1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device; and, more particularly, to a nitride semiconductor light emitting device capable of improving an ESD (Electrostatic Discharge) tolerance (withstand voltage characteristic) of a GaN based nitride light emitting device and a method for manufacturing the same.
2. Description of the Related Art
Generally, it has been known that a GaN based light emitting device has a bad static electricity characteristic in comparison with other compound light emitting devices. This reason is that a lot of crystal defects are formed in a GaN thin film due to a great lattice mismatch (16%) between a substrate and a thin film to be grown since the GaN light emitting device is formed on a sapphire substrate having a great lattice mismatch.
This crystal defect increases a leakage current of device and an active layer of the light emitting device having a lot of crystal defects is broken by a strong field when an external static electricity is applied. Generally, it has been known that the crystal defect (threading dislocations) in the order of 1010˜1012/cm2 exist at the GaN thin film.
The static electricity broken characteristic of the light emitting device is a very important issue related to the application range of the GaN based light emitting device. Specifically, a design of devices for withstanding a static electricity generated from package devices and workers of the light emitting device is a very important parameter for improving yield of a final device.
Particularly, the static electricity characteristic has been become more important since the GaN based light emitting device are recently used by being applied to a bad condition in environment such as an outdoor signboard and a vehicle light.
Generally, an ESD (Electrostatic Discharge) of a conventional GaN light emitting device withstands several thousands volts in a forward direction under HBM (Human Body Mode), whereas it does not withstand several hundreds volts in a reverse direction. The reason is that the crystal defects of the device are major reasons as described above, and also, an electrode design of the device is very important. Particularly, since the GaN light emitting device generally employs a sapphire substrate as an insulator, the ESD characteristic is further deteriorated by increasing a concentration phenomenon of currents around an N-electrode during a practical device operation as the N-electrode and a P-electrode are formed at the same plane on the structure of the device.
A conventional method for improving such ESD characteristic has been frequently approached in external aspects of the device. U.S. Pat. No. 6,593,597 teaches a technique for protecting a light emitting device from the ESD by connecting an LED and a Schottky diode in parallel by integrating the LED device and the Schottky diode on the same substrate. In addition, in order to improve the ESD tolerance, a method for connecting an LED to a Zenor diode in parallel has been proposed. However, since such methods cause complicatedness to be assembled by purchasing an additional Zenor diode or performing a Schottky junction and increase the manufacturing cost of the device, they are not preferable in point of cost or yield.
The most preferable method is that the ESD characteristic is improved by the light emitting device in itself by improving a thin film characteristic or a structure of the light emitting device. Japanese Pat. No. 3,622,562 discloses a method for improving an ESD characteristic by using a multi-stacked thin film structure using un-doped nitride indium gallium and un-doped nitride gallium on an n-type contact layer, whereas Japanese Pat. No. 3,063,756, Japanese Pat. No. 3,424,629, Japanese Pat. No. 2006-237254 and U.S. Pat. No. 6,677,619 try to reduce leakage currents and improve ESD tolerance by making a layer structure a good crystal property by reducing the density and size of defects to be generated from each layer of the light emitting device.
As like this, it is preferable that the quality of a GaN thin film is intrinsically increased to improve the ESD characteristic, but there exists a limitation to remove the defects, therefore, a method of growing the thin film has been continuously continued to improve the ESD characteristic of the GaN up to date.