Patent Publication Number: US-2013228740-A1

Title: Light-emitting diode device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a light-emitting diode (LED) device, and more particularly to a group III-Nitride LED device. 
     2. Description of Related Art 
     A conventional light-emitting diode (LED) may have a homojunction structure or a heterojunction structure. A homojunction LED primarily includes an n-doped layer and a p-doped layer, which include the same material and thus have the same energy gap. A p-n junction is formed between the n-doped layer and the p-doped layer. 
     A heterojunction LED primarily includes a bottom cladding layer, an active layer, and a top cladding layer. The bottom/top cladding layer and the active layer have different materials and thus have different energy gaps. As a result, carriers may be confined in the active layer to form a well region. 
     The heterojunction LED is the more commonly used LED. The heterojunction LED&#39;s active layer typically includes a single quantum well (SQW) or a multi quantum well (MQW) with an energy gap smaller than the energy gap of the cladding layer. The energy gap difference increases re-combination rate of electrons and holes, luminescence efficiency, and light emitting output. The MQW has a greater light emitting output than the SQW, but, however, has a greater thickness that raises serial resistivity and its forward voltage. 
     As the quantum well structure in the LED is restricted in use, a photoluminescence (PL) lifetime and a carrier overflow cannot be effectively shortened, and a radioactive recombination rate therefore cannot be effectively enhanced. 
     Accordingly, a need has arisen for a novel quantum well structure that enhances the luminescence efficiency and the light emitting output. 
     SUMMARY OF THE INVENTION 
     In certain embodiments, at least one light-emitting diode (LED) unit includes at least one LED. The LED includes an n-side nitride semiconductor layer, a p-side nitride semiconductor layer, and an active layer located between the n-side nitride semiconductor layer and the p-side nitride semiconductor layer. The active layer includes one or more well layers. At least one of the well layers has a multilayered structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a cross section of an embodiment of a light-emitting diode (LED) device. 
         FIG. 1B  shows a cross section of an embodiment of a conventional LED device. 
         FIG. 1C  shows a cross section of an embodiment of an LED device. 
         FIG. 2A  shows an exemplary energy band diagram of the conventional LED in  FIG. 1B . 
         FIG. 2B  shows an exemplary energy band diagram of the LED in  FIG. 1C . 
         FIG. 3  shows an exemplary internal quantum efficiency (IQE) comparison between the LED of  FIG. 1B  and the LED of  FIG. 1C . 
         FIG. 4  shows a cross section of another embodiment of an LED device. 
         FIG. 5  shows a perspective diagram illustrating an embodiment of an LED device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  shows a cross section of an embodiment of light-emitting diode (LED) device  100 . For better appreciating the embodiment, drawings show layers that are most pertinent to the embodiment. LED device  100  includes at least one LED unit  10 , and each LED unit  10  includes at least one LED that is a group III-nitride LED. As shown in  FIG. 1A , LED unit  10  includes n-side nitride semiconductor layer  11 , p-side nitride semiconductor layer  15 , and active layer  13  situated between the n-side nitride semiconductor layer  11  and the p-side nitride semiconductor layer  15 . N-side nitride semiconductor layer  11  may include n-type gallium nitride (GaN), and p-side nitride semiconductor layer  15  may include p-type gallium nitride. In certain embodiments, active layer  13  includes one or more well layers  131 , at least one of well layers  131  may have a multilayered structure. 
     In some embodiments, active layer  13  is made of at least one well layer  131  and at least one barrier layer  132 , which are stacked alternately. The multilayered structure of active layer  13  may be a superlattice structure. The superlattice structure may include at least one first sub-well layer  1311  and at least one second sub-well layer  1312 . First sub-well layer  1311  and second sub-well layer  1312  may be stacked alternately to result in the multilayered structure. In some embodiments, first sub-well layer  1311  includes indium gallium nitride (In x Ga 1-x N), and second sub-well layer  1312  includes indium gallium nitride (In y Ga 1-y N), where the indium concentration x is different from the indium concentration y. It is noted that the indium gallium nitride with greater concentration x has a smaller energy gap than the indium gallium nitride with lesser concentration y, and barrier layer  132  (e.g., gallium nitride) has an energy gap greater than the indium gallium nitride. In some embodiments, well layer  132  includes a quaternary III-nitride such as aluminum indium gallium nitride (Al 0.1 In 0.2 Ga 0.7 N) to result in a polarization-matched structure. 
