范德堡方法在ZnO薄膜测试中的应用

近年来,随着对宽禁带半导体材料,氧化锌薄膜研究的快速发展,对其电学性质的研究也显得尤为重要。主要介绍范德堡方法在ZnO薄膜电学性质测量中的应用,并对初步测量结果作简要的讨论。     

Van der Pauw Hall Measurement on Intended Doped ZnO Films for p-Type Conductivity

A Van der Pauw Hall measurement is performed on the intended doped ZnO films （Na doped ZnO） grown by using the molecular beam epitaxial method. All as-grown samples show n-type conductivity, whereas the annealed samples （annealing temperature 900℃） show ambiguous carrier conductivity type （n- and p-type） in the automatic Van der Pauw Hall measurement. A similar result has been observed in Li doped ZnO and in as-doped ZnO films by other groups before. However, by tracing the Hall voltage in the Van der Pauw Hall measurement, it is found that this alternative appearance of both n- and p-type conductivity is not intrinsic behavior of the intended doped ZnO films, but is due to the persistent photoconductivity effect in ZnO. The persistent photoconductivity effect would strongly affect the accurate determination of the carrier conductivity type of a highly resistive intended doped ZnO sample.     

Geometrothermodynamics of Van der Waals black hole

We study the geometrothermodynamics of a special asymptotically AdS black hole, i.e. Van der Waals \(\left( VdW\right) \) black hole, in the extended phase space where the negative cosmological constant \(\Lambda \) can be regarded as thermodynamic pressure. Analysing some special conditions of this black hole with geometrothermodynamical method, we find a good correlation with ordinary cases according to the state equation.     

Modified Van der Pauw Method of Measuring the Electrical Conductivity Tensor of Anisotropic Semiconductor Films

Russian Physics Journal - For a solution of the boundary electrodynamic problems, the paper proposes the original method of measuring the specific conductivity tensor components of anisotropic...     

Van der Pauw resistivity measurements on evaporated thin films of cadmium arsenide, Cd3As2

Cadmium arsenide is a II-V semiconductor, exhibiting n-type intrinsic conductivity with high mobility and narrow bandgap. It is deposited by thermal evaporation, and has shown the Schottky and Poole-Frenkel effects at high electric fields, but requires further electrical characterisation. This has now been extended to low-field van der Pauw lateral resistivity measurements on films of thickness up to 1.5 μm. Resistivity was observed to decrease with increasing film thickness up to 0.5 μm from about 3 × 10⁻³ Ω m to 10⁻⁵ Ω m, where the crystalline granular size increases with film thickness. This decrease in resistivity was attributed to a decrease in grain boundary scattering and increased mobility. Substrate temperature during deposition also influenced the resistivity, which decreased from around 10⁻⁴ Ω m to (10⁻⁵ to 10⁻⁶) Ω m for an increase in substrate deposition temperature from 300 K to 423 K. This behaviour appears to result from varying grain sizes and ratios of crystalline to amorphous material. Resistivity decreased with deposition rate, reaching a minimum value at about 1.5 nm s⁻¹, before slowly increasing again at higher rates. It was concluded that this resulted from a dependence of the film stoichiometry on deposition rate. The dependence of resistivity on temperature indicates that intercrystalline barriers dominate the conductivity at higher temperatures, with a hopping conduction process at low temperatures.     

Application of the van der Pauw method to conductivity measurements on mixed ionic-electronic solid conductors

The analysis of four point dc conductivity measurements by the van der Pauw method is generalized to mixed ionic-electronic solid conductors (MSC) with gas electrodes, e.g. doped and reduced CeO₂ in a controlled oxygen atmosphere. If such measurements are done with non-blocking voltage probes, the electronic and ionic currents at the probes do not vanish. The measurement then yields the total conductivity even if the current carrying electrodes are blocking for ions or electrons. By imposing a difference in the gas pressure at the two reversible voltage probes the ionic conductivity can be determined. The currents through the probes are calculated. If the probes are not reversible, then there exist voltage drops on the probes and the four point measurement cannot be used in the above manner to yield the conductivity. To overcome the difficulty that arises with non-reversible probes a four-point measurement with three blocking electrodes (two of which are the voltage probes) is suggested and analyzed. When the electrodes block ions then the electronic conductivity σ_e is measured and when they block electrons, the ionic conductivity σ_X⁺ is determined.     

