Structural Properties of Al-O Monolayers in SiO on Silicon and the Maximization of Their Negative Fixed Charge Density.

AlO on Si is known to form an ultrathin interfacial SiO during deposition and subsequent annealing, which creates a negative fixed charge ( Q) 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. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10 cm/s at 1000 °C), are deposited at a constant rate of ∼5 × 10 Al atoms/(cm cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Q but that a SiO/AlO 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/SiO dangling bonds to create a negative Q. Hence, tunneling imposes limitations for the SiO and AlO 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 of ∼5 × 10 cm (comparable to thick ALD-AlO on Si) with ∼1.7 nm tunnel-SiO and just seven ALD-AlO 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.