Light emitting devices with an electrically active top reflector contact

According to one described embodiment, a light emitting device structure includes an epitaxial contact layer disposed on an active region of the light emitting device structure, a multi-layer reflector disposed at least partially on the epitaxial contact layer, and conductive contacts abutting the epitaxial contact layer, the multi-layer reflector enclosing the conductive contacts.

BACKGROUND

This disclosure relates generally to light emitting devices. Particularly, this disclosure relates to a light emitting device with an electrically active top reflector contact.

The ability to efficiently extract light from light emitting device structures is always a key consideration in their design. For some light emitting devices, such as Ultra-Violet Light Emitting Diodes (UV LEDs), light is usually extracted from the wafer backside because one or more layers above the light-generating active layer are opaque or light-absorbing. It is therefore desirable to place a reflector above the top layers so light travelling upwards can be reflected downwards toward the bottom output. It is also important that the LED incorporate a top-side electrical contact so that flip-chipped arrays of closely spaced emitters can be formed. Unfortunately, conventional reflectors usually incorporate dielectrics that block electrical current.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The presently described embodiments disclose device structures that have highly reflective top layers to enhance the light extraction efficiency of light emitting devices such as LEDs. According to the presently described embodiments, the light extraction efficiency of backside-emitting nitride LEDs can be significantly improved if such top contact layers are made highly reflecting—without significantly compromising their current spreading ability. This disclosure proposes device structures that provide such dual functionalities.

In one form of the presently described embodiments, an LED structure utilizes a mesh or grid pattern as a part of a top metal or conductive layer. The mesh or grid pattern allows current to be distributed across the top contact area while providing open spaces between the grids on which a highly reflective mirror can be formed. High reflectivity for the top layer is achieved by improving (e.g., optimizing) the relative fill factor of open areas to alloyed mesh contact areas. The alloyed mesh or grid pattern allows electrical current to be distributed across the entire top contact area.

In another form of the presently described embodiments, a reflector of an LED structure utilizes a dielectric layer followed by a metal layer. An electrical contact is provided between the annealed LED contacts and the metal layer of the reflector. Since the metal layer of the reflector may distribute electric current across the entire top contact area, the design of the metal contact regions may be relaxed, permitting non-grid or non-mesh contact designs. For example, isolated islands of contact metal may be surrounded by a sea of reflector dielectric material. This flexibility in the design of the metal contact regions allows for greater variation in the fill factor ratio, which for purposes of this disclosure is defined as the ratio of the area of the reflector regions to the area of the contact regions.

FIG. 1is a cross-sectional diagram that illustrates an embodiment of an LED. Referring now toFIG. 1, a light emitting diode100includes a substrate layer102and a template layer104. The substrate layer102may be formed of a variety of different materials, including sapphire. Likewise, the template layer104may be formed of a variety of different materials, including aluminum nitride or an alloy of aluminum gallium nitride.

A plurality of epitaxial layers comprises a UV light emitting diode, including, but not limited to: a light-emitting active region107, an n-doped epitaxial layer106, and a p-contact layer114. The details of LED epitaxial layers such as epitaxial layers106,107, and114are known in the art (see, for example, U.S. Pat. Nos. 6,233,265 and 6,618,413, both of which are incorporated herein by reference) and so are not described here. In one embodiment, a conductive (or metal) contact or layer112comprises a layer of nickel (Ni) followed by gold (Au). The layer stack is then heated and made to alloy with an underlying GaN p-contact layer114to form an Ohmic p-contact. The n-contact metal108can be a stack comprising of titanium (T) followed by aluminum (Al). Like the p-contact, an Ohmic n-contact is formed by heating and alloying the n-contact metal108into an underlying n-doped epitaxial layer106.

Notably, the metal layer108has an n-contact pad110formed thereon to facilitate device packaging. As shown, the p-contact metal or conductive layer112is provided with at least one aperture (e.g., a plurality of apertures) that defines a mesh pattern or grid. A reflector116, aligned with the mesh pattern is also provided to the devices.

To form the device ofFIG. 1, the conductive layer112is deposited on the epitaxial layer114such that the conductive layer has defined therein a contact area having a plurality of apertures. The conductive layer112may be formed of, for example, a metal or a conductive oxide. A reflector116is then formed on the metal conductive layer112, the reflector being aligned with the plurality of apertures of the contact area. The reflector116may take a variety of forms but, in one form(s), can be a simple non-alloyed metal such as silver, aluminum or gold. The reflector116may also be comprised of a dielectric, such as SiOx, SiNxor ITO, and a metal formation, such as a cap, to further enhance reflectivity. Alternatively, Distributed Bragg Reflectors (DBRs) consisting of alternating layers of dielectric pairs can be employed.

