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

• DOI: 10.1021/acsami.8b06098
• Publication Date: 2018-08-15T18:02:41Z
• Authors: Daniel Hiller; Jörg Göttlicher; Ralph Steininger; Thomas Huthwelker; Jaakko Julin; Frans Munnik; Michael Wahl; Wolfgang Bock; Ben Schoenaers; Andre Stesmans; Dirk König
• Keywords: Dangling bond; Passivation; Acceptor; 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 low surface recombination velocities in solar cells. Various concepts were suggested explain the origin of this Qfix. In study, we investigate Al-O monolayers (MLs) from atomic layer (ALD) sandwiched between deliberately grown/deposited films. We show Al atoms have ultralow diffusion coefficient (∼4 × 10-18 cm2/s at 1000 °C), are deposited constant rate ∼5 1014 atoms/(cm2 cycle) first ALD cycle, tetrahedral O-coordinated because adjacent imprints its near-order bond length into MLs. By variation tunnel-SiO2 thickness number MLs, demonstrate coordination alone not sufficient for formation Qfix but SiO2/Al2O3 interface within tunneling distance substrate must be present. The Al-induced acceptor states these interfaces energy levels slightly below valence band edge require charging by electrons either or Si/SiO2 dangling bonds create Hence, imposes limitations thicknesses. addition, Coulomb repulsion charged results optimum i.e., separation both interfaces. achieve maximum 1012 cm-2 (comparable thick ALD-Al2O3 Si) with ∼1.7 nm just seven cycles (∼8 Å) after optimized annealing 850 °C 30 s. findings discussed context passivating, hole-selective tunnel contact high-efficiency