Atomic microstructure and electronic properties of a-SiNx:H deposited by radio frequency glow discharge

Abstract

a-SiNx:H films of various composition x were deposited by rf glow discharge (GD). The deposition rate was analyzed for three ranges of gas flow ratio R = [NH3]/[SiH4] depending on the deposition mechanism. Properties of these films were measured by means of x-ray photoelectron spectroscopy (XPS), infrared (IR) absorption, optical absorption, and the temperature dependence of electrical conductivity. The composition x was determined by XPS. For large values of R, x was found to be saturated at 1.7. The variation of H content was detected by IR absorption. The variation of coordinating atoms of Si with increasing x was deduced from the variation of XPS spectra of the Si 2p core-level and the shift of Si-H stretching vibration frequency in IR absorption. Based on the random bonding model and assuming bonding units to the central Si atom to be Si, N, and NH, probabilities of Si tetrahedra with various coordinating units were obtained. The results indicate that there are many Si—Si bonds for the stoichiometric x=1.33 and that the concentration of Si—Si bonds diminishes at around the saturation value x=1.7. These results seem to imply that the presence of Si—Si bonds to some extent is a prerequisite condition for film deposition by GD. Experimental results of optical absorption analyzed by Tauc relation revealed the presence of two kinds of x region whose properties are quite different. For x<1.5, its optical absorption is similar to a-Si:H modified by the presence of N. With increasing x, the optical band gap EO increases and B coefficient in the Tauc plot decreases. At about x=1.5, Si—Si bonding effectively disappears and the optical absorption abruptly changes to that similar to β-Si3N4. Considering these results, the change of electronic band structure with x was deduced on the basis of the atomic structure obtained above and by the tight binding approximation. For x<1.5, the optical band gap is due to Si—Si bonding, the energy gap of which increases, and the linear band tail becomes broad with increasing x. From the observed temperature dependence of conductivity, variations of the activation energy and pre-exponential factor are obtained with x up to x=1.0. For the decrease of conductivity with x, the contribution from the pre-exponential factor is much larger than that from the activation energy. This result can be understood by a transport mechanism in the electronic band structure obtained above. Finally, it is concluded that the electronic properties of a-SiNx:H deposited by GD ran.