I. Pereyra

On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films

Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperatures using nitrous oxide (N2O) and silane (SiH4) as gas precursors. The influence of the N2O/SiH4 flow ratio (varied from 0.25 up to 5) and the thickness of the films on the optical and structural properties of the material was analyzed. The films were characterized by ellipsometry, Fourier-transform infrared spectroscopy, Rutherford backscattering spectroscopy and optical absorption. Two distinct types of material were obtained, silicon dioxide-like oxynitrides SiO2−xNx and silicon-rich oxynitrides SiOxNy (x+y<2). The results demonstrate that in silicon dioxide-like material, the nitrogen concentration can be adequately controlled (within the range 0–15 at.%) with total hydrogen incorporation below 5 at.% and no appreciable SiH bonds. It is also shown that the composition remains uniform through the entire thickness of the films. Furthermore, a linear relation between the refractive index and the nitrogen concentration is observed, which makes this material very attractive for optoelectronic applications. On the other hand, silicon-rich material is similar to amorphous silicon, and presents an increasing concentration of SiH bonds, increasing refractive index and decreasing optical gap, which makes it promising for applications in light-emitting devices.

Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures

In this work we present the results on the fabrication of thick silicon oxynitride and dioxide films deposited by conventional r.f. direct plasma enhanced chemical vapor deposition (DPECVD), at temperature as low as 320°C and from (N2O+SiH4) gaseous mixtures. The samples were characterized by profile measurements, ellipsometry measurements, etching rate, Fourier transform infrared spectroscopy (FTIR), and by scanning electron microscopy (SEM). The results show that for appropriate N2O/SiH4 flow ratio and SiH4 flow, it is possible to obtain very thick SiO2 and SiOxNy films (up to ∼10 μm) at high deposition rates (∼3 μm/h) and preserving the compositional and structural properties of similar high quality thin films obtained in a previous work (I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225). These thick SiO2 and SiOxNy films, exhibit a very well controlled refractive index, in a short range between ∼1.43 and ∼1.53, which is very attractive to SiO2/SiOxNy based waveguide fabrication. Besides the large thickness, the results show that the films present an etching rate just twice the thermally grown SiO2 rate, therefore lower than the reported values for PECVD SiO2 by other authors (M.S. Haque, H.A. Naseem, W.D. Brown, J. Electrochem. Soc. 142 (1995) 3864). Also etching experiments were performed using reactive ion etching (RIE) equipment on thick silicon oxynitride film grown onto silicon substrates covered by a thick DPECVD SiO2 buffer layer, in order to simulate a waveguide structure (ridge type) fabrication. The results of these tests show that it is possible to define vertical walls in these thick SiOxNy films, which is very important for ridge type waveguides.

Local structure and bonds of amorphous silicon oxynitride thin films

Abstract This work reports on the local structure and bonds of amorphous silicon oxynitride thin films, deposited by plasma enhanced chemical vapor deposition. The dependence of the structural properties and chemical bonds with the films composition was investigated. The used analytical techniques were X-ray absorption at the Si K-edge and Fourier transform infrared spectroscopies. The coordination numbers, interatomic distances and Debye–Waller disorder factors of the Si first shell and, the bond types and the concentration of hydrogen in the films were obtained. All the analyzed data support the formation of a materials homogeneous network with a random distribution of SiO and SiN bonds. The basic structural element of the network is a tetrahedron with a central Si atom connected to N and O, consistent with a random bonding model. As the nitrogen content in the solid phase decreases the SiON 3 , SiO 2 N 2 tetrahedral units gradually change to SiO 4 , keeping the quantity of SiO 3 N tetrahedrons almost unchanged, approximately 40%. The amount of SiO 4 units is 100% for films with high oxygen content. The nitrogen is preferentially bonded to silicon and hydrogen, while the hydrogen is mostly bonded to nitrogen.

Study of nitrogen-rich silicon oxynitride films obtained by PECVD

Abstract The results of the fabrication and characterization of silicon oxynitride films deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique at low temperature and from N 2 , N 2 O and SiH 4 gaseous mixtures are reported herein. It is shown that high nitrogen concentration films with characteristics close to stoichiometric silicon nitride (Si 3 N 4 ) can be obtained. In previous experiments utilizing N 2 O and SiH 4 as precursor gases, it was demonstrated that precise control of the refractive index for silicon dioxide-like oxynitride SiO x N y ( x + y =2) material in the 1.46 (SiO 2 ) to 1.57 range can be attained. In this study, nitrogen gas (N 2 ) was added to the previously studied gaseous mixture. In this way, it was possible to control the refractive index from 1.46 (SiO 2 ) to ∼2 (Si 3 N 4 ) through the appropriate choice of the deposition parameters. The films were characterized by profilometry, ellipsometry, Rutherford backscattering spectroscopy (RBS) and Fourier transform infrared spectroscopy (FTIR).

