High efficiency light emitting diode (LED) with optimized photonic crystal extractor

A high efficiency, and possibly highly directional, light emitting diode (LED) with an optimized photonic crystal extractor. The LED is comprised of a substrate, a buffer layer grown on the substrate (if needed), an active layer including emitting species, one or more optical confinement layers that tailor the structure of the guided modes in the LED, and one or more diffraction gratings, wherein the diffraction gratings are two-dimensional photonic crystal extractors. The substrate may be removed and metal layers may be deposited on the buffer layer, photonic crystal and active layer, wherein the metal layers may function as a mirror, an electrical contact, and/or an efficient diffraction grating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending and commonly-assigned applications:

which applications are incorporated by reference herein.

STATEMENT REGARDING SPONSORED RESEARCH AND DEVELOPMENT

The present invention was made under support from the University of California, Santa Barbara Solid State Lighting and Display Center member companies, including Stanley Electric Co., Ltd., Mitsubishi Chemical Corp., Rohm Co., Ltd., Cree, Inc., Matsushita Electric Works, Matsushita Electric Industrial Co., and Seoul Semiconductor Co., Ltd.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to light emitting diodes (LEDs), and more particularly, to a high efficiency LED with an optimized photonic crystal extractor.

2. Description of the Related Art

A light emitting diode (LED) is a semiconductor device that emits light in a stimulated manner when electrically biased in the forward direction. This effect is a form of electroluminescence.

An LED is comprised of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. When biased forwardly, electrons are injected into the junction from the n-region and holes are injected from the p-region. The electrons and holes release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, depends on the bandgap energy of the materials forming the pn junction.

As semiconductor materials have improved, the efficiency of semiconductor devices has also improved and new wavelength ranges have been used. Gallium nitride (GaN) based light emitters are probably the most promising for a variety of applications. GaN provides efficient illumination in the ultraviolet (UV) to amber spectrum, when alloyed with varying concentrates of indium (In), for example.

Unfortunately, most of the light emitted within a semiconductor LED material is lost due to total internal reflection at the semiconductor-air interface. Typical semiconductor materials have a high index of refraction, and thus, according to Snell's law, most of the light will remain trapped in the materials, thereby degrading efficiency. By choosing a suitable geometry for the LED, a higher extraction efficiency can be achieved.

FIG. 1is a cross-sectional view of a homogeneous light-emitting material10that illustrates that a fraction of the light12emitted inside the material14is inside the escape cone16and can escape the material10, while a large fraction of the emitted light18is trapped and reflected within the material10. In this situation, the reflected light18is referred to as guided light modes, or guided modes, because the light18is confined within the device10and guided transversely within the material10. One method for reducing the effects of the total internal reflection is to create light scattering or redistribution through random texturing of the surface of the device, which leads to multiple variable-angle incidence at the semiconductor-air interface of the device. This approach has been shown to improve emission efficiency by 9-30%, as a result of the very high internal efficiency and low internal losses, which allows many passes for light before it escapes from the device.

FIG. 2is a cross-sectional view of a semiconductor LED20that illustrates this concept, wherein the top surface22of the LED20is textured, the bottom surface24of the LED20comprises a reflector, the air has a refractive index of n=1, and the semiconductor material of the LED20has a refractive index of n=3.5. The textured top surface22of the LED20is used to randomize light trajectories in a geometrical optics approach.

Another method to reduce the percentage of light trapped is to use a Resonant-Cavity LED (RCLED) or Micro-Cavity LED (MCLED). MCLEDs offer opportunities to create solid-state lighting systems with greater efficiencies than existing systems using “traditional” LEDs. As a result of incorporating a gain medium within a resonant cavity, MCLEDs emit a highly compact and directional light beam. The higher extraction efficiency and greater brightness of these devices are the main advantages of these technologies over conventional LEDs.

