Transmission Electron Microscopy Studies on Mixed InGaN/GaN Multiple Quantum-well Structures

In the first part of this research, we compare the nanostructures of two InGaN/GaN multiple-quantum-well (QW) light-emitting diode (LED) samples with different structeues. In one of the samples, a low indium InGaN/GaN QW is grown before five high-indium ones, which are grown under the same conditions as those for growing the five QWs in another sample. We use the techniques of High-resolution transmission electron microscopy (HRTEM) and strain state analysis (SSA) to show the material nanostructures of these two samples. 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 those QWs based on the SSA images, it is found that the QWs close to the low-indium one have higher indium contents than those in the control sample. Such an increase of indium incorporation is attributed to the pre-strain effect of the low-indium QW on the barrier layer right above it. Also, we found that the pre-strain effect diminishes along the growth of more QWs.
The second part of this research is to study the nanostructures of three InGaN/GaN multiple-QW LED samples of different structures. In these samples, QWs of two different growth conditions are stacked with different numbers and sequences. HRTEM images are obtained to show the nanostructures and distributions of the QWs in each sample. From the SSA images, we calibrate the average indium contents of the QWs. It is found that the indium contents and distributions of QWs may effect the optical properties a lot.