A CsI hodoscope on CSHINE for Bremsstrahlung {\gamma}-rays in Heavy Ion Reactions

Bremsstrahlung $\gamma$ production in heavy ion reactions at Fermi energies carries important physical information including the nuclear symmetry energy at supra-saturation densities. In order to detect the high energy Bremsstrahlung $\gamma$ rays, a hodoscope consisting of 15 CsI(Tl) crystal read out by photo multiplier tubes has been built, tested and operated in experiment. The resolution, efficiency and linear response of the units to $\gamma$ rays have been studied using radioactive source and $({\rm p},\gamma)$ reactions. The inherent energy resolution of $1.6\%+2\%/E_{\gamma}^{1/2}$ is obtained. Reconstruction method has been established through Geant 4 simulations, reproducing the experimental results where comparison can be made. Using the reconstruction method developed, the whole efficiency of the hodoscope is about $2.6\times 10^{-4}$ against the $4\pi$ emissions at the target position, exhibiting insignificant dependence on the energy of incident $\gamma$ rays above 20 MeV. The hodoscope is operated in the experiment of $^{86}$Kr + $^{124}$Sn at 25 MeV/u, and a full $\gamma$ energy spectrum up to 80 MeV has been obtained.Comment: 9 pages, 19 figure

High-κ/metal gate for advanced transistor applications

Advanced HfO2 high-κ materials have been developed to replace SiO2 as the gate dielectrics. The Electronic structures consisting of various HfO2/SiO2/Si-substrate gate stacks have been characterized. The hafnium silicate formed at the HfO2/SiO2 interface is found to play a key role in generating an internal electric field established by interface dipoles. It is found that the variation of the interface dipole with the applied gate voltage results in hysteresis between the forward and reverse CV curves. The magnitude of this hysteresis is observed to be dependent on the viscosity of the local oxide network, the dipole elasticity, the gate-oxide thickness, and the pulse ramp rate. GaN has attracted much attention in replacing silicon in high-power transistors. However, an unavoidable formation of GaOx is the source of charged states which degrade the performance of transistors. It is found that the application of trimethylaluminum-only cycles could convert GaOx into Al2O3. However, a lower limit on the thickness of the resultant Al2O3 layer is necessary to block oxidizing agents during subsequent Al2O3 deposition. Further, it is observed that GaOx layer undergoes a structural change after an elevated-temperature annealing, resulting in a reduction in the Al2O3/GaN-interface defect density.DOCTOR OF PHILOSOPHY (EEE

The role of the disordered HfO2 network in the high- κ n-MOSFET shallow electron trapping

Current understanding of the bias temperature instability degradation usually comprises two parts: (1) shallow-level component that can recover within a short time and (2) deep level traps that the emission time of the trapped carrier is extremely long. Prevenient studies of the positive bias temperature instability degradation in the high-κ n-MOSFET indicate that oxygen vacancy (VO) is the dominant defect type that responds for the shallow electron trapping. However, recent experimental results reveal that the VO defect density required to accommodate the experimental measured recoverable threshold voltage degradation (ΔVth) is much higher than that of the reasonable atomic structure in the amorphous HfO2. On the other hand, investigations on the disordered Hf-O-Hf network in the amorphous HfO2 reveal their capabilities as charge trapping centers; therefore, in this work, atomic simulation work is performed, and our results show that the disordered Hf-O-Hf networks can act as effective electron capture centers with shallow levels near the Si conduction band. Moreover, the high density of the stretched Hf-O-Hf networks in the amorphous HfO2 also significantly enriches the shallow electron traps in the oxide.MOE (Min. of Education, S’pore)Published versio

A Comprehensive Review of Recent Progress on GaN High Electron Mobility Transistors: Devices, Fabrication and Reliability

GaN based high electron mobility transistors (HEMTs) have demonstrated extraordinary features in the applications of high power and high frequency devices. In this paper, we review recent progress in AlGaN/GaN HEMTs, including the following sections. First, challenges in device fabrication and optimizations will be discussed. Then, the latest progress in device fabrication technologies will be presented. Finally, some promising device structures from simulation studies will be discussed

Investigation of AlGaN/GaN HEMTs degradation with gate pulse stressing at cryogenic temperature

Degradation on DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) after applying pulsed gate stress at cryogenic temperatures is presented in this paper. The nitrogen vacancy near to the AlGaN/GaN interface leads to threshold voltage of stress-free sample shifting positively at low temperature. The anomalous behavior of threshold voltage variation (decrease first and then increase) under gate stressing as compared to stress-free sample is observed when lowing temperature. This can be correlated with the pre-existing electron traps in SiNX layer or at SiNX/AlGaN interface which can be de-activated and the captured electrons inject back to channel with lowering temperature, which counterbalances the influence of nitrogen vacancy on threshold voltage shift

