Hydrogenated amorphous silicon nitride (a-SiN x:H) films have been grown from a SiH 4–N 2 gas mixture through very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) at 50 ℃. The films are dense and transparent in the visible region. The peak frequency of the Si–N stretching mode in the IR absorption spectrum increases with increasing N–H bond density, which is similar to the behavior of a-SiN x:H films grown from SiH 4–NH 3 gas. During storage in a dry air atmosphere, the Si–O absorption increases. A large shift in the peak frequency of the Si–N stretching mode in the initial stage of oxidation, which is higher than the shift expected from the increase in the N–H bond density, is mainly caused by the change in the sum of electronegativity of nearest neighbors around the Si–N bond due to the increase in the Si–O bond density.
Hydrogenated amorphous silicon nitride (a-SiNx:H) films are useful as barrier layers to prevent the diffusion of oxygen and water into optical devices. To protect optical devices against thermal damage during the deposition of an a-SiNx:H film, a plasma- enhanced chemical vapor deposition (PECVD) system is used at a low substrate temperature. However, these a-SiNx:H films generally have high densities of N-H bonds and Si-H bonds; consequently, the aerial oxidation of H-terminated or dangling bonds can easily occur. Therefore, it is important to clarify the mechanism of the initial oxidation process during air exposure to protect optical devices, such as organic photovoltaic devices, which require high performance barrier films.
A-SiNx:H films are usually grown from a SiH4–NH3 gas mixture. However, in order to obtain low-hydrogen-content a-SiNx:H films, it is preferable to use N2 instead of NH3 as the nitrogen source because N2 does not contain N–H bonds. Usually, a-SiNx:H films are deposited using a PECVD system at a 13.56 MHz excitation frequency. Shifting the discharge frequency into the very high frequency (VHF) region results in a higher deposition rate for PECVD in a conventional diode-type reactor for a-Si:H [
In this paper, the deposition of a-SiNx:H films at 50˚C is presented. The effects of power density on the deposition rate, optical bandgap (Eopt), and Si–H and N–H bond densities (NSi–H and NN–H) in the resulting a-SiNx:H films are investigated. The changes in the local structure of the films that occurred during storage in dry air, as detected using Fourier transform infrared spectroscopy (FTIR), were also studied to understand the stability mechanism in the film properties.
The a-SiNx:H films were grown through the VHF–PECVD of a SiH4–N2 gas mixture in a diode-type reactor, in which each electrode had a diameter of 60 mm and the electrode separation was 10 mm. The ultimate vacuum pressure of the reactor was ~10–6 Pa and the total pressure of 2% SiH4 diluted with N2 gas was 200 Pa with a flow rate of 30 sccm. Fused silica and Si(100) wafers were used as the substrates and the substrate temperature was maintained at 50˚C during the deposition. The VHF frequency was 150 MHz and the VHF power density was varied from 70 to 385 mW/cm2. The typical film thickness was 300 nm. The Tauc optical bandgap Eopt was obtained from the optical transmittance spectrum. The Si–H bond density NSi–H and N–H bond density NN–H were calculated from the FTIR absorption spectrum in transmission mode through the expression:
where X = Si or N,
In order to investigate the local structural changes that occurred in a-SiNx:H films during storage, the samples were stored in an FTIR system containing dry air for 100 days with the temperature kept at 25˚C and the relative humidity (RH) maintained below 0.5 %.
stretching mode obtained in the present study are slightly higher than those obtained from films deposited from a SiH4–NH3 gas mixture reported by Tsu et al. [
Figures 5(a)-(c) show the FTIR spectra of the as-deposited and aged a-SiNx:H films deposited at 210 mW/cm2. The as-deposited a-SiNx:H film has an Eopt of 5.2 eV, NN–H of 2.6 × 1022 cm−3, and NSi–H of 1.6 × 1021 cm−3, respectively. The sample was fixed on the sample holder in the FTIR system containing dry air (25˚C, <0.5% RH) for 100 days, during which the peak frequency of the Si–N stretching mode shifted from 880 cm−1 to 898 cm−1 (
In
For the Si–N stretching vibration at 750 - 1000 cm−1, while the intensities at 780 cm−1 (n2) and 835 cm−1 (n3) obviously decrease, a peak simultaneously appears at 940 cm−1
(n4). These peaks and valleys are the main causes for the peak shift of the Si–N stretching mode to a higher value at the initial stage of oxidation. As mentioned before, the increase in NN–H during storage is small. Therefore, the effect of increase in NN–H during storage on the shift in the peak frequency through changes in the sum of electronegativity of the nearest neighbors around the Si–N bond, is small. On the other hand, oxidation of a-SiNx:H films occurred during storage, thus, Si–O bonds increased. This increase in the concentrations of the O atom, which is more electronegative than N, back- bonded to the Si atoms of the Si–N bonds caused a new increase in absorption at the high-wavenumber side (940 cm−1, n4). For the low-wavenumber side, there are some possible causes for the generation of valleys. When a peak appears in a spectrum due to changes in electronegativity of the nearest neighbors, a valley of almost identical area appears necessarily. The valley at the low-wavenumber side (n3) is considered to be caused by such a reduction with the generation of the new peak (n4). Meanwhile, Lucovsky and co-workers pointed out that the low-wavenumber side (840 cm−1) of the Si–N stretching mode reflects the vibration of the Si–N bonds, in which at least one H atom is back-bonded to the Si atom [
As shown in
The a-SiNx:H films grown in the present study through VHF-PECVD from a SiH4–N2 gas mixture are transparent in the visible region. The peak frequency of the Si–N stretching mode increases with increasing NN–H, which is similar to the behavior of a-SiNx:H films grown from SiH4–NH3 gas. During film storage in dry air, the initial stage of oxidation was observed through in-situ IR measurement. The peak frequency of the Si–N stretching mode also increases in the initial stage of oxidation because of the increase of absorbance at the high-wavenumber side, and the decrease at the low-wavenumber side of the Si–N stretching mode caused by an increase of the concentration of O atoms back-bonded to the Si atoms of the Si–N bonds.
This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K04682 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. I wish to acknowledge helpful discussions and encouragement from Professor Y. Hoshi.
Kobayashi, S. (2016) IR Spectroscopic Study of Silicon Nitride Films Grown at a Low Substrate Temperature Using Very High Frequency Plasma- Enhanced Chemical Vapor Deposition. World Journal of Condensed Matter Physics, 6, 287-293. http://dx.doi.org/10.4236/wjcmp.2016.64027