Structural and dc conduction studies on PbTe thin films

Lead Telluride (PbTe) films of different thickness were prepared onto precleaned glass substrates under the pressure of 2?×?10^?5 Torr by thermal evaporation. X-ray diffraction technique, scanning electron microscopy, and current–voltage characteristics were used to characterize the films. The structural analysis of the films was carried using X-ray diffractometer. The surface morphology was analyzed by using scanning electron microscope. The dc electrical conduction mechanism in vacuum-evaporated Al/PbTe/Al thin film sandwich system in the thickness range 500–5,000 Å at different temperature (303–483 K) was found to be a modified Poole–Frenkel type. The results of variation of activation energy with applied voltage and thickness are discussed.     

The kinetics of the formation of a solid solution in an Ag–Pd polycrystalline film system

The kinetics of homogenization of an Ag–Pd film system with a total thickness of 120 nm and a grain size of 5–10 nm has been studied by means of in situ TEM heating. The film system has been formed by the sequential deposition of components in a vacuum on the substrate at room temperature. It has been shown that diffusion processes are activated, starting from the temperature 453 K, resulting in complete homogenization of the film system at 573 K with preservation of its fine-grained structure. The effective diffusion coefficient in the Ag–Pd system was measured as 10^?17–10^?18 m²/s at 553 K. A possible mechanism of homogenization is discussed.     

飞秒激光热反射系统测量金属薄膜中的热波

金属材料中的热波现象可以利用包含电子弛豫时间影响的双曲两步模型进行理论分析.通过飞秒激光热反射实验系统对金属薄膜材料进行了测量.利用偏振分光棱镜将飞秒激光分成抽运光和探测光,其中较强的抽运光用于加热金属薄膜而较弱的探测光用于探测薄膜表面反射率随时间的变化,两束光之间的光程差通过步进电机进行精确控制.利用金属薄膜反射率和电子温度的正比例关系就可以得到电子温度随时间的变化规律.实验发现在加热激光脉冲过后的电子温度下降区间会出现另一个较弱的电子温度波峰,并利用相同厚度的两块金属薄膜样品重复测量对实验结果进行了验证.理论上这一现象可以解释为金属薄膜中热波在背面反射的结果,并且实验结果和双曲两步模型给出的热波理论计算结果相符合.根据实验结果计算出热波传递速度约为5×105m/s,对应的电子弛豫时间为60fs.     

Nanoprecipitation resulting from the decomposition of supersaturated solid solutions in the tracks of swift heavy ions

The electron subsystem of a material is strongly excited when swift heavy ions pass through the material. The subsequent relaxation of this excitation results in considerable short-term (<10⁻⁹ s) heating of the material in the nanometer vicinity of the projectile trajectory. Nanoprecipitation stimulated by such thermal spikes in supersaturated solid solutions is studied. Nanoprecipitates are shown to form when the temperature in the track reaches a point where the characteristic time of precipitation becomes shorter than the time of cooling of the track. The region of most efficient precipitation may be offset from the track axis. The initial cylindrical nonuniformity of the spatial density of nucleating clusters may cause the formation of nanodimensional tubular heterostructures extended along the trajectory of the heavy ions. The parameters of the system that are the most favorable to the tubular mode of precipitation are found.     

Optical limitation of Mid-IR radiation in vanadium dioxide films

Theoretical and experimental results on the limitation of pulsed radiation in the spectral range 3.8–10.6 μm in vanadium dioxide films are presented. The effect of the film structure on the shape of the temperature hysteresis loop is studied. The film thickness and the structure of an interferometer with a vanadium dioxide film are optimized to improve the radiation limitation efficiency. The spatial dynamics of vanadium film switching under the action of a radiation pulse is investigated. It is shown experimentally that the radiation attenuation coefficient under the limitation conditions can be as high as 10⁴ or more.     

