| Summary: | 碩士 === 國立交通大學 === 光電工程學系 === 98 === Direct wide-bandgap gallium nitride (GaN) and other III-nitride-based semiconductors have attracted much attention for potential applications such as blue, green, and ultraviolet (UV) light-emitting diodes (LEDs) and blue laser diodes (LDs). Although InGaN-based LEDs have many excellent properties, the efficiency droop will occur under high current injection, resulting in the application of lighting was limited. In order to reduce the efficiency droop behavior in InGaN/GaN light emitting diodes, several semiconductor technologies have been used, such as non-polar material, AlInGaN barrier layer, thick DH active region or thick QW, barrier doping etc..
In this study, we tried to reduce the efficiency droop behavior in InGaN-based by using low temperature GaN (LT-GaN) pre-strained layer and graded quantum well (GQW) in which the well-thickness increases along [0001] direction. To better understand the influence of LT-GaN pre-strained layer and GQW on efficiency droop, a lot of measurement techniques were performed to investigate the optical and electrical properties of the grown specimens, including photoluminescence (PL), cathodoluminescence (CL), electroluminescence (EL), L-I-V curve and Advanced Physical Models of Semiconductor Devices (APSYS). Form power-dependent PL spectrum, the emission peak wavelength of the specimen with LT-GaN pre-strained layer and GQW exhibited relatively slight blue shift, which could be related to smaller quantum confined Stark effect (QCSE). Besides, the CL images shown that the specimen has more uniform emission area and less dark spots, resulting in the enhancement in emission efficiency. APSYS simulation analyzed that specimen with LT-GaN pre-strained layer and GQW revealed superior hole distribution as well as radiative recombination distribution. Additionally, according to the analysis of electroluminescence spectrum, specimen with LT-GaN pre-strained layer and GQW reveals additional emission peak from the following narrower wells within GQWs. These results are in good agreement with the result obtained from APSYS simulation.
Based on the results mentioned above, a high-efficiency InGaN-based LED with LT-GaN pre-strained layer and GQW has been fabricated, which demonstrated an improvement in output power of 36% at current density of 22 A/cm2 and 71% at current density of 244 A/cm2. Besides, the efficiency droop was alleviated to be about 17% from maximum at current density of 22 A/cm2 to 244 A/cm2, which is much smaller than 54% of conventional LED.
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