二元系纳米多层薄膜非线性互扩散研究

采用非均匀体系非线性动力学离散模型计算AB二元系纳米多层薄膜的非线性互扩散,研究了A、B原子间作用力之差VBB-VAA和有序能V对纳米多层薄膜浓度分布和界面结构的影响.对于V≤0的纳米多层薄膜,随着VSS-VAA由0减小到-0.05eV,扩散过程中多层薄膜浓度偏离对称分布,界面由宽化向平直转变,有序能V由0减小到-0.025eV,导致多层薄膜的扩散速度加快.当V>0时,扩散过程中多层薄膜出现相分离趋势导致的上坡扩散,初始宽化互混的界面变得平直,且随着VBB-VAA由0减小到-0.05V,多层薄膜扩散过程中形成的浓度起伏长大成为新的亚层.AB二元系纳米多层薄膜扩散过程中的浓度分布和界面结构演变与扩散非对称性系数m'、有序能V和初始时刻的浓度梯度有关.     

Structure, hardness and thermal stability of nanolayered TiN/CrN multilayer coatings

Nanolayered TiN/CrN multilayer coatings were deposited on silicon substrates using a reactive DC magnetron sputtering process at various modulation wavelengths (Λ), substrate biases (V_B) and substrate temperatures (T_S). X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the coatings. The XRD confirmed the formation of superlattice structure at low modulation wavelengths. The maximum hardness of the TiN/CrN multilayers was 3800 kg/mm² at Λ=80  Å, V_B=−150 V and T_S=400°C. Thermal stability of TiN, CrN and TiN/CrN multilayer coatings was studied by heating the coatings in air in the temperature range (T_A) of 400-800°C. The XRD data revealed that TiN/CrN multilayers retained superlattice structure even up to 700°C and oxides were detected only after T_A?750°C, whereas for single layer TiN and CrN coatings oxides were detected even at 550°C and 600°C, respectively. Nanoindentation measurements showed that TiN/CrN multilayers retained a hardness of 2800 kg/mm² upon annealing at 700°C, and this decrease in the hardness was attributed to interdiffusion at the interfaces.     

X-ray diffraction and high resolution transmission electron microscopy characterization of intermetallics formed in Fe/Ti nanometer-scale multilayers during thermal annealing

Intermetallics formation in the Fe/Ti nanometer-scale multilayers magnetron-sputtering deposited on Si(100) substrate during thermal annealing at 623-873 K was investigated by using small and wide angle X-ray diffraction and cross-sectional high-resolution transmission electron microscopy. The Fe/Ti nanometer-scale multilayers were constructed with bilayer thickness of 16.2 nm and the sublayer thickness ratio of 1:1. At the annealing temperature of 623 K, intermetallics FeTi were formed by nucleation at the triple joins of α-Fe(Ti)/α-Ti interface and α-Ti grain boundary with an orientational correlation of FeTi(110)//α-Ti(100) and FeTi[001]//α-Ti[001] to adjacent α-Ti grains. The lateral growth of intermetallics FeTi which is dependent on the diffusion path of Ti led to a coalescence into an intermetallic layer. With an increase in the annealing temperature, intermetallics Fe₂Ti were formed between the intermetallics FeTi and the excess Fe due to the limitation of Fe and Ti atomic concentrations, resulting in the coexistence of intermetallics FeTi and Fe₂Ti. It was found that the low energy interface as well as the dominant diffusion path constrained the nucleation and growth of intermetallics during interfacial reaction in the nanometer-scale metallic multilayers.     

Modeling the diffusion of solid copper into liquid solder alloys

During the soldering process, the copper atoms diffuse into liquid solders. The diffusion process determines integrity and the reworking possibility of a solder joint. In order to capture the diffusion scenarios of solid copper into liquid Sn-Pb and Sn-Cu solders, a computer modeling has been performed for 10 s. An analytical model has also been proposed for calculating the diffusion coefficient of copper into liquid solders. It is found that the diffusion coefficient for Sn-Pb solder is 2.74 × 10^− 10 m²/s and for Sn-Cu solder is 6.44 × 10^− 9 m²/s. The modeling results reveal that the diffusion coefficient is one of the major factors that govern the rate at which solid Cu dissolve in the molten solder. The predicted dissolved amounts of copper into solders have been validated with the help of scanning electron microscopic analysis.     

