## 3 Results
### Reaction Mechanisms of NH\({}_{3}\) Adsorption on TMG Adsorbed Si(100) Surface  
After the TMG adsorption step, we assumed that TMG was adsorbed on the Si\({}_{2}\)-NH and Si-NH\({}_{2}\) surfaces by consuming two-adsorption sites. Therefore, for the final products of the first-half ALD reaction, PC 2-1 and PC 2-2 structures were used as substrates in the nitridation step. In this section, we simulated the nitridation step of GaN thin film grown using TMG and NH\({}_{3}\) precursors in the ALD process. The reaction mechanisms of NH\({}_{3}\) adsorption on the residual TMG adsorbed silicon surfaces (2-1 and PC 2-2 substrates) are shown in Figure 7. The reactions started from the adsorption stages. In the adsorption 3-1 stage, the NH\({}_{3}\) molecule approached the N2-GaCH\({}_{3}\) surface. The distance between N3 and Ga was 2.15 A. In the adsorption 3-2 stage, the distance between N3 and Ga was  
Figure 6: Energy profile of NH\({}_{3}\) adsorption on residual TMG adsorbed on the Si\({}_{2}\)–NH surface (PC 1–2 substrate). The corresponding molecular structures of NH\({}_{3}\) adsorption are labeled (a)–(h), as shown at the bottom of the figure. 2.11 A, which was shorter than for adsorption on the PC 2-1 substrate, suggesting that the NH\({}_{3}\) molecule could approach the PC 2-2 substrate closer than the adsorption on the PC 2-1 substrate. During the adsorption stages, NH\({}_{3}\) was found on the top of the GaN surface, resulting in N-H bond lengths (in NH\({}_{3}\)) of approximately 1.01 A for both substrates. The H-N-H angles (in NH\({}_{3}\)) were 108\({}^{\circ}\), 107\({}^{\circ}\), and 107\({}^{\circ}\) for the PC 2-1 substrate and 107\({}^{\circ}\), 108\({}^{\circ}\), and 108\({}^{\circ}\) for the PC 2-2 substrate. These adsorption characteristics were comparable to other work. In 2015, [PERSON] et al. [47] reported the adsorption of NH\({}_{3}\) on a GaN surface. The distance between the N in the NH\({}_{3}\) and Ga at the surface site was 2.10 A. During the adsorption stage, NH\({}_{3}\) was found on the top site of the GaN surface, resulting in N-H bond lengths of 1.03 A and H-N-H angles of 108.6, 108.7, and 111.2 A. In the current work, the Ga-C1 bonds were broken in the TS 3-1 and TS 3-2 stages at distances of 2.28 and 2.23 A, respectively. Differences in the adsorption mechanisms of the two substrates were observed in the transition state. The N-H bond was broken in the TS 3-2 stage but remained in the TS 3-1 stage. The H atom released during the TS 3-2 stage attracted the CH\({}_{3}\) group and combined to generate CH4 as a by-product. Next, Ga-N bonds were formed with lengths of 1.84 and 1.95 A in the PC 3-1 and PC 3-2 structures, respectively. The crystal Ga-N bond has been reported to be 1.992 A [47]. In the final step, the CH4 molecules were eliminated, while the N2 Ga-NH\({}_{2}\) species were left as new surface adsorption sites. The NH\({}_{3}\) molecule not only eliminated the CH\({}_{3}\) group but also regenerated the NH\({}_{2}\) surface site to accommodate the new TMG adsorption in the next step of ALD. The energy pathways of NH\({}_{3}\) adsorption on the residual TMG adsorbed silicon surfaces (PC 2-1 and PC 2-2) are shown in Figure 7 by black and red lines, respectively. In the adsorption stages (adsorption 3), 0.58 and 0.94 eV were released when NH\({}_{3}\) approached the PC 2-1 and PC 2-2 substrates, respectively, indicating that NH\({}_{3}\) was more active with the PC 2-2 substrate than the other substrate. Compared to adsorption on the PC 2-1 substrate, the reaction pathway of NH\({}_{3}\) adsorbed on the PC 2-2 substrate was more thermodynamically and kinetically favorable. In the transition state, the activation barriers in the TS 3-1 and TS 3-2 stages were approximately 2.00 \(\pm\) 0.02 eV. The relative energy levels of the TS 3-1 and TS 3-2 stages were 1.38 and 1.06 eV, respectively, above the reactant level. The  
Figure 7: Comparison of reaction pathways of NH\({}_{3}\) adsorption on TMG-adsorbed Si–NH\({}_{2}\) (black line) and Si\({}_{2}\)–NH (red line) substrates.