WO2017175201A2 - Low etendue high brightness light emitting devices - Google Patents

Low etendue high brightness light emitting devices Download PDF

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WO2017175201A2
WO2017175201A2 PCT/IB2017/052036 IB2017052036W WO2017175201A2 WO 2017175201 A2 WO2017175201 A2 WO 2017175201A2 IB 2017052036 W IB2017052036 W IB 2017052036W WO 2017175201 A2 WO2017175201 A2 WO 2017175201A2
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waveguide
layer
region
extraction
optical
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WO2017175201A3 (en
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Eric Feltin
Pascal Gallo
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Novagan
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/36—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes
    • H01L33/38—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes with a particular shape
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/44—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the coatings, e.g. passivation layer or anti-reflective coating
    • H—ELECTRICITY
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    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
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    • H01L33/58—Optical field-shaping elements
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    • H01L2933/00—Details relating to devices covered by the group H01L33/00 but not provided for in its subgroups
    • H01L2933/0083—Periodic patterns for optical field-shaping in or on the semiconductor body or semiconductor body package, e.g. photonic bandgap structures
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/10—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/10—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector
    • H01L33/105—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector with a resonant cavity structure
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    • H01L33/02—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H01L33/22—Roughened surfaces, e.g. at the interface between epitaxial layers
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    • H01L33/40—Materials therefor
    • H01L33/405—Reflective materials
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    • H01L33/36—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes
    • H01L33/40—Materials therefor
    • H01L33/42—Transparent materials
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    • H01L33/44—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the coatings, e.g. passivation layer or anti-reflective coating
    • H01L33/46—Reflective coating, e.g. dielectric Bragg reflector
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    • H01L33/48—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor body packages
    • H01L33/50—Wavelength conversion elements
    • H01L33/507—Wavelength conversion elements the elements being in intimate contact with parts other than the semiconductor body or integrated with parts other than the semiconductor body

Definitions

  • TITLE Low etendue high brightness light emitting devices
  • the invention allows increasing light emitting devices luminance while improving their reliability.
  • small surface emission, high brightness light emitting devices can be fabricated. It consists in separating the region where light is generated by electrically induced spontaneous emission, from the region where light is extracted. Between these two regions is a waveguide structure. Specific features can be used to control the angular dispersion of the extracted emission and the extraction surface, for example to minimize apparent source. It is therefore possible to achieve high luminance.
  • the invention can be applied in lighting, projection, displays, polymer curing, microelectronic fabrication, telecommunications or biophotonics for example.
  • the invention relates to LED devices, arrays of LED devices and methods of manufacturing thereof. Specifically, the invention relates to, but is not limited to, LED devices having a plurality of optical components implementable with standard waferscale fabrication techniques. More specifically, the invention relates, but is not limited to, LED devices providing a high luminance (brightness).
  • the invention can be applied to fields such as, but not limited to, lighting, projection, displays, polymer curing, microelectronic fabrication, telecommunications or biophotonics.
  • the invention discloses a new solution for the realization of light emitting devices offering a high luminance.
  • the new device features an optical cavity used to guide the spontaneous emission generated by the emitting region inside the device towards a specific area where it is extracted in air. By controlling the angular dispersion of the extracted emission and the extraction surface (apparent source) it is possible to achieve high luminance.
  • luminance of the optical source is the limiting factor.
  • low luminance of displays prevents their use in sunlight environments.
  • High luminance corresponds to a high optical power density in a small etendue.
  • TV screens have luminance around 100 cd/m 2
  • a LED will have a luminance of 10 Mcd/m 2 , 10 times lower than the sun and at least 100 ⁇ 00 less than a laser diode.
  • Laser diodes are offering very high brightness as the power density in the laser output beam can reach kW/cm2 in a highly focalized beam.
  • laser emission is coherent by definition and it is an issue in many applications.
  • the laser beam projected onto a surface will be responsible of the appearance of speckles in the image which strongly degrade the image quality (sharpness, ).
  • LEDs are emitting an incoherent light.
  • the brightness of LEDs is limited by the maximum current density supported by the device (typically a few kA/cm 2 ) and the large etendue due to the isotropic spontaneous emission.
  • high power can be generated in an LED it is usually possible only for large etendue.
  • An efficient light collimation is possible with external optical elements, but luminance will still be limited by the initial output power of the LED and its initial etendue.
  • One example is point source LED in which the emission of an LED is shaped using a geometrical filter to select only a small part of the light produced by the LED. With this design, it is possible to fabricate a source with a small etendue but the output power in this etendue is reduced.
  • Luminance of LDs is much higher compared to that of an LED because of two elements: a gain medium coupled with an optical cavity.
  • the optical cavity is used to confine light. When lasing threshold is reached light emission is enhanced only in specific waveguide modes allowed by the optical cavity. It is the combination of an optical cavity and the fact that all light is generated in a limited number of modes which eventually leads to a high luminance.
  • Edge emitting ridge lasers are based on horizontal waveguides. When lasing threshold is reached light emission is enhanced only in specific waveguide longitudinal modes allowed by the optical cavity thanks to stimulated emission. Despite the fact that light is barely confined in the waveguide stimulated emission allows for a high photon density in one or several guided mode only. For edge emitting lasers the output emission is extracted over a small surface of one side of the ridge although light is initially generated over the whole ridge. In this case it is possible to have a large surface for current injection while the emission surface remains small but it is only possible because of stimulated emission which leads to speckles and still require a high current density to reach the lasing threshold.
  • the light emitting device described by the invention features an optical cavity and has a large light generation area and a small extraction area.
  • the invention discloses several designs of light emitting devices offering a high luminance and/or light mixing possibilities.
  • the device surfaces have to be as reflective as possible to form an optical cavity with a high quality factor.
  • a few solutions to form reflective surfaces are highly reflective metals (Ag, Al, Au,...), distributed Bragg mirrors, sub-wavelength grating mirrors, photonic crystals but other solutions can be used.
  • an aperture is formed in the optical cavity to allow light to escape. The aperture can be done by removing the highly reflective layer locally and/or to form a highly transmissive interface.
  • a non-exhaustive list of solutions includes surface random roughening, anti-reflection coatings, anti-reflection roughening (moth eye, %), diffractive optical elements, sub-wavelength gratings, photonic crystals.
  • the efficiency of the device can be maximized by redirecting the emitted light inside the device towards the extraction zone.
  • a non-exhaustive list of solutions to achieve this goal comprises: vertical microcavities, microdisks, ridge waveguides, tapered waveguides, photonic crystal waveguides, and membranes.
  • LEDs are usually designed to extract as much as possible of this guided emission and with a minimum of reflection inside the device medium to avoid re-absorption. It is also important to minimize the size of the device in order to decrease the fabrication cost. Therefore all LED are designed to extract the generated light in the same region where it is produced. For this reason light emitting diodes have usually a limited brightness because light extraction is done on the larger surface possible. Control of the light beam characteristics in those devices is usually done with an external lens but the luminance is still etendue limited.
  • the objective of the invention is to benefit from this guided light to direct it in a specific area where it can be extracted with the desired characteristics.
  • the extraction area can even be far away from the initial source.
  • Our invention takes advantage of the physical separation of the light generation area (active region) and the extraction area. By doing so the luminance of emitters such as LEDs is no more limited by the efficiency of the device and luminance can be dramatically increased.
  • the concept relies on the concentration of light emitted from extended surface towards a smaller area:
  • the extraction zone can be very small so the brightness (or luminance) will be very high.
  • the emission area active region
  • the luminance in the extraction area can be further increased and is only limited by optical losses of the waveguide.
  • the size of the opening should be small compared to the emitting area to increase the concentration of light in the opening of the device.
  • optical losses inside the device forming the cavity should be reduced as much as possible to maximize the chance for emitted photons to reach the opening and be extracted from the device. Localize the opening in a region of high optical power density in order to increase the probability for light to escape from the
  • n-doped substrate can be: a substrate such as GaN, ZnO, SiC, silicon, sapphire, GaN on sapphire, GaN on SiC, GaN on silicon, GaP, GaAs, AIN, or any material with a thickness sufficient to serve as support for a device
  • An n-doped layer can be: n-type lll-V alloys such as n-type Al x ln y Gai_ x _ y N, n-type Al x ln y Gai_ x _ y As, n-type Al x ln y Gai_ x _ y P, n-type ll-VI alloys, n-type ZnO...
  • a p-doped layer can be: p-type lll-V alloys such as p-type Al x ln y Gai_ x _ y N, p-type Al x ln y Gai_ x _ y As, p-type Al x ln y Gai_ x _ y P, p-type ll-VI alloys, p-type ZnO,...
  • An emitting region can be made of single layer of Al x ln y Gai_ x _ y N, Al x ln y Gai_ x _ y As, Al x ln y Gai_ x _ y P, ll-VI alloys, ZnO, ...
  • An emitting region can be made of multiple quantum wells or quantum dots planes made of InGaN/GaN, InGaN/AIGaN, GaN/AIGaN, Al z ln w Gai -z-w N/Al x ln y Gai -x-y N, Al z ln w Gai -z-w As/Al x ln y Gai -x-y As, Al z ln w Gai_ z _ w P/Al x ln y Gai_ x _ y P, ll-VI alloys, ZnO, ...
  • a transparent conductive layer can be made of one or several transparent conductive oxide such as ITO, 10, ZnO, AZO, GZO, ...
  • a highly reflective metallic contact can be made of one or several layers of metal such as: Ag, Al, Au, Ni, Ti, Ni, Pd, Pt, Cr, ...
