Near-contact Region Research Articles

Improved CZTSe solar cell efficiency via silver and germanium alloying

In this study, we report systematic investigation of the effects of Ag and Ge alloying on properties of CZTSe layers, as well as, on the performance of solar cells fabricated using these films. In this context, Ag-Ge doped CZTSe layers were produced by selenization of Cu/Sn/Zn/Cu/(Ag,Ge)/Se precursor stack structures using rapid thermal processing. All precursor stacks and the Ag-Ge doped CZTSe films obtained after selenization exhibited (Cu + Ag)-poor and Zn-rich chemical composition. XRD studies demonstrated pure kesterite phase for all reacted films. Raman spectra confirmed this finding. Cross-sectional SEMs showed large grain structure, which resulted from Ag-Se and Ge-Se liquid phase formation that assisted crystal growth during high temperature annealing. While a slight Ag-front-gradient was achieved in Ag-doped CZTSe film, the Ag gradient disappeared with incorporation of Ge into the lattice. Addition of Ge formed a gradient within the material such that near-contact region was more Ge-rich. Solar cells fabricated using films with various compositions demonstrated that double doping CZTSe with both Ag and Ge improved the device efficiency from about 5 % to over 8 %.

Hydrodynamic interaction of two droplets covered with insoluble surfactant in shear flow

Despite its significance in various applications, e.g., droplet microfluidics and enhancing oil recovery, the effects of surfactants on the hydrodynamic interactions of droplets in shear flow still need to be clarified. This paper presents a systematic investigation on the hydrodynamic interaction of two droplets covered with the insoluble surfactant in a simple shear flow, which is realized by using our own three-dimensional front-tracking code. Our results indicate that surfactants significantly affect the hydrodynamic interactions of droplets, characterized by the maximum deviation in the z-direction and the shear-induced self-diffusion. Then, we examine the effects of surfactant-relevant parameters, i.e., the surface Peclet number and the elasticity number. Both the maximum deviation in the z-direction and the shear-induced self-diffusion increase with the elasticity number and the surface Peclet number. Specifically, we find two mechanisms underlying the effects of surfactant on the hydrodynamic interaction of two droplets. First, Marangoni stress induced by surfactant sharply increases the maximum deviation in the z-direction when the surface Peclet number is large. It is because that Marangoni stress hinders liquid drainage from the near-contact region and thickens the liquid film between the droplets. Second, hydrodynamic interactions during crossing lead to considerable alterations of the shape and local surfactant concentration, resulting in an irreversible trajectory shift of the droplets. So, the shear-induced self-diffusion sharply rises at the high surface Peclet number.

Resistance and mobility functions for the near-contact motion of permeable particles

A lubrication analysis is presented for the resistances between permeable spherical particles in near contact,$h_0/a\ll 1$, where$h_0$is the minimum separation between the particles, and$a=a_1 a_2/(a_1+a_2)$is the reduced radius. Darcy's law is used to describe the flow inside the permeable particles and no-slip boundary conditions are applied at the particle surfaces. The weak permeability regime$K=k/a^{2} \ll 1$is considered, where$k=\frac {1}{2}(k_1+k_2)$is the mean permeability. Particle permeability enters the lubrication resistances through two functions of$q=K^{-2/5}h_0/a$, one describing axisymmetric motions, the other transverse. These functions are obtained by solving an integral equation for the pressure in the near-contact region. The set of resistance functions thus obtained provide the complete set of near-contact resistance functions for permeable spheres and match asymptotically to the standard hard-sphere resistances that describe pairwise hydrodynamic interactions away from the near-contact region. The results show that permeability removes the contact singularity for non-shearing particle motions, allowing rolling without slip and finite separation velocities between touching particles. Axisymmetric and transverse mobility functions are presented that describe relative particle motion under the action of prescribed forces and in linear flows. At contact, the axisymmetric mobility under the action of oppositely directed forces is$U/U_0=d_0K^{2/5}$, where$U$is the relative velocity,$U_0$is the velocity in the absence of hydrodynamic interactions and$d_0=1.332$. Under the action of a constant tangential force, a particle in contact with a permeable half-space rolls without slipping with velocity$U/U_0=d_1(d_2+\log K^{-1})^{-1}$, where$d_1=3.125$and$d_2=6.666$; in shear flow, the same expression holds with$d_1=7.280$.

