Structure and properties of hydrogenated amorphous silicon carbide thin films deposited by PECVD

a-Si_1?xC_x:H films are deposited by RF plasma enhanced chemical vapor deposition (PECVD) at different RF powers with hydrogen-diluted silane and methane mixture as reactive gases. The structure and properties of the thin films are measured by infrared spectroscope (IR), Raman scattering spectroscope and ultra violet–visible transmission spectroscope (UV–vis), respectively. Results show that the optical band gap of the a-Si_1?xC_x:H thin films increases with increasing Si–C bond fraction. It can be easily controlled through controlling Si–C bond formed by modulating deposition power. At low deposition power, the bond configuration of the a-Si_1?xC_x:H thin film is more disordered owing to the distinct different bond lengths and bond strengths between Si and C atoms. At a too high deposition power, it becomes still high disordered due to dangling bonds appearing in the a-Si_1?xC_x:H thin film. The low disordered bond configuration appears in the thin film deposited with moderate deposition power density of about 2.5 W/cm².     

Study of in-gap defects in intrinsic and B-doped a-Si1−xCx:H by photo-induced optical absorption and photoluminescence

The infrared (IR) absorption dependence on visible light illumination has been measured in doped and undoped hydrogenated amorphous silicon carbide (a-Si_1−xC_x:H) films grown by plasma enhanced chemical vapour deposition. The measurements were made by a highly sensitive technique which exploits properly designed a-Si_1−xC_x:H/ZnO test waveguides for lengthening the interaction region between the IR and visible (VIS) radiations in the material. Experimental data show that boron doping strongly enhances the VIS light induced variation of the IR absorption, whereas the increase in carbon content has a quenching effect on the phenomenon. The a-SiC:H films have been also characterized by photoluminescence measurements. The spectra are dominated by a photoluminescence band, ranging between 1.4 eV and 1.9 eV. This band is enhanced by the increase in carbon content, while is strongly quenched with increasing B-doping level. On the basis of these results, a correlation is found between the measurements of optical absorption, photo-induced absorption and photoluminescence. The type and the density of defects induced in the films by the different growth conditions have been recognized as the origin of the different behaviors observed.     

Local structural,magnetic, and optical properties of Zn1−xFexO thin films

High quality Zn_1−xFe_xO thin films were deposited on α-sapphire$\alpha \text{-sapphire}$ substrates by RF magnetron sputtering. X-ray absorption fine structure measurements showed that the chemical valence of Fe ions in the films was a mixture of 2+ and 3+ states, and Fe ions substituted mainly for the Zn sites in the films. DC-magnetization measurements revealed ferromagnetic properties from 5 to 300 K. The photoluminescence measurements at 15 K showed a sharp main transition peak at 3.35 eV along with a broad impurity peak at 2.45 eV. The structural and magnetization analyses of the Zn_1−xFe_xO films strongly suggested that the ferromagnetism was the intrinsic properties of the films.     

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500 °C by reactive rf sputtering of GaAs target in argon-nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAs_xN_1−x (x ≈ 0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.     

Molecular beam epitaxy of crystalline and amorphous GaN layers with high As content

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN_1−xAs_x alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As₂ flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from ∼3.4 to ∼1.35 eV with increasing As content. The energy gap reaches its minimum of ∼1.35 eV at x∼0.6–0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN_1−xAs_x alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN_1−xAs_x alloys have short-range ordering that resembles random crystalline GaN_1−xAs_x alloys.     

Raman scattering in amorphous films of In1−xSex alloys

Raman scattering (RS) in amorphous films of In_1−xSe_x with 0.67±x?0.38 has been studied in backscattering geometry with the use of a microscope. Recorded RS spectra are revealing a mixed vibrational density-of-states and molecular character. The spectra spread from the Rayleigh line up to 200-250 cm⁻¹. The bands superimposed on the continua are related to zone center modes of the relevant crystal counterpart, Se-Se or In-In vibrations. The RS spectra suggest the structure of the In_1−xSe_x alloys to be the continuous random network built up of In centered tetrahedral clusters with In and Se atoms at the corners. The structure of the Se-rich alloys is similar to 4-2 networks with dominant InSe_4/2 clusters and two-fold coordination of Se bridging atoms. That of the In-rich alloys is expected to resemble 4-3 network with rather strong involvement of In atoms at corner of the In-centered tetrahedral clusters and Se atoms being linked to three In ones.     

