Interface Design And Mode Modulation Of ZnO Microcavity Ultraviolet Lasers

With the advent of the post Moore Law Period,the demand for nanotechnology in opto-electronic integration is very urgent,and UV short-wave optical source is a necessary condition for realizing high spatial resolution devices.ZnO of the hexagonal wurtzite structure has a wide direct band gap（3.4 e V）and large exciton binding energy（60 me V）,which has a unique physical advantage for realizing UV lasing.Owning to their excellent optical microcavity and high refractive index,One-dimensional（1D）ZnO micro/nanowire has an excellent optical field limitation and electrical transport capability.In the last few decades,the research of ultraviolet laser based on ZnO has achieved fruitful results.However,most ZnO micro/nanostructured lasers are optically pumped lasing or electrically pumped random lasing.In comparison,small size lasing with fixed modes is more beneficial for important applications such as on-chip integration.However,such small cavity will lead to the heavy optical loss at the nano cavity surface or interface,so it is difficult to form effective oscillation.More importantly,In electrically pumped nanolaser,not only the optical loss at interfaces is still in challenge,but also the carrier transport and electrical contact are particularly important.In addition,how to realize the laser output with dynamically adjustable emission wavelength is always a key problem to be solved for practical application.At present,the commercial method of modulation mode dynamically is mainly realized through the electro-optic effect of electro-optic crystals.However,the modulation is operated passively in general.It will be great favorable for optoelectronic integration and other important applications when a gain medium with electro-optic effect is applied to design an optical microcavity integrating optical pumping and electro-optic modulation.In this paper,based on the n-ZnO/p-Ga N heterojunction,firstly the interface optical loss of the device is reduced and the carrier recombination efficiency is improved by insert a suitable interface buffer layer.The UV lasing behavior in electrically pumped nano-scaled heterojunction diode is realized.Then,the luminescence intensity of the device is effectively improved and a clear and stable electroluminescence spectrum of the oscillation mode is obtained by constructing a perfect core-shell structure.Finally,based on the excellent optically pumped laser performance of the ZnO microcavity and the electro-optic effect of ZnO,the integration of laser generation and regulation is realized in the same microcavity.The main contents are listed as following:1.It is in challenge how to overcome the heavy optical loss in ZnO/Ga N interface and how to inject charge carrier effectively for sufficient gain.Facing to the issues,an individual hexagonal ZnO nanorod was bonded on p-Ga N to construct a heterojunction through PEDOT/Hf O₂ buffer in our case.As the injection current reached 6 m A,the FWHM of the emission spectrum quickly narrowed from～30 nm to 1.4 nm,and the intensity increase dramatically.This demonstrated the UV lasing behaviors in a nano-scaled heterojunction diode.Optically,the inserted buffer with reasonable energy band configuration and low reflective index reduces the optical loss caused by the similar refractive index of ZnO and Ga N,hence decreases the lasing threshold from 1.55μW to 1.32μW of the nanocavity.Electrically,a reliable electrical contact is guaranteed by inserting the organic polymer PEDOT facilely.More importantly,PEDOT and Hf O₂ form a step energy band configuration in the p-Ga N/ZnO heterojunction for efficient charge carrier injection,thus contribute to improve optical gain and realizes the UV stimulated radiation.2.Facing to the issues of heavy optical loss and low carrier injection efficiency,the core-shell structure is constructed by the classical magnetic sputtering method,and the n-ZnO nanorod@BN/p-Ga N heterostructure device is fabricated.After the active region is coated with BN shell,the surface of ZnO nanorods is modified,the near-band-edge emission of ZnO nanorods is increased up to 85 times,the defect emission is suppressed about 63%.The results indicated that he BN shell not only improves the carrier recombination efficiency,but also suppresses the heavy optical loss at the ZnO/Ga N interface.When the injection current is 2.5 m A,the EL intensity increased up to 4.5 times and an EL spectrum with clear and stable oscillation mode is obtained.3.In order to integrate lasing generation and modulation in the same microcavity to achieve the integration of lasing generation and modulation,the ZnO microcavity with electro-optic effect is used as the gain medium to generate optically pumped UV lasing,and the dynamic modulation of lasing is achieved by applying electric field.When the electric field is parallel to the ZnO C-axis,the WGM lasing mode red-shifts with the increase of the applied bias,and the rate is～0.02 nm/V.The research results show that the refractive index variation is correlated linearly with the electric field strength,so-called the Pockels effect,and the electro-optic coefficient of 7.68×10^-9m/V is obtained near the bandgap energy.