Patent Publication Number: US-7901964-B2

Title: Method of fabricating AC light emitting device having photonic crystal structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/065,063, filed on Feb. 27, 2008, which is the U.S. national phase application of PCT International Application No. PCT/KR2006/03541, filed Sep. 6, 2006, and claims priority to Korean Patent Application No. 10-2005-0107516, filed Nov. 10, 2005, the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a light emitting device, and more particularly, to an AC light emitting device having photonic crystal structures. 
     BACKGROUND OF THE INVENTION 
     Recently, a GaN-based light emitting diode (LED) has been widely used as a light emitting device. GaN-based LEDs have significantly advanced the LED technology and have been employed in various applications including full color LED displays, LED traffic signals, white LEDs and the like. 
     Recently, high-efficiency white LEDs have been expected to replace fluorescent lamps. In particular, the efficiency of white LEDs is approaching that of ordinary fluorescent lamps. However, the LED efficiency can be further improved, thus its continuous improvement will be required. 
     Two major approaches to improve the efficiency of LEDs have been attempted. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second one is to increase the light extraction efficiency. Since the internal quantum efficiency currently reaches 70˜80%, there is little room for further improvement of the internal quantum efficiency. However, the light extraction efficiency may be further improved. The technology for improving the light extraction efficiency mainly relates to the enlargement of the “escape cone.” However, the enlargement of the escape cone cannot completely eliminate light loss due to the total internal reflection. Further, even for light radiated into the escape cone, reflection loss occurs due to the refractive index mismatch between the LED and the surrounding media. 
     A new approach to reduce total internal reflection and reflection loss and improve light extraction efficiency is disclosed in U.S. Pat. No. 5,955,749, entitled “Light emitting device 
     utilizing a periodic dielectric structure” by Joannopoulos et al. According to the U.S. Pat. No. 5,955,749, a lattice of holes is formed in semiconductor layers of a light emitting diode to create photonic crystals. The lattice creates a medium with a periodically varying dielectric constant and affects a path of light transmitting through the medium. This lattice forms a photonic band gap (PBG) and photons having energy within the PBG cannot propagate inside the photonic crystal. Therefore, if a photonic crystal is formed such that the energy of photons emitted by LED is within the PBG, the lateral propagation of photons is prohibited to allow all the photons to be exited from the LED to the outside, so that the light extraction efficiency can be improved. 
     However, the aforementioned U.S. Pat. No. 5,955,749 discloses an LED having GaAs-based n-type, p-type and active layers, and the lattice of holes is formed on these layers. Thus, during the formation of the holes by etching the GaAs-based layers, the layers can be easily damaged. In addition, GaAs has a very high surface recombination rate at which holes and electrons are recombined in a surface thereof. The surface recombination of holes and electrons does not generate light having a desired wavelength. Thus, the light quantum efficiency is seriously reduced, since the surface area of the active layer is increased as the lattice of holes is formed. 
     Furthermore, a light emitting diode is a semiconductor device which is formed into a p-n junction structure of semiconductors and emits light by the recombination of electrons and holes, and the light emitting diode is generally driven by current flowing in one direction. Thus, in a case where the light emitting diode is driven by alternating current, an A/D converter is required to convert AC into DC. As the A/D converter is utilized together with a light emitting diode, the installation cost of the light emitting diode is increased. In particular, it is difficult to utilize a light emitting diode as general household illumination. In order to replace the existing fluorescent lamps with LEDs, there is a need for a novel light emitting diode which can be driven using an AC power source without an A/D converter. 
     The present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a light emitting device of which light extraction efficiency is improved by employing a photonic crystal structure and which can be directly driven using an AC power source without an A/D converter. 
