Patent Application
20140191243
Disclosed embodiments relate to light emitting diodes (LEDs) on patterned articles including patterned substrates.
Lighting consumes nearly 25% of the world's electricity and thus is one of the largest consumers of energy and contributors to greenhouse gas emissions. In the decade 2000 to 2010 significant advancements have been made to LED-based solid state lighting systems with luminous efficiency increasing from about 20 lm/watt to nearly 100 lm/watt. Further improvement in efficiency and reduction in manufacturing cost is essential to make white LEDs cost competitive with fluorescent and incandescent bulbs.
LEDs generally include three semiconductor layers on a substrate. Between p-type and n-type semiconductor layers, an active region is provided. When the LED is forward-biased (switched on), the active region emits light when electrons and holes recombine there. GaN-based LEDs are common LEDs. One type of LED is an organic LED (OLED) where the emissive layer is a film of an organic compound that emits light.
Efficiency of light emitting devices (e.g., LEDs, OLEDs) can be increased by enhancing the internal quantum efficiency, which represents the conversion efficiency of electrons to photons, by improving the light extraction efficiency or out-coupling efficiency. External efficiency of LEDs, OLEDs and other thin film light emitting devices is known to be limited by the out-coupling efficiency (or extraction efficiency). The high refractive indices of the active layer leads to total internal reflection (TIR) and waveguiding of a significant portion of the generated light. The higher the refractive index of the active layer the smaller the escape cone defined by the critical angle for TIR.
Out-coupling efficiency has been improved by the opening of higher number of the six escapes cones for each direction (lateral and vertical) by use of thick transparent substrates, shaping of LED chips, or by reducing wave-guiding through modification of various interfaces in the device. Interface modification induces photon randomization thereby giving multiple chances to photons to escape upon subsequent reflections. Photon randomization has been achieved by simple interface roughening, or by having regular patterned structures at various interfaces, such as Bragg gratings, photonic crystals, micro-rings, microlenses, micro-pyramids, and cones.
Sapphire substrates, or sapphire wafers, are now used by the majority of the world's LED manufacturers for the production of green, blue, and white LEDs. Patterned sapphire substrates (PSS) have been shown to substantially improve the efficiency of LED devices as compared to planar surfaced sapphire substrates, essentially because of two reasons. Firstly, PSS improves the quality of epi-layer (reduces defect density) which increases the internal quantum efficiency, and secondly it increases the light out-coupling efficiency by reducing TIR.
Two types of substrate patterns are typically used for forming an epitaxial light emitting stack, patterns with recessed features such as trenches or circular holes, and patterns with protruding features. The GaN epi-growth on substrate patterns with recessed features occurs via epitaxial lateral overgrowth (ELOG). On a substrate with protruding features GaN growth is known to preferentially takes place from the (0001) flat bottom growing laterally over the protruding features in a process known as facet-controlled epitaxial lateral overgrowth. Both of these substrate patterns generally provide a reduction in defect density in the epitaxial layers.
This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.
Disclosed embodiments include patterned articles comprising a substrate support having planar substrate surface portions including a substrate material having a substrate refractive index. A patterned top surface is on the substrate support including a plurality of features lateral to the planar substrate surface portions protruding above a height of the planar substrate surface portions. At least a top surface of the plurality of features include an epitaxial (epi)-blocking layer including at least one of (i) a non-single crystal material (amorphous or polycrystalline) having a refractive index lower as compared to the substrate refractive index and (ii) a reflecting metal or a metal alloy (hereafter “reflecting material”). The epi-blocking layer can include 2 or more layers of different materials, or can be a particle-based layer. The epi-blocking layer is not on the planar substrate surface portions. Disclosed embodiments also include light emitting diodes (LEDs) on disclosed patterned articles, and methods to form the same, including chemical mechanical polishing (CMP)-based methods.
Disclosed embodiments in this Disclosure are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the disclosed embodiments. Several aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments. One having ordinary skill in the relevant art, however, will readily recognize that the subject matter disclosed herein can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring structures or operations that are not well-known. This Disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.
Disclosed embodiments include patterned articles, LEDs comprising disclosed patterned articles, and methods for forming LEDs on disclosed patterned articles.
Patterned article 100 includes a patterned top surface 120. The patterned top surface 120 includes the planar substrate surface portions 110a and a plurality of features 113 lateral to and protruding from the planar substrate surface portions 110a. Although the shape of the features 113 is shown as being rectangular having vertical sidewalls, disclosed features may have non-vertical side walls, including curved sidewalls to provide feature shapes such as conical or hemispherical. A typical height range for the plurality of features 113 is from 0.1 μm to 5 μm. The bottom diameter of the features 113 can vary from 0.5 μm to 20 μm, whereas the distance between centers of the features 113 (feature pitch) can vary from 1 μm to 20 μm.
At least a top surface of the plurality of features 113 includes an epi-blocking layer 113a on a bottom portion 113b of the feature. The epi-blocking layer 113a comprises (i) a non-single crystal (amorphous or polycrystalline) material having a refractive index lower as compared to the refractive index of the substrate support (substrate refractive index) and/or (ii) a reflecting metal or a metal alloy (reflecting material). The epi-blocking layer 113a is not on the planar substrate surface portions 110a. The thickness of the epi-blocking layer 113a is generally between 100 Å and 5 μm. The epi-blocking layer 113a may cover between 10% and 100% of the surface area of the bottom portion 113b of the features.
