Influence of a two-dimensional electron gas on current—voltage characteristics of Al0.3{Ga}0.7 N/GaN high electron mobility transistors

The J-V characteristics of AltGa1 tN/GaN high electron mobility transistors（HEMTs） are investigated and simulated using the self-consistent solution of the Schro dinger and Poisson equations for a two-dimensional electron gas（2DEG） in a triangular potential well with the Al mole fraction t = 0.3 as an example.Using a simple analytical model,the electronic drift velocity in a 2DEG channel is obtained.It is found that the current density through the 2DEG channel is on the order of 10^13 A/m^2 within a very narrow region（about 5 nm）.For a current density of 7 × 10^13 A/m62 passing through the 2DEG channel with a 2DEG density of above 1.2 × 10^17 m^-2 under a drain voltage Vds = 1.5 V at room temperature,the barrier thickness Lb should be more than 10 nm and the gate bias must be higher than 2 V.     

High-mobility two-dimensional electron gases at oxide interfaces： Origin and opportunities

Our recent experimental work on metallic and insulating interfaces controlled by interfacial redox reactions in SrTiO3-based heterostructures is reviewed along with a more general background of two-dimensional electron gas(2DEG)at oxide interfaces.Due to the presence of oxygen vacancies at the SrTiO3surface,metallic conduction can be created at room temperature in perovskite-type interfaces when the overlayer oxide ABO3has Al,Ti,Zr,or Hf elements at the B sites.Furthermore,relying on interface-stabilized oxygen vacancies,we have created a new type of 2DEG at the heterointerface between SrTiO3and a spinelγ-Al2O3epitaxial film with compatible oxygen ion sublattices.This 2DEG exhibits an electron mobility exceeding 100000 cm2·V 1·s 1,more than one order of magnitude higher than those of hitherto investigated perovskite-type interfaces.Our findings pave the way for the design of high-mobility all-oxide electronic devices and open a route toward the studies of mesoscopic physics with complex oxides.     

An X-ray imaging device based on a GEM detector with delay-line readout

An X-ray imaging device based on a triple-GEM （Gas Electron Multiplier） detector, a fast delay-line circuit with 700 MHz cut-off frequency and two dimensional readout strips with 150 μm width on the top and 250 μm width on the bottom, is designed and tested. The localization information is derived from the propagation time of the induced signals on the readout strips. This device has a good spatial resolution of 150 μm and works stably at an intensity of 105 Hz/mm2 with 8 keV X-rays.     

Influence of the channel electric field distribution on the polarization Coulomb field scattering in In0.18Al0.82N/AIN/GaN heterostructure field-effect transistors

By making use of the quasi-two-dimensional （quasi-2D） model, the current-voltage （l-V） characteristics of In0AsA10.82N/A1N/GaN heterostructure field-effect transistors （HFETs） with different gate lengths are simulated based on the measured capacitance-voltage （C-V） characteristics and I-V characteristics. By analyzing the variation of the electron mobility for the two-dimensional electron gas （2DEG） with electric field, it is found that the different polarization charge distributions generated by the different channel electric field distributions can result in different polarization Coulomb field scatterings. The difference between the electron mobilities primarily caused by the polarization Coulomb field scatterings can reach up to 1522.9 cm2/V.s for the prepared In0.38AI0.82N/A1N/GaN HFETs. In addition, when the 2DEG sheet density is modulated by the drain-source bias, the electron mobility presents a peak with the variation of the 2DEG sheet density, the gate length is smaller, and the 2DEG sheet density corresponding to the peak point is higher.     

Stability and electronic structure studies of LaAIO3/SrTiO3 （110） heterostructures

The first-principles calculations are employed to investigate the stability, magnetic, and electrical properties of the oxide heterostructure of LaAIO3/SrTiO3 （110）. By comparing their interface energies, it is obtained that the buckled interface is more stable than the abrupt interface. This result is consistent with experimental observation. At the interface of LaAIO3/SrTiO3 （110） heterostructure, the Ti-O octahedron distortions cause the Ti tzg orbitals to split into the two- fold degenerate dxz/dyz and nondegenerate dxy orbitals. The former has higher energy than the latter. The partly filled two-fold degenerate t2g orbitals are the origin of two-dimensional electron gas, which is confined at the interface. Lattice mismatch between LaA103 and SrTiO3 leads to ferroelectric-like lattice distortions at the interface, and this is the origin of spin-splitting of Ti 3d electrons. Hence the magnetism appears at the interface of LaAIO3/SrTiO3 （110）.     