     According to an aspect of the present invention for achieving the object, there is provided an AC light emitting device having photonic crystal structures. The light emitting device includes a plurality of light emitting cells and metallic wirings for electrically connecting the light emitting cells with one another. Further, each of the light emitting cells includes a first conductive type semiconductor layer, a second conductive type semiconductor layer disposed on one region of the first conductive type semiconductor layer, and an active layer interposed between the first and second conductive type semiconductor layers. In addition, a photonic crystal structure is formed in the second conductive type semiconductor layer. The photonic crystal structure prevents light emitted from the active layer from laterally propagating, such that light extraction efficiency of the light emitting device can be improved. Furthermore, the metallic wirings electrically connect a plurality of light emitting cells with one another such that an AC operated light emitting device can be provided. 
     The photonic crystal structure may have various shapes and be regularly arranged two-dimensionally. For example, the photonic crystal structure may include a lattice of holes which are formed in the second conductive type semiconductor layer and arranged two-dimensionally. Alternatively, the photonic crystal structure may include a periodic unevenness formed by partially etching the second conductive type semiconductor layer or a lattice of second conductive type semiconductor rods formed by etching the second conductive type semiconductor layer. The photonic crystal structures are periodically arranged to create a photonic band gap and also prevent light from laterally propagating. Thus, the light emitted from the active layer cannot be laterally propagated by means of the photonic crystal structure and can thus be exited to the outside. Accordingly, light extraction efficiency of the light emitting device can be improved. 
     Furthermore, first electrode pads may be formed in other regions of the first conductive type semiconductor layers. In addition, transparent electrodes cover the second conductive type semiconductor layers. Moreover, second electrode pads may be disposed on the transparent electrodes. In this case, each of the metallic wirings connects the first and second electrode pads of the adjacent light emitting cells. 
     According to another aspect of the present invention for achieving the object, there is provided a method of fabricating an AC light emitting device having photonic crystal structures. The method of the present invention comprises forming a first conductive type semiconductor layer, an active layer and a second conductive type semiconductor layer on a substrate. The second conductive type semiconductor layer, the active layer and the first conductive type semiconductor layer are patterned to form a plurality of light emitting cells. Each of the light emitting cells includes an isolated first conductive type semiconductor layer, a second conductive type semiconductor layer disposed on one region of the isolated first conductive type semiconductor layer, an active layer interposed between the isolated first conductive type semiconductor layer and the second conductive type semiconductor layer, and a photonic crystal structure formed in the second conductive type semiconductor layer. Metallic wirings for electrically connecting the light emitting cells having the photonic crystal structure are then formed. According to the present invention, since the photonic crystal structure is formed in the second conductive type semiconductor layer, a fabricating process can be simplified. Further, since the active layer and the first conductive type semiconductor layer need not be etched, the etching time can be shortened. Accordingly, etching damages which may occur in the first conductive type semiconductor layer, the active layer and the second conductive type semiconductor layer can also be reduced. In addition, since the metallic wirings electrically connect the light emitting cells with one another, an AC operated light emitting device can be provided. 
     The photonic crystal structure may be formed by etching the second conductive type semiconductor layer using photolithographic and etching processes, and an e-beam lithography or hologram technique can be used. 
     Further, transparent electrodes are formed on the second conductive type semiconductor layers having the photonic crystal structure. In addition, first electrode pads may be formed on other regions of the first conductive type semiconductor layers, and second electrode pads may be formed on the transparent electrodes. Each of the metallic wirings connects the first and second electrode pads of the adjacent light emitting cells. According to the present invention, an AC light emitting device having photonic crystal structures can be provided, in which light extraction efficiency can be significantly increased. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of an AC light emitting device having photonic crystal structures according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of the AC light emitting device taken along line A-A′ of  FIG. 1 . 
         FIGS. 3   a  and  3   b  are perspective views illustrating a photonic crystal structure according to an embodiment of the present invention. 
         FIGS. 4 to 7  are sectional views illustrating a method of fabricating an AC light emitting device according to an embodiment of the present invention. 
         FIGS. 8 and 9  are circuit diagrams illustrating the operation of an AC light emitting device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments are provided as an illustration to fully convey the spirit of the present invention to those skilled in the art. Thus, the present invention is not limited to the embodiments which will be described below, but may be implemented in other forms. In the drawings, the width, length, thickness, etc. of components may be exaggerated for the sake of convenience. Throughout the descriptions, like reference numerals designate like elements. 