Microstructural changes and elemental diffusion of sputtered NiCrAlY coating on a Ni-base SC superalloy subjected to high temperature

Microstructural changes and elemental diffusion of sputtered NiCrAlY coating on the Ni-base SC superalloy were investigated. Tests were conducted by exposing coated specimens both in vacuum and in still air at 1000 °C. XRD and SEM were used to analyze phases and microstructures. EPMA was used to characterize the distribution of elements in the coating interdiffusion layers and substrates. It was found that the as-sputtered NiCrAlY coating showed homogeneous element distribution with abrupt change of constituent at the interface between the substrate and the coating. After vacuum heat treatment, microstructural and phase changes occurred in the coating because of interdiffusion between the substrate and the coating—an interdiffusion layer and two new phases emerged. After 200 h oxidation at 1000 °C in air, the interdiffusion layer thickened and a diffusion influenced layer emerged below the interdiffusion layer in the substrate. Some of the Al in the γ′-Ni₃Al phase was replaced by Ta, resulting in an increase of the lattice parameter a₀ of γ′ phase.     

Interdiffusion effect on GaAsSbN/GaAs quantum well structure studied by 10-band k•p model

The effect of annealing on the photoluminescence (PL) in GaAsSbN/GaAs quantum well (QW) grown by solid-source molecular-beam epitaxy (SS-MBE) has been investigated. Low-temperature (4 K) PL peaks shift to higher energy sides with the increase of annealing temperature. An As-Sb atomic interdiffusion at the heterointerface is proposed to model this effect. The compositional profile of the QW after interdiffusion is modeled by an error function distribution and calculated with the 10-band k•p method. When the diffusion length equals to 1.4 nm, a corresponding transition energy blueshift of 36 meV is derived. This agrees with the experimental result under the optimum condition (750 °C at 5 min).     

Ag diffusion in ZnS thin films prepared by spray pyrolysis

ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80-400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10^− 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N₂ atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect.     

Bulk modulus and hardness of chalcopyrite structured solids

The bulk modulus and microhardness can be represented by an empirical linear relation that is a simple function of melting temperature T_m, atomic volume Ω and product of ionic charges (Z₁Z₂Z₃). Values of bulk modulus B and microhardness H of A^IB^IIIC₂^VI and A^IIB^IVC₂^V chalcopyrite semiconductors exhibit a linear relationship when plotted against the k_BT_m/Ω (k_B = Boltzmann's constant), but fall on two straight lines according to the product of ionic charges of the compounds. This correlation is similar in form to other correlations in the literature for diffusion data of materials that indicate the significance of the melting temperature as a scaling or lattice dynamic properties of materials. The calculated results are compared with available experimental data and previous calculations based on phenomenological models.     

Interfacial properties and characterization of Sc/Si multilayers

We investigate the intermixing of layers in Sc/Si and Sc/B₄C/Si/B₄C multilayers using electron and synchrotron excited soft X-ray emission and absorption spectroscopy. The multilayers are annealed at 100, 200, 300, 400 and 500 °C after preparation by magnetron sputtering. Silicon K_β emission and reflectivity measurements verify that the non-annealed multilayer systems are composed of distinct layers with only a minor interdiffusion in Sc/Si samples whereas annealing Sc/Si multilayers at 400 °C leads to a degradation of the multilayer structure and the formation of intermittent scandium silicide, ScSi. The presence of B₄C barriers in Sc/B₄C/Si/B₄C hinders this degradation from developing for the entire temperature range considered. The barrier layers continue to be effective for the entire temperature range even after an extended shelf-life.     

