A very important aspect in the discussion of penetration is the reactivity of the remaining surface reactive groups towards the precursor. Reactive bridge oxygen (RBO) on the SiO2 had been discussed before as potential nucleation sites for selectivity-lifting reactions. In a computational study on ALD of Al2O3 on NGS SiO2, RBOs are found to not be blocked by the SMI at medium densities of 2.5 SMIs nm -2 (Fig. .). TEA adsorbs on top of the SMI layer (this is the first step for overgrowth, see section 1.4.4) before penetrating the inhibitor layer with a low intralayer diffusion barrier (ΔE ‡ = 11 kJ•mol -1 ). It attaches to the RBO in an exothermic Lewis acid-Lewis base reaction (ΔE = -113 kJ•mol -1 ). The number in brackets in Fig. .a shows that the dispersion contribution to the bonding is the major factor up to this intermediate stage. This intermediate then readily converts into a surface-bonded TEA via a negligible barrier (ΔE ‡ = 3 kJ•mol -1 ) to a very stable and covalently bonded product (ΔE = -232 kJ•mol -1 ). Looking at the same process at an elevated SMI density of 3.4 SMIs nm -2 , the adsorption at the RBO turns into an endothermic reaction for both TMA and TEA highlighting again the crucial role of SMI surface coverage leading to steric blocking through adsorption prevention (Fig. .c). From this study it was concluded that a preceding water pulse removing the RBOs (Fig. .b) and creating more reactive OH-groups at the surface should be beneficial for creating a dense inhibitor layer. Such an approach of increasing OH-density has already been used experimentally for a different ALD process. The effect of precursor size on selectivity has been investigated in several studies. Kim et al. presented an explanation for the often-observed high selectivity of AS-ALD when using DMAI as a precursor. In addition to the rather large vdW-volume of the precursor, it is present in dimerized form under typical ALD conditions which increases the efficiency of steric blocking. A further study found the substitution of TMA by chlorinated precursors Al(CH3)2Cl and AlCl3 leading to higher selectivity which was again explained via dimerization trends. The role of precursor dimerization on selectivity has recently been investigated more systematically in two computational studies. One study investigated trends across a whole range of precursors with central elements B, Al, Ga and In and different ligand types using DFT methodology, while the second study focused on high accuracy calculations using CCSD(T)/CBS quality calculations for Al-precursors eliminating most error sources and serving as benchmark data. .