WO2014207662A1 - Rugosification de surfaçage multi-échelle de dispositifs optoélectroniques pour améliorer l'extraction de lumière - Google Patents

Rugosification de surfaçage multi-échelle de dispositifs optoélectroniques pour améliorer l'extraction de lumière Download PDF

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Publication number
WO2014207662A1
WO2014207662A1 PCT/IB2014/062573 IB2014062573W WO2014207662A1 WO 2014207662 A1 WO2014207662 A1 WO 2014207662A1 IB 2014062573 W IB2014062573 W IB 2014062573W WO 2014207662 A1 WO2014207662 A1 WO 2014207662A1
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Prior art keywords
light
ordered
features
roughening
micro
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PCT/IB2014/062573
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English (en)
Inventor
Songnan Wu
Stéphane TURCOTTE
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Koninklijke Philips NV
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Koninklijke Philips NV
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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/81—Bodies
    • H10H20/819—Bodies characterised by their shape, e.g. curved or truncated substrates
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/84—Coatings, e.g. passivation layers or antireflective coatings
    • H10H20/841—Reflective coatings, e.g. dielectric Bragg reflectors

Definitions

  • This invention relates to the field of optoelectronic devices, and in particular to surface roughening to enhance the light extraction efficiency of a surface of a light emitting device.
  • the invention can also be used to decrease light reflection from a surface of a detector or a solar cell device.
  • a conventional semiconductor light emitting element consists of a double
  • heterostructure that includes a light emitting ('active') region that is sandwiched between an N-type cladding layer and a P-type cladding layer.
  • charge-carriers electrosprays
  • these charge-carriers may recombine and may release a photon (generate light).
  • the generated light may travel in any direction.
  • Commercially available light emitting elements typically include reflective surfaces that redirect light towards an intended light extraction surface to increase the amount of light that strikes the light extraction surface. However, light may strike the extraction surface at virtually any angle. Light will be totally reflected if it is incident on the extraction surface at an angle that exceeds the critical angle of the extraction surface, and some portion of the light will be internally reflected even if the incident angle is below the critical angle (Fresnel reflections).
  • the critical angle of an interface between two media is determined by the indices of refraction nl and n2 of the two media, and is equal to:
  • Equation 1 arcsin (n2/nl), (Equation 1) for light traveling from the medium having an index of refraction nl into a medium having a lesser index of refraction of n2. Light that strikes the surface at an angle greater than the critical angle will be totally internally reflected, and will not escape through the surface.
  • the term "escape cone” is used to define the range of angles within which light will escape through the surface.
  • a common technique to reduce and minimize internal reflection at the light extraction surface of a device is to adopt a "patterned" or “rough” light extraction surface rather than a flat surface, allowing a greater proportion of the light to escape from the inside of the light emitting device.
  • a "patterned" or “rough” light extraction surface rather than a flat surface, allowing a greater proportion of the light to escape from the inside of the light emitting device.
  • another way to enhance light extraction is to roughen the surface of the reflector that is used to reflect light toward the light extraction surface.
  • Ordered roughening' a roughening process that creates a pattern of features that is repeated on a surface
  • a 'non-ordered roughening' a process that creates features that do not conform to a specifically defined pattern
  • An ordered roughening generally provides surface features having a particular size or shape situated on the surface in a desired spatial arrangement.
  • a common ordered roughening includes, for example, creating conic structures that are uniformly distributed on the surface. These conic structures may have a particular size, shape, and/or distribution based on the expected emission characteristics of the light emitting element. These structures may also have a particular size, shape, or distribution based on the refractive characteristics of the material on either side of the interface between these materials. Differently sized or shaped structures/features may be used to provide a combination of desired extraction effects.
  • a non-ordered roughening may provide a random distribution of features on the surface, a random distribution of shapes or dimensions of the features, or combinations of such distributions.
  • a micro-bead blasting of a surface may produce a roughened surface having particular characteristics, such as a particular texture that is characteristic of micro-bead blasting, but the fact that each micro-bead is not controlled to strike the surface at a specific location results in a "non-ordered" roughening as the term is used herein.
  • a roughening that produces features of varying sizes or shapes is a non-ordered roughening, even if the features are spatially aligned, if the size or shape of the feature at any particular location is not controlled.
  • a feature, pattern or texture may be considered non-ordered if its shape, either over a specific location or over its entire surface, cannot be predicted in advance or is the result of randomness.
  • Hao-chung Kuo discloses a combination of ordered and non-ordered roughening techniques wherein the light emitting surface is first patterned to provide uniformly spaced conic features having a particular predominant slope, then subjected to a non-ordered roughening process at a smaller scale.
  • a combination of two or more roughening processes, at substantially different scales, is used to roughen an extraction surface of a light emitting device.
