Fully transparent ultraviolet or far ultraviolet light emitting diodes

Abstract

A fully transparent ultraviolet (UV) or far-ultraviolet (FUL) LED is disclosed, wherein all semiconductor layers except the active region are transparent to radiation emitted in the active region. A key technology enabling this invention is a transparent tunnel junction, which replaces the light-absorbing p-GaN and metal mirror p-contact regions currently found in all commercially available UV LEDs. This tunnel junction also allows for the use of a second n-AlGaN current-diffusing layer above the active region (on the p-side of the device), similar to the current-diffusing layer already present below the active region (on the n-side of the device). Therefore, small-area and / or remote p-contact and n-contact regions can be used, and light can be extracted from both the top and bottom sides of the device. This fully transparent semiconductor device can then be encapsulated using a transparent material to form a fully transparent UV or FUL LED with high brightness and high efficiency.

Description

[0001] Related application citation

[0002] This application claims a claim to the following co-pending and co-assigned claims pursuant to 35 USC Section 119(e):

[0003] U.S. Provisional Application No. 63 / 049,801, entitled “Fully Transparent Ultraviolet or Far-Ultraviolet Light Emitting Diode,” was filed on July 9, 2020, by Christian J. Zollner, James S. Speck, Steven P. DenBaars, and Shuji Nakamura, with agent file number G&C30794.0781USP1 (UC 2020-725-1).

[0004] This application is incorporated herein by reference. Background Technology Technical Field

[0006] This invention relates to a novel design for a completely transparent ultraviolet (UV) or far-ultraviolet light-emitting diode (LED). In these devices, all semiconductor layers and other components, except for the active region, are transparent to the wavelength of light generated in the active region. Therefore, maximum light extraction efficiency is achieved, resulting in a high-power ultraviolet emitter. Background Technology

[0008] This invention relates to the fabrication of devices using semiconductor layers based on group III nitrides. As used herein, the term "group III nitride" or simply "nitride" refers to a semiconductor with the chemical formula Ga. w Al x In y B z Any alloy composition of N (Ga, Al, In, B)N semiconductor, wherein:

[0009] 0≤w≤1, 0≤x≤1, 0≤y≤1, 0≤z≤1, and w+x+y+z=1.

[0010] Group III nitride layers can consist of single or multiple layers with varying or gradient compositions, including layers with different (Al, Ga, In, B)N compositions. Furthermore, these layers can be doped with elements such as silicon (Si), germanium (Ge), magnesium (Mg), boron (B), iron (Fe), oxygen (O), and zinc (Zn).

[0011] Group III nitride layers can be grown in any crystallization direction, such as on a conventional polar c-plane, or on a nonpolar plane (e.g., a-plane or m-plane), or on any semi-polar plane (e.g., {20-21}, {20-2-1}, {11-22}, or {10-11}).

[0012] Deposition methods can be used to grow group III nitride layers, including metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy (MBE).

[0013] Group III nitride layers (such as gallium nitride (GaN)) and their ternary and quaternary compounds (AlGaN, InGaN, AlInGaN) incorporating aluminum and indium have been well used in the fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices.

[0014] Furthermore, the development of AlGaN for short-wavelength devices has enabled group III nitride-based light-emitting diodes (LEDs) and laser diodes (LDs) to surpass many other research projects. Therefore, AlGaN-based materials and devices have become the dominant material system for ultraviolet semiconductor applications. Summary of the Invention

[0015] This invention discloses a novel design for a completely transparent ultraviolet (UV) or far-UV LED, thus exhibiting very high efficiency. It is well known that fully transparent LEDs provide the highest light extraction efficiency for visible light devices; however, completely transparent UV LEDs do not exist. This invention discloses the first and only completely transparent UV or far-UV LED, achieved by eliminating all light-absorbing components of the UV or far-UV LED.

[0016] The semiconductor device layers of such LEDs must be completely transparent to the emission wavelength, which is common in existing technologies, except for the p-GaN hole injection layer and active region, which are light-absorbing. Fully transparent ultraviolet or far-ultraviolet LEDs incorporate transparent tunnel junctions instead of p-GaN. A tunnel junction is a highly doped pn junction that operates under reverse bias and injects holes into the p-side of the LED via interband tunneling. This tunnel junction can include polarization-enhanced structures and may incorporate novel structures, such as scandium (Sc)-containing compounds or scandium-containing nitride alloys. Furthermore, this tunnel junction allows the n-type current diffusion layer to be located above the p-side of the device, eliminating the need for a lossy metal mirror and supporting top-side emission in addition to the proven bottom-side emission through the transparent substrate. This is because the metal contact layer with the p-side of the LED can be made much smaller than the emission region, whereas existing technologies require the emission region to be completely covered by metal.

[0017] In this preferred embodiment, the device is encapsulated using a completely transparent material (e.g., quartz, sapphire, or other ultraviolet-transmitting materials) and in a manner that enables both bottom-side and top-side emission. Except for the use of ultraviolet-transmitting materials, the mounting and encapsulation of the device are similar to those of transparent visible-light LEDs in the prior art.

