Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

Crystal growth and dielectric, piezoelectric and elastic properties of Ca3TaGa3Si2O14 single crystal

Single crystals of Ca₃TaGa₃Si₂O₁₄ (CTGS) were successfully grown from stoichiometric melts by the conventional Czochralski technique. The relative dielectric constants, the piezoelectric strain constants and the elastic compliance constants of CTGS single crystal have been determined by an electric bridge and resonance-antiresonance method. At room temperature, the two piezoelectric strain constants d₁₁ and d₁₄ are −4.58×10⁻¹² coulombs per newton (C/N) and 10.43×10⁻¹² coulombs per newton (C/N), respectively. The velocities of the bulk acoustic wave are also calculated.     

Diffusion of 55Fe in Fe2O3 single crystals

The diffusion of ⁵⁵Fe has been measured parallel to the c axis of Fe₂O₃ single crystals at temperatures in the range 708–1303°C and at an oxygen activity of unity. The tracer penetration profiles were determined using sectioning techniques. For temperatures above 900°C the tracer diffusion coefficient is given byD¹(Fe) = 1.6 × 10⁹ exp[?6.0 (eV)/kT] cm² s^?1 and below 900°C by 2.8 × 10^?9 exp[?1.8 (eV)kT]. The high-temperature behaviour is probably characteristic of pure Fe₂O₃, whereas diffusion at lower temperatures may be influenced by impurities. The most likely defects responsible for diffusion of Fe are iron interstitials and, for oxygen, oxygen vacancies, and the observed activation energies are discussed in terms of the properties of these defects. The diffusion data and defect models have been used to predict the rate of growth of Fe₂O₃ and indicate that outward Fe diffusion is the dominant transport process. Previously published data for Fe₂O₃ growth in a variety of experimental situations have been corrected to a single rate constant using a model for multilayer growth. The corrected data are all in good agreement but are approximately two orders of magnitude greater than predicted from diffusion data, which suggests that grain boundary diffusion controls the growth of Fe₂O₃ in practice.     

SrBi2Ta2O9铁电薄膜微结构及其抗疲劳等铁电特性的研究

在低温Pt/Ti/SiO₂/Si衬底上用脉冲准分子激光沉积技术结合氧气氛下700℃退火获得高质量的SBT薄膜，其择优取向为（008）和（115）．薄膜厚度约为200nm．铁电性能测试显示较饱和的、方形的电滞回线，其剩余极化和矫顽电场分别为10μC/cm²和57kV/cm，在10¹⁰次开关极化后没有显示任何疲劳，在5V直流电压下的漏电流密度约为4×10^-8A/cm²，直流击穿电场约为250kV/cm 关键词：     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Electrical and optical activation studies of high dose Si-implanted Al0.18Ga0.82N

Both electrical and optical activation studies of Si-implanted Al_0.18Ga_0.82N have been made as a function of anneal time and anneal temperature to obtain maximum possible electrical activation efficiency. Silicon ions were implanted at 200 keV with doses of 5×10¹⁴ and 1×10¹⁵ cm⁻², and the samples were annealed from 1100 to 1250 ^°C for 5-25 min with a 500 Å thick AlN cap in a nitrogen environment. The electrical activation efficiency and Hall mobility increase with anneal time and anneal temperature. Nearly 100 and 95% electrical activation efficiencies were obtained for Si-implanted Al_0.18Ga_0.82N with doses of 5×10¹⁴ and 1×10¹⁵ cm⁻² and annealing at 1250 and 1200 ^°C for 25 min, respectively. The photoluminescence measurements show an excellent implantation damage recovery after annealing at these optimum anneal conditions, showing a strong near band emission. These optical results correlate well with the electrical results.     

Trapping center parameters in In6S7 crystals

Thermally stimulated current measurements were carried out on In₆S₇ single crystals in the temperature range of 10-225 K with a constant heating rate of 0.8 K/s. The study of trapping centers was accomplished by the measurements of current flowing along the c-axis of crystals. The analysis of the glow curve according to various methods, such as curve fitting, initial rise and peak shape methods, gives results in good agreement with each other and revealed two trapping centers in In₆S₇ with activation energies of 157 and 290 meV. Their capture cross sections have been determined as 7.5×10⁻²³ and 7.1×10⁻²⁰ cm²,respectively. The good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping has confirmed that retrapping is negligible in these centers.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

High-Resolution Study of the Three Lowest Infrared Bands of PHD2

The high-resolution (2.3×10⁻³ cm⁻¹) Fourier transform infrared spectrum of PHD₂ was recorded for the first time and analyzed in the region of the three lowest bands belonging to the three bending fundamentals. Between 1100 and 1800 transitions were assigned to each of the ν₃, ν₄, and ν₆ bands and 1267 upper energies were obtained thereof. These were fitted by 58 parameters, which reproduce the experimental upper energies with rms deviations of 1.3×10⁻⁴ cm⁻¹ and 2.7×10⁻⁴ cm⁻¹ for the J≤17 and J≥18 levels, respectively.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.     

