Plasma diagnostics of an Ar/NH3 direct-current reactive magnetron sputtering discharge for SiNx deposition

We have performed the deposition of silicon nitride thin films with the DC reactive magnetron sputtering technique from a silicon target in an Ar/NH₃ gas mixture. Usually, the control of the process is carried out with discharge voltage measurements, which give information on the nature of the sputtering mode: metallic or reactive. To have a more complete view of the sputtering process, we have performed X-ray photoelectron spectroscopy (XPS) to investigate the chemistry of the silicon target racetrack and optical emission spectroscopy (OES) to investigate the Ar/NH₃ gas phase near the target surface. When the NH₃ molar fraction is increased, XPS measurements reveal the progressive formation of a silicon nitride layer on the target surface, thereby demonstrating a continuous transition to the reactive mode. OES measurements have highlighted the presence of several species which, according to the literature, are believed to be directly sputtered from the surface of the target: Si, SiH and SiN. Their intensities could be related to the chemical state of the target surface and provide a better insight into the sputtering process on the target surface.     

Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Evaluation of organic perovskite photoconductors for direct conversion X-ray imaging detectors

Enhancing the sensitivity of a direct conversion flat panel X-ray imaging detector with minimum manufacturing cost has been a major dream for long decades. This criterion has been recently addressed by the usage of MAPbX₃ (MA is CH₃NH₃ and X is a halogen atom such as Cl, I, or Br) perovskite in X-ray imaging detectors. Though MAPbI₃ has shown large area deposition capability and good X-ray sensitivity, it has to fulfil other criteria such as low dark current, high spatial resolution and high signal to noise transfer capabilities. This paper evaluates the imaging performances such as X-ray sensitivity, detective quantum efficiency (DQE) and modulation transfer function (MTF) of organic perovskites (e.g., MAPbI₃ and MAPbBr₃) with comparison to amorphous selenium (a-Se). These perovskite materials have slightly higher linear attenuation coefficients than a-Se and the expected X-ray sensitivity of these two perovskite photoconductors are higher than a-Se. The mechanisms of the dark current and photocurrent gain in MAPbI₃ detector are also investigated. The MAPbI₃ detector shows some photocurrent gain, which is due to the enhanced electron injection under X-ray illumination. The expected theoretical zero spatial frequency DQE of the MAPbI₃ detectors is similar to that of a-Se while the MAPbBr₃ detector establishes a better DQE than a-Se. The expected MTF of the MAPbBr₃ detectors is similar to that of a-Se while the MAPbI₃ shows worse resolution than a-Se. Based upon our theoretical investigation, we believe that the organic perovskite can find its state of the art in near future if rigorous research for improving the charge carrier transport properties and optimizing its detector structure for low dark current were to be made.     

Effects of doping concentration on the microstructural and optoelectrical properties of boron doped amorphous and nanocrystalline silicon films

Boron doped hydrogenated amorphous silicon thin films were prepared by plasma-enhanced chemical vapor deposition technique at various flow rate of diborane (F_B). As-deposited samples were thermally annealed at the temperature of 800 °C to obtain the doped nanocrystalline silicon (nc-Si) films. The effect of boron concentration on the microstructural, optical and electrical properties of the films was investigated. X-ray photoelectron spectroscopy (XPS) measurements demonstrated the presence of the substitutional boron in the doped films. It was found that thermal annealing can efficiently activate the dopants in films accompanying with formation of nc-Si grains. Based on the temperature-dependent conductivity measurements, it was shown that the dark conductivity of doped amorphous samples increases monotonously with the increase of doping content. While the dark conductivity of doped nc-Si films is not only determined by the concentration of dopant but also the crystallinity of the films. As increasing the flow rate of diborane, the crystallinity of doped nc-Si films decreases, which causes the decrease of dark conductivity. Finally, the high dark conductivity of 178.68 S cm⁻¹ of the B-doped nc-Si thin films can be obtained.     

