Complex Formation of Trimethylaluminum and Trimethylgallium with Ammonia: Evidence for a Hydrogen-Bonded Adduct

Quantum Chemistry Study on Gas Reaction Mechanism in AlN MOVPE Growth

By using the density functional theory of quantum chemistry, the gas reaction mechanism in the AlN MOVPE process has been investigated, especially after the amide DMAlNH₂ formation. Two reaction paths are distinguished after the amide DMAlNH₂ formation and oligomerization: the intramolecular path and the intermolecular path, both involved with methane elimination. By inspections of the changes of the Gibbs energy ΔG between products and reactants, as well as the Gibbs energy of activation divided by RT, ΔG^*/RT, to account for thermal activation at different temperatures, the most probable gas reaction paths, and gas products for AlN thin film growth are determined both thermodynamically and kinetically. Our results indicate that under metal organic vapor phase epitaxy condition, for the intramolecular path, (MMAlNH)₂ is the most probable gas reaction products; for the intermolecular path, both Al(NH₂)₃ and (AlNHNH₂)₂ are the most probable gas reaction products. We also prove that (AlN)₂ and (AlN)₃ clusters are thermodynamically unfavored in the gas phase.

Composite GaN-C-Ga ("GaCN") Layers with Tunable Refractive Index

This article describes novel composite thin films consisting of GaN, C, and Ga (termed "GaCN", as an analogue to BCN and other carbonitrides) as a prospective material for future optical applications. This is due to their tunable refractive index that depends on the carbon content. The composites are prepared by introducing alternating pulses of trimethylgallium (TMG) and ammonia (NH₃) on silicon substrates to mimic an atomic layer deposition process. Because the GaCN material is hardly reported to the best of our knowledge, a comprehensive characterization is performed to investigate into its chemical nature, primarily to determine whether or not it exists as a single-phase material. It is revealed that GaCN is a composite, consisting of phase-segregated, nanoscale clusters of wurtzitic GaN polycrystals, in addition to inclusions of carbon, nitrogen, and gallium, which are chemically bonded into several forms, but not belonging to the GaN crystals itself. By varying the deposition temperature between 400 and 600 °C and the NH₃ partial pressure between 0.7 × 10^-3 and 7.25 mbar, layers with a wide compositional range of Ga, C, and N are prepared. The role of carbon on the GaCN optical properties is significant: an increase of the refractive index from 2.19 at 1500 nm (for carbon-free polycrystalline GaN) to 2.46 (for GaCN) is achieved by merely 10 at. % of carbon addition. The presence of sp²-hybridized C=N clusters and carbon at the interface of the GaN polycrystals are proposed to determine their optical properties. Furthermore, the formation of the GaN polycrystals in the composite occurs through a TMG:NH₃ surface-adduct assisted pathway, whereas the inclusions of carbon, nitrogen, and gallium are formed by the thermal decomposition of the chemisorbed TMG species.

Analysis of the Gas Phase Kinetics Active during GaN Deposition from NH3 and Ga(CH3)3

The results of a systematic investigation aimed at determining the dominant gas phase chemistry active during GaN MOVPE are reported and discussed in this work. This study was performed developing a thermodynamic database including the most stable GaN gas phase species and a gas phase mechanism that could efficiently describe their interconversion kinetics. The thermodynamic data and the kinetic mechanism were calculated combining density functional theory and ab initio simulations. Structures and vibrational frequencies of reactants and transition states were determined at the M062X/6-311+G(d,p) level, while energies were computed at the ROCBS-QB3 level. Rate constants were calculated using transition state theory using the rigid rotor - harmonic oscillator approximation and considering the possible degeneration of internal motions in torsional rotations. The thermodynamic analysis indicated that the Ga gas phase species formed in the highest concentration at the standard GaN deposition temperature (1300 K) is GaNH2, followed by GaH and Ga. The diatomic GaN gas phase species, often considered to be the main precursor to the film growth, is predicted to be unstable with respect to GaNH2. Among the gas phase species containing two Ga atoms, the most stable are GaNHGaH(NH2)3, GaNHGaH2(NH2)2, and GaNHGa(NH2)4, thus indicating that the substitution of the methyl groups of the precursor with H or amino groups is thermodynamically favored. Several kinetic routes leading to the formation of these species were examined. It was found that the condensation of Ga(R1)x(R2)3-x species, with R1 and R2 being either CH3, NH2, or H, is a fast process, characterized by the formation of a precursor state whose decomposition to products requires overcoming submerged energy barriers. It is suggested that these species play a key role in the formation of the first GaN nuclei, whose successive growth leads to the formation of GaN powders. A kinetic analysis performed using a fluid dynamic model allowed us to identify the main reactive routes of this complex system.

Feasibility of novel (H3C)nX(SiH3)3−ncompounds (X = B, Al, Ga, In): structure, stability, reactivity, and Raman characterization from ab initio calculations

Stability of the (H₃C)_nX(SiH₃)_3−ncompounds.

Theoretical Investigation of the Gas-Phase Kinetics Active during the GaN MOVPE

Quantum chemistry investigations have been performed to study the gas-phase chemistry active during the MOVPE of GaN when Ga(CH3)(3) and NH3, diluted in a H2 carrier gas, are used as precursors. Optimized molecular geometries, energies, and transition-state structures of gas-phase species have been determined with density functional theory at the B3LYP/6-311+g(d,p) level. On the basis of the similarity with the soot formation mechanism active during hydrocarbon combustion, we propose that in this system a gas-phase chemistry is active and its reactivity is enhanced by a radical chain mechanism started from methyl radicals. Initiation reactions are surface processes or the pyrolysis of Ga(CH3)(3). A propagation mechanism composed of fast radical reactions, most of which without an activation energy, was identified, and kinetic constants were determined for each step. The proposed mechanism is able to describe the formation of large GaN adducts formed by up to three R-Ga-NH units. These molecules can give fast cyclization reactions that lead to the formation of six-membered cyclic species, which, similar to benzene for combustion, are thermodynamically stable in vast temperature and pressure ranges and can thus be considered as the first GaN nuclei. We also found that the presence of H2 as a carrier gas can greatly enhance the rate of formation of gas-phase particles because it is a major source of atomic hydrogen, a promoter of gas-phase reactivity.