Polarization characteristics of semipolar (11-22) InGaN/GaN quantum well structures grown on relaxed InGaN buffer layers and comparison with experiment

Partial strain relaxation effects on polarization ratio of semipolar (11-22) InxGa1xN/GaN quantum well (QW) structures grown on relaxed InGaN buffers were investigated using the multiband effective-mass theory. The absolute value of the polarization ratio gradually decreases with increasing In composition in InGaN buffer layer when the strain relaxation ratio (0 y-y- y-y- )/0 y-y- along y-axis is assumed to be linearly proportional to the difference of lattice constants between the well and the buffer layer. Also, it changes its sign for the QW structure grown on InGaN buffer layer with a relatively larger In composition (x > 0.07). These results are in good agreement with the experiment. This can be explained by the fact that, with increasing In composition in the InGaN subsrate, the spontaneous emission rate for the y-polarization gradually increases while that for x-polarization decreases due to the decrease in a matrix element at the band-edge (k|| = 0).