Computational Study on the Growth of Gallium Nitride and a Possible Source of Oxygen Impurity Bhaskar Mondal, Debasish Mandal, Deepanwita Ghosh, and Abhijit K. Das* Department of Spectroscopy, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032 ReceiVed: January 13, 2010; ReVised Manuscript ReceiVed: March 8, 2010 The reaction pathways for the gallium nitride GaN growth by gas phase reaction of trimethylgallium (TMG) with ammonia is studied theoretically. Water is the most important impurity in ammonia, therefore its reaction with TMG is investigated as a possible source of oxygen impurity in GaN. Gallium oxide (GaO) formed by the reaction between TMG and H 2 O is predicted to be one of the possible source of oxygen impurity in GaN. The mechanisms and energetics of these reactions in the gas phase have been investigated by density functional B3LYP/[LANL2DZ-ECP + 6-31G(d,p)] method and ab initio MP2/[LANL2DZ-ECP + 6-31G(d,p)], CCSD(T)/ [LANL2DZ-ECP + 6-31G(d,p)]//B3LYP/[LANL2DZ-ECP + 6-31G(d,p)], CCSD(T)/[LANL2DZ-ECP + 6-31G(d,p)]//MP2/[LANL2DZ-ECP + 6-31G(d,p)], and CCSD(T)/[LANL2DZ-ECP + Ahlrichs-VTZP]//MP2/ [LANL2DZ-ECP + Ahlrichs-VTZP] methods. Both the reactions of TMG with NH 3 and H 2 O are modeled using pre-equilibrium charge-transfer complexes (CH 3 ) 3 Ga:NH 3 (C1) and (CH 3 ) 3 Ga:OH 2 (C2) having binding energies of 18.8 and 12.4 kcal/mol, respectively. The first step of the methane elimination reaction from the complexes proceeds through the saddle points TS1 and TS1a having activation barriers 37.0 and 22.6 kcal/ mol for C1 and C2, respectively. The first CH 4 elimination step is exothermic for both the cases, but the exothermicity is 15.0 kcal/mol greater for CH 4 elimination from C2. The next step of methane elimination from the stable reaction intermediates (CH 3 ) 2 GaNH 2 and (CH 3 ) 3 GaOH has a very high activation barrier of 76.0 and 67.8 kcal/mol via saddle points TS2 and TS2a, respectively. The calculated reaction rates at 298.15 K for both the reactions are low but are comparable to each other. The total rate constant k tot for GaN formation is 2.07 × 10 -60 cm 3 molecule -1 s -1 , and that for GaO formation is 6.85 × 10 -62 cm 3 molecule -1 s -1 . 1. Introduction Gallium nitride is a semiconductor having a wide band gap (3.4 eV) for which there has been significant interest for fabrication of blue light-emitting diodes and lasers. 1 The reaction of TMG with NH 3 at high temperature is widely used for the production of gallium nitride. 2 Films of GaN are typically synthesized by chemical vapor deposition (CVD) technique from TMG and NH 3 at temperatures around 1000 °C. Because of the complexity of the reaction mechanism that involves both gas phase and surface chemistry, with the latter being considered a more important factor in determining the quality of semicon- ductor films along with the properties of the substrate, advances in GaN growth techniques have been made largely empirically, and there is a lack of knowledge of fundamentals for the deposition processes. Several problems also arise from unwanted incorporation of hydrogen, carbon, and oxygen in the GaN films. 3 The oxygen incorporation, in particular, reduces the luminescence intensity of the material drastically. Both experi- mental and theoretical attempts have been made to explain the reaction pathways of GaN growth using the TMG/NH 3 system. 4-9 Bergmann et al studied the reactions of TMG with ammonia and water over a wide range of temperature, and they detected dimeric and trimeric gallium amides [(CH 3 ) 2 GaNH 2 ] x (x ) 2, 3) as a reaction intermediates for the GaN growth process. 10,11 They also mentioned that the formation of volatile compounds in the reaction between TMG and water might be the main reason for oxygen incorporation and enrichment in gallium nitride films. It is well established that a charge transfer complex (CH 3 ) 3 Ga:NH 3 or TMG:NH 3 is formed that decomposes to GaN at elevated temperatures. Several mechanisms have been proposed for the reaction paths to GaN. Hirako et al proposed that in a region of elevated temperature the charge transfer adduct may decompose and polymerize, but the main pathway of GaN growth has been shown to be (CH 3 ) 3 Ga:NH 3 f (CH 3 ) 2 GaNH 2 f CH 3 GaNH f GaN. 12 In a recent article Timoshkin et al have also predicted the above dissociation pathway to be a major route for the growth of GaN films. 13 But the detailed reaction mechanism and kinetics for the thermal dissociation pathways of the charge-transfer complex are not yet well understood. The reaction of organo-metallic compounds with water is very important due to their key role in synthetic organic chemistry. So, the reactions between organo-gallium compounds and water are not only important for CVD processes, but they are also interesting from a fundamental point of view. In spite of that, very little is known about these reactions. To understand the underlying chemistry of the CVD processes and also to advance the CVD technologies in future, both experi- mental techniques and theoretical modeling of the reactions are necessary. The main objective of this article is to present a concise theoretical study on the dissociation route involving CH 4 elimination for the growth of GaN films using high level DFT and ab initio methods and subsequently to predict the possible reaction of TMG with water as a possible source of oxygen impurity in GaN. The formation of GaN may involve 1:1 and 2:1 pre-equilibrium charge-transfer complexes such as (CH 3 ) 3 Ga: NH 3 and (CH 3 ) 3 Ga:(NH 3 ) 2 and some polymeric reaction inter- * To whom correspondence should be addressed. E-mail: spakd@ iacs.res.in. J. Phys. Chem. A 2010, 114, 5016–5025 5016 10.1021/jp100332t  2010 American Chemical Society Published on Web 03/23/2010