  • a non-ordered roughening is applied at a relatively large scale, and another roughening, either ordered or non-ordered, is applied at a relatively small scale.
  • the large scale roughening improves light extraction efficiency by providing an increased number of escape cones, at varying angles.
  • the small scale roughening improves light extraction efficiency by scattering the light that strikes the smaller features.
  • the large scale non-ordered roughening improves the light output distribution and may reduce the occurrence of multiple total internal reflections, while preserving a Lambertian-like radiation pattern.
  • the parameters of the process may be controlled to provide a particular effect, texture and/or morphology, such as a particular coarseness, depth, density, and the like of the features produced by the process.
  • Extraction surfaces of light emitting devices may be roughened using any of a variety of conventional techniques, including etching, micro-blasting, grinding, nano-imprinting and others. Depending upon the technique used and the parameters associated with the process, the degree of roughening may be controlled, as well as the coarseness of the roughening.
  • FIGs. 1 A-IB illustrate an example prior art roughening technique that combines ordered and non-ordered roughening.
  • FIG. 2 illustrates an example combination of a non-ordered roughening at a large scale and a non-ordered roughening at a small scale.
  • FIG. 3 illustrates an alternative example combination of a non-ordered roughening at a large scale and a non-ordered roughening at a small scale.
  • FIGs. 1 A-1B illustrate an example prior art roughening technique that combines ordered and non-ordered roughening, such as taught by Kuo, referenced above.
  • FIG. 1 A illustrates a structure 100 that includes a light emitting element 110.
  • the lower surface 102 of the structure 100 includes a reflective element that serves to reflect light from the light emitting element upwards, toward the upper surface 104, which is the light extraction surface of the structure 100.
  • the index of refraction of the structure 100 is higher than the index of refraction of the media exterior to the structure 100, and the difference between the indices of refraction determines the critical angle at the surface 104.
  • the escape cone 101 of FIG. 1A is a cone with an apex on the surface 104, normal to the surface 104, and having a solid angle determined by the TIR angle.
  • the surface 104 includes an ordered roughening that produces conic structures 120 that are spaced apart by substantially flat regions 130.
  • the light emitting element 110 provides light, illustrated by example light rays 1 11, 112, 113, 114.
  • Light 111 that strikes the substantially flat region 130 of surface 104 within the escape cone 101 exits the structure 100, and light 112 that strikes the surface 130 outside the escape cone 101 of the surface is reflected back into the structure 100.
  • the conic structures 120 serve to change the orientation of the escape cone relative to the light emitting element, as illustrated by the escape cone 10 ⁇ at the location struck by light 113 on the structure 120.
  • This light 113 is emitted at the same angle as the light 112 that is internally reflected from the surface 130, yet because the escape cone 10 ⁇ is re-oriented, the light 113 is able to escape the structure 100.
  • These structures 120 also increase the surface area of surface 104, compared to a flat surface, and thereby increase the number of possible escape cones.
  • some light may strike the surface 104 in such a manner that it is repeatedly internally reflected until it is eventually absorbed within the structure 100. That is, for example, the symmetry of the surface 104 may cause a reflected light to eventually re- strike the surface in equivalent, or near-equivalent conditions as those that led to the first total internal reflection event. In this situation, the light rays may experience multiple cycles of total internal reflection until the light eventually strikes within an escape cone; and with each cycle, the intensity of the light is diminished by absorption.
  • FIG. IB illustrates the structure 100 after being roughened by a second process.
  • the roughening is at a substantially smaller scale than the ordered roughening of FIG.1, and produces small features 140 randomly distributed over the surface 104.
  • the structure 100 of FIG. IB exhibits the predominant conic structures 120 separated by flat surfaces 130, but without the smooth profile of FIG. 1A.
  • the large features 120, 130 are termed micro-features, and the small features 140 are termed nano-features.
  • the micro-features 120, 130 have dimensions that are substantially greater than the wavelength of the light emitted by the light emitting element 110, whereas the nano-features 140 have dimensions comparable to the wavelength of the emitted light, or smaller.
  • the term size or dimension refers to a given characteristic of a feature and may include the depth, the height, the size, the density, the coarseness, and any geometric parameter that defines such feature, including but not limited to its orientation and angular nature.
  • the dimensions of the nano-features 140 are comparable to the wavelength of the emitted light, or smaller, when light 117, 118 strikes a nano-feature 140, it is scattered through several beams of lower intensities, with an angular distribution depending on that particular scattering event. As illustrated, some of the light in this pattern 145 will escape the structure 100, and some will be reflected back into the structure 100. However, this scattering effect may be substantially less dependent of the angle of incidence of the light with respect to the nominal normal to the surface at the point of incidence, and therefore light that would otherwise be internally reflected is scattered rather than reflected, and at least a portion of the scattered light 145 will escape the structure 100. Additionally, some of the light 119 that is scattered back into the structure 100 may subsequently escape the structure 100 when it again strikes the surface 104, due to the angular re-distribution of the scattered rays.