     In some embodiments, the LED device further comprises a substrate, wherein the LED unit is disposed on the substrate. The substrate includes sapphire, germanium (Ge), silicon carbide (SiC), gallium arsenide (GaAs), zinc oxide (ZnO), or lithium aluminum oxide (—LiAlO2). In some embodiments, substrate is polar substrate, semi-polar substrate, or non-polar substrate. 
     In some embodiments, the substrate is a polar substrate, at least one of the first sub-well layers and at least one of the second sub-well layers each has a thickness less than or equal to 2 nanometer (nm). In some embodiments, the substrate is a semi-polar substrate or non-polar substrate, at least one of the first sub-well layers and at least one of the second sub-well layers each has a thickness less than or equal to 10 nanometer (nm). Taking polar substrate for example, as shown in  FIG. 1A , well layer  131  includes at least three sub-well layers, which are approximately the same in thickness and each of which is less than or equal to 2 nanometer (nm) in thickness. Referring to  FIG. 1A , well layer  131  may include at least five sub-well layers, which are approximately the same in thickness and each of which is less than or equal to 2 nanometer (nm) in thickness. 
     In certain embodiments, as shown in  FIG. 1A , LED unit  10  may include first middle layer  12  placed between n-side nitride semiconductor layer  11  and active layer  13 . First middle layer  12  may or may not contact n-side nitride semiconductor layer  11  and/or active layer  13 . The LED unit may include second middle layer  14  between active layer  13  and p-side nitride semiconductor layer  15 . Second middle layer  14  may or may not contact active layer  13  and/or p-side nitride semiconductor layer  15 . In some embodiments, first middle layer  12  and second middle layer  14  have different constituents to result in an asymmetric structure. As shown in  FIG. 1A , first middle layer  12  may include aluminum gallium nitride (AlGaN) sub-layer  121  and indium gallium nitride (InGaN) sub-layer  122 . Indium gallium nitride sub-layer  122  may contact active layer  13 . Second middle layer  14  may include gallium nitride (GaN) sub-layer  141  and indium gallium nitride (InGaN) sub-layer  142 . Gallium nitride sub-layer  141  may contact active layer  13 . 
       FIG. 1B  and  FIG. 1C  show cross sections of embodiments of LED devices each having a single quantum well structure.  FIG. 1B  shows a cross section of conventional LED device  110 . LED device  110  includes active layer  13  with single well layer  131  (with a total thickness of 5 nm). Single well layer  131  has its constituents (e.g., including In 0.162 Ga 0.838 N) evenly distributed.  FIG. 1C  shows a cross section of an embodiment of LED device  120 . LED device  120  includes active layer  13  with single well layer  131  (with a total thickness of 5 nm). Single well layer  131  is a multilayered structure made of different constituents of at least one first sub-well layer  1311  (each having a thickness of 1 nm) and at least one second sub-well layer  1312  (each having a thickness of 1 nm). First sub-well layer  1311  and second sub-well layer  1312  are stacked alternately to result in the multilayered structure. 
     In some embodiments, first sub-well layer  1311  includes indium gallium nitride (In x Ga 1-x N) and second sub-well layer  1312  includes indium gallium nitride (In y Ga 1-y N), where the indium concentration x is different from (e.g., less than) the indium concentration y. For example, the concentration x may be equal to 0.14 and the concentration y may be equal to 0.18. It is noted that the indium gallium nitride with greater concentration x has a smaller energy gap than the indium gallium nitride with lesser concentration y. 
       FIG. 2A  shows an exemplary energy band diagram of conventional LED device  110  having a single quantum well as shown in  FIG. 1B .  FIG. 2B  shows an exemplary energy band diagram of LED device  120  having a multilayered single quantum well as shown in  FIG. 1C . According to the energy band diagram of  FIG. 2B , it is observed that there are five energy gaps corresponding to the five sub-well layers  1311 / 1312  of the single quantum well  131 . 
