High light extraction efficiency (LEE) light emitting diode (LED)

A light-emitting diode, comprising a substrate that has a first surface and an opposing second surface. A reflection layer is disposed on the first surface of the substrate and a light-emitting diode structure is arranged on the second surface of the substrate. The light-emitting diode structure includes a first semiconducting layer, an active layer and a second semiconducting layer disposed consecutively on the second surface. A plurality of protruding asymmetric micro-structured elements define at least a part of the second surface of the substrate such that at least a portion of a surface of each micro-structured element is disposed at an obtuse angle to the first surface of the substrate when measured from within the respective micro-structured element. The first semiconducting layer and the second semiconducting layer respectively have a first electrode and a second electrode.

FIELD OF THE INVENTION

The present invention relates in general to Light-Emitting Diodes (LEDs) and, more particularly, to a design and method for a high light-extraction efficiency (LEE) LED.

BACKGROUND OF THE INVENTION

Light-emitting diodes (LED or LEDs) are solid-state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a voltage is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.

Generally, an existing LED has been widely used as a backlight unit for a liquid crystal display device used in mobile phones, televisions (TVs), Personal Digital Assistants (PDAs), or notebook computers. Further, with the development of LED manufacturing technologies, efficiency has increased, brightness is greatly improved, and thus LEDs are not only used as light sources for large-sized LCD devices, such as TVs, but are also widely used for typical lighting, security lights, street lamps, etc. A LED has a long lifespan, environmentally-friendly characteristics, and an expectation to be widely used for normal lighting in the future via continuous efforts to improve electric-to-light conversion efficiency and reduce the cost.

Currently, concern about a global energy shortage has been raised again, and energy conservation has become an important issue. In the illumination field, LED lighting is booming as a revolutionary energy-saving illumination technology. LED-lamps, as a new type of green lighting product, are energy saving and environmental friendly, which may be the future development trend, and the twenty-first century will enter an era of adopting new illuminating sources represented by LED lighting.

Light-emitting diodes (LEDs) have a growing and a sizable market. As reported in LEDinside, the worldwide LED lighting market will grow from US $25.7 billion in 2015 to US $30.5 billion in 2016. One of the key parameters of a LED is the light-extraction efficiency (LEE). Due to the high refractive index of the LED material [e.g., n≈2.5 for gallium nitride (GaN)], a portion of the light cannot escape from the active area of a LED, which lowers the LEE. There have been continuous efforts to improve the LEE.

One prior art method is to create micro and/or nanostructured roofs or so-called overlayers on an exit surface of a LED, as illustrated inFIG. 1. According to this prior art method, a LED10is composed of a substrate101, a n-type GaN layer102, a n-type electrode103, an active layer104, a p-type GaN layer105, a p-type electrode106, and a micro and/or nanostructured roof or overlayer110, in which angles α and β are within the range of 0<α≤90° and 0<β≤90°. Light is generated when electrons and holes recombine at the active layer104. Some emitted light rays (e.g., Ray108) exit out the surface111due to the existence of overlayer110. However, there are still some light rays (e.g., Ray109) that reflect back from surface111even with the existence of overlayer110.

Another prior art method is to fabricate the LED20on a micro structured substrate, as illustrated inFIG. 2, see e.g., U.S. Pat. Nos. 8,450,776, 6,133,589, 7,633,097, 7,777,241, and 7,534,633. The LED20is composed of a micro structured substrate201(e.g., made of sapphire, silicon carbide, silicon), a reflection layer202, a n-type GaN layer203, an active layer204, a N-type electrode205, a p-type GaN layer206, and a p-type electrode207. There may also be a buffer layer between the substrate and n-type GaN layer to improve the lattice match. In all these prior art methods, the angles α and β are within the range of 0<α≤90° and 0<β≤90°. These micro structured substrates can help (1) to create a better lattice match between the substrate (e.g., sapphire) and GaN layer and (2) to increase the LEE. For example, the light ray211exits out the LED due to the reflection by the microstructured surface208. However, there are still light rays that cannot exit out and are trapped in the LED20even with the existence of microstructured substrate. For example, the ray212is first refracted by surface221, then reflected by reflector202, further refracted by surface222and finally reflected by surface223, resulting in lower light-extraction efficiency. Thus, it would be desirable to overcome the limitations of the existing LEDs and to provide a better solution.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a high-light-extraction efficiency light-emitting diode consists of a substrate with a first surface and an opposing second surface. A reflection layer is disposed on the first surface of the substrate and a light-emitting diode structure is arranged on the second surface of the substrate. The light-emitting diode structure includes a first semiconducting layer, an active layer and a second semiconducting layer disposed consecutively on the second surface. A plurality of protruding asymmetric micro-structured elements define at least a part of the second surface of the substrate. A portion of a surface of each micro-structured element is disposed at an obtuse angle with respect to the first surface of the substrate when measured from within the respective micro-structured element. Each protruding micro-structured element is asymmetric with respect to a plane substantially parallel to the first surface of the substrate and to a plane substantially perpendicular to the first surface of the substrate.

