In-plane gate transistors in AlxGa1−xN/GaN heterostructures written by focused ion beams

Focused ion beam implantation of gallium and dysprosium was used to locally insulate the near-surface two-dimensional electron gas of Al_xGa_1−xN/GaN heterostructures. The threshold dose for insulation was determined to be 2×10¹⁰ cm⁻¹ for 90 keV Ga⁺ and 1×10⁹ cm⁻¹ for 200 keV Dy²⁺ at 4.2 K. This offers a tool not only for inter-device insulation but also for direct device fabrication. Making use of “open-T” like insulating line patterns, in-plane gate transistors have been fabricated by focused ion beam implantation. An exemplar with a geometrical channel width of 1.5 μm shows a conductance of 32 μS at 0 V gate voltage and a transconductance of around 4 μS, which is only slightly dependent on the gate voltage.     

A modified electron bombardment type ion source for the study of solids

A modified electron bombardment type ion source suitable for use with mass spectrometer is described. Ion formation occurs throughout a relatively large volume in the ionisation box, since no magnetic field is used to collimate the ionising electrons. A sensitivity of 2 × 10⁻⁵ amp/torr is obtained for an ion extraction energy of 2 keV and 200 mass resolution. Trajectory tracing has been used to study the operation of the ion source. Capability of the ion source to analyse solid samples in microgram quantity was tested by studying evaporation of BaO from tungsten.     

Tokai Radioactive Ion Accelerator Complex (TRIAC)

An ISOL-based radioactive nuclear beam (RNB) facility, Tokai Radioactive Ion Accelerator Complex (TRIAC), has been jointly developed by High Energy Accelerator Research Organization (KEK) and Japan Atomic Energy Agency (JAEA). The facility started to supply RNBs for experiments in 2005 and RNBs including fission fragments with energies up to 1.1MeV/A are available in the present. Several experimental studies were performed successfully using ⁸Li beams with various energies.     

单链表在离子发动机光学系统粒子模拟中的应用

目前离子发动机光学系统数值模拟大多采用PIC方法,由于该方法需要跟踪单个粒子的运动,因此需要存储每个粒子的位置信息与运动信息。传统的粒子模拟程序中,保存粒子信息大多采用数组的方式,但是采用这种方式存在弊端,例如对粒子总数变化的自适应不好。基于此开发了一种使用链式存储结构的粒子模拟程序,该程序使用带头节点的单向链表存储粒子信息。使用基于链式存储结构的PIC方法对离子发动机光学系统进行了粒子模拟,验证了链式粒子信息存储方法在粒子模拟中的可用性。模拟表明：（1）在粒子模拟中采用链式存储结构存储粒子信息,无需预先指定最大粒子总数,程序可自适应粒子总数的变化,因此无需进行试算,节省了计算时间;（2）在粒子模拟中采用链式存储结构,由于不存在内存资源的浪费,因而可显著提高程序的存储效率与计算效率。     

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al1−xGaxAs

The use of focused ion beam implantation doping of an inverted GaAs/Al_1−xGa_xAs heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10¹¹ cm⁻² and an electron mobility of 4.8×10⁵ cm²/V s, as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.     

Estimation of electron fluxes in the Earth's geostationary orbit with Tikhonov regularization during a magnetically quiet period

Electron energy fluxes in the Earth's outer radiation belt are estimated using an inverse theory of the Tikhonov regularization based upon observations in geostationary orbits. Particle Detector (PD) experiment aboard a geostationary satellite GEO-KOMPSAT-2A (GK2A) at a geographic longitude of 128.2°E provided observations of electrons within a 150–2,400 keV energy range with an unprecedented energy resolution of ΔE/E in the range of 5–25%. Instrument response functions, calculated with Monte-Carlo simulations, are deconvoluted with electron observations. Using regularization parameters determined from Generalized Cross Validation (GCV), the Tikhonov method was applied to observations made during a geomagnetically quiet period. This Tikhonov regularization method, now possible for observations in the Earth's radiation belt for the first time, allows direct inference of electron fluxes without resorting to predetermined functional forms. Comparisons of our results with those from conventional methods indicate differences among the results as large as ∼200%.     

Fabrication and testing of the recoil mass spectrometer at Bombay Pelletron

A recoil mass spectrometer (RMS) has been designed, fabricated and installed at the 15° S beam-line of the Pelletron at TIFR. The RMS consists of a quadrupole doublet just after the target chamber followed by an ‘electrostatic deflector’, a magnetic dipole and a second electrostatic deflector. The recoils produced in the ¹²C + ⁵⁸Ni reaction using 60 MeV ¹²C beam were focussed with the help of electric and magnetic fields and detected in a strip detector placed at the focal plane of the RMS. Further testing of the spectrometer to obtain mass resolution and efficiency are in progress.     