       FIG. 1  is a plan view of an AC light emitting device having photonic crystal structures according to an embodiment of the present invention,  FIG. 2  is a sectional view taken along line A-A′ of  FIG. 1 , and  FIGS. 3  ( a ) and ( b ) is a perspective view illustrating a variety of photonic crystal structures according to the present invention. 
     Referring to  FIGS. 1 and 2 , a plurality of light emitting cells  28  are disposed on a substrate  21 . The substrate  21  may be an insulating or conductive substrate. 
     Each of the light emitting cells  28  comprises a first conductive type semiconductor layer, a second conductive type semiconductor layer  29  disposed on one region of the first conductive type semiconductor layer, and an active layer  27  interposed between the first and second conductive type semiconductor layers. Here, the first and second conductive types are of an n-type and a p-type, respectively, or vice versa. 
     Each of the first conductive type semiconductor layer  25 , the active layer  27  and the second conductive type semiconductor layer  29  may be formed of a gallium nitride semiconductor material, i.e. (B, Al, In, Ga)N. The constituents and composition of the active layer  27  are chosen in such a manner that light having a desired wavelength, e.g. ultraviolet rays or blue light can be emitted. Each of the first and second conductive type semiconductor layers  25  and  29  is formed of a material having a band gap wider than that of the active layer  27 . 
     Further, a buffer layer  23  may be interposed between the light emitting cells  28  and the substrate  21 . The buffer layer  23  reduces the effects of a lattice mismatch between the substrate  21  and the first conductive type semiconductor layer  25  to be formed on the substrate  21 . 
     Furthermore, a photonic crystal structure is formed in the second conductive type semiconductor layer  29 . The photonic crystal structure may be formed in such a manner that the second conductive type semiconductor layer  29  is etched either to form a lattice of holes  29   a  as illustrated in  FIG. 3  ( a ), or to form a lattice of rods  29   b  as illustrated in  FIG. 3  ( b ). In addition, the photonic crystal structure may be a periodic unevenness formed by partially etching the second conductive type semiconductor layer  29 . The lattice parameter and the diameter of holes or rods in the photonic crystal structure are chosen in such a manner that light energy radiated from the active layer  27  falls within the light band gap. Usually, they are determined by the light wavelength and the refractive index of the second conductive type semiconductor layer. 
     Since the photonic crystal structure is formed in the second conductive type semiconductor layer  29 , it is not necessary to etch the active layer  27  and the first conductive type semiconductor layer  25 . Thus, the active layer  27  and the first conductive type semiconductor layer  25  hardly experience damages due to the etching. In addition, since the etching time is shortened, the second conductive type semiconductor layer  29  can also experience less damage due to the etching. 
     The photonic crystal structure causes a periodic variation of dielectric constants, and thus, a photonic band gap is created. As shown in  FIG. 1 , the holes  29   a  or rods  29   b  may be arranged in a triangular or hexagonal form, but the present invention is not limited thereto. For example, the holes or rods may be arranged in other various forms such as a matrix form. 
     Further, the holes  29   a  or rods  29   b  may have a circular cross-section as illustrated in the figures, but may have other various cross sections including a rectangular one, a hexagonal one and the like. 
     Furthermore, a transparent electrode  31  is formed on the second conductive type semiconductor layer  29  having the aforementioned photonic crystal structure. The transparent electrode  31  is formed of a material capable of transmitting light emitted from the active layer  27  and ohmic-contacted with the second conductive type semiconductor layer  29 . At this time, the holes  29   a  or the empty space between the rods  29   b  may be filled with insulating materials having different refractive indexes from the second conductive type semiconductor layer  29 . 
     The insulating material is employed to prevent the transparent electrode  31  from coming into direct contact with the active layer  27 . The insulating material can be omitted, and thus, the empty space between the rods  29   b  or the holes  29   a  may be filled with air. 
