Similar admittance behavior of amorphous silicon carbide and nitride dielectrics within the MIS structure

PECVD grown a-SiNx:H and a-SiCx:H films were investigated as dielectric films in the form of metal/insulator/p-silicon (MIS) structures. AC admittance of MIS structures was measured as a function of dc gate bias voltages and frequencies (1-1000 kHz) of the superimposed ac bias voltage (10 mV). For each applied bias voltage (from accumulating to inverting bias regimes), temperature (T) dependence of both capacitance (C) and conductance (G/ω) were measured to investigate majority/minority carrier behavior under various frequencies ω (kHz-MHz) as parameters. C and G/ω-T-ω measurements reveal that observed pairs of capacitance steps and conductance peaks are related to traps lying on the same energy value, residing in the insulator and at the interface of insulator/semiconductor structure and differing only through capture cross-sections. On the other hand, surface band bending (ψ_s) of silicon and activation energy (E_A) deduced from the Arrhenius plot of the frequency vs. reciprocal temperature as a function of gate bias (V_G) seem linearly dependent, implying that E_A reflects the ψ_s variation.     

Low turn-on and high efficient oxidized amorphous silicon nitride light-emitting devices induced by high density amorphous silicon nanoparticles

The role of amorphous silicon nanoparticles (a-Si NPs) in electroluminescent characteristics of oxidized amorphous silicon nitride (a-SiN_x:O) light-emitting devices (LEDs) has been studied. A-Si NPs with a high density of 1 × 10¹² cm^− 2 are formed in the a-SiN_x:O films after rapid thermal annealing at 900 °C for 40 s. A notably enhanced electroluminescence (EL) is obtained from the a-Si-in-SiN_x:O devices and the EL peak position can be tuned from red to green-yellow by controlling the forward voltage. Compared to EL of the a-SiN_x:O device, the turn-on voltage can be reduced to 3 V and the EL power conversion efficiency can be almost six times higher. The improved performance of the LEDs is ascribed to the effective carrier injection due to introduction of high density a-Si NPs.     

Low refraction properties of F-doped SiOC:H thin films prepared by PECVD

Low refractive index materials which F-doped SiOC:H films were deposited on Si wafer and glass substrate by low temperature plasma enhanced chemical vapor deposition (PECVD) method as a function of rf powers, substrate temperatures, gas flow ratios (SiH₄, CF₄ and N₂O). The refractive index of the F-doped SiOC:H film continuously decreased with increasing deposition temperature and rf power. As the N₂O gas flow rate decreases, the refractive index of the deposited films decreased down to 1.378, reaching a minimum value at an rf power of 180 W and 100 °C without flowing N₂O gas. The fluorine content of F-doped SiOC:H film increased from 1.9 at.% to 2.4 at.% as the rf power was increased from 60 W to 180 W, which is consistent with the decreasing trend of refractive index. The rms (root-mean-square) surface roughness significantly decreased to 0.6 nm with the optimized process condition without flowing N₂O gas.     

Refractive index and etched structure of silicon nitride waveguides fabricated by PECVD

Silicon nitride (SiN_X) thin films were deposited by means of an RF plasma enhanced chemical vapor deposition (PECVD) reactor using SiH₄ and N₂ gases. The refractive index of the SiN thin films increased from 1.5652 to 2.7621 as the SiH₄/N₂ flow ratio was increased from 0.16 to 1.66, since the amount of Si in the film increased, while that of N decreased, as the SiH₄/N₂ flow ratio was increased. The core shape became circular after annealing at 1200 °C. This change is related to a decrease in the viscosity with increasing annealing temperature. This decreased viscosity causes condensation of the core layer due to surface tension, which leads to the change in shape from rectangular to circular. The thickness, refractive index and shape of the films were characterized by ellipsometry, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).     

Bonding structure and optical properties of a-CNx:H films deposited in CH4-NH3 system

Hydrogenated amorphous carbon nitride (a-CN_x:H) films were deposited by plasma enhanced chemical vapor deposition (PECVD) in CH₄-NH₃ system. The chemical composition and bonding configuration were investigated by XPS and FTIR. The results indicated that both sp²CN and sp³CN bonds generally increased with the increase of the nitrogen concentration, and the N atoms bonded to C atoms through CN, CN and CN bonds. Remarkably, for FTIR spectra, two peaks (2125 and 2200 cm⁻¹) were obviously observed, corresponding to CN bond which was found to predominantly exist in the isonitrile structure. As more nitrogen atoms were incorporated, the optical band gap was found to vary from 1.8 to 2.5 eV. Finally, the conduction mechanisms were discussed at low and high temperature, respectively.     