  • a waveguide can be made of one or several layers of low loss materials such as: dielectric materials (Si02, SiN, Zr02, Ti02, Ta205, ...) or semiconductors (GaN, AIN, ZnO, ...) or insulators (AI203, .7)
  • a rough surface is a non flat surface that can be: a surface with a random roughness, a diffusing surface, a textured surface with periodic or non-periodic features (micro-lenses, micro- domes, micro-pillars, micro/nano-rods, micro-pyramids, micro-stripes, motheye structures,...), a sub- wavelength diffraction grating structures, a photonic crystals, diffractive optical elements.
  • a highly reflective layer can be made of one or more reflective layers.
  • Highly reflective layers include distributed Bragg reflectors (DBR), sub-wavelength diffractive gratings, photonic crystals, metals
  • the emitting device is based on a planar ridge waveguide 1.
  • the waveguide comprises at least an n-doped layer, an active region and a p-doped layer.
  • Light is generated by the active layer.
  • Emission inside the device is isotropic but a large part is confined inside the waveguide by total internal reflection on the external surfaces of the waveguide.
  • Highly reflective contacts 2 and 3 are deposited on the p-doped layer and the n-doped layer respectively. Both contacts are highly reflective in order to avoid absorption of the guided light propagating inside the waveguide.
  • At one end of the waveguide an aperture is formed in the top metallic contact to extract the emission.
  • the metallic contacts have to be large to maximize the emitting surface and therefore the power generated inside the device.
  • the surfaces of the contacts can be much larger than the surface of the aperture.
  • the increase of the surface of the emitting region results in an increase of the total power produced. Therefore increasing the area of the emitting region allow an increase of the power extracted from the aperture that can be much larger than the power generated by device with the emitting surface equal to that of the aperture.
  • the luminance of the aperture can be dramatically increased by increasing the surface of the emitting region while keeping the extraction region small.
  • the waveguide comprises an n-doped layer 201, a p-doped layer 203 and an emitting layer 202 between them.
  • a transparent conductive layer (204) is deposited on the p-doped layer 203 for current spreading.
  • Layer 204 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer.
  • Highly reflective layers such as DB s can be deposited on the bottom and top side of the device surfaces (elements 205 and 206) to minimize extraction of light in unwanted area of the device as it would be lost for the targeted extraction region.
  • a small aperture is formed in layer 205 and two apertures are formed in layer 206.
  • a metallic contact 207 is contacting the surface of the n-doped layer 201 through an aperture in layer 206.
  • Another contact 208 is deposited on the surface of the p-doped layer 203 through an aperture in layer 205. The extraction is confined in aperture 209.
  • Both metallic contacts are small but it is still preferred to use contacts as reflective as possible to reduce absorption of the emission of layer 202 by the contacts 207 and 208.
  • This design allows a decrease of the optical losses in the waveguide by decreasing the surface of the absorbing metal which results in increased output power in the aperture.
  • a second advantage is the possibility to inject current in the whole emitting region.
  • an optical element can be deposited on the extraction region.
  • an anti-reflection coating (layer 210) is deposited exclusively in the extraction region to increase its extraction efficiency.
  • a lateral confinement of the emission generated inside the waveguide is added to the previous embodiment.
  • Highly reflective layers (211 and 212) are deposited on all free surfaces not covered by the metallic contacts to avoid emission extraction in unwanted area of the device (edge facets,).
  • This design adds photon recycling: photons which are not extracted through the aperture can be reflected at the edge of the device thanks to the reflective interfaces. If internal losses are sufficiently small photons can have many reflections inside the waveguide and can eventually be redirected towards the aperture to be extracted.
  • layers 205, 206, 211 and 212 can be used as layers 205, 206, 211 and 212 such as DB , a combination of an insulating material and a metal, painting, structured surface or any reflective material or element.
  • a diffusing element is added to the previous embodiment in the extraction region.
  • the emitting structure is formed by at least an n- doped layer 301, a p-doped layer 303 and an emitting layer 302 between them.
  • a transparent conductive layer 304 deposited on p-doped layer 303 is used for current spreading.
  • Layer 304 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer.
  • Highly reflective layers such as DBRs can be deposited on the bottom and top side of the device surfaces (elements 305 and 306) to minimize extraction of light in unwanted area of the device as it would be lost for the targeted extraction region.
  • a small aperture is formed in layer 305 and two apertures are formed in layer 306.
  • a metallic contact 307 is contacting the surface of the n-doped layer 301 through an aperture in layer 306.
  • Another contact 308 is deposited on the surface of the p-doped layer 303 through an aperture in layer 305. The extraction is confined in aperture 309.
  • the emission extracted through the aperture was limited due to total internal reflection at the semiconductor/air interface above a certain angle.
  • the extraction is limited to 2-4% depending on the refractive index of the waveguide.
  • Different methods can be implemented to increase the extraction efficiency of the aperture.
  • One solution consists in roughening the interface, randomly or with a defined pattern.
  • a possible implementation in the case of AIGalnN LEDs consists in roughening the n-layer surface by chemical etching. For example 30% KOH solution can be used to form hexagonal pyramids with a random dispersion of height from a flat c-plane GaN surface with N polarity (typically n-GaN layer in AIGaln N LEDs).
  • Such a roughening process could also be done by dry etching using a patterned mask or any other method.
  • the extraction efficiency through the aperture 309 is highly enhanced compared to previous embodiments as laterally guided modes which would have not been extracted through a flat interface (because of total internal reflection) can be extracted with a rough interface.
  • the extraction efficiency of the aperture was enhanced thanks to a roughened surface in the aperture 303. It is possible to obtain a similar effect by including a diffusing or diffracting element inside the waveguide as shown in Figure 4.
  • the emitting structure is formed by at least an n-doped layer 401, a p-doped layer 403 and an emitting layer 402 between them.
  • a transparent conductive layer 404 deposited on p-doped layer 403 is used for current spreading. Layer 404 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer.
  • Roughening of the surface of the n-doped layer is done on a small area to allow light scattering in the vicinity of the extraction region above (409). Roughening can have a random pattern or an organized pattern such as sub-wavelength gratings or photonic crystals, micro lenses for example to increase beam directivity.
  • the insertion of a planarization layer 413 is necessary if a DBR is used as reflector having optimal optical characteristics. Indeed, without a planarization layer the reflectivity of a DBR on it would be reduced. However, one could also replace the couple 413/407 by a single highly reflective layer.
  • a small aperture is formed in layer 406 and two apertures are formed in layer 405.
  • a metallic contact 407 is contacting the surface of the n-doped layer 401 through an aperture in layer 406.
  • Another contact 408 is deposited on the surface of the p-doped layer 404 through an aperture in layer 405. The extraction is confined in aperture 409.
  • a dome lens 410 is used to increase the extraction efficiency and directivity of the extracted beam as it is the scope of the invention to fabricate a light source with a high luminance.
  • Other refractive optical elements can be used instead of a lens for beam shaping or light extraction purpose.
  • the scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high.
  • a focalizing optical element is included in the optical cavity where light is generated to increase the optical power density in the vicinity of the extraction region.
  • the light emitting device is composed by a layer 501 which can be a substrate.
  • An emitting structure is deposited on layer 501. It is composed by an n-doped layer 502, an emitting layer 503 and p-doped layer 504.
  • On the top of layer 504 a concave mirror is formed. This mirror can be fabricated by forming a lens (layer 505) on the top of layer 504 and a highly reflective layer 506, possibly a metal.
  • Layer 505 can be a transparent conductive layer to facilitate current spreading or a transparent conductive layer can be inserted between layers 504 and 505 if necessary.
  • a highly reflective layer 510 is deposited on the bottom of layer 501 to redirect light towards the concave mirror.
  • the extraction region 509 is defined by an opening made through layer 510.
  • the opening 509 is localized at the focal point of the concave mirror to increase the optical power density on the opening.
  • light which is not extracted by the extraction region is reflected inside the device until it reaches the opening (509) or is extracted at the edges of the device.
  • high ly reflective layers (512 and 513) are deposited on the edges of the device to minimize optical losses and increase photon recycling.
  • a lens or any optical element able to improve the extraction efficiency or mod ify the farfield pattern can be used.
  • a lens (511) is placed on the aperture 509 to focalize the extracted emission.
  • the bottom side of the substrate is partly or totally textured with a random roughness or with a pattern. This roughness is used to diffuse photon not extracted directly by the aperture. Photons not escaping through the aperture can be reflected in another direction and have another possibility to be redirected towards the aperture.
  • a transparent planarization layer 607 such as spin-on-glass, is deposited on the back of layer 501.
  • the highly reflective layer 510 is deposited on the bottom of layer 607 to red irect light towards the concave mirror.
  • the extraction region 509 is defined by an opening made through layer 510.
  • the opening (509) is localized at the focal point of the concave mirror to increase the optical power density on the opening.
  • th is implementation light which is not extracted by the extraction region is reflected inside the device until it reaches the opening (509) or is extracted at the edges of the device.
  • high ly reflective layers 512 and 513 are deposited on the edges of the device to minimize optical losses and increase photon recycling.
  • a lens or any optical element able to improve the extraction efficiency or modify the farfield pattern can be used.
  • a lens (511) is placed on the aperture 509 to focalize the extracted emission. The small size of the open ing 509 and the large emitting region defined by layer 503 guarantees that the luminance of the extraction region will be dramatically enhanced .
  • a planar waveguide structure is deposited on an n-doped substrate 701.
  • a sub-wavelength grating mirror is used as a highly reflective layer deposited on layer 701.
  • Layer 702 is made of a SiOx layer with holes periodically spaced with dimensions and periodicity chosen to allow a high reflectivity at the emission wavelength when holes are filled with an n-doped material (layer 703).
  • An opening (712) is formed in layer 702 to let the emitted photons escape down to layer 701.
  • An n-doped layer 703 is deposited on layers 702.
