Patent Application
20080197378
The present invention relates to light emitting diodes, and in particular relates to light emitting diodes (LEDs) that are used in conjunction with a phosphor that converts light from the LED to produce an output that is either partially or totally a combination of the frequencies emitted by the LED and those converted by the phosphor.
Light emitting diodes are a class of photonic devices in which the application of current across the device, and most fundamentally across a p-n junction, generates recombination events between electrons and holes. In turn the events produce at least some energy in the form of emitted photons.
Because the recombination events are defined and constrained by the principles of quantum mechanics, the energy (and thus the photon) generated by the event depends upon the characteristics of the semiconductor material in which the event takes place. In this regard, the bandgap of the semiconductor material is the most fundamental characteristic with respect to the performance of light emitting diodes. Because the recombination events take place between the valence band and the conduction band of the semiconductor materials, they can never generate an amount of energy larger than the bandgap. Accordingly, materials with smaller bandgaps produce lower energy (and thus lower frequency) photons while materials with larger bandgaps can produce higher energy, higher frequency photons.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, generally low cost.
Light emitting diodes, or at least the light emitting properties of semiconductors, have been recognized for decades. A 1907 publication (H. J. Round, Electrical World 49, 309) reported that current applied through silicon carbide produced an observed but unexplained emission of light. More widespread commercial use of LEDs began in the 1970s with indicator type use that incorporated lower frequency LEDs (typically red or yellow in color) formed from smaller bandgap materials such as gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP).
In the 1990s, the development of the blue light emitting diode as a commercial rather than theoretical reality greatly increase the interest in LEDs for illumination purposes. In this regards, “indication” refers to a light source that is viewed directly as a self-luminous object (e.g. an indicator light on a piece of electronic equipment) while “illumination” refers to a source used to view other objects in the light reflected by those objects (e.g., room lighting or desk lamps). See, National Lighting Product Information Program, http://www.lrc.rpi.edu/programs/NLPIP/glossary.asp (December 2006).
Although light emitting diodes have become widely adapted for indicator purposes, their potential use for illumination includes applications such as indoor and outdoor lighting, backlighting (e.g. for displays), portable lighting (e.g., flashlights), industrial lighting, signaling, architectural and landscaping applications, and entertainment and advertising installations.
The availability of blue light emitting diodes correspondingly provides the opportunity for at least two techniques for producing white light. In one technique, blue LEDs are used in conjunction with red and green LEDs so the combination can form white light or—such as in a full-color display—any other combination of the three primary colors.
In a second technique, and one that has become commercially widely adopted, a blue light emitting diode is combined in a lamp with a yellow-emitting phosphor; i.e., a phosphor that absorbs the blue light emitted by the LED and converts and emits it as yellow light. The combination of blue and yellow light will produce a tone of white light that is useful for many illumination circumstances.
Although the terminology is used flexibly, the word “diode” (or “light emitting diode”) is most properly applied to the basic semiconductor structure that includes the p-n junction. The term “lamp” most properly refers to a packaged device in which the diode is mounted on electrodes that can connect it to a circuit and within a polymer lens that both protects the diode from environmental exposure and helps increase and direct the light output. Nevertheless, the term “LED” is often used to refer to packaged diodes that might more correctly be referred to as lamps and vice versa. Generally speaking the meaning of the terms will be clear in context.
Because the blue frequencies represent the highest energy within the visible spectrum (with red frequencies being the lowest), they must be produced by relatively high-energy recombination events. This in turn requires that the semiconductor material have a relatively wide bandgap. Accordingly, candidate materials for blue light emitting diodes, and thus for white-emitting LED lamps, include silicon carbide (SiC), the Group III nitrides (e.g., GaN), and diamond. Because of their direct emitter properties, most interest in blue light emitting diodes has focused upon the Group III nitride materials such as gallium nitride, aluminum gallium nitride (AlGaN), and indium gallium nitride (InGaN).
Illumination, however, tends to require higher quantities of light output than does indication. In this regard, the number of individual photons produced by a diode in any given amount of time depends upon the number of recombination events being generated in the diode, with the number of photons generally being less than the number of recombination events (i.e., not every event produces a photon). In turn, the number of recombination events depends upon the amount of current applied across the diode. Once again the number of recombination events will typically be less than the number of electrons injected across the junction. Thus, these electronic properties can reduce the external output of the diode.
Additionally, when photons are produced, they must also actually leave the diode and the lamp to be perceived by an observer. Although the majority of photons will leave the lamp without difficulty, a number of well-understood factors come into play that prevent the photons from leaving and that can thus reduce the external output of an LED lamp. These include internal reflection of a photon until it is re-absorbed rather than emitted. The index of refraction between the materials in the diode can also change the direction of an emitted photon and cause it to strike an object that absorbs it. The same results can occur for yellow photons that are emitted by the phosphor. In an LED lamp such “objects” can include the substrate, parts of the packaging, the metal contact layers, and any other material or element that prevents the photon from escaping the lamp.