Gas sensing performance of nanocrystalline ZnO prepared by a simple route

The nanocrystalline powders of pure and Al³⁺-doped ZnO with hexagonal structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and the microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution between Al₂O₃ and ZnO. DC electrical properties of the prepared nanoparticles were studied by DC conductivity measurements. The indirect heating structure sensors based on pure and doped ZnO as sensitive materials were fabricated on an alumna tube with Au electrodes. Gas-sensing properties of the sensor elements were measured as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure ZnO exhibited high response to NH₃ gas at an operating temperature of 200 °C. Doping of ZnO with Al³⁺ increased its response towards NH₃ and the Al³⁺-doped ZnO (3.0 wt% Al₂O₃) showed the maximum response at 175 °C. The selectivity of the sensor elements for NH₃ against different reducing gases like LPG, H₂S and H₂ was studied. The results on response and recovery time were also discussed.     

Characterization of SiNx:H films deposited by rapid thermal chemical vapor deposition

Hydrogenated silicon nitride films were deposited with NH₃, SiH₄ and N₂ gas mixture at 700 °C by rapid thermal chemical vapor deposition (RTCVD) system. The NH₃/N₂ flow ratio and deposition pressure are found to influence the film properties. The stress of SiN_x:H films deposited by RTCVD is tensile, which can reach ~ 1.5 GPa in our study. The stress of SiN_x:H films is dependent on the deposition parameters, which can be associated with chemical configuration of the film. It is suggested that the presence of hydrogen atoms will relax the Si-N network, which results in the decrease of tensile stress of the SiN_x:H film.     

On the epitaxy of aluminum nitride on silicon substrates in a chloride-hydride process

The technological conditions under which the silicon surface interacts with vapor-phase reactants present in a chloride-hydride system for the epitaxial growth of aluminum nitride are determined. The method of electron channeling patterns is used to show that the growth of single-crystal layers of AlN on silicon substrates in the chloride-hydride system is hindered by the interaction of the silicon with NH₃ in the presence of HCl at T⩽800°C, with the formation of an amorphous layer of Si₃N₄. To obtain a high-quality texture it is important that prior to deposition of the AlN layers the silicon substrates be held in an NH₃ atmosphere in order to form a dense layer of Si₃N₄. Single-crystal growth of AlN can be achieved in a chloride-hydride system of chemical deposition from the vapor phase at a reduced pressure, since the deposition temperature is then substantially lower (down to 550°C) and the chemical interaction with the substrate is hindered. Pis’ma Zh. Tekh. Fiz. 24, 52–57 (October 26, 1998)     

ITO/p+nc-Si:H contact barrier effects on n-i-p′-p silicon solar cell performances

The computer program AMPS-1D (analysis of microelectronic and photonic structures) has been used to explore the effect of front contact barrier heights for electrons (φ_b0) or holes (φ_h) on the performances of n-i-p′-p amorphous/nanocrystalline silicon based solar cell, with a p type hydrogenated nanocrystalline silicon double layer and with no back reflector. φ_b0 is the result of band bending at the indium tin oxide (ITO)/p⁺doped hydrogenated nanocrystalline silicon (p nc-Si:H) interface. This paper presents results for a n-i-p′-p device, when the p nc-Si:H layer is used as a window and the p′-nc-Si:H layer as a buffer. Band diagram at thermodynamic equilibrium and current–voltage characteristics (J–V), under dark and illumination conditions, for the considered solar cell structure, are calculated. The modeling showed that the reverse bias currents do not depend on the front contact barrier heights. However, in the forward direction, this contact barrier influences strongly the J–V characteristic in the dark and under illumination. As a result, when φ_b0 increases, output cell parameters, like open circuit voltage (V_OC), fill factor (FF) and efficiency (E_ff) increase. The best values obtained are 0.893 V, 0.757 and 8.04%, respectively. These values correspond to a front contact barrier height (φ_b0) equal to 1.65 eV. Such a value of φ_b0 can be realized experimentally by using an indium tin oxide (ITO) front contact electrode, with a work function value about 5.35 eV.     

