Characterization And Analysis Of Wide Bandgap Semiconductor Ⅲ-Nitrides Thin Films Grown By MOCVD

Wide bandgap semiconductor materials have become the key materials for developing semiconductor devices with high frequency,high power,high temperature resistance and radiation resistance due to their characteristics of high breakdown electric field,good thermal conductivity,high electron saturation rates and radiation resistance.However,manufacturing yields,device performance and long-term reliability are severely limited by material defects.In addition,the extreme electrical and thermal stresses will be caused by the inherent high electric field and high current generated during operation for high-voltage and high-power switching devices.In order to improve the material defects and stress problems,and enhance the optical properties of semiconductor films,three kinds of semiconductor epitaxial materials,Be implanted GaN,AlN and AlGaN were designed and prepared by metal organic chemical vapor deposition（MOCVD）method in this thesis.The morphology structure,stresses,optical properties,defect evolution,energy band structure,and cross-sectional correlation properties of materials were characterized by AFM,SEM,XRD,Raman,SE,TEM,UV-Vis and XPS.The main contents and conclusions are as follows:（1）The Be-implanted GaN samples were prepared by MOCVD and ion implantation,and subjected to a series of high temperature rapid thermal annealing（RTA）.The structural morphology,annealing effects and optical properties of the materials were systematically investigated.The results show that there is obvious delamination on the surface of the Be-implanted GaN samples.The damage caused by ion implantation will be repaired after high-temperature RTA,and the crystal quality of the annealed sample will be significantly improved.The bandgap difference of the Be implanted layers at different annealing temperatures is 0.14 eV.With the increase of annealing temperature,the crystal order increases,the defects gradually decrease,and the refractive index,extinction coefficient and bandgap of the samples have typical blue-shift.The bandgap value of the sample changes from 3.328 to 3.083 eV in the range of300-850 K.The refractive index,extinction coefficient and bandgap of the samples are red-shifted with increasing temperature.This study provides temperature-dependent optical parameters of the Be implanted GaN samples,which can provide certain reference indicators and new ideas for the development of high temperature,high power and high frequency microwave devices.（2）High-quality AlN samples were grown on c-plane flat sapphire substrates and nano-patterned sapphire substrates by MOCVD method.The stress and optical properties of AlN films grown on flat sapphire substrate and nano-patterned sapphire substrate were compared and analyzed.The results show that both of AlN samples have a compressive stress that becomes smaller with increasing temperature.With the lateral growth of AlN nanopillars,the nanopores at the interface provide release channels for the stresses.Nanopatterning results in reduced disorder and defects in the AlN epilayer,while high temperature thermal annealing promotes grain growth and reduces defects and residual strain.In the range of interface to surface for AlN cross-section,the compressive stress varies with the reduction in threading dislocation density.The nano-holes on the patterned substrate increase the light scattering,while the incident light is also easily trapped by the nano-holes,making the light transmission of the AlN films on the nano-patterned substrate not as good as that of the flat substrate.The refractive index and extinction coefficient of the AlN films are red-shifted with increasing temperature,and the bandgap varies linearly with temperature in the300-850 K.The experimental results enable us to predict the thermo-optical effects and optimize the optical properties of AlN-based high-power devices in the temperature up to 850 K.（3）To further analyze the surface and cross-section properties of AlN films,the PL spectrum,temperature dependent warpage,chemical state,energy band structure and variable temperature cross-section Raman of the samples were investigated.The results show that the PL spectra of the AlN films have an emission peak at 5.75 eV,there is free exciton recombination and longitudinal optical phonon（LO）replica.The dislocation density and substrate type have little effect on the PL spectra.The curvature radius of AlN film increases and the warpage of wafer decreases with the increase of temperature in 300-1350 K.The nano-patterned substrate affects the bottom-up temperature transfer,thus improving the residual internal stress in the AlN film.The binding energy of the same chemical bond in the film will be different due to the influence of some experimental variables.The FWHM of the E₂（low）in cross-sectional Raman spectra is the smallest at the interface,which shows an opposite trend to other phonon modes.With increasing depth,the AlN films become more subject to compressive stress.The smallest line-width of E₂（high）on the surface of the film indicates the best crystal quality near the surface.As the temperature increases,the Raman shifts decreases and the FWHM increases for the five phonon modes in the cross-sectional Raman spectra.The changes of temperature and depth will lead to the transformation of stress in AlN film.This study of the stress changes in the film can provide a useful reference for stress control engineering of AlN films at different temperatures.（4）In order to improve the performance of DUV-LEDs,AlN templates with high crystalline quality need to be grown on heterogeneous substrates.The effects of defect formation,evolution mechanism and dislocation density on the AlN crystal quality caused by different substrate growth mechanisms were investigated.The results show that the micro-strain of nano-patterned AlN is significantly better than that of sample grown on flat substrate,with a value of only 0.11×10^-3.The surface roughening on flat substrate has a modulating effect on the threading dislocations and facilitates the growth of nucleation layers,resulting in high quality AlN films on flat substrate.The nano-holes formed by the nano-patterned substrate are favorable for the threading dislocations to extend preferentially to the hole sidewall,inducing either merged annihilation or bending blocking of threading dislocations,but the threading dislocations concentrated at the merged interface are more likely to extend to the surface.The three different morphologies of etching pits are obtained by etching the surface of AlN with hot aqueous KOH solutions.The convex and concave defects on the surface of AlN films can be obtained by electron channeling contrast imaging（ECCI）technology.The advantages of nanopatterned growth can be demonstrated by different defect characterization methods,but the results of the different test means can diverge depending on the selection area of the sample measurement,the range of signal areas collected and other factors.（5）The AlGaN/GaN samples were grown by MOCVD system on c-face sapphire substrates with different flow rates of Alsource.The composition,dislocation density,in-plane stress,surface chemical state and optical properties of the film were analyzed.The results show that the low surface mobility of Alatom inhibits the growth of films and leads to the increase of surface roughness with the increase of Alcomponent.The increase of Alcomposition results in the inhibition of nucleation growth due to the surface migration of Alatoms is much lower than that of Gaatoms,which leads to an increase in dislocation density and a decrease in crystal quality of AlGaN films.The tilt angle of dislocation generation varies from 0.0485°to 0.0768°,indicating that the threading dislocations in the AlGaN epilayer become larger with the increase of Alcomponent.There is partial strain between the GaN and AlGaN epilayers,and24%of the Alcomponent sample has a minimum strain value of-3.34×10^-4.According to XPS analysis,the binding energy of the same chemical bond in the Al2p,Ga3d and N 1s core levels of the epilayers will become larger as the Alcomponent increases.