Electron microscope observations of interdiffusion and ordering in copper-gold thin film diffusion couples

Observation of interdiffusion and ordering in thin polycrystalline sandwich layers of copper and gold has been carried out by means of electron diffraction. Samples of total thickness 800 and 1000 Å with thickness ratios corresponding to AuCu₃, AuCu and Au₃Cu respectively were vacuum deposited onto cleavage surfaces of NaCl. The diffusion experiments were performed in a temperature range around 200 °C. A preferred formation of the disordered AuCu₃ alloy was found during diffusion, in agreement with previous X-ray investigations. The ordering was much slower than the interdiffusion process, resulting in the coexistence of ordered and disordered crystallites even after equalized concentration. More complete ordering was obtained only after extended annealing. The interdiffusion coefficient D was evaluated from the diffraction patterns using a new method and was found to be 3 × 10^-4 exp (25 000/RT).     

Electrical transport mechanisms and photovoltaic behavior of 2-(2-furanylmethylene) propanedinitrile/p-Si heterojunction

Au/2-(2-furanylmethylene) propanedinitrile/p-Si heterojunction was fabricated using conventional thermal evaporation technique. Current density–voltage (J–V) characteristics of the heterojunction were investigated at different temperatures. Tunneling conduction mechanism in the lower voltage range was identified from the forward bias (J–V) characteristics. The calculated value of the change of built-in voltage with respect to the absolute temperature is (−1.88 × 10⁻³ V K⁻¹). At higher voltages, a space charge limited current (SCLC) mechanism controlled by an exponential trapping distribution above the valence band edge was observed. The total concentration of traps was found to be 8.077 × 10²¹ m⁻³. Under reverse bias, the conduction mechanism is due to thermally generated carriers in the lower voltage range and the Poole–Frenkel effect is observed in the higher voltage range. The heterojunction showed a photovoltaic behavior under illumination with an open-circuit voltage of 0.19 V and a short-circuit current density of 102.7 mA m⁻².     

Measurements and modeling of the DC temperature dependence of electrical conductivity in thin films of lead phthalocyanine

Many of the phthalocyanines exhibit p-type conductivity, and electrical conductivity through thin films of these materials having ohmic contacts show space-charge-limited conductivity (SCLC) dominated by trap levels located within the band gap. In the present work evaporated thin films of lead phthalocyanine with ohmic gold electrodes were prepared, which showed two distinct regions in the dependence of current density J on applied voltage V. At low voltages sample conductivity was ohmic, changing at higher voltages to a square-law dependence of J on V, which is indicative of SCLC dominated by trap levels located at a single discrete energy level. The results of temperature measurements indicate three distinct regions, in each of which the hole concentrations are controlled by different activation energies. A simple model is proposed in which a single trap level is located at the same energy spacing E_t from the valence band edge as a single acceptor level. This predicts three different temperature ranges, two of which correspond to those covered by the experimental results. The experimental results indicate a trap level located at an energy E_t = 0.36 eV above the valence band edge and a thermal band gap E_g = 1.51 eV. Using the proposed model together with data from the experimental J-V characteristics, an acceptor concentration of 4.85 × 10¹⁹ m ^− 3 and a trap concentration of 5.18 × 10²⁵ m^− 3 are indicated. Measurements of mobility based on this model yield a value of 2.6 × 10^− 4 m² V^− 1 s^− 1, which is in close agreement with previous work.     

