Study of the passivation mechanism of c-Si by Al<inf>2</inf>O<inf>3</inf> using in situ infrared spectroscopy

Abstract

We present an in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy study of the passivation mechanism in the surface passivation of Si solar cells by Al 2 O 3 thin films deposited via atomic layer deposition (ALD) using TMA and O 3 as precursors. The IR measurements suggest that during the annealing stage, the Si-H bonding near the interface decreases. We have used D-terminated c-Si internalreflection crystals to differentiate the residual H atoms that may migrate from ALD Al 2 O 3 films versus the residual D atoms present at the Al 2 O 3 /c-Si interface after ALD. Within the sensitivity of the ATR-FTIR spectroscopy setup of ~10 12 cm -2 for Si-H bonds, we do not detect any migration of H from Al 2 O 3 to the c-Si interface. Therefore, we conclude that the migration of O, and the subsequent restructuring of the interface during the annealing step, primarily contributes toward the chemical passivation of the Al 2 O 3 /c-Si interface.

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

“…Thick Al 2 O 3 on Si passivates the surface best when annealed at 400−450 °C for 30 min, which is related to H or H 2 passivation of the interface 44,45,47 and/or O migration from Al 2 O 3 toward the interface. 51 In contrast, our ESR results do not indicate that hydrogen or oxygen passivation of the Si/SiO 2 interface is the origin for improved passivation. Instead, we assume that our interfacial SiO 2 is already stoichiometric apart from the inevitable, ≤6 Å thick substoichiometric transition layer between Si and SiO 2 .…”

“…for just 30 s, which are safe conditions in terms of inadvertent Al-diffusion in SiO 2 (proven by ToF-SIMS). Thick Al 2 O 3 on Si passivates the surface best when annealed at 400-450°C for 30 min, which is related to H-or H 2 -passivation of the interface44,45,47 and/or O-migration from Al 2 O 3 towards the interface 51. In contrast, our ESR-results do not indicate that hydrogen or oxygen passivation of the Si/SiO 2 interface is the origin for improved passivation.…”

“…The added defect densities [P b0 + P b1 ] for the reference sample of ∼4 × 10 12 cm −2 are ∼2 times higher than optimized thermal SiO 2 50 but no surprise for the PECVD-SiO 2 used here. We observe that the sample with a thick SiO 2 (12 between Si and ∼5.5 Å Al−O indicates that a charge carrier tunneling process that takes place here passivates the defects, rather than a chemical passivation of DBs by hydrogen 44,45,47 or oxygen 51 migrating from thick ALD-Al 2 O 3 layers toward the Si interface. Given the opportunity to discharge the singly occupied amphoteric P b -type defect (transition energy ∼0.3 eV above the Si VB edge 52 ), it appears likely that electron capture into the Al-induced acceptor states does not only happen via electrons from the Si VB but also using electrons from dangling bond defects at the Si/SiO 2 interface.…”

“…Hydrogen (both molecular H 2 and atomic H) has diffusion coefficients more than sufficient to travel through just 10 additional nanometers of SiO 2 and the same five ALD monolayers of Al–O are present as hydrogen source. The absence of defect passivation effects by adding 10 nm SiO 2 between Si and ∼5.5 Å Al–O indicates that a charge carrier tunneling process that takes place here passivates the defects, rather than a chemical passivation of DBs by hydrogen ,, or oxygen migrating from thick ALD-Al 2 O 3 layers toward the Si interface. Given the opportunity to discharge the singly occupied amphoteric P b -type defect (transition energy ∼0.3 eV above the Si VB edge), it appears likely that electron capture into the Al-induced acceptor states does not only happen via electrons from the Si VB but also using electrons from dangling bond defects at the Si/SiO 2 interface.…”

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

“…The meanwhile established explanation for chemical passivation of thick ALD-Al 2 O 3 films deposited directly on Si involves the passivation of DBs by hydrogen [17][18][19] and/or oxygen 20 diffusing from the Al 2 O 3 to the Si/SiO 2 interface. This is supported by the fact that the typical annealing to activate the surface passivation is often at 400-450°C for 15-30 min, 21 i.e., conditions well suited to H-passivate DBs at the Si/SiO 2 interface.…”