Extraction efficiency refers to the ability of the photons generated by a particular system to actually exit the system as “useful” radiation. This higher extraction efficiency is, however, limited to values in the 40% range as the micro-cavity structure also leads to very efficient emission into guided modes and leaky modes. Thus, it would be useful if these guided modes could be extracted.

As noted above, guided modes are modes that are guided in the device plane due to the index difference between the structure layers. Leaky modes are radiated through the layers, towards the air or substrate—for instance through a DBR. Leaky modes are usually lost as they undergo multiple total internal reflection at interfaces, travelling back and forth within the device, until their energy is dissipated by various loss mechanisms (e.g., metal mirror loss, free carrier absorption, re-absorption by the active layer which recycles energy by re-emission, although with some losses, etc.).

FIG. 3is a cross-sectional view of a semiconductor LED26that illustrates radiating, guided and leaky modes, wherein the LED26includes a substrate28, a buffer layer30, and an active layer32including quantum wells (QWs)34. Part of the emitted light is extracted36towards the air and forms radiative modes36, part of the emitted light leaks38through the various layers of the device26into the substrate28and forms leaky modes38, and part of the emitted light is reflected40in the active layer32(or in both the active layer32and buffer layer30) and forms guided modes40.

To obtain high efficiency LEDs, it is necessary to optimize direct mode emission36outside the structure, minimize the leaky mode emission, if such modes are present38, and, if possible, re-emit the guided mode emission40. The present invention aims at fulfilling this goal, in structures amenable to easy fabrication.

FIGS. 4A-4Band5A-5B illustrate the structures (FIGS. 4A and 5A) and simulations (FIGS. 4B and 5B) of micro-cavity emission, via a bottom metal or distributed Bragg reflector (DBR) mirror, and a single interface with air at the top, in a gallium nitride (GaN) materials system.FIGS. 4B and 5Bare angular emission diagrams inside the semiconductor, on a log scale, whereinFIG. 4Bshows the emission of the structure ofFIG. 4A, andFIG. 5Bthat ofFIG. 5A. For bothFIGS. 4B and 5B, the left half of the figure shows the transverse magnetic (TM) emission and its right half shows the transverse electric (TE) polarized emission. Only in-plane monochromatic dipoles are assumed.

InFIG. 4A, the structure includes a metal mirror41, and an active layer42including quantum wells43, wherein the structure is a 3λ/4 cavity with the quantum wells43placed at λ/4 (44) of the metal mirror41. InFIG. 4B, arrow45shows emission towards the air, while arrow46shows emission towards the substrate. Also inFIG. 4B, braces47indicate extracted light, and brace48indicates guided modes.

InFIG. 5A, the structure includes a buffer layer49, 7 period DBR mirror50and active layer51including quantum wells52, wherein the structure is a λ cavity with the quantum wells52placed at λ/2 (53) of the 7 period DBR mirror50. InFIG. 5B, arrow54shows emission towards the air, while arrow55shows emission towards the substrate. Also inFIG. 5B, braces56indicate extracted light, braces57indicate leaky modes, and brace58indicates guided modes.

Difficulties are encountered in most materials systems when attempting to obtain large micro-cavity extraction improvements of LEDs.FIGS. 4B and 5Bshow the emission diagrams from which is extracted the expected efficiency of optimized GaN micro-cavity LEDs, and illustrate the following issues:(i) The index contrast for materials epitaxially grown is quite limited, in particular for the very important nitride materials. This is why emission into many leaky modes is seen in the DBR mirror50structure ofFIG. 5A, which make the DBR mirror50structure ofFIG. 5Aless efficient than the metal mirror41ofFIG. 4A.(ii) The displayed efficiencies are only possible because very thin structures were considered, leading to low-order cavities (as described in reference 10 below). It is difficult to obtain such thin active layers. For example, it is usually necessary to grow a thick (several microns) buffer layer of nitride on a substrate before growing good quality material for the active layer. While lifting off the nitride materials (buffer layer and active layer) from the substrate is already a delicate operation, there is, in addition, extreme difficulty in obtaining the thinner layers (i.e., further removing part or all of the buffer layer) bounded by good metal mirrors, which would lead to the excellent performance of the thin metal mirror structure shown inFIG. 4A.

