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

Daniel Hiller*, Jörg Göttlicher, Ralph Steininger, Thomas Huthwelker, Jaakko Julin, Frans Munnik, Michael Wahl, Wolfgang Bock, Ben Schoenaers, Andre Stesmans, Dirk König

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    30 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Pages (from-to)30495-30505
    Number of pages11
    JournalACS applied materials & interfaces
    Volume10
    Issue number36
    DOIs
    Publication statusPublished - 12 Sept 2018

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