Vibrational Stretching Modes of the Si-H and Si-D Bonds in Amorphous Silicon Nitride
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Vibrational Stretching Modes of the Si-H and Si-D Bonds in Amorphous Silicon Nitride Shu-Ya Lin Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan
ABSTRACT The vibrational density of states for the SiH(D) and SiH2(D2) groups in amorphous silicon nitride are calculated. The Si-H(D) bond-stretching modes of the SiH2(D2) groups are found to split, with the Si-D bond-stretching mode having larger splitting. The difference between the Si-D bond-stretching frequency of the SiD group and the Si-D asymmetrical stretching frequency of the SiD2 group has about the same value as that of the Si-H bonds. The differences in the effective masses of the Si-H and Si-D bonds and the degrees of the bond-stretching splitting of the SiH2 and SiD2 groups have compensating effects. INTRODUCTION Hydrogenated amorphous silicon nitride thin films can be deposited by plasma-enhanced chemical vapor deposition (PECVD) [1 - 8]. The source gases are usually SiH4 + N2 or SiH4 + NH3. The former gases usually produce hydrogenated amorphous silicon nitride films with Si-H bonds, but no N-H bonds [1]. The latter gases can produce films with different amounts of Si-H and N-H bonds by varying the flow ratio R (= [NH3]/[SiH4]) of the source gases [1, 6]. The Si-H bond-stretching frequency, detected by the infrared (IR) absorption, changes from about 1995 to 2160 cm-1 as a-SiNx changes from an a-Si film with low H concentration to the stoichiometric a-Si3N4:H film [3, 6]. The N-H bond-stretching frequency is about 3350 cm-1 [5, 6, 8]. When the films were prepared at high NH3/SiH4 flow ratio R, two things happened [4 - 7]. The Si-H bond-stretching frequency increased to about 2190 – 2200 cm-1 and the N-H bond-stretching frequency decreased to about 3335 cm-1. The Si-H bond-stretching frequency was higher than the normal highest value of 2160 cm-1 by about 30 – 40 cm-1. By replacing hydrogen with deuterium in similar processes, the Si-D bond-stretching frequency increased by about the same value as that of the Si-H bond-stretching frequency. This study tries to address the Si-H bond-stretching frequency beyond 2160 cm-1 and the same increment of the Si-H and Si-D bond-stretching frequencies for the amorphous silicon nitride films prepared at high source flow ratio. THEORETICAL METHOD A21.5.1 1
The cluster-Bethe-lattice method [9, 10] is used in this study to calculate the local vibrational density of states (DOS) for the Si-H and Si-D bonds in amorphous silicon nitride. The tetrahedral unit of SiN4 and a four-fold rotation-reflection, S4, symmetry are used to build the Si3N4 Bethe lattice shown in Fig. 1(a). In the Si3N4 Bethe lattice as constructed, each Si atom has four N-atoms as its neighboring atoms on the corners of the tetrahedron and each N-atom has three Si-atoms as its neighbors, with near planar configuration. Among the three Si-N-Si angles, two of them are 1200 and the other one is 104.480. The Green’s function, G, is used to set up the equations of the problems. The equation of motion can be written as
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