Structural Properties of Al–O Monolayers in SiO₂ on Silicon and the Maximization of Their Negative Fixed Charge Density

Al2O3 on Si is known to form an ultrathin interfacial SiO2 during deposition and subsequent annealing, which creates a negative fixed charge (Qfix) 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 Qfix. 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 Qfix. 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 Qfix 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.