Deformation behavior of complex carbon nitride and metal nitride based bi-layer coatings

A bi-layer coating consisting of a TiAlCrN inner layer and complex carbon nitride (CN_x) plus CrCN outer layer was deposited on to a high speed steel (M2) substrate via physical vapour deposition (PVD). Detailed microstructural analysis of the coating has been performed via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XPS analysis indicated that the outer layer contained both C-C bonds, associated with an amorphous phase, and the presence of CrCN, which was confirmed by TEM analysis. Localized deformation of the coating was performed using nanoindentation with a spherical indenter. Force-displacement curves of the indentation tests exhibited a number of ‘pop-ins’ during the loading cycle, indicative of cracking and other deformation events. Cross-sectional analysis of the indents revealed extensive cracking in the TiAlCrN layer and shear steps at the steel-nitride interface, consistent with the events observed in the force-displacement curves. On the other hand minimal cracking was observed in the CN_x + CrCN layer. It is believed that the relatively ductile outer layer inhibits the propagation of cracks from the inner, brittle layer.     

Highly reflective nc-Si:H/a-CNx:H multilayer films prepared by r.f. PECVD technique

Multilayer thin films consisting of a-CN_x:H/nc-Si:H layers prepared by radio-frequency plasma enhanced chemical vapour (r.f. PECVD) deposition technique were studied. High optical reflectivity at a specific wavelength is one of major concern for its application. By using this technique, a-CN_x:H/nc-Si:H multilayered thin films (3-11 periods) were deposited on substrates of p-type (111) crystal silicon and quartz. These films were characterized using ultra-violet-visible-near infrared (UV-Vis-NIR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, field effect scanning electron microscopy (FESEM) and AUGER electron spectroscopy (AES). The multilayered films show high reflectivity and wide stop band width at a wavelength of approximately 650 ± 60 nm. The FTIR spectrum of this multilayered structure showed the formation of Si-H and Si-H₂ bonds in the nc-Si:H layer and CC and N-H bonds in a-CN_x:H layer. SEM image and AES reveal distinct formation of a-CN_x:H and nc-Si:H layers in the cross section image with a decrease in interlayer cross contamination with increasing number of periods.     

Investigation of optical and compositional properties of thin SiNx:H films with an enhanced growth rate by high frequency PECVD method

Hydrogenated thin silicon nitride (SiN_x:H) films were deposited by high frequency plasma enhanced chemical vapor deposition techniques at various NH₃ and SiH₄ gas flow ratios [R = NH₃/(SiH₄ + NH₃)], where the flow rate of NH₃ was varied by keeping the constant flow (150 sccm) of SiH₄. The deposition rate of the films was found to be 7.1, 7.3, 9 and 11 Å/s for the variation of R as 0.5, 0.67, 0.75 and 0.83, respectively. The films were optically and compositionally characterized by reflectance, photoluminescence, infrared absorption and X-ray photoelectron spectroscopy. The films were amorphous in nature and the refractive indices of the films were varied between 2.46 and 1.90 by changing the gas flow ratio during the deposition. The PL peak energy was increased and the linear band tails become broad with the increase in R. The incorporation of nitrogen takes place with the increase in R.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

Abstract

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1?xTe_x) and Fe(Se_1?xS_x). In the case of Fe(Se_1?xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1?xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1?xCo_x)AsF. The T_c of Fe(Se_1?xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

Textured silicon nitride: processing and anisotropic properties

Abstract

Textured silicon nitride (Si₃N₄) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si₃N₄, with emphasis on the anisotropic and abnormal grain growth of β-Si₃N₄, texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si₃N₄ seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si₃N₄ single crystals are presented. Various anisotropic properties of textured Si₃ N₄ and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si₃N₄ with a unique microstructure composed of oriented elongated β-Si₃N₄ and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si₃N₄ and β-Sialon.     

