Researched On Optical-Electrical Characteristics And Reliability Of Gan Led With Ito Films

Light emitting diodes (LEDs) have characteristics for the benefits of environmental conservation such as high efficiency, long lifetime, environmental friendly and so on. They are also widely used in lighting field. In fabrication of LED, current spreading effect of horizontal GaN-based LED is the main factor that influences the external quantum efficiency. Indium tin oxide (ITO) is used as current spreading layer and a contact to P-GaN, because of its characteristics such as good conductivity and high transparency in the blue wavelength range.This paper is supported by the National High Technology Research and Development Program (863program) Funded Projects (No.2009AA03A1A3) and National Key Technologies R&D Program (No.2011BAE01B14). This paper examines on the characteristics and the reliability of GaN-based LEDs with ITO. The main contents of this study are as follows:1. The characteristics of ITO after rapid thermal annealing (RTA) treatment were tested and analyzed, as well as effects of different RTA temperature on vacuum evaporated ITO. Optical transparency and resistivity of ITO for different annealing temperatures were measured.It was found that resistivity of450℃RTA was as low as1.19×10-4Ω·cm，along with a high transparency of94.17%at460nm.2. Characteristics of ITO/P-GaN contact were studied. ITO contact resistance showed a minimized value of3.9×10-3Ω·cm2after450℃annealing. AES analysis indicated the variation of atomic distribution in the ITO/P-GaN interface.3.(1) The fabricating process of conventional high-light-extraction electrode with low contact resistance for GaN was mainly investigated. Carrier concentration and hall mobility of ITO for different annealing temperatures were measured by Hall test. Ni/Au and ITO/P-GaN contact blue LEDs were fabricated and compared. With20mA current injection, it was found that forward voltage and output power were3.14V and12.57mW.(2) The GaN-based high-voltage (HV) LEDs with a5um deep isolation groove and an acceptable mesa sidewall angle of79.2°are fabricated and presented. The contact resistance of HV LEDs is also tested and was reduced by4.6Q compared to conventional LEDs, while the output power increased by5W.4.(1) Thermal characteristics of LED with ITO films were analyzed. The junction temperature of power AlGaInP (red and orange) and InGaN (blue and green) LEDs were measured at different drive current.Optical and electrical parameters under different temperature were also measured. The effects of different material and component element on thermal characteristics of LED were analyzed. It was found that photoelectric properties of LED depends on the junction temperature.(2) The effect of ITO on the reliability of GaN-based LEDs was evaluated by six thousand aging tests. It was found that the LED with an ITO film has better ohmic contacts and smaller voltage, and has a smaller change under an increasing current. The LED with a thicker ITO layer has a higher reliability. The smaller the chip size, the higher the junction temperature, and the stability is worse. The increase of junction temperature caused by voltage rise is the main reason of LED failure.(3) The current stress test was carried on12V-HV LED. The changes of luminous flux and forward voltage were analyzed. Optical microscope and FLIR were used to observe the microscopic image of the failure LEDs. Variation of junction temperature with voltage before and after aging were simulated. The failure mechanism of the sample was also analyzed.