Semiconductor light emitting device with shaped substrate and method of manufacturing the same

Embodiments of the invention include a substrate (10) and a semiconductor structure (12) grown on the substrate. The semiconductor structure includes a light emitting layer (18) disposed between an n-type region (16) and a p-type region (20). The substrate includes a first sidewall (30) and a second sidewall (32). The first sidewall and second sidewall are disposed at different angles relative to a major surface of the semiconductor structure. A reflective layer (34) is disposed over the first sidewall (30).

FIELD OF THE INVENTION

The present invention relates to a semiconductor light emitting device that may have improved light extraction.

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 known LED. After growth of the epitaxial structure on the growth substrate2, metal contacts are formed on the epitaxial structure, and the device is flipped over relative to the growth direction and attached to a mount5. InFIG. 1, the epitaxial structure and metal contacts are shown as block3. A phosphor layer4is formed over the substrate2. Phosphor layer4absorbs light emitted by the light emitting layer of the epitaxial structure and emits light of a different wavelength. A lens6is disposed over the LED and the mount5.

In the device illustrated inFIG. 1, a majority of the light extracted from the LED is extracted from the substrate side (as opposed to the epitaxial structure side). Light emitted by the light emitting layer toward the metal contacts may be partially reflected by a reflective p-contact, before exiting the structure. The p-contact may cover some portion of the epitaxial structure surface opposite the substrate. The phosphor generally emits light in all directions. Some light8absorbed by the phosphor and emitted at a different wavelength is emitted in the direction of lens6. Some light9emitted by the phosphor is emitted back toward the epitaxial structure3. Because the epitaxial structure3is not very reflective, light9is likely to be absorbed, which reduces the efficiency of the device ofFIG. 1.

SUMMARY

It is an object of the invention to provide a device that may reduce absorption of light by the epitaxial structure.

Embodiments of the invention include a substrate and a semiconductor structure grown on the substrate. The semiconductor structure includes a light emitting layer disposed between an n-type region and a p-type region. The substrate includes a first sidewall and a second sidewall. The first sidewall and second sidewall are disposed at different angles relative to a major surface of the semiconductor structure. A reflective layer is disposed over the first sidewall.

A method according to embodiments of the invention includes growing a semiconductor structure on a substrate, the semiconductor structure including a III-nitride light emitting layer disposed between an n-type region and a p-type region. The method further includes shaping the substrate to form a first sidewall and a second sidewall. The first sidewall and second sidewall are disposed at different angles relative to a major surface of the semiconductor structure. A reflective layer is formed over the first sidewall.

DETAILED DESCRIPTION

In embodiments of the invention, the growth substrate is shaped to reduce the amount of light that is directed toward the epitaxial structure.

Though in the examples below the semiconductor light emitting devices are III-nitride LEDs that emit 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.

FIG. 2illustrates a III-nitride LED that may be used in embodiments of the present invention. Any suitable semiconductor light emitting device may be used and embodiments of the invention are not limited to the device illustrated inFIG. 2. The device ofFIG. 2is formed by growing a III-nitride semiconductor structure12on 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, sapphire, 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 into the substrate.

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 barrier layers. A p-type region20may 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, a 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. 2. 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 form electrical connections to the LED, one or more interconnects26and28are formed on or electrically connected to the n- and p-contacts22and21. Interconnect26is electrically connected to n-contact22inFIG. 2. 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 growth substrate wafer then diced from the wafer of devices. The semiconductor structure, the n- and p-contacts22and21, and the interconnects26and28are represented in the figures below by block12.

FIG. 3illustrates a device with a shaped growth substrate, according to embodiments of the invention.FIG. 4is a cross sectional view of the device illustrated inFIG. 3. The device illustrated inFIG. 3may be diced from the growth substrate wafer such that the epitaxial structure12is rectangular. Along one of the long edges of the rectangular epitaxial structure12, the growth substrate10is shaped into a substantially vertical sidewall30. Along the other long edge of the rectangular epitaxial structure12, the growth substrate10is shaped into and a sloped sidewall32. Sloped sidewall32may be sloped so that it meets substantially vertical sidewall30at a vertex33at the top of growth substrate10. In the cross section illustrated inFIG. 4, the vertical sidewall30and the sloped sidewall32make a right triangle with an angle35of 60°. The angle35of the triangle may be at least 50° in some embodiments and less than or equal to 70° in some embodiments. Along the short edges of the rectangular epitaxial structure, the growth substrate10may have substantially vertical sidewalls36such that substrate10forms a triangular prism. The substrate is not limited to the right triangle shape illustrated inFIGS. 3 and 4and may be shaped into any suitable shapes and cross sections, including, for example, geometrical shapes such as trapezoidal prisms, other triangles besides right triangles, any sort of prism, and prismatoids. A prism is a polyhedron with an n-sided polygonal base, a translated copy of the base not in the same plane as the base, and n other faces (necessarily all parallelograms) joining corresponding sides of the base and the translated copy of the base. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a pentagonal prism. A prismatoid is a polyhedron where all vertices lie in two parallel planes.