Study of electrical properties of CIGS thin films prepared by multistage processes

In this work, the dispersion mechanisms affecting the electric transport in CuIn_1−xGa_xSe₂ (CIGS) thin films grown by a chemical reaction of the precursor species, which are evaporated sequentially in two and three-stage processes are analyzed. It was found, through conductivity and Hall coefficient measurements carried out as functions of temperature, that the electrical conductivity of the CuIn_1−xGa_xSe₂ films is affected by the transport of free carriers in extended states of the conduction band as well as for variable range hopping transport mechanisms, each one predominating in a different temperature range.     

SiOx-doped DLC films: Charge transport, dielectric properties and structure

In the present study, SiO_x-doped diamond-like carbon (DLC) films were synthesized by ion beam deposition on different substrates. Electrical properties, morphology and structure of the DLC films were investigated. Poole-Frenkel emission was the main carrier transport mechanism in all investigated metal-SiO_x-doped DLC-metal samples. Dielectric properties of the samples were dependent on both the bottom and top electrode metal. The trans-polyacetylene chain vibrations detected from the Raman spectra have been observed for all the SiO_x-doped DLC films. Different dielectric properties of the film deposited onto the different metal interlayers were explained both by different roughness of the metal films and by different structure of the ion beam-synthesized SiO_x-doped DLC films.     

Optical properties of ion-implanted silicon and separation by implantation of oxygen silicon-on-insulator substrates in the infrared: Study of B and P2 implantation doping

The optical properties of ion implanted silicon and silicon-on-insulator substrates have been studied by Fourier transform infrared spectroscopy. The influence of the implanted-ion mass in changing the refractive index of a silicon target has been examined by implanting 80 keV ¹¹B⁺ and ⁶²P₂⁺ ions respectively. A refractive index rise not exceeding 2% and total amorphization were observed respectively in the vicinity of the Si surface after boron and phosphorous implantations. Free carrier profiles generated after thermal annealing at 950 °C/30 min and 1150 °C/120 min were modeled by Pearson and half-Gaussian distributions respectively. The phosphorous implantation was also performed in silicon-on-insulator substrates, yielding after annealing nearly homogeneous free-carrier profiles in the top-Si layer and optical mobility values comparable to those of bulk-Si.     

Influence of incidence angle and distance on the structure of aluminium nitride films prepared by reactive magnetron sputtering

Aluminum nitride (AlN) films were reactively deposited on (100) oriented silicon substrates by reactive radio frequency (RF) magnetron sputtering for different incidence angles and distances between substrate and target.X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to consider the influence of process parameters such as reactive gas flow rate, grazing incidence angle (α), and distance (d) between substrate and target surface on the property of AlN films. XRD results showed that AlN film prepared at a constant distance (d) of 3 cm and an incidence angle of 45° revealed a mixture of AlN (002), (100), and (101) planes, while the film prepared at α = 0° revealed a strong AlN (002) orientation which has a perpendicular growth direction to the substrate surface. AFM results showed that AlN film prepared at α = 0° exhibited more flat surface morphology than that of film prepared at α = 45°.     

Structural and electronic properties of CdTe1-xSex films and their application in solar cells

The performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTe_1-xSe_x films are examined for different Se concentrations. The band gap of this compound changes with composition with a minimum of 1.40 eV for x = 0.3. We show that with increasing x, the lattice constant of CdTe_1-xSe_x decreases, which can influence the solubility of dopants. We find that alloying CdTe with Se changes the effect of Cu doping on the p-type conductivity in CdTe_1-xSe_x, reducing the achievable charge carrier concentration with increasing x. Using a front surface CdTe_1-xSe_x layer, compositional, structural and electronic grading is introduced to solar cells. The efficiency is increased, mostly due to an increase in the short-circuit current density caused by a combination of lower band gap and a better interface between the absorber and window layer, despite a loss in the open-circuit voltage caused by the lower band gap and reduced charge carrier concentration.     

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si₃N₄ ceramics were prepared by freeze casting using liquid N₂ as refrigerant. The pore structure, porosity, α → β-Si₃N₄ transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α → β-Si₃N₄ phase transformation, but was beneficial to the mechanical properties of the obtained porous Si₃N₄ due to its unique pore structure.     