The reflectivity of the reflector116could be improved, e.g. optimized, by choosing appropriate thicknesses of reflector materials. For a 325 nm UV LED, for example, various reflector materials and thicknesses may be selected. The thickness of the reflector materials may vary as a function of the presence of, in one embodiment, an absorbing 20 nm thick GaN p-contact layer114. This GaN top layer is considered by many to be essential for adequate electrical contacting.

FIG. 2is a simplified model for reflectivity calculations for a 325 nm UV LED such as the one illustrated inFIG. 1. As shown inFIG. 2, the 20 nm thick GaN p-contact layer114is disposed between the reflector116and the upper surface of the light-emitting active region107.

Different reflector designs employing a variety of different materials can be used. For example, without a reflector, only about 11% of upward-directed light is reflected. A simple reflector116consisting of an unalloyed metal capable of reflecting light of the wavelength emitted by the LED active region such as Al, Ag, or Au significantly increases reflectivity. Such a metallic reflector can be easily evaporated and has the additional advantage of being electrically conducting, so current distribution is improved. A 300 nm layer of Au, for example, boosts the reflectivity at the reflector region to about 56%.

The performance of simple metal reflectors can be improved by inserting a dielectric material transparent to the wavelength of light emitted by the LED active region such as SiO2, ZrO2, HfO2, Si3N4, TiO2, Ta2O5or Al2O3between the top metal of the reflector116and the GaN surface of the p-contact layer114. The percentages of elements in the dielectrics are nominal values. The actual compositions can vary from the nominal values depending on material deposition methods and conditions. For these reasons, SiO2is sometimes labeled as SiOx, Si3N4is sometimes labeled as SixNy, and so on. For example, a 0.4-lambda thick SiO2(54.8 nm thick for λ=325 nm LEDs) followed by a 300 nm thick Au increases the reflectivity to about 90%. The preferred dielectric material and its corresponding optimized layer thickness would differ from material to material and from design to design, so it will have to be determined for each device structure chosen. A transparent conductive film such as Indium Tin Oxide (ITO) or Zinc Oxide (ZNO) can be used in place of the dielectric to improve current spreading.

Distributed Bragg Reflectors (DBRs) consisting of pairs of alternating materials such as SiO2, ZrO2, HfO2, Si3N4, TiO2, Ta2O5or Al2O3can also be employed. Again, the percentages of elements in the dielectrics are nominal values. The actual compositions can vary from the nominal values depending on material deposition methods and conditions. A 5-pair quarter-wave thick SiO2/ZrO2DBR produces a reflectivity of about 55% at the design wavelength of 325 nm. The GaN contact layer114shifts the reflectivity spectrum of the DBR so the maximum reflectivity occurs at a slightly longer wavelength than the design wavelength. Increasing the thickness of each layer in every pair to 0.28-λ shifts the reflectivity spectrum so a maximum reflectivity of 65% results at the design wavelength of 325 nm.

Other types of reflectors116can be designed. For example, a metal cap can be placed above a dielectric DBR, or an optical phase shifting dielectric layer can be placed between the GaN contact layer and a DBR. It should also be understood that if reflectivity of the reflector116is selected to be sufficiently high, the light emitting device contemplated herein may take the form of a surface-emitting laser. Table 1, which appears below this paragraph, summarizes the reflectivity results for the reflector designs that were discussed above.

FIG. 3is a cross-sectional diagram that further illustrates the LED100ofFIG. 1. Many of the elements illustrated inFIG. 1are also shown inFIG. 3. For convenience, a description of these common elements will not be repeated here. Also for convenience, the template layer104and the GaN p-contact layer114ofFIG. 1are not shown inFIG. 3.

InFIG. 3, the mesh or grid-shaped conductive layer112is shown as being electrically connected to a p-contact pad122, which is in a region adjacent to the LED active region107. This lateral electrical connection is employed because oftentimes an electrical connection through the reflector116is not assured. Additionally, a dielectric layer120is employed to prevent electrical shorting between the p-contact pad122and the n-contact layer108.

In another form of the presently described embodiments, methods and structures are presented for improving the specific reflector design that uses a dielectric layer followed by a metal cap. As can be seen in Table 1, the reflector design that uses a layer combination of 0.4λ-thick SiO2followed by a 300 nm layer of Au offered the best reflectivity (up to 90%).

It is also conceivable that other materials may be used instead. For instance, the SiO2layer may be replaced with other dielectric materials transparent to the wavelength of light emitted by the LED active region such as ZrO2, HfO2, Si3N4, TiO2, Ta2O5or Al2O3. Additionally, in the case of UV LEDs, the Au film may be replaced with other metals that exhibit high reflectivity to UV light such as Al, Ag, or Rh. The optimal thicknesses of the reflector layers are likely to change as other materials are substituted for SiO2and Au.

