Denis Stryahilev

Amorphous silicon nitride deposited at 120 °C for organic light emitting display-thin film transistor arrays on plastic substrates

Nitrogen-rich amorphous silicon nitride (a-SiNx:H) films with [N]/[Si] ratios ranging from 1.4 to 1.7 were deposited by a 13.56 MHz plasma-enhanced chemical vapor deposition method at a temperature of 120 °C. The films’ composition, dielectric constant, electrical resistivity, and breakdown voltage were evaluated. The electrical properties of a-SiNx:H films with a [N]/[Si] ratio of more than 1.6 are superior to their lower N-content counterparts. Amorphous silicon thin film transistors (TFTs) that incorporate a-SiNx:H dielectrics were fabricated on glass and plastic substrates at a maximum processing temperature of 120 °C. The TFTs exhibit effective field effect mobility of 0.5–0.8 cm2/V s, an ON current of ∼10−5 A, an ON/OFF ratio of more than 106 and a subthreshold slope of 0.5 V/dec. The performance of the transistors seems to be compatible with application of them in active–matrix organic light emitting displays.

Dielectric performance of low temperature silicon nitride films in a-Si:H TFTs

We report on properties of amorphous silicon nitride (a-SiNx:H) films deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) at 120 C and their performance as a gate dielectric in amorphous hydrogenated silicon (a-Si:H) thin film transistors (TFTs). It was found that the films with [N]/[Si] ratio higher and lower than about 1.5 can be clearly distinguished due to different physical properties, structure of electronic defects and better electrical quality of more N-rich materials. a-Si:H TFTs incorporating N-rich a-SiNx:H dielectric layers exhibited field effect mobility of 0.9–1 cm 2 =V s, threshold voltage of 4 V, subthreshold slope of 0.5 V/dec, OFF-current � 1 pA and ON/OF ratio more than 10 6 . 2002 Elsevier Science B.V. All rights reserved.

Optical properties, statistics of bond angle deformations and density of states in Si-rich a-SiNx:H alloys

Samples of a-SiNx:H (0.0≤x≤0.72) films were obtained by glow discharge decomposition of SiH4+H2+NH3 mixture. The film composition and chemical bonding were determined from infrared absorption data. The bandgap and optical constants (refractive index in the limit of weak absorption and absorption spectra for photon energies higher than bandgap) were obtained from optical transmittance spectra. The density of valence band tail states was deduced from subgap absorption spectra measured by the constant photocurrent method. Raman scattering was measured to estimate the average Si–Si bond deformation potential in a-SiNx:H. Statistical consideration enabled us to compare this value with characteristic energy of tail states distribution. It was established that bond angle disorder in an inhomogeneous structure determined the density of valence band tail states while the approximation of homogeneous alloy with random bonding was not correct.

Investigation of inhomogeneities in a-SiNr: H alloys by infrared spectroscopy

Abstract Frequencies of subbands in the SiH stretching absorption band of a-SiNr: H alloys were calculated by means a chemical induction model for a wide range of nitrogen and hydrogen concentrations ([N]/[Si] from 0 to 1.33 and [H]/[Si] from 0 to 0.6). Films of a-SiNr: H with 0 ⩽ r ⩽ 0.40 were obtained by glow discharge decomposition of SiH4 + NH3 + H2 mixtures at temperature of 220°C, pressure of 0.37 Torr and power density of 0.3 W/cm2. Decomposition of SiH stretching absorption band into four peaks corresponding to HSi3−nNn structures with n = 0, 1, 2, 3 and comparison of peak frequencies with values calculated with the chemical induction model allow one to detect the local environment of SiH bonds.