Features of the Band Structure and the Mechanism of Lateral Photoconductivity in Hybrid Structures T/SiO-=SUB=-2-=/SUB=-/Si(T=Fe,Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-, TiO-=SUB=-2-=/SUB=-)

This paper presents the results of studying the lateral photovoltaic effect in hybrid structures based on SiO2/Si depending on the nature of the conductivity of the material of the upper layer (Fe is a metal, Fe3O4 is a semimetal, TiO2 is a semi-insulator). It is shown that this effect is observed in hybrid structures in which a depleted region with a significant band bending is formed at the SiO2/Si interface. Theoretical calculations of the lateral photovoltage based on the energy parameters of the band diagrams showed that an increase in the sensitivity of the lateral photoelectric effect is associated with an increase in the built-in potential at the SiO2/Si interface. It has been established that the transient characteristics of the lateral photoelectric effect, such as the rise and fall times of the photovoltage signal, are determined by the configuration of the RC-filter in the near-contact region, which depends on the conductivity of the upper layer. It is shown that, in the case of the lateral photovoltaic effect, current transfer occurs along the inversion layer, and the upper layer serves to generate a quasi-p-n junction at the SiO2/Si interface. Keywords:lateral photovoltaic effect, hybrid structures, iron, magnetite, titanium dioxide.

Особенности зонной структуры и механизма латеральной фотопроводимости в гибридных структурах T/SiO-=SUB=-2-=/SUB=-/Si (T=Fe, Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=-, TiO-=SUB=-2-=/SUB=-)

This paper presents the results of studying the lateral photovoltaic effect in hybrid structures based on SiO2/Si depending on the nature of the conductivity of the material of the upper layer (Fe is a metal, Fe3O4 is a semimetal, TiO2 is a semi-insulator). It is shown that this effect is observed in hybrid structures in which a depleted region with a significant band bending is formed at the SiO2/Si interface. Theoretical calculations of the lateral photovoltage based on the energy parameters of the band diagrams showed that an increase in the sensitivity of the lateral photoelectric effect is associated with an increase in the built-in potential at the SiO2/Si interface. It has been established that the transient characteristics of the lateral photoelectric effect, such as the rise and fall times of the photovoltage signal, are determined by the configuration of the RC-filter in the near-contact region, which depends on the conductivity of the upper layer. It is shown that, in the case of the lateral photovoltaic effect, current transfer occurs along the inversion layer, and the upper layer serves to generate a quasi-pn junction at the SiO2/Si interface.

Near-contact approach of two permeable spheres

An analysis is presented for the axisymmetric lubrication resistance between permeable spherical particles. Darcy's law is used to describe the flow in the permeable medium and a slip boundary condition is applied at the interface. The pressure in the near-contact region is governed by a non-local integral equation. The asymptotic limit $K=k/a^{2} \ll 1$ is considered, where $k$ is the arithmetic mean permeability, and $a^{-1}=a^{-1}_{1}+a^{-1}_{2}$ is the reduced radius, and $a_1$ and $a_2$ are the particle radii. The formulation allows for particles with distinct particle radii, permeabilities and slip coefficients, including permeable and impermeable particles and spherical drops. Non-zero particle permeability qualitatively affects the axisymmetric near-contact motion, removing the classical lubrication singularity for impermeable particles, resulting in finite contact times under the action of a constant force. The lubrication resistance becomes independent of gap and attains a maximum value at contact $F=6{\rm \pi} \mu a W K^{-2/5}\tilde {f}_c$, where $\mu$ is the fluid viscosity, $W$ is the relative velocity and $\tilde {f}_c$ depends on slip coefficients and weakly on permeabilities; for two permeable particles with no-slip boundary conditions, $\tilde {f}_c=0.7507$; for a permeable particle in near contact with a spherical drop, $\tilde {f}_c$ is reduced by a factor of $2^{-6/5}$.

