Study On The Reliability Of GaN-based Ultraviolet High Power Light Emitting Diodes

Because of the commercialization of Ga N-based blue/green light-emitting diodes(LEDs) and laser diodes, people paid more interests in the research of III-Nitride compound semiconductors. Moreover, Ga N was becoming one of the main materials to fabricate optoelectronic devices depending on its wide and direct band gap, radiation hardness and thermo stability.In this thesis, the reliability of Ga N-based ultraviolet high power LED is mainly investigated, which is primarily based on the degradation mechanisms of UV LEDs after on-state DC stress. First, the degradation mechanisms of Ga N-based blue or green LEDs, which contain electromigration of the contact metal, degradation of p-type Ohmic contact and p-type Ga N layer, the increasing of deep-level defects and the instability of p-type Mg doping, are summarized from plenty of research papers.Subsequently, on-state DC stress experiments are conducted for 395 nm UV high power LEDs, whose operation current is 350 m A and dimension is 45×45 mil2. Two groups of devices with different luminance decreases are chosen to combine and analyze the I-V performance, which is consisted of reverse region, threshold region and linear region. It’s shown that the degradation mechanism of these LEDs is deep-level defect, rather than the degradation of Ohmic contact metal. Furthermore, low frequency noise measurements are conducted for these LEDs. After analyzing the low frequency noise performance, it’s reinforced that the main reason that lead to the degradation of LEDs is the existence of plenty of deep-level defects.Afterwards, the performances of shorter wavelength LEDs, namely Al Ga N/In Ga N LED and Al Ga N/Al Ga N LED, are investigated, respectively. The wavelengths of these two LEDs are obtained through EL curve, which are 360 nm and 351 nm. First, by the I-V measurement two LEDs with different performances are analyzed. Then C-V performances at 10 KHz and 100 KHz are tested, it’s revealed that at certain voltage bias, the carriers in LED can’t respond to this frequency, which show the decreasing of capacitance in the C-V curve. Also the apparent carrier distributions are calculated by the C-V data. By combining the ACD of two different LEDs, it’s revealed the reasonsthat cause the degradation of devices are the degraded material quality. By analyzing the ACD curves of these LEDs, it can be found the parts of 360 nm and 351 nm LEDs, which have bad material characteristics, are n-type Ga N to part of MQWs and MQWs separately.