Patent Application
20130009167
The present invention relates to a light emitting diode (LED) device and a method of making the same, and in particular an LED device structure with a patterned crystalline light extraction layer obtained from epitaxial growth in contact with the current spreading layer.
There is growing popularity in the use of light emitting diodes (LED) for general lighting and backlight applications to replace conventional light sources such as incandescent bulbs, halogen bulbs, cold cathode fluorescent lamps (CCFL) and compact fluorescent lamps due to the lower power consumption and the use of non-toxic materials in LEDs. To produce white LEDs, indium-gallium nitride-based blue LED chips emitting at ˜450 nm is typically used to excite a phosphor layer to create white light. However, due to the large refractive index contrast between GaN (n˜2.5) and air, the majority of generated light has difficulty escaping the structure and is waveguided within the semiconductor layer itself, subsequently resulting in absorption. The poor light extraction efficiency (LEE) in these structures severely limits the device performance. In a blue LED chip without any special light extraction features, the LEE of the chip is only ˜25-30%. A variety of methods have been employed to increase LEE, such as surface roughening, photonic crystal, flip-chip mounting, chip-shaping, and patterned sapphire substrates. Among these, patterned sapphire substrates (PSS) [Yamada et. al, Japanese Journal of Applied Physics, vol. 41, L1431-1433, (2002)] are commonly used in commercial blue LED chips to increase LEE to ˜60%.
Besides PSS structures, a variety of methods have been employed to improve LEE further. The key feature towards increasing LEE is to reduce total internal reflection (TIR) of light at the GaN(ITO)-air interface, and by doing so light will be extracted out of the structure quicker and reduce the probability of being absorbed in the structure. Surface patterning with periodic nano/micro structures have been reported in the literature as means to reduce TIR in these structures. LED chips in commercial products are typically small area devices (i.e. 300 μm×800 μm). More than 50% of light is extracted by edge emission through the facets of a singulated chip, and the remainder is extracted through the top surface, when a reflector is placed at the bottom of the chip. If the chip size is increased, surface emission becomes dominant over edge emission. However, light extraction from the surface is challenging due to TIR at the GaN-air interface, resulting in increased absorption for large area chips compared to smaller ones. In order to not compromise chip efficiency, manufacturers are using an array of small area LED chips instead of a single large chip to form a high power LED module.
In U.S. Pat. No. 5,955,749 (published date 21 Sep. 1999), Joannopolous et al describes the use of photonic crystal structures by etching periodic nanopatterns into the p-GaN layer to improve light extraction from the surface. However, this method will also result in increased operating voltage, as reported by Chhajed et. al [Applied Physics Letters, vol. 98, 071102 (2011)]. Kim et. al [Applied Physics Letters, vol. 91, 171114 (2007)] employed surface nano-scale patterning on the indium tin oxide (ITO) current spreading layer to increase light extraction. Although an increase in LEE is observed, patterning the ITO layer reduces the effective ITO thickness and subsequently increases the sheet resistance.
From the examples above, it is evident that using surface patterning structures that does not involve etching into the p-GaN or ITO layer is preferable, since any improvement in LEE will be offset by an increase in operating voltage due to higher series resistance. An alternative method is to deposit a patterned high refractive index layer over the ITO film. Since the refractive index of ITO is ˜1.9-2.0 (450 nm), the high index film will need to have a higher refractive index than the ITO layer. Light emitted will then be coupled into the high index layer, which is patterned with micro/nano structures to extract light out. Light extraction will not be effective if the high index layer is not patterned. Using the same concept, Cho et. al [Japanese Journal of Applied Physics, vol. 49, 102103 (2010)] reported the use of nanopatterned TiO2 (n˜2.2) on top of the ITO layer to increase LEE. The author reported a 12% increase in light output power, and since the p-GaN and ITO layer is not etched, the current-voltage (I-V) characteristics is unaffected. This approach also improves light extraction through surface emission, therefore giving manufacturers the option of using large area LED chips over an array of small ones without compromising efficiency.
The high index layer approach will be more effective the higher its refractive index, and is ideally the same refractive index as the GaN layer (n˜2.5). However, it is not easy to achieve transparent films with refractive indices larger than 2.2. Conventional films such as ZnO, ZrO2, TiO2, Ta2O5, ITO, SiOxNy, SiNx, AlN, ZnS, and IZO have refractive indices of ˜2.0-2.2 at 450 nm. Attempts to increase the refractive index of these films are not trivial, since the optical transparency of the film will need to remain very high quality in an LED device. Since emitted light is reflected/refracted many times in an LED structure before escaping into air, a slight increase in absorption of the high index film can result in a sharp decrease in output power.
