The Chemical Vapor Deposition of Aluminum Nitride:  Unusual Cluster Formation in the Gas Phase

Abstract

An ab initio investigation of the chemical vapor deposition of AlN from the AlCl 3 NH 3 adduct is presented. Geometries, harmonic vibrational frequencies and relative energies for the AlCl 3 NH 3 adduct, its dissociation products AlCl n, NH n ( n = 1−3), and ring and cluster compounds [(Cl 2 AlNH 2 ) n ( n = 1, 2), (ClAlNH) n ( n = 1, 2, 3, 4, 6)] are discussed. The Al−N bond lengths in the investigated compounds are strongly dependent on the coordination numbers of the aluminum and nitrogen centers, decreasing from 2.0 Å for 4-coordinated Al/N centers to 1.79 and 1.68 Å for 3- and 2-coordinated Al/N centers, respectively. Thermodynamic analysis shows that dissociation of Cl x AlNH x ( x = 2, 3) compounds with elimination of HCl and simultaneous formation of oligomeric forms is preferable to the process of dissociation into components or simple HCl detachment. Under standard conditions gaseous 4-coordinated Al/N compounds (ClAlNH) 6 and (ClAlNH) 4 are more stable than 3-coordinated (ClAlNH) 2 and (ClAlNH) 3 compounds. In 4-membered rings and clusters, the electrostatic repulsion between nearby Al−Al and N−N atoms makes reorganization to 6-membered rings extremely favorable. The suggested mechanism of AlN deposition involving cluster formation in the gas phase is discussed.