Collision rates of permeable particles in creeping flows

Binary collision rates are calculated for the permeable particles undergoing (i) Brownian motion, (ii) gravity sedimentation, (iii) uniaxial straining flow, and (iv) shear flow. Darcy's law is used to describe the flow inside the permeable particles, and no-slip boundary conditions are applied at particle surfaces. A leading-order asymptotic solution of the problem is developed for the weak permeability regime K=k/a2≪1, where k=12(k1+k2) is the mean permeability and a=a1a2/(a1+a2) is the reduced radius; ai, ki (i = 1, 2), respectively, is the radius and permeability of each particle. The resulting collision rates are given by the quadrature of the pair mobility functions for permeable particles in the near-contact lubrication region and size-ratio-dependent parameters obtained from standard hard-sphere pair mobility functions. Collision rates in shear flow vanish below a critical value of the permeability parameter K* that increases with diminishing size ratio. The analogous problem of pair collision rates of particles with small-amplitude surface roughness δa is also analyzed. The formulas for the collision rates of rough particles provide accurate analytical approximations for the collision rates of permeable particles for all four aggregation mechanisms and a wide range of size ratios using an equivalent roughness δ=0.72K2/5.

Structure and thermal diffusivity of SiC-NbC ceramics

The structure and thermal diffusivity of SiC - NbC ceramic materials are investigated. It is shown that with a change in the composition in sintered ceramics, the lattice parameter changes, and also in the near-contact region there is a mutual diffusion of silicon and niobium. This indicates the formation of solid solutions. As the niobium carbide content and temperature increase, the thermal diffusivity of SiC - NbC ceramic materials decreases.

State of Stress in the Near-Contact Region of a Semiconductor during Metallization Track Electrodegradation

Heating of a thin metal film on a semiconductor surface and the corresponding state of stress of the near-contact region are considered. The stress estimation procedure in case of the local pulsed heating of a metallized region on the semiconductor surface is described. The theoretical and experimental sizes of the deformed silicon substrate region are compared with each other. An experiment is carried out using rectangular current pulses. The stress–strain region, which depends on the electric pulse duration and amplitude, is estimated. Experimental evidence shows a strong heterogeneity of the metallization track after pulse passage.

The singular hydrodynamic interactions between two spheres in Stokes flow

We study exact solutions for the slow viscous flow of an infinite liquid caused by two rigid spheres approaching each either along or parallel to their line of centers, valid at all separations. This goes beyond the applicable range of existing solutions for singular hydrodynamic interactions (HIs), which, for practical applications, are limited to the near-contact or far field region of the flow. For the normal component of the HI, by the use of a bipolar coordinate system, we derive the stream function for the flow as the Reynolds number (Re) tends to zero and a formula for the singular (squeeze) force between the spheres as an infinite series. We also obtain the asymptotic behavior of the forces as the nondimensional separation between the spheres goes to zero and infinity, rigorously confirming and improving upon the known results relevant to a widely accepted lubrication theory. Additionally, we recover the force on a sphere moving perpendicularly to a plane as a special case. For the tangential component, again by using a bipolar coordinate system, we obtain the corresponding infinite series expression of the (shear) singular force between the spheres. All results hold for retreating spheres, consistent with the reversibility of Stokes flow. We demonstrate substantial differences in numerical simulations of colloidal fluids when using the present theory compared with the existing multipole methods. Furthermore, we show that the present theory preserves positive definiteness of the resistance matrix R in a number of situations in which positivity is destroyed for multipole/perturbative methods.

Features of the Temperature Dependence of the Specific Contact Resistance of Au–Ti–Pd–n+–n-Si Diffusion Silicon Structures

The temperature dependences of the specific contact resistance of silicon ρc with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd–n+–n-Si structures are ohmic. It is shown that minimal ρc is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

Особенности температурной зависимости удельного контактного сопротивления диффузионных кремниевых структур Au-Ti-Pd-n-=SUP=-+-=/SUP=--n-Si

AbstractThe temperature dependences of the specific contact resistance of silicon ρ_ c with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd– n ^+– n -Si structures are ohmic. It is shown that minimal ρ_ c is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

Features of current-voltage characteristic of nonequilibrium trench MOS barrier Schottky diode