A novel fabrication technique for three-dimensional concave nanolens arrays

A novel facile technique is proposed for fabricating three-dimensional (3D) concave nanolens arrays on a silicon substrate. The technique leverages an inherent characteristic of the polymethyl methacrylate (PMMA) resist during inductively coupled plasma (ICP) etching. The tendency for plasma ions to accumulate at the edge of the PMMA resist helps create a local electric field that causes the ions to etch the sidewall of the PMMA resist. This process progressively increases the uncovered area, resulting in a graded etched depth or a concave structure in the substrate. In addition, using a given ICP etching recipe, the time required for a PMMA resist to be removed by sidewall etching is determined by its width. The use of PMMA resist of different widths enables one to achieve structures of varying etched depths and thus a 3D lens array. Optical characteristics of the fabricated nanolens were simulated using the FDTD (Finite-difference time-domain) method, and focal lengths ranging from 150 nm to 420 nm were obtained. This type of nanolens is very useful in ultraviolet optical devices and CMOS image sensors.Published versio

Development of a semi-cylindrical time projection chamber prototype for ( 3He,t^{3}{\textrm{He}},t 3 He , t ) charge exchange reaction experiment

Abstract The charge exchange (CE) reaction is an effective probe to study the structure of atomic nuclei in the isospin dimension, which has been studied for decades. To expand the range of nuclei studied by CE reactions to a wider range and research the structure characteristics of unstable nuclei, including the isospin symmetry, spin-isospin excitation, and nuclear symmetry energy, a semi-cylindrical time projection chamber (scTPC) prototype was designed and constructed to probe ( $$^{\textrm{3}}$$ 3 He,t) CE reactions in inverse kinematics. The 266 nm UV laser was used to achieve electron-drift-velocity calibration. The scTPC has an energy resolution (FWHM) of 5.6% for $$\alpha$$ α particles emitted by $$^{\textrm{241}}$$ 241 Am radioactive source. The position resolution of scTPC is described by the residual method. The spatial resolution on the pad plane is 409 \upmu μ m. And the position resolution in the drift direction is 326 \upmu μ m, equivalent to an angular resolution of 0.4 $$^\circ$$ ∘ . These performances suggest that the scTPC can measure $$\Delta E$$ Δ E and particle tracks precisely. The successful development of the scTPC prototype provides better conditions for the next step of experimental data analysis and processing

Exploration of grid scheme for Frisch-grid ionization chamber

A new Frisch-grid ionization chamber has been built to explore the appropriate choice of Frisch-grid. Detailed studies of the relationship between grid geometries and detector performance have been performed with an $$^{241}$$Am source. This paper describes and compares the energy resolution of ionization chambers with parallel-wire and mesh grids of different grid parameters. Some specific recommendations for grid selection are provided based on the data currently available. To obtain optimal energy resolution, the operating voltage of the chamber must satisfy the condition of minimum electron collection on the grid with distinct geometries and parameters, respectively. Since there is no established theory applicable to both types of grids, we have devised a careful simulation procedure incorporating the COMSOL and Garfield++ codes to search for the conditions of the minimum electron collection on the grid. The simulation results fit the experimental data well, suggesting that this simulation method successfully predicts the suitable voltage setting when using a mesh grid or parallel wires grid as the shielding electrode

New trapezoid-shaped Frisch-grid ionization chamber for low-energy particle measurements

A new trapezoid-shaped Frisch-grid ionization chamber (TFG-IC) has been built as a part of a $$\varDelta {E}-E$$ telescope system for the detection and identification of charged particles at energies down to a few MeV. To study the effect of the drift electric field uniformity, two types of sealed windows, namely a pair of SSA (split-strip aluminized mylar film) and a pair of DSA (double-sided aluminized mylar film) sealed windows have been investigated. The detector’s performances were studied using a standard $$^{241}$$Am source at different gas pressures, and the total energy-deposit resolution achieved is about 1.1%(FWHM). The $$\varDelta {E}-E$$ telescope, which was composed of TFG-IC and a DSSSD (double-sided silicon strip detector), has been tested using a three-component $$\alpha$$ source and the $$^{241}$$Am source under laboratory conditions. The results show that the energy resolution with the SSA sealed windows which provide uniform drift electric field has a smaller fluctuation than that with the DSA ones; the fluctuations are about 1% and 4% for the former and the latter, respectively. Simulations using the COMSOL software also confirmed the electric-field distortion at the edge of the detector with the DSA windows. A correlation curve between energy resolution and energy deposit of charged particles at various gas pressures and for two gas species is derived for TFG-IC with the SSA sealed windows using the measurement with the $$^{241}$$Am source. Incorporating the above results, we performed Monte Carlo simulations to evaluate the particle-identification capability of the telescope. The results show that the telescope can be extended to the identification of low-energy particles