Improvement in Bias Stress Reliability by Barrier Thickness Variation in GaN Based Light-Emitting Diodes

Abstract

For bias stress reliability testing, InGaN/GaN MQW blue LEDs with different barrier thicknesses were grown by metalorganic chemical vapor deposition. Fast-Fourier-transformed high-resolution transmission electron microscopy was used to analyze the influence of the bias stress reliability with the strain status in the barrier layer. A comparison of the (1100) planar distances showed that a thicker thickness of the barrier layer induced the relaxation of stored strain. A thinner barrier thickness led to the reduced formation of misfit dislocations, which was responsible for the improvement of bias stress reliability of LEDs. These results indicated the importance of delicate control of stored strain in nitride films for improving the reliability and lifetimes of devices.Group III-nitride-based semiconductors are some of the most promising materials for optoelectronic applications, such as lightemitting diodes (LEDs) and laser diodes in the blue and ultraviolet ranges, which can be used in full-color displays, full-color indicators, and as light sources for lamps. 1, 2 Although visible LEDs that have been recently fabricated with GaN epilayers exhibited high power and high brightness for general illumination applications, the issues relating to bias stress reliability, which is related to the efficiency drop from an increase in operating time, should be resolved to guarantee long term stability. 3, 4 Under a long term operation (long duration of applied bias), it is considered that the generation of a leakage path in the localized regions is responsible for the drop in the brightness efficiency, and it also induces an increase in the leakage current level and variation in the threshold voltage (V th ). 4-6 It is known that this leakage path results from various structural defects, such as stacking faults (SFs) and threading dislocations (TDs), and this path is also influenced by the defect generation of misfit dislocations (MDs), etc., in the MQW active region exhibiting a bright emission. [5][6][7][8] The origins of the structural defects within the active region affecting the leakage path can be categorized into the following three defects: SFs, TDs, and MDs. The first type of defect is SFs. They can be easily formed with low formation energy within MQWs, because they are located on the closed packing plane and basal plane. 9 In particular, high indium incorporation in the InGaN layers for a long wavelength emission induces the formation of high density SFs. 9, 10 However, a recently developed epi-growth technique suppresses the formation of the SFs in the active region. The second type of defect is TDs. TDs originating from the film/substrate interface that propagate into the surfaces of whole films by penetrating all the n-and p-type regions play a role in the leakage current. 5, 7 However, most of commercially available GaN epi-layers have a similar TD density of ∼10 9 /cm 2 , thus it is difficult to distinguish the effect of TD density on the leakage current. 5,11 The third type of defect is MDs generated with...