Patent Application
20050179042
In general, gallium nitride (GaN)-based light emitting devices are based on a p-n diode structure including n-type and p-type GaN-based semiconductor layers stacked on top of a substrate through a process of epitaxial or domain epitaxial growth. A GaN-based light-emitting device with a traditional single p-n diode structure has been widely utilized as an efficient light source for realizing such colors as blue, green, and white. However, as GaN-based light-emitting devices become utilized in diverse applications, it becomes challenging for a traditional single p-n diode structure to meet the requirements of these diverse applications.
One of the requirements is protection of light-emitting devices against electro-static discharge. Electro-static discharge (ESD) is the buildup of electrical energy and its sudden release. Typically, in the light-emitting devices, ESD is caused by the imbalance of electrical charges at interfaces between the light-emitting devices and other external environments. This ESD can cause a catastrophic device failure by an electrical overstress, causing a larger amount of current to flow through the device than it can tolerate. In order to minimize the failure of devices due to ESD events, a proper protection from an ESD event up to at least a certain energy level is required. A zener diode made of silicon is widely used for ESD protection in various electronic devices. A structure connecting a light-emitting diode with a silicon zener diode has been proven effective, resulting in superb ESD protection in many industry practices. Using two discrete components to provide ESD protection, however, requires an additional bonding process during the device packaging between the zener and light-emitting diodes. Conventionally, the additional bonding, e.g., between the light-emitting and zener diodes, has been achieved either by a wire-bonding connecting the electrodes of the two p-n diodes with metal wires or by a bump-bonding where the connection is made by metal bumps. However, these existing approaches increase complexity and cost of packaging processes.
Another requirement is to increase overall light output from a single light-emitting device package. To achieve this goal, one method may be to use multiple light-emitting devices in series. Another method may be to enhance light extraction out of individual light-emitting devices.
One example of the first approach in the art is to connect multiple discrete light-emitting devices in series. However, as discussed above, connecting discrete light-emitting devices by a wire-bonding process increases complexity and cost of packaging processes.
With respect to the second approach, it is challenging to increase light extraction out of a light-emitting device due to intrinsic physical properties of semiconductor materials. In general, GaN-based light-emitting devices are composed of multiple semiconductor layers grown on top of a substrate. The semiconductor layers of light-emitting devices form interfaces with surrounding materials, including the substrate, air, and the encapsulating epoxy typically used in packaging. Due to large differences in the indices of refraction between the GaN-based semiconductor layers and the surrounding materials, a majority of the light generated within the device is trapped within the semiconductor layers, being sandwiched between the substrate and the encapsulating epoxy or surrounding air. This phenomenon is described as total internal reflection. Due to this phenomenon, it is very difficult to extract light from the device effectively. Typically, efficiency of a traditional GaN-based light-emitting device is less than 10%.
An integrated light-emitting device of the present invention includes multiple p-n diodes, either all light-emitting diodes or light emitting diode(s) in combination with ESD-protection diode(s), integrated monolithically on a single substrate.
One aspect of the present invention includes an integrated device comprising an insulating substrate and multiple p-n diodes of monolithic semiconductor materials over the insulating substrate. Preferably, the monolithic semiconductor materials are GaN-based semiconductor materials.
In one embodiment, the integrated device of the invention includes at least one light-emitting device of monolithic semiconductor materials over the insulating substrate. Each light-emitting device includes a light-emitting diode and an electro-static discharge protection diode, which are formed from the monolithic semiconductor materials. The light-emitting diode and electro-static discharge protection diode are interconnected with each other through electrodes of opposite polarities of the light-emitting and electro-static discharge protection diodes.
In another embodiment, the integrated device of the invention includes a plurality of light-emitting devices of monolithic semiconductor materials over the insulating substrate. Each of the light-emitting devices includes a light-emitting diode. The light-emitting devices are connected in series through electrodes of opposite polarities of the light-emitting diode component of the light-emitting devices.