The vertical sidewall30may be covered with a highly reflective material34. Vertical sidewalls36may be covered with a reflective material such as highly reflective material34or a different material. Any suitable reflective material(s) may be used, such as, for example, reflective paint, reflective metals such as Ag or Al, dielectric materials, or white diffusers. In some embodiments, some or all of the sidewalls of the structure12are coated with a highly reflective material. Light is extracted from the substrate through the sidewall with the largest area, the sloped sidewall32in the embodiment illustrated inFIGS. 3 and 4.

A growth substrate that is shaped according to embodiments of the invention may be thicker than a growth substrate in a conventional device, such as the device illustrated inFIG. 1. For example, the growth substrate10inFIGS. 3 and 4may be at least 300 μm thick in some embodiments, no more than 1000 μm thick in some embodiments, at least 500 μm thick in some embodiments, and no more than 800 μm thick in some embodiments.

The substrate10may be shaped by any suitable method. For example, the substantially vertical sidewalls30and36may be formed by dicing the device from the growth substrate wafer. The sloped sidewall32may be formed by any suitable technique such as, for example, etching, grinding, laser scribing, ablation, or sub-surface scribing and breaking (sometimes referred to as “stealth” dicing or cutting). In some embodiments, the device12is protected during the formation of sloped sidewall32, for example, by covering it with handling tape or any other suitable material or technique. The sloped sidewall32may be formed after the semiconductor structure is grown on the substrate10in some embodiments. The sloped sidewall32may be formed before or after dicing the device from the growth substrate wafer. In some embodiments, the substrate10is thinned before forming sloped sidewall32.

FIG. 5illustrates a device with a shaped growth substrate and a wavelength converting layer. The wavelength converting layer40is formed on sloped sidewall32, the surface of the substrate10from which light is extracted. Wavelength converting layer40may be, for example, wavelength converting layer material formed into a ceramic for example by sintering, then glued to the substrate10, or a wavelength converting material mixed with a transparent material such as silicone, glass, or epoxy that is disposed on the substrate by any suitable method including, for example, molding, laminating, electrophoretic deposition, spin coating, spray coating, screen printing, or dispensing. Wavelength converting layer40is often between 50 and 100 μm thick, though it may be any thickness appropriate to the particular wavelength converting material used and to the particular method used to form the wavelength converting layer.

The wavelength converting material in wavelength converting layer40may be, for example, conventional phosphors, organic phosphors, quantum dots, organic semiconductors, II-VI or III-V semiconductors, II-VI or III-V semiconductor quantum dots or nanocrystals, dyes, polymers, or other materials that luminesce. The wavelength converting material absorbs light emitted by the LED and emits light of one or more different wavelengths. Unconverted light emitted by the LED is often part of the final spectrum of light extracted from the structure, though it need not be. Examples of common combinations include a blue-emitting LED combined with a yellow-emitting wavelength converting material, a blue-emitting LED combined with green- and red-emitting wavelength converting materials, a UV-emitting LED combined with blue- and yellow-emitting wavelength converting materials, and a UV-emitting LED combined with blue-, green-, and red-emitting wavelength converting materials. Wavelength converting materials emitting other colors of light may be added to tailor the spectrum of light emitted from the structure.

As illustrated by ray46, light that is emitted by wavelength converting layer40into substrate10may be reflected by reflective coating34, then emitted toward wavelength converting layer40where the light may escape the device. Light46emitted by the wavelength converting layer40into substrate10is unlikely to encounter the semiconductor structure12in the device ofFIG. 5, in contrast to the device ofFIG. 1, where a significant amount of light emitted by the phosphor into the substrate is absorbed by the poorly reflecting epitaxial structure. Accordingly, shaping the growth substrate may increase the extraction efficiency of the device, by reducing the amount of light generated by the wavelength converting layer40that is absorbed by the semiconductor structure.

In some embodiments, devices with shaped growth substrates are disposed on reflective mounts, as illustrated inFIG. 5. The reflective mount42may be formed from a reflective material such as a reflective metal, or may be any suitable material with a reflective top surface44. The top surface may be made reflective by, for example, coating the surface with a reflective metal such as Ag or Al, reflective paint, or any other suitable material.

Since the device illustrated inFIGS. 3, 4, and 5emits light through a single sidewall of the substrate10, (sloped sidewall32), the light emission is not symmetrical, as in the device ofFIG. 1.FIG. 6illustrates a configuration where two of the devices illustrated inFIG. 5are disposed on a reflective mount42, back to back. The reflective coating34on the vertical sidewalls30of the left side device50and the right side device52are disposed adjacent each other. Light from the left side device50is emitted largely in the direction51. Light from the right side device52is emitted largely in the direction53.