  • the likelihood of light entering a repeating cycle of total internal reflections is reduced, particularly in the case of re-striking near the original point of incidence and/or at or near the same angle of incidence, because the light may strike a nano- feature 140 during one of the multiple cycles of repeated internal reflections.
  • the scattering is directional, distributed forward and backward along the axis of the incident light ray, some of the scattered light may continue to restrike near the original point of incidence and/or at or near the same angle of incidence.
  • light that would otherwise escape by striking the surface within the escape cone may also be scattered if it strikes a nano-feature 140, and a portion of the scattered light 145 will be reflected back into the structure 100.
  • the net gain in light output (gain from otherwise internally reflected light - loss from otherwise escaping light) will be dependent upon a variety of factors including the amount of light internally reflected by the micro -roughened surface before nano-roughening, the likelihood of multiple internal reflections, the relative density of the nano-features, and so on.
  • an ordered micro-roughening as illustrated in FIGs. 1 A and IB may exhibit undesirable light output characteristics.
  • An ordered roughening for example, will generally provide a non-lambertian radation pattern that may present symmetries or ordering patterns related to the ordered nature of the extraction surface.
  • more light may exit the top of each conic structure 120 than the sides, as reflected light is directed up the cone until it escapes through the top, as illustrated by the path of light 114 in FIG. 1 A.
  • each conic structure 120 may appear as a bright spot surrounded by a darker halo.
  • this ordered light output pattern may not be detected by the naked eye, in some applications the presence of such a pattern may produce unwanted optical effects, such as Moire or other interference patterns when combined with other ordered optical elements, such as an array of pixels, and other undesirable optical artifacts.
  • the direction of the light that exits the sides of the conic structure 120 is generally at a severe angle relative to the normal to the surface of the light emitting element 1 10, which may be disadvantageous in applications that require a Lambertian light output distribution in a direction normal to the light emitting element 110.
  • FIG. 2 illustrates a dual-scale roughening technique that avoids the aforementioned undesirable optical effects that may be caused by the ordered micro-roughening process of FIGs. lA and IB.
  • FIG. 2 illustrates a structure 200 that includes a surface 204 that has undergone a non- ordered micro-roughening and a non-ordered nano-roughening.
  • the overall (micro) profile of the surface 204 includes a variety of hills and valleys of random sizes and shapes.
  • the surface 204 also includes nano-features 140 with dimensions that are comparable to the wavelength of the emitted light, or smaller.
  • Light 211, 212, 213 that strikes the surface 204 without striking a nano-feature 140 will be refracted or reflected, based on its angle of incidence relative to the escape cone 101.
  • the size and orientation of the escape cone is determined by the critical angle for total internal reflection, based on the indices of refraction of the media on either side of the surface 204, and the slope of the surface 204 at the point of incidence. Because of the micro-roughening, the total surface area 204 is greater than the surface area of a flat surface, allowing for more potential escape zones, with varying orientations. However, as contrast to the ordered micro-roughening of FIGs. 1 A-1B, the non-ordered micro-roughening of FIG. 2 will produce a more uniform radiation pattern.
  • the ordered nature of the light output patterns of the structure 100 in FIGs. 1A, IB will generally be indistinguishable to the naked eye, but may present interference patterns or other optical artifacts in particular applications, such as when combined with optical elements that also have an ordered arrangement.
  • the likelihood of another optical element having a pattern that can produce such interference with a non-ordered light output pattern is virtually nil.
  • the varying shapes of the micro-features in the structure 200 will serve to direct the refracted or reflected light in varying directions, and consequently will likely provide a light output distribution that is closer to a Lambertian distribution than a structure with an ordered set of micro-features, such as the structure 100 of FIGs. 1 A-1B.
  • the asymmetric nature of the micro-features of structure 200 in FIG. 2 will substantially reduce the likelihood of light entering a cycle of repeated strikes at or near the same point of incidence at or near the same angle of incidence.
  • creating a non-ordered micro- roughening may be substantially simpler than the process for creating an ordered micro- roughening.
  • an ordered micro-roughening for example, the size, shape, and location of each micro-feature must be controlled, often requiring a multi-step process.
  • a non-ordered micro-roughening for example, the size, shape, and location of each micro-feature will require substantially less control, and the non-ordered micro-roughened surface may be produced in a single step.
  • the micro-features on the surface 204 may be achieved by first defining patterns on the device surface using photoresist or dielectric layer followed by selective wet chemical etching or dry etching.
  • the features produced by a chemical etching will generally have a shape and texture that is determined by the particular makeup of the etchant and the material being etched, although a conic shape may be typical.
  • the size, density, and depth of the features will generally be dependent upon the relative chemical structures of the etchant and material being etched, the distribution of the etchant, and the duration of the etching.
  • a sparsely distributed strong etchant may produce widely spaced peaks with steep slopes, whereas a weaker etchant may produce shallower valleys with slighter slopes.
  • Dry etching may also provide characteristic features with controllable shapes, size, depths, and densities. Conventionally, a particular process is applied with parameters controlled to provide features with a desired nominal size, shape, depth, and density.