An embodiment of a conventional LED structure is described in U.S. Pat. No. 6,657,236 (B. Thibeault et. al, issued 2 Dec. 2003). An array of LEE features is formed on the current spreading layer to improve LEE. The LEE features are formed by evaporation, chemical vapour deposition (CVD) or sputtering. It is preferable to use GaN as the LEE features in this case, but high quality crystalline GaN films cannot be achieved using the methods described in the prior art. Crystalline GaN films can also be achieved using epitaxial growth methods, such as metal organic chemical vapour deposition (MOCVD) or molecular beam epitaxy (MBE). Furthermore, crystalline quality GaN film cannot be achieved by growing directly on the current spreading layer (i.e. ITO), since attempts to grow GaN directly on ITO or any other metallic/oxide films will result in poor quality amorphous films.
In U.S. Pat. No. 7,244,957 (N. Nakajo et. al, issued 17 Jul. 2007), a patterned niobium doped TiO2 layer is used as the light extraction layer. Niobium doping is used to improve the electrical conductivity of the film, enabling the TiO2 layer to act both as a LEE layer and also a current spreading layer. This LEE improvement is limited for this structure since it is difficult to achieve refractive index higher than 2.2 for TiO2.
US 2008/0121918 (S. P. DenBaars et. al, published 29 May 2008) describes an LED structure which is mounted p-side down and the N-polarity face of n-type GaN is wet etched with KOH to form conical light extraction features. The use of p-side down mounting is more complicated than conventional p-side up mounting and this structure requires the use of bulk GaN substrates, which is more expensive than conventional GaN on sapphire substrate structures.
From these examples, there is a need in the art for LED devices with good light extraction efficiency. An object of the present invention is to provide an LED with good light extraction through the use of high refractive index light extraction layer, and this will be key towards realisation of high efficiency nitride LEDs.
The present invention provides an LED structure with good light extraction properties. The invention includes a crystalline quality epitaxial GaN film light extraction layer grown on a separate substrate, which is mechanically bonded onto the current spreading layer of a GaN LED structure. Light extraction through surface emission will also be improved for this structure, which enables large area LED chips to be made without compromising efficiency.
According to an aspect of the invention, a light emitting diode is provided which includes an active region in combination with a current spreading layer; and a crystalline epitaxial film light extraction layer in contact with the current spreading layer, the light extraction layer being patterned with nano/micro structures which increase extraction of light emitted from the active region.
According to another aspect, the active region in combination with the current spreading layer includes a conductive n-type region on a substrate, the active region on the n-type region, a p-type region on the active region, and the current spreading layer on the p-region region.
In accordance with another aspect, the crystalline epitaxial film light extraction layer is bonded directly onto the current spreading layer.
According to still another aspect, the crystalline epitaxial light extraction layer is bonded onto the current spreading layer with an intermediate adhesive layer.
In yet another aspect, the intermediate adhesive layer is optically transparent and has a similar or higher refractive index than a refractive index of the current spreading layer.
According to another aspect, the crystalline epitaxial film light extraction layer has a doping level equal to or greater than 1018 cm−3.
According to still another aspect, a shape of the nano/micro structures is at least one of a square, circle, triangle or combination thereof.
In accordance with another aspect, the nano/micro structures have a height of between 10 nm and 10 μm, and a diameter of between 100 nm and 10 μm.
According to still another aspect, the current spreading layer is made of indium tin oxide, indium zinc oxide, zinc oxide or indium zinc tin oxide.
In accordance with yet another aspect, the crystalline epitaxial film light extraction layer is any one or more of GaN, AlGaN, InGaN, AlInGaN, or diamond.
In yet another aspect of the invention, a method of making a light emitting diode is provided. The method includes forming an active region in combination with a current spreading layer; and providing a crystalline epitaxial film light extraction layer in contact with the current spreading layer, the light extraction layer being patterned to form nano/micro structures which increase extraction of light emitted from the active region.
According to another aspect, the method includes forming the active region in combination with the current spreading layer on a first substrate; expitaxially growing the crystalline epitaxial film light extraction layer on a second substrate that is separate from the first substrate; and mechanically bonding the crystalline epitaxial film light extraction layer on the second substrate to the current spreading layer on the first substrate.
According to yet another aspect, the method includes forming a conductive n-type region on a first substrate, forming the active region on the n-type region, forming a p-type region on the active region, and forming the current spreading layer on the p-type region; epitaxially growing the crystalline epitaxial film light extraction layer on a second substrate that is separate from the first substrate; mechanically bonding the crystalline film light extraction layer on the second substrate to the p-type region on the first substrate; following the mechanical bonding, removing the second substrate; and forming the nano/micro structures on a surface of the crystalline film light extraction layer exposed by the removal of the second substrate.
In accordance with another aspect, the crystalline epitaxial film light extraction layer is mechanically bonded directly onto the current spreading layer.
According to another aspect, the crystalline epitaxial light extraction layer is mechanically bonded onto the current spreading layer using an intermediate adhesive layer.
According to yet another aspect, the intermediate adhesive layer is optically transparent and has a similar or higher refractive index than a refractive index of the current spreading layer.