  • An emitting layer 704 is deposited on layer 703 and a p-doped layer 705 is deposited on layer 704.
  • a transparent conductive layer (layer 706) is deposited over the surface of layer 705.
  • a grating structure 707 is formed inside layer 706 to diffract light.
  • a transparent layer 708 deposited on layer 706 for getting back a flat surface and a highly reflective layer 710 is deposited on the top of layer 708 to reflect all light in the direction of the extraction region 712.
  • An opening is formed in layers 708 and 710 for form a highly reflective metallic contact 709 in the opening.
  • Another metallic contact 711 is deposited on the n-doped substrate 701 but it could also be deposited on layer 703.
  • a h ighly reflective layer 713 is formed on the waveguide facet on the opposite side of the extraction region to reflect light not propagating to the extraction region.
  • This layer 713 can be a DB or a metal for example but it is not limited to those elements as any reflecting material can be used for this purpose.
  • the required emitting/extraction region surface ratio depends on the coupling between the emitting region and the extraction region 712. If the coupling is high, meaning that a large part of the light generated is guided and extracted through extraction region 712 then there is no need to use a large emitting region. On the other hand if there is most of the light produced inside the device is not extracted through extraction region 712, then it is necessary to compensate with a large emitting region. Thus, a layer 706/opening 712 surface ratio > 10 can be used.
  • the waveguide is a taper.
  • the waveguide formed by layers 703, 704, 705, 706, and 708, has a trapezoidal shape.
  • the emission is done on the whole surface to maximize the total power and the extraction region 712 is localized on the larger part of the taper.
  • Such a waveguide design is used to redirect light emitted inside the waveguide in the direction of the grating structure thanks to multiple reflections at the interfaces on the lateral edges of the taper.
  • With a long waveguide the dispersion angle of the emission in the plane will decreases when light propagate towards the extraction region as the photons align progressively. If optical losses inside the waveguide are low it is preferable to use a long waveguide.
  • a collimated beam inside the waveguide is beneficial to couple the emitted light with air with a particular beam shape but other waveguide shape can be used.
  • the planar waveguide (formed by layers 703, 704, 705, 706, and 708) has a parabolic shape with a parabolic mirror (813) on the opposite side of the extraction region.
  • This parabolic mirror is used to redirect and collimate more efficiently light towards the extraction region (707).
  • the length of the device can be very large it is possible to obtain a highly collimated light into the waveguide, without increasing the width of the waveguide. Therefore, the surface of the extraction region can be kept small while increasing both total output power and collimation of light propagating in the waveguide towards the extraction region.
  • Such an incident light source is perfect for controlling the output beam from the extraction region by using adapted sub-wavelength gratings for example.
  • the scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high.
  • a planar waveguide is used to confine light in the plane in conjunction with focalizing DB s formed in the waveguide.
  • the extraction region is placed at the focal of the focalizing DBRs.
  • the planar waveguide is formed on a highly reflective layer 902 deposited on an n-doped substrate 901.
  • Layer 902 can be a DBR but it is not limited to DBR.
  • An opening is formed in layer 902 and an n-doped layer 903 is deposited in the opening.
  • the waveguide is composed by at least an emitting layer 905 inserted between an n-doped layer 904 and a p-doped layer 906.
  • An electrical contact 909 is deposited on the surface of layer 901. As layers 901, 903 and 904 are all n-doped the current can flow easily from contact 909 to the active region 905.
  • a grating structure 907 is formed on a small part in the center of the waveguide.
  • a highly reflective contact layer 908 is deposited on the waveguide structure.
  • Focusing DBRs (910, 911, 912 and 913) are formed in the external part of the waveguide by etching through layers 904, 905 and 906 down to layer 902.
  • the periodicity and fill factor of the DBRs are designed to provide a high reflectivity and a focal point in the center of the waveguide to focalize the emission generated in the waveguide by layer 905 towards the diffraction grating 907 and assure a strong extraction efficiency by the extraction region defined by layer 903.
  • the waveguide structure is planar.
  • a highly reflective layer 1302 is deposited on an n-doped substrate 1301.
  • a light emitting structure composed by a p-doped layer 1305 deposited on an emitting layer 1304 deposited on an n-doped layer 1303.
  • the light emitting structure is deposited on layer 1302.
  • Layers 1304 and 1305 are partially etched to remove the active region in a part of the waveguide. Thus re-absorption of the emission propagating inside he waveguide by the p-doped layer 1305 and the active layer 1304 is completely suppressed.
  • An electrical contact 1307 is deposited on layers 1301, 1302 and 1303.
  • a highly reflective electrical contact 1306 is deposited on a part of layer 1305 to form an optical resonant optical cavity in the part of the waveguide with the light emitting structure inside.
  • the thickness of the cavity is a multiple of ⁇ /2 ⁇ (with ⁇ the emission wavelength and n the effective index of the cavity) and layer 1304 is at a distance of (k+1) ⁇ /4 ⁇ from layer 1306, taking into account the phase shift induced by the metal contact 1306.
  • Using highly reflective layers 1302 and 1306 allow forming a vertical resonant cavity.
  • Using highly reflective layer 1302 and 1306 to form the resonant cavity allows to redistribute light emission mainly into lateral guided modes by suppressing other optical modes of propagation in the cavity. If the total cavity thickness is reduced to a few multiple of ⁇ /2 ⁇ the number of propagating modes is further reduced which can be useful for beam shaping.
  • the shape of the waveguide in the propagating plane should enhance the propagation of the emission towards the extraction region 1308.
  • the shape is a long linear taper in order to minimize reflections of the taper.
  • Other type of waveguides can be used such as adiabatic taper to maximize the coupling with the extraction region 1308. It is obvious that waveguide shapes offering higher coupling efficiency with the extraction zone are highly preferred.
  • the emitting/extraction region surface ratio (grating surface/ resonant cavity surface) is high enough to allow an enhancement of the luminance of the extraction region 1308.
  • FIG. 11 Another preferred implementation shown in Figure 11 similar to the previous one is composed by a substrate (layer 1501), a highly reflective layer 1502 deposited on layer 1501.
  • the optical cavity is the same as the previous embodiment: an p-doped layer 1505 deposited on an emitting layer 1504 deposited on an n-doped layer 1503.
  • This optical cavity is deposited on layer 1502 and is again partially etched to prevent absorption by the emitting layer 1504. In this way it is easy to fabricate a long waveguide formed by layer 1503.
  • Highly reflective electrical contacts 1506 and 1507 are deposited on layers 1505 and 1503, respectively.
  • an anti-reflection coating (layer 1508) is deposited on the edge of the device to increase the extraction probability. Instead of an anti- reflection coating it is possible to use a rough surface or any method to increase light extraction.
  • the planar waveguide (layer 1506) is structured from the top to from a 3D waveguide.
  • Photonic crystals are used to fabricate a tapered waveguide. By designing properly the periodicity and the size of the photonic crystal can be used as a perfect mirror in the propagation plane.
  • the with of the waveguide is smaller in the direction of the exit facet (1509) in order to increase the luminance at the extraction area.
  • the shape of the waveguide is such that most of the light emitted inside the device is propagating towards the exit facet with low reflexions. High propagation efficiency reaching 90% have already been demonstrated for linear or adiabatic tapers. As the waveguide (layer 1503) is nearly loss-free is can be very long and therefore the emitting region essentially defined by the emitting layer 1504 can be also large. Using a waveguide shape with a reduced width at the exit facet to increase the luminance.
  • FIG. 12 Another preferred implementation shown in Figure 12. Internal absorption remains an issue in waveguides, especially if the waveguide incorporates an active region. In a preferred embodiment this issue is resolved by coupling an emitting structure where current is injected with an absorption- free waveguide fabricated on the same wafer.
  • the device structure is composed by a substrate 1701.
  • a highly reflective layer 1702 that can be used for electron conduction, for example an n-doped DBR is deposited on layer 1701.
  • An optical cavity is deposited on layer 1702. It is formed by an n-doped layer 1703, an emitting layer 1704 and a p-doped layer 1705.
  • a highly reflective contact layer 1706 is deposited on layer 1705. Layers 1702-1706 form a vertical resonant cavity.
  • a reflector 1707 is formed on one side of the device to reflect all light propagating in the opposite direction of the extraction region.
  • a sub-wavelength Bragg mirror 1707 is used. This mirror can have an in- plane spherical or parabolic profile to collimate light.
  • a second electrical contact 1708 is deposited on layer 1703.
  • a loss-free waveguide made for example of Si0 2 is deposited on layer 1702 and is in contact with one edge of the optical cavity (layers 1703-1705).
  • the optical cavity can have many shapes but in this preferred embodiment it has a curved shape similar to that of a focalizing lens.
  • the goal is to focalize the emission in a small region 1710 of the waveguide 1709, where the emission can be extracted efficiently from the waveguide.
  • Layer 1709 has also a triangular shape similar to that of a linear taper to further focalize light in region the extraction region 1710. It is possible to fabricate other kind of waveguides to obtain a collimated emission instead of a focalized emission reaching the extraction region.
  • a variation of the previous embodiment consists in using an emitting material instead of layer 1710.
  • a wavelength converter material Phosphors, rare-earth elements in a matrix, lumogen, quantum dots in a matrix, MQWs are a few examples for such a wavelength converter material but many other material can be used.
  • the emission generated by the active region is partially or totally absorbed by the wavelength converter material layer 1710 and the same layer 1710 is re-emitting light in the waveguide where it is guided partially towards the extraction region.
  • the waveguiding structure is a planar waveguide with an in- plane shape designed to guide light towards the extraction region.
  • the scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high.
  • the waveguide structure is a microdisk.
