Plasma-deposited silicon nitride films with low hydrogen content for amorphous silicon thin-film transistors application

Abstract

A comparative study of the vibrational properties of PECVD amorphous silicon nitride films obtained from SiH4 + NH3 and SiH4 + N2 precursor gas mixtures has been performed by FT-IR transmission spectroscopy. The bonded hydrogen, calculated from the absorption spectra, shows important quantitative and qualitative differences depending on the precursor gas mixtures used. The hydrogen content of near-stoichiometric films obtained from SiH4 + N2 mixture is 10 times lower than that of films prepared from SiH4 + NH3 mixture. In addition, hydrogen is mainly bonded to nitrogen atoms in films from SiH4 + NH3, whereas it is mainly bonded to silicon atoms in films from SiH4 + N2. These low-hydrogenated silicon nitride films, obtained from mixtures containing N2, have been applied as insulator layers in the preparation of amorphous silicon thin-film transistors (a-Si TFTs). The TFTs were of normal staggered type composed of the structure Al/a-SiN:H/a-Si:H grown on NiCr source and drain electrodes deposited on glass substrates. TFTs with a 0.2 μm thick a-Si:H layer and 10 μm channel length have on-off current ratios of 5 × 104, electron field-effect mobilities of about 1.5 cm2/V s and threshold voltages around 5 V.