     A first electrode pad  33  may be formed on other region of the first conductive type semiconductor layer  25 . The first electrode pad  33  is ohmic-contacted with the first conductive  30  type semiconductor layer  25 . Furthermore, a second electrode pad  35  may be formed on the transparent electrode  31 . 
     The first electrode pads  33  and the second electrode pads  35  are connected with each other through metallic wirings  37 . The metallic wirings  37  electrically connect adjacent light emitting cells  28  with one another such that arrays of light emitting cells connected in series can be formed. Such serial arrays are connected in reverse parallel to an AC power source to provide a light emitting device suitable for AC operation. Alternatively, a bridge rectifier can be interposed between the array and an AC power source such that the array can be operated by the AC power source. The AC operation of an array of light emitting cells will be explained later with reference to  FIGS. 8 and 9 . 
     According to the embodiment, a light emitting device with improved light extraction efficiency can be provided by forming a photonic crystal structure in the second conductive type semiconductor layer  29 . In addition, an AC light emitting device can be provided by allowing the light emitting cells to be electrically connected with each other through the metallic wirings  37 . 
       FIGS. 4 to 7  are sectional views illustrating a method of fabricating an AC light emitting device according to an embodiment of the present invention. 
     Referring to  FIG. 4 , a first conductive type semiconductor layer  25 , an active layer  27  and a second conductive type semiconductor layer  29  are formed on a substrate  21 . In addition, a buffer layer  23  may be formed on the substrate  21 , before forming the first conductive type semiconductor Layer  25 . 
     The substrate  21  may be formed of sapphire (Al 2 O 3 ), silicon carbide (SiC), zinc oxide (ZnO), silicon (Si), gallium arsenide (GaAs), gallium phosphide (GaP), lithium-alumina (LiAl 2 O 3 ), boron nitride (BN), aluminum nitride (AlN) or gallium nitride (GaN), which can be selected depending on materials of semiconductor layers to be formed on the substrate  21 . In a case where a GaN semiconductor layer is formed, the substrate  21  is usually formed of sapphire or silicon carbide (SiC). 
     The buffer layer  23  is formed to buffer the lattice mismatch between the substrate  21  and the semiconductor layer  25  which will be formed on the substrate  21 . For example, the buffer layer  23  may be formed of GaN or AlN. In a case where the substrate  21  is conductive, the buffer layer  23  is preferably formed of an insulating or semi-insulating layer. 
     Alternatively, the buffer layer may be formed of AlN or semi-insulating GaN. 
     Each of the first conductive type semiconductor layer  25 , the active layer  27  and the second conductive type semiconductor layer  29  may be formed of a GaN based semiconductor material, i.e. (B, Al, In, Ga)N. Constituents and composition of the active layer  27  are chosen in such a manner that light having a desired wavelength can be emitted. Each of the first and second conductive type semiconductor layers  25  and  29  is formed of a material having a band gap wider than that of the active layer  27 . The first and second conductive type semiconductor layers  25  and  29  and the active layer  27  may be intermittently or continuously grown using a metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE) technique. 
     Here, the first and second conductive types may be an n-type and a p-type, respectively, or vice versa. In case of a GaN compound semiconductor layer, the n-type semiconductor layer can be formed by doping silicon as an impurity and the p-type semiconductor layer can be formed by doping magnesium as an impurity. 
     Referring to  FIG. 5 , the second conductive type semiconductor layer  29 , the active layer  27  and the first conductive type semiconductor layer  25  are patterned to isolate the first conductive type semiconductor layer  25 . At this time, the buffer layer  23  may be patterned and isolated together with the above layers. The first and second conductive type semiconductor layers  25  and the active layer  27  may be patterned through photolithographic and etching processes. 
     In the meantime, the second conductive type semiconductor layer  29  and the active layer  27  are patterned to be placed on one region of the isolated first conductive type semiconductor layer  25 , as shown in the figure. Accordingly, a portion of the isolated first conductive type semiconductor layer  25  is exposed. 