The present invention also includes a method of producing an integrated device. The method comprises depositing a set of semiconductor layers over an insulating substrate to produce a monolithic p-n junction structure. From the monolithic p-n junction structure, multiple electrically-isolated p-n diode structures are formed. Electrodes on the p-n diodes are formed on the p-n diode structures to produce p-n diodes. The method also includes interconnecting electrodes of opposite polarities of the p-n diodes.
In one embodiment, the method of the invention produces at least one light-emitting device that includes a light-emitting diode and an ESD-protection diode. The light-emitting and electro-static discharge protection diodes of the light-emitting device are interconnected with each other through electrodes of opposite polarities of the light-emitting and electro-static discharge protection diodes.
In another embodiment, the method of the invention produces a plurality of light-emitting devices that includes a light-emitting diode. The light-emitting devices are connected in series through electrodes of opposite polarities of each of the light-emitting diodes.
Yet another aspect of the present invention includes a light-emitting device comprising a substrate and a light emitting diode over the substrate, where at least one sidewall of the light-emitting diode is beveled.
The present invention also includes a method of producing a light-emitting device having at least one beveled sidewall. The method comprises depositing multiple semiconductor layers over a substrate to produce a p-n junction structure. Using the p-n junction structure, a light-emitting diode structure is formed. A bevel is formed on at least one sidewall of the light-emitting diode structure. A light-emitting diode having at least one beveled sidewall is produced by forming electrodes on the light-emitting diode structure. In some embodiments, beveling at least one sidewall of the light-emitting diode may be performed after the electrodes of the light-emitting diode structure are formed.
With the integrated light-emitting devices of the invention, multiple p-n diodes, either all light-emitting diodes or light-emitting diode(s) in combination with ESD protection didoe(s), are monolithically integrated on a single insulating substrate. The present invention, thus, provides much simpler and more reliable solution than the use of multiple discrete p-n diodes connected by wire-bonding or bump-bonding. Also, the present invention is advantageous over the conventional integration of discrete p-n diodes by wire-bonding or bump-bonding, because multiple p-n diodes are integrated monolithically before packaging, reducing the total number of terminals connected in the package.
In addition, by beveling at least one sidewall of light-emitting device, enhanced light extraction can be achieved. In particular, the integrated device of one embodiment of the invention, where multiple light-emitting devices that includes a light-emitting diode and an ESD protection diode, are monolithically integrated in series on a single substrate can incorporate the beveled sidewall(s). With such a device, efficient protection against ESD and enhanced overall light output can be achieved not only due to enhanced light extraction from individual light-emitting diodes but also due to multiplied light output by the number of total light-emitting diodes. Also, with the method of the present invention, beveling the sidewall(s) of light-emitting diode(s) can be performed while forming electrically-isolated p-n diodes (e.g., light-emitting diode(s) and ESD protection diode(s)) or p-n diode structures. That is, while isolating p-n diodes or p-n diode structures electrically from each other, beveling the sidewall(s) of light-emitting diode(s) can be made simultaneously, reducing processing steps.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
In general, a light-emitting device contains multiple semiconductor layers grown epitaxially on a substrate, such as sapphire. Alternatively, the semiconductor layers can be grown domain-epitaxially as described in U.S. 2004/0072381 A1, the entire teachings of which are incorporated herein by reference. The growth of semiconductor layers can be achieved by a number of widely-known crystal growth techniques in the art, including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). The epitaxial layers are stacked in such a way to form a vertical p-n junction structure, which is typically achieved by stacking n-type layers and then p-type layers in sequence on top of a substrate. The device described in the present invention can have light emission either from the top surface of the semiconductor layers or from the bottom of the substrate. Preferably, the device of the present invention is a gallium nitride (GaN)-based device.
