Patent Publication Number: US-7910154-B2

Title: Nitride-based light emitting devices and methods of manufacturing the same

Description:
PRIORITY STATEMENT 
     This non-provisional U.S. patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2006-0001393, filed on Jan. 5, 2006, in the Korean Intellectual Property Office (KIPO), the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Example embodiments of the present invention relate to light emitting devices and methods for manufacturing the same. At least one example embodiment of the present invention provides a nitride-based light emitting device having increased power and/or life span and a method of manufacturing the same. 
     2. Description of the Conventional Art 
     Conventional light emitting devices, such as light emitting diodes (LED), are semiconductor PN junction diodes. PN junctions of III-V group compound semiconductor may be created by combining III-group and V-group elements from the periodic table. A III-V compound semiconductor may have a higher luminous efficiency, for example, the luminous efficiency may be close or substantially close to 100% which may be higher than silicon. Thus, LEDs may be used in, for example, diode lasers. In addition, because electrons move at higher speeds in LEDs and LEDs may operate at higher temperatures, LEDs may be used in higher-speed and/or higher-power electronic devices. For example, several III-group and V-group elements may be combined to manufacture semiconductors having a variety of material composition and/or characteristics may be manufactured. 
     Characteristics of LEDs may be luminosity, brightness and/or radiant flux in a visible ray region may. Luminosity may be represented by light velocity per unit cubic angle or Candelas (cd), and brightness may be indicated by luminosity per unit area. A photometer may be used to measure luminosity. Radiant flux represents power radiated from all wavelengths of an LED, and may be represented by energy radiated per unit time or Watts (W). 
     Luminous efficiency may be used in determining a visible ray LED performance. Luminous efficiency may be represented by Lumens per Watt (lm/W). This corresponds to wall-plug efficiency (e.g., optical output/input electric power quantity) considering the human eyes&#39; luminosity factor. Luminous efficiency of an LED may be determined by three factors, such as internal quantum efficiency, extraction efficiency and operating voltage. 
     In accordance with the conventional art, LEDs may have a sapphire/n-GaN/MQW/p-GaN structure. However, in such LEDs, defect density may be higher due to manufacturing limitations. Thus, internal quantum efficiency of an MQW layer and/or manufacturing higher-power LEDs may be limited. In higher-power LEDs, due to the defects, a leakage current may be larger, which may increase a driving voltage of a device and/or reduce the life span of the device. 
     An example, conventional LED may have a nano rod array structure with an InGaN quantum well. In this example, a method of manufacturing the nano rod may be more complicated and/or it may be difficult to actually utilize such a technology in industry. 
     SUMMARY OF THE INVENTION 
     Example embodiments of the present invention provide nitride-based light emitting devices having higher power and/or longer life span and methods of manufacturing the same. 
     According to an example embodiment of the present invention, a light emitting device (LED) may include an n-clad layer formed on a crystalline wafer. A porous layer may be formed by processing the n-clad layer (e.g., from its top surface to a depth) in a mixed gas atmosphere of HCl and NH 3 . An active layer and a p-clad layer may be formed (e.g., sequentially) on the porous layer. The porous layer may be formed to a thickness of about 1 μm to about 10 μm, inclusive. 
     In at least some example embodiments of the present invention, the crystalline wafer may be a Si, GaAs, SiC, GaN or sapphire substrate. The n-clad layer may be an n-GaN layer and the p-clad layer may be a p-GaN layer. The active layer may have a multi-quantum well (MQW) structure such as a GaN/InGaN/GaN MQW or GaN/AlGaN/GaN MQW structure. 
     According to another example embodiment of the present invention, a method of manufacturing a light emitting device may include forming an n-clad layer on a crystalline wafer. The n-clad layer may be processed (e.g., from its top surface to a depth) in a mixed gas atmosphere of HCl and NH 3  to form a porous layer. An active layer and a p-clad layer may be formed (e.g., sequentially) on the porous layer. 