     The single quantum well of LED  120  device shown in  FIG. 1C  may include a superlattice structure (by respectively adjusting the energy gaps of the sub-well layers). The superlattice structure may substantially increase overlapping between an electron wave function and a hole wave function, effectively shorten a photoluminescence (PL) lifetime and a carrier overflow, and effectively enhance a radioactive recombination rate and an optical gain, thereby greatly increasing luminescence efficiency.  FIG. 3  shows an exemplary internal quantum efficiency (IQE) comparison between LED device  110  of  FIG. 1B  and LED device  120  of  FIG. 1C . Curve  31  represents conventional LED device  110  having a single quantum well and curve  32  represents LED device  120  having a multilayered single quantum well. According to  FIG. 3 , the LED device associated with the curve  32  has a larger efficiency than the LED device associated with the curve  31 . 
     Although a single LED is exemplified in the aforementioned embodiments, it is appreciated that the LED unit described herein may include a plurality of LEDs that are stacked.  FIG. 4  shows a cross section of an embodiment of LED device  130 . LED device  130  includes at least one LED unit  135 , and each LED unit includes stacked LEDs. In certain embodiments, LED unit  135  includes first LED  1  and second LED  2 , which are stacked via tunnel junction  44 . First LED  1  may include n-side nitride semiconductor layer  41 , active layer  42 , p-side nitride semiconductor layer  43 , and first electrode  40 . In certain embodiments, active layer  42  is situated between the n-side nitride semiconductor layer  41  and p-side nitride semiconductor layer  43 . First electrode  40  may be electrically connected with n-side nitride semiconductor layer  41 . For example, n-side nitride semiconductor layer  41  includes n-type gallium nitride (GaN), active layer  42  includes indium gallium nitride (InGaN), and p-side nitride semiconductor layer  43  includes p-type gallium nitride. Similarly, second LED  2  may include n-side nitride semiconductor layer  51 , active layer  52 , p-side nitride semiconductor layer  53   n  and second electrode  50 . In certain embodiments, active layer  52  is situated between n-side nitride semiconductor layer  51  and p-side nitride semiconductor layer  53 . Second electrode  50  is electrically connected with the p-side nitride semiconductor layer  53 . For example, n-side nitride semiconductor layer  51  includes n-type gallium nitride (GaN), active layer  52  includes indium gallium nitride (InGaN), and p-side nitride semiconductor layer  53  includes p-type gallium nitride. In some embodiments, active layer  42  of first LED  1  and active layer  52  of second LED  2  include the same constituent material, thereby emitting lights of the same wavelength. In other embodiments, active layer  42  of first LED  1  and active layer  52  of second LED  2  include different constituent materials, thereby emitting lights of distinct wavelengths. Relevant details may be referred, for example, to U.S. Pat. No. 6,822,991 to Collins et al., entitled “Light emitting devices including tunnel junctions,” disclosure of which is incorporated by reference as if fully set forth herein. 
       FIG. 5  shows a perspective diagram illustrating an embodiment of LED device  140 . LED device  140  includes a plurality of LED units  20  that are arranged on substrate  24  in an array form. LED device  140 , as shown in  FIG. 5 , may therefore be called an LED array. First electrode  25  of one LED unit  20  and second electrode  27  of a neighboring LED unit  20  may be electrically connected via solder wire  22  or an interconnect line. Thus, the LED units may be connected in series or parallel sequences. Taking the series connected sequence as an example, first electrode  25  of the most front LED unit  20  and second electrode  27  of the most rear LED unit  20  in the sequence are respectively connected to two ends of power supply  29 . LED unit  20 , as shown in  FIG. 5 , may be the single LED of  FIG. 1A  or the vertically stacked LEDs of  FIG. 4 . 
     It is to be understood the invention is not limited to particular systems described which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification, the singular forms “a”, “an” and “the” include plural referents unless the content clearly indicates otherwise. Thus, for example, reference to “a device” includes a combination of two or more devices and reference to “a material” includes mixtures of materials. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.