As used herein, “monolithic semiconductor materials” means that the semiconductor materials are formed as single-crystalline materials on a common substrate. A “monolithic p-n junction structure,” as used herein, means a p-n junction structure that is formed from the monolithic semiconductor materials. Similarly, a “monolithically integrated” device, as used herein, means a device where multiple sub-devices, which are fabricated from the same epitaxially or domain-epitaxially grown semiconductor layers and share the same substrate, are fabricated into a single chip.
To fabricate multiple p-n diodes monolithically on a single substrate, a set of monolithic multiple semiconductor layers are grown on a single substrate by the techniques discussed above to produce a p-n junction structure, and the p-n junction structure is fabricated to produce multiple p-n diode structures, as discussed below. The junction area of each of the p-n diode structures is separately defined on the surfaces of the semiconductor layers. The individual p-n diode structures are then electrically isolated from each other. The electrical isolation of p-n diode structures can be achieved by removing the conductive semiconductor layers between the p-n diode structures. Electrodes can be formed on the electrically-isolated p-n diode structures. Alternatively, electrodes can be formed on the p-n junction structure before producing multiple electrically-isolated diode structures. Electrodes of those isolated diodes are then electrically interconnected. Since the substrate is insulating, there is no electrical path from one device to the other unless an intentional connection is made between the electrodes, i.e., anodes and cathodes, of the p-n diodes.
Removal of selected portions of the conductive semiconductor layers can be achieved by various etching techniques widely used in the semiconductor industry. Typical etching techniques include wet or dry etching techniques. In wet etching, the material is dissolved when immersed in a chemical solution. In dry etching, the material is sputtered or dissolved using reactive ions or a vapor phase etchant. Typically, drying etching involves the plasma chemistry that is used in various types of conventional systems such as RIE (Reactive Ion Etching), ICP (Inductively Coupled Plasma)-RIE (called just ICP in this document), and ECR (Electron Cyclotron Resonance)-RIE.
Once every individual p-n diode is completely isolated from each other, monolithic integration can be achieved by selectively connecting the electrodes of individual diodes, implementing a desired function of the integrated device comprising multiple diodes. Preferably, the connection is via a metal connection. An insulating layer may be deposited between the p-n diode structure of at least one of the p-n diodes and the metal connection.
Semiconductor layers 124 and 132 represent p-type semiconductor layers grown on the n-type semiconductor layer. Thus, electrodes 120 and 128 are p-type electrodes (anodes) in electrical contact with p-type semiconductor layers 124 and 132, respectively. Electrodes 122 and 130 are n-type electrodes (cathodes) in electrical contact with n-type semiconductor layer or stack of p-type and n-type semiconductor layers where the top layer of the stack is an n-type semiconductor layer, 118 and 126, respectively.
Unless otherwise specified, it is assumed that integrated device 100 and all later embodiments (integrated devices 200, 300, 400 and 500) thereof, are made according to the principles of the present invention, as described above.
In one embodiment, the integrated device of the invention includes at least one light-emitting device that includes a light-emitting diode and an ESD-protection diode. The light-emitting diode and ESD-protection diode are made from the same monolithic semiconductor materials on a common insulating substrate. With the ESD-protection diode, the integrated device can be protected from high energy ESD events. As discussed above, ESD can potentially cause a catastrophic device failure due to a very large amount of current flowing through the device.
Depending upon the interface conditions of an electrical device, an ESD stimulus can occur in two different polarities, positive or negative, resulting in a forward or a reverse ESD current through the device, respectively. Generally, in a GaN-based light-emitting device, the forward ESD is less likely to cause damage, as the large amount of current is almost uniformly distributed across the whole p-n junction area. In practice, protection of a light-emitting device from negative ESD is more important, as higher energy ESD can be endured in a forward direction.
The monolithic integration of the configuration shown in
Depending upon the size of the ESD-protection diode to be integrated with a light-emitting diode, there is a trade-off in the final performance of the light-emitting device between the ESD endurance and other electro-optical parameters, such as forward operating voltage and total light output. ESD endurance becomes better with a larger size protection diode. Bigger protection diodes, however, result in a higher forward operating voltage and a lower light output from the light-emitting device, which are not desirable in typical applications. Therefore, an optimum size has to be chosen to meet the required ESD endurance level, while minimizing the penalty in forward voltage and light output.
