Emission Efficiency Improvement of GaN-based Light-emitting Diodes

• Main Authors: Chih-Feng Lu, 呂志鋒
• Other Authors: Chih-Chung Yang
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/63507784391404310165

博士 === 臺灣大學 === 光電工程學研究所 === 98 === In this dissertation, we have demonstrated the dependencies of output spectral overall red shift and spectral blue shift in increasing injection current on the prestrained barrier thickness in an InGaN/GaN QW LED of prestrained growth. It was found that a thinner prestrained barrier led to a larger general spectral red shift and a smaller blue shift in increasing injection current because of the stronger prestrain effect. Also, it was found that in terms of device resistance and saturation current, the LED performances of prestrained samples were better than that of a conventional LED. An LED of a thinner prestrained barrier had a better performance. These observations were attributed to the higher average indium content and stronger indium-rich clustering behavior in a sample of stronger prestrain. These attributions were supported by the SSA results in the TEM measurements. Furthermore, a green LED was fabricated based on the prestrained growth technique to compare with an LED of the same emission wavelength based on the conventional growth method. Then, the prestrained underlying growth technique was used to grow three yellow-emitting QWs of high efficiency. The yellow photons were mixed with blue light from an overgrown blue-emitting QW to produce white light. The improved properties of the phosphor-free monolithic white-light LED have been discussed. The reduction of the EQE droop of an LED through the SP-QW coupling mechanism has been demonstrated. With a current spreading grid pattern on the mesa surface, it was found that a smaller grid period led to more effective carrier transport into the QW regions under Ag deposition for stronger SP-QW coupling such that the droop effect was more significantly reduced, including the increase of injection current density of maximum EQE and the decrease of drooping slope. The observation of the SP-QW coupling effect in the samples of thin p-GaN was also supported by the different droop behaviors of the LED samples fabricated with the epitaxial structure of thick p-GaN, in which the SP-QW coupling effect is weak. Besides, we have demonstrated the dependence of the output spectrum on the junction temperature of a blue/green two-color InGaN/GaN QW LED. By decreasing the metal thickness of the p-type Ohmic contact on the device, the contact resistance was increased and hence the junction temperature was also increased. With the junction temperature increased, the probability for the deeper QWs to capture holes became higher such that blue emission could be enhanced to compete with the dominating green emission from the first QW. The conclusion of a higher junction temperature in a sample of a thinner p-type metal layer was consistent with the measurements of the output power versus the injection current, the current versus the applied voltage, and the direct measurement of contact resistance.