[0018] Multiple devices can be connected together on a transparent substrate to achieve new functionality. In the preferred embodiments given herein, many devices can be connected in series or in a bridge circuit configuration to operate efficiently with standard wall-plug AC power without requiring expensive and bulky conversion electronics and ballasts. Attached Figure Description

[0019] Please now refer to the accompanying drawings, where the same reference numerals always denote corresponding parts:

[0020] Figure 1 This is a flowchart illustrating the steps of manufacturing a transparent ultraviolet LED or far-ultraviolet LED according to an embodiment of the present invention.

[0021] Figure 2A and 2B This is a schematic diagram of a conventional ultraviolet LED.

[0022] Figure 3A , 3B The diagram shows a transparent ultraviolet LED device without any p-GaN or lossy metal mirrors.

[0023] Figure 4A and 4B This is a schematic diagram of a transparent ultraviolet LED mounted on a transparent plate, which is able to emit light from the top and bottom.

[0024] Figure 5A and 5B This is a schematic diagram of a filament-type ultraviolet LED that utilizes a fully transparent ultraviolet LED and supports very high light extraction efficiency.

[0025] Figure 6 This is a schematic diagram of a diode bridge circuit that allows ultraviolet LEDs to use AC power.

[0026] Figure 7A This is a comparison graph showing the relationship between voltage and output power and injection current for deep ultraviolet LEDs packaged using conventional geometry and vertical geometry. Figure 7B It is a photograph of the vertical geometry of an ultraviolet LED; Figure 7C These are photomicrographs of the ultraviolet LED emission pattern taken in a conventional planar (on-chip) geometry, showing a metal contact layer that occupies less than 50% of the emission area. Detailed Implementation

[0027] The following description of preferred embodiments is made with reference to the accompanying drawings, which form part of the description and illustrate specific embodiments in which the invention can be practiced. It should be understood that other embodiments may also be utilized, and structural changes may be made without departing from the scope of the invention.

[0028] Overview

[0029] This invention describes a high-efficiency ultraviolet or far-ultraviolet LED device that is completely transparent, thereby achieving maximum light extraction efficiency. Specifically, the LED has an emission wavelength below 400 nm (UV-A LED), more preferably below 300 nm (UV-B LED), below 280 nm (UV-C LED), and below 230 nm (far-ultraviolet LED).

[0030] Existing technologies in the ultraviolet LED industry utilize various light-absorbing elements, which reduce device efficiency and thus power output. Furthermore, far-ultraviolet LEDs, which hold great promise for skin and eye-safe disinfection applications, are extremely inefficient and unavailable on the market, partly due to the harmful light-absorbing properties of many device elements. This invention addresses these problems by introducing a novel, completely transparent device element to replace the light-absorbing elements in the prior art.

[0031] A fully transparent ultraviolet or far-ultraviolet LED is placed on or above a transparent substrate. In this preferred embodiment, the LED is fabricated on a sapphire substrate because sapphire substrates are low in cost, have excellent optical and structural quality, and are optically transparent throughout the target spectral region. In an alternative embodiment, a semiconductor layer for the LED can be grown on some other substrate and then transferred onto the sapphire substrate.

[0032] High-quality aluminum nitride (AlN) layers can be grown on or over sapphire substrates using various techniques, detailed in the literature and well-established in the industry. The sapphire substrate can be planar, patterned, or nanopatterned, and the AlN or AlGaN buffer layer can include a nanoporous buffer layer to enhance structural properties or achieve lattice-matched layers. The AlN and AlGaN layers can include conventional c-planes or novel semi-polar or non-polar orientations. Semi-polar and non-polar orientations can improve light extraction efficiency, carrier injection efficiency, and quantum efficiency.

[0033] Next, except for the active region, all semiconductor layers of a UV or far-UV LED are optically transparent. Existing technologies typically include substantially transparent layers; however, the p-side of the device usually has a light-absorbing hole injection layer. Almost all currently commercially available UV LED devices contain an absorptive p-GaN hole injection layer because it cannot form good electrical contacts with p-AlGaN, and current diffusion does not occur in p-AlGaN.

[0034] In this invention, hole injection occurs through interband tunneling within the tunnel junction. Tunnel junctions show great promise in ultraviolet LED applications because they eliminate the need for p-GaN (and, as described below, enable more efficient current diffusion architectures). Tunnel junctions can include pn junction structures with heavily doped p-type and n-type layers, superlattices, or graded layers on both sides of the pn junction to improve performance through polarization and bandgap engineering.

[0035] An n-AlGaN current diffusion layer can be deposited above the n-side of the tunnel junction (above the p-side of the LED). The superior electrical properties of transparent n-AlGaN (as opposed to p-AlGaN) allow most of the top side of the device to be completely transparent, with only a small area contacted by a metal ohmic contact layer in a comb or mesh contact structure.