喷气式Z箍缩等离子体发射离子束能谱的研究

使用紧凑型汤姆生离子谱仪对喷气式Z箍缩（Z-pinch）等离子体发射的离子束能谱进行了实 验研究.紧凑型汤姆生离子谱仪由入射窗、偏转电磁场、后置针孔及CR-39探测板组成.等离 子体发射的离子束经前置针孔、谱仪入射窗准直后进入偏转电磁场偏转,由后置针孔射出轰 击探测板形成可分辩的抛物线簇.对抛物线簇进行分析处理,得到了等离子体辐射的离子束 能谱及能谱随箍缩状况的变化趋势. 关键词： Z箍缩等离子体 紧凑型汤姆生谱仪 离子束能谱     

Dressed ion acoustic solitary waves in quantum plasmas with two polarity ions and relativistic electron beams

A theory for dressed quantum ion acoustic waves (QIAWs), which includes higher-order corrections when QIAWs are investigated by the reductive perturbation method, is presented for unmagnetized plasmas containing positive and negative ions and weakly relativistic electron beams. The properties of the QIAWs are investigated using a quantum hydrodynamic model, from which a Korteweg–de Vries equation is derived using the reductive perturbation method. An equation including higher-order dispersion and nonlinearity corrections is also derived, and the physical parameter space is discussed for the importance of these corrections.     

二次电子对离子束流品质影响的动态研究

基于粒子云网格的计算方法,建立了具有外电路的加速系统模型,模拟了氘离子束轰击靶面产生二次电子的过程,动态地分析了二次电子的产生对离子加速电压的影响以及对氘离子束束流品质和氘离子束轰击靶面能量的影响。结果表明,当有0.06 A的二次电子电流产生时,离子加速电压将会下降45%,从而导致氘离子束束流品质下降,参与氘氚反应的氘离子数减少,相应的氘离子束轰击靶面的能量下降43.8%。     

Post-accelerator LINAC design for the VECC RIB project

Variable Energy Cyclotron Centre (VECC) is presently developing an ISOL post-acclerator type of RIB facility. The scheme utilises the existing K=130 room temperature variable energy cyclotron machine as the primary accelerator for the production of RIBs and radio frequency quadrupole (RFQ) and LINAC modules for the post-acceleration. The design aspects of these post-accelerator LINAC modules will be discussed in this paper.     

Interaction of charged particles with nanotubes

We have studied the interaction of charged particles with single-walled metallic nanotubes by solving Poisson’s equation with appropriate boundary conditions. Numerical results for the energy dispersion relations as a function of the wave vector are presented when the charged particle is inside or outside the nanotube.     

A comparative study of secondary ion yield from model biological membranes using Aun and C60 primary ion sources

Au_n⁺ and C₆₀⁺ primary ion sources have been used to acquire spectra from phospholipids, symmetric liposomes and asymmetric liposomes. We demonstrate that when using different ion beams different chemical information can be obtained. Symmetric and asymmetric liposomes, with 95% asymmetry, were produced and analysed with Au⁺, Au₃⁺ and C₆₀⁺ primary ion beams. C₆₀⁺ gave the greatest yield from the symmetric liposome but after correcting for the yield effects on the data obtained from the asymmetric liposome it has been shown that C₆₀⁺ is the most surface sensitive, providing the least information from the inner leaflet of the liposome. Au_n⁺ provides the greatest amount of information from the inner leaflet. The results present the possibility of designing ToF-SIMS experiments that selectively probe specific regions of a (bio)molecular surface.     

Status report on the folded tandem ion accelerator at BARC

The folded tandem ion accelerator (FOTIA) facility set up at BARC has become operational. At present, it is used for elemental analysis studies using the Rutherford backscattering technique. The beams of¹H, ⁷Li, ¹²C, ¹⁶O and ¹⁹F have been accelerated up to terminal voltages of about 3 MV and are available for experiments. The terminal voltage is stable within ±2 kV. In this paper, present status of the FOTIA and future plans are discussed.     

