Size dependent electron momentum density distribution in ZnS

In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nanoparticles of size 3.8 nm and 2.4 nm are synthesized using the chemical route and characterized by X-ray diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as bulk ZnS samples employing 59.54 keV gamma-rays from ²⁴¹Am source. The results reveal that the valence electron density in momentum space becomes narrower with reduction of particle size. To evaluate the charge transfer on compound formation, the ionic model based calculations for a number of configurations of Zn^+xS^−x (0.0≤x≤2) are also performed utilizing free atom Compton profiles. These results suggest different amounts of charge transfer in these materials varying from 1.2 to 2.0 electron from Zn to S atom.     

Photoluminescence measurements in Be-δ-doped back-gated quantum well

We measured the photoluminescence (PL) spectra of a two-dimensional electron system induced in a Be-δ-doped GaAs/AlGaAs quantum well (QW) with a back gate. The electron density is controlled by means of the back-gate voltage. We estimated the electron density using the magneto-optical method and the PL linewidth, and also by undertaking transport measurements. We show that a uniform 2DES as large as 1 mm² is induced by the back-gate operation from 2.5 × 10¹⁰ cm⁻². This experiment indicates that optical measurement with a back-gated QW is advantageous for studying the low-density 2DES.     

Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

High resolution electron density mapping for LiF and NaF by maximum entropy method (MEM)

High resolution maximum entropy method (MEM) electron density maps have been elucidated for LiF and NaF using reported X-ray structure factors. The ionic nature of the bonding between constituent atoms in both the systems is found to be well pronounced and clearly seen from the electron density maps. The resolution of the present MEM maps is 0.063 Å per pixel for LiF and 0.072 Å per pixel for NaF along the three crystallographic axes. The electron density at the middle of the bond along [111] is found to be 0.0673 e/ų for LiF and 0.003 e/ų for NaF showing the different ionic strengths of the bonding. The electron density along [100] and [110] has also been drawn and analyzed. The inequality in the ionicity for the individual atoms and the electron content for different ionic radii have also been analyzed and compared with already published results. The wR_MEM obtained from MEM analysis is 0.3% for LiF and 0.79% for NaF.     

Influence of AlN interfacial layer on electrical properties of high-Al-content Al0.45Ga0.55N/GaN HEMT structure

Unintentionally doped high-Al-content Al_0.45Ga_0.55N/GaN high electron mobility transistor (HEMT) structures with and without AlN interfacial layer were grown by metal-organic chemical vapor deposition (MOCVD) on two-inch sapphire substrates. The effects of AlN interfacial layer on the electrical properties were investigated. At 300 K, high two-dimensional electron gas (2DEG) density of 1.66 × 10¹³ cm⁻² and high electron mobility of 1346 cm² V⁻¹ s⁻¹ were obtained for the high Al content HEMT structure with a 1 nm AlN interfacial layer, consistent with the low average sheet resistance of 287 Ω/sq. The comparison of HEMT wafers with and without AlN interfacial layer shows that high Al content AlGaN/AlN/GaN heterostructures are potential in improving the electrical properties of HEMT structures and the device performances.     

Field electron emission improvement of ZnO nanorod arrays after Ar plasma treatment

Vertically well-aligned single crystal ZnO nanorod arrays were synthesized and enhanced field electron emission was achieved after radio-frequency (rf) Ar plasma treatment. With Ar plasma treatment for 30 min, flat tops of the as-grown ZnO nanorods have been etched into sharp tips without damaging ZnO nanorod geometrical morphologies and crystallinity. After the Ar ion bombardment, the emission current density increases from 2 to 20 μA cm⁻² at 9.0 V μm⁻¹ with a decrease in turn-on voltage from 7.1 to 4.8 V μm⁻¹ at a current density of 1 μA cm⁻², which demonstrates that the field emission of the as-grown ZnO nanorods has been efficiently enhanced. The scanning electron microscopy (SEM) results, in conjunction with the results of transmission electron microscopy (TEM), Raman spectroscopy and photoluminescence observation, are used to investigate the mechanisms of the field emission enhancement. It is believed that the enhancements can be mainly attributed to the sharpening of rod tops, and the decrease of electrostatic screening effect.     

