Free Surface State Research Articles

Numerical Simulation of the Instability of a Non-Isothermal Liquid Film in Film Devices

This work presents results of numerical simulation of the instability of vertical liquid film flow at moderate Reynolds numbers in the framework of partial differential equation of the state of the free surface of the liquid film. Equation coefficients include different physicochemical factors such as surface tension parameter and Marangoni parameter. Python scripts are used for computation of wave characteristics, instability regions of liquid films and liquid films free surface state. Computational experiments were carried out for the vertical water and alcohol films. Results could be used in the design or modernization of existing equipment and in the design of technological processes in liquid films.

Giant Zeeman-type spin splitting of free electron/hole states on quasi-2D perovskite niobates: a theoretical prediction

We study the spin-orbit interaction of two-dimensional electron/hole gas (2DEGs/2DHGs) on quasi-2D potassium niobates (KNs) via first-principles calculations. The strong surface polarity changes the free surface states from 2DEGs to 2DHGs. The in-plane dipole maintained on 2D models leads to giant Zeeman-type spin splitting, as high as 566 meV for the (001)c facet KN and 1.21 eV for the (111)c facet KN. The thickness-dependent Zeeman-type spin splitting shows a linear relation with respect to 1/r, while the corresponding in-plane polarization quantum has a linear relation of 1/(2^0.5)with respect to a decrease in thickness. Interestingly, the 2DHGs with molecular-like orbital character is solely constituted by O 2p states, showing logic switchable behavior at extremely thin samples with enormous Zeeman-type splitting that can switch between insulator and conductor by opposite spin polarization.

Nonlinear Development of the Marangoni Instability in Liquid Films

A nonlinear mathematical model of the state of the free surface of a nonisothermal liquid film is presented. The basic wave characteristics and the liquid film free surface state have been calculated for moderate Reynolds numbers in the presence of Marangoni instability. A nonlinear parabolic equation for the amplitude of the envelope of wave train and the results of computational experiment on the nonlinear development of Marangoni instability in liquid films are presented.

Spin to Charge Conversion at Room Temperature by Spin Pumping into a New Type of Topological Insulator: α-Sn Films.

We present results on spin to charge current conversion in experiments of resonant spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) α-Sn. By angle-resolved photoelectron spectroscopy (ARPES), we first check that the Dirac cone (DC) at the α-Sn (0 0 1) surface subsists after covering Sn with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into α-Sn layers induces a lateral charge current that can be ascribed to the inverse Edelstein effect by the DC states. Our observation of an inverse Edelstein effect length much longer than those generally found for Rashba interfaces demonstrates the potential of TIs for the conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of the TI for spin to charge conversion and the conditions to reach it.

Enhanced efficiency of dye-sensitized solar cells based on Mg and La co-doped TiO2 photoanodes

The efficiency of Dye-Sensitized Solar Cells (DSSC) can be significantly enhanced by optimizing the concentration of surface trap states and by tailoring the band gap energy in the semiconductor oxides used as photoanodes. Magnesium (Mg) and Lanthanum (La) ions have been co-doped onto TiO2 and used for the fabrication of photoanodes for DSSC. The doped TiO2 particles were characterized by SEM, XRF, XRD, Raman, XPS and UV–vis diffuse reflection spectroscopy. The fabricated cells were characterized by I-V and EIS techniques. XRD and Raman measurements performed on the doped powders showed only peaks ascribed to pure anatase. The particle sizes were significantly reduced by doping with the double metal ions, La and Mg, as calculated by the Scherrer equation using the anatase XRD 101 peaks. Current - voltage measurements on the fabricated DSSC presented maximum photoelectric conversion efficiency (PCE) of 8.04% from cells with 0.5mol.% Mg and La co-doped TiO2 anodes (0.5MgLa-TiO2), which gave a 20% improvement over cells with pure TiO2 anodes that had 6.7% PCE. The DSSC performance reduced gradually to 6.67 and 5.45 as the concentration of the co-dopants, Mg and La increased to 1 and 2mol.% respectively. DSSC with 1mol.% Mg doped TiO2 photoanodes (1Mg-TiO2) and 1mol.% La doped TiO2 photoanodes (1La-TiO2) reached 6.8% and 7.6% PCE, respectively, lower than that of 0.5MgLa-TiO2. EIS analysis showed that the DSSC with co-doped TiO2 anodes indicated an overall stronger resistance to charge recombination with the electrolyte than those with the single doped and pure TiO2 photoanodes. 0.5MgLa-TiO2 was found to be the optimum concentration for DSSC efficiency with reduced charge recombination. This is attributed to the elevation of free surface state energy and the presence oxygen vacancy (trap states) as compared to pure TiO2.

