Abstract:
A manufacturing method and a thus produced light-emitting structure for a white colored light-emitting device (LED) and the LED itself are disclosed. The white colored LED includes a resonant cavity structure, producing and mixing lights which may mix into a white colored light in the resonant cavity structure, so that the white colored LED may be more accurately controlled in its generated white colored light, which efficiently reduces deficiency, generates natural white colored light and aids in luminous efficiency promotion. In addition to the resonant cavity structure, the light-emitting structure also includes a contact layer, an n-type metal electrode and a p-type metal electrode.

Description:
RELATED APPLICATIONS 
   This application is a Division of application U.S. Ser. No. 10/720,063, now U.S. Pat. No. 7,279,347 entitled “GALLIUM NITRIDE BASED LIGHT-EMITTING DIODE” and filed on Nov. 25, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to a light emitting device and a manufacturing method for a light-emitting device (LED), and particularly to a highly efficient light-emitting structure and a manufacturing method for an LED. In particular, the light-emitting structure proposed herein is based on the Group-III GaN-based materials and has a resonant cavity structure used to enhance luminous efficiency of the generated light therefrom. 
   2. Description of Related Art 
   Light-emitting devices (LEDs) have been developed and on the market for years and are useful in providing lights as generally recognized. The use of LEDs in digital watches and calculators are well known. As we see, it may also find other important applications in communications and other areas, such as mobile phone and some appliances. Recently, there is a trend that LEDs be further applied to ordinary human living utilization, such as large panels, traffic lights and lighting facilities and the perspective thereof are looking good. Therefore, LEDs are increasingly playing an important role in our daily life and deserving more efforts. As is transparent to those skilled in the art, LEDs are produced based on some semiconductor materials and emits lights by dint of the behaviors aroused in the semiconductor materials in the presence of an applied electrical bias. 
   In particular, an LED gives off a light by a light-emitting structure therein generally composed of some Group-III (compound) semiconductor owing to its stronger inclination of recombination of electrons and holes. In principle, an LED is basically a well-known p-n junction structured device, i.e., a device having a p region, an n region and a transient region therebetween. With a forward voltage or current bias applied, the majority of carriers in the p or n regions drift respectively towards the other region (through the transient region) in the device due to the energy equilibrium principle and a current is accounted for, in addition to the general thermal effects. When electrons and holes jumped into a higher value of energy band with the aid of electrical and thermal energy, the electrons and the holes recombine there and give off lights when they randomly and spontaneously fall back to a reduced energy state owing to thermal equilibrium principle, i.e. spontaneous emission. 
   Afterwards, the concept and structure widely used in semiconductor device of the multi-quantum well (MQW) layers are introduced into an LED structure. Generally, the MQW layers are formed between the p and the n regions in the above-mentioned p-and-n structure, which forms the so-called “PIN” structure. With the aid of the MQW active layers, the possibility of recombination of the electrons and holes in the p-n junction based device are efficiently enhanced and the luminous efficiency thereof is upgraded considerably. Further, the color of a light emitted from the LED may be controlled through a choice of the materials, dopant concentration and layer thickness in the MQW layers. 
   However, the current LEDs are still not sufficient in brightness in serving as some light supplying facilities, and which has long been the common issue that all researchers in the field concern and desire to address. 
   In view of the foregoing problem, the inventors of the present invention provides a novel colored light emitting diode with a different structure so as to increase luminous efficiency of the currently used LED. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide a light-emitting structure and its manufacturing method for an LED which may usefully enhance its luminous efficiency without largely increasing cost. 
   To achieve the object, the present invention provides a light-emitting structure for an LED, wherein the light-emitting structure comprises a resonant cavity. In one embodiment, the resonant cavity bordered by a lower reflecting component, p-GaN based distributed Bragg reflector (DBR) and an upper reflecting component, a metal reflector or an n-GaN based distributed Bragg reflector (DBR). Owing to the light resonation and the thus self-exciting of the emitted light in the LED device, the light out of the LED device is efficiently enhanced with a fixed electric power source. 
   To achieve the above-mentioned LED, the present invention also provides a manufacturing method for the light-emitting structure. In one embodiment, the method comprises forming a buffer layer over a substrate; forming an GaN based epitaxial layer over the buffer layer; forming an MQW active layer over the n-GaN based layer; forming a p-DBR over the MQW active layer; forming a p-GaN based epitaxial layer over the p-DBR and etching away a portion of the n-GaN based layer, the MQW active layer, the p-type DBR and the p-GaN based layer whereby an exposing region is formed on the n-GaN layer; and coating a metal reflector over a bottom side of the substrate. 