De Haas-van Alphen Effect of Free Electron Gas Revisited

The free electron gas in a uniform magnetic field at low temperature is restudied. The grand partition function previously obtained by Landau＇s quantitative calculation contains three parts, which are all approximate. An improved calculation is presented, in which two of the three parts are obtained in exact forms. A simple remedy for Landau and Lifshitz＇s qualitative calculation in the textbook is also given, which turns the qualitative result into the same one as obtained by the improved quantitative calculation. The chemical potential is solved approximately and the thermodynamic quantities are caiculated explicitly in both a weak field and a strong field. The thermodynamic quantities in a strong field obtained here contain both non-oscillating and oscillating corrections to the corresponding results derived from Landau＇s grand partition function. In particular, Landau＇s grand partition function is not sufficiently accurate to yield our nonzero results for the specific heat and the entropy. An error in the Laplace-transform method for the problem is corrected. The results previously obtained by this method are also improved.     

The effect of k-cubic Dresselhaus spin–orbit coupling on the decay time of persistent spin helix states in semiconductor two-dimensional electron gases

We study the theoretical effect of k-cubic （i.e, cubic-in-momentum） Dresselhaus spin-orbit coupling on the decay time of persistent spin helix states in semiconductor two-dimensional electron gases. We show that the decay time of persistent spin helix states may be suppressed substantially by k-cubic Dresselhaus spin-orbit coupling, and after taking the effect of k-cubic Dresselhaus spin-orbit interaction into account, the theoretical results obtained accord both qualitatively and quantitatively with other recent experimental results.     

Anomalous Kondo-Switching Effect of a Spin-Flip Quantum Dot Embedded in an Aharonov-Bohm Ring

We theoretically investigate the Kondo effect of a quantum dot embedded in a mesoscopic Aharonov-Bohm （AIR） ring in the presence of the spin flip processes by means of the one-impurity Anderson Hamiltonian. Based on the slave-boson mean-field theory, we find that in this system the persistent current （PC） sensitively depends on the parity and size of the AB ring and can be tuned by the spin-flip scattering （R）. In the small AB ring, the PC is suppressed due to the enhancing R weakening the Kondo resonance. On the contrary, in the large AB ring, with R increasing, the peak of PC firstly moves up to max-peak and then down. Especially, the PC phase shift of π appears suddenly with the proper value of R, implying the existence of the anomalous Kondo effect in this system. Thus this system may be a carldidate for quantum switch.     

Electron Acceleration and Bunch Generation by Intense Femtosecond Laser Pulse in Preplasma of a Target

We present analytical studies of electron acceleration in the low-density preplasma of a thin solid target by an intense femtosecond laser pulse. Electrons in the preplasma are trapped and accelerated by the ponderomotive force as well as the wake field. Two-dimensional particle-in-cell simulations show that when the laser pulse is stopped by the target, electrons trapped in the laser pules can be extracted and move forward inertially. The energetic electron bunch in the bubble is unaffected by the reflected pulse and passes through the target with small energy spread and emittance. There is an optimal preplasma density for the generation of the monoenergetic electron bunch if a laser pulse is given. The maximum electron energy is inverse proportion to the preplasma density.     

