CO2 plasma treatment of tin oxides

By combining in situ ellipsometry and XPS depth profiling, the effects of a CO2 plasma treatment on tin oxides have been investigated. Tin oxide, used as the transparent front contact in hydrogenated amorphous silicon (a-Si:H) solar cells, is chemically reduced by the impact of the silane plasma during the preparation of a-Si:H. In this report a detailed study will be presented how tin oxides can be protected by a specific CO2 plasma treatment which prevents this reduction process. In situ ellipsometry proves that no reduction occurs during the deposition of a-Si:H on tin oxides after a pre-treatment by CO2 plasma. Moreover these pre-treated tin oxides are resistant to a pure hydrogen plasma, which causes significant chemical reduction of untreated ones and substantially deteriorates their optical transmission. XPS analysis of CO2 treated tin oxides reveals the formation of a SiO2 layer, since silicon is chemically transported from the reactor walls to the surface of the substrate by means of the plasma. XPS depth profiling of a-Si:H/SiO2/SnO2 interfaces confirms the interpretation of the in situ ellipsometry measurements. With the appropriate setting of CO2 plasma parameters, no metallic tin is formed at the a-Si:H/SnO2 interface. Employing inadequate parameters, however, leads to a reduction of the tin oxide, and to the formation of metallic tin at the interface. The investigation of these different situations convincingly demonstrates how in situ ellipsometry and XPS depth profiling are able to provide complementary information on this chemical reduction process. The deposition of the samples has been monitored in real time by ellipsometry, while the erosion of these layers by sputtering and simultaneous XPS analyses quantitatively reveals their chemical composition.