Electrical characterisation of SrTiO3/Si interfaces

Deposition of SrTiO₃ (STO) thin films by ultra-high vacuum rf magnetron sputtering was performed in order to produce high-quality STO/p-Si (1 0 0) interfaces and STO insulator layers with high dielectric constants. The deposition temperatures were in the range from room temperature to 550 °C. Capacitance-voltage (C-V) and conductance-frequency measurements showed that the dielectric constant of the films ranges from 55 to 120. C-V measurements on Al/STO/p-Si structures clearly revealed the creation of metal-insulator-semiconductor diodes. The interface state densities (D_it) at the STO/p-Si interfaces were obtained from admittance spectroscopy measurements. The samples deposited at lower temperatures revealed values of D_it between 2×10¹¹ and 3.5×10¹² eV⁻¹ cm⁻² while the higher temperature deposited samples had a higher D_it ranging between 1×10¹¹ and 1×10¹³ eV⁻¹ cm⁻². The above results were also well correlated to X-ray diffraction measurements, Rutherford backscattering spectroscopy, and spectroscopic ellipsometry.     

Compositional dependence of the optical properties of amorphous Sb2Se3−xSx thin films

Thin films of Sb₂Se_3−xS_x solid solutions (x = 0, 1, 2, and 3) were deposited by thermal evaporation of presynthesized materials on glass substrates held at room temperature. The films compositions were confirmed by using energy dispersive analysis of X-rays (EDAX). X-ray diffraction studies revealed that all the as-deposited films as well as those annealed at T_a < 423 K have amorphous phase. The optical constants (n, k) and the thickness (t) of the films were determined from optical transmittance data, in the spectral range 500-2500 nm, using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. An analysis of the optical absorption spectra revealed an Urbach’s tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy increases quadratically as the S content increases.     

Role of Sb incorporation on the electrical and photoelectrical properties of Se-In glassy alloy

Amorphous thin films of Se₉₀In_10-xSb_x (x = 0, 2, 4 and 6 at.%) have been prepared by vacuum evaporation technique. The dark and photo-conductivity measurements were carried out in all the thin films under study. Temperature dependence of conductivity in dark as well as in presence of light show that conduction is through a thermally activated process in both the cases. A discontinuity in various photoelectrical parameters is observed at 2 at.% of Sb concentration which could be correlated with average coordination number at this particular composition. The obtained results are explained on the basis of chemically ordered network model and the topological model.     

SiOx films prepared using RF magnetron sputtering with a SiO target

In this study, SiO_x thin films were prepared using reactive radio frequency magnetron sputtering at room temperature with a SiO sintered target. The obtained SiO_x films were identified using X-ray diffraction, transmission electron microscopy, infrared absorption spectroscopy, X-ray photoelectron spectroscopy and ultraviolet-visible transmittance measurement. The x in SiO_x film was controlled from 0.98 to 1.70 by changing the oxygen flow ratio at deposition. Increasing the oxygen flow ratio increased the optical gap of the SiO_x films from 3.7 to greater than 6 eV.     

Structure of partially reduced xPbO (1 − x)SiO2 glasses: combined EXAFS and MD study

We have studied the structure of partially reduced lead-silicate glasses using combined EXAFS (extended X-ray absorption fine structure) and MD (molecular dynamics) methods. The analysis was performed for glasses of x[(1 − p)Pb pPbO] (1 − x)SiO₂ composition, x = 0.3, 0.5, 0.7, where parameter (1 − p) describes the degree of reduction, i.e. the content of the granular metallic phase, appearing as the result of the reduction process (e.g. annealing in hydrogen atmosphere). In the EXAFS experiment (1 − p) was expressed via the time of reduction realized at 400 °C (1.5 h, 24 h, 70 h), whereas in the MD simulations it was determined precisely by using proper numbers of particles (corresponding to (1 − p) = 0.0, 0.25, 0.5, 0.75 and 1.0). In the paper we describe in detail the local structure around lead atoms and its changes in the function of glass composition and reduction degree. The tendency for agglomeration of Pb⁰ into clusters, the formation of the granular metallic phase, and continuity of silica and lead oxide subnetworks are discussed. A good agreement between EXAFS-extractcd and MD-extracted parameters of the short-range structure encouraged us to preform a medium-range order analysis, based on the MD simulations only. Moreover, combining the EXAFS and MD methods we could correlate the reduction time (technological parameter) with the degree of reduction (1 − p) and the actual state of the granular structure. The latter relation may be useful for controlled production of reduced glasses of pre-requcstcd physical properties.     

