Light emitting device on a mount with a reflective layer

Embodiments of the invention include a semiconductor light emitting diode (LED) attached to a top surface of a mount. A multi-layer reflector is disposed on the top surface of the mount adjacent to the LED. The multi-layer reflector includes layer pairs of alternating layers of low index of refraction material and high index of refraction material. A portion of the top surface in direct contact with the multi-layer reflector is non-reflective.

FIELD OF THE INVENTION

The present invention relates to a light emitting device disposed on a mount with a reflective layer disposed next to the light emitting device.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.

FIG. 1illustrates a packaged phosphor-converted light emitting device described in more detail in U.S. Pat. No. 8,680,556.FIG. 2illustrates the composite reflective layer ofFIG. 1in more detail. The device ofFIG. 1includes an LED302disposed on package300and covered by an encapsulant304that includes phosphor. Light emitted out of the LEDs302, light emitted from phosphors in the encapsulant304, and light which reflects off of the exit surface of the encapsulant304may be emitted towards the substrate300. At least a portion of this light is reflected off a composite layer362and redirected so that it can exit the package.

FIG. 2shows one possible configuration of a reflective composite layer362. In this configuration, a set of dielectric layers308is formed over a set of metal layers. The portion of the dielectric layers308adjacent to the metal layer310may be an adhesion layer312. The portion of metal layer310adjacent to the substrate300may also be an adhesion layer312. In the embodiments described in U.S. Pat. No. 8,680,556, the material in the composite layer closest to substrate300is silver.

SUMMARY

It is an object of the invention to provide an LED disposed on a mount with a reflective layer disposed next to the LED on the mount.

Embodiments of the invention include a semiconductor light emitting diode (LED) attached to a top surface of a mount. A multi-layer reflector is disposed on the top surface of the mount adjacent to the LED. The multi-layer reflector includes layer pairs of alternating layers of low index of refraction material and high index of refraction material. A portion of the top surface in direct contact with the multi-layer reflector is non-reflective.

Embodiments of the invention include a semiconductor light emitting diode (LED) attached to a top surface of a mount. A layer is disposed on the top surface of the mount adjacent to the LED. A lens is disposed over the LED and the layer. The layer has an index of refraction lower than the index of refraction of the lens.

DETAILED DESCRIPTION

In embodiments of the invention, a lighting device such as a semiconductor light emitting diode is disposed on a package with at least one reflective layer.FIG. 3illustrates one example of a III-nitride LED. Any suitable semiconductor light emitting device may be used and embodiments of the invention are not limited to the LED illustrated inFIG. 3.

In the device ofFIG. 3, a majority of light is extracted from the LED through the growth substrate. Such a device may be referred to as a flip chip device. The LED ofFIG. 3is formed by growing a III-nitride semiconductor structure on a growth substrate10as is known in the art. The growth substrate is often sapphire but may be any suitable substrate such as, for example, a non-III-nitride material, SiC, Si, GaN, or a composite substrate. A surface of the growth substrate on which the III-nitride semiconductor structure is grown may be patterned, roughened, or textured before growth, which may improve light extraction from the device. A surface of the growth substrate opposite the growth surface (i.e. the surface through which a majority of light is extracted in a flip chip configuration) may be patterned, roughened or textured before or after growth, which may improve light extraction from the device.

The semiconductor structure includes a light emitting or active region sandwiched between n- and p-type regions. An n-type region16may be grown first and may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, which may be n-type or not intentionally doped, and n- or even p-type device layers designed for particular optical, material, or electrical properties desirable for the light emitting region to efficiently emit light. A light emitting or active region18is grown over the n-type region. Examples of suitable light emitting regions include a single thick or thin light emitting layer, or a multiple quantum well light emitting region including multiple thin or thick light emitting layers separated by bather layers. A p-type region20may then be grown over the light emitting region. Like the n-type region, the p-type region may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers.

