Abstract:
A semiconductor light emitting device for emission of light having a predetermined bandwidth in a primary direction of emission includes a light generating region for the generation of light; and a 1-dimensional photonic crystal structure having a photonic bandgap covering at least a segment of said bandwidth. The 1-dimensional photonic crystal structure is located such that upon incident of light from the light generating region, light having a wavelength within the bandgap of the 1-dimensional photonic crystal structure is reflected in the primary direction of emission.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the structure and fabrication of semiconductor light emitting devices. More particularly, the present invention relates to improvements in light emission efficiency of semiconductor light emitting devices. 
     BACKGROUND OF THE INVENTION 
     Light emitting diodes (LEDs) are semiconductor light emitting devices that emit lights when an electrical current is supplied thereto. Typically, an LED is formed of multiple layers of materials having a layer of p-doped material or p-type semiconductor layer (“p-layer”), a layer of n-doped material or an n-type semiconductor layer (“n-layer”), and a light generating region or p-n junction. When powered, the p-n junction emits lights in a primary direction towards one of the p- and n-layers creating a field of illumination. 
     To improve light emission efficiency of an LED device, various techniques have been proposed by the prior art, examples of which follow. 
     U.S. Pat. No. 6,784,462, entitled “Light-emitting diode with planar omni-directional reflector” and issued to Fred E. Schubert on Aug. 31, 2004, discloses an omni-directional reflector disposed between a light-emitting region and a conductive holder. The reflector has a transparent layer, an array of ohmic contacts and a reflective conductive film, arranged in sequence. The ohmic contacts increase the portion of light that reaches and is reflected by the underlying reflective film, and the increased reflection, in turn, increases the light extraction efficiency of the LED. However, the LED of &#39;462 patent has a relatively high requirement on the material of the reflective film, which needs to be electrically conductive and have a high reflectivity. Furthermore, the disposal of the reflector between the light-emitting region and the conductive holder may make the fabrication process unnecessarily complicated. 
     In U.S. Pat. No. 6,958,494, entitled “Light emitting diodes with current spreading layer” and issued to Lin, et al. on Oct. 25, 2005, a conductive and transparent Indium-Tin Oxide (ITO) film and an ultra-thin composite metallic layer, serving as a good ohmic contact and current spreading layer, are firstly attached onto a semiconductor cladding layer. Thereafter, holes may be etched into the semiconductor cladding layer to form a Photonic Band Gap structure to improve LED light extraction. However, the process of forming the Photonic Band Gap structure may be unnecessarily complicated. Furthermore, the etching process may cause damages to the contact layer above the semiconductor cladding layer and consequently may make the electrical contact instable. 
     It is an object of the present invention to provide a semiconductor light-emitting device, which overcomes at least some of the deficiencies exhibited by some of those of the prior art. 
     SUMMARY OF THE INVENTION 
     In a first aspect, there is provided a semiconductor light emitting device for emission of light having a predetermined bandwidth in a primary direction of emission. The device includes a light generating region for the generation of light; and a 1-dimensional photonic crystal structure having a photonic bandgap covering at least a segment of said bandwidth. The 1-dimensional photonic crystal structure is located such that upon incident of light from the light generating region, light having a wavelength within the bandgap of the 1-dimensional photonic crystal structure is reflected in the primary direction of emission. 
     In a second aspect, there is provided a semiconductor light emitting device for emission of light having a predetermined bandwidth in a primary direction of emission, the device including:
         a multi-layer stack of materials including a layer of p-doped material, a layer of n-doped material, and a light generating region therebetween; and   a 2-dimensional photonic crystal structure located at or adjacent an upper surface of the multi-layer stack and distal of the light generating region in the primary direction of emission, wherein the 2-dimensional photonic crystal structure has a photonic bandgap covering at least a segment of said bandwidth for extracting lights from the device.       

     In a third aspect, there is provided a flip-chip semiconductor light emitting device for emission of light having a predetermined bandwidth in a primary direction of emission, the device including:
         a light generating region;   a substantially transparent substrate on which the light generating region is formed, the substrate having a top emitting surface; and   a 2-dimensional photonic crystal structure formed at or adjacent an upper surface of the substrate such that the 2-dimensional photonic crystal structure is positioned in front of the light generating region in the primary light emitting direction,   wherein the 2-dimensional photonic crystal structure has a photonic band gap covering at least a segment of said bandwidth for extracting light from the device.       

     In a fourth aspect, there is provided a vertical semiconductor light emitting device for emission of light having a predetermined bandwidth in a primary direction of emission, the device including:
         a light generating region; and   a 1-dimensional photonic crystal structure located behind the light generating region in the primary light emitting direction and acting as a reflective layer, wherein the 1-dimensional photonic crystal structure has a photonic band gap covering at least a segment of said range of wavelength.       

