Study On Gas Phase Reaction Kinetics Of AlGaN Films Grown By High Temperature MOCVD

Due to serious parasitic reactions,low surface mobility of Al atoms,and complex film growth methods,AlGaN films grown by conventional metal organic chemical vapor deposition(MOCVD)technology have defects such as low growth rate,surface defects,and poor uniformity.Experiments have proved that increasing the substrate temperature can effectively improve the quality of the film,so the high-temperature MOCVD growth of AlGaN films has become the mainstream process method in the industry.Since the quality of the film is affected by the chemical reaction inside the reaction chamber,it is important to perfect the reaction path at high temperature and to understand the reaction mechanism in depth.Based on density functional theory and transition state theory,this thesis uses Gaussian software to perform quantum chemical calculations on the three gas phase reaction paths inside the reaction chamber at different temperatures,and analyzes the gas phase reaction mechanism through thermodynamics and chemical reaction kinetics.Considering that the activity of H₂ in the carrier gas increases at high temperatures and the V-III ratio inside the reaction chamber is greatly excessive.Quantum chemical calculations and analysis are carried out for the influence of the two on the reaction path,and the gas-phase reaction path inside the reaction chamber at high temperature is further improved.The main research results of the thesis are as follows:(1)Based on density functional theory,three reaction paths for MOCVD growth of AlGaN films are studied,including the pyrolysis path,the addition path,and the polymerization path to generate oligomers.The calculation results show that the occurrence of the pyrolysis pathway requires high temperature activation,the addition pathway can proceed spontaneously at low temperatures,and the main product is an amino compound,which provides a reaction source for the subsequent polymerization reaction.This result is consistent with the experimental conclusion.By comparing the energy barriers,it is found that the amino compounds are more prone to polymerization.In the polymerization path,the polymerization ability of Al-containing amino compound DMAl NH₂ is stronger than that of Ga-containing amino compound DMGa NH₂,and the presence of DMAl NH₂ will promote the polymerization of DMGa NH₂to form dimers(DMAl NH₂)(DMGa NH₂),resulting in the consumption of Ga source.At high temperatures,the dimer can spontaneously shed methane molecules into smaller rings,and its main product is(MMXNH₂)₂(X=Al/Ga),which provides a precursor for film growth.(2)Based on the density functional theory,the influence of H₂ in the carrier gas on the pyrolysis path and polymerization path is studied.The calculation results show that both H₂and the H free radicals produced by the reaction with methyl CH₃ promote the pyrolysis path,reducing the spontaneous reaction temperature from close to the substrate temperature(1673K)to low temperature(298 K).H free radicals can also promote the reaction of dimers to remove methane molecules,reduce the conditions for spontaneous reaction,and change the main product from(MMXNH₂)₂ to(XNH₂)₂.At high temperatures,the presence of H₂ in the carrier gas greatly promotes the reaction process inside the reaction chamber,which is consistent with the experimental observation that the growth rate increases with the increase of H₂.(3)Based on the density functional theory,the effect of excessive V-III ratio on the addition path and the polymerization path is studied.The calculation results show that TMX can spontaneously combine with two NH₃ at low temperature and generate the amino compound DMXNH₂:NH₃.After the temperature rises to 873 K,it decomposes back into TMX:NH₃and NH₃.Based on thermodynamics,the effect of NH₃ on the reaction of dimers to remove methane molecules is less than that of H free radicals.However,the actual amount of NH₃inside the reaction chamber is much larger than H free radicals,the promotion of NH₃ on the removal of methane molecules from the dimer is also very important.