     The surface processing may be performed at a temperature of about 950° C. to about 1200° C., inclusive. For example, surface processing may be performed at a temperature of about 1065° C. The surface processing may be performed in a halide or hydride vapor phase epitaxy (HVPE) system. In this example, a mixture ratio of HCl and NH 3  may be about 1:10. The mixed gas may further include a carrier gas including an inert gas, for example, at least one of N 2 , Ar and H 2 . In this example, a mixture ratio of HCl, NH 3  and the carrier gas may be about 1:10:80. The n-clad layer, the active layer and/or the p-clad layer may be formed using vapor deposition. The vapor deposition may be one of halide or hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more apparent by describing in detail some example embodiments with reference to the attached drawings in which: 
         FIG. 1  is a schematic cross-sectional view of a light emitting device according to an example embodiment of the present invention; 
         FIGS. 2A through 2G  are flowcharts illustrating a method of manufacturing a light emitting device according to an example embodiment of the present invention; and 
         FIG. 3  is a sectional scanning electron microscope (SEM) photo showing a porous GaN layer manufactured according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE PRESENT INVENTION 
     Various example embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which some example embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
     Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. This invention may, however, may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     Accordingly, while example embodiments of the invention are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the invention to the particular forms disclosed, but on the contrary, example embodiments of the invention are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element or layer is referred to as being “formed on” another element or layer, it can be directly or indirectly formed on the other element or layer. That is, for example, intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly formed on” to another element, there are no intervening elements or layers present. Other words used to describe the relationship between elements or layers should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the FIGS. For example, two FIGS. shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
       FIG. 1  is a schematic cross-sectional view of a light emitting device according to an example embodiment of the present invention. As shown, the light emitting device may include an n-clad layer  20 , a porous layer  22 , an active layer  30  and/or a p-clad layer  40 . The n-clad layer  20 , the porous layer  22 , the n active layer  30  and/or the p-clad layer  40  may be formed (e.g., sequentially) on a crystalline wafer  10 . An n-electrode  50  and a p-electrode  60  may be formed on an etched surface of the n-clad layer  20  and on the p-clad layer  40 , respectively. The single crystalline wafer  10  may be, for example, a Si, GaAs, SiC, GaN, sapphire or any other suitable substrate. The n-clad layer  20  may be formed of a nitride semiconductor layer, for example, an n-GaN-based III-V-group nitride semiconductor layer (e.g., an n-GaN layer, an n-GaN/AlGaN layer, etc.). The p-clad layer  40  may be formed of a nitride semiconductor layer, for example, a p-GaN-based III-V-group nitride semiconductor layer (e.g., a p-GaN layer, a p-GaN/AlGaN layer, etc.). Because the structure and material of the n-clad layer  20  and the p-clad layer  40  and a method of forming the same are well-known in the art, a detailed description will be omitted for the sake of brevity. 
     The n-clad layer  20  may be processed from a top surface to a desired or given depth in a mixed gas atmosphere of, for example, HCl and NH 3  to form the porous layer  22 . The porous layer  22  may be formed of, for example, a plurality of protrusion pillars. When the porous layer  22  is formed to a thickness of about 1 μm to about 10 μm, inclusive, an optical power characteristic of the light emitting device may be improved. A method of forming the porous layer  22 , according to an example embodiment of the present invention, will be described in detail with regard to  FIGS. 2A-2G . 
     The active layer  30  may be formed of a nitride semiconductor layer, for example, a GaN-based III-V-group nitride semiconductor layer. In at least one example embodiment of the present invention, the active layer  30  may be formed of In x Al y Ga 1-x-y N (0≦x&lt;1, 0≦y&lt;1 and 0≦x+y&lt;1), for example, an InGaN layer, an AlGaN layer or any other similar layer. In at least this example embodiment, the active layer  30  may have a multi-quantum well (MQW) or a single quantum well (SQW) structure. For example, the active layer  30  may have a GaN/InGaN/GaN MQW, a GaN/AlGaN/GaN MQW or any other similar structure. 
