The role of nickel impurity in the crystallization of a-Si thin films has been studied using x-ray absorption fine structure spectroscopy (XAFS). The growth process involves two geometries (1) the deposition of 50 nm Ni films on fused silica substrates followed by deposition of 400 nm amorphous Si (a-Si) films at different substrate temperatures and (2) 400 nm a-Si deposition followed by 50 nm Ni deposition. X-ray Absorption studies were carried out at Ni K-edge using the BM08 GILDA Beamline of the European Synchrotron Research Facility (ESRF, Grenoble) in fluorescence mode at two different angles of incidence. To understand the evolution of the local structure of Ni diffusing from bottom to top and from top to bottom, in Ni on top and Ni bottom geometries respectively, total reflection (to study top layers) and the 2 degree incidence angle (to study bottom layers) were employed. In case of Ni bottom geometry, a clear sign of pure metallic Ni is observed in the bottom layers of the film for the deposition carried up to 300 °C. However the top layers have combined features of metallic Ni and nickel rich silicide. XAFS analysis of the Ni indicates that the film deposited up to 350 °C contains mostly unreacted metallic Ni in both top and bottom layers. When Ni is on top, no traces of metal oxide or silicide are observed in the near edge (XANES) as well as extended x-ray absorption (EXAFS) parts of the XAFS spectra. However, a sudden change to high metal oxidation is observed in case of the film deposited at 400 °C. Some traces of metallic Ni and silicides are also observed in the EXAFS part of the spectra. The quantity of metal oxidation is found to be higher in the top layers than the bottom layers, indicating that the top layers are highly oxidized. For the film deposited beyond 500 °C, Ni shows pure NiSi2 phase in both geometries. The appearance of metallic Ni on the top of 400 nm a-Si strongly supports the argument of metal diffusion mechanism in the crystallization process of a-Si thin films. At further higher deposition temperatures, the bottom layers have the combined features of metallic Ni and NiSi2, and the top layers have the combined contribution of both NiSi2 and NiO.
"X-ray absorption fine structure study of nickel induced crystallization of amorphous silicon thin films"
F Rocca;
2012
Abstract
The role of nickel impurity in the crystallization of a-Si thin films has been studied using x-ray absorption fine structure spectroscopy (XAFS). The growth process involves two geometries (1) the deposition of 50 nm Ni films on fused silica substrates followed by deposition of 400 nm amorphous Si (a-Si) films at different substrate temperatures and (2) 400 nm a-Si deposition followed by 50 nm Ni deposition. X-ray Absorption studies were carried out at Ni K-edge using the BM08 GILDA Beamline of the European Synchrotron Research Facility (ESRF, Grenoble) in fluorescence mode at two different angles of incidence. To understand the evolution of the local structure of Ni diffusing from bottom to top and from top to bottom, in Ni on top and Ni bottom geometries respectively, total reflection (to study top layers) and the 2 degree incidence angle (to study bottom layers) were employed. In case of Ni bottom geometry, a clear sign of pure metallic Ni is observed in the bottom layers of the film for the deposition carried up to 300 °C. However the top layers have combined features of metallic Ni and nickel rich silicide. XAFS analysis of the Ni indicates that the film deposited up to 350 °C contains mostly unreacted metallic Ni in both top and bottom layers. When Ni is on top, no traces of metal oxide or silicide are observed in the near edge (XANES) as well as extended x-ray absorption (EXAFS) parts of the XAFS spectra. However, a sudden change to high metal oxidation is observed in case of the film deposited at 400 °C. Some traces of metallic Ni and silicides are also observed in the EXAFS part of the spectra. The quantity of metal oxidation is found to be higher in the top layers than the bottom layers, indicating that the top layers are highly oxidized. For the film deposited beyond 500 °C, Ni shows pure NiSi2 phase in both geometries. The appearance of metallic Ni on the top of 400 nm a-Si strongly supports the argument of metal diffusion mechanism in the crystallization process of a-Si thin films. At further higher deposition temperatures, the bottom layers have the combined features of metallic Ni and NiSi2, and the top layers have the combined contribution of both NiSi2 and NiO.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