The trench MOS barrier Schottky diodes (TMBS diode) under the influence of the voltage drop of the additional electric field (AEF) appearing in the near-contact region of the semiconductor are in a nonequilibrium state and their closed external circuit flows currents in the absence of an external voltage. When an external voltage is applied to the TMBS diode, the current transmission is described by the thermionic emission theory with a specific feature. Both forward and reverse I-V characteristics of the TMBS diode consist of two parts. In the initial first part of the forward I-V characteristic there are no forward currents, but reverse saturation currents flow, in its subsequent second part the currents increase exponentially with the voltage. In the initial first part of the reverse I-V characteristic, the currents increase in an abrupt way and in the subsequent second part the saturation currents flow under the action of the image force. The mathematical expressions for forward and reverse I-V characteristic of the TMBS diode and also narrow or nanostructure Schottky diode are proposed, which are in good agreement with the results of experimental and calculated I-V characteristics.

A hybrid lattice Boltzmann and finite difference method for droplet dynamics with insoluble surfactants

Droplet dynamics in microfluidic applications is significantly influenced by surfactants. It remains a research challenge to model and simulate droplet behaviour including deformation, breakup and coalescence, especially in the confined microfluidic environment. Here, we propose a hybrid method to simulate interfacial flows with insoluble surfactants. The immiscible two-phase flow is solved by an improved lattice Boltzmann colour-gradient model which incorporates a Marangoni stress resulting from non-uniform interfacial tension, while the convection–diffusion equation which describes the evolution of surfactant concentration in the entire fluid domain is solved by a finite difference method. The lattice Boltzmann and finite difference simulations are coupled through an equation of state, which describes how surfactant concentration influences interfacial tension. Our method is first validated for the surfactant-laden droplet deformation in a three-dimensional (3D) extensional flow and a 2D shear flow, and then applied to investigate the effect of surfactants on droplet dynamics in a 3D shear flow. Numerical results show that, at low capillary numbers, surfactants increase droplet deformation, due to reduced interfacial tension by the average surfactant concentration, and non-uniform effects from non-uniform capillary pressure and Marangoni stresses. The role of surfactants on the critical capillary number ($Ca_{cr}$) of droplet breakup is investigated for various confinements (defined as the ratio of droplet diameter to wall separation) and Reynolds numbers. For clean droplets,$Ca_{cr}$first decreases and then increases with confinement, and the minimum value of$Ca_{cr}$is reached at a confinement of 0.5; for surfactant-laden droplets,$Ca_{cr}$exhibits the same variation in trend for confinements lower than 0.7, but, for higher confinements,$Ca_{cr}$is almost a constant. The presence of surfactants decreases$Ca_{cr}$for each confinement, and the decrease is also attributed to the reduction in average interfacial tension and non-uniform effects, which are found to prevent droplet breakup at low confinements but promote breakup at high confinements. In either clean or surfactant-laden cases,$Ca_{cr}$first remains almost unchanged and then decreases with increasing Reynolds number, and a higher confinement or Reynolds number favours ternary breakup. Finally, we study the collision of two equal-sized droplets in a shear flow in both surfactant-free and surfactant-contaminated systems with the same effective capillary numbers. It is identified that the non-uniform effects in the near-contact interfacial region immobilize the interfaces when two droplets are approaching each other and thus inhibit their coalescence.

Two touching spherical drops in a uniaxial compressional flow: The effect of interfacial slip

This study presents a semi-analytical solution for the problem of two touching drops with slipping interfaces pushed against each other in a uniaxial compressional flow at low capillary and Reynolds numbers. The jump in the tangential velocity at the liquid-liquid interface is modeled using the Navier slip condition. Analytical solutions of the contact force, the drop-scale stresses, and the drop-scale pressure are provided as functions of the slip coefficient α, the viscosity ratio κ, and the drop size ratio k. Since unequal drop sizes are considered, two problems are solved in the tangent sphere co-ordinate system to determine the steady state position: a pair of touching drops with its contact point at the origin of an axisymmetric straining flow, and two touching drops placed in a uniform flow parallel to the axis of symmetry of the drops. A general observation is that the effect of slip is manifested most strongly for drops whose viscosity is much greater than the suspending fluid κ≫1. For highly viscous drops, the flow and stress fields transition from those corresponding to solid particles for ακ ≪ 1, to those for inviscid drops in the limit ακ ≫ 1. The analytical expressions provided here for the contact force and the stress distributions will serve to provide the restrictions that complete the definition of the lubrication flow problem in the thin film between the two colliding drops. While the contact force that drains fluid out of the thin film is relatively unaffected by slip, the tangential stress and pressure in the near-contact region are mitigated significantly for ακ ≫ 1. The latter is expected to assist coalescence at high capillary numbers.

