Transmission Electron Microscopy Studies on Pre-strained InGaN/GaN Multiple Quantum-well Structures and Overgrown GaN Films on GaN Nano-columns

In this research, we compare the nanostructures and optical characterizations of three InGaN/GaN multiple quantum well light-emitting diode samples with different structures. The first sample is grown with the onventional technique, the second sample is grown with a low-indium QW before the high-indium QWs based on a prestrained technique. From the HRTEM images, different degrees of indium aggregation and composition fluctuation between QWs are observed. From the calibrations of the average indium contents of these QWs based on the strain state analysis images, we can see the higher indium contents and stronger clustering behaviors in the second sample. From the EL spectrum, we can see the smaller blue shift in increasing injection current level in the second sample, which is based on the prestrained growth technique when compared with the first sample, which is based on the conventional growth technique. It is found that the quantum-confined Stark effect (QCSE) under the prestrained growth condition is actually enhanced. The smaller blue shift in the prestrain sample is attributed to the injection-current dependence of hole distribution among QWs and the enhanced carrier localization in the QWs of increased indium contents. Carrier localization can reduce the QCSE and its screening effect. The third sample is a blue/yellow dual-wavelength white-light InGaN/GaN QWs LED epitaxial structure with its EL spectrum close to the ideal condition in the Commission International de l'Eclairage (CIE) chromaticity based on the prestrained growth technique. The screening effect of the QCSE in the implemented white-light LED is quite small.
Then, we compare the nanostructures and optical characterizations of two samples of overgrown GaN films on GaN nano-columns at different growth temperatures. GaN nano-columns are grown on silicon substrate with molecular beam epitaxy. GaN films are directly overgrown on nano-columns with metalorganic chemical vapor deposition. From the plane-view scanning electron microscopy images, the overgrown GaN film in the sample of high-temperature growth (900 ℃) has a flatter surface morphology. From the X-ray diffraction results, the crystal quality of the sample of high-temperature growth (900 ℃) is generally better than the one grown at the low temperature (800 ℃).