Numerical simulation of the extraction of ions from a magnetized plasma

The extraction of ions from a magnetized plasma in laser isotope separation systems is analyzed using one-dimensional, time-dependent equations of two-fluid magnetohydrodynamics. It is suggested that electron emission from the cathode may be used to compensate for the ion space charge which limits the rate of extraction. It is shown that at an accelerating voltage of 25 V and an emission current of 2.5 mA/cm², a plasma with a density of 10¹¹ cm⁻³ is accelerated to a velocity of 1.5×10⁵ cm/s in 3 μs. In this case, the maximum temperature of the plasma electron component does not exceed 1.5 eV. Pis’ma Zh. Tekh. Fiz. 23, 42–46 (April 26, 199)     

Self-absorption of Hβ line in pellet ablation clouds in devices with magnetic confinement of high-temperature plasma

We have determined the optical absorption depth for H_β line in clouds of secondary plasma near pellets ablating in high-temperature plasma of devices with magnetic confinement. It is established that, for all values of the electron concentration (10¹⁶–3 × 10¹⁷ cm^?3) and electron temperature (2–5 eV) measured in typical impurity pellet clouds, the absorption is negligibly small. At the same time, it can be significant in clouds of higher density (10¹⁷–5 × 10¹⁷ cm^?3) with a temperature of 1–2 eV, which are typical of cryogenic fuel pellets.     

Study of microwave components for an electron cyclotron resonance source: Simulations and performance

A high power (2 kW, CW) magnetron-based microwave system operating at 2.45 GHz has been designed, tested, characterized, and used to produce plasma. The system consists of a microwave source, an isolator, a directional coupler, a three-stub tuner, a high voltage break, a microwave vacuum window, and a microwave launcher. These microwave components were simulated using microwave studio software. The low power and full term characterization of the microwave system has been done using vector network analyzer. The system was tested for 2 kW continuous wave of microwave power using glass-water load. The microwave system has been developed to study the microwave interaction with plasma at different operation regimes (Gases: Nitrogen, argon and hydrogen; Gas pressure : 10^?5–10^?3 mbar; Microwave power : 300–1000 W; Magnetic field: 875–1000 G) and to extract the proton beam current with hydrogen produced plasma. A plasma density ～5 × 10¹¹ cm^?3 and average electron temperature of ～13 eV was obtained. This article describes various aspects of the microwave system including design, fabrication, characterization and performance studies of the microwave components.     

Dense PLZT films grown on nickel substrates by PVP-modified sol–gel method

We have successfully grown ferroelectric Pb_0.92La_0.08Zr_0.52Ti_0.48O₃ (PLZT) films on base metal foils by chemical solution deposition using sol–gel solutions containing polyvinylpyrrolidone. Under zero-bias field, we measured a dielectric constant of ≈820 and dielectric loss of ≈0.06 at room temperature, and a dielectric constant of ≈1250 and dielectric loss of ≈0.03 at 150 °C. In addition, leakage current density of ≈1.5 × 10^?8 A/cm², remanent polarization of ≈11.2 μC/cm², and coercive field of ≈40.6 kV/cm were measured at room temperature on a ≈3-μm-thick PLZT film grown on LaNiO₃-buffered nickel substrate. Finally, energy density ≈25 J/cm³ was measured from the P–E hysteresis loop at an applied field of 2 × 10⁶ V/cm.     

A study of the electrical characteristics,before electroforming,of thin SiOx films with laterally-spaced electrodes

The sample structure comprises two laterally-spaced electrodes joined by a thin dielectric film. The electrodes make a blocking contact to the dielectric with a barrier height ～ 0.75 eV. The voltage-current characteristic is studied between 213 to 413 K. Below room temperature the effect of hopping conduction on such contacts is explained. At room temperature and at high fields where hopping conduction is expected to be less effective, the conduction is controlled by the contact. Above room temperature and at low fields localized state conduction (hopping) becomes effective again with activation energy ～ 0.07 eV and an estimated charge carrier mobility of 1.3×10^?2cm²V^?1sec^?1. The density of ionizable impurities is estimated to be in the range 1 to 6×10¹⁸cm^?3 and the density of surface states is of the order of 1×10¹³cm^?2.     

