Realization Of Deep Ultraviolet AlGaN-based Vertical-cavity Surface-emitting Lasers

The group-Ⅲ-nitride direct bandgap semiconductor has various advantages of high thermal stability,radiation resistance,corrosion resistance,high breakdown voltage,and high thermal conductivity.It is thus suitable for light-emitting devices.Among these devices,the vertical-cavity surface-emitting laser(VCSEL)has attracted significant research interest.Electrically pumped VCSELs have been demonstrated in the visible-light range longer than 400 nm.However,several studies of VCSELs lasing in the sub-400-nm regime have been reported.The shortest wavelength of 363 nm was reported by Joan in 1996.The AlGaN semiconductor bandgap varies from 3.4 to 6.0 eV,nearly covering the entire ultraviolet(UV)range.Recently,with the improvement of the epitaxial crystal quality of AlGaN material by metal-organic chemical vapor deposition,AlGaN is expected to be the first choice for constructing the active region of UV VCSELs.In the present work,a deep UV(DUV)planar microcavity with an AlGaN active region was designed and fabricated,and optical loss in the cavity was studied.Effective methods for suppressing the optical loss were developed and used to smooth the AlGaN surface after laser lift-off(LLO).Finally,a DUV VCSEL was successfully demonstrated.The main details of this study are the following.(1)Considering the needs of the LLO process,two structure epilayers grown on sapphire substrate were employed:an AlGaN quantum dot(QD)active region grown on the GaN buffer layer,and a AlGaN quantum well(QW)active region grown on the AlGaN(average Al composition<60%)buffer layer.Photoluminescence(PL)measurements for the two epilayers were performed.Three distinct emission peaks were observed at 300,316,and 338 nm from the epilayer with QD structure at room temperature(RT),which originated from different energy levels in the QDs.At RT,the polarization-field-induced quantum confined Stark effect(QCSE)in the QDs was observed.However,at low temperature,the polarization field was weak and the QCSE was not obvious.The internal quantum efficiency(IQE)of the QDs was estimated to be 55.53%,measured by temperature-dependent PL(TDPL).In the PL measurements of the epilayer with QW structure,the emission center of the epilayer was at 274.71 nm at RT.Emission intensity varied linearly with excitation energy,indicating that radiative recombination dominated the emission at RT,which suggests low dislocation density in the QW active region.The IQE of the QWs was estimated to be 62%by TDPL measurements.(2)The LLO process was optimized for the two structure epilayers separately,and substrate transference was demonstrated for both epilayers.The LLO process for the QW epilayer was performed by decomposing the n-Al0.6Ga0.4N layer.The optimized laser energy density used in LLO was 2.5 and 3.7 J·cm-2/pulse for QD and QW epilayers,respectively.Then,a DUV planar microcavity with dielectric distributed Bragg reflectors(DBRs)was successfully fabricated to study the optical loss in the cavity.Four cavity modes,that is,at 305,314,323,and 335 nm,were observed.Owing to the extreme high roughness of the surface after LLO,optical scattering loss at the interface was strong,and the optical loss in the cavity was on the order of 103 cm-1.It is proposed that the quality value of the cavity can be improved by reducing the roughness of the interface or setting the rough interface at the node of the optical field.(3)The process for reducing the optical loss was studied and the AlGaN surface after LLO was smoothed.A photo-assisted chemical etching(PCE)method was proposed to smooth the AlGaN surface after LLO.Under optimized conditions[(0.3 X 10-3-mol/L KOH solution combined with 0.5-W/cm2(at 360 nm)irradiation intensity],the root-mean-square roughness of the QD epilayer surface after LLO decreased from 30.5 to 5.6 nm.After applying the PCE process in the fabrication of DUV planar microcavity,the Q value increased from 174 to 270.Another method employed was optimizing the chemical mechanical polishing(CMP)process,which was used to smooth and thin the QW epilayer surface after LLO.After CMP,stimulated radiation was observed from the epilayer.The emission peak was at 278.7 nm with a linewidth of 3 nm.This suggests the optimized CMP process is beneficial to obtaining a smooth epilayer surface,decreasing optical loss,and,hence,facilitating stimulated radiation.(4)An optically pumped DUV VCSEL with dielectric DBR structure was demonstrated using the AlGaN QW epilayer.Before the deposition of the top DBR,stimulated emission peaks at 280 nm were observed.After finishing the fabrication of the device,lasing at 275.91 nm was observed with a linewidth of 0.78 nm and threshold power density of 1.21 MW/cm2.This is the shortest wavelength VCSEL and the first lasing in the UVC range(200-280 nm)for VCSELs.