   Along with the high luminous efficiency, the resonant cavity utilizing the metal reflector as the lower reflecting element may efficiently reduce cost and simplify manufacturing process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To better understand the other features, technical concepts and objects of the present invention, one may read clearly the description of the following preferred embodiment and the accompanying drawings, in which: 
       FIG. 1  depicts schematically a manufacturing method of a preferred embodiment according to the present invention; 
       FIGS. 2 and 2A  depict schematically a perspective diagram of a structure of a light-emitting structure of a preferred embodiment according to the present invention; 
       FIGS. 3 and 3A  represent a particular example of the epitaxial structure shown in  FIG. 2 ; 
       FIG. 4  depicts schematically a manufacturing method of a second embodiment according to the present invention; 
       FIG. 5  depicts schematically a structure of a light-emitting structure of a second embodiment according to the present invention; 
       FIG. 6  depicts a particular example of the epitaxial structure shown in  FIG. 5 ; 
       FIG. 7  depicts schematically a manufacturing method of a third embodiment according to the present invention; 
       FIG. 8  depicts schematically a manufacturing method of a fourth embodiment according to the present invention; 
       FIG. 9  depicts schematically a perspective diagram of a structure of a fourth embodiment according to the present application; 
       FIG. 9A  depicts schematically a structure of a device of the fourth embodiment according to the present application; 
       FIGS. 10 and 10A  depict schematically a particular example of the epitaxial structure shown in  FIG. 9 ; 
       FIG. 11  depicts schematically a manufacturing method of a fifth embodiment according to the present invention; 
       FIG. 12  depicts schematically a structure of a fifth embodiment according to the present invention; 
       FIG. 13  depicts a particular example of the epitaxial structure shown in  FIG. 12 ; and 
       FIG. 14  depicts schematically a structure of a six embodiment according to the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention relates to an improved light-emitting structure for an LED in terms of luminous efficiency, wherein the light-emitting structure comprises a resonant cavity. In a preferred embodiment, the manufacturing method for a high efficiency light-emitting device (LED) device according to the present invention comprises the following steps. In appreciating the preferred embodiment, please refer directly to  FIG. 1 to 3 , wherein the reference numerals given in the corresponding device are also used in the recitation of the steps. 
   Step  1 : forming a buffer layer  11  over a substrate  10 , i.e., forming a buffer layer  11  over an upper surface  10   a  of the substrate  10 . The substrate  10  may be such as sapphire, silicon carbide (SiC) and gallium nitride (GaN) for the consideration that a GaN based material is chosen thereon, The buffer layer  11  may be composed of some layers depending on choice of design, such as a coarse grain nucleation layer made of GaN and an undoped GaN layer. The nucleation layer is a low temperature layer, i.e. formed under a low temperature condition, about 500-550° C.; has a thickness of 200-400 Å and will be referred to as an LT-GaN layer herein. The undoped GaN is a high temperature layer, formed under a temperature of 1020-1040° C. and has a thickness of 0.2-2 μm, and will be termed as an HT-GaN layer. These buffer layers may be formed by molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD) and some other suitable technologies, currently in existence or set forth in the future. The application of the buffer layer  11  is aimed to lattice matching between the substrate and the epitaxial layer formed thereon, and some other reasons. 
   Step  2 : forming an n-GaN based epitaxial layer  13  over the buffer layer  11 . It may be executed by such as MBE and MOCVD. In forming such n-GaN based epitaxial layer, the temperature is 1020° C.-1040° C. and the formed thickness is 2-8 μm. 
   Step  3 : forming an MQW active layer  14  over the n-GaN based epitaxial layer  13 , wherein the MQW active layer  14  is chosen so that the MQW active layer  14  may generate a light with a wavelength from 380 nm to 600 nm. 
   Step  4 : forming a p-type distributed Brag reflector (DBR)  15  over the MQW active layer  14 . As well known to those persons skilled in the art, a DBR is a multi-layer structure formed for reflection of a light. In a preferred embodiment of the present invention, the p-type DBR  15  is AlGaN/GaN. The thickness thereof is 0.1-0.5 μm and the process temperature therefor is 960-1000° C. The reflectance of the p-type DBR may be chosen between 50 and 80%. 