Synthesis of metal oxide nanomaterials by plasma treatment - A SEM investigation of Nb2O5 nanowires

Plasma techniques for rapid synthesis of large quantities of metal oxide nanoparticles are presented. Suitable metal foils are exposed to cold oxygen plasma with a large density of neutral oxygen atoms of the order of 10²¹ m⁻³ and the ion density of the order of 10¹⁶ m⁻³. Depending on the flux of neutral atoms onto the sample surface as well as other plasma parameters, such as the positive ion kinetic energy and the sample temperature, different types of nanoparticles are synthesized, such as nanowires, nanocones and nanobelts. Nanowires are synthesized at extremely large flux of neutral oxygen atoms on the sample surface, often exceeding 1 × 10²⁴ m⁻² s⁻¹. Other features are synthesized at moderate flux of atoms of the order of 10²³ m⁻² s⁻¹. The phenomenon is explained by rapid nucleation of metal oxide on the sample surface followed by a one -dimensional growth of the nanowires due to balanced supply of neutral atoms and ions to the surface. Controlling the plasma parameters, a fair control on the nanostructure size as well as shape is achieved. The technique allows for synthesis of nanowires at the rate of almost 1 g/h, and therefore assures huge quantities of this material. The synthesis of niobium oxide nanowires is studied to some details by scanning electron microscopy.     

Mechanistic study of Al electrodeposition from EMIC–AlCl3 and BMIC–AlCl3 electrolytes at low temperature

Aluminum was electrodeposited from two different ionic liquid electrolytes namely; 1-ethyl-3-methylimidazolicum chloride and AlCl₃ (EMIC–AlCl₃) and 1-butyl-3-methylimidazolium chloride (BMIC–AlCl₃) at 90 ± 2 °C. Nodular morphology of aluminum was obtained at the copper substrate. To evaluate the reaction mechanism, chronoamperometric study was adopted. From the chronoamperometric analysis, it was observed that the reaction was controlled by instantaneous three-dimensional diffusion process. The diffusion coefficient (D) of Al₂C₇⁻ was found to be 5.2–6.6 × 10⁻¹¹ m² s⁻¹ and 2.2 × 10⁻¹¹ m² s⁻¹ for EMIC–AlCl₃ and BMIC–AlCl₃ ionic liquid electrolytes, respectively. The nuclei density of electrodeposited aluminum on the copper was also confirmed by scanning electron microscope wherein uniformly distributed aluminum nodules were observed.     

Direct current electrical conduction mechanism in plasma polymerized thin films of tetraethylorthosilicate

The plasma polymerized tetraethylorthosilicate (PPTEOS) thin films were deposited on to glass substrates at room temperature by a parallel plate capacitively coupled glow discharge reactor. The current density-voltage (J-V) characteristics of PPTEOS thin films of different thicknesses have been observed at different temperatures in the voltage region from 0.2 to 15 V. In the J-V curves two slopes were observed — one in the lower voltage region and another in the higher voltage region. The voltage dependence of current density at the higher voltage region indicates that the mechanism of conduction in PPTEOS thin films is space charge limited conduction. The carrier mobility, the free carrier density and the total trap density have been calculated out to be about 2.80 × 10^− 15m²V^− 1s^− 1, 1.50 × 10²²m^− 3 and 4.16 × 10³³m^− 3 respectively from the observed data. The activation energies are estimated to be about 0.13 ± 0.05 and 0.46 ± 0.07 eV in the lower and higher temperature regions respectively for an applied voltage of 2 V and 0.09 ± 0.03 and 0.43 ± 0.10 eV in the lower and higher temperature regions respectively for an applied voltage of 14 V. The conduction in PPTEOS may be dominated by hopping of carriers between the localized states at the low temperature and thermally excited carriers from energy levels within the band gap in the vicinity of high temperature.     

Electrical characterisation of stearic acid/calix[4]amine Langmuir–Blodgett thin film

Within this work we deposited 16 monolayers of stearic acid alternated with 15 monolayers of calix[4]amine to form a non-centrosymmetric Langmuir–Blodgett (LB) thin film onto an aluminized (50 nm coated) glass microscopic slide. Dielectric constant and dielectric loss for the film were determined using C-f and tan (δ-f) measurements. The value of the pyroelectric figure of merit was determined as 1.73 μC m⁻² K⁻¹. To elucidate the conduction mechanism of stearic acid/calix[4]amine LB film, DC current–voltage measurements between −4 and +4 V were carried out. The I(V) behaviour shows a symmetrical and highly non-linear behaviour. Analysis of this behaviour of the stearic acid/calix[4]amine LB film showed a conductivity value of 1.12 × 10⁻¹³ S m⁻¹ for ohmic region. The exponential part of I(V) dependence obeyed the Schottky conduction mechanism with a barrier height of 1.67 eV. This LB film structure shows a typical insulating behaviour for low voltage values and the Schottky effect becomes dominant when the voltage increases. The frequency dependence of conductivity shows a power law relationship between conductance and frequency.     

