Abstract:
An LED device ( 90 ) includes: an epitaxial structure ( 100 ) having a plurality of layers of semiconductor material and forming an active light-generating region ( 120 ) which generates light in response to electrical power being supplied to the LED device ( 90 ); and, a substrate ( 200 ) that is substantially transparent in a wavelength range corresponding to the light generated by the active light-generating region ( 120 ). The substrate has first and second opposing end faces ( 202, 206 ) and a plurality of side walls ( 210 ) extending therebetween, including a first side wall having a first portion thereof that defines a first surface ( 212, 214, 216, 218 ) which is not substantially normal to the first face ( 202 ) of the substrate ( 200 ). The epitaxial structure ( 100 ) is disposed on the first face ( 202 ) of the substrate ( 200 ).

Description:
BACKGROUND  
       [0001]     The present inventive subject matter relates to the lighting arts. It is particularly applicable to high light output green, blue and/or ultraviolet (UV) gallium nitride (GaN) based light emitting diodes (LEDs) and LED arrays, and will be described with particular reference thereto. However, application is also found in connection with other types of LEDs and in other LED applications.  
         [0002]     GaN based LEDs, as are commonly known in the art, are suitable for many illumination applications. GaN based LEDs typically emit light in the green, blue and/or UV wavelength ranges. At times, GaN based LEDs employ wavelength-converting phosphors to produce white or other colored light for illumination. Such LEDs have a number of advantages over other types of illuminators, including, e.g., compactness, low operating voltages, and high reliability.  
         [0003]     However, GaN based LEDs for lighting applications can suffer from low luminous output. For example, a typical GaN based LED may generate about 100 lumens of light output. In contrast, a typical incandescent light source may generate about 1,000 lumens of light output. One obstacle to high light output in GaN based LEDs is extraction of the light from the device.  
         [0004]     With reference to  FIG. 1 , in a flip chip LED arrangement, an epitaxial structure  10  typically including multiple layers of semiconductor material and forming an active light-generating region  12  (e.g., a double heterostructure, multiple quantum well (MQW), or other suitable light-generating configuration), is usually disposed on a substrate  20  that is substantially transparent or transmissive to light at the wavelength generated. A pair of electrodes and/or electrical contacts  30  (e.g., a p-type and an n-type) are also arranged on the LED in operative electrical communication with the light-generating region  12  so that electrical power supplied to the LED therethrough drives the same to generate light. In a so called lateral current flip chip LED device, the electrodes  30  are commonly located on the same side of the epitaxial structure  10  generally opposite the substrate  20 , as opposed to a so called vertical current LED device where the pair of electrodes are usually arranged on two sides of the LED, each on a side opposite from the other.  
         [0005]     Commonly, the LED is mounted to a support (e.g., a sub-mount, printed circuit board (PCB), reflector cup, etc.) in flipped orientation, that is, with the light-generating region  12  proximate to the support and the substrate  20  distal from the support. In the flip chip arrangement, the goal is generally to extract a substantial amount of light from the LED through the light-transmissive substrate  20 . However, some conventional lateral current flip chip configurations can be disadvantageous in terms of light extraction efficiency.  
         [0006]     For example, a refractive index mismatch at an interface  40  between the substrate  20  and epitaxial structure  10  can hinder the light from finding its way into the substrate in the first place, e.g., due to total internal reflection (TIR). Light so trapped is more likely to be absorbed through wave guiding in the epitaxial structure  10  thereby reducing the overall lumens output by the LED. The thickness of the substrate  20  can also contribute to light loss. Additionally, extraction of light from the substrate  20  may also be inhibited by its shape. For example, the side walls  22  of the substrate  20  are typically substantially normal to the opposing end faces of the substrate  20 , namely, the end face forming interface  40  with the epitaxial structure  10  and the opposing end face  24 . This normal arrangement of the side walls  20  tends to result in light generated by the LED having an angle of incidence therewith that produces TIR, thereby impeding light extraction from the substrate  20 .  
         [0007]     The present inventive subject matter contemplates a new and improved LED device and/or method for producing and/or using the same that overcomes the above-mentioned limitations and others.  
       BRIEF SUMMARY  
       [0008]     In accordance with one aspect, an LED device is provided. It includes: an epitaxial structure having a plurality of layers of semiconductor material and forming an active light-generating region which generates light in response to electrical power being supplied to the LED device; and, a substrate that is substantially transparent in a wavelength range corresponding to the light generated by the active light-generating region, the substrate having first and second opposing end faces and a plurality of side walls extending therebetween, including a first side wall having a first portion thereof that defines a first surface which is not substantially normal to the first face of the substrate. The epitaxial structure is disposed on the first face of the substrate.  
         [0009]     Numerous advantages and benefits of the present inventive subject matter will become apparent to those of ordinary skill in the art upon reading and understanding the present specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting. Further, it is to be appreciated that the drawings are not to scale.  
         [0011]      FIG. 1  is a diagrammatic illustration showing a lateral current spread flip chip LED in accordance with a prior art design.  
