Anisotropic optical gain in m -plane Inx Ga1-x N/GaN multiple quantum well laser diode wafers fabricated on the low defect density freestanding GaN substrates

The threshold power density for the stimulated emission (SE) at 400 nm of m -plane In0.05 Ga0.95 N/GaN multiple quantum well (QW) laser diode (LD) wafer excited with a stripe along the c -axis was found to be lower than along the a -axis, although the SEs exhibited transverse electric field mode for both configurations. The result was explained according to the polarization selection rules for the lowest and the second lowest energy interband transitions in anisotropically strained m -plane InGaN QWs. In case of the LD wafer lased at 426 nm, SE was observed only along the c -axis, where pronounced broadening of the gain spectrum was found. Because the equivalent internal quantum efficiency was only 44%, further reductions in nonradiative defect density and the width of gain spectrum are essential to realize longer wavelength LDs.