Nanoscale Manipulation Of GaN Porous Structures And Applications In Optical Engineering

At present,the semiconductor industry has penetrated into every corner of life,supporting the development and progress of the information technology science.In the past half century,the development of the first and second generation semiconductor materials has changed people’s way of life,production,communication and thinking,thus having a profound impact on human society.In recent years,the third-generation semiconductor materials such as GaN and SiC have attracted more and more attention and are considered as "new generation information functional materials and devices" and"strategic emerging industries".GaN possess excellent properties such as wide band-gap,high breakdown electric field,high saturation velocity,high temperature resistance,anti-radiation and chemical inertness,showing the huge application prospects in semiconductor lighting,5G communication,power electronics,aerospace,and so on.At present,GaN epitaxial films are mostly grown on heterogeneous substrates such as sapphire(Al2O3)and Si wafers by metal-organic chemical vapor deposition(MOCVD).Due to the high lattice mismatch and thermal mismatch with the foreign substrates,the GaN single crystal films have large residual stress and defect density(108-109 Cm-2),which affect the performance of devices.In addition,due to the small specific surface area of film materials,it is difficult to show excellent properties in the fields of energy absorption/storage,and optical engineering.Therefore,the construction of new structural morphologies of GaN films is conducive to improving the properties and extending their application.Nano-sized pores can be introduced into GaN single-crystal films by means of electrochemical(EC)etching technique.Porosification of GaN can release the residual stress of film,increase the specific surface area,and improve the optoelectronic properties.Meanwhile,EC etching,as a defect etching technique,can reduce the defect density of the films.The thesis will first study etching mechanism of GaN nanopores,and then focus on the preparation of GaN porous structures and expand the application of nanoporous GaN(NP-GaN)in optical engineering.The main researches of the thesis are as follows:1.The effect of electrolyte type on GaN pore morphology was systematically studied,and the growth mechanism of GaN nanopores was illuminated.Using EC etching technology,Si-doped n-type GaN epitaxial films were etched in NaOH,HNO3,NaNO3 and NaCl electrolytes,respectively,to prepare NP-GaN films.SEM images shows that the average diameters of the GaN nanopores prepared in NaOH,HNO3,NaNO3 and NaCl solutions were～20.5,～14.7,～10.5 and<5 nm,respectively,and the pores grew along the vertical,inclined,and parallel to the film/substrate interface,respectively.The variation of GaN pores morphology was analysed by the depletion model.The effects of pore spacing in film surface and thickness of space charge region on the nanopores morphologies were discussed.Furthermore,we proposed that the diversity of pore growth directions can be explained by the comprehensive actions of GaN[0001]and<112 0>etching rates.Electrochemical impedance spectroscopy was used to investigate the electrode reaction and kinetics at film/electrolyte interface.The study found that:(1)The etching mechanism of NaOH electrolyte is slightly different from that of HNO3,NaNO3 and NaCl electrolytes.(2)In HNO3 solution,the charge transfer resistance Rct between electrode and solution interface is the smallest,and the electrode reaction rate is the fastest.Because the oxide layer is the main barrier to charge exchange during the electrode reactions,higher H+concentration can accelerate the dissolution of oxide.(3)In the NaCl electrolyte,Cl-and Ga3+tend to form a stable Ga-Cl bonds on the GaN(0001)plane,which inhibit the further progress of the reaction.Therefore,it is difficult to prepare high performance NP-GaN DBR mirror.2.A series of NP-GaN DBR mirrors in the visible-near-infrared region were designed and fabricated by combining software simulation with EC etching technique.On this basis,a new method to realize exfoliation and transfer of a large-area,crack-free,high-quality NP-GaN DBR film is proposed.