In some embodiments, the first semiconducting layer has a first electrode and/or the second semiconducting layer has a second electrode. The plurality of protruding asymmetric micro-structured elements each may have a curved surface. In some embodiments, the plurality of protruding asymmetric micro-structured elements of the substrate are created by a dry-etching or direction wet-etching method.

To enhance the light extraction efficiency of the light-emitting diode, an overlayer may be deposited on the second semiconducting layer. The overlayer has a plurality of protruding asymmetric micro-structured elements. At least a portion of a surface of each micro-structured element of the overlayer is disposed at an obtuse angle with respect to the second semiconducting layer when measured from within the respective micro-structured element. Each protruding micro-structured element of the overlayer is asymmetric with respect to a plane substantially parallel to the second semiconducting layer and to a plane substantially perpendicular to the second semiconducting layer.

In some embodiments, a plurality of micro and/or nano structured members are disposed on each of the plurality of protruding asymmetric micro-structured elements of the overlayer. In other embodiments, the plurality of protruding asymmetric micro-structured elements of the overlayer each has a curved surface.

In another embodiment in accordance with the present invention, a high-light-extraction efficiency light-emitting diode has a substrate with a first surface, an opposing second surface, and a reflection layer disposed on the first surface of the substrate. A light-emitting diode structure is arranged on the second surface of the substrate. The light-emitting diode structure includes a first semiconducting layer, an active layer, a second semiconducting layer and an overlayer disposed consecutively on the second surface. The overlayer has a plurality of protruding asymmetric micro-structured elements. To increase the light extraction efficiency, at least a portion of a surface of each micro-structured element of the overlayer is disposed at an obtuse angle with respect to the second semiconducting layer when measured from within the respective micro-structured element. Moreover, each protruding micro-structured element of the overlayer is asymmetric with respect to a plane substantially parallel to the second semiconducting layer and to a plane substantially perpendicular to the second semiconducting layer. The overlayer is transparent within an emission wavelength range of the light-emitting diode.

In some embodiments, the first semiconducting layer and the second semiconducting layer has a first electrode or a second electrode, respectively. The plurality of protruding asymmetric micro-structured elements of the overlayer each may have a curved surface, which enhances the light extraction efficiency of the light emitting diode. Some embodiments have a plurality of micro and/or nano structured members disposed on each of the plurality of protruding asymmetric micro-structured elements of the overlayer.

In yet another embodiment of the present invention, a high-light-extraction efficiency light-emitting diode includes a first semiconducting layer with a first surface and an opposing second surface. A first electrode is arranged on the first semiconducting layer and a light-emitting diode structure is arranged on the second surface of the first semiconducting layer. The light-emitting diode structure includes an active layer, a second semiconducting layer and a second electrode disposed consecutively on the second surface. A plurality of protruding asymmetric micro-structured elements define at least a part of the first surface of the first semiconducting layer. At least a portion of a surface of each micro-structured element is disposed at an obtuse angle with respect to the second surface of the first semiconducting layer when measured from within the respective micro-structured element, wherein each protruding micro-structured element is asymmetric with respect to a plane substantially parallel to the second surface of the first semiconducting layer and to a plane substantially perpendicular to the second surface of the first semiconducting layer. The plurality of protruding asymmetric micro-structured elements each may have a curved surface.