Nanocrystalline silicon fabrication by conventional plasma enhanced chemical vapor deposition for bottom gate thin film transistor

Nanocrystalline silicon was successfully fabricated using conventional plasma enhanced chemical vapor deposition (PECVD) for bottom gate thin film transistor. This was accomplished by promoting nucleation rate in the initial stage of silicon growth by H₂ or SF₆ plasma treatment of the surface of silicon nitride (SiN_x) films. Microstructure of hydrogenated nanocrystalline silicon (nc-Si:H) films confirmed the crystallization of silicon, and nanocrystalline silicon thin film transistor exhibited excellent stability.     

Investigation of Magnetic Entropy Change and Griffiths-like Phase in La0.65Ca0.35MnO3 Nanocrystalline

In this paper, we reported a detailed study of magnetic properties and magnetic entropy change of La _0.65Ca_0.35MnO₃ nanocrystalline, which was prepared by using the sol–gel method. The structural analysis shows that the nanocrystalline sample crystalizes in orthorhombic perovskite structure and the average size is about 30 nm. Based on the measurements of magnetization, a larger effective magnetic moment was obtained and an obvious deviation of the inverse magnetic susceptibility was observed, indicating the presence of Griffiths-like phase in paramagnetic region. Around the temperature of paramagnetic–ferromagnetic phase transition, the magnetocaloric effect (as represented by the magnetic entropy change) was determined from isothermal magnetization and calculated with Maxwell relation. Compared with bulk polycrystalline, the obtained magnetic entropy change in nanocrystalline is small. This result clearly reveals that the decrease of the sample’s size to nanoscale is detrimental for the increase of magnetocaloric effect of magnetic materials. Besides the particle size and surface effect, the paramagnetic–ferromagnetic phase transition driven from first to second order should be a main reason for the small magnetocaloric effect in La _0.65Ca_0.35MnO₃ nanocrystalline.     

Reactively sputtered MoO3 films for ammonia sensing

We report the gas-sensing properties of ion-beam sputter deposited MoO₃ thin-films. The change in the DC conductivity was measured in dry N₂ with 10% O₂ in the presence of up to 490 ppm of NH₃, NO, NO₂, C₃H₆, CO and H₂. At ∼440 °C the film was found to be very sensitive to NH₃, with 490 ppm increasing the conductivity by approximately a factor of 70. This was approximately 17 times greater than the response to the other gases. The NH₃ response was strongly affected by the accompanying levels of O₂, NO₂ and H₂O. For example, changing the accompanying O₂ levels from 1% to 20% decreased the NH₃ response by approximately a factor of 20. Similarly, the presence of 100 ppm NO₂ (in 10% O₂) decreased the NH₃ response by approximately a factor of three, and 1% water vapor decreased it by more than a factor of two. The NH₃ response, however, was relatively unaffected by 100 ppm of accompanying NO, C₃H₆, CO or H₂. XPS measurements show that the increased conductivity in the presence of NH₃ was also accompanied by a partial reduction of the surface MoO₃. We observed an increase in the resistance of the films after extended time at elevated temperatures.     

Improved visible-light responsive photocatalytic activity of N and Si co-doped titanias

Thermal reaction of titanium tetraisopropoxide and tetraethyl orthosilicate in 1,4-butanediol afforded nanocrystalline silica-modified titanias having large surface area and superior thermal stability. In this study, the thus-obtained silica-modified titanias were treated in an NH₃ flow at high temperatures, and their physical and photocatalytic properties were investigated. Compared with NH₃-treated TiO₂ without silica modification, the NH₃-treated silica-modified titanias showed a stronger absorption in the visible region (400–500 nm) and had a larger peak at 396 eV in the N 1s XPS spectrum. These results indicate that a larger amount of nitrogen was stably doped in the silica-modified titania. The obtained products exhibited a high photocatalytic activity for degradation of Rhodamine B and decomposition of acetaldehyde under visible light irradiation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electrical,structural and gas sensing properties of zinc phthalocyanine thin films