     Referring to  FIG. 6 , photonic crystal structures are formed in the patterned second conductive type semiconductor layers  29 . The photonic crystal structures are formed by patterning the second conductive type semiconductor layers  29  using photolithographic and etching processes. The photonic crystal structure may have either a lattice of holes  29   a  arranged two dimensionally as shown in  FIG. 3  ( a ) or a lattice of rods  29   b  arranged two dimensionally as shown in  FIG. 3  ( b ). In addition, the photonic crystal structure may be a periodic unevenness formed by partially etching the second conductive type semiconductor layer  29 . 
     When etching the second conductive type semiconductor layer  29  to form the holes  29   a  or rods  29   b , the active layer  27  and the first conductive type semiconductor layer  25  may be etched. However, since it is not necessary to etch the active layer  27  and the first conductive type semiconductor layer  25 , the etching time can be shortened as compared with the prior art. In addition, in a case where the photonic crystal structure is confined within the second conductive type semiconductor layer  29 , the increase of a surface area due to the photonic crystal structure can be reduced and thus the surface recombination rate at which holes and electrons are recombined in a surface can also be reduced. 
     The second conductive type semiconductor layer may be patterned using, for example, an e-beam lithography or hologram technique and other various etching techniques, for example, dry etching techniques. 
     Here, although it has been described in the present invention that the photonic crystal structures have been formed after isolating the light emitting cells  28  from one another, the photonic crystal structures can be formed before isolating the light emitting cells  28  from one another. That is, as explained above with reference to  FIG. 4 , the photonic crystal structures may be formed by patterning the second conductive type semiconductor layer  29  after forming the first conductive type semiconductor layer  25 , the active layer  27  and the second conductive type semiconductor layer  29  on the substrate  21 . 
     Referring to  FIG. 7 , transparent electrodes  31  are formed on the second conductive type semiconductor layers  29  having the photonic crystal structures. The transparent electrodes  31  may be formed using a lift-off technology. The transparent electrode  31  may be made of any material so long as the material can transmit light emitted from the active layer  27 . For example, Ni/Au or ITO (Indium Tin Oxide) film may be employed. Furthermore, before forming the transparent electrode  31 , the holes  29   a  or the empty space between the rods  29   b  may be filled with insulating materials. The insulating material has a different refractive index from the second conductive type semiconductor layer  29  and is selected to prevent the transparent electrode  31  from being electrically connected to the active layer  27 . In a case where the transparent electrode  31  is not brought into direct contact to the active layer  27  without the insulating material, the insulating material can be omitted. 
     Further, first electrode pads  33  are formed on other regions of the isolated first conductive type semiconductor layers  25 . The first electrode pads  33  may be formed using a lift-off technique. In addition, second electrode pads  35  are formed on a certain region of the transparent electrodes  31 . The second electrode pads  35  may also be formed using a lift-off technique. Since the second electrode pad  35  absorbs light, the photonic crystal structure needs not be formed in the region of the second conductive type semiconductor layer  29  where the second electrode pad  35  will be formed. The first and second electrode pads  33  and  35  may be formed at the same time. The first electrode pads  33  and/or the second electrode pads  35  may be formed of Ti/Au. 
     Then, metallic wirings  37  are formed to electrically connect the first and second electrode pads  33  and  35  on adjacent light emitting cells  28 . Consequently, the light emitting device of  FIG. 2  is completed. The metallic wirings  37  may be formed using the conventional air-bridge or step-cover process. 
       FIGS. 8 and 9  are circuit diagrams illustrating the AC operation of a light emitting diode chip according to the embodiment of the present invention. Here,  FIG. 8  is a circuit diagram illustrating the AC operation of two arrays of serially connected light emitting cells which are connected with each other in a reverse parallel.  FIG. 9  is a circuit diagram illustrating the AC operation of an array of serially connected light emitting cells using a bridge rectifier connected thereto. 
