers 17 and 19, and the cathode electrode 21 by the insulating layer 23. Similar to the reflective layers 17 and 19, the reflective film 25 is also made of, for example, a metal such as Ag or Al, and reflects the light L1 generated from the active layer 11.
The phosphorescent section 5 includes a phosphorescent plate 27 and an adhesive layer 29. The phosphorescent plate 27 is molded from a phosphor in solid form that receives the light L1 and subsequently generates light L2 in a second band of wavelengths longer than that of the light L1. In accordance with the present embodiment, the second band of wavelengths is that of, for example, yellow light (500 nm to 700 nm), and the phosphor is, for example, ZnSSe. The phosphorescent plate 27 is glued to the side surface of the semiconductor laminate 6 by the adhesive layer 29. In accordance with the present embodiment, the phosphorescent plate 27 is provided on the one surface, out of the four surfaces, on which no optically reflective film 25 is provided (in other words, the light-emitting surface 6a). It will be appreciated that the thickness of phosphorescent plate 27 along the direction intersecting with the side surface of the semiconductor laminate 6 is adjusted so that the luminous energy of the luminescence (yellow light) L2 generated in the phosphorescent plate 27, will by means of that yellow light L2 and blue light L1 turn into white light of a desired hue.
The operation of an LED 1 having the above described configuration is as follows. When a drive voltage is applied from the exterior of the LED 1 to a region between the cathode electrode 21 and the reflective layer 17, which serves also as the anode electrode, an electric field is generated between the cathode electrode 21 and the reflective layer 17, and carriers are concentrated in the well layers 39a and 39b within the active layer 11. Accordingly, blue light L1 is generated in the active layer 11 and concentrated on the light-emitting surface 6a after being reflected by the reflective layers 17 and 19 and the reflective film 25. A portion of the blue light L1 pumps the phosphorescent plate 27 and turns into yellow light L2, while the rest transmits through the phosphorescent plate 27 as blue light. As a result, the generated blue light L1 and yellow light L2 combine to become white light that is emitted to the exterior from the LED 1.
Next, with reference to
Firstly, the wafer-shaped substrate 7 made of conductive GaN is prepared as shown in
Next, resists 50a and 50b are respectively deposited on the cathode electrode 21 and the reflective layer 17 as shown in
Subsequently the resists 50a and 50b as represented in
In accordance with the present embodiment described above, the LED 1 has the following effects. Firstly, since the blue light L1 from the active layer 11 is concentrated on the side surface of the semiconductor laminate 6 and the phosphorescent plate 27 is also provided on the side surface thereof, the intensities of the yellow light L2 and blue light L1 can be easily matched by adjusting the thickness of the phosphorescent plate 27 accordingly, thereby creating white light with a stabilized hue.
Furthermore, it is preferable that the semiconductor laminate 6 in the LED 1 includes the substrate 7, which is provided between the n-type cladding layer 9 and the reflective layer 19, as described in the present embodiment. The substrate 7 is sufficiently thicker than a semiconductor layer formed by, for example, epitaxial growth. Thus, owing to the n-type cladding layer 9, the active layer 11, and the p-type cladding layer 13 being provided on the substrate 7, current supplied to the layers diffuses within the substrate 7, thereby causing the portion of the active layer 11 that generates blue light L1 to spread in an orientation that intersects with the lamination orientation of the active layer 11 (in other words, towards the sides of the semiconductor laminate 6). As a result, the LED 1 enables blue light L1 to be more efficiently extracted from the side surface of the semiconductor laminate 6.
It is also preferable that the LED 1 be furnished with the phosphorescent plate 27 that serves as a phosphor as described in the present embodiment. Inasmuch as the intensity of the yellow light L2 can be easily regulated by adjusting the thickness of the phosphorescent plate 27, by means of an LED 1 according to the present embodiment white light in which mottling is minimized can be readily produced.
As described in the present embodiment, it is preferable that the reflective film 25 is provided on three of the four side surfaces of the semiconductor laminate 6 in the LED 1. By doing so, blue light L1 can be concentrated on one specific side surface of the semiconductor laminate 6, thereby restricting the direction in which the white light is emitted and contributing to enhancing the efficiency with which the light is taken out.
In addition, because the LED 1 according to the present embodiment differs from the conventional surface-emitting type of LED in that light is extracted from a side face, a plurality of the LEDs 1 can be stacked while retaining a relatively small dimension. Therefore, providing a plurality of the LEDs 1 to generate a desired light intensity is a simple matter. Moreover, the LED 1 according to the present embodiment utilizes a solid phosphor (phosphorescent plate 27) instead of the phosphor-intermixed synthetic polymer used in conventional LEDs, and therefore the phosphor can be easily mounted on the semiconductor laminate 6, which improves productivity and yields of the LED 1.
The substrate 8 is made of a conductive GaN-based compound. In addition, the substrate 8 includes the luminescent sources 20 that receives light (blue light L1) in a first band of wavelengths and generates light (yellow light L3) in a second band of wavelengths. In growing the GaN substrate 8 by a process such as vapor phase epitaxy, impurities such as oxygen and carbon can be added or crystal defects (nitrogen holes) introduced into the substrate. The impurities such as oxygen and carbon, or crystal defects such as nitrogen holes serve as the luminescent sources 20, receiving blue light L1 and generating luminescence (yellow light L3). For example, when irradiated with light having a wavelength shorter than 480 nm, the luminescent sources 20 generate luminescence with a wide wavelength range of 520 nm to 650 nm. The central wavelength and emission spectrum of the luminescence are controlled by the types and quantity of impurities added or the quantity of crystal defects.