After growth of the semiconductor structure, a reflective p-contact is formed on the surface of the p-type region. The p-contact21often includes multiple conductive layers such as a reflective metal and a guard metal which may prevent or reduce electromigration of the reflective metal. The reflective metal is often silver but any suitable material or materials may be used. After forming the p-contact21, a portion of the p-contact21, the p-type region20, and the active region18is removed to expose a portion of the n-type region16on which an n-contact22is formed. The n- and p-contacts22and21are electrically isolated from each other by a gap25which may be filled with a dielectric such as an oxide of silicon or any other suitable material. Multiple n-contact vias may be formed; the n- and p-contacts22and21are not limited to the arrangement illustrated inFIG. 3. The n- and p-contacts may be redistributed to form bond pads with a dielectric/metal stack, as is known in the art.

In order to electrically and physically attach the LED to another structure, one or more interconnects26and28are formed on or electrically connected to the n- and p-contacts22and21. Interconnect26is electrically connected to n-contact22inFIG. 3. Interconnect28is electrically connected to p-contact21. Interconnects26and28are electrically isolated from the n- and p-contacts22and21and from each other by dielectric layer24and gap27. Interconnects26and28may be, for example, solder, stud bumps, gold layers, or any other suitable structure. Many individual LEDs are formed on a single wafer then diced from the wafer of devices. The substrate10may be thinned after growth of the semiconductor structure or after forming the individual devices. In some embodiments, the substrate is removed from the device ofFIG. 3. A majority of light extracted from the device ofFIG. 3is extracted through the substrate10(or the surface of the semiconductor structure exposed by removing the substrate10).

FIGS. 4, 5, 6, and 7illustrate packaged LEDs, with different reflective layers formed on the mount. In each ofFIGS. 4, 5, 6, and 7, an LED1is electrically and physically connected to the top surface42of a mount40.

The LED1may be electrically connected to electrically conductive structures44formed on the surface42. The electrically conductive structures44are typically metal pads. The metal pads may be substantially the same size as the LED1, as illustrated in, for example,FIG. 4, or may cover all or a portion of the top surface42that is not covered by LED1, as illustrated in, for example,FIG. 5. The metal pads44electrically connect to bonding pads (not shown in the figures) formed, for example, on the top surface42, or on the bottom surface of the mount40. Bonding pads on the bottom surface of mount40may connect to the metal pads44for example through vias, not shown, formed within the mount or through any other suitable structure.

The mount40may be, for example, a ceramic structure, a plastic structure, a metal structure with one or more electrical isolation layers, a lead frame, or any other suitable structure.

In each ofFIGS. 4, 5, 6, and 7, a transparent cover46is disposed over LED1and mount40. The cover46is often, as illustrated in4,5,6, and7, an optic such as a dome lens, a Fresnel lens, or any other suitable structure. In some embodiments, the cover46is simply a conformal, transparent sheet of material. The cover46may be formed separately from the LED and mount and attached by gluing or any other suitable technique, or formed in situ, for example by laminating, molding, or any other suitable process. In some embodiments, one or more materials are mixed with the transparent material that forms the cover46. Examples of suitable materials mixed with the transparent material include one or more wavelength converting materials such as powder phosphors, materials to adjust the index of refraction, particles that cause scattering, or any other suitable material.

In each ofFIGS. 4, 5, 6, and 7, all or a portion of the top surface42of mount40that is not covered by LED1is covered by one or more reflective layers. The reflective layers may prevent light from being absorbed by the mount40, or may reduce the amount of light absorbed by the mount40, by reflecting unconverted light emitted by the LED1and/or converted light emitted by a phosphor in cover46. The reflective layers may increase extraction from the cover46, which may increase the efficiency of the device.

InFIGS. 4 and 5, a multi-layer reflector48is disposed on the top surface42of the mount40. The multi-layer reflector may be, for example, multiple layer pairs of alternating layers of high index and low index materials. The low index material may have a refractive index of at least 1.2 in some embodiments and no more than 1.6 in some embodiments. The high index material may have a refractive index of at least 2 in some embodiments and no more than 3 in some embodiments. The low index material may be SiO2or any other suitable material. The high index material may be TiO2, Ta2O5, or any other suitable material.

