The effects of annealing on the formation and stability of the microcrystalline phase in neutron irradiated amorphous Si : H thin films

Abstract

Isothermally annealed neutron irradiated hydrogenated amorphous silicon thin films showed a phase transformation from amorphous to microcrystalline (μc-phase) at 473 K. Infrared vibrational spectroscopy and analytical electron microscopy (TEM) were used to follow the transformation. The stability of the μc-phase is strongly dependent upon the stress level in the thin films. The microcrystalline thin films showed a neutron induced defect state in the IR spectra at 530 cm −1, a transverse optical phonon mode of SiSi at 522 cm −1 which was IR active, and smaller wavenumber sharp peaks below 520 cm −1. Annealing the thin films at 473 K in a hydrogen atmosphere for 30 min produced a large fraction of microcrystalline phase. The neutron defect state of 530 cm −1 and the states below 520 cm −1 shifted during annealing. Overlapped peaks were observed centered at 525 cm −1. Transmission electron microscopy studies indicated that large amounts of uniform (≈ 10 nm in diameter) μc-phase formed after 30 min at 473 K. The increased IR signal intensity of a grain boundary SiH stretching mode at 2019 cm −1 also indicated the presence of large amounts of μc-phase in the film. However, the μc-phase was unstable under the electron beam. 15N-hydrogen profiling showed that only a slight loss of hydrogen occurred during the annealing process. The hydrogen profile after annealing indicates that most of the μc-phase is likely located near the surface of the thin film and at the interface between the thin film and fused quartz substrate.