(1)The optical properties of DBR mirror are simulated and calculated by software.It is found that the stopband width is basically linearly related to the Bragg wavelength.The more stacked layers of DBR,the greater the peak reflectance(when the number of stacked layers exceed 15 pairs,the peak reflectivity is>99.9%).In the preparation experiments,a series of NP-GaN DBRs with high reflectance(>95%)were prepared by changing the layer thickness and controlling the etching bias.The prepared DBR mirror can cover the wavelength range of 570-1720 nm,and the NP-GaN DBR mirror in the near-infrared region was prepared for the first time.When the Bragg wavelengths of samples are～615,～815,～1100,and～1625 nm,the corresponding stop-band widths are～80,～110,～120,and～180 nm,respectively,which are approximately consistent with the calculated results.(2)GaN epitaxial layer with porous structure were annealed at high temperature in NH3 atmosphere.From the SEM images,it is found that the pore morphology of the porous layers changed obviously before and after annealing,from uniformly distributed and denser columnar pores to sparse spherical pores,and the combination of adjacent pores appeared.Based on this,the GaN epitaxial structure was redesigned:a heavily doped n+-GaN sacrificial layer was embedded under the NP-GaN DBR layer.Combined with EC etching and high-temperature annealing,the high quality,self-standing NP-GaN DBR was prepared.From the optical microscope,the self-supporting NP-GaN DBR film surface is flat and smooth without obvious cracks and fragments.The root mean square roughness(RRMS)measured by AFM was 0.934 nm,which is slightly higher than the RRMS value of the etched GaN sample(0.632 nm).In Raman and PL spectra,the positions of the E2 phonon peak and the emission peak of the exfoliated GaN films showed obvious red shift compared to the unetched samples,indicating the residual stress in the self-standing film is released.3.Using pulsed laser deposition(PLD)technique,Eu-doped Lu2O3(Lu2O3:Eu)single crystal films were grown on NP-GaN DBR and GaN vertical nanopore arrays,respectively.The effects of temperatures and substrates on the structure,crystallinity,morphology and luminescence properties of Lu2O3 films were systematically studied,and the enhancement mechanism of GaN ordered porous structure on optical extraction efficiency is illustrated.(1)According to the emission characteristic of Eu3+,NP-GaN DBR films with peak reflectance>95%,stop-band at～560-～660 nm and RRMS value of 0.737 nm were designed and prepared.XRD patterns shows that the films grown on DBR substrates are single-crystal Lu2O3(111)at different temperatures(380-580℃).When the substrate temperature is 480℃,the crystalline quality is the best.The film has a double domain structure,and the out-of-plane and in-plane epitaxial relationships between the film and the substrate are Lu2O3(111)‖GaN(0001)and Lu2O3[110]‖GaN[1210],respectively.XPS results show that the atomic ratio of Eu/(Eu+Lu)in the film is about 5.4 at.%,which is close to the stoichiometric setting value of the target.The PL intensity(using the strongest emission at 613 nm as a reference)of the Lu2O3:Eu film deposited on the DBR substrate is～3.4 times higher than that of the film on the unetched GaN substrate.Multi-reflection model analysis shows that the enhancement factors of excitation and emission light are～1.4 and～2.6 times,respectively,and the total PL enhancement factor is～3.64 times.The results demonstrate that the NP-GaN DBR substrate can be used as a large-area,broadband optical coupling platform for heteroepitaxial films.(2)The Lu2O3:Eu single crystal films were grown on GaN vertical nanopore arrays prepared by EC etching technique.The surface pore densities/sizes of the samples prepared at 8 V,12 V and 16 V are～1.2 × 1010 cm-2(～10 nm),～1.3 × 1010 cm-2(～15 nm),～4×1011 cm-2(～18 nm),respectively.Because the surface damage of substrate caused by EC etching has a negative effect on the growth of Lu2O3:Eu films,the crystalline qualities decrease slightly with the increase of the etching voltage of substrates.Compared with the PL intensity of the Lu2O3:Eu film on the reference substrate,the PL intensity of the film deposited on GaN nanopore arrays is increased by 47%,which can be attributed to the scattering and reflection of the nanopores and photoluminescence enhancement by a resonant cavity formed between substrate and film surface.In order to further improve the luminescence efficiency,GaN nanopore arrays was annealed at high temperature in NH3 atmosphere,resulting in the GaN transformation of the inner wall from amorphous to single crystal.Compared with the unannealed substrate,the PL intensity of film deposited on annealed substrate increased by 34%.This is because the smooth single crystal inner wall of nanopore has a lower photon capture capacity.