The plurality of protruding asymmetric micro-structured elements of the first semiconducting layer may be created by a dry-etching or direction wet-etching method. The light-emitting diode may be fabricated by using a flip-chip light-emitting diode manufacturing technique. A plurality of micro and/or nano structured members may be disposed on each of the plurality of protruding asymmetric micro-structured elements of the first surface of the first semiconducting layer. In some embodiments, the first electrode is arranged on the first surface or the second surface of the first semiconducting layer. In some embodiments, the first semiconducting layer and the second semiconducting layer comprise a p-type layer and a n-type layer respectively. In other embodiments, the first semiconducting layer and the second semiconducting layer may comprise a n-type layer and a p-type layer respectively.

The following features may also be incorporated in the above described embodiments of the high-light-extraction efficiency light-emitting diode. The plurality of protruding asymmetric micro-structured elements have a base and the base may be selected from the group consisting of a hex shape base, a triangular shape base, a square shape base, a circular shape base, an elliptical shape base and a polygon shape base. The substrate of the light-emitting diode may consist of sapphire, crystalline silicon, crystalline silicon carbide, gallium nitride, gallium arsenide, indium phosphor, or an organic material. The first semiconducting layer and the second semiconducting layer may consist of a n-type layer and a p-type layer respectively. In some light-emitting diodes, the first semiconducting layer or the second semiconducting layer are formed of a doped (AlxGa1-x)yIn1-yP (where 0≤x, y≤1), doped AlyInxGa1-x-yN (where 0≤x, y≤1), doped AlxGa1-xAs (0≤x≤1), or doped organic material, wherein Al is aluminum, Ga is gallium, In is indium, P is phosphor, As is arsenide, and N is nitride. The first semiconductor layer or the second semiconducting layer may be doped with Silicon (Si) or Magnesium (Mg).

There are several techniques to grow the semiconducting layers to fabricate a LED. In some embodiments, the doped (AlxGa1-x)yIn1-yP or the doped AlyInxGa1-x-yN is grown by a metal oxide chemical vapor deposition (MOCVD) method. In other embodiments, a liquid phase epitaxy (LPE) method may be used to grow the doped semiconducting layer AlxGa1-xAs.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the present invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings and refer to the same or like parts. Furthermore, it is required that the present invention is understood, not simply by the actual terms used, but by the meaning of each term laying within. Additional advantages, objects, and features of the invention will be set forth in part in the description that follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To overcome the limitations of the prior art micro-structured substrate, one embodiment in accordance with the present invention is a LED30that is fabricated from a novel substrate301that has a plurality of protruding asymmetric micro-structured elements, as illustrated inFIGS. 3A, 3B, and 3C.FIG. 3Ashows a two-dimensional (2D) drawing andFIG. 3Bshows the three-dimensional (3D) drawing of an embodiment of the present invention.FIG. 3Cillustrates examples of possible shapes of the protruding asymmetric micro-structured elements, including the micro-structured elements having a hex shape base, a triangular shape base, a square shape base, a circular shape base, and an elliptical shape base. There may also be other shapes such as polygon shape base, or a base with other irregular shapes. The LED30is composed of a substrate301with a first surface341and a second surface351. A light-emitting diode structure is arranged on the second surface351. The light-emitting diode structure consists of a first semiconducting layer303, an active layer305, and a second semiconducting layer306disposed consecutively on the second surface351. Some embodiments may have the second surface351with a plurality of protruding asymmetric micro-structured elements, a reflection layer302, a n-type GaN layer303, a n-type electrode304, an active layer305, a p-type layer306, and a p-type electrode307. There may also be a buffer layer between substrate301and n-type GaN layer303. The active quantum well layer305may emit light when an electron and a hole recombine. A plurality of protruding asymmetric micro-structured elements define at least a part of the second surface351of the substrate301. At least a portion of a surface of each microstructured element is disposed at an obtuse angle with respect to the first surface341when measured from within the respective micro-structured element (i.e., one of the angles α or β is larger than 90°). It should be noted that the angles α or β are the internal angles between the two surfaces, respectively, of a micro-structured element and a plane shown by the dotted line at the base of the micro-structured elements. As will be clear to those of skill in the art, these angles may also be defined as the internal angle between the first surface341and the surfaces of the micro-structured element. The shape of each protruding micro-structured element is such that it is asymmetric with respect to a plane substantially parallel to the first surface341and also to a plane substantially perpendicular to the first surface341.