     In a light emitting device having the above structure, if a voltage is applied between the n-electrode  50  and the p-electrode  60 , electrons and holes may be injected into the active layer  30  from the n-clad layer  20  and the p-clad layer  40 , respectively, and combined in the active layer  30  so that light may be output from the active layer  30 . 
     According to at least some example embodiments of the present invention, the surface of the n-GaN layer  20  may be processed to a desired or given thickness to form the porous layer  22 . Defects such as edge dislocation, included in the n-GaN layer  20 , may be removed while performing this processing, and an MQW layer having a lower defect density may be formed on the porous layer  22 . This may result in a light emitting device having improved electrical characteristics, reduced leakage current and/or improved internal quantum efficiency. 
     According to at least the above described example embodiment, the active layer  30  may be formed of a plurality of areas (e.g., dots) which may be formed on each of the protrusion pillars of the porous layer  22 . The areas may be aligned in a line. However, according to another example embodiment of the present invention, the areas (e.g., dots) may be connected to one another, for example, continuously. For example, when forming the active layer  30 , a relative size of a vertical growth G v  velocity and/or a lateral growth G L  velocity may be adjusted to control the shape of the active layer  30 . The active layer  30  may be formed into various shapes. 
       FIGS. 2A through 2G  are flowcharts illustrating a method of manufacturing a light emitting device according to an example embodiment of the present invention. Referring to  FIG. 2A , the n-clad layer  20  may be formed on the crystalline wafer  10 . The clad layer  20  may be formed using the same or substantially the same stacking method, for example, a GaN-based crystalline layer growth on a GaN substrate, a GaN-based crystalline growth on a sapphire substrate or any other suitable stacking method. The n-clad layer  20  may be formed of a nitride semiconductor layer (e.g., an n-GaN-based III-V-group nitride semiconductor layer, such as, an n-GaN layer, an n-GaN/AlGaN layer, etc.). The n-clad layer  20  may be formed using, for example, vapor deposition, such as, halide or hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or any other suitable deposition technique. Because the methods are well-known in the art, a detailed description will be omitted for the sake of brevity. 
     Referring to  FIGS. 2B and 2C , the n-clad layer  20  may be processed from a top surface to a given or desired depth in a mixed gas atmosphere of HCl and NH 3  to form the porous layer  22 . In this example, the porous layer  22  may be formed of a plurality of protrusion pillars, and/or to a thickness of about 1 μm to about 10 μm, inclusive. A mixture ratio of HCl and NH 3  may be about 1:10, and the mixed gas may further include a carrier gas. The carrier gas may include at least one of N 2 , Ar, H 2 , or any other similar inert gas. A mixture ratio of HCl, NH 3  and the carrier gas may be about 1:10:80. The surface processing may be performed at a temperature of about 950° C. to about 1200° C., inclusive, for example, about 1065° C. The surface processing may be performed in a reaction chamber, for example, an HVPE system. 
     While the process of forming the porous layer  22  is performed, defects such as edge dislocation, included in the n-GaN layer  20  may be removed. The mixed gas atmosphere of HCl and NH 3  may act as a vapor etchant. Defect regions in the n-GaN layer  20  may be etched and changed into a porous shape. 
     Referring to  FIGS. 2D and 2E , the active layer  30  and the p-clad layer  40  may be formed (e.g., sequentially) on the porous layer  22 . The active layer  30  and the p-clad layer  40  may be formed using vapor deposition, such as, HVPE, MOCVD, MBE, etc. 
     The active layer  30  may be formed of a nitride semiconductor layer, for example, a GaN-based III-V-group nitride semiconductor layer, such as, In x Al y Ga 1-x-y N (0≦x&lt;1, 0≦y&lt;1 and 0≦x+y&lt;1) (e.g., an InGaN layer, an AlGaN layer, etc.). In this example, the active layer  30  may have a multi-quantum well (MQW) or a single quantum well (SQW) structure. For example, the active layer  30  may be formed of GaN/InGaN/GaN MQW or GaN/AlGaN/GaN MQW. 
