Fabrication and characteristics of the InGaN/GaN multiple quantum well blue LEDs
by Liang Hu
M.Phil. Electrical and Electronic Engineering
xv, 81 leaves : ill. ; 30 cm
In this work, processing procedures to fabricate InGaN/GaN MQW blue LEDs were developed and optimized. A blue LED, with an emission wavelength of 457.6 nm and a CW output power of about 4 mW at 50 mA driving current in chip form, was fabricated. At the typical test current of 20 mA, the output power was 1.7 mW under 3.5V forward bias before packaging....[ Read more ]
In this work, processing procedures to fabricate InGaN/GaN MQW blue LEDs were developed and optimized. A blue LED, with an emission wavelength of 457.6 nm and a CW output power of about 4 mW at 50 mA driving current in chip form, was fabricated. At the typical test current of 20 mA, the output power was 1.7 mW under 3.5V forward bias before packaging.
Current-voltage and capacitance-voltage characteristics were measured and studied. Temperature dependent I-V measurement results indicated that the defects assistant tunneling current dominated at low forward bias, caused by the high density defects inside the p-n junction region of InGaN/GaN based LEDs. Although injection current will dominate at larger forward bias when energy band across the p-n junction is pulled flat, these defects may affect the light emitting efficiency of the LED. The doping concentration profile near the p-n junction interface was deduced from the C-V measurement. High doping concentration about 2x l018cm -3 was observed in the n+-GaN region near the interface. Using fine voltage scanning step, the MQW structure was clearly observed. I-V and C-V measurements provide valuable information and feedback for sample growth and fabrication.
The spatial inhomogeneity images of electroluminescence of LEDs were taken using an optical microscopy. It was observed that light emitted only at some sporadic spots on the device surface at very low injection current. When the driving current was gradually increased, more spots and areas began to light up. However, some dark areas remained dark even at very high current injection. A large amount of fine structures with size smaller than 100nm were directly observed under a 1000x magnification microscope. The spatial inhomogeneity of light emitting was attributed to quantum dot like structures, with different sizes and different In compositions, inside the MQW structure. With increasing of the injection current, injected carriers gradually fill the quantum dots in the sequence of narrow to wide bandgap. The density of the fine QD like regions was very high. The light emission was contributed mainly by these fine QD like structures, which prevent the carriers from reaching the nonradiative defects outside the QDs. We believe this is the leading reason why GaN based materials can be used to fabricate high brightness LEDs in spite of their high dislocation density. Electroluminescence and photocurrent spectra were also measured and studied to investigate the light emitting mechanism. The peak position of electroluminescence spectra had a large red shift compared to the absorption edge obtained by photocurrent spectra for all samples. Such large Stokes' like shifts are typical for InGaN/GaN MQW structures as reported in literature suggesting the existence of QDs structures with minimum energy band.