Thus, there is a need in the art for improved LED structures that provide increased light extraction efficiency. In addition, there is a need in the art for improved LED structures that optimize direct emission outside the structure, minimize leaky mode emission, and re-emit guided mode emission. Moreover, there is a need to provide such improved LEDs while retaining a planar structure, in order to render such structures amenable to easy fabrication. The present invention satisfies these needs, and aims especially at the case of LEDs that support numerous guided modes (e.g., LEDs whose thickness can not easily be made thin enough to make them support only one or a few modes). Finally, the present invention can enhance the directionality of light emission, which is a highly desirable property for some applications, such as LCD displays.

SUMMARY OF THE INVENTION

The present invention discloses a high efficiency, and possibly highly directional, light emitting diode (LED) with an optimized photonic crystal extractor. The LED is comprised of a substrate, a buffer layer grown on the substrate (if such a layer is necessary), one or more optical confinement layers, an active layer including emitting species, and one or more diffraction gratings, wherein the diffraction gratings are two-dimensional photonic crystal extractors. The substrate may be removed and metal layers may be deposited on the buffer layer and active layer, wherein the metal layers may function as a mirror, an electrical contact and/or a diffraction grating.

DETAILED DESCRIPTION OF THE INVENTION

Overview

The present invention describes new LED structures that provide increased light extraction efficiency while retaining a planar structure. The new LED structures provide direct emissions outside the structure and, in addition, convert guided light into extracted light using a diffraction grating. This grating may be placed outside the current-injected region of the active layer, or current may be injected into the grating region. Moreover, the diffraction grating is comprised of an array of holes, which may be pierced into the emitting species of the active layer, or only in other layers of the LED. The diffraction grating is a two-dimensional photonic crystal extractor, and the present invention provides improvements over previous implementations of photonic crystal extractors.

The LED is comprised of a substrate, a buffer layer grown on the substrate (if such a layer is needed), an active layer including emitting species, one or more optical confinement layers that tailor the structure of the guided modes in the LED, and one or more diffraction gratings, wherein the diffraction gratings are two-dimensional photonic crystal extractors. The substrate may be removed and one or more additional layers (such as metal or a DBR) may be deposited one or both sides of the LED, e.g., on the buffer layer, photonic crystal and active layer, wherein the additional layers may function as a mirror, electrical contact, and/or diffraction grating.

In order to efficiently excite modes that will be extracted by the photonic crystal, one or more light confining layers are placed around the active layer. The efficiency of the new LED structure is due to the fact that guided light is only (or mostly) emitted into guided modes that will be interacting with the photonic crystal, so that the many guided modes that are usually lost are diffracted outside the device. This is especially important in the case of an LED that supports numerous guided modes, such as a nitride-materials-based LED (which usually has to be several microns thick due to material growth considerations). The new LED structure retains a planar single layer structure making it easily manufacturable at low cost.

In some configurations, the new LED structure also displays highly directional light emission.

Technical Description

FIGS. 6 and 7are top plan and cross-section side views, respectively, of a photonic crystal extractor64, that illustrates the principles of operation for such extractors64. The photonic crystal extractor64includes an active layer or excited region66and a 2-dimensional photonic crystal68having a plurality of holes70. In this extractor64, light extraction72of the guided modes74is performed by diffraction using the 2-dimensional photonic crystal68.