  • the microdisk structure is made of at least an emitting layer 2104 inserted between an n-doped layer 2103 and a p-doped layer 2105.
  • An n-type doped pedestal 2102 is supporting the microdisk structure and layer 2102 is also in contact with an n-doped layer 2101 which can be a substrate.
  • An electrical contact 2109 is deposited on the surface of layer 2101. As layers 2101, 2102 and 2103 are all n- doped the current can flow easily from contact 2109 to the active region 2102.
  • a transparent conductive contact 2106 is deposited on a large part of the surface of layer 2105.
  • the large surface of this contact layer allows increasing the output power generated inside the microdisk by layer 2104.
  • the second small pad contact 2108 is deposited on layer 2106.
  • the surface of contact 2108 is preferably small in order to minimize its effect (absorption, diffusion, ....) on the optical guiding of the emission inside the microdisk.
  • Light is generated inside the microdisk and is reflected by the semiconductor/air interfaces.
  • Optical whispering modes build up in the microdisk.
  • Such optical structure can demonstrate very high quality factor (>2000 and much higher), so light is highly confined into the device.
  • a defect is used to extract light in a specific area.
  • An extraction region 21010 is formed on the top surface of layer 2105 by patterning it with a sub-wavelength grating structure.
  • Random roughness or any non-flat surface can be used to diffuse or diffract light in this specific area.
  • This extraction region could also be formed in layer 2106 or any additional layer deposited on layer 2105 or 2106.
  • the size of the extraction region 21010 has to be small to maximize the luminance of this area as most of the light is extracted by the extraction region.
  • the size of contact 2106 has to be large to further increase the luminance.
  • the extraction zone is coupled with the guided mode propagating in the microdisk to maximize the extraction efficiency of the device.
  • the waveguiding structure is a microdisk only supported by a pedestal. Such a device structure is difficult to fabricate but similar structures can be easily fabricate and can be compatible with the concept of the invention.
  • the waveguide is a microdisk deposited on a highly reflective layer 2202 deposited on an n-doped substrate 2201.
  • Layer 2202 can be a DB but it is not limited to DBR.
  • layer 2202 is n-type doped.
  • An opening is formed in layer 2202 and an n-doped layer 2203 is deposited in the opening.
  • the microdisk is deposited on layers 2202 and 2203 and it is composed by at least an emitting layer 2205 inserted between an n-doped layer 2204 and a p-doped layer 2206.
  • An electrical contact 2209 is deposited on the surface of layer 2201. As layers 2201-2204 are all n-doped the current can flow easily from contact 2209 to the active region 2205.
  • a transparent conductive contact 2207 is deposited on a large part of the surface of layer 2206. The large surface of this contact layer allows increasing the output power generated inside the microdisk by layer 2205. Light is generated everywhere inside layer 2205 thanks to the transparent conductive layer 2207 and is reflected by the TCO/air, semiconductor/DBR and semiconductor/air interfaces.
  • Optical whispering modes build up in the microdisk.
  • Such optical structure can demonstrate very high quality factor (>1000 and much higher), so light is highly confined into the device. Finally a defect is used to extract light in a specific area.
  • a grating structure 2210 is formed on the top surface of layer 2207 by patterning. This grating structure is used to diffract light and redirect it in a vertical direction (up and down).
  • the second small pad contact 2208 is deposited on layer 2207, above the extraction region defined by the aperture in layer 2202 (i.e. layer 2203).
  • the surface of contact 2208 is preferably small in order to minimize its effect (absorption, diffusion, ....) on the optical guiding of the emission inside the microdisk.
  • Contact 2208 is highly reflective in order to reflect the vertical light beam created by the diffraction grating 2210.
  • the size of the extraction area (2203) is small in comparison to the size of the contact layer 2207 to maximize the luminance of this area.
  • the size of the contact 2207 has to be large to further increase the luminance.
  • Light generated by layer 2205 is guided inside the microdisk and diffracted by the grating 2210, reflected by contact 2208 and redirected towards layer 2203. In this concept the extraction region is linked to the pedestal (layer 2203) but there could be two separate pedestal if needed.
  • the extraction zone is coupled with the guided mode propagating in the microdisk to maximize the extraction efficiency of the device.
  • a monolithic light emitting device which includes a waveguide, one or more emitting regions inside the waveguide and an extraction region smaller than the emitting region
  • a device according to claim 1 and 2 or 3, wherein DB s are used to confine light inside the waveguide
  • a device according to claim 1 and 2 or 3, wherein photonic crystal structures are used to confine light inside the waveguide
  • a device according to claim 1 and 2 or 3, wherein grating structures are used to confine light inside the waveguide
  • a device according to claim 1 and 2, 3, 4, 5 or 6, wherein the waveguide incorporates one or a plurality of resonant cavities
  • a device according to claim 1 and 7, wherein grating structures are used to confine light inside the resonant cavity
  • a device according to claim 1 and 23 wherein the sub-wavelength structure is a grating structure
  • a device wherein the waveguide is separated in two waveguides.
  • the first waveguide includes the emitting region and the second waveguide is a low optical loss waveguide where the extraction region is localized.

Abstract

The invention allows increasing light emitting devices luminance while improving their reliability. In particular, thanks to the invention, small surface emission, high brightness light emitting devices can be fabricated. It consists in separating the region where light is generated by electrically induced spontaneous emission, from the region where light is extracted. Between these two regions is a waveguide structure. Specific features can be used to control the angular dispersion of the extracted emission and the extraction surface, for example to minimize apparent source. It is therefore possible to achieve high luminance. The invention can be applied in lighting, projection, displays, polymer curing, microelectronic fabrication, telecommunications or biophotonics for example.

Description

TITLE: Low etendue high brightness light emitting devices
ABSTRACT
The invention allows increasing light emitting devices luminance while improving their reliability. In particular, thanks to the invention, small surface emission, high brightness light emitting devices can be fabricated. It consists in separating the region where light is generated by electrically induced spontaneous emission, from the region where light is extracted. Between these two regions is a waveguide structure. Specific features can be used to control the angular dispersion of the extracted emission and the extraction surface, for example to minimize apparent source. It is therefore possible to achieve high luminance. The invention can be applied in lighting, projection, displays, polymer curing, microelectronic fabrication, telecommunications or biophotonics for example.
FIELD OF THE INVENTION
The invention relates to LED devices, arrays of LED devices and methods of manufacturing thereof. Specifically, the invention relates to, but is not limited to, LED devices having a plurality of optical components implementable with standard waferscale fabrication techniques. More specifically, the invention relates, but is not limited to, LED devices providing a high luminance (brightness). The invention can be applied to fields such as, but not limited to, lighting, projection, displays, polymer curing, microelectronic fabrication, telecommunications or biophotonics.
BACKGROUND
[0001] The invention discloses a new solution for the realization of light emitting devices offering a high luminance. The new device features an optical cavity used to guide the spontaneous emission generated by the emitting region inside the device towards a specific area where it is extracted in air. By controlling the angular dispersion of the extracted emission and the extraction surface (apparent source) it is possible to achieve high luminance.
[0002] In many applications luminance of the optical source is the limiting factor. For example low luminance of displays prevents their use in sunlight environments. High luminance corresponds to a high optical power density in a small etendue. Typically TV screens have luminance around 100 cd/m2, a LED will have a luminance of 10 Mcd/m2, 10 times lower than the sun and at least 100Ό00 less than a laser diode.
Laser diodes are offering very high brightness as the power density in the laser output beam can reach kW/cm2 in a highly focalized beam. However, laser emission is coherent by definition and it is an issue in many applications. In projection systems the laser beam projected onto a surface will be responsible of the appearance of speckles in the image which strongly degrade the image quality (sharpness, ...).
[0003] Thus, there is still a need for light emitters combining an incoherent emission with a high luminance. LEDs are emitting an incoherent light. Unfortunately the brightness of LEDs is limited by the maximum current density supported by the device (typically a few kA/cm2) and the large etendue due to the isotropic spontaneous emission. Although high power can be generated in an LED it is usually possible only for large etendue. An efficient light collimation is possible with external optical elements, but luminance will still be limited by the initial output power of the LED and its initial etendue. One example is point source LED in which the emission of an LED is shaped using a geometrical filter to select only a small part of the light produced by the LED. With this design, it is possible to fabricate a source with a small etendue but the output power in this etendue is reduced.
[0004] The main reason behind relatively small luminescence of LEDs is that the surface used for extraction in light emitting devices corresponds to the larger surface of the device, usually with an area comparable in size to the area where light is generated electrically. [0005] Luminance of LDs is much higher compared to that of an LED because of two elements: a gain medium coupled with an optical cavity. The optical cavity is used to confine light. When lasing threshold is reached light emission is enhanced only in specific waveguide modes allowed by the optical cavity. It is the combination of an optical cavity and the fact that all light is generated in a limited number of modes which eventually leads to a high luminance.
[0006] In VCSEL light is confined vertically inside an optical cavity and light is extracted over the area where it is produced. When lasing occurs the intensity at the exit facet is very high leading to a high luminance but it is still limited by the emission area.
[0007] Edge emitting ridge lasers are based on horizontal waveguides. When lasing threshold is reached light emission is enhanced only in specific waveguide longitudinal modes allowed by the optical cavity thanks to stimulated emission. Despite the fact that light is barely confined in the waveguide stimulated emission allows for a high photon density in one or several guided mode only. For edge emitting lasers the output emission is extracted over a small surface of one side of the ridge although light is initially generated over the whole ridge. In this case it is possible to have a large surface for current injection while the emission surface remains small but it is only possible because of stimulated emission which leads to speckles and still require a high current density to reach the lasing threshold.
