Quantum chemical study of gas-phase reactions of trimethylaluminium and triethylaluminium with ammonia in III–V nitride semiconductor crystal growth

Fundamental chemistry and modeling of group-III nitride MOVPE

“…3. The experimental activation energy for this step is in good agreement with theoretical calculations by Mihopolous et al [8] and Ikenaga et al [34]. Based on analogy with the TMAl:NH 3 chemistry, it has often been suggested that TMGa+NH 3 mixtures would react to yield (CH 3 ) 2 Ga-NH 2 +CH 4 with eventual dimerization and/or trimerization.…”

Quantum chemical study on substituent effect of gas‐phase reactions in III–V nitride semiconductor crystal growth

“…2, and the relative energies of the reaction species are listed in Table 2. Similarly as for R = CH 3 and C 2 H 5 in the previous papers [9][10][11][12][13][14], both the activation energy and the heat of reaction for the Al source system are lower than those for the Ga source system in reaction (2). The relative energies of 3a and 3g are respectively lower than those of corresponding amino adducts for R = CH 3 and C 2 H 5 because the M-N interaction in MR 2 NH 2 is not the coordination interaction but the covalent bond where the strength of interaction is affected seriously by the ability for electron donation due to alkyl groups, and therefore the stronger M-N interaction for R = t-C 4 H 9 reduces the relative energies of 3 more remarkably according to the same reason that the stronger Al-N interaction in the Al source system reduces the heat of reaction more strikingly.…”

“…From the viewpoint of the substituent effect, the Mulliken charges of the alkyl groups R clearly show the well-known order of the ability for electron donation, -t-C 4 H 9 > -C 2 H 5 > -CH 3 . The ability for electron donation contributes to suppression of the intramolecular polarization that prevents the coordination interaction [9][10][11]. However, the M-N bond orders and the quantities of the Mulliken charge transfer from NH 3 to MR 3 inform us that 2 has the same degree of M-N coordination interaction as (C 2 H 5 ) 3 M⋅NH 3 for each M though it has a stronger one than (CH 3 ) 3 M⋅NH 3 , because three bulky t-butyl groups prevent the metal atom from configuring the distinct tetrahedral coordination observed in (CH 3 ) 3 M⋅NH 3 and (C 2 H 5 ) 3 M⋅NH 3 [9,10]: the tetrahedral coordination of M is quite easy to receive the lone pair of NH 3 .…”

“…Furthermore, the steric repulsion due to bulky substituents such as t-butyl works effectively in other reaction mechanisms, e.g. the bimolecular mechanism [10], R 3 M⋅NH 3 + M′R′ 3 → R 3 M⋅NH 2 M′R′ 2 + R′H, and reactions in the presence of excess ammonia [9,10]. For R = R′ = CH 3 , we have reported that the activation energies along normal unimolecular reactions are reduced by way of these reaction mechanisms [9].…”

“…On the other hand, the growth of AlGaN layers at atmospheric-pressure MOVPE is inhibited by gas-phase reactions among precursors leading to adduct formation, called "parasitic reactions" [3 -5]. We have studied the mechanism of gas-phase reactions in MR 3 /H 2 /NH 3 (M = Al, Ga, In; R = CH 3 , C 2 H 5 ) systems by a quantum chemical approach [9 -17], and have clarified that the Al source gases and their deviates enhance reactivity to the parasitic reactions [9][10][11][12][13][14]. In order to suppress the parasitic reactions, we have to consider another Al source gas for the growth of AlGaN.…”