<i>In Situ</i> Studies of Atomic Layer Deposition of Hafnium Oxide on (Ag,Cu)(In,Ga)Se₂ for Thin Film Solar Cells

Ambient pressure X-ray photoelectron spectroscopy is employed to study in real time the chemical reactions occurring on (Ag,Cu)(In,Ga)Se2 (ACIGSe) surfaces during the first atomic layer deposition (ALD) cycle of HfOx under realistic synthesis conditions by using tetrakisdimethylamido-hafnium (TDMA-Hf) and H2O precursors. We find that the initial deposition due to surface reactions of HfOx ALD on ACIGSe depends on the pretreatment of the ACIGSe surface. While the growth of HfOx occurs directly upon exposure to the metal precursor for the nontreated (i.e., as-deposited) ACIGSe surface through chemical reactions, the growth is slower for the ACIGSe surface pretreated by postdeposition treatment by RbF. In the latter case, the diffusion of alkali and fluorine elements at the surface is observed during the ALD growth, thus leaving less reactive sites for the TDMA-Hf molecules to adsorb on. The results indicate that an optimization of the ALD of HfOx on ACIGSe needs to be taken into consideration for alkali metal fluoride-treated ACIGSe.