Electrostatic simulations of the radio frequency heating of a non‐neutral plasma in an electron trap

The electrostatic simulations of the radio frequency (RF) heating mechanism, excitations, and ionization process of an electron plasma are carried out using a two‐dimensional (2D) particle‐in‐cell (PIC) code. RF drives with excitation frequencies of 1–15 MHz and amplitudes of 5 and 10 V were applied at two different axial positions, to the centre and to one end on the electrode stack of the ELTRAP device, at ultra‐high vacuum conditions. It is observed that the axial kinetic energy (eV) profile of the confined electrons increases with an increase of the RF excitation amplitudes, and densities from 5 × 10⁷ to 10¹² m^?3 for all cases under consideration. The simulation results indicate that with continuous RF excitations, the electron heating in the beginning is higher at the trap wall of the device and extends towards the central region of the trap over a simulation time of up to 100 µs. These results on the electron heating are in good agreement with the experimental findings (optical diagnostics of ELTRAP). The heating effect is larger when the RF power is applied from the position close to one end of the trap in comparison to the central position. Monte–Carlo PIC simulations with hydrogen as a background gas are also performed to evaluate the ionization process at pressures of 10^?8, 10^?7, and 10^?6 torr using the same electron plasma densities. The results show that at increasing pressures, the electron‐neutral collisions rate increases linearly with the background gas pressure. Increased collision frequency is obtained at higher RF drive amplitudes, which proportionally increases electron temperature, so that more ionization and secondary electrons are generated.     

固体C60/Si异质结的电学表征——整流特性、能带模型与温偏效应

研究了非掺杂固体C₆₀与n-Si和与p-Si接触的电学性质,电流-电压(J-V)特性测量表明两种接触的导电极性相反,且都具有很强的整流作用,表明在两种接触界面附近存在着阻挡载流子输运的、性质不同的势垒.电流-温度(J-T)测量表明,电流与温度的倒数呈指数依赖关系,从中估算出C₆₀/n-Si和C₆₀/p-Si异质结的有效势垒高度分别为0.30和0.48eV.引进异质结的能带模型,成功地解释了上述测量结果,由能带模型和测量数据估算出以硅为衬底的 关键词：     

Improved structural ordering in sexithiophene thick films grown on single crystal oxide substrates

We report on sexithiophene films, about 150-nm thick, grown by thermal evaporation on single-crystal oxides and, as comparison, on Si/SiO₂. By heating the entire deposition chamber at 100°C we obtain standing-up oriented molecules all over the bulk thickness. Surface morphology shows step-like islands, each step being only one monolayer in height. The constant and uniform warming of the molecules obtained by heating the entire deposition chamber allows a stable diffusion-limited growth process. Therefore, the regular growth kinetics is preserved when increasing the thickness of the film. Electrical measurements on differently structured films evidence the impact of the inter-island separation region size on the main charge-transport parameters.     

Development of MWCNTs-based wideband photodetector in the visible range: wavelength and power-dependent response studies

Photosensitive detector has been reproducibly fabricated using MWCNTs random network-based film grown by chemical vapor deposition technique. For the first time, excellent photoconductive response is reported over the entire green to red band (508–680 nm) of the visible light with maximum efficiency. Wavelength-dependent studies as well as white light exposure confirm the achievement of significant response in the entire visible region which is in commensurate with the absorption spectra of the detection element. The other remarkable novelties of this work are its fast response and recovery time, excellent repeatability and no drift over a time span of 1 year. Besides, there is no local heating effect observed due to photon flux density up to 1,800 lux/m². The sensing mechanism is explained by interband transition of free carriers, and there is no heat-induced e–h pair generation. The results have opened up the possibility of reproducible fabrication of such device applicable for optical detector or CCD for visible light detection.     

