Optimal Design Of VCSEL Sub-wavelength Grating P-plane Mirror

The resonant cavity of VCSEL is short,which requires the number of distributed Bragg reflector(DBR)pairs to reach 20-40,so as to meet the lasing demands of the device.While when the number of DBR pairs is large,it not only increases the series resistance and processing difficulties,but also introduces barriers in miniaturization,especially for the high resistance characteristics and light absorptions of P-type DBR,which will greatly affect characteristics of devices.Therefore,optimizing P-plane reflecting mirrors is necessary in improving performances of vertical cavity surface emitting lasers.In recently years,with developments of micromachining technologies and theoretical researches,subwavelength gratings have attracted more and more attentions.VCSEL’s P-plane reflecting mirrors can be replaced by subwavelength gratings to reduce high series resistances and large absorption losses caused by multilayer DBRs,through which,at the mean time,output quality and polarization characteristics of VCSELs can be improved and miniaturizations of devices can be achieved.This thesis mainly deals with analogue simulations and fabrications of high-contrast sub-wavelength grating(HCG)mirrors in VCSELs.Firstly,diffraction characteristics of subwavelength grating mirrors were theoretically analyzed through rigorously coupled wave theory(RCWA)and equivalent medium theory(EMT),then a simulation software based on rigorously coupled wave theory was adopted to design a 850 nm TM polarized high-contrast subwavelength grating mirror,which was of the same material system as the GaAs-based VCSEL.Through theoretical analyses and numerical simulations,influences of different grating parameters on reflectance was analyzed when light was incident perpendicular to the substrates,and the optimal value as well as tolerance range were given.The reflectance under TM polarization for normal incidence to the surface was also investigated.Besides,considering the limiting factors in practical preparations,influences of the grating shape on the reflectance were studied.Processes and conditions were optimized during fabrications of HCG,and particularly,studies on key technologies were performed,such as electron beam exposure and ICP etching.In conclusion of this study,the exposure dose had a great influence on the duty cycle and morphology of HCG.Thus,high-precision and good-profile images could be obtained through choosing a suitable exposure dose.Effects of the groove width set on the duty cycle,those of the HCG duty cycle and other factors on the grating etch rate as well as morphologies during ICP etching were studied in depth.Based on the above researches,HCG mirrors with structural parameters that could meet design requirements were fabricated through epitaxial growth,ultraviolet lithography,wet etching,oxidation and other processing technologies.Testing results show that broadband high reflections could be achieved through sub-wavelength grating mirrors in the 850 nm band.The results provide theoretical and technical supports for applications of HCG in GaAs-based VCSELs.