  • microbead blasting may also provide features of semi- spherical in shape, with a texture that is dependent upon the material used for the blasting relative to material being roughened.
  • the size of the features will be dependent upon the size of the microbeads, and depth and density of the features will generally be dependent upon the force with which the microbeads are projected onto the surface and the duration of the blasting, respectively.
  • the nano-features may be achieved in a similar way as the micro-features but with different process parameters.
  • the nano-features may also be achieved by adopting only wet or dry etching or both without a photoresist or dielectric pattern transfer. Such a technique may be dependent upon the specific device surface composition and may require accurate process control to control the density and size of the nano-features.
  • Nano-imprint techniques may also be used to generate the nano-features.
  • Nano-features may also be created by applying a specific material upon the interface surface that is porous, amorphous or rough in nature, or may become so after certain treatment, such as UV exposure or annealing.
  • micro- features and nano-features may be produced by combining a PEC (photo electrochemical) wet etch and an ICP (inductively coupled plasma) dry etch of surface 104.
  • the exposed epi surface may be roughened by PEC wet etch to create microfeatures on the scale of lum and above.
  • the resulting epi surface may be roughened again by ICP dry etch that creates nano- features on the order of lOOnm on the micro-roughened surface 104.
  • ICP dry etch inductively coupled plasma
  • the nano-features 140 on the surface 204 of structure 200 serve the same purpose as the nano-features 140 on the surface 104 of structure 100. Because the dimensions of the nano-features 140 are comparable to the wavelength of light emitted by the light emitting element 110, or smaller, when light 216, 217 strikes a nano-feature, it will be scattered 145. This scattering 145 will occur regardless of size or orientation of the escape cone at the point of incidence. Accordingly, light that would otherwise be totally internally reflected will be scattered, allowing at least a portion of this light to escape through the surface 104. Similarly, light within an escape cone, which would otherwise exit the surface 104, is also scattered, and a portion of this light will be reflected back into the structure 200.
  • the net gain in light output (gain from otherwise internally reflected light - loss from otherwise escaping light) will be dependent upon a variety of factors including the amount of light internally reflected by the micro-roughened surface before nano-roughening, the likelihood of multiple internal reflections, the relative density of the nano-features, and so on.
  • the likelihood of cycles of multiple internal reflections in the structure 200 of FIG. 2 can be expected to be substantially less than the likelihood of cycles of multiple internal reflections in the ordered (symmetric) features of the structure 100 of FIG. IB.
  • FIG. 3 illustrates an alternative arrangement of a structure 300 with a roughened surface 304 on a reflective interface 320 between medium 306 and medium 308. In this example structure, light is extracted through the upper surface 301 of the structure 300.
  • Surface 301 is illustrated as being a flat surface for ease of illustration and explanation, but one of skill in the art will recognize that this surface 301 may also be roughened to enhance the light extraction efficiency using any of a variety of roughening techniques.
  • the surface 304 includes an ordered micro-roughening and a non-ordered nano- roughening that produces nano-features 340.
  • Light 311, 312, 313 that strikes the surface 304 without striking a nano-feature 340 will be reflected from the reflective interface 320.
  • This reflective interface 320 may be a reflective element that is situated between the media 306, 308, or the media 306 may be a reflective material.
  • the media 306, 308 may be selected to have indices of refraction that provide for a very small critical angle, so that a substantial majority of the emitted light will be totally internally reflected at the interface 320.
  • the lower surface 302 of the structure 300 may also include a reflective material so that light that may enter the media 306 will be reflected back to the interface 320.
  • Pattern 345 illustrates only the back-scattered light (assuming very little forward-scattered light in the case of a metal reflector), for ease of illustration, one of skill in the art will recognize that the intensities of both forward and backward-scattered light depend on the nature of the reflector used, the slope of the interface 320 at the point of incidence, as well as other factors.
  • the reflected light from the interface 320 will strike the surface 301 and will either exit the structure 300, or be internally reflected back into the structure 300.
  • the surface 301 may also be roughened to increase the likelihood of the light exiting the surface 301.
  • the asymmetry of non-ordered micro-features on the surface 304 serves to reflect the light toward the light extraction surface 301 at varying angles, thereby avoiding the creation of an ordered, periodic or symmetric light output pattern, and minimizing the likelihood of light entering a repeating cycle of multiple internal reflections.
  • the reflections from the asymmetric non-ordered micro-features, as well as the scatterings from the nano-features, also serve to produce a more Lambertian-like light output distribution than would be produced by ordered micro-features. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.
  • the active region is a light absorbing element instead of a light emitting element, such as may be used in light detection devices or solar cell devices. That is, this technique may be applied to light emitting surfaces, light receiving surfaces, light reflector surfaces, or other interfaces where better light transmission through the interface can lead to better device performance, based on the metrics of colorimetry, photometry and LED metrology.