ECR ion source based low energy ion beam facility

Mass analyzed highly charged ion beams of energy ranging from a few keV to a few MeV plays an important role in various aspects of research in modern physics. In this paper a unique low energy ion beam facility (LEIBF) set up at Nuclear Science Centre (NSC) for providing low and medium energy multiply charged ion beams ranging from a few keV to a few MeV for research in materials sciences, atomic and molecular physics is described. One of the important features of this facility is the availability of relatively large currents of multiply charged positive ions from an electron cyclotron resonance (ECR) source placed entirely on a high voltage platform. All the electronic and vacuum systems related to the ECR source including 10 GHz ultra high frequency (UHF) transmitter, high voltage power supplies for extractor and Einzel lens are placed on a high voltage platform. All the equipments are controlled using a personal computer at ground potential through optical fibers for high voltage isolation. Some of the experimental facilities available are also described.     

Optimized FIB silicon samples suitable for lattice parameters measurements by convergent beam electron diffraction

The aim of this paper is to check the effect of artefacts introduced by focused ion beam (FIB) milling on the strain measurement by convergent beam electron diffraction (CBED). We show that on optimized silicon FIB samples, the strain measurement can be performed with a sensitivity of about 2.5 × 10⁻⁴ which is very close to the theoretical one and we conclude that FIB preparation can be suitable for such measurements in microelectronic devices.

To achieve this, we first used CBED and electron energy loss spectroscopy (EELS) which provide a procedure permitting an exact knowledge of the sample geometry, i.e. the thickness of both amorphous and crystalline layers. This procedure was used in order to measure the FIB-amorphized sidewall layer. It was found that if the FIB preparation is optimized one can reduce this amorphous layer down to around 7 nm on each side. Secondly different preparation techniques (cleavage, Tripod™ and FIB) permit to check if the surface damaged layer introduced by FIB influences the strain state of the sample. Finally, it was found that the damaged layer does not introduce measurable strain in pure silicon but reduces appreciably the quality of the CBED patterns.     

Segmented linear radiofrequency quadrupole/laser ion source project at TRIUMF

Reactions such as ²⁵Al(p,γ)²⁶Si are the key to understand the production of ^26g Al and ^26m Al in our galaxy. Experimental results could provide important constraints on nova nucleosynthesis and modelling where ²⁶Al is believed to be produced. To achieve such measurements, high-intensity and high-purity radioactive beams are required. However, production targets at ISOL-type facilities such as ISAC at TRIUMF produce high-intensity alkali beams by surface ionization on hot transfer tubes hampering the measurement of isotopes of interest. To overcome this issue, an ion source combining a segmented linear radiofrequency quadrupole (RFQ) to a laser ion source is being built. Its main function is to suppress alkali impurities whilst allowing for fast-release of short-lived isotopes. The beam production method, the RFQ/laser ion source and the removal of alkali contaminants are discussed in this paper.     

Carbon-cluster mass calibration at SHIPTRAP

A carbon-cluster ion source has been installed and tested at SHIPTRAP, the Penning-trap mass spectrometer for precision mass measurements of heavy elements at GSI. Carbon-cluster ions ¹²C_n ⁺, 5 ≤n ≤23, were produced by laser-induced desorption and ionization from a carbon sample. They were tested for the first time as reference ions in an on-line mass measurement of the radionuclides ¹⁴⁴Dy, ¹⁴⁶Dy and ¹⁴⁷Ho. In addition, carbon clusters of various sizes were used for an investigation of the systematic uncertainty of SHIPTRAP covering a mass range from 84 u to 240 u. The mass-dependent uncertainty was found to be negligible for the case of (m-m _ref)< 100 u. However, a systematic uncertainty of 4.5 ×10^-8 was revealed.     

Self-assembly of well-ordered and highly uniform nanoripples induced by focused ion beam

Well-ordered and highly uniform nanoripple structures on the surface of single crystal LaAlO₃ (1 0 0), SrTiO₃ (1 0 0) and Al₂O₃ (0 0 0 1) were formed via self-assembly (not by beam writing) by focused ion-beam bombardment. The morphology and topography of nanoripple structures were characterized using in-situ focused ion-beam/scanning electron microscope, as well as ex-situ atomic force microscopy. Under off-normal bombardment without sample rotation, the characteristic wavelength of nanoripples varying from 248 to 395 nm on the LaAlO₃ (1 0 0) surface can be obtained by changing ion fluence and incident angle. When all sputtering parameters except the ion fluence are constant, the wavelength of nanoripples is increased with the enhanced ion fluence. These results demonstrate the potential application of using ion sputtering method for fabricating the well-ordered and highly uniform nanoripples which can be used in nanodevices.