         [0012]      FIG. 2  is a diagrammatic illustration showing an exemplary lateral current spread flip chip LED die or chip with a shaped substrate that embodies aspects of the present inventive subject matter.  
         [0013]      FIG. 3  is a diagrammatic illustration showing another exemplary lateral current spread flip chip LED die or chip with a shaped substrate that embodies aspects of the present inventive subject matter.  
         [0014]      FIG. 4  is a diagrammatic illustration showing yet another exemplary lateral current spread flip chip LED die or chip with a shaped substrate that embodies aspects of the present inventive subject matter.  
         [0015]      FIGS. 5A and 5B  are diagrammatic illustrations showing a cross-section view and perspective view, respectively, of an exemplary shaped substrate with recesses for a lateral current spread flip chip LED die or chip that embodies aspects of the present inventive subject matter.  
         [0016]      FIG. 6  is a diagrammatic illustration showing the LED die or chip of  FIG. 2  arranged in exemplary packaging such that the same embodies aspects of the present inventive subject matter.  
         [0017]      FIG. 7  is a diagrammatic illustration showing the LED die or chip of  FIG. 3  arranged in exemplary packaging such that the same embodies aspects of the present inventive subject matter. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]     With reference to  FIGS. 2-4 , a lateral current spread flip chip LED die or chip  90  in accordance with suitable embodiments of the present invention includes an epitaxial structure  100  disposed on a shaped substrate  200 . The epitaxial structure  100  includes multiple layers of semiconductor material and forms an active light-generating region  120 , e.g., a double heterostructure, MQW, or other appropriate light-generating configuration. Suitably, the epitaxial structure  100  comprises a GaN based semiconductor material device that emits light in the green, blue and/or UV wavelength ranges when supplied with electrical power. However, other semiconductor material LEDs are also contemplated.  
         [0019]     A pair of electrodes and/or electrical contacts  130  (e.g., a p-type and an n-type) are arranged on the LED chip  90  in operative electrical communication with the light-generating region  120  so that electrical power supplied to the LED chip  90  therethrough drives the same to generate light. Suitably, the devices is a lateral current device and the electrodes  130  are located on the same side of the epitaxial structure  100  opposite the substrate  200 .  
         [0020]     In a suitable embodiment, preferably the light-generating region  120  is arranged between cladding layers  122 , at least one of which is an n-type cladding layer. To achieve an efficient lateral device, the n-type cladding layer is preferably a layer of GaN material having good conductivity, i.e., preferably a conductivity at or below 30 Ohm/sq, and more preferably at or below 20 Ohm/sq. Nevertheless, achieving the desired conductivity can be a challenge with respect to growth. This challenge is, however, preferably overcome by having the layer sufficiently thick (e.g., around 1.5 μm or greater) and/or by using special doping techniques (e.g., delta doping, superlattices (SLs), and/or the like).  
         [0021]     The substrate  200  is substantially transparent or transmissive to light of the wavelength generated by the active light-generating region  120  such that at least some portion of the generated light enters the substrate  200  from the epitaxial structure  100 , passes through the substrate  200 , and is extracted or emitted therefrom through a backside face  206  and/or side walls  210 . Suitable materials for the substrate  200  include sapphire (Al 2 O 3 ), silicon carbide (SiC) and gallium nitride (GaN). Optionally, the substrate  200  comprises silicon carbide with an absorption coefficient less than 5.0 cm −1 . Alternately, the substrate  200  comprises a nitride material with a refractive index not lower than 2.2 and an absorption coefficient less than 5.0 cm −1 . Of course, other suitable transparent substrate materials are also contemplated.  
         [0022]     The substrate  200  is suitably a solid mass having a primary thickness t measured as the shortest distance between to two opposing end faces, namely, an epi-side face  202  that forms an interface  204  with the epitaxial structure  100  and the backside face  206  opposite therefrom. Suitably, the end faces are substantially planar and parallel to one another. The end faces optionally have square, rectangular or other polygonal areas that are different in size from one another. As shown in  FIG. 2 , the epi-side face  202  has an area that is greater than the area of the backside face  206 . Alternately, as shown in  FIG. 3 , the epi-side face  202  has an area that is less than the area of the backside face  206 .  
         [0023]     A plurality of side walls  210  are disposed and/or extend between the end faces  202  and  206 . At least a portion of at least one of the side walls  210  is not substantially normal to the substrate end faces. For example,  FIG. 2  shows sides walls  210  having portions that define surfaces  212  (suitably, planar surfaces) that are inclined with respect to the epi-side face  202  to form acute angles therewith. Alternately,  FIG. 3  shows sides walls  210  having portions that define surfaces  214  (suitably, planar surfaces) that are inclined with respect to the epi-side face  202  to form obtuse angles therewith. As shown in  FIGS. 2 and 3 , the shaped substrate  200  substantially takes the form of a truncated pyramid, comparatively in opposite orientations with respect to the epitaxial structure  100 .  