     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which description illustrates by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention now will be described, by way of example only, and with reference to the accompanying drawings in which: 
         FIG. 1  shows a cross sectional view of a first embodiment of a semiconductor light emitting device according to the present invention; 
         FIG. 2   a  is a perspective view of a 1-dimensional photonic crystal structure suitable for use in conjunction with the semiconductor light emitting device of  FIG. 1 ; 
         FIG. 2   b  is a perspective view of a 2-dimensional photonic crystal structure suitable for use in conjunction with the semiconductor light emitting device of  FIG. 1 ; 
         FIG. 3  shows a cross sectional view of a second embodiment of a semiconductor light emitting device according to the present invention; 
         FIG. 4  shows a cross sectional view of a third embodiment of a semiconductor light emitting device according to the present invention; 
         FIG. 5  shows a cross sectional view of a fourth embodiment of a semiconductor light emitting device according to the present invention; and 
         FIG. 6  shows a cross sectional view of a fifth embodiment of a semiconductor light emitting device according to the present invention 
     
    
    
     DETAILED DESCRIPTION 
     The following description refers to exemplary embodiments of a semiconductor light emitting assembly according to the present invention. Reference is made in the description to the accompanying drawings whereby the semiconductor light emitting assembly is illustrated in the exemplary embodiments. Similar components between the drawings are identified by the same reference numerals. 
     In  FIG. 1 , an exemplary embodiment of a top-emitting semiconductor light emitting device  100  includes a multi-layer stack  101  of materials formed on a substrate  103 ; the multi-layer stack  101  includes a layer of p-doped material or p-type semiconductor layer  105 , a layer of n-doped material or an n-type semiconductor layer  107 , and a light generating region or p-n junction  109  as generally understood in the art. When powered, the p-n junction  109  emits lights in all directions, but a primary amount of light emissions will exit the top-emitting semiconductor light emitting device  100  in a primary light emitting direction indicated by arrow  111 , as could be understood in the art. 
     In this embodiment, the substrate  103  is a transparent substrate, and the top-emitting semiconductor light emitting device  100  further includes a 1-dimensional photonic crystal structure  113  attached to a bottom surface of the substrate  103  by, for example, gluing or deposition. Alternatively, the 1-dimensional photonic crystal structure  113  can be sandwiched between the substrate  103  and the n-layer  107 . 
     In the exemplary embodiment, the 1-dimensional photonic crystal structure  113  acts as an omnidirectional reflector for reflecting lights that exit through the substrate  103  so as to improve the light emission efficiency in the primary light emitting direction  111 . As shown in  FIG. 2   a , the 1-dimensional photonic crystal structure  113  has two sets of alternating layers  201 ,  203 , each set being formed from a high and a low refractive index material respectively, such that the structure  113  exhibits periodicity in one dimension substantially parallel to the primary light emitting direction  111 . In the exemplary embodiment, the number of the layers of the 1-dimensional photonic crystal structure is in a range of 2 to 128, preferably 8 to 64. In the exemplary embodiment, the first and second materials are selected from a group of TiO 2 , SiO 2 , Si 3 N 4 , and Ta 2 O 5 , and preferably have a relatively high refractive index contrast. 
     Furthermore, the 1-dimensional photonic crystal structure  113  has a lattice constant of approximately one fourth of the photonic band gap of the 1-dimensional photonic crystal structure, and the photonic band gap of the 1-dimensional photonic crystal structure  113  is designed to cover at least a segment of the wavelength range of the light emissions from the p-n junction  109 , not shown in  FIG. 2   a , and is in the range of 400-800 nm in the exemplary embodiment, preferably in the range of 440-470 nm, 500-540 nm or 600-650 nm. It will be understood by a skilled person in the art that lights which penetrate through the substrate  103 , again not shown in  FIG. 2   a , will be reflected substantially in a lossless manner by the 1-dimensional photonic crystal structure  113 , regardless of their incident angles at which the lights penetrates into the 1-dimensional photonic crystal structure  113 . Thereby, a substantial amount of light from the p-n junction  109  in a direction opposite to the primarily light emitting direction  111  is reflected by the 1-dimensional photonic crystal structure  113  such that light emission efficiency in the primary light emitting direction  111  is increased. 
     As shown in  FIG. 1 , the top-emitting semiconductor light emitting device  100  further includes a 2-dimensional photonic crystal structure  115  attached to a top surface  117  of the p-layer  105  by, for example, gluing, deposition, evaporation or imprinting. The 2-dimensional photonic crystal structure is in front of the multi-layer stack  101  in the light emitting direction  111  for extracting lights out of the light emitting device  100 . The photonic band of the 2-dimensional photonic crystal structure  115  also covers at least a segment of the range of wavelength the light emissions from the p-n junction  109  and is in the range of 400-800 nm in the exemplary embodiment, preferably in the range of 430-480 nm. Such a 2-dimensional photonic crystal structure  115  assist extraction of lights out of the light emitting device  100  in the primary light emitting direction  111  as will be understood by a skilled person in the art. 