Ion irradiation-induced modifications of diamond nanorods synthesised by microwave plasma chemical vapour deposition

Diamond nanorods (DNRs) synthesised by the high methane content in argon rich microwave plasma chemical vapour deposition (MPCVD) have been implanted with nitrogen ions. The nanorods were characterised by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The DNRs consist of single-crystalline diamond cores of 3–5?nm in diameter and several tens of nanometres in length. For purification from non-diamond contents, hydrogen plasma etching of DNRs was performed. Structural modifications of etched DNRs were studied after irradiating with 50?keV nitrogen ions under the fluence of 5?×?10¹⁴, 1?×?10¹⁵, 5?×?10¹⁵ and 1?×?10¹⁶?ions?cm^?2. Nitrogen-ion implantation changes the carbon–carbon bonding and structural state of the nanocrystalline diamond (NCD). Raman spectroscopy was used to study the structure before and after ion irradiation, indicating the coexistence of diamond and graphite in the samples. The results indicated the increase in graphitic and sp²-related content, at the expense of decrease in diamond crystallinity, for ion implantation dose of 5?×?10^15?cm^?2 and higher. The method proves valuable for the formation of hybrid nanostructures with controlled fractions of sp³–sp² bonding.     

Preparation of a strip structure for quantum-cascade lasers

A method for overgrowing laser stripe structures with high-ohmic indium phosphide to bound the domain of current flow and to improve heat dissipation is presented. Overgrowing was performed using metal-organic chemical-vapor deposition. It is shown that the method makes it possible to obtain high-quality epitaxial layers and defectless overgrowth boundaries without special processing of the structures after photolithography. All the InP layers were n-conductive, the specific resistance was ρ ～ 5 × 10⁴ Ω cm, and the carrier concentration was n ～ 5 × 10¹⁰ cm^?3. The characteristics of the grown InP layers permit one to obtain high-quality quantum-cascade lasers.     

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×10¹⁶ cm^?3. The thermionic emission was identified for moderate dopant concentration of 5×10¹⁷ cm^?3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×10¹⁷ cm^?3). High dopant concentration of 5×10¹⁸ cm^?3 leads to almost pure field emission transport within the whole investigated temperature range.     

Latent spatial instability of current in high-power bipolar switches

A theory describing the injection instability in high-power bipolar semiconductor switches at a current density of J ～ 1 kA/cm² and a collector field strength of E > 5 × 10⁴ V/cm is proposed. It is established that, if the efficiency of cathode injection source increases with J and E, fluctuations in the current distribution over wafer area with a wavelength on the order of λ ～ 100 μm (not related to the wafer transverse size) can develop in such devices. The corresponding volume fluctuations in the charge carrier densities, potentials, field strength, and current density are localized in the bulk, at a depth that is much smaller than the plate thickness. This latent instability is not related to the sign of the external differential resistance, so that internal damping factors are necessary in order to prevent its development.     

Direct Measurements of the Strong Refractive Nonlinearity of Impurity Origin in Heavily-doped GaAs

Abstract

A strong dispersive nonlinearity below the band gap in heavily doped bulk n-GaAs is observed for differnet donor impurities. Negative refractive index changes of up to ?5 × 10^?3 are obtained in the spectral range 880–900 nm, induced by light of the same wavelength at an incident intensity of about 5 × 10⁵ W cm^?2. Since the lifetime of the nonlinearity is ～ 10^?10 s, it is suggested that a bistable device exploiting this effect could be constructed with a switching energy of (1?5) × 10^?14 J μm^?2.     

Structure transition and multiferroic properties of Mn-doped BiFeO3 thin films

Pure BiFeO₃ (BFO) and Mn-doped BiFe_1?yMn_yO₃ thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel method. The effects of Mn-doping on the structure and electric properties of the BFO thin films were studied. The X-ray diffraction (XRD) analysis reveals a structure transition in the Mn-doped BiFe_0.96Mn_0.04O₃ (BFMO) thin film. The Rietveld refined XRD patterns conform the trigonal (R3c: H) and tetragonal (P422) symmetry for the BFO and BFMO thin films, respectively. The structure transition and the mixed valences of Mn ions substantially improve the electric properties of the BFMO thin film. The remnant polarization (P _r) of the BFMO thin film was 105.86 μC/cm² at 1 kHz in the applied electric field of 865 kV/cm. At an applied electric field of 150 kV/cm, the leakage current density of BFMO thin film is 1.42 × 10^?5 A/cm². It is about two orders of magnitude lower than that of the pure BFO thin film (1.25 × 10^?3 A/cm²). And the enhanced saturated magnetization of the BFMO thin film is 4.45 emu/cm³.     