Electric and adhesion properties of an interface between Sn1 − x Mn x Te single crystals and Bi–Sn alloys

The adhesion and electric properties of an interface between Sn1 − xMnxTe single crystals and a 57 wt % Bi and 43 wt % Sn alloy in a temperature range of ∼77–300 K are studied. It is shown that the Bi–Sn alloy and the above single crystals form an ohmic contact that exhibits fairly high work of adhesion and strength of adhesion, along with low contact resistance. The deposition of the Bi–Sn alloy on the end faces of the crystals results in the formation of such intermediate phases as Bi2Te3 and SnTe at the interface, the doping of the near-contact region of the crystal, and the filling of vacancies in the tin sublattice in this region with diffusing atoms of the contact alloy components.

Method of separate determination of high-ohmic sample resistance and contact resistance

A method of separate determination of two-pole sample volume resistance and contact resistance is suggested. The method is applicable to high-ohmic semiconductor samples: semi-insulating gallium arsenide, detector cadmium-zinc telluride (CZT), etc. The method is based on near-contact region illumination by monochromatic radiation of variable intensity from light emitting diodes with quantum energies exceeding the band gap of the material. It is necessary to obtain sample photo-current dependence upon light emitting diode current and to find the linear portion of this dependence. Extrapolation of this linear portion to the Y-axis gives the cut-off current. As the bias voltage is known, it is easy to calculate sample volume resistance. Then, using dark current value, one can determine the total contact resistance. The method was tested for n-type semi-insulating GaAs. The contact resistance value was shown to be approximately equal to the sample volume resistance. Thus, the influence of contacts must be taken into account when electrophysical data are analyzed.

S1620105 ニアコンタクト領域での浮上ヘッドスライダの剛性・減衰特性

The slider/disk spacing of magnetic disk drives has been reduced to less than 2nm by using the thermal flying height control system in order to increase the recording density. Thus, the slider/disk interface has come to the near contact phase where the contact of both surface frequently occurs. It is important to improve the damping characteristics for suppressing the vibration caused by the slider/disk contact. A dynamic analysis method, which employs perturbation technique and is applicable in the near-contact region, is introduced. We applied the method to investigate the effects of recesses, which are arranged on the air bearing surface, on the improvement of damping characteristics.

Modification of the Van der Pau method for measuring electrophysical parameters of high-resistance semiconductors

It is shown that the Van der Pau method can be used under illumination of the sample near-contact region with monochromatic radiation of variable intensity with a quantum energy that exceeds the investigated-material band gap. A modified Van der Pau technique for measuring such high-resistance semiconductor parameters as the electrical resistivity and the free-carrier concentration and mobility is proposed. This technique was tested on a semi-insulating n-type gallium arsenide sample with a free-electron concentration at room temperature of ∼107 cm−3.

Binding and interlayer force in the near-contact region of two graphite slabs: Experiment and theory

Via a novel experiment, Liu et al. [Phys. Rev. B 85, 205418 (2012)] estimated the graphite binding energy, specifically the cleavage energy, an important physical property of bulk graphite. We re-examine the data analysis and note that within the standard Lennard-Jones model employed, there are difficulties in achieving internal consistency in the reproduction of the graphite elastic properties. By employing similar models which guarantee consistency with the elastic constant, we find a wide range of model dependent binding energy values from the same experimental data. We attribute some of the difficulties in the determination of the binding energy to: (i) limited theoretical understanding of the van der Waals dispersion of graphite cleavage, (ii) the mis-match between the strong bending stiffness of the graphite-SiO2 cantilever and the weak asymptotic inter-layer forces that are integrated over to produce the binding energy. We find, however, that the data do support determination of a maximum inter-layer force that is relatively model independent. We conclude that the peak force per unit area is 1.1 ± 0.15 GPa for cleavage, and occurs at an inter-layer spacing of 0.377 ± 0.013 nm.