[0036] This method of creating a "buried tunnel junction" structure maintains the p-type conductivity of the p-type layer by preventing passivation or by activating the buried p-type layer after growth. For example, holes can be etched or formed by selectively masking the regeneration of the n-AlGaN current-diffusing layer to allow gas exchange with the buried tunnel junction layer. Utilizing the transparent n-AlGaN current-diffusing layer above the tunnel junction, highly efficient top-side emission is added to the already highly efficient bottom-side emission through the transparent substrate, without the need for a lossy metal mirror.

[0037] Finally, the transparent ultraviolet or far-ultraviolet LED is encapsulated and / or packaged in a completely transparent material, such as quartz, sapphire, zinc oxide (ZnO), or any other desired transparent material. The LED can then be encapsulated and configured in various configurations similar to visible light LEDs to maximize light extraction efficiency. In one possible embodiment achieved with a completely transparent substrate and device architecture, multiple ultraviolet or far-ultraviolet LEDs can be connected in series or in a bridged configuration to directly utilize the AC voltage provided by a conventional wall socket. Another possible embodiment of the transparent design is a filamentary configuration, which enables maximum light extraction in all directions.

[0038] Currently, the availability of UV-compatible sealant encapsulation materials is limited, and their performance and lifespan are unknown. In particular, there are no commercially available and reputable UV sealants that have been proven to withstand high luminous power and high temperatures (above 50°C). Therefore, in this preferred embodiment, no sealant or other encapsulation material comes into contact with the UV LED; instead, the UV LED and the transparent growth substrate are mounted in a transparent fixture, such as a quartz (or other transparent) housing filled with an inert gas that dissipates heat and maintains the reliability of the UV LED.

[0039] Technical Specifications

[0040] Using a transparent substrate allows light to be emitted through the bottom of the substrate. In this preferred embodiment, a sapphire substrate is used. High-quality aluminum nitride (AlN) layers can be grown on or over the sapphire substrate using various techniques detailed in academic literature and well-established in the industry. The sapphire substrate can be planar, patterned, or nanopatterned, and the AlN or AlGaN buffer layer can include a nanoporous buffer layer to enhance structural properties or achieve a lattice-matched layer. For example, nanoporous AlGaN can be used to achieve device layers with low through-dislocation densities, while also serving as a conformal pseudo-substrate layer for lattice-matched growth of active region layers. This reduces the piezoelectric field in the active region, which is considered to degrade device efficiency. Unlike bulk AlN substrates, which are both expensive and absorb light due to impurities, AlN or AlGaN buffer layers containing nanoporous layers are completely transparent. However, these products can also be used if fully transparent AlN substrate wafers are produced in the future. In an alternative embodiment, an absorbing substrate (e.g., AlN or SiC) can be used for growth, and the epitaxial semiconductor layer can then be transferred onto a transparent substrate via wafer bonding.

[0041] Unlike these alternative substrate options, sapphire is currently the preferred option due to its transparency and low cost, and is therefore considered a preferred embodiment of the invention. The back side of the substrate may be roughened before or after growth to increase light extraction from the bottom of the substrate. In this disclosure, the phrase "growth substrate" or "native substrate" is used to refer to the preferred embodiment of a sapphire substrate used as a growth template for semiconductor device layers and also as the final mounting component of an LED in a fixture. This simplifies the process and eliminates the need for light-absorbing adhesives, metal adhesives, or other destructive components. In an alternative embodiment, the sapphire mounting component or base may be a separate sapphire wafer or chip, rather than a sapphire piece used for semiconductor layer growth.

[0042] One of the key technologies of the fully transparent ultraviolet LED or far-ultraviolet LED of the present invention is the fully transparent tunnel junction. The tunnel junction is a heavily doped pn junction that operates under reverse bias, in which electrons tunnel from the valence band on the p side to the conduction band on the n side, thereby injecting holes into the p side of the device.

[0043] A fully transparent tunnel junction can consist of AlGaN and AlN layers, or a very thin GaN layer. Due to carrier confinement, a properly designed GaN layer thinner than a few nanometers cannot efficiently absorb light, thus remaining completely transparent at the target wavelength.

[0044] Furthermore, heavily p-doped graded AlGaN or AlN can be used to form the p-side of the tunnel junction. Due to the difference between spontaneous and piezoelectric polarization between Al(Ga)N layers of different compositions, such layers utilize strong polarization fields to generate two-dimensional or three-dimensional hole-gas regions. These regions are known to produce very good p-AlN ohmic contact regions and are also expected to produce very good tunnel junction layers.

[0045] It is important to design the tunnel junction to be fully transparent and highly electrically efficient, so that the tunnel junction region can include multiple uniform, superlattice, or graded composition layers with various doping levels and thicknesses.

[0046] The most important aspect of the tunnel junction is that, in addition to efficiently injecting holes into the p-side of the device, it allows for the addition of an additional n-type current diffusion layer above the tunnel junction. Since n-AlGaN is highly conductive and completely transparent, this new device design allows LED devices to contain transparent current diffusion layers both below (on the "n-side") and above (on the "p-side") the active region. For the buried tunnel junction to remain effective, the p-type material must remain conductive. Therefore, the n-AlGaN current diffusion layer above the tunnel junction can be patterned (either through masked dry etching followed by growth, or through patterned regrowth above the p-type layer of the tunnel junction) to have openings that allow gas exchange, thereby activating the p-AlGaN. This is another key technology enabling the extraction of light through both the top and bottom of the device.

