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.
Wet etching has been widely used in defect evaluation for Ga-face GaN and surface roughness for N-face GaN dodecagonal pyramids has been fabricated on laser-lift-off N-face GaN by hot phosphor acid etching.The dodecagonal pyramid shows twelve facets including six{20-2-3}and six{22-4-5}planes.From cross-sectional TEM image,it is shown that the pyramid corresponds to the top of the edge dislocation.Compared with hexagonal pyramid-surface light emitting diodes(LEDs)etched by commonly used photoelectrochemical(PEC)process in KOH aqueous,the dodecagonal pyramid-surface LEDs show improved light extraction efficiency because of more facets,which effectively reduces the total internal reflection.
QI ShengLi,CHEN ZhiZhong,SUN YongJian,FANG Hao,TAO YueBin,SANG LiWen,TIAN PengFei,DENG JunJing,ZHAO LuBing,YU TongJun,QIN ZhiXin&ZHANG GuoYi State Key Laboratory for Artificial Microstructure and Mesoscopic Physics,School of Physics,Peking University,Beijing 100871,China
We have investigated the transverse mode pattern and the optical field confinement factor of gallium nitride （GaN） laser diodes （LDs） theoretically. For the particular LD structure, composed of approximate 4 μm thick n-GaN substrate layer, the maximum optical confinement factor was found to be corresponding to the 5^th order transverse mode, the so-called lasing mode. Moreover, the value of the maximum confinement factor varies periodically when increasing the n-side GaN layer thickness, which simultaneously changes and increases the oscillation mode order of the GaN LD caused by the effects of mode coupling. The effects of the thickness and the average composition of Al in the AlGaN/GaN superlat.tice on the optical confinement factor are also presented. Finally, the mode coupling and optimization of the layers in the GaN-based LD are discussed.
