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 Al2O3 was studied using in situ infrared spectroscopy and photoconductance decay measurements. Al2O3 was deposited on high-lifetime, float-zone c-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 the c-Si/Al2O3 interface, as well as the bulk of the Al2O3 film during the entire process. Our results show that some surface Si–H bonds are preserved after the ALD of Al2O3 on H-terminated Si. During the annealing step at 400 °C, restructuring occurs at the c-Si/Al2O3 interface to form interfacial SiOx. Isotope labeling was used to differentiate interfacial SiD bonds on the c-Si surface from H incorporated in Al2O3. Within the sensitivity of our infrared setup (∼1013 cm–2), we did not observe any net migration of atomic H or D from Al2O3 to the c-Si/Al2O3 interface. To isolate the effects of chemical and field-effect passivation of Al2O3 thin films, we carried out surface passivation studies on c-Si/SiO2/Al2O3 stacks. We also annealed these stacks in different atmospheres to test the influence of annealing atmospheres on the chemical passivation of c-Si by Al2O3 and observed that O2-containing atmosphere led to the best surface chemical passivation.