Indium-Induced Effect on Polarized Electroluminescence from InGaN/GaN MQWs Light Emitting Diodes

Polarization-resolved edge-emitting electroluminescence （EL） studies of In GaN/GaN MQWs of wavelengths from near-UV （390nm） to blue （468nm） light-emitting diodes （LEDs） are performed. Although the TE mode is dominant in all the samples of InGaN/GaN MQW LEDs, an obvious difference of light polarization properties is found in the InGaN/GaN MQW LEDs with different wavelengths. The polarization degree decreases from 52.4% to 26.9% when light wavelength increases. Analyses of band structures of InGaN/GaN quantum wells and luminescence properties of quantum dots imply that quantum-dot-like behavior is the dominant reason for the low luminescence polarization degree of blue LEDs, and the high luminescence polarization degree of UV LEDs mainly comes from QW confinement and the strain effect. Therefore, indium induced carrier confinement （quantum-dot-like behavior） might play a major role in the polarization degree change of InGaN/GaN MQW LEDs from near violet to blue.     

Strain effects on the polarized optical properties of InGaN with different In compositions

Strain effects on the polarized optical properties of c-plane and m-plane InxGa1-xN were discussed for different In compositions (x=0, 0.05,0.10,0.15) by analyzing the relative oscillator strength (ROS) and energy level splitting of the three transitions related to the top three valence bands (VBs). The ROS was calculated by applying the effective-mass Hamiltonian based on k·p perturbation theory. For c-plane InxGa1-xN, it was found that the ROS of |X and |Y -like states were superposed with each other. Especially, under compressive strain, they dominated in the top VB whose energy level also went up with strain, while the ROS of the |Z -like state decreased in the second band. For m-plane InxGa1-xN under compressive strain, the top three VBs were dominated by |X,|Z, and |Y -like states, respectively, which led to nearly linearly-polarized light emissions. For the top VB, ROS difference between |X and |Z -like states became larger with compressive strain. It was also found that such tendencies were more evident in layers with higher In compositions. As a result, there would be more TE modes in total emissions from both c-plane and m-plane InGaN with compressive strain and In content, leading to a larger polarization degree. Experimental results of luminescence from InGaN/GaN quantum wells (QWs) showed good coincidence with our calculations.