Abstract:
The present invention relates to a semiconductor LED device comprising a pumping layer with high light emitting efficiency and an active layer with smaller bandgap converting the absorbed light into any kinds of light of wavelength as required, which generates light from the AlGaInN pumping layer containing less In, projects the rays of light on the active layer containing more In, lets the required light of wavelength emit and decreases the blue shift caused by electric current, thereby increasing the light emitting efficiency and emitting lights with more than two wavelengths from one Led device. This invention enables to obtain various light of wavelength from one device and form the element through only one epitaxy process, thereby increasing reproductivity, yield, and efficiency by not using the fluorescent materials lowering the efficiency when forming white light.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a semiconductor LED (light emitting diode; hereinafter, referred to as ‘LED’) device, and, in particular, to an AlGaInN-based LED device. The LED stacks a pumping-layer, which has a high light exchange efficiency of a current, with a low efficient active-layer, which has a smaller bandgap. As a result, a LED device with multi-wavelengths can be embodied in a device by transforming wavelengths in the active-layer. Also, white light can be achieved in a device by mixing two wavelengths.  
           [0003]    2. Description of the Background Art  
           [0004]    Generally, switching devices such as a metal semiconductor field effect transistor or a high electron mobility transistor HEMT, and light devices such as a semiconductor laser or a light emitting diode, are used as compound semiconductor devices.  
           [0005]    A semiconductor laser has a smaller driving current and a smaller size than other lasers. The semiconductor laser can directly transform light intensity and frequency according to variation of a current. The semiconductor laser has a high laser efficiency of more than 70% usually, and has a wide wavelength range. As a result, the semiconductor laser is widely used as optical communication, office appliances, such as a laser printer or a bar-code reader, and other small-sized appliances.  
           [0006]    [0006]FIG. 1 is a cross-sectional view illustrating a first example of the conventional LED device, wherein an insulating and transparent substrate is used in an AlGaInN LED.  
           [0007]    A buffer layer  11 , an n-type AlGaInN layer  12 , an AlGaInN active layer  13 , a p-type AlGaInN layer  14  and a transparent electrode  15  are sequentially stacked on an insulating substrate  10 . A p-type metal electrode  16  is formed on one side of the transparent electrode  15 .  
           [0008]    An n-type metal electrode  17  is formed on an exposed portion of the n-type AlGaInN layer  12  formed by removing a portion of the p-type AlGaInN layer  14 , the active layer  13 , the n-type AlGaInN layer  12  on the other side of the substrate  10 . Thereby light is emitted into the upper and lower portions of the insulating transparent substrate  10 .  
           [0009]    [0009]FIG. 2 is a cross-sectional view illustrating a second example of the conventional LED device, wherein a conductive semiconductor substrate is used in an AlGaInN LED device.  
           [0010]    A buffer layer  21 , an n-type AlGaInN layer  22 , an AlGaInN active layer  23  and a p-type AlGaInN layer  24  are sequentially stacked on one side of a conductive semiconductor substrate  20  such as Si or SiC. A p-type metal electrode  26  is formed on a portion of the p-type AlGaInN layer  24 . An n-type metal electrode  27  is formed on the other side of the conductive semiconductor substrate  20 .  
           [0011]    As described above, in general, compound semiconductor light emitting devices have structures in which holes are transported from a p-type metal electrode combine with electrons transported through a n-type metal electrode in a single or multi layered active layer to emit a light corresponding to a bandgap of an active layer composition. Most of the light is emitted from the p-n junction type active layer to upper and lower surfaces of the active layer.  
           [0012]    Generally, major characteristics of the LED such as an emission and wavelength are determined by an active layer. Accordingly, a crystalline composition of an active layer is very important.  
           [0013]    In a semiconductor device having a conventional AlGaInN-based LED structure, a light with one wavelength is emitted from an active layer so that either fluorescent materials or various types of LEDs were required to obtain various kinds of colors.  
           [0014]    In addition, an LED package having fluorescent phosphor in the epoxy mold shown in FIG. 3 is required to obtain white light because a light with one wavelength is emitted from an active layer in a semiconductor device having a conventional AlGaInN-based LED structure.  
           [0015]    An LED device  31  of FIG. 1 is mounted on an upper portion of a metal lead frame  30 . Metal electrodes of the LED device are connected to the metal lead frame with a wire  32 . The LED device  31  is covered with fluorescent material  33 . A body  34  molded with a transparent resin is formed.  