[0047] Metallic contact layers must be formed at the n-type current diffusion layer on either side of the active region (i.e., adjacent to the emitter region, but not directly above or below it). These metallic contact layers are, of course, light-absorbing. However, due to the current diffusion characteristics of n-AlGaN (as opposed to p-AlGaN or p-GaN), these contact areas can be made small and can be located on the side of the device or designed to minimize light absorption. In this preferred embodiment, the p-side contact area (the metal above the emitter region) is fabricated to be much smaller than the emitter region of the device, making light absorption negligible. This can be achieved using a single very small p-contact pad or by using mesh contacts. In a preferred embodiment of a transparent UV LED, the size of both contact metallization regions (including the n-side and p-side contact areas) should be minimized.

[0048] The fully transparent ultraviolet (UV) or far-UV LEDs of this invention also allow for novel devices comprising many LEDs integrated into a single device. For example, many UV or far-UV LEDs can be connected in series or in a diode bridge configuration to utilize high-voltage AC power typically found in wall-plug power supplies. These series configurations can include planar or filamentary configurations, the latter maximizing light extraction in all directions. Alternatively, the fully transparent device can be encapsulated within an optical waveguide or "light guide" structure for use as a highly efficient point light source in disinfection applications requiring a point light source. However, preferred embodiments do not use encapsulation or adhesive materials in contact with the LEDs, so only the transparent growth substrate contacts the UV LEDs. The use of a sapphire growth substrate avoids the need for encapsulation or adhesives, which can exhibit poor performance or lifespan under high-power UV irradiation and high temperatures.

[0049] In a preferred embodiment, the fully transparent LED is also mounted within a transparent housing, such as a light source or bulb, or other housing. This transparent housing can be made of quartz, specialized UV-grade glass, or any other transparent material. The housing may also be filled with an inert gas, such as argon, nitrogen, or any other desired filling gas, which carries heat away from the device through convection without causing material degradation at high temperatures.

[0050] Process Steps

[0051] This section describes the process steps for one possible embodiment of producing a fully transparent ultraviolet LED. It should be understood that this method can be used to produce other similar devices, or different methods can be used to produce the same device, without departing from the scope of this teaching.

[0052] Figure 1This is a flowchart illustrating the steps involved in manufacturing the fully transparent ultraviolet LED disclosed herein. Similar steps can also be used to produce far-ultraviolet LEDs. The growth method used in the preferred embodiment is MOCVD; however, other methods, including HVPE, MBE, or any other desired growth method, can also be used.

[0053] Box 100 represents the step of growing a transparent buffer layer on a substrate using MOCVD or some other desired technique, which will serve as a template for a subsequent UV LED layer. In one embodiment, the LED layer is grown on a sapphire substrate, wherein the sapphire substrate includes a planar sapphire substrate, a micropatterned sapphire substrate, or a nanopatterned sapphire substrate, or the back side of the sapphire substrate may be roughened. In another embodiment, an alternative substrate may be used, provided that (1) the substrate is completely transparent, or (2) if the substrate is absorbent, it is removed in a later processing step. In one embodiment, the transparent buffer layer may include an AlN buffer layer, or include an AlGaN layer disposed on or in place of an AlN buffer layer.

[0054] Box 102 represents an optional step of electrochemically porousening an AlN or AlGaN buffer layer, which results in the LED layer comprising one or more porous AlN or AlGaN layers. This can be achieved by applying a voltage to the layer while immersing it in a suitable electrolyte solution. It has recently been demonstrated that porous layers improve device quality by serving as conformal layers for lattice-matched device layers, with the porous AlN or AlGaN layer acting as a conformal pseudo-substrate for subsequent growth of relaxation or lattice-matched device layers. It can also improve material quality by reducing dislocation density, with the porous AlN or AlGaN layer serving as a dislocation density reduction structure. This process can improve the structural quality of subsequent device layers without requiring a bulky, light-absorbing AlN substrate. It can also allow for lattice-matched or relaxation pseudo-substrate.

[0055] Box 104 represents the step of growing subsequent device layers, wherein the group III nitride-based ultraviolet LED consists of one or more group III nitride layers, and each group III nitride layer contains at least some aluminum (Al) and nitrogen (N). Multiple different nitride layers can be grown to produce high-efficiency LED devices, including doped layers, active layers, polarization enhancement layers, superlattice or graded layers, or any other desired type of layer.

[0056] For example, consider the following conventional layer sequence: an n-AlGaN current-diffused layer, an AlGaN multiple quantum well (MQW) active region layer, a p-AlGaN or AlN electron blocking layer (EBL), a p-type AlGaN superlattice or a graded or polarized enhanced p-type hole supply layer, a tunnel junction including a heavily doped and / or polarized enhanced p+ tunneling layer and a heavily doped and / or polarized enhanced n+ tunneling layer, and an n-AlGaN current-diffused layer.