The dark conductivity of zinc phthalocyanine (ZnPc) has been studied as a function of material purity, crystal phase transformation and temperature with particular regard to gas sensitivity in air, O₂, N₂, argon, NH₃ and NH₃-air mixtures. α-ZnPc was found to grow in the form of randomly oriented micro- crystallites but the β form showed oriented needle-like and whisker growth. The electrical properties were found to be dependent on material purity. Entrainer- sublimed ZnPc showed higher conductivity than impure material and displayed reproducible linear characteristics with less drift and hysteresis.The conductivity of both α- and β-ZnPc is found to be critically dependent on the presence of O₂. The sensitivity to other gases differs for the α and β forms but in both cases NH₃ causes a large dark conductivity decrease, possibly owing to catalytic behaviour, effectively removing oxygen acceptors.Conductivity-temperature data indicate a transition from extrinsic to non-extrinsic conduction for most cases.The conductivity of β-ZnPc is found to be greater than that of α-ZnPc, in contrast with other phthalocyanines.The relative sensitivities to the various gases suggests that ZnPc may be a viable material for selective gas sensing devices.     

Hydrothermal Synthesis and Characterization of PEG Stabilized Co3O4 nanoparticles

Polyethylene glycol stabilized Cobalt oxide, (Co₃O₄), nanoparticles were prepared via simple, one-step, inexpensive hydrothermal method. In this process, polyethylene glycol was used as a solvent and surfactant; gaseous NH₃ was used as an alkalinity additive. Investigation of the structural, morphological, thermal, and magnetic properties were carried out using X-ray diffraction (XRD), infrared spectroscopy (FT-IR), transmission electron spectroscopy (TEM), thermal analysis (TGA), and vibrating sample magnetometer (VSM), respectively. The nanocrystalline nature of the sample was confirmed by XRD and TEM. FT-IR measurement revealed that the O from C?CO coordinates with the surface of Co₃O₄ NP??s. Room temperature VSM measurement showed the ferromagnetic behavior of the product.     

Chemical interaction of the Ti<Subscript>90</Subscript>Mg<Subscript>10</Subscript> alloy with ammonia

We have studied the chemical interaction of the Ti₉₀Mg₁₀ alloy with ammonia in the presence of NH₄Cl at temperatures from 150 to 500°C and identified conditions for the formation of fine-particle hydrides and nanocrystalline nitrides of titanium and magnesium.     

Low temperature NH3 adsorption on clean amorphous silicon and on crystalline silicon surfaces and nitrogen bonding in hydrogenated amorphous silicon nitride films: A comparative X-ray photoelectron spectroscopy study

Careful X-ray photoelectron spectroscopy studies of the nitrogen core levels were used to compare, in the same substrate temperature T_s range (between room temperature and 450 °C), the first interaction stages of amorphous and crystalline silicon surfaces (a-Si and c-Si(111)-(7 × 7)) with ammonia and the uptake of nitrogen on reactive evaporation of silicon in an ammonia ambient, i.e. during the growth of hydrogenated amorphous silicon nitride (a-SiN_x:H) films. In all cases we observed strongly correlated behaviours: the N 1s binding energy E_B decreased with increasing T_s, reflecting the presence of more dissociated species, probably NH₂ or NH and finally nitrogen, either on the silicon surfaces or in the bulk of the a-SiN_x:H films. The total N 1s core level intensities corresponding to hydronitride complexes NH_X decreased with increasing T_S up to 300 °C. Above this temperature the contribution of completely dissociated molecules relevant to nitride environments became prominent. The nitrogen coverage of the silicon surfaces at room temperature as a function of exposure was also compared with the nitrogen content x of a-SiN_x:H films as a function of NH₃ pressure during evaporation. Converted into dynamical exposures for a given evaporation rate, this pressure dependence is very well explained in terms of variations in the sticking coefficient deduced from the adsorption studies.     