           [0016]    In the white light LED package, a light with a first wavelength  35  is emitted from the LED device by current supply to the LED device. When this light excites fluorescent materials, a light with a second wavelength  36  is generated by fluorescent materials, and a mixed light of the lights with the first and the second wavelength  35  and  36  is obtained. When the colors of these two lights are complementary to each other, white light can be obtained.  
           [0017]    For example, if a yellow light with wavelength of 590 nm, which is a complementary color of a blue light with wavelength of 450 nm, is generated by exciting fluorescent material which contains a YAG using the blue LED light with wavelength of 450 nm, an LED package of white light which is a mixture of the two colors is obtained. Here, the white light is obtained by adjusting composition of a YAG fluorescent layer to tune a yellow wavelength, thereby regulating a ratio of quantity of two lights.  
           [0018]    The white light LED package can substitute a light bulb or a back light source of display. However, while the conventional white light LED package has a simple structure and manufacturing process, the reliability of fluorescent materials is inferior to an LED device. As a result, if used for a long period, its color fades, or its luminous efficiency is deteriorated.  
           [0019]    In addition, it is difficult to adjust composition and the thickness of fluorescent materials according to a chip-to-chip variation of a wavelength and a power in a first blue wavelength of LED so that a white light tuning is difficult according to mass production.  
         SUMMARY OF THE INVENTION  
         [0020]    Accordingly, an object of the present invention is to provide a semiconductor LED device from which a desired wavelength light can be efficiently obtained by embodying a high-powered pumping-layer wherein light generated from the pumping-layer is absorbed in an active layer and the absorbed light is recombined in the active layer to obtain a light with a desired wavelength.  
           [0021]    In order to achieve the above-described objects of the present invention, there is provided a semiconductor LED device, comprising: an active layer consisting of AlGaInN, wherein the active layer absorbs and transforms a light into a light of a desired wavelength; and an pumping layer consisting of AlGaInN formed perpendicularly to the active layer, wherein the pumping layer has a larger bandgap than that of the active layer and emits light by recombination of electrons and holes.  
           [0022]    In one aspect of the present invention, a semiconductor LED device also includes: a first conductive type AlGaInN layer formed on a substrate; an active layer consisting of AlGaInN formed on the first conductive type AlGaInN layer; a first AlGaInN layer formed on the active layer; an pumping layer consisting of AlGaInN formed on the first conductive type AlGaInN layer to expose one side of the first conductive type AlGaInN layer, the pumping layer having a larger bandgap than the active layer; a second conductive type AlGaInN layer formed on the pumping layer; a second conductive type metal electrode formed on the second conductive type AlGaInN layer; and a first conductive metal electrode formed on the exposed portion of the first conductive AlGaInN layer.  
           [0023]    In another aspect of the present invention, a semiconductor LED device is characterized in that the active layer comprises two or more Al x Ga y In z N/Al x1 Ga y1 In z1 N layers repeatedly stacked, the pumping layer comprises two or more Al a Ga b In c N/Al a1 Ga b1 In c1 N layers repeatedly stacked, a bandgap Eg(Al x Ga y In z N) of an Al x Ga y In z N layer corresponding the active layer is smaller than a bandgap Eg(Al x1 Ga y1 In z1 N) of an Al x1 Ga y1 In z1 N layer corresponding to the barrier layer, and a bandgap Eg(Al a Ga b In c N) of an Al a Ga b In c N layer corresponding to the pumping layer is smaller than a bandgap Eg(Al a1 Ga b1 In c1 N) of an Al a1 Ga b1 In c1 N layer corresponding to the barrier layer, wherein x+y+z=1, x1+y1+z1=1, a+b+c=1 and a1+b1+c1=1. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein;  
         [0025]    [0025]FIG. 1 is a cross-sectional view illustrating a first example of the conventional LED device.  
         [0026]    [0026]FIG. 2 is a cross-sectional view illustrating a second example of the conventional LED device.  
         [0027]    [0027]FIG. 3 is a cross-sectional view illustrating an LED package wherein the LED device of FIG. 1 is mounted.  
         [0028]    [0028]FIG. 4 is a cross-sectional view illustrating an LED device in accordance with a first preferred embodiment of the present invention.  
         [0029]    [0029]FIG. 5 is a band diagram illustrating the LED device of FIG. 4.  
         [0030]    [0030]FIG. 6 is a cross-sectional view illustrating a LED device in accordance with a second preferred embodiment of the present invention.  