   Step  5 : forming a p-GaN based layer  16  over the p-type DBR  15  and etching away a portion of the n-GaN layer  13 , the MQW active layer  14 , the p-type DBR  15  and the p-GaN based layer  16  whereby the n-GaN layer  13  has an exposing region  13   a  and an n-type electrode  17  may be disposed over the exposing region  13   a  and a p-type electrode  18  may be disposed over the p-GaN layer  16 . The p-GaN based layer  16  can be formed by such as MBE and MOCVD, under the process conditions of a temperature of 1020° C.-1040° C. and a thickness is 0.2 to 0.5 μm. On the other hand, the—and p-type electrodes  17  and  18  may be formed by such as sputtering, vaporizing and E-gun technologies, and the adopted electrode material may be well-conductive metal of all appropriate kinds, such as aluminum and copper, and may preferably have good light transparency (to the light generated from the device, i.e., 380 nm to 600 nm), such as a thin Ni/Au layer (with the Ni layer formed first and the Au layer atop the Ni layer). It is to be noted that although the formations of the p-type and n-type electrodes  17 ,  18  are not recited in this step and  FIG. 1 , they are in effect formed successively. In terms of the p-type and n-type electrodes, all embodiments explained here will not present them in the corresponding drawings and description. As for the etching, it is not presented in the corresponding drawing,  FIG. 1 . The suitable etching method may be dry etching, such as chlorine plasma etching. 
   Step  6 : coating a metal reflector  19  over a bottom side of the substrate  10 . The coating method may be such as sputtering, vaporizing and E-gun technologies. In undertaking such a coating step, the bottom side of the substrate  10  may be polished to a reduced thickness, 50 μm to 300 μm, from a larger thickness and then coated with the metal reflector  19 . The metal reflector  19  is made of a suitable metal so that a specified reflector, such as one having a desired reflectivity, may be achieved and the reflectivity may be over 90%. The metal coating layer  19  has a thickness of 50 Å to 10 μm and may be performed by electroplating, sputtering and some other suitable technologies. 
   In  FIGS. 2 and 2A , a light-emitting structure according to the preferred device embodiment of the present invention is recited which corresponds to the preferred method embodiment shown in  FIG. 1 . The light emitting device comprises a metal reflector  19 , a substrate  10 , a buffer layer  11 , an n-GaN based layer  13 , an MQW active layer  14 , a p-type DBR  15 , a contact layer  16 , an n-type metal electrode  17  and a p-type metal electrode  18 , wherein the region bordered by the two reflecting components, the metal reflector  19 , and the p-type distributed Bragg reflector (DBR)  15  forms a resonant cavity. In the figure, the circle with arrows indicates the behavior of the light resonation in the resonant cavity, and that will hold for all drawings in the present invention. In the device, the substrate  10  may be such as sapphire, gallium nitride (GaN) and silicon carbide (SiC). The metal reflector  19  coated on a lower surface  10   b  of the substrate  10  has a reflectance of larger than 90%. The buffer layer  11  is provided as an intermediate layer between the substrate  10  and the MQW active layer  12  for some reasons, such as better lattice matching. As also described in the above, the buffer layer  11  may be composed of some layers. The MQW active layer  14  is chosen so that the layer  14  may generate a light having a wavelength of 380 nm to 600 nm once an electrical bias is fed into the LED device. The contact layer  16  is a p-GaN based layer and formed over the p-type DBR  15  for contact with a corresponding electrode  18 . The p-type metal electrode  18  is disposed over the p-GaN layer  16  for electricity feed, while the n-type metal electrode  17  is disposed over an exposing region  13   a  of the n-GaN layer  13 . The n-GaN based layer  13 , the MQW layer  14  and the P-DBR layer  15  jointly form a P-I-N light generating unit, which is familiar to those persons skilled in the art and will not be explained here. 
   To obtain a specific color of the emitted light from the LED device, the MQW active layer  14  should be carefully chosen. In accordance with the generally known chromaticity diagram, when the MQW active layer emits a light with a wavelength of 465 nm to 485 nm upon an applied electric bias, the LED is a blue colored LED. When the MQW active layer  14  emits a light with a wavelength of 495 nm to 540 nm upon an applied electric bias, the LED is a green colored LED. When the MQW active layer  14  emits a light with a wavelength of 560 nm to 580 nm upon an applied electric bias the LED is a yellow colored LED. Of course, the MQW active layer  14  may emit a light having a wavelength between 380 nm-600 nm but other than the above range and become some other colored LED, which depends upon the choice of the MQW layer  14 . 