Improved activation and hydrogen storage properties of a Mg85Ni15 melt-spun alloy via surface treatment with NH4 solution

An amorphous Mg₈₅Ni₁₅ melt-spun hydrogen storage alloy, processed by submersion in an aqueous solution of NH₄⁺, is able to absorb nearly 5 mass% hydrogen at 473 K during the first hydrogenation cycle. The nanocrystalline microstructure formed during devitrification of the metallic glass is preserved by the lower required activation temperature of the NH₄⁺-treated material compared to the as-spun material; and the kinetics of subsequent absorption/desorption cycles at 573 K are dramatically improved. The material activated at 473 K exhibits a decrease in hydride decomposition temperature by 30 K, observed via DSC and TPD experiments, compared to a sample activated at 573 K. The NH₄⁺-treatment of a glassy alloy presented here provides a practical alternative to ball milling for forming a nanocrystalline material and facilitating activation, requiring much less time and a more commercially scalable option.     

First-principle calculations of anomalous spin-state excitation in LaCoO3

We investigate the different spin states of LaCoO₃ employing the state-of-the-art ab initio band structure calculations within a rotationally invariant formulation of local density approximation (LDA) + U approach. The various magnetically ordered spin states of different supercells have been studied, including the low-spin state (LS), intermediate-spin state (IS), high-spin state (HS) Co³⁺ ions, as well as all combinations among these three states. The ground state is correctly predicted to be an insulator nonmagnetic state. Our calculations, together with previous susceptibility measurements for IS excitations in the LS ground state, lead to the conclusion that the nonmagnetic–paramagnetic transition in LaCoO₃ at 90 K is caused by a gradual population of IS Co³⁺ ionic states. Our results show that the first thermally excited spin-state occurs from LS to an LS (Co_LS³⁺ = 87.5%)–IS (Co_IS³⁺ = 12.5%) ordered state, which can be distinguished from the LS–HS or IS state. We find that the mixture of LS–IS, LS–HS, and HS–IS spin states may develop an orbital ordering.     

Microstructure and magnetic properties of plasma-sprayed Fe73.5Cu1Nb3Si13.5B9 coatings

Amorphous and nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ coatings were formed by plasma-spraying micron-sized powders onto H62 brass substrates and aluminum pipes. The coatings are about 0.2-0.3 mm in thickness with fully dense and low porosity. The microstructure of the coatings is classified into two regions, namely, a full amorphous phase region and homogeneous dispersion of α-Fe nanoscale particles with a scale of 30-70 nm. The hardness of the amorphous and nanocrystalline coatings is about 960 HV_100g. Coercivity (Hc), saturation induction (B₈₀₀), and initial relative permeability (μ_i) of the coatings are 144 A/m, 0.27 T, 249, respectively, under 800 A/m direct current (DC) magnetic field. The magnetic shielding performance is good under DC magnetic field and its magnetic shielding effectiveness (SE) is 10-12 dB at coating thickness of 0.45 mm under static magnetic field of 2-40 Oe. The SE increases by increasing the coating thickness when the magnetic field frequencies are 50, 100 and 200 Hz with an intensity of 0.85 Oe. The results indicate that the amorphous and nanocrystalline alloy coatings can be good for some magnetic shielding applications.     

Silicon nitride layers of various n-content: Technology, properties and structure

Two series of amorphous silicon nitride layers (a-SiN_x:H) were formed with Radio Frequency Chemical Vapor Deposition method (13.56 MHz) from a NH₃/SiH₄ gas mixture: the first one on Si (001) and the second on glass. The deposition process was repeated at various [NH₃]/[SiH₄] ratios, while the other parameters (pressure, plasma generator power, substrate temperature, total gas flow, and time) were kept constant. It has been confirmed in optical measurements that the refractive indexes decrease for the layers obtained at increasing [NH₃]/[SiH₄] ratios. Simultaneously, the position of the band assigned to Si-H stretching vibrations (at about 2100 cm^− 1) shifts towards higher frequencies. The observed dependencies were applied in evaluation of nitrogen and hydrogen contents in the respective layers. It has been shown that when [NH₃]/[SiH₄] increases from 0 (no silane flow) to 0.2 then the a-SiN_x:H layers of x = [N]/[Si] increasing between 0 and nearly 1.4 may be obtained. The obtained layers have the refractive indexes higher than 2.1 and lower than 2.7 which make them good materials for antireflective coatings on crystalline and multicrystalline silicon solar cells.