In another embodiment, the integrated device of the invention includes multiple light-emitting devices that includes a light-emitting diode. The light-emitting diodes are formed from multiple p-n diode structures made from a p-n junction structure that is monolithically grown as described above.
Since individual light-emitting diodes perform light-emitting function while connected with each other or with one another, this serially-connected device provides light output substantially equal to that of a single light-emitting diode multiplied by the number of total diodes connected in series. The monolithic integration of the invention provides a simpler and more reliable solution than the use of the same number of discrete devices connected by a wire-bonding or bump-bonding known in the art. Because multiple light-emitting devices are integrated monolithically over a common insulating substrate prior to packaging, the total number of terminals connected in the package can be reduced. Thus, with the integrated device of the invention, the substantially same level of performance can be achieved from a single light-emitting device package as that of the same number of discrete packages.
In yet another embodiment, the integrated device of the invention includes a plurality of light-emitting devices, where each of the light-emitting devices includes a light-emitting diode and an ESD-protection diode. The light-emitting and ESD-protection diodes are formed using a monolithic p-n junction structure, as discussed above. Electrodes of the light-emitting and ESD-protection diodes are interconnected with each other through electrodes of opposite polarities of the light-emitting and ESD-protection diodes of the light-emitting devices. Each of the light-emitting devices are also interconnected with each other or with one another through electrodes of opposite polarities of the light-emitting diode of the light-emitting devices.
As shown in
The electrodes of the light-emitting and/or ESD-protection diodes can be formed prior to depositing the connection metals. Alternatively, when the same materials are used for both the electrodes of diodes and connection metals, the electrodes and connection metals can be formed simultaneously. For example, in the embodiment of
Any metals or combinations thereof, which are electrically conductive, can be used as the connection metals. Examples of the connection metals include gold, palladium and platinum.
P-side (anode) and n-side (cathode) electrodes can be formed on the p-type and n-type semiconductor layers, respectively, by methods known to those skilled in the art (see, for example, U.S. Pat. No. 6,734,091 and U.S. Patent Application Publication Nos. U.S. 2004/0000670A1 and U.S. 2004/0262621A1, and U.S. Patent Application filed on even date herewith under Attorney Docket Number 0717.2048-001, “Methods of Forming P-type Electrodes in Gallium Nitride-Based Light-Emitting Devices,” by Tchang-hun Oh, et. al., the entire teachings of which are incorporated herein by reference). The p-side and n-side electrodes are in electrical contact with the p-type and n-type semiconductor layers, respectively. Suitable materials for the electrodes of the p-n diodes in the invention can be found, for example, U.S. Pat. No. 6,734,091, U.S. 2004/0000670 A1 and U.S. 2004/0262621 A1.
For the integrated device of the invention, the insulating substrate is preferably sapphire. Because the sapphire substrate is electrically insulative, electrodes must be formed directly on the n-type and p-type semiconductor layers. In addition, since p-type semiconductor layers have only moderate conductitvity, a p-electrode typically is formed to cover substantially the entire surface of the p-type semiconductor layer, requiring the p-electrode substantially transparent. Thus, in a preferred embodiment, the light-emitting device of the invention includes a substantially transparent p-electrode, such as a nickel-oxide (see U.S. Pat. No. 6,734,091) or indium-oxide based p-electrode (see U.S. Patent Application filed on even date herewith under Attorney Docket Number 0717.2048-001).
In general, as most of light generated from semiconductor layers of light-emitting device is totally reflected at the interfaces that the semiconductor layers make either with the substrate or the encapsulating material, a significant portion of the generated light is guided and traveling laterally within the semiconductor layers. Since a considerable portion of this laterally traveling light eventually reaches the edge of the semiconductor layers exposed in the side of the device, the enhancement of light extraction can be achieved by introduction of more favorable surface conditions of the sidewall at the edge of the semiconductor layers.