Langmuir probe and spectroscopic studies of RF generated helium-nitrogen mixture plasma

This paper reports the effect of helium percentage variation in a capacitive RF helium-nitrogen mixture plasma on various plasma parameters and concentration of nitrogen active species (N₂(C³Π _u) and N₂ ⁺(B²Σ _u ⁺)). Langmuir probe is used for determination of electron energy distribution functions, effective electron temperature, plasma potential and electron density. Optical emission spectroscopy is used for determination of electron temperature from Boltzmann's plot of He–I lines and the relative changes in the concentration of active species by measuring the emission intensities of nitrogen (0-0) bands of the second positive and the first negative systems. The results demonstrate that electron temperature, electron density and concentration of active species increase significantly with increase in helium percentage in the mixture and RF power.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

Surface electron accumulation in indium nitride layers grown by high pressure chemical vapor deposition

Surface termination and electronic properties of InN layers grown by high pressure chemical vapor deposition have been studied by high resolution electron energy loss spectroscopy (HREELS). HREEL spectra from InN after atomic hydrogen cleaning show N-H termination with no indium overlayer or droplets and indicate that the layer is N-polar. Broad conduction band plasmon excitations are observed centered at 3400 cm⁻¹ in HREEL spectra with 7 eV incident electron energy which shift to 3100 cm⁻¹ when the incident electron energies are 25 eV or greater. The shift of the plasmon excitations to lower energy when electrons with larger penetration depths are used is due to a higher charge density on the surface compared with the bulk, that is, a surface electron accumulation. These results indicate that surface electron accumulation on InN does not require excess indium or In-In bonds.     

Schlieren diagnostics of pulsed electron beam ablation of polymethyl-methacrylate

The investigation of the interaction of pulsed electron beams with PMMA (polymethylmethacrylate) targets is reported. The electron beam of some 10^–8 s in duration is produced in a pulsed low-pressure gas discharge. The beam power density of up to 10⁸ W/cm² leads to a surface plasma formation similar to that of the pulsed laser ablation process. The propagation of the ablated material and the shock wave inside the PMMA target are observed by means of Schlieren diagnostics. An electron density gradient of over 3×10¹⁹ cm^–4 has been observed in the expanding plasma up to 1.5 s after the plasma formation. During the early stage of expansion, the expansion velocity of the plasma plume as determined by the steep electron density gradient is around 10⁵ cm/s. The pressure behind the shock front inside the PMMA target as determined from the shock velocity exceeds 0.3 Gpa.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Electrochemical synthesis of Cu/ZnO nanocomposite films and their efficient field emission behaviour

The Cu/ZnO nanocomposite films have been synthesized by cathodic electrodeposition and characterized using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), photoluminescence (PL) and field emission microscope (FEM). The XRD pattern shows a set of well defined diffraction peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. In addition, characteristic diffraction peaks corresponding to Cu and Zn are also observed. The SEM image shows formation of two-dimensional (2D) hexagonal sheets randomly distributed and aligned almost normal to the substrate. Uniformly distributed small clusters of Cu nanoparticles possessing average diameter of ∼25 nm, as revealed from the TEM image, are seen to be present on these 2D ZnO sheets. The selected area electron diffraction (SAED) image confirms the nanocrystalline nature of the Cu particles. From the field emission studies, carried out at the base pressure of ∼1 × 10⁻⁸ mbar, the turn-on field required for an emission current density of 0.1 μA/cm² is found to be 1.56 V/μm and emission current density of ∼100 μA/cm² has been drawn at an applied field of 3.12 V/μm. The Cu/ZnO nanocomposite film exhibits good emission current stability at the pre-set value of ∼10 μA over a duration of 5 h. The simplicity of the synthesis route coupled with the better emission properties propose the electrochemically synthesized Cu/ZnO nanocomposite film emitter as a promising electron source for high current density applications.     

Electron density distribution in GaAs using MEM

Electron density distribution of GaAs is determined by means of the maximum entropy method (MEM) using reasonably good X-ray data collected at room temperature and 200 K. The bonding electron distributions are clearly visible in the MEM map and the mixed covalent-ionic character in GaAs is evidenced. The density distributions at 200 K show more condensed electronic clouds as compared to the room temperature map, preserving the trend in the bonding characters. The electron densities at the middle of the bond are 0.79 and 0.70 e/ų at 200 K and at room temperature, respectively. The refined harmonic thermal factors are in good agreement with the published values.     

Electronic structure of Ni2TiAl: Theoretical aspects and Compton scattering measurement

In this paper, we report electron momentum density of Ni₂TiAl alloy using an in-house 20 Ci ¹³⁷Cs (661.65 keV) Compton spectrometer. The experimental data have been analyzed in terms of energy bands and density of states computed using linear combination of atomic orbitals (LCAO) method. In the LCAO computations, we have considered local density approximation, generalized gradient approximation and recently developed second order generalized gradient approximation within the frame work of density functional theory. Anisotropies in theoretical Compton profiles along [1 0 0], [1 1 0] and [1 1 1] directions are also explained in terms of energy bands.     

