The present invention relates generally to light emitting diodes and more specifically to contacts for light emitting diodes.
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. These LED device structures can also be transferred to a transparent substrate by wafer bonding. Often, an n-type layer (or layers) is deposited on the substrate, then an active region is deposited on the n-type layers, then a p-type layer (or layers) is deposited on the active region. The order of the layers may be reversed such that the p-type layers are adjacent to the substrate by either epitaxial growth or wafer bonding.
The design scheme of the flip chip LED 10 forces lateral current injection, which results in current crowding under the n-contact 24 and near the p contact area 18 as illustrated by the arrows in
One manner of solving the non-uniform current injection problem in the n-side is to use full sheet n-metal contact. However, because the n-metal contact has to be annealed at high temperature, e.g., greater than 420° C., to achieve a good ohmic contact, the metal surface is rough. As a result, the reflectively of the full sheet n-metal contact is poor and thus, decreases light extraction.
Thus, it is highly desirable to improve the contacts used with LEDs reduce the non-uniform current injection problem without decreasing light extraction.
In accordance with one embodiment, a light emitting device includes a stack of layers bonded to an undoped substrate with a doped layer between the stack of layers and the undoped substrate. The stack of layers include a layer of first conductivity type over the doped layer, an active region overlying the layer of first conductivity type, and a layer of second conductivity type overlying the active region. In one embodiment, the doped substrate is part of the stack of layers and is bonded to the undoped substrate. The doped layer and undoped substrate may be formed from the same semiconductor material, such as GaP. First and second electrical contacts are coupled to the device on a side opposite the undoped substrate. The doped layer may provide electrical contact between the first electrical contact and the layer of first conductivity type.
In accordance with another embodiment, a method of forming a light emitting device includes providing a transparent undoped substrate and forming a stack of layers including a layer of first conductivity type, an active region over the layer of first conductivity type, a layer of second conductivity type over the active region. The method includes bonding the stack of layers to the undoped substrate with a doped layer between the stack and the undoped substrate. In one embodiment, the doped layer is part of the stack of layers and may be formed on a sacrificial substrate prior to bonding to the undoped substrate. The method further includes removing a portion of the layer of first conductivity type, the active region, and the layer of second conductivity type to expose the doped layer and forming a first electrical contact to contact the layer of first conductivity type and forming a second electrical contact to contact the exposed doped layer. The first and second electrical contacts are on the same side of the doped layer opposed the undoped substrate.
As shown in
The ODRM structure 101 is formed over the capping layers 112 from a full sheet conductive transparent film 114 of, e.g., indium tin oxide (ITO), and a high reflective mirror 116 of, e.g., Ag or Au. The term “transparent” is used herein to indicate that an optical element so described, such as a “transparent film,” a “transparent layer,” or a “transparent substrate,” transmits light at the emission wavelengths of the LED with less than about 50%, preferably less than about 10%, single pass loss due to absorption or scattering. One of ordinary skill in the art will recognize that the conditions “less than 50% single pass loss” and “less than 10% single pass loss” may be met by various combinations of transmission path length and absorption constant. The conductive transparent film 114 is sometimes referred to herein as an ITO layer 114, but it should be understood that other conductive and transparent films may be used. The conductive transparent film 114 serves as the n contact for the LED 100 and the mirror 116 overlies the conductive transparent film 114. Where indium tin oxide is used as the conductive transparent film 114, the ITO layer 114 has a thickness that is, e.g., a quarter of the wavelength produced by the LED 100. By example, the ITO layer 114 is approximately 73 nm thick at a wavelength of 615 nm and has a refractive index of 2.1. The contact resistance of the ITO layer 114 is expected to be 1.5 e−5Ω cm2 or lower, with a transmission of approximately 95% or better around 600 nm.
The ODMR structure 101 provides high reflection for the light reaching the ODRM structure 101 over all incident angles. For example, the ODRM structure 101 with a quarter wavelength ITO layer 114 and an Ag mirror 116 is expected to have a reflectively of over 90% for a wide range of incident angles. Moreover, using the ITO layer 114 as a full sheet n-contact provides a uniform current injection from the n-side into the active region 108, eliminating the current crowding problem at the n-layer 110 found in conventional devices. Accordingly, the ODMR structure 101 reduces the forward voltage Vf and series resistance while increasing the extraction efficiency of the LED 100 compared to conventional devices.
It should be understood that, while the LED 100 of the present embodiment is described as a flip chip AlInGaP type device, the present ODRM structure may be used with difference devices if desired. For example, the ODRM structure may be used with a flip chip InGaN LED devices. It has been demonstrated that the ITO layer 114 can be used as a transparent contact on a p-GaN layer. The ITO layer 114 can also be applied on top of p-GaAs or P-InGaN contact layers.
With the use of the ODRM structure 101, a uniform current injection is provided at the n side of the active region. The current injection at the p side of the active region, however, may still be problematic due to the lateral contact scheme in a wide mesa structure such as that shown in
Thus, in accordance with another embodiment of the present invention, a distributed p-contact array is used, along with the ODRM structure 101, to improve current spreading and increase the junction area of the LED. The distributed contact array may be similar to that disclosed in U.S. 2003/0230754, entitled “Contacting Scheme for Large and Small Area Semiconductor Light Emitting Flip-Chip Devices”, by Daniel A. Steigerwald et al., filed Jun. 13, 2002, which has the same assignee as the present disclosure and is incorporated herein by reference.
As can be seen in
As illustrated in
By way of example, for a 500 μm×500 μm square LED chip, a 4×4 distributed p-contact array, such as that shown in
It should be understood, that the other dimensions or other materials may be used with the present invention if desired. Moreover, while the device illustrated in
The ITO layer 214, mirror layer 216 and the capping layer 212 are patterned as shown in
A dielectric layer 218, such as for example silicon nitride or silicon oxide, is deposited, as shown in
The p contact layer 220 is then deposited over the dielectric layer 218 and in via 217. The interconnect 222, which connects the p-metal deposited in each via 217, may also be deposited at this time. The p contact layer 220 is patterned to remove a portion of the material covering the mirror layer 216 as shown in
Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.
The present application is a continuation of and claims priority to U.S. patent application Ser. No. 10/960,391, filed Oct. 6, 2004, entitled “Contact and Omnidirectional Reflective Mirror for Flip Chipped Light Emitting Devices”, by Decai Sun, which is incorporated herein by reference.
Number | Date | Country | |
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Parent | 10960391 | Oct 2004 | US |
Child | 11860502 | US |