One of the steps towards the embodiments of various types of monolithically-integrated light-emitting devices is the device isolation process, as all the p-n diodes have to be electrically isolated from each other before selective connections are made. With a carefully-designed isolation process, the light output from an individual light-emitting device can be increased, benefiting from enhanced light extraction through the etched sidewall(s) of the individual light-emitting device.
Accordingly, in any of the embodiments discussed above, any one of the light-emitting devices can have a light-emitting diode having at least one sidewall that is beveled.
In the isolation process, a device area is defined as an area including the p-n diode structures and any top surface area necessary for positioning the metal electrodes. Typically the device area is defined by a polygon with more than four sides. The area outside the device area is etched from the surface of the semiconductor layers of the p-n diode structures. The etching can proceed up to a complete removal of the semiconductor layers, exposing the substrate outside the device area as required for device isolation. The etching of the semiconductor layers, however, can proceed to any depth outside the device area for the enhancement of the light extraction from the etched sidewall, although a deeper etch is preferred in order to maximize the enhancement. The etching can be carried out in more than one side the polygon forming the device area to enhance the light extraction. The etching on all the sides of the polygon as required for device isolation is preferred to maximize the benefit.
The etching process can be carried out by patterning an etch mask whose pattern is to be transferred to a top surface of the semiconductor layers. To enhance the light extraction further from the sidewall(s), the edge of the etched surface can be made undulated as shown in
Light-emitting device 212j of the invention can be used to enhance the light extraction where light emission is from either the bottom of the substrate or from the top surface of the semiconductor layers opposite the substrate. Depending upon the light-emitting sides, optionally, the etched sidewall(s) can be coated with a dielectric layer, such as silicon dioxide or silicon nitride, or a metal layer, such as a reflection layer, known in the art to increase light extraction.
A gallium nitride-based semiconductor material is a material having the formula InxAyGa1-x-yN, wherein x+y<1, 0≦x<1, and 0≦y<1. Gallium nitride-based semiconductor materials are usually grown by a vapor phase growth method such as metalorganic chemical vapor deposition (MOCVD or MOVPE, hydride chemical vapor deposition (HDCVD), or molecular beam epitaxy (MBE). Generally, a gallium nitride-based semiconductor material is an n-type material even when no n-type dopant is included in the material since nitrogen lattice vacancies are created during crystal growth. Thus, an n-type gallium nitride-based semiconductor material may not include an n-type dopant. However, an n-type gallium nitride-based semiconductor typically exhibits better conductivity when the material includes an n-type dopant. n-Type dopants for gallium nitride-based semiconductor materials include Group IV elements such as silicon, germanium and tin, and Group VI elements such as selenium, tellurium and sulfur.
A p-type gallium nitride-based semiconductor material is a gallium nitride-based semiconductor material that includes a p-type dopant. The p-type dopants (also called an acceptor) for gallium nitride-based semiconductor materials include Group II elements such as cadmium, zinc, beryllium, magnesium, calcium, strontium, and barium. Preferred p-type dopants are magnesium and zinc. Typically, during growth of the gallium nitride-based semiconductor material gaseous compounds containing hydrogen atoms are thermally decomposed to form the semiconductor material. The released hydrogen atoms, which are present mainly as protons, become trapped in the growing semiconductor material, and combine with p-type dopant, thereby inhibiting their acceptor function. To improve the conductivity of a p-type gallium nitride-based semiconductor material, the material may be placed in a high electric field, typically above 10,000 volts/cm for about 10 minutes or more. The protons trapped in the semiconductor material are drawn out of the material to the negative electrode, thereby activating the function of the p-type dopants (see, for example, U.S. Publication No. 2003/0199171, the entire teachings of which are incorporated herein by reference). Alternatively, the conductivity of the p-type gallium nitride-based semiconductor material can be improved by annealing the material at a temperature above 600° C. in a nitrogen environment for 10 minutes or more (see, for example, U.S. Pat. No. 5,306,662, the entire teachings of which are incorporated herein by reference).