Electrical and microstructural investigation of Au/Pd/ri ohmic contacts for AIGaAs/GaAs heterojunction bipolar transistors

Abstract

The microstructure and electrical properties of as deposited and annealed Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaAs, C doped with a concentration of 5 × 10¹⁸ and 5 × 10¹⁹ cm^?3, have been investigated using transmission electron microscopy, and current-voltage measurements as afunction of temperature in the range 198–348 K. The specific contact resistivities have also been measured using the transmission line method. It was found that increasing the epilayer doping level by an order of magnitude, from 5 × 10¹⁸ to 5 × 10¹⁹ cm^?3, caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generationrecombination within the depletion region was found to be the dominant current transport mechanism for temperatures below 289 K. The contacts to the more highly doped epilayers (C doped, 5 × 10¹⁹ cm^?3) had specific contact resistivities of 0·08 ± 0·03 Ωmm and 0·05 ± 0·06 Ωmm, respectively. These values, together with a minimal metal penetration in the semiconductor of <15 nm, indicate that these contacts are suitable for heterojunction bipolar device applications.

MST/3325     

Shear performance of microscale ball grid array structure Sn–3.0Ag–0.5Cu solder joints with different surface finish combinations under electro-thermo-mechanical coupled loads

The shear performance and fracture behavior of microscale ball grid array structure Sn–3.0Ag–0.5Cu solder joints with different substrate surface finishes (Cu with organic solderability preservatives and electroless Ni/immersion Au) combinations under electro-thermo-mechanical (ETM) coupled loads with increasing current density (from 1.0?×?10³ to 6.0?×?10³ A/cm²) were systematically investigated by experimental characterization, theoretical analysis, and finite element simulation. The results reveal that the shear strength varies slightly with different surface finish combinations, initially increasing and then decreasing as the current density is increased. Moreover, the increase in current density shifts the fracture location from the solder matrix to the interface between solder and intermetallic compound (IMC) layer, resulting in a ductile-to-brittle transition. The interfacial fracture is triggered by electric current crowding at the groove of the IMC layer and driven by the mismatch strain at the solder/IMC layer interface.

     

Energy supply to the region of colliding plasma flows with opposite polarization fields

The interaction of two colliding deuterium plasma flows propagating in opposite directions across the magnetic field and having opposite directions of the polarization fields has been experimentally studied. The plasma flows with densities up to 10¹⁶ cm^?3 and a velocity of up to 2 × 10⁷ cm/s were formed using discharges initiated in crossed E × H fields. Each discharge operated at an electric power of up to 300 mW and a discharge current up to 100 kA. The results of measurements of the equivalent capacitance of polarized plasma flows were used to estimate the transverse permittivity of plasma. The rate of depolarization in colliding flows was determined. The frequency and energy characteristics of a plasma LC circuit formed by colliding flows were estimated. The temporal modulation of plasma density in the flows was measured.     

Phosphorus-containing cesium compounds of pollucite structure. Preparation of high-density ceramic and its radiation tests

Multicomponent oxides of pollucite structure, containing Cs and Ba, were synthesized as powders and ceramics. Their chemical compositions, Cs[MgAl_0.5P_1.5O₆] and Cs_0.875Ba_0.125[Li_0.125Zn_0.875Al_0.5P_1.5O₆], were modeled on the basis of the known structural features, taking into account the principles of iso- and heterovalent isomorphism of cations. From powdered samples synthesized using sol-gel process, a ceramic was prepared by spark plasma sintering (SPS). The sintering time was 3–4 min in the temperature interval 600–850°C. The relative densities were 97 and 99%. To evaluate the radiation resistance of the ceramics, the samples were irradiated with ¹³²Xe²⁶⁺ ions (E = 167 MeV) in the fluence interval from 6 × 10¹⁰ to 1 × 10¹³ cm^?2 (ion flux density ～10⁹ s^?1 cm^?2). The amorphization took place at fluences of (1.2–1.3) × 10¹² cm^?2. This fact suggests the decisive role of the ion energy loss for ionization in the generation of radiation defects. Conditions were found for the transition of the metamict form into the crystalline form on heating.     