[0057] A tunnel junction is a group III nitride tunnel junction used to inject holes into the p-side of an LED. A tunnel junction can include a superlattice, interface, or composition-gradient region that generates spatially varying polarization. The polarization effect of spatially varying polarization enhances the performance of the p-type layer within the tunnel junction; for example, a magnesium-doped AlN layer can be used to form the hole-gas tunnel junction layer. The polarization effect of spatially varying polarization enhances the performance of the n-type layer within the tunnel junction. The polarization effect of spatially varying polarization allows the use of undoped semiconductor layers within the tunnel junction through polarization doping or modulation doping. To enhance polarization, tunnel junction performance, or LED performance, other elements, such as boron (B), scandium (Sc), or any other novel elements, can be introduced into the group III nitride material of the LED.

[0058] There may be one or more holes or openings on the surface of the LED, which expose one or more p-type layers beneath the LED surface, including the p-type layers of tunnel junctions. These holes or openings can activate the p-type layers.

[0059] A transparent current diffusion layer (e.g., n-AlGaN) can be grown on or above the tunnel junction. This transparent current diffusion layer supports remote n-contact regions, thereby allowing light to be emitted not only through the bottom of the LED and the transparent substrate, but also through the top of the LED.

[0060] Box 106 represents the steps involved in manufacturing a UV LED using various processing techniques, including mesa etching, sidewall or surface passivation using oxide or nitride film deposition (e.g., depositing silicon oxide or aluminum oxide layers by sputtering or atomic layer deposition (ALD)), and metal contact deposition, patterning, and annealing, which may be employed as needed.

[0061] For example, a common contact area can be used to form a planar parallel array of diodes. In another possible embodiment, the metallization layer is patterned to form a series or diode bridge configuration.

[0062] Preferably, the total area of ​​the contact metal on the LED is less than 50% of the LED's emitting area. In one example, the total area of ​​the contact metal on or above the p-type layer of the LED comprises an area less than 50% of the LED's emitting area. In another example, the total area of ​​the contact metal on the n-type layer of the LED comprises an area less than 50% of the LED's emitting area.

[0063] In one embodiment, the top and / or bottom surfaces of the LED may be roughened to enhance light extraction from the LED.

[0064] In one embodiment, an LED layer can be grown on a substrate, which is then removed later during device processing.

[0065] Box 108 represents the steps of packaging a device, for example, by dicing the wafer into small pieces (which may include individual LED dies, multi-LED planar arrays, multi-LED filament arrays, or any other desired configuration) and packaging the LED device with a completely transparent package.

[0066] In one embodiment, multiple interconnected LEDs are arranged in parallel, series, or diode bridge configurations while remaining on a transparent growth substrate. LEDs can be connected in parallel to operate at high power and low voltage, or connected in series to operate at high voltage and low current. A diode bridge configuration allows for direct use of high-voltage AC power. Planar geometry can be used to achieve high power output. Linear or filamentary geometry can be used to maximize light output in all directions.

[0067] Finally, if necessary, this step may also include encapsulating the LED in a transparent material, such as quartz, transparent resin, or other transparent materials, and the transparent material may contain an inert gas, including but not limited to argon or nitrogen. The shape of the transparent material can be designed to enhance light extraction; for example, the shape of the transparent material may be an inverted cone or an inverted truncated cone.

[0068] Box 110 represents the final product, namely, at least one fully transparent group III nitride-based LED with an emission wavelength of less than 400 nanometers, wherein the LED layer other than the active region layer is transparent to the emission wavelength.

[0069] In various embodiments, the LED has an emission wavelength of less than 300 nanometers and includes a UV-B LED; and / or the LED has an emission wavelength of less than 280 nanometers and includes a UV-C LED; and / or the LED has an emission wavelength of less than 230 nanometers and includes a far-ultraviolet LED.

[0070] This framework also includes operating such devices in various applications, such as using light emitted by an LED with a specific wavelength and power to act as a germicidal radiation source.

[0071] It should be noted that this embodiment allows for modification, omission, repetition, or addition of steps as needed.

[0072] Device Structure

[0073] Figure 2A and 2B This is a schematic diagram of a UV LED, showing the substrate, semiconductor layer, metal contact area, and base chip. Figure 2A This is a cross-sectional view of a UV LED. Figure 2B This is a planar view of a UV LED. Label 200 indicates a transparent mounting plate or substrate. Label 202 indicates an n-AlGaN current diffusion layer that allows the remote contact region 204 to reach the n-side of the LED (i.e., adjacent to the emitter region, rather than directly above it). Label 206 indicates the active region. Label 208 indicates the p-contact region of the LED, including a light-absorbing p-GaN contact layer that needs to be electrically contacted with the p-side of the device, and a p-side metal mirror (i.e., a metal layer located above the emitter region) whose reflectivity is significantly less than 100%, resulting in light power loss. Because there is no current diffusion layer, a remote contact region cannot be formed, and therefore light cannot be emitted from the p-side (downward) of the device. In other words, the p-side or top-side contact region 208 almost covers the entire emitter region of the device. Label 210 indicates a substrate wafer required in flip-chip processing, which is frequently used. Marker 212 indicates ultraviolet light absorbed at p-contact region 208, and marks 214 and 216 indicate light, where light 214 is reflected by mirror 208, and light 216 is emitted directly upwards. Furthermore, light 214 and 216 can only be extracted in one direction (e.g., upwards), therefore most of the emitted light is not single-pass light extraction, but rather light that has been reflected multiple times, incorporating light absorption losses from the mirror and p-GaN 208.