The multi-layer reflector may be a distributed Bragg reflector (DBR). Any suitable number of layer pairs may be used; at least 2 pairs in some embodiments, no more than 50 layer pairs in some embodiments, at least 5 pairs in some embodiments, not more than 40 pairs in some embodiments, and not more than 12 pairs in some embodiments. The total thickness of the DBR is at least 100 nm in some embodiments, not more than 2 μm in some embodiments, at least 500 nm in some embodiments, and no more than 5 μm in some embodiments. In some embodiments, one or more layers in a DBR is a polymer.

In the device ofFIG. 4, the metal pads44are confined to an area that is substantially the same size as LED1, such that multi-layer reflector48is disposed on and in direct contact with the top surface42of mount40. In some embodiments, the top surface42of mount40may be a surface that is not reflective. In the device ofFIG. 5, the metal pads cover a portion of the top surface42larger than the footprint of LED1, such that the multi-layer reflector48is disposed on and in direct contact with the metal pads44. The metal pads may be metals that are substantially not reflective of light emitted by LED1. Examples of such non-reflective metals include Cu, Al, Au, combinations thereof, or any other suitable metal or conductive material.

InFIG. 6, a multi-layer reflector48is combined with a low index layer50. The multi-layer reflector48, which may be any of the multi-layer reflectors described above in the text accompanyingFIGS. 4 and 5, is disposed in direct contact with the top surface42, as illustrated, or in direct contact with metal pads44, as illustrated inFIG. 5. A low index layer50is formed over multi-layer reflector48, such that multi-layer reflector48is disposed between the mount40and the low index layer50.

The low index layer50may be a material with an index of refraction less than 1.4 in some embodiments, no more than 1.3 in some embodiments, no more than 1.2 in some embodiments, and at least 1.1 in some embodiments. In some embodiments, the low index material50is air, which typically has an index of refraction of 1. At the interface between the low index layer50and the cover46, the contrast in index of refraction between these two layers may cause at least some light incident on the interface to be reflected by total internal reflection. The cover46is often silicone, which generally has an index of refraction between 1.4 and 1.6. Materials may be added to the silicone forming cover46, in order to increase the index of refraction of cover46, which would increase the contrast between the low index layer and the cover, which may increase the amount of light that is reflected. For example, the index of refraction of cover46may be at least 1.5 in some embodiments, at least 1.8 in some embodiments, at least 2 in some embodiments, and no more than 2.5 in some embodiments.

Suitable low index materials include low index glasses such as MgF or other fluoride glasses, low index polymers, and porous materials such as porous SiO2. The low index material may be formed by any suitable process. A porous or other low index material may be deposited using, for example, a sol gel process.

InFIG. 7, a low index layer50is used without a multi-layer reflector. The low index layer50may be any of the low index layers described above in the text accompanyingFIG. 6, may be disposed in direct contact with the top surface42, as illustrated, or in direct contact with metal pads44, as illustrated inFIG. 5.

The reflective layers illustrated inFIGS. 4, 5, 6, and 7may be formed by any suitable technique. For example, the reflective layers may be formed after the LED1is attached to the mount and before the cover is formed/attached, for example by masking the LED1or otherwise preventing the reflective layer from being formed on LED1. The reflective layers may be formed before LED1is attached to the mount. The reflective layers may be not formed in the area where LED1will later be placed, for example by selective growth or any other suitable technique, or altered to form an opening where LED1will be placed, for example by conventional photolithography techniques. There may be a gap between LED1and the reflective layers, or the LED1and the reflective layers may be positioned with no gap, as illustrated inFIGS. 4, 5, 6, and 7. The top surface of the reflective layers may be at the same elevation as the top surface of LED1, or the top surface of the reflective layers may be higher or lower than the top surface of LED1.

In some embodiments, more than one LED may be disposed on a mount with reflective layers. In some embodiments, devices other than LEDs are disposed on a mount with reflective layers.

Though in the examples above the semiconductor light emitting device are III-nitride LEDs that emits blue or UV light, semiconductor light emitting devices besides LEDs such as laser diodes and semiconductor light emitting devices made from other materials systems such as other III-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, or Si-based materials may be used.

Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. For example, any of the features described in any of the embodiments described herein may be included in or omitted from any of the embodiments described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.