1. Field of the Invention
The present invention generally relates to a light-emitting diode and a method for fabricating the same, and more particularly, to a light-emitting diode with enhanced brightness and a method for fabricating such a device, in which some portions of the highly doped layer are removed to form windows to be filled with a transparent material layer so that the light-emitting efficiency can be improved.
2. Description of the Prior Art
The light-emitting diodes (to be abbreviated as LED hereinafter) has received considerable attention for its advantages such as long lifetime, small size, low heat generation, low power consumption, high response speed, monochromic lighting ability, etc. and has been widely used in applications such as computer peripherals, clock displays, display panels, and many other 3C (computer, communication and consumer) products since the 1960s. Particularly, the development in high brightness LEDs has successfully facilitated the LED applications to extend from indoors to outdoors.
To further improve the brightness if an LED, there have been disclosed lots of new techniques such as the U.S. Pat. No. 5,153,889, entitled xe2x80x9cSemiconductor light emitting devicexe2x80x9d (filed by Kabushiki Kaisha, Toshiba). This prior art is described as shown in FIG. 1, which comprises: a current diffusion layer 18 formed on an epitaxial LED structure composed of at least a top limiting layer 17, a light-emitting active layer 16, and a bottom limiting layer 15; wherein the current diffusion layer 18 includes a isolator 100 for dividing the operation current into two separate current paths on both sides. There is also a reflective layer 13 inserted between the bottom limiting layer 15 and the substrate 11 for reflecting the light. Of course, a back electrode 102 can be deposited on the bottom surface of the substrate 11 and also, an opposed electrode 101 can be deposited on the top surface of the current diffusion layer 18.
Even though, in the prior art, enhanced-brightness can be achieved by using the current diffusion layer 18, the isolator 100, and the reflective layer 13, there still exist some drawbacks as follow:
1. When the epitaxial LED structure is grown on the substrate, a highly doped layer 155 may be unintentionally formed between the reflective layer 13 and the bottom limiting layer 15. Such a highly doped layer 155 may significantly absorb the incoming light, which may adverse affect the light intensity from the LED.
2. The epitaxial LED structure is limited by the material selecting, required to be lattice-matched to the employed substrate. However, some of the materials are not suitable for use as a substrate for LEDs. For example, a GaAs substrate may absorb the light related to the energy gap of GaAs, which leads to decreased lighting efficiency, and a GaP substrate may display the orange color, which affects the color precision of the light.
3. The isolator 100 adds considerable complexity to the fabrication process when it is to be aligned with the bottom surface of the opposed electrode 101, which results in poor flexibility in the structure design as well as considerable trouble in the complicated fabrication process.
Therefore, a permanent substrate rather than a temporary substrate has been disclosed in, for example, the U.S. Pat. No. 6,258,699 entitled xe2x80x9cLight emitting diode with a permanent substrate of transparent glass or quartz and the method for manufacturing the samexe2x80x9d (filed by Visual Photonics Epitaxy Co., Ltd.) and the U.S. patent application Ser. No. 09/384,053 entitled xe2x80x9cLight emitting diode with enhanced brightness and method for manufacturing the samexe2x80x9d, to overcome the above problems of the prior art.
Please refer to FIG. 2, which is the main structure of an LED disclosed in the U.S. Pat. No. 6,258,699. As shown in the figure, an epitaxial LED structure 26 is formed on a temporary substrate made of GaAs or InP. Then, the temporary substrate is split off and replaced by a permanent substrate 21 made of transparent glass or quartz. A metal adhesive 24 is formed on the top surface of the permanent substrate 21 so as to adhere to the bottom surface of the planar epitaxial LED structure 26. A metal reflective layer 23 is also formed on the bottom surface of the permanent substrate 21. In this manner, when a voltage is applied across diodes 201 and 202, formed on the planar epitaxial LED structure 26, the planar epitaxial LED structure 26 emits light from the p-n junction downward through the transparent glass or quartz layer and the light is reflected by the metal reflective layer 23 such that enhanced brightness of the LED can be achieved. However, there still exist some drawbacks as follow:
1. The problems related to the highly doped layer 265 still cannot be solved, which affects the lighting efficiency of the LED.
2. The reflected light has to travel through the transparent glass or quartz layer, which also affects the lighting efficiency of the LED due to long traveling path of the light.
3. The reflecting layer is formed on the bottom surface of the substrate. Therefore, the permanent substrate has to be formed of a transparent material, which limits the choice of the materials as well as the design of the device.
4. The problems due to the high temperature during operation cannot be solved, which may reduce reliability as well as the lifetime of the device.
Therefore, there is need in providing a light-emitting diode with enhanced brightness and a method for fabricating the same so as to improve the light intensity and operational reliability and further prolong the lifetime.
Accordingly, it is the primary object of the present invention to provide a light-emitting diode with enhanced brightness, in which some portions of the highly doped layer are removed to form windows to be filled with a transparent material layer so that the problems related to the highly doped layer can be overcome and the light-emitting efficiency can be improved.
It is another object of the present invention to provide a light-emitting diode with enhanced brightness, in which a permanent substrate is used to replace the temporary substrate so that the permanent substrate for LEDs can be made of a thermal conductor, an electrical conductors or a material for changing colors. In this manner, the application field is broadened and the lifetime of the device is prolonged.
It is still another object of the present invention to provide a light-emitting diode with enhanced brightness, in which the metal contacts are placed to effectively program the current path as well as the current density so that the current congestion can be prevented and better light-emitting efficiency can be achieved.
It is still another object of the present invention to provide a method for fabricating a light-emitting diode with enhanced brightness. The method employs a simplified fabrication procedure to overcome the problems related to the highly doped layer. Such a simplified fabrication procedure does not only facilitate mass production but also improve the reliability, resulting in a reduced fabrication cost.
It is still another object of the present invention to provide a method for fabricating a light-emitting diode with enhanced brightness. The method is suitable not only for use in fabricating a vertical LED but also for a planar LED.
In order to achieve the foregoing objects, the present invention provides a light-emitting diode with enhanced brightness, comprising: an epitaxial LED structure having at least one light-emitting active layer with a plurality of windows formed below the light-emitting active layer in a highly doped layer: at least one conductive contact is formed on the bottom surface of the highly doped layer; a transparent material layer formed in the windows; an adhesion layer formed between the transparent material layer and a permanent substrate; a bottom electrode formed on the bottom surface of the permanent substrate; and an opposed electrode formed on the top surface of the epitaxial LED structure.
The present invention further provides a method for fabricating a light-emitting diode with enhanced brightness, comprising the steps of: forming an epitaxial LED structure having at least one lighting-emitting active layer on a temporary substrate, wherein a highly doped layer is naturally formed at the bottom of the lighting-emitting active layer; splitting off the temporary substrate; forming at least one conductive contact on the bottom surface of the highly doped layer; removing portions of the highly doped layer that are not connected to the conductive contact so that a window is formed between two portions of the highly doped layer; filling the window with a transparent material layer; and attaching a permanent substrate to the bottom of the transparent material layer.
Other and further features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.