The Studies On Electrically Pumped ZnO Ultraviolet Lasers

Zinc oxide(ZnO)as a wide bandgap semiconductor with large exciton binding energy(60 meV),has been considered a promising material for efficient and low threshold lasing in ultraviolet(UV)region.Quite a few attempts have been made to realize ZnO based electrically pumped lasers.However,the performance of the ZnO lasers needs still much improving.In this thesis,we aimed to enhance the performance of ZnO lasers,and the main results achieved were listed as follows:(1)Piezophototronic effect can be used to tune the carrier transportation,seperation and recombination,thus has been utilized to improve the performance of many optoelectronic devices such as solar cells,photodetectors and light-emitting devices.It has been stated that ZnO has the highest piezoelectric tensor among the tetrahedrally bonded semiconductors.If the two unique characteristics of ZnO,large exciton binding energy and high piezoelectric coefficient can be employed simultaneously,remarkable results will be achieved.According to the above speculation,we utilized piezophototronic effect to enhance the performance of electrically pumped lasers for the first time.The threshold voltage of the lasers has been reduced from 48 to 20 V by applying a pressure of 16.0 kPa on the device,and the emission efficiency and injection current at a fixed applied voltage of 50 V have been enhanced by a factor of 3.95 and 1.25,respectively.Meanwhile,the output power of the device has been enhanced by 496%.The improvements of the lasing characteristics can be attributed to the improved current injection and enhanced emission efficiency via piezophototronic effect.The results may provide a route toimprove the performance of laser devices based on piezoelectric semiconductors.(2)Plasmonic lasers,which utilize surface plasmon polaritons to achieve optical confinement,feedback and amplification,have strong electric field confinement and high spontaneous emission factor.The strong electric field confinement enables smaller physical size and faster operation speed of the plasmonic lasers.Furthermore,the high spontaneous emission factor can significantly reduce the threshold.In the UV region,the electromagnetic field frequency is close to the surface plasmon frequency,maximizing both confinement and Ohmic loss of the metal.The high loss impedes realization of plasmonic lasers in the UV region.Hybrid plasmonic waveguides consisting of gain medium,separated from metal films by low loss spacer layers,have been proposed to reduce the Ohmic losses.This hybrid plasmonic structure leads to a strong optical confinement in the thin gap region with relatively low propagation loss.We realized the first electrically driven UV plasmonic laser,with an ultralow threshold of 9.1 A/cm~2,based on p-GaN/MgO/ZnO/MgO/Ag structures.Here ZnO was chosen as the gain media to match the surface plasmon frequency of Ag.More importantly,the large exciton binding energy of ZnO(60 meV)is beneficial to the occurring of lasing.In this structure,electric field is strongly confined in the MgO gap region,which enhances the spontaneous emission factor and reduces the damping loss of Ag.The results may address a step towards UV electrically driven plasmonic lasers and provide a way to improve the performance of ZnO based UV lasers.