FIG. 7illustrates the far-field emission pattern from the device illustrated inFIG. 1. The emission pattern ofFIG. 1is a substantially symmetrical Lambertian pattern.FIG. 8illustrates the far-field emission pattern from the device illustrated inFIG. 6. The structure ofFIG. 6emits light in two lobes, lobe51A from the left side device50and lobe53A from right side device52. The structure ofFIG. 6extracts substantially more light from the sides of the structure than the device ofFIG. 1, which extracts substantially more light from the top of the structure. The structure illustrated inFIG. 6may be beneficial in lighting applications that do not require directionality but rather favor significant side emission, for example to enhance light diffusion.

FIG. 9illustrates an embodiment which may be suitable for applications requiring directionality of light emission. In the device ofFIG. 9, a cavity or opening66is formed in a mount64. The cavity66is shaped to accommodate a device with a shaped growth substrate10, such as the device illustrated inFIG. 3. The shape of cavity66may be substantially the same as the shape of the device. For example, cavity66may be shaped such that the sidewall32is substantially parallel to one of the surfaces of mount64. In some embodiments, cavity66may be shaped so that sidewall32is coincident with a surface of mount64, or inset to accommodate a wavelength converting member. In the alternative, the cavity66may be shaped so that sidewall32protrudes above a surface of mount64.

The sidewall30of the substrate10that is coated with a reflective material34is disposed proximate the cavity66. In some embodiments, one or more of the internal surfaces of cavity66are reflective, such that a separate reflective material layer34may be omitted. Likewise, if one or more of the internal surfaces of cavity66is reflective, a separate reflective material layer may be omitted from sidewalls36. The sidewall32from which light is extracted is disposed substantially even with the top surface of mount64(as used in this context, the “top” surface of mount64refers to the surface from which the cavity extends). A wavelength converting member60, which may be, for example, a luminescent ceramic or any other suitable material, may be attached to sidewall32by a transparent adhesive layer62such as silicone, epoxy, glass, or any other suitable material. Reflective material68may be disposed around the wavelength converting member60. The structure illustrated inFIG. 9emits a similar far-field pattern as the structure illustrated inFIG. 1. A majority of light is extracted from the structure through the top of the structure in the orientation illustrated inFIG. 9.

In some embodiments, the reflective layer68that surrounds the wavelength converting member60and/or the mount64in the device ofFIG. 9may be made from materials that conduct heat. The mount may be made of any suitable material such as, for example, metal, ceramic, AlN, or silica. In some embodiments, the surface of the mount is coated with any suitable reflective material (not shown inFIG. 9), including, for example, silver metal. Reflective layer68may be, for example, any suitable material such as a white reflector paste. The mount64and/or reflective layer68may conduct heat away from the LED and/or the wavelength converting member60, which may increase the efficiency of the structure and/or improve the lifetime of the device.

The shape of substrate10is not limited to the shapes illustrated inFIGS. 3, 4, 5, 6, and 9.FIG. 10illustrates the cross section of a shaped substrate10according to embodiments of the invention. A plane70of the semiconductor structure12is illustrated inFIG. 10. In the device ofFIG. 4, the sidewall30on which a reflective layer is disposed forms a right angle with the equivalent of plane70. The sidewall32from which light is extracted forms an acute angle35with the equivalent of plane70. The substrate10ofFIG. 4has four sidewalls and no other surface. Light is extracted from the sidewall32which is sidewall with the largest area.

InFIG. 10, the sidewall30on which a reflective layer is disposed forms an acute angle72with plane70. The sidewall32from which light is extracted forms an acute angle74with plane70. Acute angle74is small than acute angle72(i.e., sidewall30is closer to vertical than sidewall32). In other words, sidewall30is arranged at a different angle relative to major plane70than sidewall32(this is also the case for the device ofFIG. 4). As inFIG. 4, in the device ofFIG. 10, light is extracted from the sidewall32. In some embodiments, the sidewall from which light is extracted is the sidewall with the largest area.

The substrate illustrated inFIG. 10also has a top surface76. Top surface76may be coated with a reflective layer. The reflective layer is similar or identical to those described above. Little or no light is extracted from the substrate through top surface76. The substrate illustrated inFIG. 10has a top surface and four sidewalls, two long sidewalls30and32which are opposite each other, and two short sidewalls36which are opposite each other. The short sidewalls36may be vertical or angled at an acute angle relative to major plane70. The area of sidewall32is larger than the area of top surface76. Sidewalls30and36and top surface76may be covered with a reflective layer. A wavelength converting layer may be disposed on sidewall32, as described above.

In some embodiments, in addition to or instead of being coated with a reflective material, the reflective sidewalls of the substrate may be textured, for example by roughening or patterning with a random or repeating pattern, to scatter and/or direct light toward the sidewall32, from which light is extracted from the substrate.

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. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.