A double toroidal analyzer for scanning probe electron energy spectrometer

An ultra-high vacuum(UHV) compatible electron spectrometer employing a double toroidal analyzer has been developed. It is designed to be combined with a custom-made scanning tunneling microscope(STM) to study the spatially localized electron energy spectrum on a surface. A tip–sample system composed of a piezo-driven field-emission tungsten tip and a sample of highly ordered pyrolytic graphite(HOPG) is employed to test the performance of the spectrometer.Two-dimensional images of the energy-resolved and angle-dispersed electrons backscattered from the surface of HOPG are obtained, the performance is optimized and the spectrometer is calibrated. A complete electron energy loss spectrum covering the elastic peak to the secondary electron peaks for the HOPG surface, acquired at a tip voltage of-140 V and a sample current of 0.5 pA, is presented, demonstrating the viability of the spectrometer.     

Physical design of a wavelength tunable fully coherent VUV source using a self-seeding free electron laser

In order to meet the requirements of the synchrotron radiation users, a fully coherent VUV free electron laser （FEL） has been preliminarily designed. One important goal of this design is that the radiation wavelength can be easily tuned in a broad range （70 170 nm）. In the light of the users＇ demand and our actual conditions, the self-seeding scheme is adopted for this proposal. Firstly, we attempted to fix the electron energy and only changed the undulator gap to vary the radiation wavelength; however, our analysis implies that this is difficult because of the great difference of the power gain length and FEL efficiency at different wavelengths. Therefore, we have considered dividing the wavelength range into three subareas. In each subarea, a constant electron energy is used and the wavelength tuning is realized only by adjusting the undulator gap. The simulation results show that this scheme has an acceptable performance.     

Charge ordering modulations in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using the selected-area electron diffraction （SAED） technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy （HRTEM） images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi0.4Ca0.6MnO3/SrTiO3 interface from those in the region a little further from the Bi0.4Ca0.6MnO3/SrTiO3 interface, and the possible reasons for this are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

Study of RPC gas composition using Daya Bay RPCs

This study uses Daya Bay RPCs operating in streamer mode to investigate gas mixtures of at least 50% argon, at most 6% isobutane, and with small amounts of SF6. Isobutane is reduced to 2% without degradation of performance, and SF6 reduces the noise rate and current, as well as the signal size. This study provides quantitative relationships between basic RPC operating parameters and various gas compositions.     

Observation and analysis of halo current in EAST

Plasma in a typically elongated cross-section tokamak （for example, EAST） is inherently unstable against vertical displacement. When plasma loses the vertical position control, it moves downward or upward, leading to disruption, and a large halo current is generated helically in EAST typically in the scrape-off layer. When flowing into the vacuum vessel through in-vessel components, the halo current will give rise to a large J × B force acting on the vessel and the in-vessel components. In EAST VDE experiment, part of the eddy current is measured in halo sensors, due to the large loop voltage. Primary experimental data demonstrate that the halo current first lands on the outer plate and then flows clockwise, and the analysis of the information indicates that the maximum halo current estimated in EAST is about 0.4 times the plasma current and the maximum value of TPF × Ih/Ip0 is 0.65, furthermore Ih/Ipo and TPF × Ih/Ipo tend to increase with the increase of Ip0. The test of the strong gas injection system shows good success in increasing the radiated power, which may be effective in reducing the halo current.     

Optimization and numerical simulation for the extraction system of the H- multicusp ion source

A new ion source has been designed and manufactured for the CYCLONE30 accelerator, which has a much advanced performance compared with the original. It is expected that the newly designed ion source extraction system will transport a very large percentage of the beam without deteriorating the beam optics, which is designed to deliver an H- beam at 30 keV. The accelerator assembly consists of three circular aperture electrodes made of copper. The simulation study was focused on finding parameter sets that raise the beam perveance as large as possible and which reduce the beam divergence as low as possible. Ion beams of the highest quality are extracted whenever the half-angular divergence is minimum, for which the perveance current intensity and the extraction gap have optimum values. The triode extraction system is designed and optimized by using CST software （for Particle Beam Simulations）. The physical design of the extraction system is given in this paper. From the simulation results, it is concluded that it is possible to achieve this goal by decreasing the thickness of the plasma electrode, shortening the first gap, and adjusting the acceleration electrode voltage.     