The problem usually encountered is that, to obtain good extraction efficiency, it is necessary to use a very thin active layer with good optical confinement, so that only a few modes are excited by the luminescent species in the excited region66, which are typically semiconductor quantum wells (but may also be other species, such as quantum dots, polymers, metallic clusters, etc.) and then the light is primarily emitted into modes that interact strongly with the photonic crystal68, with which they have a good overlap, so that guided modes that are usually lost are diffracted outside the LED. This is particularly difficult to achieve by direct growth in those materials where a thick buffer layer is required to achieve growth of device quality material, such as nitride materials widely used in blue, green and ultraviolet (UV) LEDs.

In that case, several original measures are needed to counter the degraded extraction performance originating in the multimode properties of the optical structure. These measures aim at ensuring that light is preferentially emitted in those modes that are sufficiently extracted by the photonic crystal. Indeed, in a multimode structure, a number of modes (those with a large effective index) usually have little overlap with the photonic crystal, so that they are weakly extracted.

FIG. 8is a cross-sectional side view that illustrates a structure76comprised of a substrate78, a buffer layer80, an optical confinement layer82, an active layer84including emitting species86and a top metallic contact88, as well as a2-dimensional photonic crystal68having a plurality of holes70. The structure supports various optical modes90,92and94, wherein mode90comprises a guided mode localized in the active layer84region, mode92comprises a guided mode localized in the buffer layer80region and mode94comprises a delocalized guided mode. In this embodiment, the use of the optical confinement layer82(e.g., AlGaN in GaN) can help to: (1) only excite those modes principally localized above the optical confinement layer82(modes90and94); (2) excite modes not localized fully above the optical confinement layer82, but still significantly localized above that optical confinement layer82(mode94). Here, the optical confinement layer82should have an optical index smaller than that of the material forming the LED. The optical confinement layer82may comprise a homogeneous material or a heterostructure (such as a superlattice or even a DBR).

In structures with a substrate78, some other modes are still guided in the structure between the substrate78interface and the optical confinement layer82, but should be only weakly excited (such as mode92) as their overlap with the emitting species86is small. Thus, they carry a smaller fraction of the emitted light, which is a good thing in view of their much poorer extraction efficiency by the photonic crystal68.

Typically, the portion of the active layer84above the emitting species86can be one to a few optical lengths thick (wherein the optical length is the wavelength in the material of the active layer84) from a waveguide that supports such modes as mode90(which means that tunneling of these modes across the layer82is weak). The photonic crystal68must be close enough to the emitting species86(e.g., either the photonic crystals68cross the light emitting species86in the active layer84or the photonic crystals68are situated within one or a few optical lengths from the light emitting species86in the active layer84), so that the modes which receive most emitted light are also strongly extracted by the photonic crystal68.

Depending on the hole70pattern and sizes, and also on the use of mirror layers (either metallic or DBR) placed on top or at the bottom of the structure76, emission can occur to the top or the bottom of the device. Deposition of metal on the photonic crystal68may also enhance significantly its diffractive properties with respect to a simple dielectric photonic crystal.

Alternately, the current-injected active layer region84may coincide or overlap with the photonic crystal68region, as in the structure96shown inFIG. 9. This case offers the advantage that a light generation region coincides or overlaps with the photonic crystals68, so that the guided light is immediately emitted in modes adapted to the photonic crystal68, and does not suffer from reflexion or scattering at an interface of the photonic crystal68region.

Various geometries can be used as for the perforations in the photonic crystal68. The simplest geometries are square or rectangular arrays, illustrated as98and100, respectively, inFIG. 10. More complex geometries also lead to more efficient light extraction, such as Archimedean tilings, illustrated as102inFIG. 10. Finally, even random patterns with a characteristic correlation length in the vicinity of the wavelength can also act as efficient light scatterers. A figure of merit for the grating is that it should diffract light in a large range of incident directions—ideally in all directions. Choice of the photonic crystal lattice can help fulfill this purpose.

