| Summary: | 碩士 === 國立交通大學 === 光電科技學程 === 106 === Abstract
The objective of this study is to analyze the influence of GaN LEDs with different quantum well pairs. The changed number of MQW pairs are 1, 3, 5, 7, 9, 11, 13 (LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13). We aimed at finding out the optoelectronic characteristics during quantum well pairs changes, and improve efficiency droop at high current injection.
By using photo-luminescence (PL) measurement, we discovered that as the quantum well pairs increases, the radiative recombination efficiency of the MQW pairs increases as well. The PL intensity is therefore increased, causing the PL intensity to reach saturation when the quantum well number reaches 11. It was found the forward voltages (at 60mA) of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-P11, and LED-P13 are 3.04V, 2.92V, 2.86V, 2.89V, 2.88V, 2.89V, and 2.88V, respectively. When comparing LED-P1 and LED-P5, it was also found that more quantum well pairs around the n-GaN enhance the current spreading, which indirectly reduce the current crowding effect. When measured at a reverse voltage at -10V,it wad found that the leakage current increases as the MQW pair increase. The differences between the leakage current might have been caused by the low quality of the GaN film, which is related to the amount of quantum well pairs.
The Light Output Power at 60mA injection current of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13 are 37.4mW, 44.1mW, 47.8mW, 48.2mW, 48.7mW, 49.8mW, and 45.5mW, respectively. When using LED-P1 as the reference in calculation, the LOP of LED-P11 is more than 33.1%. The carrier confinement and radiative recombination improve as the quantum well pairs increase, thus increasing light output power. The droop comparison result derives from the calculation within 200mA using 60mA as the reference point. The droop efficiency of LED-P1, LED-P3, LED-P5, LED-P7, LED-P9, LED-11, and LED-P13 are13.5%, 13.1%, 14.8%, 12.9%, 12.7%, 11.2%, and 11%, respectively. The droop might have decreased since the carrier confinement have improved and the electron overflow has also decreased.
We simulated the energy band diagram of each quantum well pairs, using the ‘matlab2012a’ software invented by Professor Wu from Taiwan University. It was found that the energy band diagrams of 60mA and 200mA, the energy band has become less slope as the MQW pairs increase. This means that the increase in quantum well MQW pairs may be able to prevent the energy band from tilting, thus reducing the piezoelectric field and making sure that the free electrons and holes are distributed in the well evenly. This results in higher radiative recombination and higher LOP, therefore preventing droop effect. It is in accordance with our experiment results.
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