Furthermore, in addition to emitting light, the epitaxial layers also absorb incoming light (and for some of the same quantum mechanic reasons). Thus from a general and comparative standpoint, a quantum well will re-absorb more light than a p-type epitaxial layer of gallium nitride, and a p-type layer of gallium nitride will re-absorb more light than an n-type epitaxial layer of gallium nitride.
To date, bulk crystal growth of large Group III nitride crystals remains difficult. Accordingly, in order to form the thin, high quality epitaxial layers that produce p-n junctions in LEDs, the Group III nitride materials must typically be grown on a substrate. When, as in some constructions, the substrate remains as part of the eventual light emitting lamp, it can provide one more opportunity to absorb photons emitted by the junction, thus reducing the external quantum efficiency of the overall diode.
The lens or encapsulant portion of most diode packages are typically formed of a low index epoxy resin, but these resins are generally subject to degradation in the presence of the higher frequency emissions of Group III nitride-based diodes. Additionally, a number of packages presently include a mirror layer (for example commonly assigned and co-pending application Ser. No. 11/082,470 filed Mar. 17, 2005 and now published as No. 20060060874) which in turn creates a more specular emission. Where a Lambertian pattern is desired, some type of diffuser must be included to reduce the specular character of the mirror. The presence of the diffuser, however, lowers the overall efficiency of the diode lamp. Furthermore, substrates that are generally convenient for manufacturing the diodes tend to be “dark”; i.e., they absorb a certain percentage of the photons produced by the diode. Such absorption similarly reduces the external quantum efficiency of a diode lamp.
Light emitting diodes typically include multiple layers of different materials. As a result, light emitted from the active portion must typically pass through or across one or more of such layers before exiting the diode. Additionally, when the diode is packaged as a lamp, the light leaving the diode must travel into, through, and out of the lens material. In each of these circumstances, Snell's law dictates that the photons will be refracted as they pass from one material to the next. The amount that the photons are refracted will depend upon the difference between the refractive indexes of the two materials and the angle of incidence at which the light strikes the interface.
In a diode or a diode lamp, although some reflected light will still escape the diode at some other location, a certain percentage will be totally internally reflected, never escape the diode or the lamp, and will thus functionally reduce the external quantum efficiency of the diode and of any lamp that includes the diode. Although the individual reduction in the percentage of photons escaping may appear to be relatively small, the cumulative effect can be significant and diodes that are otherwise very similar can have distinctly different performance efficiencies resulting from even these small percentage losses.
Accordingly, a continuing need exists to increase the external efficiency of light emitting diode and diode lamps.
In one aspect the invention is a light emitting diode that includes a layer of p-type Group III nitride and a layer of n-type Group III nitride on a transparent carrier substrate that has an index of refraction lower then the layer of Group III nitride adjacent said carrier substrate and respective ohmic contacts to the p-type Group III nitride layer and to the n-type Group III nitride layer.
In yet another aspect, the invention is a method of forming a light emitting diode. The method includes the steps of forming respective p-type and n-type layers of Group III nitride on a compatible substrate, separating the compatible substrate from the Group III nitride epitaxial layers, and joining the Group III nitride epitaxial layers to a transparent carrier substrate that has an index of refraction lower than the index of refraction of the adjacent Group III nitride layer.
In yet another aspect, the invention is a light emitting diode lamp that includes a reflector and a light emitting diode on the reflector. The diode includes at least respective layers of n-type and p-type Group III nitride on a transparent carrier substrate that has a refractive index lower than the refractive index of the adjacent Group III nitride layer. An encapsulant covers the light emitting diode and the encapsulant has a refractive index within 0.2 of the refractive index of the transparent carrier substrate.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings.
The present invention is a light emitting diode, a first embodiment of which is broadly designated at 10 in
The low refractive index transparent carrier substrate typically has a refractive index of between about 1.35 and 1.65. Representative materials useful for diode carrier substrates include, but are not limited to, quartz, fused quartz, and glass. Sapphire can also be used as the transparent carrier substrate but has a slightly higher refractive index of about 1.8. The term “transparent” is generally well understood in this art, but representative transparent carrier substrates will transmit at least about 70 percent of incident light and preferably between about 90 and 100 percent of incident light of the relevant frequencies generated by the active portions of the diode.
Stated differently, by matching the refractive index of the substrate to the refractive index of the encapsulant, the invention enhances the transmission of light from the substrate to the encapsulant and thus enhances the external quantum efficiency.
