The surface structures ofwurtzite ZnO(0001) and(0001) surfaces are investigated by using a first-principles calculation of plane wave ultra-soft pseudo-potential technology based on density functional theory(DFT).The calculated results reveal that the surface energy of ZnO-Zn is bigger than that of ZnO-O,and the ZnO-Zn surface is more unstable and active.These two surfaces are apt to relax inward,but the contractions of the ZnO-Zn surface are smaller than the ZnO-O surface.Due to the dispersed Zn4s states and the states of stronger hybridization between the Zn and O atoms,the ZnO-Zn surface shows n-type conduction,while the O2p dangling-bond bands in the upper part of the valence cause the ZnO-O surface to have p-type conduction.The above results are broadly consistent with the experimental results.
GaN-based multiple quantum well light-emitting diodes(LEDs) with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop.The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.
