Al₂O₃ on Si is known to form an ultrathin interfacial SiO₂ during deposition and subsequent annealing, which creates a negative fixed charge (Q_fix) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Q_fix. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO₂ films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10^-18 cm²/s at 1000 °C), are deposited at a constant rate of ∼5 × 10¹⁴ Al atoms/(cm² cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO₂ imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO₂ thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Qfix but that a SiO₂/Al₂O₃ interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO₂ dangling bonds to create a negative Q_fix. Hence, tunneling imposes limitations for the SiO₂ and Al₂O₃ layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Q_fix of ∼5 × 10¹² cm^-2 (comparable to thick ALD-Al₂O₃ on Si) with ∼1.7 nm tunnel-SiO₂ and just seven ALD-Al₂O3 cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.