HVPE生长GaN气相反应路径的研究

Study on gas reaction path in GaN HVPE

采用量子化学的密度泛函理论,对HVPE生长GaN的气相反应路径进行系统的理论计算分析,特别针对GaCl与NH3的反应、GaCl3:NH3消去H2和Cl2的反应、液Ga的气化导致H自由基的产生及其后续的反应.通过计算各反应的Gibbs自由能变和过渡态能垒,分别从热力学和动力学上判断反应进行的可能性.研究发现,在HVPE反应器中, GaCl与NH3可能发生的六种反应均有ΔG>0,表明GaC l与NH3的反应在热力学上不利于发生.对于GaCl3:NH3消去H2或Cl2的三种反应,也同样在热力学上不利于发生.通过热力学相平衡理论计算发现,当T>1200 K时, Ga蒸气压迅速上升,因此在气相反应中不能忽略气态Ga的影响.气态Ga与HCl反应将生成H自由基,并最终生成氨基物GaClNH2和GaCl(NH2)2.这两种氨基物均不能继续与NH3发生反应.因此得出结论,在HVPE反应器中,除了GaCl外,还可能存在另外两种含Ga气相分子,即GaClNH2和GaCl(NH2)2,它们将同时提供GaN的表面反应以及纳米粒子的生长前体.

By density functional theory of quantum chemistry,the gas reaction mechanism in GaN HVPE growth is extensively studied,especially the reaction of GaCl with NH3,the elimination of H2 or Cl2 from GaCl3:NH3,and the formation of H radicals caused by the evaporation of liquid gallium and its subsequent reactions.By calculations of Gibbs free energies and energy barriers at different temperatures,the reaction probabilities are determined both thermodynamically and kinetically.The results show thatΔG>0 for the whole six possible reactions between GaCl and NH3,which indicates that those reactions are unfavorable.For the three reactions with elimination of H2 or Cl2 from GaCl3:NH3,the reactions are also unfavorable.By thermodynamic calculation of Ga phase change,when T>1200 K,the Ga vapor pressure rises rapidly.Thus,the gaseous Ga in gas phase reaction cannot be neglected.The gaseous Ga reacts with HCl to form GaCl and H radicals,and H radicals will finally result in the amino compounds GaClNH2 and GaCl(NH2)2,which are also unfavorable to further react with NH3.Thus,we conclude that,in addition to GaCl,GaClNH2 and GaCl(NH2)2 are also the most likely Ga-containing molecules in the HVPE reactor,which will provide both GaN surface growth and nanoparticles.