[0074] Figure 3A , 3B The diagram shows a transparent ultraviolet LED without any p-GaN or lossy metal mirrors, where 3C... Figure 3A This is a cross-sectional view of a transparent ultraviolet LED. Figure 3B This is a plan view of a transparent ultraviolet LED. Figure 3C This is a side view of a transparent ultraviolet LED. Although the device shown is not processed in a flip-chip manner, it is similar to... Figure 2A and 2BIn contrast, it is drawn in an inverted form. Markings 300-306 respectively indicate... Figure 2A and 2B The components indicated by markings 200-206 are the same as those in the diagram, namely, the transparent mounting plate or substrate 200, the n-AlGaN current diffusion layer 202, the n-contact region 204, and the active region 206. Marking 308 indicates a tunnel junction that supports hole injection to the p-side of the device without any light-absorbing layer. Marking 310 indicates an n-AlGaN current diffusion layer that can be grown over the tunnel junction 308.

[0075] Designation 312 denotes the p-side contact region (which is the metal contact region of the n-AlGaN current diffusion layer 310). Due to the current diffusion characteristics of the n-AlGaN current diffusion layer 310, the metal of the p-side contact region 312 (i.e., the metal above the active region 306) can be much smaller than the emitter region, including remote contact pads (as shown) or mesh contact patterns. In an alternative embodiment, the electrical contact region can be made directly at the n-AlGaN current diffusion layer 310 of the tunnel junction 308.

[0076] Because of the tunnel junction 308 and the n-AlGaN current diffusion layer 310, all electrical contact areas 312 can be fabricated remotely (not shown in the figure), and light is emitted from both the top and bottom of the device. Labels 314 and 316 indicate light emitted through the p-side and n-side of the device, and the figure shows that there are no absorption or loss elements in either of the main light emission directions. Although there is some reflection, most of the light is emitted on its first pass, and the light extraction efficiency is very high.

[0077] Figure 3C The side view of the ultraviolet LED includes a semiconductor device 320, a wire bonding region 318, and a sapphire substrate or mounting component 300. The semiconductor device 320 includes, for example, all of the elements 302-312. The wire bonding region 318 can be replaced by photolithographically defined metal leads, indium or other metals, solder-based metallization, or any other desired electrical contact mechanism.

[0078] Figure 4A and 4B This is a schematic diagram of a lamp using fully transparent ultraviolet LED devices, in which... Figure 4A and 4BThese are all cross-sectional views of a fully transparent ultraviolet LED. The device is encapsulated or contained in a transparent container 400 made of quartz, ultraviolet-grade resin, or some other transparent material, filled with an inert gas, such as argon 402. In this preferred embodiment, the ultraviolet LED 404 is retained on a sapphire growth substrate 406, which becomes a transparent mounting plate, thus eliminating the need for adhesives to bond the device 404 to the sapphire growth substrate 406. Metallic wiring can be secured by wire bonding 408, directly patterned into the sapphire growth substrate 406, or implemented using some combination of wire bonding, photolithographic metallization, and soldering. Electrical connections are made using wires 410, and these connections should include direct current for the operation of the individual device.

[0079] exist Figure 4A In this process, light is extracted in two directions, as indicated by label 412. Figure 4B In this configuration, the geometry of the housing 416 causes light to be reflected, achieving unidirectional emission 414. This can be achieved, for example, by optimizing the angles of the walls of the housing 416. In a preferred embodiment, the geometry of the housing 416 is an inverted frustocone, allowing light emission to be directed in a single preferred direction.

[0080] Figure 5A and 5B This is a schematic diagram of a filamentary ultraviolet LED that utilizes a fully transparent ultraviolet LED and supports very high light extraction efficiency. Figure 5A and 5B These are cross-sectional views of filament-type ultraviolet LEDs. In this type of device, many LEDs are connected in series, parallel, or in a diode bridge configuration, allowing the use of any power supply (including high-voltage AC) without the need for a driver circuit. The designations 500-510 respectively indicate... Figure 4A and 4B The components are similar to those shown by markings 400-410, namely, the fixture, container or housing 500 filled with inert gas 502, the ultraviolet LED 504 retained on the native sapphire substrate 506 (the native sapphire substrate 506 becomes a transparent mounting plate), the wire bonding area 508 and the lead 510.

[0081] Each strip of LED filament strips, each containing a plurality of LEDs 504, is placed within a fixing device 500, arranged in such a manner that ultraviolet light emitted from one device 504 is not absorbed by the active regions of adjacent devices 504. Therefore, Figure 5A and 5B The two stripes shown should be arranged in an interlaced geometry (in the direction of the page) so that they do not directly block each other. Furthermore, no ultraviolet-absorbing materials that would hinder light extraction from the transparent casing were used.

