Study On Gas Reaction Mechanism Of AlN-MOCVD

AlN is an important wide-band gap semiconductor material,which has the characteristics of high temperature resistance,high breakdown field strength and so on,It is the key material for preparing ultraviolet and deep ultraviolet devices.Metal organic chemical vapor deposition?MOCVD?is the main method for the growth of AlN films.In the MOCVD growth of AlN,the strong Al-N bond leads to serious gas parasitic reaction,and a large number of source gases are transformed into nanoparticles,resulting in low growth efficiency,poor film quality and slow growth rate.Therefore,it is important to understand the mechanism of gas-phase chemical reactions during the growth peocess of AlN-MOCVD.The gas-phase chemical reaction paths for the growth of AlN by MOCVD,including the adduct path,pyrolysis path and hydrogenolysis pathway,were studied by combining density functional theory?DFT?and transition state theory.By Optimizing the molecular configuration and potential energy surface of each reaction path in AlN-MOCVD.The enthalpy difference and Gibbs free energy difference at different temperatures were calculated to determine the probability of reaction occurrence,the direction of reaction and the reaction rate.The main conclusions are as follows:?1?Gibbs free energies of bimolecular reactions of NH₃ with[DMAlNH₂]₂,[MMAlNH]₂,[MMAlNH]₃ at different temperatures were calculated sperately.According to the criterion of free energy,at high temperatures,the bimolecular reaction probability of[DMAlNH₂]₂ with NH₃ is very small,and it tends to go to the intramolecular reaction path of[DMAlNH₂]₂ with lower energy barrier,which eliminates methane to produce[MMAlNH₂]₂.Both[MMAlNH]₂ and[MMAlNH]₃ can react with NH₃ with low energy barrier.The products[AlNHNH₂]₂ and[AlNHNH₂]₃ may be important precursors for surface reaction and nanoparticles in the gas phase reaction of AlN-MOCVD.?2?[MMAlNH]_?n=2,3?can react with and NH₃ to form a coordination adduct and the reaction can proceed spontaneously at lower temperature.However,with the increasing of temperature,the coordination adduct will be dissociate into[MMAlNH]_?n=2,3?and NH₃.It is unfavorable for the reaction at high temperature.?3?It is found that both TMAl and DMAlNH₂ require very high barriers.Although the ethylene elimination reaction of TMAl or DMAlNH₂ can be carried out spontaneously at high temperature.It requires high activation free energy.Thermodynamics,the pyrolysis path is disadvantageous both thermodynamically and kinetically.?4?There is a competitive mechanism between the reaction of TMAl and NH₃ and the reactions of TMAl and H₂.According to the criterion of activation free energy,it is found that the activation free energy of the adduct reaction of TMAl and NH₃ is lower and the adduct path is dominant.The activation free energy of the adduct reactions of DMAlNH₂ with NH₃are smaller than those of DMAlNH₂ with H₂ reactions,which explore that the AlN-MOCVD reactor mainly takes the adduct path rather than the hydrolysis path.