PECVD a-Sic:H Young’s modulus obtained by MEMS resonant frequency

In this paper we study the Young’s modulus of PECVD obtained silicon rich (x > 0.5) a-Si_xC_1?x:H thin films through the study of the resonance frequency of free standing cantilevers. These structures are fabricated based on front side bulk micromachining of Si substrate and actuated thermally. In this approach, an alternating electric current passes through a photolithography patterned metallic film deposited on the cantilever, heating the structure by Joule effect and inducing vibrations on the cantilever. This method of actuation is independent of the separation between the structure and substrate, which is its main advantage, because it allows the actuation of cantilevers that are bent upwards or downwards, which is an aspect of particular importance in the characterization of PECVD materials for MEMS applications. The work is focused on low stress silicon rich amorphous hydrogenated silicon carbide films obtained by PECVD at low temperatures (320 °C). The measurements were carried out in groups of cantilevers with different length (between 550 and 200 μm) and utilizing a-SiC:H films obtained with three different compositions. The results show that the films exhibit modulus of elasticity in the range of 20–35 GPa, low residual stress (～90 GPa) and maintain excellent chemical inertness in KOH and HF solutions.     

Effects of high-temperature annealing on electron spin resonance in SiOx films prepared by R. F. sputtering system

Electron spin resonance (ESR) spectra are investigated in order to analyze paramagnetic defects in amorphous SiO_x films with 0.8 ≤ x ≤ 1.87 prepared by a co-sputtering of Si-wafer chips and a SiO₂ disk target. Effects of the thermal annealing at 900 °C and 1100 °C on the ESR spectra are also investigated. Four types of silicon dangling bond centers with forms of •Si ≡ Si_3 − nO_n (n = 0, 1, 2 or 3) are assumed in order to simulate the ESR spectra. The random bonding model appears to describe the network structure of the films with x ~ 2, that is, near the stochiometric composition of SiO₂. It is suggested that the structural fluctuation around silicon dangling bonds is larger in the sputtered SiO_x films used in the present work in comparison with those prepared by plasma-enhanced chemical vapor deposition.     

Bismuth germanate and bismuth silicate glasses in cryogenic detectors

We present and discuss the results of measurements of conductivity and secondary electron emission coefficient of Bi_xGe_1−xO_2−0.5x (where x=0.13, 0.23, 0.33, 0.47) and Bi_xSi_1−xO_2−0.5x (where x=0.47, 0.57, 0.67) glasses reduced in hydrogen. The surface conductivity of reduced glasses strongly depends on reduction temperature, reduction time and bismuth content. The temperature dependence of the surface conductivity in a high temperature regime is well described by σ∼exp[−(T₀/T)ⁿ] law where n=1/4. The secondary electron emission coefficient of reduced glasses is practically independent of the degree of reduction but increases when binary glasses are modified by alkali ions. The research results confirmed that bismuth silicate and bismuth germanate glasses modified by alkali ions may be good materials for detectors working in cryogenic temperature.     

Microstructure and percolation threshold characteristics of reactive sputtered Au-TiO2 granular composite films

The Au/TiO₂ composite films were prepared by using reactive co-sputtering technique. The size and shape of the embedded Au particles and the absorption spectra of the composite films were investigated by using SEM, XRD, and UV-VIS-NIR spectrophotometer, respectively. The average size of Au particles and the electrical conductivity decrease as the sputtering pressure increases. The normalized conductivity of the films deposited at five different pressures with the Au concentration in the range of 0.15-0.91 were measured. The percolation threshold increases from 0.21 to 0.90 as the sputtering pressure increases from 2 × 10⁻² Torr to 9.5 × 10⁻² Torr.     