As described above, a gallium nitride-based semiconductor structure includes an p-type gallium nitride-based semiconductor layer and n-type gallium nitride-based semiconductor layer. The p-type gallium nitride-based semiconductor layer is generally grown over the n-type gallium nitride-based semiconductor layer. The n-type and p-type semiconductor layers can be in direct contact with each other or, alternatively, an active region can be sandwiched between the n-type and p-type gallium nitride-based semiconductor layers. An active region can have a single quantum-well structure or a multiple quantum-well structure. An active region having a single quantum-well structure has a single layer (i.e., the well layer) formed of a gallium nitride-based semiconductor material having a lower band-gap than the n-type and p-type gallium nitride-based semiconductor layers sandwiching it. An active region having a multiple quantum-well structure includes multiple well layers alternately stacked with multiple layers that have a higher band-gap than the well layers (i.e., barrier layers). The outermost layer of the active region closest to the n-type gallium nitride-based semiconductor layer is a well layer and has a smaller band-gap than the n-type gallium nitride-based semiconductor layer. The outermost layer of the active region closest to the p-type gallium nitride-based semiconductor layer may be a well layer or a barrier layer and may have a band-gap that is larger or smaller than the p-type gallium nitride-based semiconductor layer. Typically, the thickness of a well layer in a quantum-well structure is about 70 Å or less, and the barrier layers are about 150 Å or less. Generally, the well layers and barrier layers in a quantum-well structure are not intentionally doped.
Semiconductor layers were grown in a c-sapphire substrate by low-pressure MOCVD. The first deposited layer was a 20 nm-thick GaN nucleation layer, which was followed by a 4 μm-thick, silicon-doped (doping concentration of about 1019 cm−3) n-type GaN layer. The next layers were multiple quantum well active layers made of InxGa1-xN/GaN (0<x<0.5) layers. The last layer was a 0.6 μm-thick Mg-doped p-type GaN top layer. The estimated concentration of the activated Mg dopants was approximately 3×1017 cm−3, as determined by the Hall measurement. After the MOCVD growth of the epitaxial layers, the device fabrication was carried out using conventional semiconductor processing techniques commonly used in industry.
Using the semiconductor layers grown as described in Example 1, an integrated light-emitting device that includes a light-emitting diode and an ESD protection diode was monolithically fabricated. The integrated light-emitting device was fabricated following the schematic of the integration shown
Finished devices had a total area of 300×300 μm2 with two p-n diodes, i.e., a light-emitting diode and an ESD-protection diode, which were integrated as shown in
Many testing methodologies known in the art can be used for testing tolerance of the integrated device of the invention against ESD. One of the most popular procedures is based on a human body model (HBM), simulating the impact of an ESD induced by human. In this example, the JEDEC HBM standard was used for the ESD test. During the test of the ESD tolerance against HBM, about 90% of the single diode devices failed at the ESD voltage level of 600V or below. On the other hand, 100% of the devices with two diodes that were tested showed endurance against HBM up to 6000V. The devices that included the nickel-oxide based contact layer and the devices that included the indium-tin-oxide based contact layer showed similar ESD protection against HBM. Hundred samples were tested for each of the integrated devices of the invention and control device. This result confirms a significant improvement in the ESD endurance as a result of the monolithic integration.
Light-emitting device whose structure was as shown in the schematic of
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/544,577, filed on Feb. 13, 2004. This application also claims the benefit of U.S. Provisional Application Nos. 60/553,718 and 60/553,717, both of which were filed on Mar. 15, 2004. The entire teachings of the above applications are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60544577 | Feb 2004 | US | |
| 60553718 | Mar 2004 | US | |
| 60553717 | Mar 2004 | US |