In accordance with still another aspect, the second substrate is one of a sapphire, silicon or silicon carbide substrate.
According to another aspect, a shape of the nano/micro structures is at least one of a square, circle, triangle or combination thereof.
In still another aspect, the nano/micro structures have a height of between 10 nm and 10 μm.
According to yet another aspect, the nano/micro structures have a diameter of between 100 nm and 10 μm.
According to another aspect, the current spreading layer is made of indium tin oxide, indium zinc oxide, zinc oxide or indium zinc tin oxide.
In accordance with another aspect, the crystalline epitaxial film light extraction layer is any one or more of GaN, AlGaN, InGaN, AlInGaN, or diamond.
According to another aspect, the crystalline epitaxial film light extraction layer has a doping level equal to or greater than 1018 cm−3.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In the annexed drawings, like references indicate like parts or features:
The present invention provides an LED device with good optical light extraction efficiency. The LED structure includes a periodic nano/micro structure on a crystalline quality epitaxial film, formed on top of the current spreading layer. The crystalline film preferably has a higher refractive index than the current spreading layer, in order to enhance light extraction efficiency. Surface emission of light will also be enhanced, which enables large area LED chips to be used instead of an array of small LED chips without affecting chip efficiency. The invention will be detailed through the description of embodiments, wherein like reference numerals are used to refer to like elements throughout.
According to the invention, an LED with good light extraction efficiency may be obtained. As illustrated in
Referring again to
According to this embodiment, the general structure in
As broadly described herein, the crystalline film light extraction layer in the present invention can be doped or undoped. For example, the crystalline film light extraction layer may be intrinsically undoped or include only unintentional background doping. Alternatively, the crystalline film light extraction layer may be intentionally doped in order to alter properties of the layer.
In the case of Embodiment 3 of the invention, the structures in
According to this embodiment, the method of making the structure described in Embodiments 1-3 is described.
The nano or micro structures can be formed using conventional methods such as optical contact lithography, electron-beam lithography, stepper lithography, nanoimprint lithography, deep UV lithography and other known methods.
The present invention provides an LED structure with good light extraction properties. The invention includes a crystalline quality epitaxial GaN film light extraction layer grown on a separate substrate, which is mechanically bonded onto the current spreading layer of a GaN LED structure. Light extraction through surface emission will also be improved for this structure, which enables large area LED chips to be made without compromising efficiency.
The invention provides for the light extraction layer to be optically transparent, of crystalline quality and obtained from epitaxial growth. The light extraction layer will preferably have a higher refractive index than the current spreading layer and preferably similar or higher than that of the GaN film. The light extraction layer is patterned with micro/nano structures to act as light extraction features.
The crystalline film light extraction layer is a highly doped material, which improves the current spreading properties of the LED device, in addition to its enhanced light extraction features.
The light extraction layer is mechanically bonded directly onto the current spreading layer using direct wafer bonding, i.e., without an intermediate layer.
The light extraction layer may be mechanically bonded onto the current spreading layer using an optically transparent intermediate layer, and this intermediate (or adhesive) layer preferably has a similar or higher refractive index than that of the current spreading layer.
The invention provides a light emitting diode (LED) which includes a conductive n-type region; a p-type region formed on the active region; a current spreading layer formed on top of the p-type region; an optically transparent intermediate layer; and a crystalline epitaxial film light extraction layer at the surface; a plurality of nano/micro structures are formed on the crystalline epitaxial film light extraction layer for light extraction from the active region, and the nanostructures having a diameter of at least 100 nm and up to 10 μm.
The shape of the nano/micro structures may be at least one of a square, circle, triangle or combination thereof.
The nano/micro structures may have a height of between 10 nm and 10 μm.
The current spreading layer may be made of indium tin oxide.
The crystalline epitaxial film light extraction layer may be any one or more of GaN, AlGaN, InGaN, AlInGaN, or diamond.
The invention provides a method of making a light emitting diode (LED. The method includes forming a conductive n-type region on a substrate; forming an active region on the n-type region; forming a p-type region on the active region (thereby referred to as the host wafer); and preparing a crystalline epitaxial film light extraction layer on a separate substrate (thereby referred to as the sacrificial substrate); mechanically bonding the crystalline epitaxial film light extraction layer on the sacrificial substrate onto the p-type region on the host wafer; removing the sacrificial substrate; and forming nano/micro structures onto an exposed surface of the crystalline epitaxial film light extraction layer.
The crystalline epitaxial film light extraction layer on the sacrificial substrate is grown on a sapphire, silicon or silicon carbide substrate.
Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
The invention thereby provides an LED device with nano/micro structures formed on a crystalline epitaxially grown film on top of the current spreading layer on an LED device structure. By selecting a high refractive index crystalline film, high light extraction efficiency can be achieved with this structure. The invention further provides a method of producing the same.