Besides the luminescent sources 20, the substrate 8 in the LED 1b also includes the semiconductor laminate 6, such that a portion of the blue light L1 output from the active layer 11 pumps the luminescent sources 20 in the substrate 8 to generate yellow light L3 within the substrate 8. Blue light L1 and yellow light L3 are then reflected by the reflective layers 17 and 19 and the reflective film 25 and mix together with each other to generate white light, which in turn is emitted from the light-emitting surface 6a that is a side surface of the semiconductor laminate 6.
According to the present embodiment, blue light L1 emitted from the active layer 11 and yellow light L3 generated within the substrate 8 are concentrated at the side surface (light-emitting surface 6a) of the semiconductor laminate 6, so that the intensities of the blue light rays L1 and the yellow light rays L2 emitted from the side surface of the semiconductor laminate 6 can readily be made to conform with each other. As a result, white light in which mottling is kept under control can be emitted from the LED 1b.
Furthermore, a protrusion 45 formed integrally with the case 41 is provided on the bottom of the groove 41a, and the LED 1 is positioned so that either the cathode electrode 21 or the anode electrode (light-emitting layer 17, cf.
In both the semiconductor light-emitting device 2 and the semiconductor light-emitting device 2a, drive voltage applied from the exterior of the semiconductor light-emitting device through the lead pins 47a and 47b is directed to a region between the anode electrode (light-emitting layer 17) and the cathode electrode 21 of the LED(s) 1. Accordingly, as described in Embodiment 1, white light generated by means of blue light and yellow light is emitted from one side surface of the LED 1, that is, the semiconductor laminate 6 side on which the phosphorescent plate 27 is provided. The white light is emitted heading towards the exterior of the semiconductor light-emitting device 2 (or 2a).
According to the present embodiment, since both semiconductor light-emitting devices 2 and 2a include the LED 1 described in the Embodiment 1, white light with a stabilized hue can be emitted from the semiconductor light-emitting devices. Moreover, white light having a relatively large intensity can also be easily obtained since the semiconductor light-emitting device 2a includes a plurality of LEDs 1. Furthermore, since the LEDs 1 are configured so that light is emitted from the side face, the LEDs 1 can be stacked along the thickness direction, which allows the semiconductor light-emitting device to be fabricated in a relatively small size even though the device is furnished with a plurality of LEDs 1.
The light guide 37 is a white-light-guiding, surface-emitting component. The light guide 37 is preferably made of a material that diffuses white light, such as acrylic or polycarbonate. The light guide 37 receives white light L4 from the LED 1d, which is mounted on the side surface of the light guide 37, and subsequently emits the white light L4 from a principal surface 37a of the guide. In other words, the principal surface 37a serves as a light-emitting surface of the light guide 37. A rear surface 37b opposite the principal surface 37a of the light guide 37 is composed of a metal film 35 made of, for example, Al and reflects the white light L4. The rear surface 37b of the light guide 37 serves as a reflective surface that reflects the white light L4 composed of blue light and yellow light.
The white light L4 emitted from the LED 1d passes through the interior of the light guide 37, with a portion being reflected at the metal film 35, and reaches the principal surface 37a of the light guide 37. At that moment the white light L4 is diffused in the light guide 37 interior. Thus, with white light L4 being emitted approximately uniformly from the principal surface 37a (light-emitting surface) of the light guide 37, the light guide 37 surface-emits.
According to the present embodiment, the semiconductor light-emitting device 2b has the following effects. Firstly, since light from the side surface of the semiconductor laminate 6 in the LED 1d is presented to the side surface of the light guide 37, the light guide 37 may be of the same thickness as that of the semiconductor laminate 6. Thus the thickness of the light guide 37 is reduced as compared to the conventional semiconductor light-emitting device such as a backlight.
In the conventional backlight 110 shown in
Furthermore, since the semiconductor light-emitting device 2b according to the present embodiment includes the LED 1d, which has the same configuration as the LED 1 described in Embodiment 1, white light L4 with a stabilized hue can be reflected by the rear surface 37b and emitted from the principal surface 37a (light-emitting surface). Although the present embodiment describes a semiconductor light-emitting device 2b including only one LED 1d, multiple LEDs with a common light-emitting direction can also be attached to the side surface of the light guide 37.
The LEDs and the semiconductor light-emitting devices of the present invention are not limited to the above described embodiments and modified examples, and in addition various modifications are possible. For example, although the LED in each embodiment described above is furnished with a substrate made from a GaN-based compound, the substrate is not limited to that material, and may be made of another electroconductive compound, such as a SiC, AIN, Si, etc.
In addition, the materials used as the reflective layers and the reflective films are also not limited only to metals; other materials such as photonic crystals in which the refractive index is adjusted to reflect light can also be used. Additional reflective films can also be provided on the side surfaces of the phosphorescent plate along the thickness direction, thereby preventing the light from escaping through the side surfaces of the phosphorescent plate, and thus further stabilizing the hue of the white light.
Only selected embodiments have been chosen to illustrate the present invention. To those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for limiting the invention as defined by the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP-2003-390882 | Nov 2003 | JP | national |