Ferroelectric properties of Pr6O11-doped Bi4Ti3O12

Praseodymium doped Bi₄Ti₃O₁₂ (BIT) ceramics with composition Bi_2.9Pr_0.9Ti₃O₁₂ (BPT) were prepared by solid state reaction. These samples have polycrystalline Bi-layered perovskite structure without preferred orientation, and consist of well-developed plate-like grains with random orientation. Pr doping into BIT causes a large shift of the Curie temperature (T_C) of the BIT from 675 to 398 °C. At an electric field of 87 kV/cm, the remanent polarization and the coercive field of the BPT ceramics are 30 μC/cm² and 52 kV/cm, respectively. Furthermore, the dielectric permittivity and the dissipation factor of the BPT ceramics are 300 and 0.003 at 1 MHz, 1 V, and room temperature. Ferroelectric properties of the BPT ceramics are superior to V-doped Bi₄Ti₃O₁₂ (∼20 μC/cm² and 80 kV/cm) and (Sr, Ta)-doped Bi₄Ti₃O₁₂ (∼12 μC/cm² and 71 kV/cm) ceramics. In addition, the dense ceramics of praseodymium-doped B₄Ti₃O₁₂ were obtained by sintering at 1100 °C, about 100-200 °C lower than those of the SrBi₂Ta₂O₉ system.     

Magnetic properties of the garnets Bi0·8Ca2χT2·2-2.χFe5-χVχ]O12

The behaviour of the magnetization, Curie temperature, Mössbauer spectra, and lattice parameter is studied in the garnet series Bi_0·8Ca_2.χT_2·2-2.χFe_5-χV_χ]O₁₂. The shape of magnetization vs temperature curves shows only a minor dependence on x. The hyperfine field at the octahedral ⁵⁷Fe nuclei at 5°K decreases linearly with x (12·5kOe per substituted V neighbour), while that at the tetrahedral ⁵⁷Fe nuclei is not affected. The dependence of the Curie temperatures and hyperfine fields on x is discussed in relation to the Fe-O-V-O-Fe exchange. The influence of Bi substitution is consistent with the idea of a geometric effect.     

Kaonic atoms measurements at the DAΦNE accelerator

Radioactive ⁵⁷Mn⁺(T _1/2?= 1.5 min) ions have been implanted at the ISOLDE facility at CERN with 60 keV energy to fluences <10¹²/cm² into p-type Si_1???x Ge _x (x < 0.1) single crystals held at 300–600 K. The implantation and annealing processes result in the majority of the implanted Mn ions occupying substitutional lattice sites. In the subsequent ⁵⁷Mn nuclear β ^???-decay to the 14.4 keV Mössbauer state of ⁵⁷Fe (T _1/2?= 100 ns), an average recoil energy of 40 eV is imparted to the ⁵⁷Fe daughter atoms which results in a large fraction being expelled into tetrahedral interstitial sites and the creation of a vacancy. The remainder occupies substitutional sites. This technique of recoil production of ^57m Fe_I thus allows for the study of the diffusion characteristics of interstitial Fe. From the temperature dependent line broadening, the activation energies have been determined and decrease with increasing Ge concentration which contributes significantly to the increase of the jump frequency. A similar result has been obtained in n-type SiGe but there the values for the activation energies were much higher.     

Electrical properties of La2O3 thin films grown on TiN/Si substrates via atomic layer deposition

The electrical as well as the structural properties of La₂O₃ thin films on TiN substrates were investigated. Amorphous stoichiometric La₂O₃ thin films were grown at 300 °C via atomic layer deposition technique by using lanthanum 2,2,6,6-tetramethyl-3,5-heptanedione [La(TMHD)₃] and H₂O as precursors. Post-annealing of the grown film induced dramatic changes in structural and the electrical properties. Crystalline phases of the La₂O₃ film emerged with the increase of the post-annealing temperature. Metal-insulator-metal (MIM) capacitor was fabricated to measure the electrical properties of the grown film. The dielectric constant of the La₂O₃ thin films increased with annealing temperature to reach the value of 17.3 at 500 °C. The leakage current density of the film post-annealed at 400 °C was estimated to be 2.78 × 10⁻¹⁰ and 2.1 × 10⁻⁸ A/cm² at ±1 V, respectively.     

