dc.description.abstract | This dissertation includes the growth and characterization of InGaN/GaN quantum well and laser diode structures grown by metalorganic chemical vapor deposition. The main work can be divided into the following three parts.
First, the luminescence mechanism of InGaN/GaN multiple quantum wells (MQWs) is studied using photoluminescence (PL) measurements combined with theoretical calculations. A large piezoelectric field of 1.92 MV/cm is deduced from the well width dependence of PL emission energy for the InGaN/GaN MQWs with In composition of ~0.23. Significant spectral blueshift accompanied by linewidth reduction is observed as increasing the excitation power at low temperature. While the emission energy with temperature follows the empirical Varshini equation under high excitation power, the so-called S-shaped spectral shift is observed under low excitation power. A model, taking into account photogenerated and thermally activated carrier screening effects on the polarization field in the InGaN quantum well, is developed to account for this phenomenon. Competition of inhomogeneous and homogeneous PL linewidth broadening with temperature is discussed. Besides, two carrier loss channels are deduced from the temperature quenching of PL intensity. One is the dissociation of exciton and the other one is the escape of electron from quantum well.
Second, the interdiffusion coefficient of In and Ga cations is extracted by self-consistent calculation of diffusion, Schrödinger and Poisson’s equations. The interdiffusion process is characterized by a single activation energy of about 3.4 eV and governed by vacancy-controlled second-nearest-neighbor hopping. Phase transformation in InGaN/GaN MQWs with high indium content (~40%) is observed upon thermal annealing. Cross-sectional transmission electron microscopy shows the existence of quantum dot-like islands within the quantum wells for the as-grown sample but these islands are significantly reduced after thermal annealing at 900 ºC. Two dominant high- and low-energy peaks are assigned to be from the quantum well state and localized state transitions, respectively. A huge piezoelectric field of ~3.2 MV/cm is also deduced from the Stoke shift energy. Besides, the effect of composition inhomogeneity on the luminescence is investigated. The PL peak exhibits a redshift followed by a blueshift with increasing the annealing time. A diffusion model, including the in-plane and out-plane diffusion of In-rich dot-like structures, is proposed to account for the spectral shift. The suppression of phase separation is attributed to the elastic strain in the pseudomorphic InGaN quantum well.
Finally, the electroluminescence characteristics of blue-violet laser diode structures are explored. Two emission peaks with different behavior are observed. Significant blueshift and linewidth broadening is measured for the low-energy peak with injection current, while slight blueshift and moderate linewidth narrowing occurs for the high-energy peak. Accordingly, these two peaks are assigned to be from the localized state and quantum well state emissions, respectively. The quantum well state emission exhibits a biexciton feature in contrast to the localized excitons. Besides, the emission associated with the localized state shows injection current dependent thermal quenching behavior. A multiple carrier escaping mechanism is proposed to account for this phenomenon. | en_US |