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

Abstract

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.

Kinetic Analysis of GaN-MOVPE via Thickness Profiles in the Gas Flow Direction with Systematically Varied Growth Conditions

“…Here TMGa is considered to be representative of the chemical species that can be formed in the gas phase following TMGa decomposition, most notably GaNH 2 . 39 Because GaNH 2 and TMGa have similar diffusion coefficients, we believe that this approximation is reasonable. We calculated D for TMGa in NH 3 and in H 2 individually using ChapmanEnskog theory, and then found D in a NH 3 /H 2 mixture as follows: Table II lists the Lennard-Jones parameters of constituent species necessary for the Chapman-Enskog theory calculations.…”

“…The mass transfer coefficient can be expressed as k d = D/δ, where D is the diffusion coefficient of the growth species, and δ is the thickness of the boundary layer. 55 A hot zone that appears in the vicinity of the heated substrate 32,39 may affect both C gs and D for the growth species; however, the temperature dependence of D is weak, 56 and so the assumption that the composition profile in the upstream part is independent of temperature is reasonable if the growth is controlled by mass transport. The temperature of the hot zone will increase with increasing substrate temperature, increasing the rate of gas-phase reactions.…”

“…[26][27][28][29][30][31][32][33] Several plausible candidate growth species that contribute to layer growth have been reported, including TMGa:NH 3 adducts, [(CH 3 ) 2 GaNH 2 ] 3 , [34][35][36] diatomic GaN, 37,38 and GaNH 2 . 39 However, a model that can describe the growth behavior consistently in all reactor configurations and for all process conditions remains elusive. This is our concern, and necessitates comprehensive investigation on the behavior of GaN growth under a wide variety of conditions, from both experimental and theoretical standpoints.…”

Methylamines as Nitrogen Precursors in Chemical Vapor Deposition of Gallium Nitride

“…1 The reaction pathways of the trimethyl gallium (Ga(CH3)3)/NH3 system has been studied earlier, both experimentally and theoretically. 2,3,4,5,6,7 The studies show a complex gas phase chemistry where both Ga(CH3)3 and NH3 could decompose to more reactive species that could deposit on the surface or react in the gas phase to form species containing both gallium and nitrogen. Ga(CH3)3 is a Lewis acid due to the empty p-orbital on the metal center and ammonia exists with a free electron pair on the nitrogen, making it a Lewis base; for this reason, Lewis adduct formation of H3N:Ga(CH3)3 is very likely, especially with the Le Chatelier's driving force provided by the large surplus of ammonia in the CVD gas mixture.…”

“…8,9,10 The formation of H2N-M(CH3)2 could contribute to the growth of GaN by deposition, but might be also detrimental to the CVD process by leading to the formation of parasitic nitride nanoparticles in the gas phase. 3,9,11,12,13 Despite the fact that CVD processes for 13-Ns have reached a level of maturity that allows them to be used for industrial production of 13-N based electronics, they typically require a very high NH3 /13 precursor ratio, typically 100-1000:1 for AlN and GaN 1 , and up to 10 5 :1 for InN 14 , reflecting poorly tuned CVD chemistry. From a purely thermodynamic point of view, NH3 will mainly decompose under deposition conditions to highly stable dinitrogen and dihydrogen via the reactive species NH2 and NH.…”