In the case where the current-injected active layer84region of the LED is not pierced by the photonic crystal68, as shown inFIG. 8, it is also desirable to design the photonic crystal68extractor to optimally couple the guided light emitted in the excited region to the extracting photonic crystals68. Light reflection is to be avoided, as light re-entering the non perforated region is prone to be reabsorbed. Thus, the photonic crystals68may include a taper comprised of variable hole lines, which can be placed in front of the extracting photonic crystal68region to smooth out the transition. The taper may comprise one or more periods of modified holes, wherein the modified holes have a variable hole depth, as shown in theFIG. 11, which is a cross-sectional side view of the taper effect for the guided modes74achieved by a progressive increase of hole70depths at the beginning of a photonic crystal extractor68, or a variable hole70period, or a variable hole70diameter (air filling factor).

Adjusting the parameters of the holes70allows the device to emit preferentially towards the top or bottom of the structure, as desired. Additionally, reflectors may be placed on one or more sides of the device, so that the device cannot emit light from that side and to re-direct the light emitted towards a desired direction.

Possible modifications of the present invention include the following:The position of the emitting species can be finely tuned inside the structure (on a length scale that is small as compared to the optical length), so as to precisely control its emission properties, such as directionality, spectral lineshape, fraction of light emitted in radiative or guided modes, etc.Additional optimization can be achieved as regards electrical injection, for example, by the use of suitable material layers. The present invention describes only the enhancement of optical properties, and does not describe in detail the design required for proper electrical properties.The photonic crystal characteristics can be varied along the structure, so as to locally modify the emission properties of the photonic crystals. For example, wavelength and extraction properties can be tailored as desired.Additional layers can be added on top of the structure in order to achieve multi color or white light emission, by energy conversion by the fluorophores contained in that layer, as described in cross-referenced applications identified above.Several optical confinement layers can be added in order to tailor further the structure of guided modes in the LED. These layers may have an optical index smaller or higher than that of the main material forming the LED. A smaller index may define separate optical regions in the LED, as was illustrated above, while a higher index may modify the emission pattern of the emitting species (as described in Reference14identified below).

Experimental results are shown inFIGS. 12 and 13.FIG. 12presents an angular-resolved photoluminescence experiment, which illustrates the band structure of a photonic crystal extractor formed on a waveguide supporting numerous guided modes.FIG. 13presents a comparison with a simulation (e.g., the dots onFIG. 13), which shows that some of the bands of the photonic crystal are not extracted (they do not appear in the measurement). With the structure suggested by the present invention, little light is emitted in these bands.

As can be seen inFIGS. 12 and 13from the width of the diffracted lines, diffraction by the photonic crystal is very directional for a given mode and wavelength. Therefore, if the emission lineshape of the emitting species is narrow enough and by suitable tailoring of the guided modes structure, light diffraction by the photonic crystals can occur in a given range of directions, thereby forming a highly directional light source.

Finally,FIG. 14is a flowchart illustrating the fabrication steps performed according to a preferred embodiment of the present invention.

Block104represents the step of forming a buffer layer on a substrate.

Block106represents the step of forming an active layer on the buffer layer, wherein the active layer includes of one or more light emitting species.

Block108represents the step of forming one or more optical confinement layers under or around the active layer, wherein the optical confinement layers tailor a structure of guided modes within the LED.

Block110represents the step of forming one or more diffraction gratings on the optical confinement layer, wherein each diffraction grating is a two-dimensional photonic crystal and the diffraction gratings direct emissions outside the LED and convert guided modes into extracted light. The optical confinement layers enhance light extraction by the photonic crystals, and help to excite those modes localized above the optical confinement layers.

Block112represents the (optional) step of removing the substrate.

Block114represents the (optional) step of depositing metal layers on the buffer layer and/or the active layer, wherein the metal layers may function both as a mirror and/or an electrical contact

The end result of these steps is a high efficiency LED with an optimized photonic crystal extractor. The LED retains a planar single layer structure.

REFERENCES

The following references are incorporated by reference herein:

CONCLUSION