Structural properties of N-rich a-Si-N:H films with a low electron-trapping rate

Hydrogenated amorphous silicon nitride (a-SiNx:H) films with a high nitrogen content were prepared by RF plasma-enhanced chemical vapour deposition from a mixture of SiH4 and NH3 at 300 degrees C. The influence of the main operating variables on the quality of the material was examined. Characteristics and properties of the films were investigated using optical absorption, infrared spectroscopy, x-ray photo-electron spectroscopy and capacitance-voltage measurements. From infrared transmission and reflection measurements, it was found most of the IR peaks obtained by direct transmission in the range 850-1040 cm(-1), which have often been mentioned in the literature, are caused not by absorption but rather by a strong reflection due to the Reststrahlen effect. IR studies carried out with polarized light revealed an anisotropic beam-sample interaction probably due to short-range phonon vibration modes: a parallel 'LO mode' (1040-1070 cm(-1)), a parallel and perpendicular 'TO mode' (840 cm(-1)). For sufficiently high values of the N/Si ratio, a decreasing percentage of SiH4 or an increasing RF power leads to a decrease in the trapped charge density which should be related simultaneously to an increase in the N/Si ratio and a decrease in the Si-H bond density in the deposited films. It is suggested that, under our experimental conditions, the charged particle bombardment due to the RF discharge does not play an important role in the charge trapping and that the trapping rate seems to be controlled only by the nature of the traps.