[0008] Other solutions to fabricate a light emitter with a high luminance consist in using light couplers to combine several light beams in a single planar waveguide or an optical fiber. In this approach the output power travelling through the optical fiber increases with each added source and eventually exits at the fiber end facet. Of course the coupling efficiency of the couplers is not 100% (especially for not collimated sources) so coupling losses highly reduce the overall efficiency. In addition the combined size of the couplers, the optical guide and each additional emitter result in a very bulky device and is not compatible with wafer-scale technology.
[0009] In conclusion there is currently no all integrated light emitter offering a high luminance with a coherent light emission that can be produced with simple wafer-scale technology.
 BRIEF SU MMARY OF TH E INVENTION
[00010]The scope of the invention to fabricate a light source with a high luminance.
[00011]The light emitting device described by the invention features an optical cavity and has a large light generation area and a small extraction area. The invention discloses several designs of light emitting devices offering a high luminance and/or light mixing possibilities.
[00012]The device surfaces have to be as reflective as possible to form an optical cavity with a high quality factor. A few solutions to form reflective surfaces are highly reflective metals (Ag, Al, Au,...), distributed Bragg mirrors, sub-wavelength grating mirrors, photonic crystals but other solutions can be used. Then an aperture is formed in the optical cavity to allow light to escape. The aperture can be done by removing the highly reflective layer locally and/or to form a highly transmissive interface.
[00013] A non-exhaustive list of solutions includes surface random roughening, anti-reflection coatings, anti-reflection roughening (moth eye, ...), diffractive optical elements, sub-wavelength gratings, photonic crystals.
[00014]The efficiency of the device can be maximized by redirecting the emitted light inside the device towards the extraction zone. A non-exhaustive list of solutions to achieve this goal comprises: vertical microcavities, microdisks, ridge waveguides, tapered waveguides, photonic crystal waveguides, and membranes.
DETAILED DESCRIPTION AN D BEST MODE OF IMPLEMENTATION
[00015] LEDs are usually designed to extract as much as possible of this guided emission and with a minimum of reflection inside the device medium to avoid re-absorption. It is also important to minimize the size of the device in order to decrease the fabrication cost. Therefore all LED are designed to extract the generated light in the same region where it is produced. For this reason light emitting diodes have usually a limited brightness because light extraction is done on the larger surface possible. Control of the light beam characteristics in those devices is usually done with an external lens but the luminance is still etendue limited.
[00016] In a completely different approach the objective of the invention is to benefit from this guided light to direct it in a specific area where it can be extracted with the desired characteristics. The extraction area can even be far away from the initial source. Our invention takes advantage of the physical separation of the light generation area (active region) and the extraction area. By doing so the luminance of emitters such as LEDs is no more limited by the efficiency of the device and luminance can be dramatically increased. The concept relies on the concentration of light emitted from extended surface towards a smaller area:
the extraction zone can be very small so the brightness (or luminance) will be very high. the emission area (active region) can be very large the total output power and therefore the luminance in the extraction area can be further increased and is only limited by optical losses of the waveguide.
In order to increase the luminance of the extraction area different parameters have to be optimized:
Decreasing the extraction area/emission area ratio. The size of the opening should be small compared to the emitting area to increase the concentration of light in the opening of the device.
The optical losses inside the device forming the cavity should be reduced as much as possible to maximize the chance for emitted photons to reach the opening and be extracted from the device. Localize the opening in a region of high optical power density in order to increase the probability for light to escape from the
Improve light extraction efficiency of the extraction region by using a rough extraction surface, anti-reflection coatings or refractive optical elements such as lenses.
Improve directivity by using diffractive elements on the extraction region such as sub- wavelength gratings or photonic crystals.
[00017] In the following embodiments of the invention, different elements can be made of substructures or materials such as that in the non-exhaustive list below: or n-doped substrate can be: a substrate such as GaN, ZnO, SiC, silicon, sapphire, GaN on sapphire, GaN on SiC, GaN on silicon, GaP, GaAs, AIN, or any material with a thickness sufficient to serve as support for a device
An n-doped layer can be: n-type lll-V alloys such as n-type AlxlnyGai_x_yN, n-type AlxlnyGai_x_yAs, n-type AlxlnyGai_x_yP, n-type ll-VI alloys, n-type ZnO...
A p-doped layer can be: p-type lll-V alloys such as p-type AlxlnyGai_x_yN, p-type AlxlnyGai_x_yAs, p-type AlxlnyGai_x_yP, p-type ll-VI alloys, p-type ZnO,...
An emitting region can be made of single layer of AlxlnyGai_x_yN, AlxlnyGai_x_yAs, AlxlnyGai_x_yP, ll-VI alloys, ZnO, ...
An emitting region can be made of multiple quantum wells or quantum dots planes made of InGaN/GaN, InGaN/AIGaN, GaN/AIGaN, AlzlnwGai-z-wN/AlxlnyGai-x-yN, AlzlnwGai-z-wAs/AlxlnyGai-x-yAs, AlzlnwGai_z_wP/AlxlnyGai_x_yP, ll-VI alloys, ZnO, ...
A transparent conductive layer can be made of one or several transparent conductive oxide such as ITO, 10, ZnO, AZO, GZO, ... A highly reflective metallic contact can be made of one or several layers of metal such as: Ag, Al, Au, Ni, Ti, Ni, Pd, Pt, Cr, ...
A waveguide can be made of one or several layers of low loss materials such as: dielectric materials (Si02, SiN, Zr02, Ti02, Ta205, ...) or semiconductors (GaN, AIN, ZnO, ...) or insulators (AI203, ....)
A rough surface is a non flat surface that can be: a surface with a random roughness, a diffusing surface, a textured surface with periodic or non-periodic features (micro-lenses, micro- domes, micro-pillars, micro/nano-rods, micro-pyramids, micro-stripes, motheye structures,...), a sub- wavelength diffraction grating structures, a photonic crystals, diffractive optical elements.
A highly reflective layer can be made of one or more reflective layers. Highly reflective layers include distributed Bragg reflectors (DBR), sub-wavelength diffractive gratings, photonic crystals, metals
[00018] The basic concept of the invention is presented on Figure 1. The emitting device is based on a planar ridge waveguide 1. The waveguide comprises at least an n-doped layer, an active region and a p-doped layer. Light is generated by the active layer. Emission inside the device is isotropic but a large part is confined inside the waveguide by total internal reflection on the external surfaces of the waveguide. Highly reflective contacts 2 and 3 are deposited on the p-doped layer and the n-doped layer respectively. Both contacts are highly reflective in order to avoid absorption of the guided light propagating inside the waveguide. At one end of the waveguide an aperture is formed in the top metallic contact to extract the emission. The metallic contacts have to be large to maximize the emitting surface and therefore the power generated inside the device. The surfaces of the contacts can be much larger than the surface of the aperture. In a waveguide having low optical losses the increase of the surface of the emitting region (essentially the surface of the device) results in an increase of the total power produced. Therefore increasing the area of the emitting region allow an increase of the power extracted from the aperture that can be much larger than the power generated by device with the emitting surface equal to that of the aperture. Thus, the luminance of the aperture can be dramatically increased by increasing the surface of the emitting region while keeping the extraction region small.
However, although any increase of the emitting region surface will result in an increase of the luminance of the extraction region the increases is limited by many factors:
Optical losses due to extracted light outside the extraction region
Internal absorption of the emission by the waveguide structure
Internal emission not coupled with the extraction region
Extraction efficiency of the extraction region
In the following paragraphs many implementations of the invention that can suppress or reduce those limiting factors are disclosed. [00019] Although the structure of Figure 1 allows an increase of the luminance the efficiency of the approach relies on a waveguide structure with low optical losses. Optical losses are coming from internal absorption, unwanted extraction of light and absorption by any layer in contact with the waveguide. Optical waveguide can demonstrate low absorption/diffusion losses but metal contacts are partially absorbing. One way to minimize this absorption is to replace the metallic contacts with transparent contacts such as transparent conductive oxide (ITO, ZnO, 10, ...) for example. In that case the contact can even be over the aperture as it is transparent, allowing a direct emission from the emitting region below the aperture. This approach is illustrated in Figure 2.
[00020] In this preferred embodiment the waveguide comprises an n-doped layer 201, a p-doped layer 203 and an emitting layer 202 between them. A transparent conductive layer (204) is deposited on the p-doped layer 203 for current spreading. Layer 204 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer. Highly reflective layers such as DB s can be deposited on the bottom and top side of the device surfaces (elements 205 and 206) to minimize extraction of light in unwanted area of the device as it would be lost for the targeted extraction region. A small aperture is formed in layer 205 and two apertures are formed in layer 206. A metallic contact 207 is contacting the surface of the n-doped layer 201 through an aperture in layer 206. Another contact 208 is deposited on the surface of the p-doped layer 203 through an aperture in layer 205. The extraction is confined in aperture 209.
[00021] Both metallic contacts are small but it is still preferred to use contacts as reflective as possible to reduce absorption of the emission of layer 202 by the contacts 207 and 208. This design allows a decrease of the optical losses in the waveguide by decreasing the surface of the absorbing metal which results in increased output power in the aperture. A second advantage is the possibility to inject current in the whole emitting region. Finally an optical element can be deposited on the extraction region. In a preferred embodiment an anti-reflection coating (layer 210) is deposited exclusively in the extraction region to increase its extraction efficiency. [00022] In a further preferred embodiment a lateral confinement of the emission generated inside the waveguide is added to the previous embodiment. Highly reflective layers (211 and 212) are deposited on all free surfaces not covered by the metallic contacts to avoid emission extraction in unwanted area of the device (edge facets,...).
[00023] This design adds photon recycling: photons which are not extracted through the aperture can be reflected at the edge of the device thanks to the reflective interfaces. If internal losses are sufficiently small photons can have many reflections inside the waveguide and can eventually be redirected towards the aperture to be extracted.