On the surface trapping parameters of polytetrafluoroethylene block

Surface flashover phenomena under high electric field are closely related to the surface characteristics of a solid insulating material between energized electrodes. Based on measuring the surface potential decaying curve of polytetrafluoroethylene (PTFE) block charged by a needle-plane corona discharge, its surface trapping parameters are calculated with the isothermal current theory, and the correlative curve between the surface trap density and its energy level is obtained. The maximum density of electron traps and hole traps in the surface layer of PTFE presents a similar value of ∼2.7 × 10¹⁷ eV⁻¹ m⁻³, and the energy level of its electron and hole traps is of about 0.85-1.0 eV and 0.80-0.90 eV, respectively. Via the X-ray photoelectron spectroscopy (XPS) technique, the F, C, K and O elements are detected on the surface of PTFE samples, and F shows a remarkable atom proportion of ∼73.3%, quite different from the intrinsic distribution corresponding to its chemical formula. The electron traps are attributed to quantities of F atoms existing on the surface of PTFE due to its molecular chain with C atoms surrounded by F atoms spirally. It is considered that the distortions of chemical and electronic structure on solid surface are responsible for the flashover phenomena occurring at a low applied voltage.     

Site specific effects in alloy surface reactivity probed by Li ions

We investigated site specific reactivity of the α-Cu-Al(17 at.%)(1 0 0) alloy surface versus that of Al and Cu by measuring neutralisation of Li⁺ ions. Experiments show that Li⁺ neutralisation on the α-CuAl(1 0 0) alloy surface does not occur efficiently on Al and is somewhat more efficient on Cu as compared to a pure Cu sample, indicating depletion of electron density on Al and transfer to neighboring Cu. It can thus be concluded that Cu sites in presence of Al are more reactive in the sense that processes involving electron capture from the surface would proceed more efficiently on these.     

Molecular interactions in methylimidazolium tetrafluoroborate ionic liquid ([Mim][BF4]): Structures, binding energies, topological properties and NMR one- and two bonds spin-spin coupling constants

Molecular interactions in methylimidazolium tetrafluoroborate ionic liquid ([Mim⁺][BF₄⁻]) have been investigated using B3LYP, B3PW91 and MP2 methods with a wide range of basis sets. Binding energy, topological properties of electron density, charge transfer, nucleus-independent chemical shift (NICS) and NMR one- and two bonds spin-spin coupling constants were calculated. With five preferential binding sites in the vicinity of the Mim⁺ ring, five ion pairs (A-E) with three intermolecular hydrogen bonds were found on the potential energy surface. The most stable ion pairs are formed via N-H and C-H bonds of Mim⁺ and B-F bonds of BF₄⁻. Ion pairs have electronic binding energies (BEs) in the range of − 335.6 to − 402.9 kJ/mol at MP2/6-311++G(2d,2p) level and − 328.1 to − 383.6 kJ/mol at B3LYP/AUG-cc-pVTZ level. NBO analysis confirms that the charge transfer takes place from BF₄⁻ to Mim⁺. The NICS values reveal the aromaticity of imidazole ring. The results show a correlation between absolute value of ^1hJ(H?F) and electron density at H?F bond critical point.     

Formation of functional groups on graphite during oxygen plasma treatment

Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 × 10¹⁶ m⁻³, the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 × 10²¹ m⁻³ for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112° down to about 1°. The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy.     

Field emission property of copper nitride thin film deposited by reactive magnetron sputtering

Copper nitride (Cu₃N) thin film was deposited on silicon (Si) substrate by reactive magnetron sputtering method. X-ray diffraction measurement showed that the film was composed of Cu₃N crystallites with anti-ReO₃ structure and exhibited preferential orientation of [1 0 0] direction. The field emission (FE) result showed that Cu₃N film had a turn-on electric field of about 3 V/μm at a current density of 1 μA/cm² and a current density of 700 μA/cm² was obtained at the electric field of 24 V/μm. The emission mechanism inferred by Fowler-Nordheim (FN) plot is shown as following: thermal electron emission at low field region and tunneling electron emission at high field region.     

Effects of SnO2 surface coating on the degradation of ZnS thin film phosphor

Pulsed laser deposited ZnS bare and SnO₂ coated ultra thin films were subjected to prolonged electron beam bombardment with 2 keV energy and a steady 44 mA/cm² current density, in 1 × 10⁻⁶ Torr O₂ pressure backfilled from a base pressure of 3 × 10⁻⁹ Torr at room temperature. Auger electron spectroscopy (AES) was used to monitor changes of the surface chemical composition of both the bare and coated phosphor films during electron bombardment. Degradation was manifested by the decrease of sulphur and accumulation of oxygen on the surface of the bare phosphor. However, with the SnO₂ coating this phenomenon was delayed until the protective SnO₂ was depleted on the surface through dissociation and reduction.