Substrate301may be made of sapphire, crystalline silicon, crystalline silicon carbide, gallium nitride, gallium arsenide, indium phosphor, an organic substrate, or other suitable materials. In a non-limiting example, substrate301with a plurality of asymmetric micro-structured elements may be fabricated by dry-etching or directional (e.g., UV light assistant) wet-etching. During dry etching, the material of substrate301may be removed, typically using a masked pattern, to obtain the plurality of asymmetric micro-structured elements by exposing the material to a bombardment of ions. During bombardment, plasma of reactive gases such as fluorocarbons, oxygen, chlorine, and boron trichloride is used. Sometimes nitrogen, argon, helium and other gases are also added to these reactive gases to dislodge portions of the material of substrate301from the exposed surface. During wet-etching, substrate301with the plurality of asymmetric micro-structured elements may be fabricated by chemically removing layers from the second surface351of the substrate301. Part of the surface of the substrate301may be protected from the etchant by a “masking” material that resists etching. Ultraviolet (UV) photo-assisted wet etching may also be used on the surface of substrate301.

This novel substrate with a plurality of asymmetric micro-structured elements offers the high LEE due to the following unique features. First, with the same height-to-base aspect ratio (i.e., h/b as illustrated inFIG. 3A), substrate301with the plurality of asymmetric micro-structured elements has a larger surface area than that of the micro-structured substrate that does not contain an obtuse angle i.e.FIG. 2. Second, the light ray is better randomized when hitting the plurality of asymmetric micro-structured elements because the deflection angle can be a function of location. For example, when the light rays311and312propagate in the same direction and hit the location A and B, respectively on substrate301with the plurality of asymmetric micro-structured elements, they deflect differently. In location A, it is only deflected by surface321. However, in location B, it is deflected by both surfaces321and322due to the existence of an obtuse angle β. Thus, the output direction of rays311and312become different after the deflection by the asymmetric micro-structured elements of substrate301. Via multiple reflections and refractions by the surfaces321,322and bottom reflector302, the light ray (e.g.,313) can emit out of the surface323.

FIG. 4is a graphical presentation of the calculated light-extraction efficiency (LEE) as a function of an oblique angle β. The maximum LEE of the LED that uses the prior art micro-structured substrate that does not contain the obtuse angle is 64.7% at β=90°. However, the LEE of the LED that employs the novel obtuse angle micro-structured substrate in accordance with the present invention may be as high as 70.7% at β=140°, representing a 6.0 percentage point increase. This increase is substantial for the LED market. Embodiments of the LED according toFIG. 3Amay have the oblique angle β within a range of 110° to 150°.

Furthermore, such a substrate with a plurality of asymmetric micro-structured elements may also improve the overlap between the electron wave function and the hole wave function due to the semiconductor bandgap structure, which results in a higher internal quantum conversion efficiency.

Another embodiment of the present invention is illustrated inFIG. 5, wherein LED40contains an overlayer507with a plurality of asymmetric micro-structured elements525. Similar to the asymmetric micro-structured elements of substrate301, at least a portion of a surface of each micro-structured element525is disposed at an obtuse angle with respect to a second semiconducting layer505, when measured from within the respective micro-structured element525. Similarly, each protruding micro-structured element525has a shape such that it is asymmetric with respect to a plane substantially parallel to the second semiconducting layer505and to a plane substantially perpendicular to the second semiconducting layer505. Overlayer507is transparent within an emission wavelength range of light-emitting diode40.

The LED40is composed of a substrate501with a first surface541and a second surface551. A reflection layer508is disposed on the first surface541and a light-emitting diode structure is arranged on the second surface551. The light-emitting diode structure includes a n-type GaN layer502, a n-type electrode503, an active quantum well layer504, a p-type GaN505, a p-type electrode506, and an overlayer507with a plurality of asymmetric micro-structured elements, in which one of the angles α or β is larger than 90°. The active quantum well layer504emits light when an electron and a hole recombine. The overlayer507having a plurality of asymmetric micro-structured elements may be fabricated on a coated layer (e.g., made of photoresist) by micro and/or nano lithography (such as direct-writing laser lithography). The asymmetric micro-structured element overlayer507may also be realized by harnessing a flip-chip light-emitting diode architecture.

