Amorphous hydrogenated silicon films produced by an expanding argon-silane plasma investigated with spectroscopic IR ellipsometry

We have produced amorphous hydrogenated silicon (a-Si:H) films from silane with an unconventional deposition technique, a supersonically expanding d.c. arc plasma. The deposited films are analysed mainly by using spectroscopic IR ellipsometry. Further analysis has been performed with scanning electron microscopy, IR absorption spectroscopy and in situ He-Ne ellipsometry. The film structure appears to be strongly linked to the degree of ionization of the expanding beam, the injection location of silane gas, the degree of dissociation and the percentage of the injected hydrogen gas. Deposition at low arc power results in films of polysilane, which are very sensitive to oxidation during air exposure. Without hydrogen injection, films with a high refractive index and low hydrogen content are obtained (below the detection limit of the IR transmission spectrometer). Hydrogen injected in the middle of the plasma arc results in a-Si:H films with a refractive index of 3.75 at 632 nm; this value is close to the indices of the best films obtained with plasma-enhanced chemical vapour deposition (PECVD). In these films, the strength of the vibrational absorption at 2000 cm-1, which can be assigned to SiH stretch bonds, is equal to the strength of a vibration at 2085 cm-1. Because the bending absorptions of SiH2 at 860 and 890 cm-1 are not detected in the films produced, it is concluded that this 2085 cm-1 absorption peak in our films is caused by bond stretching of SiH rather than by that of SiH2. As in PECVD, the optimum substrate temperature at which films of good quality are obtained is in the range from 525 to 575 K. The deposition rate is of the order of several nanometers per second