| Ⅲ-nitride is one of the most potential materials for the short wavelength, high speed optoelectronic devices owing to their wide and broad direct band gaps. As the nitride technology has gradually been well developed, those crucial problems remaining unresolved appear to be even more harsh and tough. Nowadays, the mainstream of the structure of optoelectronic devices still relay on the realization of their functions by means of electrical drive and epitaxial heterostructures. Regarding these two fundamental structures, it is obvious that the substrate materials, the quality of epitaxial layers and the electrode materials play the key roles in the material science and device manufacture and become the most widely concerned research topics. Therefore, our work in this thesis aimed to investigate projects including the free-standing GaN substrate, the epitaxial growth of AIN films, the transparent electrodes of Cu nanosilk. Significant achievements and important results obtained in this work are the following:Firstly, in situ self-release of thick GaN wafer from sapphire substrate. Release energies ER of a-, m-,(nonpolar) and c-(polar) planes of wurtzite GaN are systematically calculated under different biaxial strains by using first-principles method. The results reveal that:(i) cracks along different nonpolar directions are related with the sign and magnitude of the misfit stress fields. Cracks on m-and oplanes prefer to occur at high temperature during the epitaxial growth and the cooling stage, respectively. Based on these facts, the optimized buffer layer and substrate pretreatment were proposed to avoid the happening of cracks and the intact high-quality GaN thick film could be obtained,(ii) The trend of the ER curve for c plane comes out in an asymptotical characteristic and has a critical turning point of release energy where c plane will be favorable to release. According to the above analysis, we come up with the idea of modulating the stress field gradient to meet the turning point. It is found that by controlling the uniformity and the thickness, the curvature radius of the thick GaN wafer will decrease and the steeperly graded strain field could be achieved. When the thickness exceeds400μm, the curvature stress will cover the turning point and leads to the in situ self-release of thick GaN wafer. The free-standing GaN shows smooth surface and good crystal quality.Secendly, growth of high-quality AIN films by MOVPE at-1100℃. To investigate the AIN heteroepitaxial growth technique and mechanism, the sapphire substrate nitridation, nucleation buffer layer, high temperature AIN epitaxy and crystal doping techniques have been studied. The results show that:(i) The time of nitridation and high temperature nucleation will reduce the mismatch stress between AIN and sapphire and switch the tensile stress to compressive.(ii) Optimized MMEE method has been realized. As the pulse period becomes longer, the two-dimensional growth with better grain orientation will be promoted, however, the strain by the grain coalescence increases. In contrast, the continuous growth mode helps enhance the three-dimensional growth and reduce the dislocation density, however, the grain orientation is deteriorated. By combining these two modes and optimizing the cycle time, the quality of the AIN epilayers is largely improved. The FWHM of (0002) and (10-12) are59aresec and610aresec, respectively,(iii) Mg-doping and Mg-Si δ-doping methods are proposed to improve the AIN quality. Since the Mg atomic radius is larger than that of Al, the residual strain in the epilayer as well as screw dislocation density could be effectively reduced and meanwhile, the migration of Al atoms is enhanced beacause of the pre-reaction preventing. As a result, the surface smoothness of AIN epilayer has been improved up to0.47nm (RMS).Thirdly, synthesis of Cu nanosilk as transparent ohmic electrodes for GaN-based LED. Through the solution method with Ni+2as the catalyst, oleylamine as dispersant, CuCl2· H2O as precursor, Cl-as sidewall passivation agent, we successfully synthesized superlong (>40μm) and ultrafine (-16nm) Cu nanosilks in the world. Then, with Cu nanosilk ink the transparent conductive film is fabricated by the percolation with vacuum filtration and the imprinting transfer. The Cu nanosilk transparent electrode achieves the low sheet resistance at high transparency (93%@51Ω2/sq), whose performance is already comparable to the traditional ITO electrodes. The excellent transmission from ultraviolet to infrared indicates potential and broad applications. Finally, after low-temperature annealing at vacuum, the Cu nanosilk electrodes achieve the good ohmic contact to the n-GaN and p-GaN layers on the GaN-based blue LED chip. Bright EL light emission shows excellent light transmission characteristic as well as electrical stability. These results indicate that the replacement of ITO is around the corner and broad applications could be provided in advanced optoelectronic devices. |
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