Study On Unidirectional Emission Of GaN-on-Si Microring Lasers

Gallium nitride(GaN)-based microdisk(microring)lasers grown on Si combine with both advantages of Ⅲ-Ⅴ nitride semiconductor materials and features of whispering gallery mode microcavities,which have advantages of high luminescence efficiency,small footprints,high quality factor,low power consumption,in-plane light coupling and large modulation bandwidth,as well as are expected to be promising onchip light sources with great application potential in the fields of optoelectronic integration,on-chip optical interconnect and visible light communication.However,the highly rotational symmetry of circular microcavity usually leads to in-plane isotropic emission and hence inefficient light collection.Besides,this isotropic light interferes with other adjacent devices,which is not beneficial to optical signal transmission and limits their applications.In order to solve the above problems,we carefully designed and prepared GaN-on-Si microring lasers based on a near-fully clad structure,aiming to simultaneously realize the unidirectional emission and efficient coupling output of microring lasers.The main research contents and results are as follows:1.The effects of dielectric films deposited by plasma-cnhanced atomic layer deposition(PEALD)and plasma-enhanced chemical vapor deposition(PECVD)on the optical and electrical properties of the GaN-on-Si microring laser were investigated in comparison.It was found that the microring laser passivated with the SiO2 layer grown by PEALD(PEALD-SiO2)had lower lasing threshold,higher luminous efficiency and weaker edge size effect dependence,which was mainly because dielectric films grown by PEALD could better passivate sidewall defect states.The trap-state density of the microring laser with PEALD-SiO2 passivation was reduced by 70%compared to that of the device passivated with the Si3N4 layer deposited by PECVD(PECVD-Si3N4).In addition,the step coverage of the metal films deposited by electron beam evaporation and magnetron sputtering was also compared.The experimental results showed that the metal films were basically unattached on the microring sidewall by electron beam evaporation,while the growth rate ratio of Ag/Ti/Au on the sidewall and surface of the microring laser was obtained by magnetron sputtering of about 0.33/0.36/0.45.2.The unidirectional emission of GaN-on-Si microring laser with a near-fully clad structure was successfully prepared by coating a dielectric layer and a metal layer on the microring sidewall in turn,in which the lateral confinement of the optical field was enhanced by taking advantage of the high reflectivity of metallic silver to visible light;and by integrating a direct coupled waveguide at the microring rim to achieve the coupled light output.The GaN-on-Si microring laser realized room-temperature electrically pumped lasing with a similar threshold current as compared with the conventional microring laser without metal layer coating.More importantly,the light emission around the microring laser coated with the metal layer was efficaciously confined and outputted only from the coupled waveguide.The optical power collected along the waveguide direction was effectively increased,and the optical crosstalk to the adjacent devices was reduced by 81.6%,which successfully achieved the unidirectional emission and efficient coupling output of the GaN-based microring laser.3.By integrating another microdisk at the other end of the coupled waveguide as photodetector,and the preliminary on-chip integration of GaN-based microring laser,waveguide and photodetector was demonstrated.Moreover,the GaN-on-Si microring laser had a RC-limited bandwidth of 10.8 GHz,showing great potential applications for high-speed in-plane data transmission,visible light communication and optical integration on a GaN-on-Si platform.