         [0031]    [0031]FIG. 7 is a cross-sectional view illustrating a LED device in accordance with a third preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    A semiconductor LED device of the present invention will be described in detail with reference to the accompanying drawings.  
         [0033]    Generally, the efficiency of an AlGaInN-based light device is deteriorated with In composition of AlGaInN layer. A light emitted from an InGaN active layer containing In of about 35% has a wavelength of about 470 nm. The power output in this case is about 35 mW depending on a structure of a device. On the other hand, light of an active layer containing In of about 5% or about 22% have wavelengths of 380 nm or 430 nm, respectively. In this case, power output of more than about 10 mW can be obtained even with the same structure. This is due to deterioration of the quality of crystal as the quantity of In in an active layer increases, resulting in low efficiency. If the quantity of In is increased, a phenomenon of a light being transformed into a light with a shorter wavelength during the process of injection of current into the active layer, i.e. a blue shift phenomenon is increased, thereby deteriorating the efficiency.  
         [0034]    A noble structure according to the present invention takes advantages of high quality of a pumping layer with low In composition and an active layer with high In composition, particularly the active layer absorbing light to transform into light as is instead of recombining electrons and holes by a current to emit light. Thus, light with a desired wavelength can be obtained from the same conductive type n-n active layer rather than from the conventional p-n structure.  
         [0035]    According to the principle described above, inefficient lights with wavelengths of 470 nm(blue), 525 nm(green) and 635 nm(red) can be easily obtained by using a pumping layer emitting high efficient light having short wavelength such as lights having wavelengths of 380-430 nm.  
         [0036]    In addition, by utilizing this principle, light having more than one wavelength can be obtained from one LED device. Lights having two or more wavelengths can be properly combined to obtain Lights with new colors. For example, if a deep blue light having a wavelength of 450 nm generated from a pumping layer is absorbed in an active layer to generate a yellow light having a wavelength of 590 nm, which is a complementary color of blue, white light can be obtained. The wavelength of the emitted light can be controlled by adjusting a thickness of an active layer and the number of stacked layers. Also a white light can be obtained by adjusting the amount of light absorbed in and penetrating through the active layer, which is emitted from a pumping layer.  
         [0037]    As an alternate method, light with two wavelengths can also be obtained by emitting a portion of light in the pumping layer to combine with light from an active layer.  
         [0038]    Since fluorescent materials are not used in an LED of multi-wavelengths or white light, the reliability of a device is improved. In addition, since epitaxial wafer obtained from an epitaxial growth process is applicable to the conventional blue LED manufacturing process, the manufacturing process is simplified and a device with superior uniformity can be obtained to improve yield.  
         [0039]    A semiconductor LED device of the present invention comprises an active layer which determines a wavelength and a power output of an LED, and a pumping layer perpendicular to the active layer, which supplies energy to the active layer, wherein a light generated from the pumping layer is absorbed in the active layer.  
         [0040]    [0040]FIG. 4 is a cross-sectional view illustrating a semiconductor LED device in accordance with a first preferred embodiment of the present invention.  
         [0041]    Referring to FIG. 4, a buffer layer  41 , an n-type Al x1 Ga y1 In z1 N layer  42  (x1+y1+z1=1), an active layer  43  comprising a stacked structure of Al x Ga y In z N and Al x1 Ga y1 In z1 N layers repeatedly stacked, an n-type AlGaInN layer  44 , a pumping layer  45  comprising a stacked structure of Al a Ga b In c N and Al a1 Ga b1 In c1 N layers repeatedly stacked, a p-type AlGaInN layer  46  and an opaque p-type metal electrode  47  are sequentially staked on an insulating transparent substrate  40  consisting of alumina, sapphire or quartz, using a preferred MOCVD method. Here, an upper portion of the n-type AlGaInN layer  44  on one side of the transparent substrate  40  and layers formed thereon are removed. An n-type metal electrode  48  is formed on the exposed portion of the n-type AlGaInN layer  44 .  
         [0042]    The active layer  43  and the pumping layer  45  respectively comprises a stacked structure of two or more layers with different bandgaps repeatedly stacked although they are shown as single layers in FIG. 4. The active layer  43  and the pumping layer  45  comprise repeatedly stacked multiple Al x Ga y In z N/Al x1 Ga y1 In z1 N layers and multiple Al a Ga b In c N/Al a1 Ga b1 In c1 N layers. However, they can also be formed of single layers.  
         [0043]    The reason for employing multi-layers is to utilize characteristics of an AlGaInN-based device such as high luminous efficiency superior to that of single layer.  