Lattice distortion and room-temperature ferroelectric properties of (Sm, Cr) co-doped BiFeO3 thin films

Pure BiFeO₃ (BFO) and Bi_0.85Sm_0.15Fe_0.97Cr_0.03O₃ (BSFCO) thin films were prepared on FTO/glass (SnO₂: F) substrates by using a chemical solution deposition method. The effects of (Sm, Cr) co-doping on the microstructure and ferroelectric properties of the BSFCO thin films were studied. The X-ray diffraction and Raman scattering spectra proved that the co-doped BSFCO thin film has a lattice distortion compared with the pure BFO thin film. The remnant polarization (2P _r) of the BSFCO thin film was 153.67 μC/cm² at 1 kHz in the applied electric field of 1,270 kV/cm. At an applied electric field of 100 kV/cm, the leakage current density of the co-doped BSFCO thin film (2.12 × 10^?6 A/cm²) was 3 orders lower than that of the pure BFO thin film (3.8 × 10^?3 A/cm²). The improved properties of the co-doped thin film could be attributed to lattices distortion, more grain boundaries, higher binding energy of Sm–O and the mixed-valence states of Cr³⁺ and Cr ⁶⁺.     

Conduction and field induced degradation in thin ZrO2 films sputtered in nitrogen containing plasma on silicon

This paper deals with the electrical and stress induced degradation of reactively sputtered ZrO₂/Si interface deposited in N₂ containing plasma and pure argon ambient. MOS C–V and I–V techniques were used for interface characterization. Leakage current and flat band shifts were compared for ZrO₂ films deposited with and without N₂ containing plasma. The effect of current stress and post deposition annealing carried out on the samples deposited in different ambient was investigated. The annealed devices showed better electrical and reliability characteristics. The flat band voltage shifts towards negative value in annealed devices on being stressed, indicating positive charge trapping in the high-k dielectric layer. The flat band voltage saturates faster when stressed with higher current density. The oxide charge density increases from 4.5 × 10¹² cm^?2 for as deposited samples to 5.6 × 10¹² cm^?2 on application of stress. The samples grown in pure argon ambient showed enhanced leakage when compared with samples grown in nitrogen ambient on application of stress .     

Structural,optical, and electrical properties of ZnTe:Cu thin films by PLD

In this work, ZnTe and ZnTe:Cu films were obtained by pulsed laser deposition using the co-deposition method. ZnTe and Cu₂Te were used as targets and the shots ratio were varied to obtain 0.61, 1.47, 1.72, and 3.46% Cu concentration. Doping of ZnTe films with Cu was performed with the purpose of increasing the p-type carrier concentration and establishing the effect of concentration of Cu on structural, optical, and electrical properties of ZnTe thin films to consider their potential application in electronic devices. According to X-ray diffraction, X-ray photoelectron spectroscopy, UV–visible spectroscopy, and Hall effect results, ZnTe and ZnTe:Cu films correspond to polycrystalline zinc–blende phase with preferential orientation in (111) plane. Optical characterization results indicate that as-deposited films (band gap?=?2.16 eV) exhibit a band gap decrease as function of the increase of Cu concentration (2.09–1.64 eV), while, annealed films exhibit a decrease from 1.75 to 1.46 eV, as the Cu concentration increases. Lastly, Hall effect results show that ZnTe films correspond to a p-type semiconductor with a carrier concentration of 3?×?10¹³ cm^?3 and a resistivity of 1.64?×?10⁵ Ω?cm. ZnTe:Cu films remain like a p-type material and present an increasing carrier concentration (from 3.8?×?10¹⁵ to 1.26?×?10¹⁹ cm^?3) as function of Cu concentration and a decreasing resistivity (from 7.01?×?103 to 2.6?×?10^?1 Ω cm). ZnTe and ZnTe:Cu thin films, with the aforementioned characteristics, can find potential application in electronic devices, such as, solar cells and photodetectors.