Fabrication And Characterizations Of Nanostructured GaN Material And Optoelectronic Devices

After the semiconductor materials represented by the first generation Ge and Si,and the compound semiconductor materials represented by the second generation GaAs and InP,the third generation semiconductor materials which is represented by GaN and SiC has become the forefront and hot spot of semiconductor research all over the world.Due to the high breakdown voltage,high electron drift velocity,stable physical and chemical performance,high thermal conductivity and dielectric constant in GaN material make it popular in the fabrication of microelectronics and optoelectronic devices and other fields.Currently,GaN-based monocrystalline films are grown on sapphire substrate by heterogeneous epitaxial metal-vapor deposition(MOCVD).However,there is a high lattice mismatch rate between gallium nitride and sapphire substrates which makes GaN-based films have high defect density and high residual stress,which affects the stability and reliability of GaN-based devices.Therefore,it is necessary to find a simple and effective method to reduce defect density and residual stress of GaN-based films.Because the nanoporous GaN-based films fabricated by wet etching can better solve the problems of high defect and high stress and further improve the quality of crystals,the research on the fabrication and properties of nanoporous GaN films has attracted more and more attention.After years of efforts,metal-assisted chemical etching,photo-assisted electrochemical etching and electrochemical etching have been widely used.In this thesis,electrochemical etching technology and MOCVD technology were used to grow heteroepitaxial on surface c sapphire substrate and fabricate porous GaN DBR,LED based on porous GaN DBR and GaN-based detector.By means of DBR structure design,process development and optimization,distributed Bragg mirror(DBR)with nano porous GaN,nanoporous GaN-based LED and GaN-based photodetector with high reflectivity were fabricated.The main research contents of this thesis include the following parts:1.Electrochemical etching technology was used to etch heavily doped GaN periodic films in oxalic acid and sodium nitrate(NaNO3)etch solution to fabricate porous GaN DBR films.The formation conditions of high quality GaN DBR were studied systematically by measuring the structure and reflection characteristics of etched samples.Compared with oxalic acid etch solution,DBR mirror etched by NaNO3 has the advantages of higher reflectance and larger window.In order to study the etching uniformity,we prepared 2-inch porous GaN DBR in NaNO3 etching solution,and found that the DBR fabricated in this way had better uniformity by measuring the reflectivity and the difference of half height and width.By simulating the reflectivity of porous GaN DBR by the method of transmission line model(TLM),we can determine the refractive index change of the etched porous GaN layer and the influence of the thickness and number of layers on the performance of GaN DBR.According to the simulation results,increasing the thickness of the layer and changing the refractive index of the layer can increase the reflectivity of GaN DBR and the width of stop band.2.After the epitaxial growth of GaN LED structure with heavy doped GaN periodic structure on sapphire substrate by using MOCVD technology,the LED with high reflectivity DBR was fabricated by electrochemical etching.2-inch LED sample were etched in NaNO3 etching solution at different voltages through different regions.After measuring and comparing the luminous intensity and luminous power of LED samples before and after etching,it was found that the luminous power of LED samples fabricated under the etching voltage of 18 V reached the highest which was 4.7 mW,and the crystal quality was relatively stable.This may be attributed to the high reflection effect of DBR on the emission of light from the active layer(InGaN/GaN multi-quantum well layer)after 18 V etching,the low etching voltage will not cause damage to the LED structure,and the stress relaxation after etching leads to the increase of its internal quantum efficiency.3.After the fabrication of GaN MSM UV photodetector by MOCVD,annealing,electron beam evaporation and thermal evaporation,the HfO2 insulation layer was deposited by ALD.By comparing the performance parameters of GaN detectors before and after the deposition of the insulating layer,it was found that the dark current of the device decreased by 17 times after the deposition of the insulating layer of HfO2,which could be attributed to the formation of a high barrier between the metal and the upper surface of the GaN device by the insulating interface layer of the high-k medium HfO2.By comparing the response time under 405nm laser irradiation,it can be found that the detectors before and after the deposition insulation layer show good repeatability and time-varying stability,and the response time is short.The responsivity and detection rate before and after the deposition insulation layer are slightly reduced,but the change is not significant at the same order of magnitude.These results show that GaN photodetector has great application potential after deposition of HfO2 insulation layer.