Femtosecond optical Kerr effect study of Ge10As40S30Se20 film

The results of the femtosecond optical heterodyne detection of optical Kerr effect at 805 nm with the 80 fs ultrafast pulses in amorphous Ge₁₀As₄₀S₃₀Se₂₀ film is reported in this paper. The film shows an optical non-linear response of 200 fs under ultrafast 80 fs-pulse excitation, and the values of real and imaginary parts of non-linear susceptibility χ⁽³⁾ were 9.0×10⁻¹² and −4.0×10⁻¹² esu, respectively. The large third-order non-linearity and ultrafast response are attributed to the ultrafast distortion of the electron orbits surrounding the average positions of the nucleus of Ge, As, S and Se atoms. This Ge₁₀As₄₀S₃₀Se₂₀ chalcogenide glass would be expected as a promising material for optical switching technique.     

Impact of 100 MeV Au on the surface of relaxed Si0.5Ge0.5 alloy films studied by atomic force microscopy

We demonstrate a gradual surface modification process of relaxed Si_0.5Ge_0.5 alloy films by 100 MeV Au beam with fluence varying between 5 × 10¹⁰ and 1 × 10¹² ions/cm² at 80 K by means of atomic force microscopy (AFM). Presence of Ge quantum dots (QDs) was found in the virgin sample. The disappearance of the QDs were noticed when the samples were irradiated with a fluence of 5 × 10¹⁰ ions/cm². Craters were found developing at a fluence of 1 × 10¹¹ ions/cm². Apart from the evolution of the craters, blisters were also detected at a fluence of 1 × 10¹² ions/cm². Variation of the average root mean square value of the surface roughness as a function of fluence was examined.     

Ferroelectric properties of neodymium-doped Sr2Bi4Ti5O18 thin film prepared by solgel route

Sr₂Bi₄Ti₅O₁₈ (SBTi) and Nd-modified SBTi (SBNT) thin films were deposited on Pt/Ti/SiO₂/Si (1 0 0) substrates using a sol-gel method. Structure, morphology and electric properties were investigated systematically. These films were randomly oriented and composed of rod-like grains. The remanent polarization (2P_r) and coercive field (E_c) of SBNT films were 30 μC/cm² and 55 kV/cm, respectively. This value of 2P_r was much higher than the reported value of SBTi prepared by pulsed-laser deposition. More importantly, the SBNT films showed high fatigue resistance against continuous switching up to 3×10⁹ cycles and excellent charge-retaining ability up to 3×10⁴ s.     

Synthesis and characterization of thermoluminescent Li2B4O7 nanophosphor

Lithium borate (Li₂B₄O₇) is a low Z_eff, tissue equivalent material that is commonly used for medical dosimetry using the thermoluminescence (TL) technique. Nanocrystals of lithium borate were synthesized by the combustion method for the first time in the laboratory. TL characteristics of the synthesized material were studied and compared with those of commercially available microcrystalline Li₂B₄O₇. The optimum pre-irradiation annealing condition was found to be 300 °C for 10 min and that of post-irradiation annealing was 300 °C for 30 min. The synthesized Li₂B₄O₇ nanophosphor has very poor sensitivity for low doses of gamma up to 10¹ Gy whereas from 10¹ to 4.5×10² Gy this phosphor exhibits a linear response and then from 4.5×10² to 10³ Gy it shows supralinearity. Thermoluminescence properties of Li₂B₄O₇ nanophosphor doped with Cu has also been investigated in this paper. It shows low fading and a linear response over a wide range of gamma radiation from 1×10² to 5×10³ Gy. Therefore the synthesized lithium borate nanophosphor doped with Cu may be used for high dose measurements of gamma radiations.     

High spin states in 57Co

High spin states of ⁵⁷Co have been studied via prompt γ-ray spectroscopy in the reactions ⁴⁸Ti(¹²C, p2n) and ⁵⁴Fe(α, p) at 26–48 MeV and 12–24 MeV, respectively. The energies and decay modes of these levels were determined from the analysis of γ-ray singles and γ-γ coincidence spectra, excitation functions, angular distributions and correlations. The relevant lifetimes were measured by the Doppler-shift attenuation method. The new levels established in this work are at 4037, 4814 and 5918 keV with the most probable J^π assignment of $\text{15}\text{2}{}^{\mathrm{?}}$, if $\text{17}\text{2}{}^{\mathrm{?}}$ and $\text{19}\text{2}{}^{\mathrm{?}}$, respectively. The previously known level at 2524 keV was assigned to have $\text{J}{}^{\mathrm{\pi }}\text{=}\text{13}\text{2}{}^{\mathrm{?}}$. These together with the known $\text{9}\text{2}{}^{\mathrm{?}}$(1224 keV) and $\text{11}\text{2}{}^{\mathrm{?}}$(1690 keV) levels constitute the yrast states of ⁵⁷Co. The measured lifetimes of the above six levels are (in order of increasing energies) 0.085±0.030, 0.32±0.10, 0.16±0.06, 0.10_?0.07^+0.06, 1.5_?0.5⁴ and 0.17_?0.07^+0.08 ps, respectively. Comparisons with some theoretical calculations are presented.