Van der Waals interaction between an atom and a surface defect

A continuum dielectric theory is used to investigate the Van der Waals interaction between an atom and a point defect on a planar surface. A vacancy (or adsorbed atom) is modelled by a hemispherical pit (or bump) which perturbs the dielectric response of the defect-free surface. The anisotropy of the interaction is found to be qualitatively different for the vacancy and adsorbate situations. The estimated magnitude of the interaction seems consistent with recent experimental results.     

The Van der Waals interaction between an atom and a solid

Measurements of the scattering of neutral atoms by metal cylinders have indicated a serious discrepancy between theory and experiment for the strength of the long-range Van der Waals interaction. To clarify the content of the theory, we present here a formal analysis of the Van der Waals interaction between an atom and a solid, based on the approach of McLachlan. A key quantity in the theory is the response of the solid to an external, time-dependent dipole. This response is amenable to classical analysis, and we calculate it for a variety of model systems. When the solid may be treated as a continuum with a flat surface, the concept of surface impedance allows a convenient parameterization and we may incorporate thereby magnetic effects, non-local dielectric response, and the influence of finite layers in the solid. We also study the modifications induced by a rough surface, finding the average change in the coupling strength in the presence of weak roughness.     

Van der Waals force derived from a quantum statistical mechanical path integral method

The main purpose of this paper is to apply the quantum statistical mechanical model for polarizable fluids, used by Høye-Stell and others, to the calculation of the van der Waals force (dispersion force) between monatomics. The results are in agreement with those obtained by quantum perturbation theory. The quantum statistical method implies considerable formal simplifications.The introductory sections of the paper show, for illustrative purposes, how the dispersion forces in limiting cases also can be obtained from macroscopic quantum electromagnetic theory when two dilute dielectric media with plane-parallel surfaces are separated by a vacuum (Casimir effect).     

范德瓦尔斯方程的几点探析

分析了学生在范德瓦尔斯方程学习中容易产生误解或者忽视的几个重要问题,以加强学生对范德瓦尔斯方程的准确理解.     

Van der waals constants of rare gases in an interpolation model

The possibility is investigated of determining the dispersion constants for pairs of rare-gas atoms directly from the homoatomic and heteroatomic interaction potentials. A simple interpolation model is proposed which makes it possible to find the constant C₆ from the known values of the potentials close to the minimum point. Calculations of the van der Waals constants C₆ performed using the potentials obtained by different authors demonstrated the good accuracy of the model and a definite correlation between the accuracy of the calculated van der Waals constants and the closeness to experiment of the pair potential employed in the model. Kabardino-Balkarian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 70–73, August, 1997.     

Power delivered by an array of Van der Pol oscillators coupled to a resonant cavity

An array of Van der Pol oscillators coupled to an RLC load is considered both theoretically and experimentally. It is found that the oscillators are active when the capacitance of the capacitor coupling the array to the load is below a critical value increasing with the number of oscillators. The power delivered to the load by the array of active oscillators increases with the number of oscillators till a limiting value increasing with the quality factor of the load. Good agreement is obtained between the theoretical and experimental results.     

Influence of Noise on Mode Change in a Van der Pol System

In this paper we study the additive influence of white noise on the position of bifurcation curves in the parameter plane of a Van der Pol system.     

Self-diffusion in Kr compared with Van der Waals model

The predictions of the Van der Waals model for the self-diffusion in Kr are compared with the experimental results at 220 K and for 0.7ρ_c ? ρ ? 2.1ρ_c. The Van der Waals model fails to represent the Kr data while at the same temperature in the case of CH₄ this model works quite well for ρ_c < ρ ? 3ρ_c.     

Anisotropic spheres with Van der Waals-type equation of state

We study static spherically symmetric space-time to describe relativistic compact objects with anisotropic matter distribution and derive two classes of exact models to the Einstein–Maxwell system with a modified Van der Waals equation of state. We motivate a Van der Waals-type equation of state to physically signify a high-density domain of quark matter, and the generated exact solutions are shown to contain several classes of exact models reported previously that correspond to various physical scenarios. Geometrical analysis shows that the physical quantities are well behaved so that these models may be used to describe anisotropic charged compact spheres.     

Regular and random dynamics of a Van der Pol oscillator with retarding feedback

A comparative analysis is made of the properties of certain mathematical models that have been proposed to describe the nonlinear dynamics of narrow-band oscillating systems with a lag. A modified model of a Van der Pol oscillator with a retarding feedback loop is studied numerically, and an analysis is made of the ways in which periodic motion in such systems becomes unstable and random oscillations are generated. A physical interpretation is given of the numerical results. Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 104–113, February, 1998.