         [0044]    Here, the active layer  43  and the pumping layer  45  satisfy the equations x+y+z=1, x1+y1+z1=1, a+b+c=1 and a1+b1+c1=1, respectively. A bandgap Eg(Al x Ga y In z N) of an Al x Ga y In z N layer included in the active layer  43  is smaller than a bandgap Eg(Al x1 Ga y1 In z1 N) of an Al x1 Ga y1 In z1 N layer corresponding to a barrier layer. A bandgap Eg(Al a Ga b In c N) of an Al a Ga b In c N layer included in the pumping layer  45  is smaller than a bandgap Eg(Al a1 Ga b1 In c1 N) of an Al a1 Ga b1 In c1 N layer corresponding to a barrier layer. The Eg(Al x Ga y In z N) is larger than the Eg(Al a Ga b In c N).  
         [0045]    The principle of operation of the semiconductor LED device according to the present invention will be described referring to FIG. 5.  
         [0046]    [0046]FIG. 5 is a band diagram illustrating the LED device of FIG. 4, wherein barrier structures of the active layer  43  and the pumping layer  45  are simplified as an AlGaInN layer, i.e. when x1=0, y1=1, z1=0, a1=0, b1=1, c1=0. A bandgap  50  of the well portion, Eg(Al a Ga b In c N), of the pumping layer is larger than a bandgap  51  of the well portion, Eg(Al x Ga y In z N), of the active layer  43 .  
         [0047]    Holes  53  provided through a p-type AlGaInN layer  46  by a current supplied from p-electrodes are captured in the pumping layer  45  and electrons  52  provided through an n-type AlGaInN layer  42  in the pumping layer  45 . The electrons and the holes are combined in the pumping layer  45  to emit a light  54  corresponding to the bandgap Eg  40  of the pumping layer  45 . A light  55  emitted into the lower portion of the pumping layer  45  is absorbed in the active layer  43 . A light-emitted into the upper portion of the pumping layer  45  is reflected in a p-type metal electrode  48  and reabsorbed in the active layer  43 . The absorbed light generates electrodes  56  and holes  57 . Electrons  56  and holes  57  are captured by the active layer  43  and they are recombined in the active layer  43  to emit a light  58  corresponding to the bandgap Eg  41  of the active layer. This light has the lowest energy possible in an LED structure and can be emitted entirely through a substrate without being absorbed in any layer. When the amount of Al increases, a bandgap increases while when the amount of In increases, a bandgap decreases, bandgaps of the active layer  53  and the pumping layer  55  can be adjusted by utilizing these characteristics.  
         [0048]    The semiconductor layers can be formed using the methods such as MOCVD (Metal-Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy) or VPE (Vapor Phase Epitaxy).  
         [0049]    [0049]FIG. 6 is a cross-sectional view illustrating a semiconductor LED device in accordance with a second preferred embodiment of the present invention. The semiconductor LED device has a similar structure to that of the LED of FIG. 4. However, the semiconductor LED device comprises a window for emitting a light formed by patterning one side of a p-type metal electrode.  
         [0050]    A transparent substrate  40 , a buffer  41 , an n-type AlGaInN layer  42 , an active layer  43  comprising a stacked structure of Al x Ga y In z N/Al x1 Ga y1 In z1 N layers repeatedly stacked, an n-type AlGaInN layer  44 , a pumping layer  45  comprising a stacked structure of Al a Ga b In c N/Al a1 Ga b1 In c1 N layers repeatedly stacked, a p-type AlGaInN layer  46  and a p-type metal electrode  49  are sequentially formed. The p-type metal electrode  49  is partially removed to form a window W exposing the p-type AlGaInN layer  46  and emitting a light therethrough. An n-type metal electrode  48  is formed on one side of the n-type AlGaInN layer  44 .  
         [0051]    The semiconductor LED device of FIG. 6 can be configured to control the amount of light emitted through the window by adjusting the size of a window W. An open width of the window W is preferably 0˜300 m. The wavelength of light emitted from each side of a substrate can be adjusted to be different.  
         [0052]    A conductive substrate can be used instead of a transparent substrate. In the case of using a transparent substrate a transparent electrode is used for a p-type electrode so that a light can be emitted therethrough.  
         [0053]    The substrates in the above embodiment are all transparent substrates such as sapphire, alumina or quartz substrates, but conductive substrates such as SiC or Si substrates can be used.  