[00024] Different materials can be used as layers 205, 206, 211 and 212 such as DB , a combination of an insulating material and a metal, painting, structured surface or any reflective material or element.
[00025] In a further preferred embodiment illustrated in Figure 3 a diffusing element is added to the previous embodiment in the extraction region. The emitting structure is formed by at least an n- doped layer 301, a p-doped layer 303 and an emitting layer 302 between them. A transparent conductive layer 304 deposited on p-doped layer 303 is used for current spreading. Layer 304 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer. Highly reflective layers such as DBRs can be deposited on the bottom and top side of the device surfaces (elements 305 and 306) to minimize extraction of light in unwanted area of the device as it would be lost for the targeted extraction region. A small aperture is formed in layer 305 and two apertures are formed in layer 306. A metallic contact 307 is contacting the surface of the n-doped layer 301 through an aperture in layer 306. Another contact 308 is deposited on the surface of the p-doped layer 303 through an aperture in layer 305. The extraction is confined in aperture 309.
[00026] In the previous embodiments the emission extracted through the aperture was limited due to total internal reflection at the semiconductor/air interface above a certain angle. For a flat surface the extraction is limited to 2-4% depending on the refractive index of the waveguide. Different methods can be implemented to increase the extraction efficiency of the aperture. One solution consists in roughening the interface, randomly or with a defined pattern. A possible implementation in the case of AIGalnN LEDs consists in roughening the n-layer surface by chemical etching. For example 30% KOH solution can be used to form hexagonal pyramids with a random dispersion of height from a flat c-plane GaN surface with N polarity (typically n-GaN layer in AIGaln N LEDs). Such a roughening process could also be done by dry etching using a patterned mask or any other method. The extraction efficiency through the aperture 309 is highly enhanced compared to previous embodiments as laterally guided modes which would have not been extracted through a flat interface (because of total internal reflection) can be extracted with a rough interface.
[00027] In the previous embodiment the extraction efficiency of the aperture was enhanced thanks to a roughened surface in the aperture 303. It is possible to obtain a similar effect by including a diffusing or diffracting element inside the waveguide as shown in Figure 4. The emitting structure is formed by at least an n-doped layer 401, a p-doped layer 403 and an emitting layer 402 between them. A transparent conductive layer 404 deposited on p-doped layer 403 is used for current spreading. Layer 404 may not be necessary if current spreading is good but in the case of p-type GaN it is preferable to use a current spreading layer. Roughening of the surface of the n-doped layer is done on a small area to allow light scattering in the vicinity of the extraction region above (409). Roughening can have a random pattern or an organized pattern such as sub-wavelength gratings or photonic crystals, micro lenses for example to increase beam directivity. The insertion of a planarization layer 413 is necessary if a DBR is used as reflector having optimal optical characteristics. Indeed, without a planarization layer the reflectivity of a DBR on it would be reduced. However, one could also replace the couple 413/407 by a single highly reflective layer.
It is possible to use a similar design where a surface of one layer inside the device is rough or textured. Highly reflective layers such as DB s can be deposited on the bottom and top side of the device surfaces (elements 405 and 406) to minimize extraction of light in unwanted area of the device as it would be lost for the targeted extraction region.
A small aperture is formed in layer 406 and two apertures are formed in layer 405. A metallic contact 407 is contacting the surface of the n-doped layer 401 through an aperture in layer 406. Another contact 408 is deposited on the surface of the p-doped layer 404 through an aperture in layer 405. The extraction is confined in aperture 409.
All emissions can only escape from the device through the small aperture 409 made in layer 405. A dome lens 410 is used to increase the extraction efficiency and directivity of the extracted beam as it is the scope of the invention to fabricate a light source with a high luminance. Other refractive optical elements can be used instead of a lens for beam shaping or light extraction purpose.
[00028] The scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high. In a preferred embodiment illustrated in Figure 5 a focalizing optical element is included in the optical cavity where light is generated to increase the optical power density in the vicinity of the extraction region.
The light emitting device is composed by a layer 501 which can be a substrate. An emitting structure is deposited on layer 501. It is composed by an n-doped layer 502, an emitting layer 503 and p-doped layer 504. On the top of layer 504 a concave mirror is formed. This mirror can be fabricated by forming a lens (layer 505) on the top of layer 504 and a highly reflective layer 506, possibly a metal. Layer 505 can be a transparent conductive layer to facilitate current spreading or a transparent conductive layer can be inserted between layers 504 and 505 if necessary. A highly reflective layer 510 is deposited on the bottom of layer 501 to redirect light towards the concave mirror. The extraction region 509 is defined by an opening made through layer 510. The opening 509 is localized at the focal point of the concave mirror to increase the optical power density on the opening. In this implementation light which is not extracted by the extraction region is reflected inside the device until it reaches the opening (509) or is extracted at the edges of the device. For this reason, in a preferred embodiment high ly reflective layers (512 and 513) are deposited on the edges of the device to minimize optical losses and increase photon recycling.
Finally a lens or any optical element able to improve the extraction efficiency or mod ify the farfield pattern can be used. In this embod iment a lens (511) is placed on the aperture 509 to focalize the extracted emission.
[00029] In another preferred embodiment shown in Error! Reference source not found, the bottom side of the substrate is partly or totally textured with a random roughness or with a pattern. This roughness is used to diffuse photon not extracted directly by the aperture. Photons not escaping through the aperture can be reflected in another direction and have another possibility to be redirected towards the aperture. A transparent planarization layer 607, such as spin-on-glass, is deposited on the back of layer 501. The highly reflective layer 510 is deposited on the bottom of layer 607 to red irect light towards the concave mirror. The extraction region 509 is defined by an opening made through layer 510. The opening (509) is localized at the focal point of the concave mirror to increase the optical power density on the opening. In th is implementation light which is not extracted by the extraction region is reflected inside the device until it reaches the opening (509) or is extracted at the edges of the device. For this reason, in a preferred embodiment high ly reflective layers (512 and 513) are deposited on the edges of the device to minimize optical losses and increase photon recycling. Finally a lens or any optical element able to improve the extraction efficiency or modify the farfield pattern can be used. In this embodiment a lens (511) is placed on the aperture 509 to focalize the extracted emission. The small size of the open ing 509 and the large emitting region defined by layer 503 guarantees that the luminance of the extraction region will be dramatically enhanced .
[00030] In a preferred embodiment shown in Figure 7, a planar waveguide structure is deposited on an n-doped substrate 701. A sub-wavelength grating mirror is used as a highly reflective layer deposited on layer 701. Layer 702 is made of a SiOx layer with holes periodically spaced with dimensions and periodicity chosen to allow a high reflectivity at the emission wavelength when holes are filled with an n-doped material (layer 703). An opening (712) is formed in layer 702 to let the emitted photons escape down to layer 701. An n-doped layer 703 is deposited on layers 702. An emitting layer 704 is deposited on layer 703 and a p-doped layer 705 is deposited on layer 704. A transparent conductive layer (layer 706) is deposited over the surface of layer 705. A grating structure 707 is formed inside layer 706 to diffract light. A transparent layer 708 deposited on layer 706 for getting back a flat surface and a highly reflective layer 710 is deposited on the top of layer 708 to reflect all light in the direction of the extraction region 712. An opening is formed in layers 708 and 710 for form a highly reflective metallic contact 709 in the opening. Another metallic contact 711 is deposited on the n-doped substrate 701 but it could also be deposited on layer 703. A h ighly reflective layer 713 is formed on the waveguide facet on the opposite side of the extraction region to reflect light not propagating to the extraction region. This layer 713 can be a DB or a metal for example but it is not limited to those elements as any reflecting material can be used for this purpose.
[00031] Surface of the grating structure is small enough compared to the surface of layer 706 to allow an enhancement of the luminance. The required emitting/extraction region surface ratio (layer 706/open ing 712 in this embodiment) depends on the coupling between the emitting region and the extraction region 712. If the coupling is high, meaning that a large part of the light generated is guided and extracted through extraction region 712 then there is no need to use a large emitting region. On the other hand if there is most of the light produced inside the device is not extracted through extraction region 712, then it is necessary to compensate with a large emitting region. Thus, a layer 706/opening 712 surface ratio > 10 can be used.
The scope of the invention is to increase as much as possible the brightness of a light source. In a preferred embodiment the waveguide is a taper. In the propagation plane the waveguide (formed by layers 703, 704, 705, 706, and 708) has a trapezoidal shape. The emission is done on the whole surface to maximize the total power and the extraction region 712 is localized on the larger part of the taper. Such a waveguide design is used to redirect light emitted inside the waveguide in the direction of the grating structure thanks to multiple reflections at the interfaces on the lateral edges of the taper. With a long waveguide the dispersion angle of the emission in the plane will decreases when light propagate towards the extraction region as the photons align progressively. If optical losses inside the waveguide are low it is preferable to use a long waveguide. A collimated beam inside the waveguide is beneficial to couple the emitted light with air with a particular beam shape but other waveguide shape can be used.
[00032] In another preferred embodiment illustrated in Figure 8 the planar waveguide (formed by layers 703, 704, 705, 706, and 708) has a parabolic shape with a parabolic mirror (813) on the opposite side of the extraction region. This parabolic mirror is used to redirect and collimate more efficiently light towards the extraction region (707). As the length of the device can be very large it is possible to obtain a highly collimated light into the waveguide, without increasing the width of the waveguide. Therefore, the surface of the extraction region can be kept small while increasing both total output power and collimation of light propagating in the waveguide towards the extraction region. Such an incident light source is perfect for controlling the output beam from the extraction region by using adapted sub-wavelength gratings for example. [00033] The scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high. In one possible implementation of the invention a planar waveguide is used to confine light in the plane in conjunction with focalizing DB s formed in the waveguide. The extraction region is placed at the focal of the focalizing DBRs.