         [0024]     In yet another embodiment shown in  FIG. 4 , the shaped substrate  200  substantially takes the form of two truncated pyramids combined in opposite orientations with respect to one another. That is to say, the side walls  210  includes portions that define two surfaces which are not substantially normal to the substrate end faces. Specifically, the surfaces  216  (suitably, planar surfaces) are inclined with respect to the epi-side face  202  to form acute angles therewith, and the surfaces  218  (suitably, planar surfaces) are inclined with respect to the epi-side face  202  to form obtuse angles therewith.  
         [0025]     Optionally, to achieve a desired light extraction benefit, the substantially non-normal portions of the side walls  210  account for more than 50% of the thickness t. That is to say, with respect to  FIG. 2 , the distance a is more than 50% of t; with respect to  FIG. 3 , the distance b is more than 50% of t; and with respect to  FIG. 4 , the combined distance of c plus d is more than 50% of t. Light extraction from the substrate  200  may be further enhanced by optionally roughening and/or texturing any one or more of the side walls  210  or end faces  202  and  206  so as to inhibit TIR at those surfaces.  
         [0026]     With reference, to  FIGS. 5A and 5B , optionally one or more recessed regions are formed in the substrate  200  such that a thickness t′ of the substrate measured in the recessed regions is less than the thickness t measured in the non-recessed regions. Suitably, one or more recesses  220  are formed in the backside face  206  of the substrate  200 . While the recesses  220  are shown in conjunction with the side wall configuration of  FIG. 4 , they are likewise optionally employed with either of the side wall configurations shown in  FIGS. 2 and 3 . The recesses  220  reduce the effective or mean thickness of the substrate  200  thereby reducing the likelihood of generated light getting absorbed in the substrate  200  insomuch as the effective or mean distance traveled therethrough is reduced. Optionally, the recesses  220  may take any suitable shape or form. However, as shown, the recesses  220  are pyramid shaped. Having the recess walls  222  substantially non-normal or inclined with respect to the backside face  206  further supports light extraction by inhibiting TIR.  
         [0027]     The LED chip  90  is mounted to a support, e.g., a sub-mount, PCB, reflector cup, etc., in flipped orientation, that is, with the light-generating region  120  proximate to the support and the substrate  200  distal from the support. With reference to  FIGS. 6 and 7 , the LED chip  90  is arranged within a suitable reflector cup  300  so as to reflect the light emitted by the LED chip  90  outward. An appropriate LED encapsulant  310  encapsulates the die or chip  90 . Suitably, the encapsulant  310  is substantially transparent or transmissive to light of the wavelength generated and/or emitted by the LED die or chip  90 , and it optionally forms a lens for focusing the light passing therethrough. Optionally, the encapsulant  310  is an epoxy with a refractive index higher than 1.5. However, other appropriate encapsulant materials are also contemplated, e.g., various resins or the like. Additionally, phosphors and/or other like wavelength-converting material are optionally employed to convert at least some portion of the light emitted from the LED die  90  from one wavelength to another, e.g., to produce a composite luminous output that appears as white light or some other color light. Suitably, the phosphor is dispersed in the encapsulant  310  and/or coated on the substrate  200 .  
         [0028]     Optionally, in production, the LED chip  90  is mounted and/or otherwise arranged in the reflector cup  300  prior to being coated with phosphor and/or encapsulated by the encapsulant  310 , which is generally poured or otherwise deposited into the reflector cup  300  in an initially liquid or flowing state. Notably, in this case, the embodiment of  FIG. 6  has certain advantages. For example, the inclined surfaces  212  of the substrate side walls  210  slope away from, or in the opposite direction of, the inclined surfaces  302  of the reflector cup. Accordingly, there is easy and/or uninhibited access to regions around the LED chip  90  for application of phosphors and/or flowing of the encapsulant  310 . Contrastingly, the embodiment of  FIG. 7 , wherein the inclined surfaces  214  of the substrate side walls  210  slope toward, or in the same direction as, the inclined surfaces  302  of the reflector cup, the gap  304  formed therebetween may restrict flowing of the encapsulant  310  to underlying regions and/or inhibit coating of the same with phosphors.  
         [0029]     Also with respect to production, optionally an array of epitaxial structures  100  are deposited on a single substrate wafer that is then diced to form a plurality of individual LED devices  90 . Suitably, the dicing is performed with one ore more angled side cuts, e.g., via sawing, laser-cutting or other like separation techniques, to shape the side walls  210 . Accordingly, in some instances, e.g., particularly where a high device yield per substrate wafer is desired, the embodiment of  FIG. 2  has certain advantages. For example, modeling on otherwise similar devices with square epitaxial structures having a side dimension of 976 μm and substrates in accordance with the embodiments of  FIGS. 2 and 3  suggests that the embodiment of  FIG. 2  has an increased per wafer chip yield compared to the embodiment of  FIG. 3  without substantial expense to the light extraction properties. For achieving effective side wall inclination, a 240 μm street width used in the  FIG. 3  embodiment demonstrates a light extraction value of around 38.7% on transparent SiC. A 70 μm street width used in the  FIG. 2  embodiment demonstrates a light extraction value of around 35%. However, the larger street width implies a higher loss of active area and a lower number of chips per wafer.  
         [0030]     The present inventive subject matter has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.