Conventionally, amorphous materials such as glass have been considered less favorable for growth of epitaxial layers, and as a result conventional thought avoided using glass as a growth substrate for epitaxial layers. If desired, and provided the materials are otherwise compatible with the structure and function of the other elements of the invention, the carrier substrate can include phosphors or other particles, such as those included to favorably scatter or diffuse light. Furthermore, although described herein as a single element, it will be understood that the carrier substrate is not limited to a single layer, and could be a multilayer structure provided it meets the structural and functional requirements of the invention.
In exemplary embodiments of the invention, however, and as will be discussed later herein with respect to the method of manufacturing, the substrate does not represent the substrate upon which the layers of 11 and 12 are grown. Instead, a carrier 13 substrate having the desired optical properties is joined to the epitaxial layers 11, 12 by a low refractive index transparent adhesive schematically illustrated as the layer 14.
As used herein, the term “carrier substrate” refers to a substrate other than the one on which the epitaxial layers were originally grown. As is known to those familiar with the art, semiconductor epitaxial layers are best grown on compatible substrates that are most amenable for such growth. Typical factors include a good lattice match, coefficient of thermal expansion, chemical compatibility between the growth substrate and the epitaxial layers, and stability at chemical vapor deposition (CVD) growth temperatures.
For a number of reasons, once the epitaxial layers are grown, they can be removed from the growth substrate and placed on a carrier substrate. Such reasons include, but are not limited to, the desire to have a substrate in the final structure that has a more desired index of refraction than did the growth substrate.
In flip chip embodiments (i.e., those in which the substrate forms the emitting face of the final diode) or in embodiments where the growth substrate is replaced with the carrier substrate, some of the growth substrate can remain as a residue adjacent one of the epitaxial layers, but normally does not. In other cases, the carrier substrate can be the same material as the growth substrate, but added to the epilayers after the growth substrate has been removed.
The low index transparent adhesive that joins the carrier substrate to the active portions of the diode (typically the epitaxial layers) has a refractive index of between about 1.35 and 1.65, is photochemically stable with respect to electromagnetic radiation in the ultraviolet, blue, and green portions of the spectrum, and is thermally stable at temperatures of at least about 100° C.
Adhesives meeting this criteria, and that likewise avoid any undesired reactions with, or negative effect upon, the other portions of the diode are acceptable for joining the carrier substrate 13 to the epitaxial layers 11 and 12. One set of materials that meet this criteria include polysiloxane compositions, which are often referred to as “silicones.” Polysiloxanes have high optical transmittance in the UV and high energy visible region, can be tailored to have a desired refractive index within a range of about 1.38-1.62, have excellent photo-thermal stability, can be cured in a variety of techniques making processing easy, are available in high purity, and can be cured over a range of hardness from gels to hard resins. Such polysiloxanes are available from numerous sources and are generally well understood in the art and will not be otherwise described in detail herein.
Other candidate materials for the adhesive include the bisbenzocyclobutene-based (“BCB”) resins, examples of which are available under the CYCLOTENE brand from Dow Chemical, Midland, Mich. 48674, USA. These BCB resins are formulated as high-solids, low-viscosity solutions and have favorable electronic and mechanical properties. In the visible wavelengths they have a refractive index ranging from about 1.62 (at 400 nanometers) to about 1.55 (at 800 nanometers). They accordingly tend to match well with the transparent carrier substrates described herein.
The diode 10 represents an embodiment in which the n-type layer 12 is adjacent to the carrier substrates 13. This positions the p-type layer 11 closer to the face of the diode 10. Accordingly the ohmic contact to the n-type 12 is designated at 15 and the ohmic contact to the p-type layer 11 is designated at 16. Each of the ohmic contacts may include a respective bond pad 17 and 20, because in many circumstances, the metals that makes the best ohmic contacts are different from the metals that make the best contact to the remainder of the circuit or other devices.
In exemplary embodiments, the diode 10 further includes an active portion such as a multiple quantum well 21, or another emitting structure such as a single or double heterostructure. These structures are generally well understood in the art and will not be described further herein in detail.
In exemplary embodiments the p-type layer 11 and the n-type layer 12 are formed of gallium nitride (GaN) and the multiple quantum well 21 is formed of alternating layers of gallium nitride and indium gallium nitride (InGaN). As is well understood in the art, adjusting the atomic fraction of indium in InGaN (i.e., InxGa1-xN) can tailor emission frequency of the diode to a certain extent, a factor that in practice is balanced against the tendency of increasing amounts of indium to form less stable nitrides.
