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

Al 2 O 3 on Si is known to form an ultrathin interfacial SiO 2 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 SiO 2 films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10 –18 cm 2 /s at 1000 °C), are deposited at a constant rate of ∼5 × 10 14 Al atoms/(cm 2 cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO 2 imprints its tetrahedral near-order and bond length into the Al–O MLs. By variation in the tunnel-SiO 2 thickness and the number of Al–O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Q fix but that a SiO 2 /Al 2 O 3 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 2 dangling bonds to create a negative Q fix . Hence, tunneling imposes limitations for the SiO 2 and Al 2 O 3 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 × 10 12 cm –2 (comparable to thick ALD-Al 2 O 3 on Si) with ∼1.7 nm tunnel-SiO 2 and just seven ALD-Al 2 O 3 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.