Insulator-like electrical transport in structurally ordered Al65Cu20+xRu15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5) icosahedral quasicrystals

Low-temperature resistivities, in zero-field and 8 T field, and magnetoresistance have been measured down to 1.4-300 K for stable icosahedral quasicrystals Al₆₅Cu_20+xRu_15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect (τ_so ∼ 10⁻¹² s). But the sample with x = −0.5 shows anomalous magnetoresistance and the anti weak-localization effect breaks down (τ_so to be 10⁻¹⁵ s). The in-field σ-T between 5 K and 20 K, for x = 1.0, 0.5, 0.0 and −0.5 samples, and between 1.4 K and 40 K for x = 1.5 sample, follow a power-law behavior with an exponent of 0.5 and above ∼30 K the exponent ranges from 1.17 to 1.58. The observed power-laws basically characterize the presence of critical regime of the metal-insulator (MI) transition, dominated by electron-electron and electron-phonon inelastic scattering events respectively. In samples with x = 1.0, 0.5, 0.0 and −0.5 the in field σ-T has been found to follow ln σ-vs-T^1/4 below 5 K, which indicates the presence of variable range hopping. The observed transport features indicate the occurrence of proximity of metal-insulator transition in these Al-Cu-Ru quasicrystal samples.     

Structure and dynamics of liquid Al1−xSix alloys by ab initio molecular dynamics simulations

First-principles molecular dynamics (MD) simulations are performed to study the structure and dynamics of liquid Al_1−xSi_x (x = 0.0, 0.12, 0.2, 0.4, 0.6, 0.8) at the temperature of 1573 K. The composition dependence of static structure factors, pair correlation functions, and diffusion constants are investigated. We found that the structure of the liquid Al_1−xSi_x alloys is strongly dependent on the composition. From our simulation and analysis, we can see that although liquid Al_1−xSi_x is metallic, there are some degrees of covalent tetrahedral short-range order in the liquid. The degree of tetrahedral short-range order increases linearly as the Si concentration in the liquid increased. The diffusion coefficients of both Al and Si atoms in liquid Al_1−xSi_x alloys at 1573 K are not very sensitive to the composition.     

Glassy state and structure of Sn–Sb–Se chalcogenide alloy

The effect of Sn addition on the glass transition and structure of c-Sb₂₀Se₈₀ chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe₄] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on Sn_xSb₂₀Se_80−x (8 ⩽ x ⩽ 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 ⩽ x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb₂Se₃ and SnSe₂ phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model.     

XAFS investigations of nitrided NbN–SiO2 sol–gel derived films

This work presents the results of the structural analysis of xNbN–(100-x)SiO₂ (x = 100, 80, 60 mol%) thin films by X-ray absorption spectroscopy (XAS). To prepare the films, thermal nitridation of sol–gel derived coatings have been performed. The resulting films have a granular structure with NbN grains distributed in the SiO₂ matrix. The size of the grains depends on the NbN/SiO₂ molar ratio. A detailed X-ray absorption fine structure (XAFS) data analysis shows that in all the samples both nitrogen and oxygen atoms are present as nearest neighbours of Nb. The intra-granular phase is an ordered NbN phase, whereas the shells around the grains are formed mainly by an oxide phase and, possibly, by other niobium nitride phases (probably with low nitrogen content). Two possible origins of the inter-granular oxide phase were considered: incomplete nitridation of Nb₂O₅ and addition of SiO₂. Both of them are connected with the sample preparation method. The obtained XAS results allowed us to correlate the thickness and stoichiometry of the films under study with the electronic structure of the Nb ions and with the local geometric structure in their environment.     

Sol-gel chemistry approach in the preparation of precursors for the substituted superconducting oxides

Sol-gel processing of ceramic materials for advanced applications involves several steps starting from precursor synthesis and ending up with multicomponent metal oxides. A simple sol-gel synthesis technique has been refined to prepare the precursors for the superconducting (Y_1−xSc_x)Ba₂Cu₄O₈ and (Y_1−xGa_x)Ba₂Cu₄O₈ compounds. The amorphous gel powders were characterized by powder X-ray diffraction, infrared spectroscopy, thermal analysis and elemental analysis. A systematic characterization of precursor gels led us to predict the approximate composition and the chemical reactions involved during gelation. The stability and high level of homogeneity obtained for the gels make them suitable as processable precursors to substituted (Y_1−xSc_x)Ba₂Cu₄O₈ and (Y_1−xGa_x)Ba₂Cu₄O₈ superconducting compounds.