Annealing effect of platinum-incorporated nanowires created by focused ion/electron-beam-induced deposition

Focused ion-beam-induced deposition(FIBID) and focused electron-beam-induced deposition(FEBID) are convenient and useful in nanodevice fabrication. Since the deposition is from the organometallic platinum precursor, the conductive lines directly written by focused ion-beam(FIB) and focused electron-beam(FEB) are carbon-rich materials. We discuss an alternative approach to enhancing the platinum content and improving the conductivity of the conductive leads produced by FIBID and FEBID, namely an annealing treatment. Annealing in pure oxygen at 500?C for 30 min enhances the platinum content values from ～18% to 30% and ～ 50% to 90% of FIBID and FEBID, respectively. Moreover, we find that thin films will be formed in the FIBID and FEBID processes. The annealing treatment is helpful to avoid the current leakage caused by these thin films. A single electron transistor is fabricated by FEBID and the current–voltage curve shows the Coulomb blockade effect.     

Investigation of a method to calculate spontaneous radiation spectra from relativistic electrons in undulators

Undulators are key devices to produce brilliant synchrotron radiation at the synchrotron radiation facilities. In this paper we present a numerical computing method, including the computing program that has been developed to calculate the spontaneous radiation emitted from relativistic electrons in undulators by simulating the electrons＇ trajectory. The effects of electron beam emittance and energy spread have also been taken into account. Comparing with other computing methods available at present, this method has a few advantages with respect to several aspects. It can adopt any measured or arbitrarily simulated 3D magnetic field and arbitrary electron beam pattern for the calculation and it＇s able to analyze undulators of any type of magnetic structure. It＇s expected to predict precisely the practical radiation spectrum. The calculation results of a short period in-vacuum undulator and an Elliptically Polarized Undulator （EPU） at Shanghai Synchrotron Radiation Facility （SSRF） are presented as examples.     

Commissioning of electron cooling in CSRe

The 400 MeV/u 12C6＋ ion beam was successfully cooled by the intensive electron beam near 1 A in CSRe.The momentum cooling time was estimated near 15 s.The cooling force was measured in the cases of difierent electron beam profiles,and the difierent angles between the ion beam and electron beam.The lifetime of the ion beam in CSRe was over 80 h.The dispersion in the cooling section was confirmed as positive close to zero.The beam sizes before cooling and after cooling were measured by the moving screen.The beam diameter after cooling was about 1 mm.The bunch length was measured with the help of the signals from the beam position monitor.The difiusion was studied in the absence of the electron beam.     

Forward and reverse electron transport properties across a CdS/Si multi-interface nanoheterojunction

The electron transport behavior across the interface plays an important role in determining the performance of op- toelectronic devices based on heterojunctions. Here through growing CdS thin film on silicon nanoporous pillar array, an untraditional, nonplanar, and multi-interface CdS/Si nanoheterojunction is prepared. The current density versus voltage curve is measured and an obvious rectification effect is observed. Based on the fitting results and model analyses on the forward and reverse conduction characteristics, the electron transport mechanism under low forward bias, high forward bias, and reverse bias are attributed to the Ohmic regime, space-charge-limited current regime, and modified Poole-Frenkel regime respectively. The forward and reverse electrical behaviors are found to be highly related to the distribution of inter- facial trap states and the existence of localized electric field respectively. These results might be helpful for optimizing the preparing procedures to realize high-performance silicon-based CdS optoelectronic devices.     

Experimental studies of THGEM in different Ar/CO2 mixtures

In this paper, the performance of a type of domestic THGEM （THick Gaseous Electron Multiplier） working in Ar/CO2 mixtures is reported in detail. This kind of single THGEM can provide a gain range from 100 to 1000, which is very suitable for application in neutron detection. In order to study its basic characteristics as a reference for the development of a THGEM based neutron detector, the counting rate plateau, the energy resolution and the gain of the THGEM have been measured in different Ar/CO2 mixtures with a variety of electrical fields. For the Ar/CO2（90%/10%） gas mixture, a wide counting rate plateau is achieved from 720 V to 770 V, with a plateau slope of 2.4%/100 V, and an excellent energy resolution of about 22% is obtained at the 5.9 keV full energy peak of the 55Fe X-ray source.