     According to this example embodiment of the present invention, the active layer  30  may be formed of a plurality of areas (e.g., dots) which may be formed on each of the protrusion pillars of the porous layer  22  and aligned in a line. However, according to at least some other example embodiments of the present invention, the areas may also be connected to one another continuously. For example, when the forming the active layer  30 , a relative size of a vertical growth G v  velocity and a lateral growth G L  velocity may be adjusted to control the shape of the active layer  30 . In this example, velocity of G v  and/or G L  may be controlled by adjusting a relative composition ratio of a source material, such as, a GaN-based III-V-group source material. In at least some example embodiments of the present invention, a relative composition ratio of Trimethylgallium (TMGa) and NH 3  may be controlled. Because this is well-known in the art, a detailed description will be omitted for the sake of brevity. 
     The p-clad layer  40  may be formed of a nitride semiconductor layer, for example, a p-GaN-based III-V-group nitride semiconductor layer, such as, a p-GaN layer or a p-GaN/AlGaN layer. Similarly, velocity of G v  and G L  of the p-clad layer  40  may be controlled so that the p-clad layer  40  may be connected to the active layer  30 , for example, continuously. 
     Referring to  FIGS. 2F and 2G , a top surface of the p-clad layer  40  may be etched to a desired or given depth of the n-clad layer  20  to form an etched surface on the n-clad layer  20 . The n-electrode  50  and the p-electrode  60  may be formed on the etched surface of the n-clad layer  20  and the p-clad layer  40 . The n-electrode  50  and the p-electrode  60  may be formed of a conductive material, such as Ag, Au, indium tin oxide (ITO), etc. to produce a light emitting device, according to at least some example embodiments of the present invention. 
     Experimental results of an example embodiment of the present invention will now be described. To form a porous GaN layer used in the experiment, a GaN substrate was mounted in an HVPE system, and the temperature of the system was raised to 1065° C. 100 sccm of HCl gas, 1000 sccm of NH 3  gas, and 8000 sccm of N 2  gas were injected into the HVPE system. Pressure of the HVPE system was adjusted to a normal pressure, and the porous GaN layer was formed. A thickness of the porous GaN layer or an air gap in the porous GaN layer was controlled by adjusting process parameters such as temperature, gas flux and process time. The stack structure was placed in an MOCVD chamber and an MQW layer and a p-GaN layer were formed on the porous GaN layer. After forming the MQW layer, a vertical growth G v  velocity was higher than a lateral growth G L  velocity, and when the p-GaN layer was grown, the lateral growth G L  velocity was higher than the vertical growth G v  velocity so that p-GaN layers were connected and adhered to each other. The n-electrode and the p-electrode were then formed as described above.  FIG. 3  is a sectional scanning electron microscope (SEM) photo showing a porous GaN layer manufactured in an experimental example as described above. 
     At least some example embodiments of the present invention provide a light emitting device having increased power and/or life span. A given or desired thickness of the n-clad layer, for example, the surface of the n-GaN layer may be processed to a given or desired thickness forming the porous layer. While the process is performed, defects included in the n-GaN layer may be removed, and the MQW layer may have a lower defect density so that the nitride-based light emitting device may have improved electrical characteristics, reduced leakage current and/or improved internal quantum efficiency. As a result, the production yield of the light emitting device may be improved, and/or the light emitting device, according to at least some example embodiments of the present invention, may be manufactured more simply, easier and/or manufacturing costs may be reduced. Thus, according to at least some example embodiments of the present invention, a higher-quality light emitting device may be manufactured with lower costs. 
     While example embodiments of the present invention have been discussed herein with regard to particular III-group and V-group elements, any suitable III-group or V-group (e.g., III-nitride semiconductors, etc.) may be used. 
     While the present invention have been particularly shown and described with reference to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims.