Selective etching of dislocations in GaN and quantitative SEM analysis with shape-reconstruction method

Due to the differences in etch-pit morphologies, chemical etching offers a possibility to determine densities of dislocations in respect to their type. In the present paper we propose a method, which implements a simple shape-from-shading procedure, i.e. with results derived from image brightness dependence on surface slope. It allows estimation of etch-pit depth distributions from scanning electron microscopy micrographs. This method is used to obtain depth distributions from GaN surface after etching in molten KOH-NaOH eutectic mixture. Depth distributions are used to estimate densities of etch-pits related to a given dislocation type. The distributions are compared with dislocation densities determined with transmission electron microscopy.     

Local state density and the reflectivity of electrons at a crystal surface

Local densities of electron states at and near the surface of a semi-infinite solid are discussed and compared with the corresponding ones in the infinite solid. The reflectivity coefficients for electrons, incident from both sides on the surface, are shown to be closely related to these densities. Illustrating results are given for the one-dimensional model crystal. The surface Green function (SGF) method is used.     

The Fermi surface of [Formula: see text

The heavy-fermion compound [Formula: see text] has been studied using the fully relativistic spin-polarized mean muffin-tin orbital method within the local density approximation. Two separate calculations, one where the f electron is treated as a valence electron and the other where it is treated as part of the core, have been performed and the Fermi surface is obtained. The angular-dependent de Haas - van Alphen (dHvA) frequencies are calculated in both cases and they are compared with the experimental dHvA frequencies. We also calculated the electron momentum densities and compared them with the electron - positron momentum densities measured from the two-dimensional angular correlation of electron - positron annihilation radiation. The spin polarization of the Fermi surface is analysed and we present a new interpretation of the experimental data of Harrison et al.     

Extreme accumulation layers on ZnO surfaces due to He+ ions

Accumulation layers with excess surface electron densities of up to 10¹⁴ cm^?2 have been produced on the (001) face of ZnO single crystals by an electrical discharge in helium atmosphere. Hall effect measurements show that at these extreme surface electron densities both the electron density and the mobility are temperature independent from room temperature down to 1.6 K.     

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.     

大气压等离子体炬电子密度的光谱诊断

利用空心针-板放电装置产生了大气压等离子体炬,采用光谱法测量了其内部及表面的电子密度. 向空心针中通入氩气,在大气环境中产生了长度为1cm的等离子体炬.实验分别测量了Hα谱线和ArⅠ(696.54nm)谱线,通过反卷积方法分离出其相应的Stark展宽,并由此计算了电子密度.结果发现,采用Hα谱线和ArⅠ(696.54nm)谱线Stark展宽计算得到的等离子体的电子密度分别为1.0×10¹⁵cm^－3和3.78×10^{15关键词： 等离子体炬 电子密度 气体温度 Stark展宽}     

用LMTO方法对清洁Nb(100)面电子结构的研究

用LMTO-ASA方法计算了铌的体态密度和清洁Nb(100)表面的局域态密度以及有关的表面能。所计算的体态密度与用其它方法计算的态密度符合得很好,表面能则支持了低能电子衍射(LEED)动力学计算的结果,并且通过分析表面局域态密度,提出了清洁Nb(100)表面存在表面态,表面态位于费密能级的上方。 关键词：     

Plasmons in Finite Three-Tile Quasiperiodic Superlattices: On-Site Model

We investigate the plasmon spectrum of a finite semiconductor superlattice in which the quantum wells are located at periodic sites but the electron densities are arranged in a three-tile quasiperiodic sequence. It is found that the branching rule of the on-site model is different from that of the transfer model. The quasiperiodic arrangement of the electron densities results in a number of surface modes. The localization of the modes is discussed.     

Cyclotron resonance of two-dimensional electrons forming wigner crystal

The cyclotron resonance peak shape is analysed for two-dimensional electrons interacting with surface vibrations of a medium. Explicit expressions for the broadening and shift of the peak are obtained at low and high electron densities.     

Surface properties of electron-hole droplets in semiconductors

The leading gradient terms in the kinetic energy densities of the electrons and holes in Ge and Si type semiconductors are calculated including band structure effects. Band structure effects are found to increase these gradient terms substantially over those for isotropic bands. The surface properties of electron—hole drops in Ge are studied using these gradient terms with several local energy densities, and band structure effects in the gradient terms are shown to increase the surface thickness, surface energy and dipole barrier over isotropic band results. The difference in the chemical potentials of the electrons and holes obtained from these approximate energy functionals indicates that the drop becomes positively charged, which is opposite in sign to that proposed recently by Rice.     

Cyclotron dynamics of electron wave packets in topological insulators in an external magnetic field

The evolution of electron wave packets formed from surface states in topological insulators located in an external magnetic field is studied. The electron and spin densities of the wave packets under consideration are calculated analytically and are visualized. The influence of the main characteristics of the wave packets (the spin polarization) on the splitting and the change in their forms with time is considered.     

Measurements of vacuum field emission from bio-molecular andsemiconductor-metal eutectic composite microstructures

Designs, calculations, and initial data are presented from a program investigating the use of biomolecular and eutectic composite microstructures as vacuum field-emission cathodes. Calculations, supported by the initial data, indicate that these electron sources should be capable of macroscopic beam current densities, J⩾100 A/cm², without the formation of a surface plasma. The absence of a plasma allows these cathodes to operate as a long-pulse to DC electron sources. The designs avoid the formation of surface plasma by forcing the current emitted from the tips to transit a thin layer of silicon. The natural high-field current-limiting nature of the silicon serves to prevent the ablation of tip structures and the subsequent formation of cathode plasma. Initial data obtained with these microstructures has demonstrated Fowler-Nordheim characteristic emission. Coated bio-molecular composite cathodes have demonstrated stable DC current densities of ~100 mA/cm², and the eutectic composite cathodes have demonstrated similarly stable DC current densities of ~1 A/cm²     

First principles study of the electron density distribution in a pair of bare metallic electrodes

Self-consistent calculations of electron density distribution from first principles for a series of semi-infinite metals show that the electron density almost drops to zero at 8.5 a.u. away from a metal surface. The electron densities in a series of bimetallic-electrode systems with a distance between the two electrodes of 21.7 a.u. are further investigated. Spin-polarized calculations of electron density for nonmagnetic and magnetic bimetallic-electrode systems are compared. Our work is helpful for first principles investigation of spin-dependent metal–molecule–metal tunneling junctions.     

Investigation of the Anode Boundary Layer of an Atmospheric Pressure Argon Arc

This paper is concerned with the thermodynamic state of the plasma in the anode region of a high intensity arc. Spectrometric studies of the boundary layer in front of a plane anode perpendicular to the axis of a wall-stablized, high-intensity arc in argon atmosphere indicate substantial deviations from LTE. Data taken at distances of 0.5 to 2 mm from the anode surface are evaluated with two different but not entirely independent non-equilibrium data reduction techniques in order to obtain radial distributions of electron temperature and electron density. The electron temperature distribution displays an off-axis peak which increases in magnitude with decreasing distance from the anode. Both electron temperatures and electron densities are considerably higher than the corresponding LTE values.     

Computer controlled techniques for high emission density mapping of thermionic cathodes

Some of the techniques commonly used (e.g. SLEEP and thermionic emission microscope) for measuring emission or work function uniformity of thermionic cathode surfaces require the use of very low or near zero current densities, thus the cathode is characterized at current densities and temperatures much lower than that of a normally operating cathode. The system reported on here uses a high voltage pulse technique and is capable of measuring emission densities in the range 1 to 80 A/cm² at normal cathode operating temperatures. The cathode surface is scanned with an anode having a 0.025 mm aperture whose position is controlled by computer operated stepping motors. The current through the aperture to a collector electrode is measured using a sample-and-hold amplifier. Pulsing and sampling are computer synchronized with the scanning, and data for each pulse are accumulated and can be processed and displayed in several ways using the computer, including a detailed “three-dimensional” map of either the electron emission density or work function variations. The entire surface of the cathode or any portion of it can be mapped in steps as small as 0.001 mm (1μm), but typically steps of 5–100 μm were used. Measurements are presented illustrating the uniformity or nonuniformity of the electron emission densities and work functions for type-B and type-M cathodes.     

Experimental Study of Planar Langmuir Probe Characteristics in Electron Current-Carrying Magnetized Plasma

Experimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current-free magnetized plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface.     

The positive charging effect of dielectric films irradiated by a focused electron beam

Space charge and surface potential profiles are investigated with numerical simulation for dielectric films of SiO₂ positively charged by a focused electron beam. By combining the Monte Carlo method and the finite difference method, the simulation is preformed with a newly developed comprehensive two-dimensional model including electron scattering, charge transport and trapping. Results show that the space charge is distributed positively, like a semi-ellipsoid, within a high-density region of electrons and holes, but negatively outside the region due to electron diffusion along the radial and beam incident directions. Simultaneously, peak positions of the positive and negative space charge densities shift outwards or downwards with electron beam irradiation. The surface potential, along the radial direction, has a nearly flat-top around the center, abruptly decreases to negative values outside the high-density region and finally increases to zero gradually. Influences of electron beam and film parameters on the surface potential profile in the equilibrium state are also shown and analyzed. Furthermore, the variation of secondary electron signal of a large-scale integration sample positively charged in scanning electron microscopic observation is simulated and validated by experiment.     

Electron density and electron redistribution in alloys—I: Electron density in elemental metals

The changes in electronic structure around each metal atom in an alloy are more accurately described in terms of electron redistribution than electron transfer. The important elements in a model for such electron redistribution are the electron configuration and radius of the positive ion cores and the conduction electron density and its variation in the interstitial regions. Positive ion core radii for metals are derived from an empirical relation between ion core radii and intermetallic distance with electronic configurations of the core from positron annihilation data. These derived ion core radii are in good agreement with those calculated for metal electron densities by Welch and Lynn from Hartree-Fock wave functions. Average conduction electron densities can be directly calculated from these radii and the experimental atomic volume; the variation of conduction electron density in the interstitial region is calculated by assigning electrons to octahedral and tetrahedral interstices according to the Interstitial Electron Model. The calculated electron densities and the interstitial variation agree very well with the experimental electron densities of Be as determined from X-ray diffraction data as well as with calculated electron densities for Na, Mg and Al. Ion core configurations are compared with those derived from band theory and also those of the Engle-Brewer Correlation and the Samsonov-Pryadkov-Pryadkov Configurational Model.     

Modification of the magnetoconductance of a two-dimensional electron gas by altering the boundary conditions in the third dimension

Substrate bias can be used as a powerful diagnostic tool for studying the magneto-transport in an inversion layer. One can accurately determine the mobile carrier threshold by such a study. Marked changes in the mobility occur with increasing confinement to the surface; a decrease for high electrons densities and an increase for low electron densities. The former was expected while the latter was not.     

High density measurements of the Stark broadened profile of the He(II)4686 Å line

The Stark broadened profile of the He(II)4686 Å line has been scanned in the electron density range of 1–3 × 10¹⁸ cm^-3. The electron temperature, as determined from the line to continuum ratio, ranged from 9.4 to 19.9 eV. The plasma which emitted the He(II)4686 Å line was created in a 60-kJ theta pinch operated with a high fill pressure (3 and 5 torr) of pure helium. Electron densities were calculated from the half-half widths of the He(II)4686 Å line using two Stark broadening theories. These electron densities are compared with the electron densities determined from the absolute value of the continuum intensity and a total sweep up compressional model. The two theoretical models predict electron densities in good agreement with the electron densities from the absolute value of the continuum intensity.