     As shown in  FIG. 2   b , in the exemplary embodiment, a plurality of holes  213  are created on a conductive and transparent film, for example, ITO (Indium Tin Oxide) thin film  211 , and are periodically arranged in two dimensions defined by the ITO film  211  to form the 2-dimensional photonic crystal structure  115 , such as by etching, electron beam lithograph process, nano-imprinting process or holography technology. The 2-dimensional photonic crystal structure  115  also functions as a contact layer for the p-type semiconductor layer  105  in the exemplary embodiment. 
       FIG. 3  illustrates a second embodiment of a top-emitting semiconductor light emitting device  300  according to the present invention. The light emitting device has a structure similar to the embodiment as depicted in  FIG. 1 , however, mounted onto an LED housing holder  301  by solders  303 , and the 1-dimensional photonic crystal structure  113  is attached onto a top surface  305  of the LED housing holder  301  for reflecting light that has passed through the substrate  103 . By forming the 1-dimensional photonic crystal structure  113  on the LED housing holder, fabrication of such a light emitting device can be simplified. 
     In  FIG. 4 , a further embodiment of the present invention is show in the form of a flip-chip semiconductor light emitting device  400 . The flip-chip light emitting device  400  has a multi-stack  401  including a layer of p-doped material or p-type semiconductor layer  403 , a layer of n-doped material or an n-type semiconductor layer  407 , and a light generating region or p-n junction  405  as generally understood in the art. When powered, the p-n junction  405  emits lights in all directions, but a primary amount of light emissions will exit the flip-chip semiconductor light emitting device  400  through a substantially transparent substrate  409  attached to a top surface of the n-layer  407  in a primary light emitting direction indicated by arrow  411 . In the exemplary embodiment, a transparent conductive film, for example, an ITO film  413  is attached to the p-layer  403  for improving electrical connections between the p-layer and p-electrode  415 . A 1-dimensional photonic crystal structure  417  is attached to the ITO film  413  acting as an omnidirectional reflector for reflecting light that exits through the p-layer  403  and ITO film  413  so as to improve the light emission efficiency in the primary light emitting direction  411 . Furthermore, a 2-dimensional photonic crystal structure  419  is formed at a top surface  421  of the substrate  409  for extracting light from the flip-chip semiconductor light emitting device  400  in the primary light emitting direction  411 , such as by etching the substrate  409  to create periodically arranged holes thereon. Alternatively, the 2-dimensional photonic crystal structure can be an individual patterned transparent layer attached onto the substrate  409 . In addition, the 2-dimensional photonic crystal structure  419  may also be sandwiched between the substrate  409  and the n-layer  407 . 
       FIG. 5  illustrates another embodiment of a flip-chip semiconductor light emitting device  500 , in which the 1-dimensional photonic crystal structure  501  extends into the mesa region  503  between the p-contact  505  and n-contact  507  and also acts as a dielectric passivation layer for isolation purpose. 
     In  FIG. 6 , an exemplary embodiment of a vertical semiconductor light emitting device  600  includes a multi-layer stack  601  having a p-layer  603 , n-layer  605 , and a p-n junction  607  therebetween which when powered emits light in a primary direction of emission as indicated by arrow  609 . An n-electrode  611  is provided and is in electrical connection with the n-layer  605  for supplying power thereto. A transparent conductive layer, for example, an ITO film  613 , can be provided between the n-layer  605  and the n-electrode  611  for improving the electrical connections therebetween. 
     Furthermore, similar to the exemplary embodiments depicted in FIGS.  1  and  3 - 5 , a 1-dimensional photonic crystal structure  615  is provided on or adjacent a bottom surface of the ITO film  613  for reflecting light that exits through the n-layer  605  and ITO film  613  so as to improve the light emission efficiency in the primary light emitting direction  609 , and a 2-dimensional photonic crystal structure  617  is formed at an upper surface of the p-layer  603  for extracting lights from the vertical semiconductor light emitting device  600  in the primary light emitting direction  609 . The 2-dimensional photonic crystal structure  617  can be formed by etching a transparent conductive layer, which can also function as a p-electrode in the exemplary embodiment. 
     It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. The foregoing describes an embodiment of the present invention and modifications, apparent to those skilled in the art can be made thereto, without departing from the scope of the present invention. 
     Although the invention is illustrated and described herein as embodied, it is nevertheless not intended to be limited to the details described, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     Furthermore, it will be appreciated and understood that the words used in this specification to describe the present invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but also to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus, if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result, without departing from the scope of the invention.