PO_x/Al₂O₃ Stacks for c‑Si Surface Passivation: Material and Interface Properties

Phosphorus oxide (PO_x) capped by aluminum oxide (Al₂O₃) has recently been discovered to provide excellent surface passivation of crystalline silicon (c-Si). In this work, insights into the passivation mechanism of PO_x/Al₂O₃ stacks are gained through a systematic study of the influence of deposition temperature (T_dep = 100–300 °C) and annealing temperature (T_ann = 200–500 °C) on the material and interface properties. It is found that employing lower deposition temperatures enables an improved passivation quality after annealing. Bulk composition, density, and optical properties vary only slightly with deposition temperature, but bonding configurations are found to be sensitive to temperature and correlated with the interface defect density (D_it), which is reduced at lower deposition temperature. The fixed charge density (Q_f) is in the range of + (3–9) × 10¹² cm^–2 and is not significantly altered by annealing, which indicates that the positively charged entities are generated during deposition. In contrast, D_it decreases by 3 orders of magnitude (∼10¹³ to ∼10¹⁰ eV^–1 cm^–2) upon annealing. This excellent chemical passivation is found to be related to surface passivation provided by hydrogen, and mixing of aluminum into the PO_x layer, leading to the formation of AlPO₄ upon annealing.