   To completely form a marketed LED, wire bonding and packaging are necessary on the light-emitting structure. Since these steps are well known to those persons skilled in the art, the description of the related technology is omitted here. 
   In  FIGS. 3 and 3A , a particular example of the device depicted in  FIG. 2  is shown. In the example, the first and second layers  111  are LT-GaN/HT-GaN buffer layers, in which the former has a thickness of 30-500 Å while the latter 0.2-0.5 μm. The third layer  131  is an n-GaN based semiconductor layer with a thickness of 2-6 μm. The fourth layer  141  is an InGaN/GaN MQW layer. The fifth layer  151  is a p-AlGaN/GaN DBR. The sixth layer  161  is p + -GaN based semiconductor with a thickness of 0.2-0.5 μm, wherein the heavy dopant concentration of the sixth layer  161  is aimed at better ohmic contact with the upper metal electrode (not shown). 
   Lower to the above layers are a substrate  101  and a metal reflector  191 , wherein the metal reflector  191  is coated below the substrate  101 . Specifically, the substrate  101  may be sapphire, SiC or GaN. In manufacturing process, the substrate  101  first has a thickness of 300-500 μm in the process of the growth of those epitaxial layers over the substrate  101 . After the epitaxial layers are formed, the substrate  101  is polished at its bottom side to a thickness of 50-300 μm and a metal reflector  191  is coated thereon. The metal reflector  191  may be Ag/Al, i.e., first coated with Ag and then Al so that Ag material will not expose, or Ag, or any other metal, and may have a thickness of 50 Å to 10 μm. 
   Now the description will be made to a second embodiment according to the present application, and please refer directly to  FIG. 4 . The second method embodiment is the same as the preferred embodiment except to the step, Step  6 ′. Step  6 ′: coating a transparent contact layer (TCL) with a suitable thickness over the contact layer, p-GaN based layer, succeeding to Step  5 . In terms of material used, the TCL may be made of Ni/Au and other suitable transparent (for the generated light from the light-emitting structure, such as a light with a wavelength of 380-600 nm) and conductive material and may be an n-TCL (n-doped) or a p-TCL (p-doped). In fact the TCL may be a doped metal, such as doped ZnO, which may be referenced to U.S. Pat. No. 6,992,331, published on Jan. 31, 2006, which is assigned to the same assignee as the present invention. 
   The second device embodiment according to the present invention is manufactured by the second method embodiment and provided schematically as  FIG. 5 . It is to be noted that the TCL  20  is added for compensating for the lower mobility of the majority of carriers, holes and uniformly spreading the electrical charges in the neighborhood of the p-type electrode  18  to the entire contact layer, p-GaN based layer  16 , and thus promoting luminous efficiency of the device. Referring to  FIG. 6 , it illustrates a particular example of  FIG. 5 . As is with the p + -GaN based layer  161  of  FIG. 3 , the p-GaN based layer  161  is also heavily doped for better ohmic contact with the upper metal electrode (not shown) and may be a p-InGaN or a p-AlInGaN layer. 
   Referring to  FIG. 7  illustrating a third method embodiment of the present invention, which is composed by adding the second method embodiment with a step, Step  8 . Step  8 : subjecting the TCL  20  to a surface treatment at its upper surface. Step  8  is executed for minimizing the portions of the generated light back off into the light-emitting structure. The surface treatment applied may be forming a roughened surface or some particularly texturized surface on the TCL surface, and the light extraction efficiency may be increased. 
   It is to be noted that Step  6 ′ and Step  7  in the second embodiment can be executed in different sequence, and so can Step  6 ′ and Step  7  in the third embodiment. 
   The fourth to the sixth embodiments according to the present invention are different with the former three embodiments in design of the resonant cavity. Referring to  FIG. 8 to 10A , a fourth embodiment according to the present invention is illustrated therein, wherein  FIG. 8  shows a method thereof,  FIGS. 9 and 9A  show a device thereof, and  FIGS. 10 and 10A  are a particular example of the device shown in  FIGS. 9 and 9A . In the embodiment, an n-DBR layer  32  is used as the lower reflecting component in replace of the metal reflector in the above-mentioned embodiments, and the method comprises the following steps. 
