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

Abstract

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.

Kinetics of Bulk Lifetime Degradation in Float‐Zone Silicon: Fast Activation and Annihilation of Grown‐In Defects and the Role of Hydrogen versus Light

“…The bulk lifetimes (measured here after stripping the coatings and SA‐passivation, unlike ref. [6]) of both dielectrics in the fully degraded state do not differ significantly, despite a factor of ≈30 lower amount of H in Al 2 O 3 (≈7 × 10 15 H cm − 2 with [H] ≈2.9 at% at 200 °C‐ALD) [ 28–30 ] versus SiN x :H (≈2.11 × 10 17 H cm −2 with [H] = 20.5 at% as measured by elastic recoil detection analysis (ERDA)). Note that ERDA quantifies the total amount of H in the dielectric film irrespective of different bond configurations, and differences in these configurations could affect the concentration of hydrogen which has entered the Si bulk.…”

Deactivation of silicon surface states by Al-induced acceptor states from Al–O monolayers in SiO2

“…The conditions for Coulomb repulsion between ionized Al-acceptors in SiO 2 are virtually identical for both samples, resulting in ∼1.4% of ionized acceptors. 12,31 Figure 7 shows schematic band structures of sample W for d tÀOx values of 1, 3, and 5 nm illustrating the discussion above. For ultra-thin tunnel-SiO 2 of 1 nm [ Fig.…”

“…16 In addition to electrons from the Si-substrate, the P b -type defects also represent a source of electrons to charge the acceptor states, as shown before by electron spin resonance (ESR). 12 Therefore, it is conceivable that their removal by H 2 results in less Q fix .…”

“…1. We presume for simplicity the ionized acceptor states to be at one depth position in the center of the 6 Al-O MLs instead of being located at both SiO 2 /Al-O interfaces 12 (due to the ultrathin Al-O layer, the discrepancy is considered negligible) and having a relaxation energy of FIG. 4.…”

“…3 Examples for the latter electrons from the Si substrate and that act as hopping sites for holes. 11,12 In this contribution, the correlation between negative Q fix and minority carrier lifetimes (τ eff ) as a function of the tunnel-SiO 2 thickness and the role of H 2 -passivation are investigated. We show that the interface trap density at the Si/SiO 2 interface depends decisively on the thickness of the tunnel-SiO 2 between the wafer and the ALD Al-O MLs.…”