Study of the Internal Quantum Efficiency in InGaN Light Emitting Diodes

• Main Authors: Chang-Hua Hsieh, 謝昌樺
• Other Authors: Chia-Feng Lin
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/37259753643763224772

碩士 === 國立中興大學 === 材料科學與工程學系所 === 99 === In this thesis, the internal quantum efficiencies of InGaN-based light-emitting diodes (LEDs) with and without ITO layers were analyzed through the power-dependent photoluminescence measurement. In the first part of this thesis, the different chip size of the InGaN LED structure with 250nm-thick ITO layer were fabricated through the laser scribing process. At 10K, the PL intensity of the LED structure with the flat ITO layer was increased by adding laser excitation power that was similar the LED structure without ITO layer. It’s due to photo-induced carriers were frozen at the p-GaN and ITO layers at low temperature. At 300K, the PL intensity of the LED structure was decreased by increasing ITO conductive area. Because of the thermal activation in InGaN quantum well structure and the increasing conductivity of the ITO, the carriers diffused away from the laser excitation spot that the number of carriers’ recombination were reduced. By increasing the laser excitation power, the PL peak wavelengths of LED structure without the ITO layer had the red-shift phenomenon at low excitation power and the blue-shit phenomenon at high excitation power. Nevertheless, LED devices with maximum ITO shows red-shift phenomenon at higher excitation power. This peak red-shift phenomenon of LED devices with maximum ITO layer at high excitation power was similar to the LED devices without ITO layer at low power. The internal quantum efficiencies (integrated PL intensity ratios, I300K/I10K) were calculated as 61.5% for devices without ITO and 14.5% for devices with maximum ITO. In the second part of this thesis, the LED structures were grown on the flat-form (FPSS) and the pyramid (PPSS) pattern sapphire substrate. In the reverse-bias PL spectra, the flat-band voltages were measured at -12V and -8V for FPSS and PPSS structures, respectively. The piezoelectric fields were calculated at -1.12 and -0.92 MV/cm for FPSS-LED and PPSS-LED, respectively. In the power dependent PL measurement, we find steady IQE value at high excitation power that the IQE values were measured at 76.1% and 83.8% for the FPSS-LED and the PPSS-LED, respectively. At 10K, the PL peak wavelength blue-shifted of FPSS-LED and PPSS-LED were measured as 2.0nm and 0.7nm. The light output power of PPSS-LED had a 23.6% enhancement at 20mA. The wavelength blue-shift of the EL spectra were measured as 1.7nm and 1.3nm for FPSS-LED and PPSS-LED, respectively. It could be caused by larger electric field induced band tilted of the InGaN well layer that induced the low electron-hole wave function overlap and large band filling effect. The larger Isc and the higher photovoltaic efficiency were observed in the FPSS-LED compared with the FPSS-LED structure.