         [0054]    [0054]FIG. 7 is a cross-sectional view of a semiconductor LED device in accordance with a third preferred embodiment of the present invention. The semiconductor LED device has a similar structure to that of the LED of FIG. 4. However, an active layer is stacked on the upper portion of a pumping layer.  
         [0055]    A buffer layer  41 , an n-type AlGaInN layer  42 , an active layer  43  comprising a stacked structure of Al x Ga y In z N/Al x1 Ga y1 In z1 N layers repeatedly stacked, an n-type AlGaInN layer  44 , a pumping layer  45  comprising a stacked structure of Al a Ga b In c N/Al a1 Ga b1 In c1 N layers repeatedly stacked, a p-type AlGaInN layer  46 , an active layer  43   a , a p-type AlGaInN layer  46   a  and a p-type metal electrode  47  are sequentially formed On a transparent substrate  40 . An n-type metal electrode  48  is formed on the n-type AlGaInN layer  44  at one side of the substrate  40 .  
         [0056]    As described above, light with multiple wavelengths or white light can be obtained from one device by adjusting composition and thickness of the active layers  43  and  43   a . Since the active layers  43  and  43   a  are provided on both sides of the pumping layer  45  to emit a light to the both sides.  
         [0057]    Also light of different wavelengths can be emitted into both directions by forming the p-type metal electrode  59  with a transparent electrode.  
         [0058]    As described earlier, a semiconductor LED device according to the present invention comprises a pumping layer with high light efficiency and an active layer comprising multiple layers with homo-junction which has smaller bandgaps and can transform a received light into light of a desired wavelength. Light is generated in a pumping layer consisting of AlGaInN, and then emitted into the active layer with more In than that of the pumping layer to emit a light of a desired wavelength. As a result, a blue shift by a current is reduced to improve luminous efficiency. Light of two or more wavelengths can be obtained from one LED device. Since the LED device is formed using one continuous growth process, reproducibility of a device is improved, thereby improving yield. Since fluorescent materials for with white light with low efficiency is not used, efficiency is improved.  
         [0059]    In addition, a mono-colored LED of single wavelength in which a light from a pumping layer is transformed into a light with a wavelength of an active layer can be embodied by adjusting the thickness and the number of the active layer to increase absorption in the active layer. Mounting of an LED device using bumps is possible to simplify a mounting process. Al a Ga b In c N/Al a1 Ga b1 In c1 N layers. However, they can also be formed of single layers.  
         [0060]    The reason for employing multi-layers is to utilize characteristics of an AlGaInN-based device such as high luminous efficiency superior to that of single layer.  
         [0061]    Here, the active layer  43  and the pumping layer  45  satisfy the equations x+y+z=1, x1+y1+z1=1, a+b+c=1 and a1+b1+c1=1, respectively. A bandgap Eg(Al x Ga y In z N) of an Al x Ga y In z N layer corresponding to an active layer is smaller than a bandgap Eg (Al x1 Ga y1 In z1 N) of an Al x1 Ga y1 In z1 N layer corresponding to a barrier layer. A bandgap Eg(Al a Ga b In c N) of an Al a Ga b In c N layer corresponding a pumping layer is smaller than a bandgap Eg (Al a1 Ga b1 In c1 N) of an Al a1 Ga b1 In c1 N layer corresponding to a barrier layer. The Eg(Al x Ga y In z N) is larger than the Eg(Al a Ga b In c N).  
         [0062]    The principle of operation of the semiconductor LED device according to the present invention will be described referring to FIG. 5.  
         [0063]    [0063]FIG. 5 is a band diagram illustrating the LED device of FIG. 4, wherein barrier structures of the active layer  43  and the pumping layer  45  are simplified as an AlGaInN layer, i.e. when x1=0, y1=1, z1=0, a1=0, b1=1, c1=0. A bandgap  50  of the well portion, Eg(Al a Ga b In c N), of the pumping layer is larger than a bandgap  51  of the well portion, Eg(Al x Ga y In z N), of the active layer  43 .  
         [0064]    Holes  53  provided through a p-type AlGaInN layer  46  by a current supplied from p-electrodes are captured in the pumping layer  45  and electrons  52  provided through an n-type AlGaInN layer  42  in the pumping layer  45 . The electrons and the holes are combined in the pumping layer  45  to emit a light  54  corresponding to the bandgap Eg  40  of the pumping layer  45 . A light  55  emitted into the lower portion of the pumping layer  45  is absorbed in the active layer  43 . A light