Improved microstructure and flux pinning properties of Gd-substituted (Bi,Pb)-2212 superconductor sintered between 846 and 860 °C

The influence of sintering temperature on microstructural and superconducting properties of Gd-substituted (Bi,Pb)-2212 superconductor was investigated. It is found that the microstructure of the system can be tailored and refined so as to yield the best superconducting properties in terms of critical current density (J_C) and flux pinning by heat treating at a temperature in the range 846-860 °C. The samples sintered at 858 °C show the maximum self-field J_C of 31,960 kA m^{− 2} and those sintered at 846 °C show the maximum flux pinning force [F_P = 1698 kN m^{− 3}] as against 1860 kA m^{− 2} and 16 kN m^{− 3}, respectively for the undoped sample sintered at its optimum sintering temperature. The changes in microstructure followed by very high enhancement of J_C, J_C-B characteristics and F_P due to Gd-substitution within a small temperature range, is of great scientific and technological significance.     

The analysis of the current–voltage characteristics of the high barrier Au/Anthracene/n-Si MIS devices at low temperatures

The Au/Anthracene/n-Si/Al MIS device was fabricated on the basis of anthracene film covalently bonded to a Si substrate. The MIS device showed Schottky behavior with barrier heights of 0.85 eV and ideality factors of 1.88 at 300 K. The barrier height of the Au/n-Si has increased after deposition of the anthracene layer onto Si. Temperature dependent current–voltage (I–V) measurements were performed on the Au/Anthracene/n-Si/Al MIS diodes in the range 140–300 K. From the temperature dependence of forward bias I–V, the barrier height was observed to increase with temperature. However, the ideality factor decreased with increasing temperature. The values of activation energy (E_a) and Richardson constant (A*) were determined as 0.24 eV and 7.57 × 10⁻⁶ A cm⁻² K⁻² from the slope and the intercept at ordinate of the linear region of Richardson plot, respectively. The increase of the series resistance R_s with the fall of temperature was attributed to lack of free carrier concentration at low temperatures.     

Investigation of phase stability and oxide ion performance in new perovskite-type bismuth vanadate

Samples of the BICDVOX system, formulated as Bi₄Cd_xV_2−xO_{11−(3x/2)−δ} in the Cd substitution range 0 ≤ x ≤ 0.25 were synthesized using the standard solid state reaction.The correlation between phase stability and oxide ion performance were investigated by variable temperature XRPD, DSC and AC impedance spectroscopy. The substitution of V⁵⁺ by Cd²⁺ exhibited different phase transitions upon varying composition. For compositions with x ≤ 0.05, two successive transitions; α↔β↔γ are evident, while the β↔γ transition exists in the composition range 0.05 < x < 0.175. However, some temperature dependent phenomena confirmed the exixtence of the γ′↔γ transition, coupled with the tetragonal symmetry stabilization for x ≥ 0.175. The maximum oxide ion conductivity at lower temperatures was observed for x = 0.20. It has also been found that the slow V⁴⁺ → V⁵⁺ re–oxidation results in increased defect trapping effects in the system at higher temperatures.     

Effects of laser scans on the diffusion depth and diffusivity of gallium in n-type 4H-SiC during laser doping

An n-type 4H-SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 10²⁰ cm⁻³) at the substrate surface was found to exceed the solid solubility limit (1.8 × 10¹⁹ cm⁻³) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10⁻⁷ cm²/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10⁻² cm²/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.