In accordance with the flip-chip architecture, the following process may be employed to manufacture LED50, as illustrated inFIG. 6. First, a substrate601that includes a plurality of asymmetric micro-structured elements625(e.g., made of sapphire, crystalline silicon, crystalline silicon carbide) is created by micro and/or nanolithography such as dry-etching or directional (e.g., UV light assistant) wet-etching. Then, a n-type semiconducting layer (e.g., GaN)602, active quantum well layer604, and p-type quantum well layer605are grown. The n-type semiconducting layer (e.g., GaN)602has a first surface641and an opposing second surface651. For the aluminum (Al), indium (In), gallium (Ga), nitride (N) [AlyInxGa1-x-yN (where 0≤x, y≤1)] or aluminum (Al), gallium (Ga), indium (In), phosphor (P) (AlGaInP) LEDs, the n-type layer602, the active layer604and p-type layer605may be grown by the metal organic chemical vapor deposition (MOCVD) method. For the aluminum (Al), gallium (Ga), arsenide (As) (AlGaAs) LEDs, the n-type layer602, the active layer604and p-type layer605may be grown by the liquid phase epitaxy (LPE) method. Furthermore, the n-type electrode603and p-type electrode606may also be fabricated on the grown n-type layer602and p-type layer605such as by sputtering or evaporation. Moreover, the metallic p-type electrode606may also serve as a reflection layer to increase the light extraction efficiency. Finally the substrate layer601and n-type layer602may be separated (e.g., by the laser lift-off process) so that the light ray610can emit out of the n-type layer containing asymmetric micro-structured elements. Now the first surface641has a plurality of asymmetric micro-structured elements with surfaces i.e.621and622.

Similar to the asymmetric micro-structured elements of substrate301, at least a portion of a surface of each micro-structured element is disposed at an obtuse angle with respect to a second surface651, when measured from within the respective micro-structured element, such that an angle β is larger than 90°. Similarly, each protruding micro-structured element has a shape such that it is asymmetric with respect to a plane substantially parallel to the second surface651and also to a plane substantially perpendicular to the second surface651.

Again, similar to LED30inFIG. 3Athat contains the substrate301with the plurality of asymmetric micro-structured elements, we also compute the LEE in the flip-chip LED50containing n-type layer602with the plurality of asymmetric micro-structured elements.FIG. 7shows the calculated light-extraction efficiency (LEE) as a function of angle β. One can see that the LEE reaches a maximum (LEE=72%) at the obtuse angle β=110°. This again shows the advantage of harnessing the plurality of asymmetric micro-structured elements. Embodiments of the LED according toFIG. 6may have the oblique angle β within a range of 90° to 130°.

Another embodiment of the invention is illustrated inFIG. 8, wherein a LED60includes micro/nano-structured members801created (e.g., by colloidal lithography) on top of overlayer507or n-type substrate602having the plurality of asymmetric micro-structured elements, as illustrated inFIG. 8. These micro/nano-structured members further increase the LEE of a LED.

Yet another embodiment of the present invention is illustrated inFIG. 9as LED70. LED70includes both a substrate301with a plurality of asymmetric micro-structured elements and an overlayer507with a plurality of asymmetric micro-structured elements, a reflection layer901, a n-type semiconductor layer902, a n-type electrode903, an active quantum well layer904, a p-type layer905, and a p-type electrode906.

FIG. 10depicts LED80, which is another embodiment of the present invention. LED80has curved surfaces (i.e.1011, and/or1012, and/or1013, and/or1014) for overlayer507and for n-type substrate1001. LED80is also made of a substrate1001, a n-type semiconductor layer1002, a n-type electrode1003, an active quantum well layer1004, a p-type semiconductor layer1005, and a p-type electrode1006. In some embodiments, LED80may also have a reflection layer1007.

As shown inFIG. 11, some embodiments of the present invention may have micro/nano-structured members801created on top of the curved surfaces1013and/or1014of LED90.

Having described the invention in detail, those skilled in the art will appreciate that, given the disclosure herein, modification may be made to the invention without departing from the spirit of the invention concept. It is not intended that the scope of the invention be limited to the specific and preferred embodiments illustrated and described. All documents referenced herein are hereby incorporated by reference, with the understanding that where there is any discrepancy between this specification and the incorporated document, this specification controls.

REFERENCES