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Abstract

L'invention concerne une combinaison de deux procédés de rugosification ou plus, à des échelles sensiblement différentes, utilisés pour rugosifier une surface d'interface d'un dispositif émetteur de lumière. Une rugosification non ordonnée est appliquée à une échelle relativement grande, et une autre rugosification, soit ordonnée soit non ordonnée, est appliquée à une échelle relativement petite. La rugosification à grande échelle améliore le rendement d'extraction de lumière en fournissant un nombre accru de cônes d'échappement, à des angles variables. La rugosification à petite échelle améliore le rendement d'extraction de lumière en dispersant la lumière qui heurte les plus petits éléments. La rugosification non ordonnée à grande échelle améliore la répartition de sortie de lumière et peut réduire l'occurrence de multiples réflexions internes locales, tout en préservant un motif de rayonnement de type Lambertien.
PCT/IB2014/062573 2013-06-27 2014-06-25 Rugosification de surfaçage multi-échelle de dispositifs optoélectroniques pour améliorer l'extraction de lumière Ceased WO2014207662A1 (fr)

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US201361840032P 2013-06-27 2013-06-27
US61/840,032 2013-06-27

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Citations (4)

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Publication number Priority date Publication date Assignee Title
US3739217A (en) 1969-06-23 1973-06-12 Bell Telephone Labor Inc Surface roughening of electroluminescent diodes
US20090179209A1 (en) * 2008-01-11 2009-07-16 Industrial Technology Research Institute Light emitting device
US20120164770A1 (en) * 2006-04-13 2012-06-28 Berthold Hahn Radiation-Emitting Body and Method for Producing a Radiation-Emitting Body
WO2012174311A2 (fr) * 2011-06-15 2012-12-20 Sensor Electronic Technology, Inc. Dispositif émetteur avec extraction améliorée

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3739217A (en) 1969-06-23 1973-06-12 Bell Telephone Labor Inc Surface roughening of electroluminescent diodes
US20120164770A1 (en) * 2006-04-13 2012-06-28 Berthold Hahn Radiation-Emitting Body and Method for Producing a Radiation-Emitting Body
US20090179209A1 (en) * 2008-01-11 2009-07-16 Industrial Technology Research Institute Light emitting device
WO2012174311A2 (fr) * 2011-06-15 2012-12-20 Sensor Electronic Technology, Inc. Dispositif émetteur avec extraction améliorée

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
HAO-CHUNG KUO: "Recent Progress of GaN Based High Power LED", L4TH OPTOELECTRONICS AND COMMUNICATION CONFERENCE, 2009

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