Silicon rich silicon oxynitride films for photoluminescence applications

Abstract In this work results on the study of the physical and optical properties of silicon rich SiOxNy thin films are presented. The films were deposited by the plasma enhanced chemical vapor deposition technique at low temperature (≈320 °C) using nitrous oxide and silane as precursor gases. The films were thermally annealed at 750 and 1000 °C, at low pressures (10−2 Pa) and in N2 ambient for different annealing times. The samples were characterized through Fourier-transform infrared spectroscopy, Raman scattering and photoluminescence (PL). Raman spectra for all samples show a band approximately at 480 cm−1, related to amorphous silicon. The spectrum for the sample with the highest silicon content, heat treated at 1000 °C also presents a band at 520 cm−1, related to microcrystalline silicon. Finally, PL experiments showed the presence of visible luminescence for the as-deposited samples in the region between 1.5 and 2 eV, attributed to defects (at higher energies) and to the presence of small variable size amorphous silicon clusters embedded in the dielectric matrix. High temperature annealing substantially decreases the PL intensity, result attributed to increased cluster size and crystallization.

Evidence of quantum size effects in a-Si:H/a-SiCx:H superlattices. Observation of negative resistance in double barrier structures

Abstract Variations in the optical gap, in the parallel conductivity and in the conductivity activation energy were observed in A-Si:H/a-SiCx:H multilayers. These results are consistent with the existence of bounded states in the a-Si wells. Also, non linearities in the current vs. voltage curves of multiple a-SiCx:H barriers embedded in the intrinsic layer of n-i-n a-Si:H structures were studied. Several cases of negative resistence, even at room temperature, were found. These results are consistent with a “sequential tunneling” phenomenon, although a bulk effect cannot be ruled out.

Chemical and morphological properties of amorphous silicon oxynitride films deposited by plasma enhanced chemical vapor deposition

Abstract The deposition of amorphous hydrogenated silicon oxynitride thin films, varying the nitrogen and oxygen content in the solid phase, is reported. The films were deposited by plasma enhanced chemical vapor deposition at different nitrous oxide/silane flow ratios, keeping constant the silane flow and the deposition temperature at 320°C. The composition of the thin films was determined by Rutherford backscattering spectroscopy (RBS) and the morphological properties were investigated by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The composition data showed that the oxygen content increases and the nitrogen content decreases, inside the films, as the ratio between the nitrous oxide flow and silane flow goes toward larger values. The oxygen ( x ) plus nitrogen ( y ) content in the chemical formula (a-SiO x N y ) is always close to two, suggesting that these atoms share the same atomic positions around the silicon atoms in a local structure similar to SiO 2 . The SAXS results revealed the presence of scatterers with an average radius 〈 R 〉 that varies from small values, like 10 A, up to 100 A. The TEM results showed the formation of particles with a circular cross-section, composed of Si, N and O spread in a matrix with the same elemental composition. These particles have a radius larger than 50 A.

Mechanical properties of boron nitride thin films obtained by RF-PECVD at low temperatures

This work describes the thermomechanical and structural modifications induced by the deposition conditions, in RF-PECVD-produced boron nitride thin films. The samples were prepared at low temperatures (< 400°C) using B 2 H 6 , N 2 and H 2 as gaseous precursors. The B 2 H 6 /N 2 flow ratio, the B 2 H 6 flow, the N 2 flow, the substrate temperature, the r.f. power and the H 2 dilution were varied. The structure and composition of the films as well as the thermomechanical properties, such as stress and hardness, were studied and correlated with each other and with the deposition parameters. It was found that the films present a mixture of hexagonal and amorphous phases and, under certain conditions, evidences of cubic phase. The fraction of these phases as well as the crystallite size, deduced by Raman spectroscopy, depend strongly on the B 2 H 6 flow, the B 2 H 6 /N 2 flow ratio and on the H 2 dilution. A boron/nitrogen ratio, close to stoichiometry was obtained for all the studied samples.

Crystalline silicon oxycarbide: Is there a native oxide for silicon carbide?

Using variable cell ab initio molecular dynamics, we have investigated hypothetical crystalline phases of silicon oxycarbide (Si1−xCxO2). We found that silicon oxide remains energetically stable with carbon incorporation, and the resulting oxycarbide material has a moderately large bulk modulus. Our results also indicated that there are at least two possible, and competing, crystalline phases for the Si2CO6. We discuss the possibility of those phases forming near the SiC/SiO2 interfaces.

Distribution of Pores in a-Si1−xCx:H Thin Films

The aim of this paper is to compare the optical, compositional and morphological properties of a-Si1 − xCx: H films deposited by plasma enhanced chemical vapour deposition (PECVD) using different mixtures of silane (SiH4) and methane (CH4) under minimum attainable deposition pressure. Films deposited at lower silane flow present a higher carbon content and larger optical gap. The morphology of the films was investigated by small-angle X-ray scattering (SAXS) using two different light sources: (i) conventional tube and (ii) synchrotron radiation. The analysis of the data from both experiments was performed in order to determine a size distribution for spherical pores. The results obtained with both light sources are consistent: the increase in the CH4 concentration implies broader size distribution functions, with an increase of the pore size up to 10 nm. Larger pores are found in films deposited at lower silane flow. For all samples, the density of the smaller pores dominates the size distribution. The relative microvoid density is not proportional to the carbon concentration but presents a maximum for the low carbon content films.