Oxidant-Dependent Conduction in Early-Cycle Atomic-Layer-Deposited Al ₂ O ₃ and Its Impact on the Dielectric Behavior of Al ₂ O ₃ /TiO ₂ Nanolaminates

Nanolaminates─atomically layered stacks of dissimilar oxides─are widely used to engineer dielectric behavior at the nanometer scale. Among these, atomic layer deposition (ALD)-grown Al 2 O 3 /TiO 2 stacks have been reported to exhibit “giant” dielectric constant (κ ∼ 10 3 ) at subnanometer periods. Here we show that the apparent high-κ is not an intrinsic dielectric enhancement but a measurement artifact arising from electrically percolative Al 2 O 3 /TiO 2 stacks treated as ideal dielectrics. Using Al 2 O 3 /TiO 2 nanolaminate metal–insulator–metal (MIM) capacitors and a combination of electrical ( C – f, I – V, EIS), spectroscopic (XPS, HAXPES), and structural (TEM) measurements, we found that TMA/H 2 O growth yields Al-deficient Al 2 O 3 on TiO 2 that appears morphologically continuous yet lacks full atomic closure, forming electronically percolative pathways that recover insulating behavior only after a fully coalesced overlayer develops. In contrast, TMA/O 3 deposition produces fully continuous Al 2 O 3 that remains insulating even at subnanometer thickness. These results establish practical design rules, showing that oxidant chemistry and film thickness together govern film continuity and insulating behavior, thereby clarifying when ultrathin ALD Al 2 O 3 functions as a true insulator versus a quasi-conductive layer. The insights extend directly to applications in microelectronics (gate stacks, MIM/DRAM capacitors), quantum devices (2DEGs, tunnel barriers), and electrochemical/photovoltaic systems (battery electrodes, electrocatalysts, and perovskite interface engineering), where reliable subnanometer coatings are essential.