Size effect of the longitudinal magnetoresistance of thin films of n- type Ge

In a study of the longitudinal magnetoresistance as a function of the magnetic field in samples of n-type Ge of various thicknesses at 300°K and 77°K, the surface of the samples was oriented parallel to the (001) plane. The directions of the current and the magnetic field were along either the <100> direction or the <110> direction. As the thickness of the samples was reduced, regardless of the orientation of the current and the magnetic field with respect to the crystallographic directions, a decrease was observed in the magnetoresistance over the entire range of magnetic fields studied, 1–280 kOe. The experimental results are attributed to a screening-length size effect and to “sliding” orbits in the enriched surface layers, which lead to an increase in the electron mobility in a strong magnetic field.     

Struktur und Widerstand sehr stark gestörter Kupferschichten bei tiefen Temperaturen

Copper and silver films were condensed with admixtures of SiO, LiF, Cu₂O and Fe onto a substrate at 4? K. For a film with 16 atomic-% SiO, electron diffraction pictures at 4? K show a nominal crystallite size of 7 å. This means that, even over small regions, there is no well defined lattice order. Such films have resistivities about 1500 times higher than that of the bulk metal at room temperature. The resistivity decreases on heating. Its temperature coefficient is negative also when the film is cooled again. The films are not so strongly disordered if only 9 atomic-% SiO is admixed. In addition, a dependence on evaporation temperature has been observed. LiF and Cu₂O produce less disorder than SiO. Different interpretations of these observations are discussed. The results with Fe show that solid solutions of high concentration can be obtained by condensing films onto very cold substrates, even when the solution components are only slightly soluble in one another at thermal equilibrium.     

Electronic excitation of the matrix during drift of excess electrons through solid xenon

It is shown experimentally that the exciton luminescence λ=172 nm) quantum yield excited by excess electrons drifting through solid xenon at 77 K in fields of 10 kV/cm amounts to 20±5 per electron and that luminescence takes place during the entire drift process. A CW bulky discharge through solid xenon (with a current up to 20 A/cm²) is realized, and intense visible luminescence due to excitation of impurities by electron impacts is observed. The prospects for using solid rare gases as matrices for studying processes in low-temperature plasmas and for creating effective electric energy converters in the vacuum ultraviolet range are discussed.     

Formation of ultrasmall germanium nanoislands with a high density on an atomically clean surface of silicon oxide

The experimental results of an investigation into the initial stages of growth of a germanium film on an atomically clean oxidized silicon surface are reported. It is shown that the growth of the germanium film in this system occurs through the Volmer-Weber mechanism. Elastically strained nanoislands with a lateral size of less than 10 nm and a density of 2 × 10¹² cm^?2 are formed on the oxidized silicon surface. In germanium films with a thickness greater than 5 monolayers (ML), there also arise completely relaxed germanium nanoislands with a lateral size of up to 200 nm and a density of 1.5 × 10⁹ cm^?2.     

Droplet migration: Quantitative comparisons with experiment

An important practical feature of simulating droplet migration computationally, using the lubrication approach coupled to a disjoining pressure term, is the need to specify the thickness, H^*, of a thin energetically stable wetting layer, or precursor film, over the entire substrate. The necessity that H^* be small in order to improve the accuracy of predicted droplet migration speeds, allied to the need for mesh resolution of the same order as H^* near wetting lines, increases the computational demands significantly. To date no systematic investigation of these requirements on the quantitative agreement between prediction and experimental observation has been reported. Accordingly, this paper combines highly efficient Multigrid methods for solving the associated lubrication equations with a parallel computing framework, to explore the effect of H^* and mesh resolution. The solutions generated are compared with recent experimentally determined migration speeds for droplet flows down an inclined plane.     

