FIELD OF THE DISCLOSURE

The present disclosure relates to solid-state lighting devices including light-emitting diodes and more particularly to light-emitting diodes with reflective layers having high reflectivity.

BACKGROUND

Solid-state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with a long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are being increasingly utilized for general illumination applications, often replacing incandescent and fluorescent light sources.

LEDs are solid-state devices that convert electrical energy to light and generally include one or more active layers of semiconductor material (or an active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet emissions. An active region may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, and/or gallium arsenide-based materials and/or from organic semiconductor materials. Photons generated by the active region are initiated in all directions.

Typically, it is desirable to operate LEDs at the highest light emission efficiency, which can be measured by the emission intensity in relation to the output power (e.g., in lumens per watt). A practical goal to enhance emission efficiency is to maximize extraction of light emitted by the active region in the direction of the desired transmission of light. Light extraction and external quantum efficiency of an LED can be limited by a number of factors, including internal reflection. According to the well-understood implications of Snell's law, photons reaching the surface (interface) between an LED surface and the surrounding environment are either refracted or internally reflected. If photons are internally reflected in a repeated manner, then such photons eventually are absorbed and never provide visible light that exits an LED.

To increase the opportunity for photons to exit an LED, it has been found useful to pattern, roughen, or otherwise texture the interface between an LED surface and the surrounding environment to provide a varying surface that increases the probability of refraction over internal reflection and thus enhances light extraction. Despite the availability of such methods, their practical employment has been limited in at least certain contexts. For example, mechanical methods may introduce stress in or cause breakage of LED material and may also be limited in terms of the position in a fabrication sequence in which they can be employed. Chemical (e.g., photolithographic etching) methods may also be limited in terms of their position in a fabrication sequence to avoid misalignment or microfeature damage during subsequent LED chip fabrication and/or to avoid chemical incompatibility with LED chip layers if etching is performed after chip fabrication.

Another way to increase light extraction efficiency is to provide reflective surfaces that reflect generated light so that such light may contribute to useful emission from an LED chip. In a typical LED package10illustrated inFIG. 1, a single LED chip12is mounted on a reflective cup13by means of a solder bond or conductive epoxy. One or more wire bonds11can connect the ohmic contacts of the LED chip12to leads15A and/or15B, which may be attached to or integral with the reflective cup13. The reflective cup may be filled with an encapsulant material16, which may contain a wavelength conversion material such as a phosphor. At least some of light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip12. While the reflective cup13may direct light in an upward direction, optical losses may occur when the light is reflected. Some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces. Some metals can have less than 95% reflectivity in the wavelength range of interest.

FIG. 2shows another LED package in which one or more LED chips22can be mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate, or submount23. A metal reflector24mounted on the submount surrounds the LED chips22and reflects light emitted by the LED chips22away from the package20. The reflector24also provides mechanical protection to the LED chips22. One or more wire bond connections11are made between ohmic contacts on the LED chips22and electrical traces25A,25B on the submount23. The mounted LED chips22are then covered with an encapsulant26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector24is typically attached to the carrier by means of a solder or epoxy bond. The metal reflector24may also experience optical losses when the light is reflected because it also has less than 100% reflectivity.

The reflectors shown inFIGS. 1 and 2are arranged to reflect light that escapes from the LED. LEDs have also been developed that have internal reflective surfaces or layers to reflect light internal to the LEDs.FIG. 3shows a schematic of an LED chip30with an LED32mounted on a submount34by a metal bond layer36. The LED further comprises a p-contact/reflector38between the LED32and the metal bond36, with the p-contact/reflector38typically comprising a metal such as silver (Ag). This arrangement is utilized in commercially available LEDs such as those from Cree® Inc., available within the EZBright™ family of LEDs. The p-contact/reflector38can reflect light emitted from the LED chip's active region toward the submount back toward the LED's primary emitting surface. The reflector also reflects total internal reflection light back toward the LED's primary emitting surface. Like the metal reflectors above, the p-contact/reflector38reflects less than 100% of light and in some cases less than 95%.

FIG. 4shows a graph40showing the reflectivity of silver on gallium nitride (GaN) at different viewing angles for light with a wavelength of 460 nm. The refractive index of GaN about 2.4, while the complex refractive index for silver is taken from the technical literature. [See Handbook of Optical Constants of Solids, edited by E. Palik.] The graph shows the p-polarization reflectivity42, s-polarization reflectivity44, and average reflectivity46, with the average reflectivity46generally illustrating the overall reflectivity of the metal for the purpose of LEDs where light is generated with random polarization. The reflectivity at 0 degrees is lower than the reflectivity at 90 degrees, and this difference can result in up to 5% or more of the light being lost on each reflection. In an LED chip, in some instances total internal reflection light can reflect off the mirror several times before it escapes and, as a result, small changes in the mirror absorption can lead to significant changes in the brightness of the LED. The cumulative effect of the mirror absorption on each reflection can reduce the light intensity such that less than 75% of light from the LED's active region actually escapes as LED light.

The art continues to seek improved light-emitting diodes and solid-state lighting devices having reduced optical losses and providing desirable illumination characteristics capable of overcoming challenges associated with conventional lighting devices.

SUMMARY

The present disclosure relates to solid-state lighting devices including light-emitting diodes and more particularly to light-emitting diodes with reflective layers having high reflectivity.

In some embodiments, a light-emitting diode (LED) chip includes an active LED structure comprising an active layer between an n-type layer and a p-type layer and a first reflective layer adjacent the active LED structure and comprising a plurality of dielectric layers. The LED chip includes a second reflective layer on the first reflective layer, a barrier layer on the second reflective layer, and a passivation layer on the barrier layer. In some embodiments, each dielectric layer of the plurality of dielectric layers comprises a different thickness. In some embodiments, the plurality of dielectric layers comprises a plurality of first dielectric layers and a plurality of second dielectric layers. In some embodiments, the plurality of dielectric layers comprises an aperiodic Bragg reflector. In some embodiments, the second reflective layer includes an electrically conductive path through the first dielectric layer.

In other embodiments, a light-emitting diode (LED) chip includes an active LED structure including an active layer between an n-type layer and a p-type layer, and a first reflective layer adjacent the active LED structure and including a plurality of first dielectric layers and a plurality of second dielectric layers wherein each layer of the plurality of first dielectric layers includes a different thickness. The thickest layer of the plurality of first dielectric layers is between other layers of the plurality of first dielectric layers.

In other embodiments, a light-emitting diode (LED) chip includes an active LED structure including an active layer between an n-type layer and a p-type layer and a first reflective layer adjacent the active LED structure and including a plurality of first dielectric layers and a plurality of second dielectric layers. An average thickness of the plurality of first dielectric layers is greater than an average thickness of the plurality second dielectric layers and at least one layer of the plurality of second dielectric layers comprises a thickness greater than at least one layer of the plurality of first dielectric layers.

In other embodiments, an LED chip comprises: an active LED structure comprising an active layer between an n-type layer and a p-type layer; a first reflective layer adjacent the active LED structure; a second reflective layer on the first reflective layer; and an adhesion layer between the first reflective layer and the second reflective layer, wherein the adhesion layer comprises a metal oxide. In certain embodiments, the first reflective layer may comprise a plurality of dielectric layers. In certain embodiments, the metal oxide comprises aluminum oxide or anodic aluminum oxide.

In another aspect, any one or more aspects, embodiments, or features described herein may be combined with any one or more other aspects or features for additional advantage.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

