## 3 Results
### Effects of Surface Adsorption Site (NH and NH2 Adsorption Sites)
#### 3.2.2 TMG Adsorption on Si\({}_{2}\)-NH Surface  
The reaction pathway of TMG adsorption on Si\({}_{2}\)-NH passivated Si(100) substrate is shown in Figure 5 (red line). In the adsorption stage, the TMG molecule appeared to be relatively inert to the NH surface. Only 0.02 eV were released in this stage. The molecule of TMG was closer to the NH\({}_{2}\) surface (2.03 A) than to the NH surface (2.24 A) during the adsorption. In the transition stage, TMG was adsorbed on the NH surface with a substantially lower activation barrier (1.11 eV). This may explain why the CH\({}_{3}\) binding capacity of the Ga atom was reduced when TMG was adsorbed on the Si\({}_{2}\)-NH surface. This can be seen from the shortening of the breaking distance of the Ga-C1 bond from the TS stage. The breaking distances of the Ga-C1 bond at TS stages were 2.24 and 2.73 A when TMG was adsorbed on Si\({}_{2}\)-NH and Si-NH\({}_{2}\) surfaces, respectively. In addition, we found that the Ga-N bond formed in the TS stage of the Si\({}_{2}\)-NH adsorption site (2.09 A) was slightly shorter than for the Si-NH\({}_{2}\) adsorption site (2.13 A), indicating that TMG was attracted by the Si\({}_{2}\)-NH adsorption site with a higher reactive force than for attraction to the Si-NH\({}_{2}\) adsorption site. We assumed that a nitrogen (N1) atom at the Si\({}_{2}\)-NH surface has a greater electron density than at the Si-NH\({}_{2}\) surface because the N1 atom at the Si\({}_{2}\)-NH surface site is connected to the 2-Si atoms and 1-H atom, while the N1 atom at the Si-NH\({}_{2}\) surface site is connected to the 1-Si atom and the 2-H atoms. For the Si\({}_{2}\)-NH surface, the Si atom has less electronegativity than the H atom, so most electrons spend more time in the N region, resulting in the stronger and easier formation of the Ga-N1 bond. When a Ga-N1 bond is formed, electrons are transferred from the Ga atom to the N atom, resulting in decreased ability of the Ga atom to hold CH\({}_{3}\) groups. As a result, TMG adsorption on the Si\({}_{2}\)-NH surface is easier than on the Si-NH\({}_{2}\) surface. In the first adsorption, TMG adsorption on the Si\({}_{2}\)-NH surface site tends to be more facile and can occur at a lower temperature due to its lower relative energy and smaller activation barrier in the TS 1-2 stage. However, in the second adsorption, the TS 2-2 stage has a higher relative energy and activation barrier compared with the TS 2-1 stage (black line). In addition, the relative energy at the TS 2-2 stage is higher than at TS 1-2. Thus, the reaction could not proceed through the TS 2-2 stage at the same temperature as for TS 1-2. Furthermore, the relative energy in the PC 2-2 stage is higher than the relative energy in the PC 1-2. This is an endothermic reaction (compared with PC 1-2 structure formation) that is a reversible reaction at higher temperature. Therefore, TMG adsorption on Si\({}_{2}\)-NH surface tends to stop at the PC 1-2 structure by consuming a one-surface adsorption site rather than continuing to a PC 2-2 structure. This work was elucidated on the conditions of TMG adsorption, whether it is adsorbed on surfaces by consuming one surface adsorption site or two. In the case that the reaction stops at the PC 1-2 structure, the reaction mechanism of NH\({}_{3}\) adsorption on this structure (nitridation step) is discussed in Section 3.3. In 2015, Pansila reported the use of NH\({}_{3}\) and NH\({}_{3}\) plasma to generate Si-NH\({}_{2}\) and Si\({}_{2}\)-NH adsorption sites on cleaned Si(100) substrates [32]. After the exposure of NH\({}_{3}\) or NH\({}_{3}\) plasma, Si-NH\({}_{2}\) and Si\({}_{2}\)-NH adsorption sites coexisted on the surface at RT. The ratio of Si\({}_{2}\)-NH to Si-NH\({}_{2}\) was substantially increased with NH\({}_{3}\) plasma exposure compared to non-plasma NH\({}_{3}\) exposure. We considered that the use of NH\({}_{3}\) plasma to create a Si\({}_{2}\)-NH surface was highly beneficial for the deposition of GaN thin films in low-temperature ALD processes.