Defect Chemistry Engineered SnO2 Thin Films via Thermal ALD: Unraveling Defect-Controlled Electronic Structure for Charge Transport

Atomic layer deposition (ALD) was utilized to fabricate SnO₂ thin films using tetrakis(dimethylamino)tin (TDMASn) and H₂O over a temperature range of 50-200 °C, with the goal of elucidating the interplay between growth temperature, defect chemistry, and electronic structure. Comprehensive characterization via photoelectron spectroscopy, ion beam analysis (IBA), and band structure mapping (UPS and LEIPS) reveals that the electronic properties of SnO₂ are not solely dictated by bandgap size but are profoundly influenced by defect-induced electronic restructuring. At low deposition temperatures (50-100 °C), films exhibit oxygen-rich compositions, residual organic ligands, and hydrogen incorporation, resulting in midgap states, pseudo-Sn²⁺ features, and shallow acceptor levels that enhance surface electron emission but hinder bulk charge transport. Higher deposition temperatures (150-200 °C) promote ligand decomposition and efficient oxidation, reducing impurity levels and enabling better crystallinity. Notably, 150 °C yields optimal balance, with a high work function (4.56 eV), high electron affinity (3.31 eV), and favorable conduction band position (E_CB = 1.25 eV), driven by hydrogen-related electron trap states. Conversely, at 200 °C, oxygen vacancies and reemergent Sn²⁺ states lower the injection barrier into the bulk but diminish surface conductivity. Deuterium labeling and ERDA confirm hydrogen incorporation, with distinct depth distributions. Surface-sensitive HER measurements and electrical transport analyses further disentangle the competing effects of surface and bulk defects on conductivity. This work advances defect-aware synthesis strategies and provides mechanistic insights for tuning ALD-grown SnO₂ in applications such as electron transport layers, catalysts, and transparent conductors.