Deep Traps and Parasitic Effects in Al0.25Ga0.75N/GaN/SiC Heterostructures with Different Schottky Contact Surfaces

Hysteresis phenomenon in the capacitance-voltage characteristics under reverse-biased Schottky gate has been investigated for Al0.25Ga0.75N/GaN/SiC structures having three different gate surfaces. This parasitic effect was correlated with the presence of deep levels in our samples. Indeed, we have noticed the presence of two traps named H1 and A1; their respective activation energies, which are determined using capacitance deep level transient spectroscopy (DLTS) are respectively 0.74 and 0.16 eV. The H1 hole trap was associated to extended defect in the Al0.25Ga0.75N/ GaN heterostructure such as threading dislocations and was responsible of capacitance hysteresis phenomenon. The A1 electron trap appears only in the HEMT (1), which has the smaller Schottky contact area. This trap was related to a punctual defect and attributed to free surface states in the access region between the gate and the source.

Probing the spatial and momentum distribution of confined surface states in a metal coordination network.

The Shockley surface state on Cu(111) reacts sensitively to the perturbation by molecular adsorbates on the surface. In the porous structure of a metal-coordinated molecular network on Cu(111), the surface state is confined to a series of discrete states. Energy and momentum of eigenstates in the pores are related to both the energy dispersion of the free surface state and the geometric and energetic details of the confining barrier formed by the molecular network. The penetration of the confined state into the barrier is found to be sensitive to the constituting architectural elements.

A mathematical model for unsteady mixed flows in closed water pipes

We present the formal derivation of a new unidirectional model for unsteady mixed flows in nonuniform closed water pipes. In the case of free surface incompressible flows, the FS-model is formally obtained, using formal asymptotic analysis, which is an extension to more classical shallow water models. In the same way, when the pipe is full, we propose the P-model, which describes the evolution of a compressible inviscid flow, close to gas dynamics equations in a nozzle. In order to cope with the transition between a free surface state and a pressured (i.e., compressible) state, we propose a mixed model, the PFS-model, taking into account changes of section and slope variation.

Features of the stress field at the surface of a flush shrink-fit shaft

The state of stress induced by a shrink-fitted circular shaft in an elastically similar half-space, positioned so that the end of the shaft is flush with the remainder of the free surface, is studied. At deep interior points the classical plane strain (Lamé) solution obtains, and the transition to the free surface state is found. It is found that a residual interfacial axial shear develops and the coefficient of friction needed to ensure adhesion along the interface is found. The effect of slip when a lower coefficient of friction is present is also found as a solution to an integral equation.

Impact of free-surface passivation on silicon on insulator buried interface properties by pseudotransistor characterization

It has been reported previously [N. Bresson et al., Proceedings of the ECS Seventh International Symposium on Silicon-on-Insulator Technology and Devices, 2005 (unpublished), pp. 317–324; F Allibert et al., Proceedings of the IEEE International SOI Conference, Honolulu, HI, 2002 (unpublished)] that the film thickness strongly impacts the parameters extracted using the pseudo metal oxide semiconductor field effect transistor (pseudo-MOSFET) with the usual FET equations [S. Cristoloveanu and S. S. Li, Electrical Characterization of SOI Materials and Devices (Kluwer, Boston, MA, 1995)]. In this paper, we investigate the influence of top free-surface states on the pseudo-MOSFET characteristics by comparing passivated versus nonpassivated samples. The parameters of concern, investigated here, are carrier mobility, density of interface states, threshold (VT), and flatband (VFB) voltages. Based on systematic measurements and existing models [H. J. Hovel, Solid-State Electron. 47, 1311 (2003)] for VT, VFB, and subthreshold slope, we show how the free-surface impact increases as the film thickness is reduced. Comparison of extracted results with simulated curves demonstrates that, in state-of-the-art ultrathin silicon on insulator structures, the preparation and properties of the free surface are no longer negligible.

Electrostatics of conducting nanocylinders

This article calculates the capacitance and transverse polarizability of nonideal conducting nanocylinders. It employs a semiclassical theory that characterizes the cylinders with a nonzero screening length ℓ and a dielectric constant ε≠1. Cylinders both with and without free surface states are considered. The results should be relevant to cylinders with a quasicontinuous distribution of energy levels such as nanowires and nanotubes. The theory yields very acceptable agreement with random phase approximation calculations for the radius dependence of the polarizability of single-wall carbon nanotubes. The same type of theory has already been applied to the capacitance of nanospheres, and in a companion article is applied to static screening by nanospheres.