[0082] Diode bridge circuit

[0083] Figure 6 This is a schematic diagram of a diode bridge circuit 600, which allows diodes (LEDs) to utilize AC power supply 602 because the two branches of the bridge are alternately switched on. If multiple diodes are connected in series on each branch, such that the total operating voltage of the series circuit is similar to that provided by a high-voltage power supply, the diodes can operate simultaneously from a high-voltage wall-plug power supply to provide high-power output without requiring any power conversion or drive circuitry. The number of diodes in each bridge, the number of parallel bridges, and all other details of circuit 600 may differ from those shown in this diagram, which should be understood as a conceptual sketch for illustrative purposes and not as a circuit diagram or design.

[0084] Experimental data

[0085] Figure 7A , 7B And 7C showed with Figure 4A Experimental data for devices similar to the one shown are presented. This device includes a semi-transparent p-side metallization region instead of a tunnel junction to demonstrate the benefits of this novel device geometry. Using a vertical mounting scheme with bidirectional light emission, the output power of the deep ultraviolet device is increased by two times.

[0086] Specifically, Figure 7A This is a comparison of the voltage and output power versus injection current for deep ultraviolet LEDs packaged using conventional and vertical geometries. The new vertical geometry increases light output power by 100%. Both devices in this data set use thin, semi-transparent metal contacts for demonstration purposes; with the fully transparent tunnel junction contacts and / or advanced packaging disclosed below, the enhancement in light extraction is expected to be much greater.

[0087] Figure 7B It is a photo of the vertical geometry of a UV LED. Figure 7C These are photomicrographs of the ultraviolet LED emission pattern taken in a conventional planar (on-chip) geometry, showing that the metal contact area accounts for less than 50% of the emission area.

[0088] Advantages and improvements

[0089] This invention discloses a fully transparent ultraviolet LED or far-ultraviolet LED device. Existing ultraviolet LEDs do not use a fully transparent device layer, nor do they use a fully transparent electrical contact layer or encapsulation material.

[0090] Light absorption by UV LED components is harmful for two reasons: first, because it reduces light extraction efficiency, thereby reducing the overall wall-mount efficiency of the device; and second, because all light absorption processes result in: (1) heat generation, which must be controlled at the system level, or (2) structural and photodegradation, as is the case with conventional organic encapsulation materials, when exposed to UV light, or (3) a combination of heating and degradation.

[0091] Organic materials exist in ultraviolet devices and are used as adhesives or sealants, but their lifespan and performance are limited. If these organic materials are removed and only completely transparent inorganic materials (such as sapphire, quartz, or other highly transparent materials) are used, then a significant reduction in light absorption and an improvement in device reliability can be achieved.

[0092] Another detrimental region of light absorption is on the p-side of the diode structure, and it limits the performance of all currently commercially available ultraviolet LED devices. As mentioned earlier, light-absorbing p-contact elements are necessary for hole injection in conventional structures, but they cannot generate effective hole injection in far-ultraviolet devices, which is why far-ultraviolet LEDs are not yet commercially available.

[0093] There is a need for a fully transparent ultraviolet LED to improve device efficiency and lifespan, and a fully transparent far-ultraviolet LED to enable the technology to achieve much-needed market applications in skin and eye-safe disinfection. The key technology for realizing fully transparent ultraviolet or far-ultraviolet LEDs is the transparent tunnel junction. The tunnel junction replaces the light-absorbing p-GaN and metal mirror contact structure with a completely transparent and highly conductive n-AlGaN layer. The highly conductive n-AlGaN is the material that provides current diffusion on the n-side of the device, supporting the remote n-contact region. Therefore, in addition to efficiently injecting holes, the tunnel junction allows the introduction of n-AlGaN on the p-side of the device, enabling current diffusion and a very small remote p-contact region (i.e., the metal above the emitter region).

[0094] The contact metal absorbs LED light from the emitting region, therefore, the smaller the area of ​​the contact metal, the better. The area of ​​the contact metal refers to both the n-type and p-type ohmic contact areas. The total area of ​​the contact metal in both the n-type and p-type ohmic contact areas should be minimized to minimize the absorption of LED light by the metal. This is especially true for the metal contact area in the p-type region located above or above the emitting layer, which should be minimized as much as possible.

[0095] Finally, fully transparent ultraviolet and far-ultraviolet LED devices enable novel device architectures that incorporate arrays of planar or filamentary devices. For example, devices can be connected in series or in a diode bridge configuration to directly utilize the high-voltage AC power supplied to most conventional wall sockets, without requiring expensive and bulky electronics for AC-DC conversion, thermal management, etc.

[0096] References

[0097] The following patents are incorporated herein by reference:

[0098] (1) U.S. Patent No. 7,687,813B2, entitled “STANDING TRANSPARENTMIRRORLESS LIGHT EMITTING DIODE”, was granted to Nakamura et al. on March 30, 2010.