Because p-type gallium nitride tends to be more resistive than n-type gallium nitride, the ohmic contact 16 to the p-type layer 11 is generally relatively large in order to enhance current flow through the p-type layer 11. Because the contact 16 is relatively large, in the invention it is formed to have a transmittance of at least about 70 percent and preferably as high as 90-100 percent. Accordingly, metal oxide compounds such as indium tin oxide (ITO) are useful for this purpose. As used herein transmittance refers to the difference, expressed as a percent, between the intensity of light that strikes an object and the intensity of the light that emerges after the original light passes through the object.
Of all of the items in the diode that tend to absorb light and thus reduce external quantum efficiency, the bond pads represent the highest absorption. Accordingly, even though in most circumstances reflecting light back into the epitaxial layers is undesired, the epitaxial layers will allow more light to escape then will the bond pads. As a result, reflecting light from the bond pads is almost always preferable, even if the reflected light returns to the epitaxial layers.
The term “lenticular” is used in a relatively broad sense to include carefully patterned surface features such as those set forth in commonly assigned and co-pending applications Publication No. 20060060874 and Ser. No. 11/461,018, filed Jul. 31, 2006 for “Method of Forming 3D Features for Improved Light Extraction,” as well as the more randomly generated features described in commonly assigned and co-pending application Ser. No. 11/343,180 filed Jan. 30, 2006 and published as No. 20060186418. The contents of each of these applications are incorporated entirely herein by reference.
Generally speaking, light tends to travel more readily from a lower index material to a higher index material. Accordingly, it can be counterintuitive to expect the light to go from a high index materials such as a Group III nitride layer to a lower index material such as the carrier substrate.
For this reason, a lenticular surface or interface (whether geometrically structured or chemically developed) tends to offer a greater advantage when light travels from a higher index material to a lower index material. Although not disadvantageous, the lenticular surface has less of a noticeable effect when the refractive index of the material on each side of the interface is about the same or when the light is traveling from a low index material to a high index material. Thus, in general the lenticular interface enhances the transmission of light from a higher refractive index material to a lower refractive index material and as a result is almost always preferred between the Group III nitride epitaxial layers and the low index of refraction carrier substrate.
Accordingly, the invention provides the opportunity to enhance the movement of light across a high-to-low interface by using the lenticular surface at positions that are most favorable toward the desired light extraction. This can, of course depend upon a number of design factors including packaging. Thus, the favorable positions for lenticular surfaces are not absolute, but rather are selected to enhance the light extraction from a given diode in a given circumstance.
The invention is not limited to the incorporation of the lenticular surface, however, because matching the index of refraction of the carrier substrate to that of the encapsulant, standing alone, has benefit. Stated in partial summary, enhancing the movement of light from the epitaxial layers to the transparent substrate (e.g., with a lenticular surface) enhances the external efficiency of the diode. Matching the index of refraction of the substrate to the index of refraction of the encapsulant (lens) likewise enhances the movement of light from the substrate to the encapsulant and thus enhances the external efficiency of the diode. Doing both—i.e., increasing the movement of light from the epitaxial layers to the substrate and matching the refractive indices of the substrate and the encapsulant-further enhances the external efficiency of the diode.
Accordingly,
A first bond pad 41 covers the ohmic contact 36 to the n-type layer 33 and a second bond pad 42 is in contact with the p-type ohmic contact 37.
As in the other embodiments, other portions of the device can also include lenticular surfaces in a manner described with respect to the other drawings.
In
In yet another embodiment, the invention is a method of forming a light emitting diode. In this embodiment, the invention includes the steps of forming (typically by epitaxial growth) respective p-type and n-type layers of Group III nitride on a compatible substrate. Silicon carbide (SiC) is particularly useful as the compatible substrate because it has a closer lattice match to the Group III nitrides then do other substrates such as sapphire or silicon. The invention is not, however, limited to silicon carbide and if other substrate materials are advantageous for purposes other than lattice matching, they can be incorporated provided they do not otherwise interfere with the production of the diode or its function when complete.
Following growth of the epitaxial layers, the compatible substrate is removed from the Group III nitride epitaxial layers and then the Group III nitride epitaxial layers are joined to the transparent carrier substrate with the index of refraction lower than the index of refraction of the adjacent Group III nitride layer.
In exemplary embodiments, a low index transparent adhesive as described previously is used to join the carrier substrate to the epitaxial layers. The invention is not, however, limited to the use of low index adhesives, and other techniques can be used to join the carrier substrate to the epitaxial layers provided they are otherwise consistent with the structure and function of the invention.
As in the other embodiments, the transparent carrier substrate with the low index of refraction comprises a material selected from the group consisting of quartz, fused quartz, glass, and sapphire.
In order to produce the structures described herein, additional features such as multiple quantum wells, superlattices, or single or double heterostructures can be added between the steps of growing the respective p-type and n-type layers on the original compatible substrate.
In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms have been employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.