Amorphous Hydrogenated Silicon Films for Solar Cell Application Obtained with 55 kHz Plasma Enhanced Chemical Vapor Deposition

In this work, a-Si:H films with good electronic properties in spite of an inhomogeneous structure were prepared by the 55 kHz plasma enhanced chemical vapor high-rate deposition technique. The structural analysis using infrared spectroscopy and atomic force microscopy has shown that these films possess two dominant types of microstructural inhomogeneities, which differ by size. To analyze the influence of a 55 kHz plasma on the properties of intrinsic a-Si:H film, the density of states in the a-Si:H mobility gap was estimated by modeling of the temperature dependence of the photoconductivity and from electron paramagnetic resonance measurements. Investigated capacitance-voltage characteristics showed that a-Si:H/c-Si heterostructures have low interface density of states and can be considered as an ideal abrupt heterojunction.

EPR spectra of amorphous silicon nitride films grown by low-temperature PECVD

Electron paramagnetic resonance (EPR) study of silicon nitride films prepared at 120 °C by plasma enhanced chemical vapor deposition (PECVD) is reported. The complex EPR line with contribution from surface defects and hyperfine structure (HFS) is observed in N-rich samples. The HFS spectrum with peak-to-peak separation ∼8 G cannot be attributed to known defect models. However, it is well described by spin-Hamiltonian of unpaired electron interacting with three equivalent nuclei having nuclear spin of 1. The structural identification of the defect responsible for the HFS is discussed.

Growth process, structure and thermal stability of a-Si1−xNx:H films

The influence of nitrogen content on structure, properties, and thermal stability of a-Si:H and a-Si 1− x N x :H films have been investigated. Films were prepared by a rf (13.56 MHz) glow discharge decomposition of gas mixtures with 10% SiH 4 + 90% H 2 and various amounts of NH 3 . The infrared, Raman scattering, and electron paramagnetic resonance spectra, dark electrical conductivity and optical absorption of the films have been measured. The structural inhomogeneity was determined by infrared transmission line at 2020 cm −1 . It is found that material with low nitrogen content ( x = 0.06) possesses higher conductivity and better thermal stability in comparison with a-Si:H. In addition, the structure of such material is shown to be more homogeneous. It is shown that a small amount of ammonia in gaseous phase during film deposition prevents the formation of structural inhomogeneities.

The properties of powder particles incorporated in a-Si:H films

We have found powder particles incorporated in device quality a-Si:H films. Investigations carried out by atomic force microscopy (AFM) have shown that these particles are of ∼200 nm in diameter and are more fragile than the rest of the film. SiH-stretching absorption bands in a-Si:H films consisted of two bands with frequencies of about 2020 and ∼2100 cm−1. Both peaks shift to higher frequencies with the increase of a volume fraction of powder particles. Taking into account the predictions of the chemical induction model, we conclude that this shift is due to high local hydrogen content in the powder particles which also explains their low hardness. It was found that the density of valence band tail states increases with the enlargement of particle surface areas. At the same time, the Urbach energy and the optical gap remain in the ranges of 50 to 70 meV and of 1.7 to 1.8 eV, respectively, indicating the good electronic properties of investigated films.

The Valence Band Tail Density of States and Bond Angle Distortion in a-SiN x : H Alloys

Films of a-SiN x :H with x = 0.0..0.62 were deposited by glow discharge decomposition of (10% SiH 4 +90%/H 2 )+ NH 3 mixture. The chemical bonding and composition of films were investigated with using of infrared spectroscopy. The deformation energy per Si atom connected to bond bending V kθ was calculated from data of Raman scattering. Characteristic energy of valence band tail (VBT) states distribution, E 0v , were determined fromrsubgap absorption spectra. The dependencies of E 0v and V kθ on film composition, x, were considered in order to estimate the influence of the bond angle disorder on the distribution of VBT states. The essential difference in behavior of E 0v and V kθ dependencies on x was found for Si-rich (x kθ value increase with x, the E 0v parameter stays almost constant; while at x>0. 15 the E 0v increase with x as well as the VKO. It means that bond angle disorder in the bulk of the material contributes to VBT characteristic energy, but it is not the only source. Another factors as it was shown may be connected with valence states of Si atoms, localized near inner boundaries.