   Step  1   a : forming a buffer layer  31  over a substrate  30 , i.e., forming a buffer layer  31  over an upper surface  30   a  of the substrate  30 . The substrate  30  may be such as sapphire, SiC or GaN. Step  2   a : forming an n-DBR  32  over the buffer layer  31 . Step  3   a : forming an n-GaN based layer  34  over the n-DBR  32 . Step  4   a : forming an MQW active layer  35  over the n-GaN layer  34 , wherein the MQW active layer  35  is chosen so that the layer  35  may emit a light having a wavelength of 380-600 nm. Step  5   a : forming a p-DBR  36  over the MQW active layer  35 . Step  6   a : forming a p-GaN based layer  37  (for example, a p-GaN layer, a p-InGaN layer or a p-AlInGaN layer) over the p-DBR  36  and etching away a portion of the n-GaN based layer  34 , the MQW active layer  35 , the p-DBR  36  and the p-GaN layer  37  whereby an exposing region  34   a  is formed on the n-GaN based layer  34 , an n-type electrode  38  may be disposed over the exposing region  34   a , and a p-type electrode  39  may be disposed over the p-GaN layer  37 . In the method embodiment, the n-type DBR and the p-type DBR are chosen below 90% in reflectance. 
   The device of the fourth embodiment according to the present invention,  FIGS. 9 and 9A , includes a substrate  30 , an n-DBR  32 , an n-GaN layer  34 , an MQW active layer  35 , a p-DBR  36 , a contact layer  37 , an n-type metal electrode  38  and a p-type electrode  39 . 
   As compared to the former three embodiments, the fourth device embodiment is different in the resonant cavity, which is formed between the n-DBR  34  and the p-DBR  36  (the metal reflector  19  and the p-DBR  15  in the afro-mentioned embodiments), and the substrate  30  is not included in the resonant path. In this case, the substrate  30  may be transparent or not transparent, such as silicon, which is contrary to the transparent substrate  10  in the above embodiments. 
   Referring to  FIGS. 10 and 10A , a particular example of  FIGS. 9 and 9A  is shown there. In the example, the first and second layer  311  is an LT-GaN/HT-GaN buffer layer, the third layer  321  is an n-AlGaN/GaN DBR, the fourth layer  341  is an n-GaN semiconductor layer having a thickness of 2-6 μm, the fifth layer  351  is an InGaN/GaN MQW layer, the sixth layer  361  is a p-AlGaN/GaN DBR and the seventh layer  371  is a p+-GaN based semiconductor layer having a thickness of 0.2-0.5 μm. These epitaxial layers are formed over the substrate  301  having a thickness of 300-500 μm. 
   Referring to  FIG. 11 to 13  illustrating a fifth embodiment according to the present invention. As shown in  FIG. 11 , the fifth method embodiment has an extra step, Step  7   a , as compared to the fourth embodiment. Step  7   a : forming a metal oxide layer  40  over the p-GaN layer  37 , wherein the layer  40  has a suitable thickness and is transparent to a visible light having a wavelength of such as 380-600 nm.  FIG. 12  shows a fifth device embodiment of the present invention, which corresponds to the method in  FIG. 11 . As mentioned in the above, some metal oxides may be used as the TCL. Accordingly, Step  7   a  provides such a TCL. 
   Referring to  FIG. 13 , it illustrates a particular example of the fifth embodiment. In the example, all layers are the same as the corresponding ones in the fourth embodiment except for the ZnO metal oxide layer  401 , which may also be Al doped ZnO and has a thickness of 50 Å-50 μm. 
   It is to be noted that the metal oxide  40  may further be In x Zn 1-x O, Sn x Zn 1-x O or In x Sn y Zn 1-x-y O based materials, wherein 0≦X≦1, 0≦Y≦1 and 0≦X+Y≦1; or a metal oxide having an index of refraction of all least 1.5; or n-type conductive or p-type conductive metal oxide; or rare earth element doped metal oxide. 
   Referring to  FIG. 14  illustrating a sixth embodiment of the present invention. In the embodiment, there is an extra step, Step  8 , as compared to the fifth embodiment. Step  8   a : subjecting the metal oxide layer  40  to a surface treatment. That is, the region of the metal oxide layer  40  not contacted with the p-type metal  39  is subject to a surface treatment so as to have a roughened surface  41  or a particularly texturized surface. 
   It is to be noted that the epitaxial layers in the present invention may be formed by self-texturing by sputtering, physical vapor deposition, ion plating, pulsed laser evaporation, chemical vapor deposition, molecular beam epitaxy technologies or some other suitable technologies. 
   While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto since those skilled in the art may easily deduce some associated modifications. For example, GaAs may be utilized in the PIN structure of the present invention and render the corresponding light emitting device to emit a red colored light and the corresponding LED as a red colored LED. In fact, the present invention is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.