[0099] (2) U.S. Patent No. 7,781,789B2, entitled “TRANSPARENT MIRRORLESSLIGHT EMITTING DIODE”, was granted to DenBaars et al. on August 24, 2010.

[0100] (3) U.S. Patent No. 8,294,166B2 entitled “TRANSPARENT LIGHTEMITTING DIODES”, granted to Nakamura et al. on October 23, 2012.

[0101] in conclusion

[0102] The description of preferred embodiments of the invention is summarized herein. The foregoing description of one or more embodiments of the invention is given for illustrative purposes. This description is not exhaustive and is not intended to limit the invention to the precise forms disclosed. Many modifications and variations can be made based on the foregoing teachings. The scope of the invention is not limited by this detailed description, but only by the appended claims.

Claims

1. A light-emitting device, comprising: At least one group III nitride-based ultraviolet (UV) light-emitting diode (LED) having an emission wavelength of less than 400 nanometers, wherein: LEDs consist of a group III nitride layer comprising at least some aluminum (Al) and nitrogen (N); Except for the active layer, the group III nitride layer of the LED is transparent to the emission wavelength; and A tunnel junction composed of a p-type AlN layer and an n-type AlGaN layer injects holes into the p-side of an LED, wherein the p-type AlN layer forms the hole-gas tunnel junction layer of the tunnel junction.

2. The light-emitting device as claimed in claim 1, wherein the total contact area of ​​the LED with the metal is less than 50% of the emitting area of ​​the LED.

3. The light-emitting device as claimed in claim 1, wherein the total area of ​​the contact metal on the p-type layer of the LED includes an area less than 50% of the emitting area of ​​the LED.

4. The light-emitting device as claimed in claim 1, wherein the total area of ​​the contact metal on the n-type layer of the LED includes an area less than 50% of the emitting area of ​​the LED.

5. The light-emitting device of claim 1, wherein the tunnel junction includes a region that produces spatially varying polarization.

6. The light-emitting device of claim 5, wherein the polarization effect of the spatially varied polarization enhances the performance of the p-type layer within the tunnel junction.

7. The light-emitting device of claim 5, wherein the polarization effect of the spatially varied polarization enhances the performance of the n-type layer within the tunnel junction.

8. The light-emitting device of claim 5, wherein the polarization effect of the spatially varying polarization enables the use of an undoped semiconductor layer within the tunnel junction through polarization doping or modulation doping.

9. The light-emitting device of claim 1, wherein one or more holes or openings on the surface of the LED expose one or more p-type layers beneath the surface of the LED, including the p-type layers of a tunnel junction.

10. The light-emitting device of claim 1, wherein a transparent current-diffusion layer composed of n-AlGaN is grown on top of the tunnel junction.

11. The light-emitting device of claim 10, wherein the transparent current diffusion layer supports a remote p-contact region, thereby enabling light to be emitted through the top of the LED in addition to through the bottom of the LED and the transparent substrate.

12. The light-emitting device of claim 1, wherein the LED layer is grown on a sapphire substrate.

13. The light-emitting device of claim 12, wherein the sapphire substrate comprises a planar sapphire substrate or a patterned sapphire substrate.

14. The light-emitting device of claim 12, wherein the back side of the sapphire substrate is roughened.

15. The light-emitting device of claim 1, wherein the top and / or bottom surfaces of the LED are roughened.

16. The light-emitting device of claim 1, wherein the LED layer is grown on another substrate, which is removed during device processing.

17. The light-emitting device of claim 1, wherein the LED layer comprises one or more porous AlN or AlGaN layers.

18. The light-emitting device of claim 1, wherein the at least one LED comprises a plurality of interconnected LEDs.

19. The light-emitting device of claim 18, wherein the plurality of interconnected LEDs are connected to a diode bridge circuit so that a high-voltage AC power supply can be directly used for the plurality of interconnected LEDs.

20. The light-emitting device of claim 1, wherein the LED is mounted within a transparent material and the transparent material contains an inert gas.

21. A semiconductor manufacturing method, comprising: Fabricate at least one group III nitride-based ultraviolet (UV) light-emitting diode (LED) with an emission wavelength of less than 400 nanometers, wherein: LEDs consist of a group III nitride layer comprising at least some aluminum (Al) and nitrogen (N); Except for the active layer, the group III nitride layer of the LED is transparent to the emission wavelength; and A tunnel junction composed of a p-type AlN layer and an n-type AlGaN layer injects holes into the p-side of an LED, wherein the p-type AlN layer forms the hole-gas tunnel junction layer of the tunnel junction.

22. A semiconductor manufacturing method, comprising: Operate at least one group III nitride-based ultraviolet (UV) light-emitting diode (LED) with an emission wavelength of less than 400 nanometers, wherein: LEDs consist of a group III nitride layer comprising at least some aluminum (Al) and nitrogen (N); Except for the active layer, the group III nitride layer of the LED is transparent to the emission wavelength; and A tunnel junction composed of a p-type AlN layer and an n-type AlGaN layer injects holes into the p-side of an LED, wherein the p-type AlN layer forms the hole-gas tunnel junction layer of the tunnel junction.

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