Chemical Passivation of Crystalline Si by Al2O3 Deposited Using Atomic Layer Deposition: Implications for Solar Cells

The atomistic-level mechanism for the chemical passivation of the monocrystalline Si (c-Si) surface with thermally annealed Al₂O₃was studied using in situ infrared spectroscopy and photoconductance decay measurements. Al₂O₃was deposited on high-lifetime, float-zonec-Si substrates using atomic layer deposition (ALD) from trimethylaluminum (TMA), and H₂O or O₃. Surface-sensitive attenuated total reflection Fourier transform infrared spectroscopy was used to monitor thec-Si/Al₂O₃interface, as well as the bulk of the Al₂O₃film during the entire process. Our results show that some surface Si-H bonds are preserved after the ALD of Al₂O₃on H-terminated Si. During the annealing step at 400 °C, restructuring occurs at thec-Si/Al₂O₃interface to form interfacial SiO_x. Isotope labeling was used to differentiate interfacial SiD bonds on thec-Si surface from H incorporated in Al₂O₃. Within the sensitivity of our infrared setup (∼10¹³cm^-2), we did not observe any net migration of atomic H or D from Al₂O₃to thec-Si/Al₂O₃interface. To isolate the effects of chemical and field-effect passivation of Al₂O₃thin films, we carried out surface passivation studies onc-Si/SiO₂/Al₂O₃stacks. We also annealed these stacks in different atmospheres to test the influence of annealing atmospheres on the chemical passivation ofc-Si by Al₂O₃and observed that O₂-containing atmosphere led to the best surface chemical passivation.