Effect of plasma treatment on the density of defects at an amorphous Si:H-insulator interface

Interface state density between a-Si:H and an insulating film (a-SiN1.7:H or a-SiO2.0) was measured by photothermal deflection (PD) spectroscopy and electron spin resonance. While the interface state density in a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density on a-Si:H, that in a-SiO2.0:H on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens was discussed in terms of plasma surface reaction. The effect of plasma reaction was examined by treating the surface of the a-Si:H layer by the plasma of NH3 or N2O gas which were dominant constituents of the source gases used to deposit the insulating layers. The PD spectral shape of a-SiO2.0 on a-Si:H was similar to N2O plasma-treated a-Si:H and that of the a-SiN1.7:H on a-Si:H structure was similar to NH3 plasma-treated a-Si:H. These results indicate that the interface defects in the a-SiO2.0 on a-Si:H structure were induced by the plasma reaction of the source gas with the surface of a-Si:H at the initial stage of deposition. The interface state densities in a-Si:H on a-SiN1.7:H and a-Si:H on a-SiO2.0 interfaces were smaller than those of a-SiN1.7:H on a-Si:H and a-SiO2.0 on a-Si:H, respectively. These differences in the interface defect density were due to the difference in the precursor used to deposit the upper layer. Chemical reaction on the surface of a-Si:H with a source gas induces interface defects at the initial stage of the deposition of the insulating layer. This surface chemical reaction was investigated by varying the condition of plasma species at the surface of a-Si:H.

Study of the photodissociation of a CdSe nanocrystal beam by means of photoluminescence and Raman scattering

We developed an apparatus that enables us to perform optical measurements of nanocrystals suspended in vacuum. CdSe nanocrystals were produced by a gas evaporation method, and nanocrystal beams were then formed using an inert-gas flow with differential pumping. We measured photoluminescence spectra of the nanocrystal beams with excitations of various photon energies and powers. For a low excitation power, edge emission of the CdSe nanocrystal beam was observed. With increase of the laser power, Raman lines of Se dimers emitted due to the photodissociation of CdSe nanocrystals were observed. It was found that the thresholds of the excitation laser fluence for the photodissociation of CdSe nanocrystals were much smaller than the thresholds of laser fluence for the laser-induced emission of Se atoms from bulk CdSe. The electronic process is dominant in the photodissociation of CdSe nanocrystals whose surfaces are completely free. We suggest that the effective supply of carriers confined in nanocrystals to the surface state, and the excitations of the completely free surface state induce the photodissociation of nanocrystals by excitation with much weaker irradiation than is required for dissociation of the bulk surface.

Order of the singular stress fields of through-thickness cracks

Using the principle of superposition, the singular stress fields of the mode I through-thickness cracks are obtained by combining the following two singular stress solutions. The first singular stress solution is in the form of r-1/2g(θ, z) in cylindrical coordinates system (r, θ, z). It can be evaluated by Hartranft's three-dimensional crack solution. Based on the loading similarity, the second singular stress solution is in the form of R-1/2f (θ, ϑ) in spherical coordinates system (R, θ, ϑ) and ν is the Poisson's ratio. A full three-dimensional finite element analysis without imposing any assumption on the order of singularity is executed to check the assumptions. We show that the solution can not be simply separated either in cylindrical or spherical coordinates, but rather, a combination of them. The variation of stress intensity factors in the vicinity of the free surface perpendicular to the crack plane from free surface state to plane strain state is also discussed.

Effect of Plasma Damage on Interface State Density Between a-Si:H and Insulating Films

ABSTRACTThe interface state density between a-Si:H and insulating film is very important in characteristics of a-Si:H thin film transistors. In this study, the interface state density was measured by photothermal deflection spectroscopy (PDS). Layered structures of a-SiN, 1.7:H on a-Si:H and a-SiO2.0 on a-Si:H were deposited by P-CVD method. The a-SiN1.7:H layer was grown from a gas mixture of SiH4 and NH3 and the a-SiO2.0 layer was grown from a gas mixture of SiH4 and N2O. While the interface state density of a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density of a-Si:H, that of a-SiO2.0 on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens is discussed in terms of plasma damage of a-Si:H surface by the source gases during deposition of insulating layer. Surface of the a-Si:H was treated by plasma of NH3 or N2o gas which is dominant constituent of source gases of insulating layer. Although the surface state density of the N2o plasma-treated sample increases, that of NH3 plasma treated sample does not increase. The shape and intensity of the spectra of N2o plasma-treated sample is similar to that of the a-Sio2.0 on a-Si:H structure. These results indicate that the interface defect between a-Sio2.0and a-Si:H layer was induced by plasma damage of the a-Si:H surface.

Surface-depletion effect correction to nonuniform carrier distributions by Hall measurements

Significant errors in the determination of carrier profiles obtained by the Hall-effect method result if the depletion effect at a semiconductor surface is not taken into account. A practical procedure for correcting the apparent measured carrier profiles for this surface-depletion effect is described for nonuniformly doped semiconductors. This correction method is then applied to Si-implanted GaAs and the results are compared with those of capacitance-voltage measurements. The number of total trapped charges in free-surface states are estimated to be about 1×1012 cm−2.