In this preferred embodiment illustrated in Figure 9 the planar waveguide is formed on a highly reflective layer 902 deposited on an n-doped substrate 901. Layer 902 can be a DBR but it is not limited to DBR. An opening is formed in layer 902 and an n-doped layer 903 is deposited in the opening. The waveguide is composed by at least an emitting layer 905 inserted between an n-doped layer 904 and a p-doped layer 906. An electrical contact 909 is deposited on the surface of layer 901. As layers 901, 903 and 904 are all n-doped the current can flow easily from contact 909 to the active region 905. A grating structure 907 is formed on a small part in the center of the waveguide. A highly reflective contact layer 908 is deposited on the waveguide structure. Focusing DBRs (910, 911, 912 and 913) are formed in the external part of the waveguide by etching through layers 904, 905 and 906 down to layer 902. The periodicity and fill factor of the DBRs are designed to provide a high reflectivity and a focal point in the center of the waveguide to focalize the emission generated in the waveguide by layer 905 towards the diffraction grating 907 and assure a strong extraction efficiency by the extraction region defined by layer 903.
[00034] In a preferred embodiment shown in Figure 10, the waveguide structure is planar. A highly reflective layer 1302 is deposited on an n-doped substrate 1301. A light emitting structure composed by a p-doped layer 1305 deposited on an emitting layer 1304 deposited on an n-doped layer 1303. The light emitting structure is deposited on layer 1302. Layers 1304 and 1305 are partially etched to remove the active region in a part of the waveguide. Thus re-absorption of the emission propagating inside he waveguide by the p-doped layer 1305 and the active layer 1304 is completely suppressed. An electrical contact 1307 is deposited on layers 1301, 1302 and 1303.
A highly reflective electrical contact 1306 is deposited on a part of layer 1305 to form an optical resonant optical cavity in the part of the waveguide with the light emitting structure inside.
The thickness of the cavity is a multiple of λ/2η (with λ the emission wavelength and n the effective index of the cavity) and layer 1304 is at a distance of (k+1) λ/4η from layer 1306, taking into account the phase shift induced by the metal contact 1306. Using highly reflective layers 1302 and 1306 allow forming a vertical resonant cavity. Using highly reflective layer 1302 and 1306 to form the resonant cavity allows to redistribute light emission mainly into lateral guided modes by suppressing other optical modes of propagation in the cavity. If the total cavity thickness is reduced to a few multiple of λ/2η the number of propagating modes is further reduced which can be useful for beam shaping.
The shape of the waveguide in the propagating plane should enhance the propagation of the emission towards the extraction region 1308. In this preferred embodiment the shape is a long linear taper in order to minimize reflections of the taper. Other type of waveguides can be used such as adiabatic taper to maximize the coupling with the extraction region 1308. It is obvious that waveguide shapes offering higher coupling efficiency with the extraction zone are highly preferred.
The emitting/extraction region surface ratio (grating surface/ resonant cavity surface) is high enough to allow an enhancement of the luminance of the extraction region 1308.
[00035] Another preferred implementation shown in Figure 11 similar to the previous one is composed by a substrate (layer 1501), a highly reflective layer 1502 deposited on layer 1501. The optical cavity is the same as the previous embodiment: an p-doped layer 1505 deposited on an emitting layer 1504 deposited on an n-doped layer 1503. This optical cavity is deposited on layer 1502 and is again partially etched to prevent absorption by the emitting layer 1504. In this way it is easy to fabricate a long waveguide formed by layer 1503. Highly reflective electrical contacts 1506 and 1507 are deposited on layers 1505 and 1503, respectively. Finally an anti-reflection coating (layer 1508) is deposited on the edge of the device to increase the extraction probability. Instead of an anti- reflection coating it is possible to use a rough surface or any method to increase light extraction.
The planar waveguide (layer 1506) is structured from the top to from a 3D waveguide. Photonic crystals are used to fabricate a tapered waveguide. By designing properly the periodicity and the size of the photonic crystal can be used as a perfect mirror in the propagation plane. The with of the waveguide is smaller in the direction of the exit facet (1509) in order to increase the luminance at the extraction area. The shape of the waveguide is such that most of the light emitted inside the device is propagating towards the exit facet with low reflexions. High propagation efficiency reaching 90% have already been demonstrated for linear or adiabatic tapers. As the waveguide (layer 1503) is nearly loss-free is can be very long and therefore the emitting region essentially defined by the emitting layer 1504 can be also large. Using a waveguide shape with a reduced width at the exit facet to increase the luminance.
[00036] Another preferred implementation shown in Figure 12. Internal absorption remains an issue in waveguides, especially if the waveguide incorporates an active region. In a preferred embodiment this issue is resolved by coupling an emitting structure where current is injected with an absorption- free waveguide fabricated on the same wafer. The device structure is composed by a substrate 1701. A highly reflective layer 1702 that can be used for electron conduction, for example an n-doped DBR is deposited on layer 1701. An optical cavity is deposited on layer 1702. It is formed by an n-doped layer 1703, an emitting layer 1704 and a p-doped layer 1705. A highly reflective contact layer 1706 is deposited on layer 1705. Layers 1702-1706 form a vertical resonant cavity. A reflector 1707 is formed on one side of the device to reflect all light propagating in the opposite direction of the extraction region. In this embodiment a sub-wavelength Bragg mirror 1707 is used. This mirror can have an in- plane spherical or parabolic profile to collimate light. A second electrical contact 1708 is deposited on layer 1703.
A loss-free waveguide (layer 1709) made for example of Si02 is deposited on layer 1702 and is in contact with one edge of the optical cavity (layers 1703-1705). In the propagating plane the optical cavity can have many shapes but in this preferred embodiment it has a curved shape similar to that of a focalizing lens. The goal is to focalize the emission in a small region 1710 of the waveguide 1709, where the emission can be extracted efficiently from the waveguide. Layer 1709 has also a triangular shape similar to that of a linear taper to further focalize light in region the extraction region 1710. It is possible to fabricate other kind of waveguides to obtain a collimated emission instead of a focalized emission reaching the extraction region.
By using a sub-wavelength grating properly designed it is possible to control the output beam. It is for example possible to obtain a collimated beam or a focused beam, which will enhance the brightness of the extraction area 1710. By focalizing the emission generated in the large optical cavity (layers 1703-1705) in the small extraction region 1710 the efficiency of the device and the luminance of the extraction zone are both highly increased.
A variation of the previous embodiment consists in using an emitting material instead of layer 1710. In particular one can use a wavelength converter material. Phosphors, rare-earth elements in a matrix, lumogen, quantum dots in a matrix, MQWs are a few examples for such a wavelength converter material but many other material can be used. In a preferred embodiment the emission generated by the active region is partially or totally absorbed by the wavelength converter material layer 1710 and the same layer 1710 is re-emitting light in the waveguide where it is guided partially towards the extraction region.
[00037] In the previous embodiments the waveguiding structure is a planar waveguide with an in- plane shape designed to guide light towards the extraction region. However, many other solutions exist to confine light in the device that can be used for the invention. The scope of the invention being to extract as much light as possible in a small area it is preferable to concentrate the light generated in the waveguide towards the extraction region or to place the extraction region in an area of the waveguide where the power density is high.
In a preferred embodiment shown on Figure 13 the waveguide structure is a microdisk. The microdisk structure is made of at least an emitting layer 2104 inserted between an n-doped layer 2103 and a p-doped layer 2105. An n-type doped pedestal 2102 is supporting the microdisk structure and layer 2102 is also in contact with an n-doped layer 2101 which can be a substrate. An electrical contact 2109 is deposited on the surface of layer 2101. As layers 2101, 2102 and 2103 are all n- doped the current can flow easily from contact 2109 to the active region 2102. A transparent conductive contact 2106 is deposited on a large part of the surface of layer 2105. The large surface of this contact layer allows increasing the output power generated inside the microdisk by layer 2104. The second small pad contact 2108 is deposited on layer 2106. The surface of contact 2108 is preferably small in order to minimize its effect (absorption, diffusion, ....) on the optical guiding of the emission inside the microdisk. Light is generated inside the microdisk and is reflected by the semiconductor/air interfaces. Optical whispering modes build up in the microdisk. Such optical structure can demonstrate very high quality factor (>2000 and much higher), so light is highly confined into the device. Finally a defect is used to extract light in a specific area. An extraction region 21010 is formed on the top surface of layer 2105 by patterning it with a sub-wavelength grating structure. Random roughness or any non-flat surface can be used to diffuse or diffract light in this specific area. This extraction region could also be formed in layer 2106 or any additional layer deposited on layer 2105 or 2106. The size of the extraction region 21010 has to be small to maximize the luminance of this area as most of the light is extracted by the extraction region. The size of contact 2106 has to be large to further increase the luminance. [00038] In a further preferred embodiment the extraction zone is coupled with the guided mode propagating in the microdisk to maximize the extraction efficiency of the device.