Generation of uniform atmospheric pressure argon glow plasma by dielectric barrier discharge

In this paper, atmospheric pressure glow discharges (APGD) in argon generated in parallel plate dielectric barrier discharge system is investigated by means of electrical and optical measurements. Using a high voltage (0–20 kV) power supply operating at 10–30 kHz, homogeneous and steady APGD has been observed between the electrodes with gap spacing from 0.5 mm to 2 mm and with a dielectric barrier of thickness 2 mm while argon gas is fed at a controlled flow rate of 1 l/min. The electron temperature and electron density of the plasma are determined by means of optical emission spectroscopy. Our results show that the electron density of the discharge obtained is of the order of 10¹⁶ cm^???3 while the electron temperature is estimated to be 0.65 eV. The important result is that electron density determined from the line intensity ratio method and stark broadening method are in very good agreement. The Lissajous figure is used to estimate the energy deposited to the glow discharge. It is found that the energy deposited to the discharge is in the range of 20 to 25 μJ with a discharge voltage of 1.85 kV. The energy deposited to the discharge is observed to be higher at smaller gas spacing. The glow discharge plasma is tested to be effective in reducing the hydrophobicity of polyethylene film significantly.     

Thickness dependent magnetic and structural properties of Co films grown on GaAs (100)

The structural, magnetic and transport properties measurements carried out on Co thin films deposited by electron beam evaporation on GaAs substrate as a function of layer thickness ranging from 50 Å to 1000 Å are presented here. Structural measurements show the film to be amorphous in nature at lower thickness which becomes crystalline at higher thickness. Magnetic measurements show an increase in saturation magnetization (M_S) with film thickness. M_S values are found to vary from 521 emu/cm³ to 1180 emu/cm³ for thicknesses ranging from 50 Å to 1000 Å. The coercivity and saturation field value shows a systematic decrease up to 600 Å thickness and increase thereafter. Various microstructural parameters were also calculated using GIXRR technique. A clear grain growth is observed in AFM technique with film thickness and its influence on transport properties was also seen. Different surface morphology and magnetic domain structures were obtained on different thin film samples by AFM and MFM techniques, respectively. XPS measurements reveal formation of CoAs phase at the interface between Co and GaAs. All these results are discussed and interpreted in detail in this communication.     

Atomic and electron structure of reconstructed (111) surface in ZnSe and CdSe crystals

The atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 Å in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.

     

Measurement of the impedance of very thin superconducting YBaCuO films at 10 GHz with a cavity perturbation method between 4 and 300 K

We have measured the complex film impedance 1/σd (σ conductivity, d film thickness) of three YBaCuO thin films with d = 44, 115, and 168 nm on MgO substrates at 10.2 GHz in the temperature range between 300 and 4 K. Below T_c, the experimental results are discussed in terms of the two-fluid model and the BCS theory. The residual resistance decreases with the film thickness. The thinnest film has a residual surface resistance of 3 · 10^?4 Ω. For this film, the complex microwave conductivity is calculated and compared with the models. Apart from the residual resistance, the measured conductivity is in agreement with the peak caused by the energy gap of the BCS theory. All measurements were performed with a cavity perturbation method which we have to our knowledge applied for the first time to superconducting thin films. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thicknesses small compared to the skin depth δ or the London penetration depth λ can be measured. Therefore, we are able to measure the impedance both in the normal and superconducting state.     

Study on the temperature distribution of heated falling liquid films

A 2-D theoretical model was derived to present the temperature distribution of falling liquid films flowing over a vertical heated/cooled plate with constant temperature. And the temperature gradients for different flow rates and fluids were also discussed for different liquid films. The temperature distributions for liquid films of water, ethanol aqueous solutions and glycerol aqueous solutions were experimentally investigated with a sensitive thermal imaging system. It is found that the surface temperature of a film flowing over a vertical heated solid plate has a characteristic relationship with the film flow distance. A lower flow rate of the film or a higher temperature of the wall generally leads to a higher surface temperature in the film inception. For films of glycerol aqueous solutions under the same heating conditions, a lower glycerol concentration causes a higher surface temperature of the film, due to the decrease of the liquid viscosity, whereas the ethanol concentration is found to have little influence on the temperature distribution of the film surface. Comparisons of the experimental data and the theoretical model show that the model can adequately describe the surface temperature distribution of a heated falling liquid film.