Thermal stability of sputtered Mo/polyimide films and formation of MoSe2 and MoS2 layers for application in flexible Cu(In,Ga)(Se,S)2 based solar cells

Molybdenum (Mo) films with a thickness of about 800 nm were room temperature sputtered onto flexible polymeric substrates. Upilex® films were chosen as substrates on the basis of their high thermal endurance and reduced coefficient of thermal expansion. Thermal stability of Mo films has been proved by heat treatment of the Mo/Upilex® structures at a temperature comparable to that used in the preparation of the Cu(In,Ga)(Se,S)₂ absorber layer. A combination of high optical reflectance (maximum values of 75-80%), low electrical resistivity (about 30 μΩ cm) and a smooth surface free of cracks for heated films highlights their good thermal stability. The formation of MoSe₂ and MoS₂ layers, after selenization/sulfurization of the Mo/Upilex® structures, has been further investigated in view of their application as back contact layers in flexible CIGS based solar cells.     

Gold catalysts for the abatement of environmentally harmful materials: Modeling the structure dependency

FeO_x, TiO₂ and CeO_x layers were deposited by pulsed laser deposition (PLD) technique onto Au films or Au nanoparticles supported on SiO₂/Si(100). The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and their reactivity was studied in catalytic CO oxidation. Comparison was made with reference samples of FeO_x/SiO₂/Si(100), TiO₂/SiO₂/Si(100), CeO_x/SiO₂/Si(100) and Au/SiO₂/Si(100) layers. The catalytic activity of the metal-oxide/Au/SiO₂/Si(100) samples must be attributed to active sites located on the metal-oxides overlayer modified by gold underneath, since no Au was exposed to the surface according to the XPS and SIMS. We found a promoting effect of gold on the catalytic activity of the FeO_x overlayer and an inhibiting effect of gold on the TiO₂ and CeO_x overlayers. These findings are discussed in terms of electronic interactions at the Au/metal oxide interface.     

ZnO复合石墨相氮化碳纳米片的制备及其光催化性能研究

为了提高石墨相氮化碳光催化性能,本文以尿素、硫脲、醋酸锌为前驱体,通过氧化热剥离与共混煅烧法分别制备g-C₃N₄纳米片和ZnO/g-C₃N₄异质结复合材料,并采用TEM、FTIR、XRD、UV-Vis DRS、BET等表征手段对制备的催化剂进行结构表征。以罗丹明、大肠杆菌为探针,考察了催化剂的光催化降解性能和抑菌活性。结果表明：以尿素和硫脲为前驱体,经过氧化热剥离处理后能得到的g-C₃N₄ 2D纳米片,其比表面积更大、光催化性能更加优异,且其对罗丹明的降解率较未剥离的g-C₃N₄提高了21.2%。在40 min氙灯照射下,纯g-C₃N₄并未表现出良好的抑菌性能,而通过ZnO复合制备的ZnO/g-C₃N₄异质结复合材料,在光催化降解率和抑菌活性方面均有很大提高,其中复合20%ZnO制得的ZnO异质结复合材料表现出最佳的光催化性能...     

Analysis of electrical properties of CIGSSe and Cd-free buffer CIGSSe solar cells

We investigated the influence of different buffer layers to the electrical parameters (J_sc, V_oc, QE and efficiency η) of solar cells. The cells with an In₂S₃ and a ZnMgO buffer layer were compared with a reference cell with a CdS buffer layer. We performed temperature and light dependent current-voltage measurements, temperature dependent capacitance measurements and quantum efficiency measurements.The cells with In₂S₃ and ZnMgO buffers differ not too much in J_sc, but they do differ in V_oc and their electrical properties — fill factor FF, diode saturation current J₀ and efficiency η. They also do differ in their spectral response, both at short and long wavelengths, and in their ideality factor. This indicates a different current transport mechanism. The device simulation program SCAPS is used for further interpretation of the measurements. After exploring the parameters we found an acceptable agreement between simulated and measured J-V and QE(λ) curves. The simulated QE curves fit well over the whole spectrum, except for the CdS buffer cell, where there is an overestimation for the intermediate wavelengths. Because of this the simulated J_sc is higher than the measured one. The simulated V_oc agrees well for all cells. For the ZnMgO buffer cell it was necessary to include a buried homo-junction.