[00039] In the previous embodiments shown on Figure 13 the waveguiding structure is a microdisk only supported by a pedestal. Such a device structure is difficult to fabricate but similar structures can be easily fabricate and can be compatible with the concept of the invention. In a preferred embodiment shown in Figure 14 the waveguide is a microdisk deposited on a highly reflective layer 2202 deposited on an n-doped substrate 2201. Layer 2202 can be a DB but it is not limited to DBR. In a further preferred embodiment layer 2202 is n-type doped. An opening is formed in layer 2202 and an n-doped layer 2203 is deposited in the opening. The microdisk is deposited on layers 2202 and 2203 and it is composed by at least an emitting layer 2205 inserted between an n-doped layer 2204 and a p-doped layer 2206. An electrical contact 2209 is deposited on the surface of layer 2201. As layers 2201-2204 are all n-doped the current can flow easily from contact 2209 to the active region 2205. A transparent conductive contact 2207 is deposited on a large part of the surface of layer 2206. The large surface of this contact layer allows increasing the output power generated inside the microdisk by layer 2205. Light is generated everywhere inside layer 2205 thanks to the transparent conductive layer 2207 and is reflected by the TCO/air, semiconductor/DBR and semiconductor/air interfaces. Optical whispering modes build up in the microdisk. Such optical structure can demonstrate very high quality factor (>1000 and much higher), so light is highly confined into the device. Finally a defect is used to extract light in a specific area. In this preferred embodiment a grating structure 2210 is formed on the top surface of layer 2207 by patterning. This grating structure is used to diffract light and redirect it in a vertical direction (up and down). The second small pad contact 2208 is deposited on layer 2207, above the extraction region defined by the aperture in layer 2202 (i.e. layer 2203). The surface of contact 2208 is preferably small in order to minimize its effect (absorption, diffusion, ....) on the optical guiding of the emission inside the microdisk. Contact 2208 is highly reflective in order to reflect the vertical light beam created by the diffraction grating 2210. The size of the extraction area (2203) is small in comparison to the size of the contact layer 2207 to maximize the luminance of this area. The size of the contact 2207 has to be large to further increase the luminance. Light generated by layer 2205 is guided inside the microdisk and diffracted by the grating 2210, reflected by contact 2208 and redirected towards layer 2203. In this concept the extraction region is linked to the pedestal (layer 2203) but there could be two separate pedestal if needed.
[00040] In a preferred embodiment the extraction zone is coupled with the guided mode propagating in the microdisk to maximize the extraction efficiency of the device.
[00041] What is claimed is:
1. A monolithic light emitting device which includes a waveguide, one or more emitting regions inside the waveguide and an extraction region smaller than the emitting region
2. A device according to claim 1, wherein the waveguide is a SLAB
3. A device according to claim 1, wherein the waveguide is a microdisk
4. A device according to claim 1 and 2 or 3, wherein DB s are used to confine light inside the waveguide
5. A device according to claim 1 and 2 or 3, wherein photonic crystal structures are used to confine light inside the waveguide
6. A device according to claim 1 and 2 or 3, wherein grating structures are used to confine light inside the waveguide
7. A device according to claim 1 and 2, 3, 4, 5 or 6, wherein the waveguide incorporates one or a plurality of resonant cavities
8. A device according to claim 1 and 7, wherein photonic crystal structures are used to confine light inside the resonant cavity
9. A device according to claim 1 and 7, wherein grating structures are used to confine light inside the resonant cavity
10. A device according to claim 1 and 7, wherein DBRs are used to confine light inside the resonant cavity
11. A device according to claims 1, wherein the area of the extraction region is smaller than 1/2 of the emitting region
12. A device according to claims 1, wherein the area of the extraction region is smaller than 1/10 of the emitting region
13. A device according to claims 1, wherein the area of the extraction region is smaller than 1/100 of the emitting region
14. A device according to claims 1, wherein the area of the extraction region is smaller than 20μιη2
15. A device according to claims 1, wherein the area of the extraction region is smaller than 10μιη2
16. A device according to claims 1, wherein the area of the extraction region is smaller than 5μιτ)2
17. A device according to claims 1, wherein the area of the extraction region is smaller than 1μιη2
18. A device according to claims 1, wherein the waveguide region is a taper
19. A device according to claims 1, wherein the waveguide region is a taper with a larger width in the direction of the extraction region
20. A device according to claims 1, wherein the waveguide region is a taper with a narrower width in the direction of the extraction region
21. A device according to claims 1, wherein the extraction area has a rough surface
22. A device according to claims 1, wherein an anti-reflection coating is deposited on the extraction area
23. A device according to claims 1, wherein a sub-wavelength structure is formed in the extraction area 24. A device according to claim 1 and 23 wherein the sub-wavelength structure is a photonic crystal structure
25. A device according to claim 1 and 23 wherein the sub-wavelength structure is a grating structure
26. A device according to claim 1 and 23 wherein the sub-wavelength structure is a diffractive optical element
27. A device according to claims 1, wherein the extraction area incorporates a refractive optical element
28. A device according to claim 1 and 27 wherein the refractive optical element is a lens
29. A device according to claim 1 and 27 wherein the refractive optical element is an array of micro-lenses
30. A device according to claims 1, wherein a diffusion element is inserted into the waveguide
31. A device according to claims 1, wherein the waveguide is separated in two waveguides. The first waveguide includes the emitting region and the second waveguide is a low optical loss waveguide where the extraction region is localized.
32. A device according to claims 1 and 31 wherein the second waveguide includes a wavelength- conversion material
33. A device according to claims 1, wherein the emission is incoherent
34. A device according to claims 1, wherein the device is a micro LED
35. A projection system incorporating the device according to claims 1 to 34
36. A display incorporating the device according to claims 1 to 34
37. A photolithographic system incorporating the device according to claims 1 to 34
Method 1:
1. Form an optical confinement layer on a substrate
2 Form an optical cavity with an active layer on the optical confinement layer
3 Form a diffusing element inside the optical cavity
4. Form a second confinement layer to form an optical cavity with a high quality factor
5. Form a small aperture inside the cavity
6 Form electrical contacts
7. Form optical elements on the optical cavity aperture
Method 2:
1. Form an optical confinement layer on a substrate
2 Form an active layer on a substrate
3 Form a second confinement layer to form an optical cavity with a high quality factor
4. Form a aperture inside the cavity
5. Form electrical contacts
6 Form optical elements in the optical cavity aperture

Claims

CLAIMS [00041] What is claimed is:
1. A monolithic light emitting device which includes a waveguide, one or more emitting regions inside the waveguide and an extraction region smaller than the emitting region
2. A device according to claim 1, wherein the waveguide is a SLAB
3. A device according to claim 1, wherein the waveguide is a microdisk
4. A device according to claim 1 and 2 or 3, wherein DB s are used to confine light inside the waveguide
5. A device according to claim 1 and 2 or 3, wherein photonic crystal structures are used to confine light inside the waveguide
6. A device according to claim 1 and 2 or 3, wherein grating structures are used to confine light inside the waveguide
7. A device according to claim 1 and 2, 3, 4, 5 or 6, wherein the waveguide incorporates one or a plurality of resonant cavities
8. A device according to claim 1 and 7, wherein photonic crystal structures are used to confine light inside the resonant cavity
9. A device according to claim 1 and 7, wherein grating structures are used to confine light inside the resonant cavity
10. A device according to claim 1 and 7, wherein DBRs are used to confine light inside the resonant cavity
11. A device according to claims 1, wherein the area of the extraction region is smaller than 1/2 of the emitting region
12. A device according to claims 1, wherein the area of the extraction region is smaller than 1/10 of the emitting region
13. A device according to claims 1, wherein the area of the extraction region is smaller than 1/100 of the emitting region
14. A device according to claims 1, wherein the area of the extraction region is smaller than 20μιη2
15. A device according to claims 1, wherein the area of the extraction region is smaller than 10μιη2
16. A device according to claims 1, wherein the area of the extraction region is smaller than 5μιτ)2
17. A device according to claims 1, wherein the area of the extraction region is smaller than 1μιη2
18. A device according to claims 1, wherein the waveguide region is a taper
19. A device according to claims 1, wherein the waveguide region is a taper with a larger width in the direction of the extraction region
20. A device according to claims 1, wherein the waveguide region is a taper with a narrower width in the direction of the extraction region
21. A device according to claims 1, wherein the extraction area has a rough surface
22. A device according to claims 1, wherein an anti-reflection coating is deposited on the extraction area
23. A device according to claims 1, wherein a sub-wavelength structure is formed in the extraction area
24. A device according to claim 1 and 23 wherein the sub-wavelength structure is a photonic crystal structure
25. A device according to claim 1 and 23 wherein the sub-wavelength structure is a grating structure
26. A device according to claim 1 and 23 wherein the sub-wavelength structure is a diffractive optical element
27. A device according to claims 1, wherein the extraction area incorporates a refractive optical element
28. A device according to claim 1 and 27 wherein the refractive optical element is a lens
29. A device according to claim 1 and 27 wherein the refractive optical element is an array of micro-lenses
30. A device according to claims 1, wherein a diffusion element is inserted into the waveguide
31. A device according to claims 1, wherein the waveguide is separated in two waveguides. The first waveguide includes the emitting region and the second waveguide is a low optical loss waveguide where the extraction region is localized.
32. A device according to claims 1 and 31 wherein the second waveguide includes a wavelength- conversion material
33. A device according to claims 1, wherein the emission is incoherent
34. A device according to claims 1, wherein the device is a micro LED
35. A projection system incorporating the device according to claims 1 to 34
36. A display incorporating the device according to claims 1 to 34
37. A photolithographic system incorporating the device according to claims 1 to 34
Method 1:
1. Form an optical confinement layer on a substrate
2 Form an optical cavity with an active layer on the optical confinement layer
3 Form a diffusing element inside the optical cavity
4. Form a second confinement layer to form an optical cavity with a high quality factor
5. Form a small aperture inside the cavity
6 Form electrical contacts
7. Form optical elements on the optical cavity aperture
Method 2:
1. Form an optical confinement layer on a substrate
2 Form an active layer on a substrate
3 Form a second confinement layer to form an optical cavity with a high quality factor
4. Form a aperture inside the cavity
5. Form electrical contacts
6 Form optical elements in the optical cavity aperture
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