     Referring to  FIG. 8 , light emitting cells  51   a ,  51   b  and  51   c  are serially connected to form a first serial light emitting cell array  51 , while other light emitting cells  53   a ,  53   b  and  53   c  are serially connected to from a second serial light emitting cell array  53 . Here, the “serial light emitting cell array” means an array of light emitting cells connected in series with one another. Both ends of each of the first and second serial arrays  51  and  53  are connected to an AC power source  55  and a ground, respectively, through lead terminals. The first and second serial arrays are connected in a reverse parallel between the AC power source and the ground. That is, both ends of the first and second serial arrays are electrically connected to each other and are arranged such that the light emitting cells are driven by means of currents flowing in opposite directions to each other. That is, as shown in the figure, the anodes and cathodes of the light emitting cells included in the first serial array  51  are arranged in opposite direction to those in the second serial array  53 . 
     Thus, when the AC power source  55  has a positive phase, the light emitting cells included in the first serial array  51  are turned on to emit light and the light emitting cells included in the second serial array  53  are turned off. On the contrary, when the AC power source  55  has a negative phase, the light emitting cells included in the first serial array  51  are turned off and the light emitting cells included in the second serial array  53  are turned on. 
     Consequently, the first and second serial arrays  51  and  53  are alternately turned on and off to allow a light emitting diode chip including the first and second serial arrays to continuously emit light. 
     In the meantime, the circuit of  FIG. 8  has been configured in such a manner that both ends of the first and second serial arrays are connected to the AC power source  55  and the ground, respectively. However, the circuit may be configured in such a manner that both ends are connected to both terminals of the AC power source. In addition, although each of the first and second serial arrays is illustrated as including three light emitting cells, the present invention is not limited thereto. The number of the light emitting cells can be increased, if desired. Further, the number of the serial arrays can be further increased. 
     Referring to  FIG. 9 , a serial light emitting cell array  61  is comprised of light emitting cells  61   a ,  61   b ,  61   c ,  61   d ,  61   e  and  61   f . Furthermore, a bridge rectifier including diode cells D 1 , D 2 , D 3  and D 4  is disposed between the AC power source  65  and the serial light emitting cell array  61  and between the ground and the serial light emitting cell array  61 . The diode cells D 1 , D 2 , D 3  and D 4  may have the same structure as the light emitting cells, but the present invention is not limited thereto. That is, the diode cells may not emit light. An anode terminal of the serial light emitting cell array  61  is connected to a node between the diode cells D 1  and D 2 , and a cathode terminal thereof is connected to a node between the diode cells D 3  and D 4 . Further, a terminal of the AC power source  65  is connected to a node between the diode cells D 1  and D 4 , and the ground is connected to a node between the diode cells D 2  and D 3 . 
     When the AC power source  65  has a positive phase, the diode cells D 1  and D 3  of the bridge rectifier are turned on and the diode cells D 2  and D 4  are turned off. Thus, current flows to the ground via the diode cell D 1  of the bridge rectifier, the serial light emitting cell array  61  and the diode cell D 3  of the bridge rectifier. 
     On the other hand, when the AC power source  65  has a negative phase, the diode cells D 1  and D 3  of the bridge rectifier are turned off and the diode cells D 2  and D 4  are turned on. Thus, current flows to the AC power source via the diode cell D 2  of the bridge rectifier, the 20 serial light emitting cell array  61  and the diode cell D 4  of the bridge rectifier. 
     Consequently, a bridge rectifier is connected to the serial light emitting cell array  61  such that the serial light emitting cell array can be continuously operated using the AC power source  65 . Here, both terminals of the bridge rectifier are connected to the AC power source and the ground, respectively, but they may be connected to both terminals of the AC power source. In the meantime, in a case where AC power source is used to drive the serial light emitting cell array  61 , ripples may occur. In order to avoid the ripples, an RC filter (not shown) can be employed. 
     According to this embodiment, a single serial light emitting cell array can be operated by means of an AC power source electrically connected thereto, and thus, the efficiency of light emitting cells can be increased as compared with the light emitting diode chip of  FIG. 8 . 
     The connection of light emitting cells described with reference to  FIGS. 8 and 9  is merely to explain the AC operation of light emitting cells and can be implemented in various ways.