The excellent reflectivity provided by dielectric/metal design and the ease of deposition when compared with the multiple layer configuration of DBR mirrors, makes this a preferred choice for forming a top reflector on LEDs. However, this reflector design should be used in combination with annealed metal contacts that provide an ohmic contact to the LED active region107. According to presently described embodiments, the dielectric/metal reflector design is improved by providing an electrical contact between the annealed metal contacts and the metal layer of the reflector116. This simplifies the delivery of current to the contacts while preserving the characteristics of the reflector116on the top surface of the LED.

FIG. 4is a cross-sectional diagram that illustrates an alternative embodiment of an LED400. Many of the elements illustrated inFIG. 1are also shown inFIG. 4. For convenience, a description of these common elements will not be repeated here. Also for convenience, the template layer104and the GaN p-contact layer114ofFIG. 1are not shown inFIG. 3.

Referring toFIG. 4, the reflector116includes a dielectric layer132and a conductive layer134disposed on the dielectric layer. Thus, the reflector116can achieve the excellent reflectivity characteristics as indicated, for example, by the third row of Table 1. Additionally, annealed metal contacts136penetrate the dielectric layer132to contact the top surface of the LED active region107. The upper portions of the annealed metal contacts136are covered by the conductive layer134. Therefore, an electrical contact exists between the annealed metal contacts136and the conductive or metal layer134of the reflector.

Arranging the metal contacts136in the manner shown inFIG. 4permits electrical contact to be established from the top surface of the metal layer134rather than from an adjacent electrical contact region as seen inFIG. 3, which provides denser LED packing on a chip because valuable chip real estate is not taken up by a lateral contact line and contact electrode placed to the side of the LED. Additionally, because the conductive layer134of the LED400does not extend over the n-contact metal108, there is no need for the dielectric layer120as shown inFIG. 3to prevent electrical shorts. Making electrical contact from the top surface of the LED also improves compatibility with flip-chip packaging processes, reduces the line resistance, and enhances heat extraction from the LED active region.

FIGS. 5 and 6are flow diagrams illustrating some processes included in embodiments of methods of fabricating the LED reflector illustrated inFIG. 4. Referring toFIG. 5, a method500begins with the process510of depositing a metal layer on a LED active region. Next, the metal layer is patterned to form the conductive LED contacts in process520. The LED contacts are annealed in process530, after which the dielectric layer is deposited on the resulting structure in process540. The dielectric layer is then etched in an etch-back process550until the upper surfaces of the LED contacts are exposed. Next, in process560, another metal layer, corresponding to the conductive layer134ofFIG. 4, is deposited on the dielectric layer and the LED contacts.

As an alternative to method500, method600ofFIG. 6begins with depositing a dielectric layer on the LED active region in process610. Next, in process620, the dielectric layer is patterned down to a top surface of the LED active region, and in process630, a contact metal layer is deposited on the patterned dielectric layer. In process640, the contact metal layer is patterned such that regions of dielectric material are disposed between regions of contact metal, creating the desired pattern of LED contacts on the LED active region. The LED contacts are annealed in process650, and another metal layer corresponding to the conductive layer134ofFIG. 4is deposited on the dielectric layer and the LED contacts in process660. The various available processes for depositing, etching, patterning, and annealing material layers are well known to those of skill in the art and will not be described in additional detail here.

The structures and methods according to the latter-described embodiments simplify the electrical contacting process while permitting light reflection from the top contact. As opposed to the embodiments that utilize the mesh or grid shaped contacts, the latter-described embodiments give greater latitude in the design of the contact regions as they need not be connected to one another in the plane of the contact because they are connected by posts to the conductive layer134of the reflector116.

Because the latter-described embodiments give greater flexibility in the design of the contact regions, the contact regions may be arranged in any number of ways to achieve different fill factor ratios.FIGS. 7A,7B, and7C are schematic top-view diagrams of example reflectors that illustrate a few of the embodiments in which the fill factor ratio can be modified. In each of theFIGS. 7A,7B, and7C, the clear regions within the extent of the conductive layer134represent areas that are reflective regions of the reflector116, while the cross-hatched regions represent the contact regions136of the reflector116. Accordingly, each of the reflectors116illustrated inFIGS. 7A,7B, and7C have fill factor ratios of substantially 1:1, 3:1, and 1:3, respectively. Of course, many other fill factor ratios can be achieved using different patterns or feature dimension for the contact regions136of the reflector116.