Fabrication And Characteristics Of 3D Patterned Surplane Structure Of GaN Epitaxy Materials Of Xidian University

GaN and its related compound materials(In GaN/Al GaN and In Al GAN)have direct bandgap across the spectrum from infrared light to UV-lightand and they are ideal materials for future solid-state lighting applications.GaN-based LED,very small in size,is a highly-efficient,energy-saving light emitting device.Recently,with the improvement of the GaN growth techniques,LED has witnessed a rapid development and played a more and more important role in our daily life.In the past few years,regular GaN-based LED grown on the plane structure c-plane GaN epi layer had relatively low luminescence efficiency due to quantum-confined stark effect.Compared with the regular GaN materials with plane structure,with both polar and nonpolar surplanes has larger specific surface and thus the LED grown on 3D material has higher lighting efficiency,lower electrical current density under same electrical current.Thus,3D materials show great potential in achieving higher internal quantum efficiency and light extraction efficiency.In this thesis,we presented stripe mesa structure and column mesa structure fabrication method and used wet etching in order to produce the column mesa with controllable side wall tilt angle.And the morphology and optical property was studied by SEM/AFM and PL/Raman spectrum respectively.(1)Stripe mesa structure was fabricated by lithography and reactive ion etching(RIE).The characterization results show that the stripe mesa has very steep side wall.The compressive stress existed in the planar GaN layer is relieved after etching.The band edge PL intensity is lower than that of planer material and the band edge peak position is red-shifted.We assume that the vertical sidewall may decrease the light absorption and radiation.And the smooth top plane of the mesa and the smooth area between each mesa will enhance the total reflection.Thus the PL intensity became lower after etching.(2)Column mesa structure was fabricated by microsphere mask and RIE on GaN epi layer of different plane.The results show that the etching process is anisotropic with high etching rate along c axis and relatively low etching rate horizontal to the c axis.The compressive stress existed in the planar GaN layer is relieved after etching.C-plane and a-plane GaN have almost the same etching rate while the etching rate of r-plane is higher.For c-plane and a-plane GaN,the band edge PL intensity are weakened and the band edge emission peaks are red-shifted due to etching.The deeper etching depth corresponds with lager red shift amount.For r-plane GaN,the band edge PL intensity is increased and the longer etching time corresponds with stronger intensity.The change of the purple and yellow light intensity also follow this trend.(3)The column mesa structure is treated with TMAH solution.For c-plane GaN,closely-packed round top surface is etched to be hexagon and didderent etching pits begin to exist.The etching rate along Ga plane and N plane are different.The band edge PL intensity increases after wet etching.But the intensity decreases after long time wet etching because the longer etching time makes the side wall become vertical which weakens the light absorption.For a-plane GaN,the round top surface disappears after wet etching and triangle wet etching pits of different sizes begin to exist.The columnar structure is etched to be cone structure with tilted side wall.After wet etching,the band edge PL intensity increased and yellow light luminescence intensity decreases.For r-plane GaN,the high etching rate is along the height direction.The round top surplane disappears and etching morphology along the c axis begins to exist.The band edge PL intensity increases after wet etching while the purple light luminescence intensity almost remains the same and the yellow light luminescence intensity is slighted decreased.