Effective passivation of silicon surfaces by ultrathin atomic-layer deposited niobium oxide

\u3cp\u3eThis letter reports on effective surface passivation of n-type crystalline silicon by ultrathin niobium oxide (Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e) films prepared by atomic layer deposition (ALD) and subjected to a forming gas anneal at 300 °C. A champion recombination parameter J\u3csub\u3e0\u3c/sub\u3e of 20 fA/cm\u3csup\u3e2\u3c/sup\u3e and a surface recombination velocity S\u3csub\u3eeff\u3c/sub\u3e of 4.8 cm/s have been achieved for ultrathin films of 1 nm. The surface pretreatment was found to have a strong impact on the passivation. Good passivation can be achieved on both HF-treated c-Si surfaces and c-Si surfaces with a wet-chemically grown interfacial silicon oxide layer. On HF-treated surfaces, a minimum film thickness of 3 nm is required to achieve a high level of surface passivation, whereas the use of a wet chemically-grown interfacial oxide enables excellent passivation even for Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e films of only 1 nm. This discrepancy in passivation between both surface types is attributed to differences in the formation and stoichiometry of interfacial silicon oxide, resulting in different levels of chemical passivation. On both surface types, the high level of passivation of ALD Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e is aided by field-effect passivation originating from a high fixed negative charge density of 1-2 × 10\u3csup\u3e12\u3c/sup\u3e cm\u3csup\u3e-3\u3c/sup\u3e. Furthermore, it is demonstrated that the passivation level provided by 1 nm of Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e can be further enhanced through light-soaking. Finally, initial explorations show that a low contact resistivity can be obtained using Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e-based contacts. Together, these properties make ALD Nb\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e a highly interesting building block for high-efficiency c-Si solar cells.\u3c/p\u3