The epi-blocking layer 113a can be a single layer, or as shown in
Patterned article 100 (and patterned articles 130 and 160 described below relative to
The refractive index of the non-single crystal material which may be referred to as a “capping layer” can vary from 1 (essentially that of air, which is a minimum value) to less than that of the material of the substrate support 110 (e.g., about 1.7 for sapphire, about 2.4 to 2.6 for silicon carbide, and about 2.4 for gallium nitride). The refractive index of a material is known to vary with the wavelength of light. The refractive index values provided are generally quoted herein in the visible range. Light extraction performance generally improves the lower the refractive index is for the capping layer.
Although not shown in
Disclosed embodiments include methods of forming patterned articles. The methods generally include providing a substrate support having planar substrate surface portions 110a comprising a substrate material having a substrate refractive index. As noted above, the substrate material may comprise crystalline materials, such as sapphire, silicon carbide, gallium nitride and silicon. An epi-blocking layer is deposited on the substrate support including (i) a non-single crystal material having a refractive index lower as compared to said the substrate refractive index or (ii) a reflecting metal or a metal alloy (reflecting material), and where the epi-blocking layer is not on the planar substrate surface portions. The epi blocking layer is patterned to form a patterned top surface on the substrate support including a plurality of features lateral to the planar substrate surface portions protruding above the planar substrate surface portions. At least a top surface of the plurality of features include the non-single crystal material or reflecting material, and the dielectric material and reflecting material (if present) are not on the planar substrate surface portions.
The substrate support can have a planar top surface throughout (see
In another embodiment the method can further comprise before the depositing and patterning, forming a masking layer on the substrate material, wherein the masking layer exposes a part (5-80%) of a top surface of the substrate material. At least one of a wet etch and a dry etch process can then be performed to remove an exposed part of the top surface of the substrate material to form a patterned substrate surface including planar substrate surface portions and features lateral to the planar substrate surface portions. A sacrificial layer can then be deposited on the patterned substrate surface.
Chemical mechanical polishing (CMP) can then be used for selectively removing the sacrificial layer from raised portions of the plurality of features while preserving the sacrificial layer over the planar substrate surface portions. CMP can alone thus create a recessed sacrificial layer. In this embodiment the polishing condition for CMP can be selected such that CMP polishes only the sacrificial layer and not the substrate. The recessed sacrificial layer can also be created by wet/dry etching depending on the sacrificial layer. Dry etching technique, such as reactive ion etching (RIE), can partially or fully etch the sacrificial layer from the top of the patterned area. Etching after CMP is not necessary, but can be included. The sacrificial layer can comprise either an organic or an inorganic film, such as a photoresist, a polymer, or various oxides, such as silica.
Disclosed patterned articles provide high efficiency light sources due to enhanced light extraction and improved epi-growth. As disclosed above, patterned articles can be fabricated by inserting an epi-blocking layer comprising a low refractive index and/or metallic reflective capping film layer on substrates including traditional patterned sapphire substrates (PSS), which is expected to result in up to about a 20% enhancement in light extraction efficiency. In the case of disclosed curvilinear layers, such curvilinear layers are expected to significantly enhance the random reflectivity of the surface, thereby increasing the light extraction efficiency for light emitting devices.
Disclosed patterned articles are significantly different structurally as compared to conventional PSS structures which only include high refractive index sapphire based features. It should be noted that even though >30% of worldwide high brightness LED production is generally based on use of standard PSS substrates, the use of capping low refractive index layers as described herein is not believed to have been disclosed before this disclosure. Significant advantages of disclosed patterned articles include (i) high efficiency and (ii) improved epi-growth (i.e., lower defect density) as compared to conventional dry/wet-etched PSS.
Significant advantages of the disclosed patterned articles are provided by introduction of a low refractive index non-single crystal layer (e.g., SiO2) on a curvilinear surface substrate surface (with or without a reflecting material layer thereunder) can lead to reflection of a higher percentage of generated photons towards the emitting top surface of the light source instead of travelling in the substrate. Since the low refractive index non-single crystal layer/film will generally cover >70% of the substrate surface, and >90% in some embodiments, most of the photons generated will be reflected randomly at this interface. Thus, higher random reflection from a non-absorbing curved surface will lead to a significant increase in extraction efficiency.
Disclosed embodiments also provide for improved GaN epi-growth. GaN does not nucleate on disclosed epi-blocking layers (e.g., an amorphous SiO2 film), which can cap the patterned area of the substrate. As noted above, disclosed patterned articles can have <10% of pristine substrate (e.g., sapphire) surface for GaN nucleation and growth using the epitaxial lateral overgrowth technique. Since >70%, such as >90% of the area of the substrate will have low defect density laterally grown GaN, disclosed methods lead to a significant reduction in defect density. In conventional wet etched PSS with trapezoidal shape, there are two (0001) oriented crystal substrate surfaces for GaN growth. Disclosed methods can cover one of these surfaces, such as with silica thus enabling GaN growth from only one pristine area of the substrate.
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
Thus, the breadth and scope of the subject matter provided in this Disclosure should not be limited by any of the above explicitly described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
