| Summary: | 碩士 === 國立交通大學 === 光電工程學系 === 100 === In this thesis, the high performance InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on a homoepitaxial GaN substrate by using metal-organic chemical vapor deposition (MOCVD). The same LED structures were also grown on sapphire substrate and patterned sapphire substrate for comparison. The material and optical properties were investigated in detail.
First, the material properties were investigated by high resolution transmission electron microscopy (HRTEM), high resolution X-ray Diffraction (HRXRD) and Raman spectroscopy. The results show that by using GaN substrate, the compressive strain, crystalline quality and dislocation density can be effectively improved, which result in the reduction of nonradiative centers in epilayer and the increase of LEDs light output efficiency.
In addition, the excitation power dependent PL, the temperature dependent PL, wavelength dependent TRPL and microscale PL measurements were employed to investigate the optical properties of InGaN/GaN MQWs grown on various substrates. We use the excitation power dependent PL at low temperature and room temperature to investigate the carrier recombination mechanism of InGaN/GaN MQWs LEDs grown on various substrates. The variations of wavelength and FWHM reflect the quantity of nonradiative centers, the Quantum confined Stark effect (QCSE) and band filling effect in MQWs. The temperature dependent PL and the wavelength dependent TRPL were also used to analyze the degree of localized states and the depth of localized states in InGaN/GaN MQWs on various substrates. The results indicate that fewer indium clusters in the InGaN/GaN MQWs on GaN substrate due to it suffers less compressive strain. As a result, we observed less degree of localized states and the depth of localized states in InGaN/GaN MQWs on GaN substrate. However, because of the superior crystalline quality and abrupt MQWs strctures for LEDs on GaN substrate, the overall internal quantum efficiency (IQE) is also higher than LEDs on PSS and sapphire substrate, especially for high carrier injection. Moreover, by using microscale PL measurement, we observed that the MQWs on GaN substrate had better spatial wavelength stability.
Finally, the EL measurements show that the light output efficiency of LEDs grown on GaN substrate is comparable to LEDs grown on PSS. The former has a superior IQE; while the latter has an advantage in light extraction efficiency (LEE). Consequently, the homoepitaxial technology for LEDs grown on GaN substrate plays an important role in solid-state lighting.
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