First principle density functional theory (DFT) was used to investigate the adsorption of NH3 during the 2 nd step of the thermal ALD cycle using the Vienna Ab initio Simulation Package (VASP) , with optimizers extended by the VTST package. The General Gradient Approximation (GGA) functional PBE together with the 3 rd version of Grimme's dispersion correction was used with a planar wave basis set with cut-off energy of 700 eV and Projector augmented wave (PAW) potentials. The valence electron configuration used for the different elements were 1s 1 for H, 2s 2 2p 2 for C, 2s 2 2p 3 for N, 3d 10 4s 2 4p 1 for Ga and 4d 10 5s 2 5p 1 for In. The electronic convergence was set to 10 -6 eV for geometry optimization and 10 -8 eV for phonon calculations. All structures were optimized until the forces on all atoms were less than 10 -2 eV Å -1 . For surface structures, the positions of atoms at the lower five layers were kept fixed after optimization of the initial structure. Transition State (TS) structures were found by the Nudged Elastic Band (NEB) method as implemented in VTST. After structural optimization, harmonic phonon calculations were preformed using finite displacements.