Description of non-equilibrium phenomena in pulsed MOS capacitors

A theoretical investigation of the non-equilibrium phenomena of pulsed MOS capacitors is presented. Unlike treated previously, a time-dependent minority carrier generation rate of bulk traps is considered and an improved model for the generation width is applied. A differential equation is found, which describes the change of the space-charge region in the non-equilibrium transients. The dependence of MOS capacitance and current on the width of the space-charge region is ascertained. In such a manner we can describe the non-equilibrium transients of pulsed MOS capacitors more accurately than in previous works.     

Comparative study of electrical characteristics for n-type 4H–SiC planar and trench MOS capacitors annealed in ambient NO

The interface properties and electrical characteristics of the n-type 4H-SiC planar and trench metal–oxide–semiconductor(MOS) capacitors are investigated by measuring the capacitance voltage and current voltage. The flat-band voltage and interface state density are evaluated by the quasi-static method. It is not effective on further improving the interface properties annealing at 1250℃ in NO ambient for above 1 h due to the increasing interface shallow and fast states.These shallow states reduce the effective positive fixed charge density in the oxide. For the vertical MOS capacitors on the(1120) and(1100) faces, the interface state density can be reduced by approximately one order of magnitude, in comparison to the result of the planar MOS capacitors on the(0001) face under the same NO annealing condition. In addition, it is found that Fowler–Nordheim tunneling current occurs at an oxide electric field of 7 MV/cm for the planar MOS device.However, Poole–Frenkel conduction current occurs at a lower electric field of 4 MV/cm for the trench MOS capacitor. This is due to the local field crowded at the trench corner severely causing the electrons to be early captured at or emitted from the SiO_2/Si C interface. These results provide a reference for an in-depth understanding of the mobility-limiting factors and long term reliability of the trench and planar SiO_2/Si C interfaces.     

Intracavity laser absorption spectroscopy of platinum fluoride, PtF

Two vibrational bands of an electronic transition of PtF occurring at 11 940 cm⁻¹ and 12 496 cm⁻¹ were recorded and analyzed. These transitions are identified as the (0,0) and (1,0) bands of an [11.9] Ω = 3/2 − XΩ = 3/2 electronic transition. Gas phase PtF was produced in a copper hollow cathode lined with platinum foil using a trace amount of SF₆, and the spectrum was recorded at Doppler resolution by intracavity laser absorption spectroscopy. This work represents the first published spectroscopic data on PtF. Molecular constants for the ground and excited electronic states are presented.     

Investigation of the characteristic energy losses of platinum (111)

With the help of a combined LEED- and Auger-investigation, the surface of a platinum (111) crystal was cleaned first. Then, the spectrum of the characteristic energy losses for both contaminated and cleaned surfaces is studied. On the cleaned surface the following losses were found: ΔE ₁=7.4 eV, ΔE ₂=13.5 eV, ΔE ₃=24.8 eV, ΔE ₄=31.8 eV, ΔE ₅=45.1 eV, ΔE ₆=54.1 eV, ΔE ₇=71.2 eV. The present results are compared with the measurement of other investigators. In particular, in good agreement with optical measurements we identify ΔE ₁ and ΔE ₂ as interband transitions, and ΔE ₃ and ΔE ₄ as surface and volume plasma loss, respectively.     

Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Extended embedded-atom method for platinum nanoparticles

We present a new technique to extend the embedded-atom method (EAM) for the simulations of non-bulk systems down to the atomic cluster level. To overcome the limitation of the traditional bulk-fit EAM interatomic potentials, bond characteristics from first-principles calculations are systematically included by introducing a local structure dependent prefactor with three additional parameters to the conventional EAM many-body term. The additional parameters improve the local potential landscape virtually for the entire range of atomic configuration space in a quantitative sense. The proposed scheme is applied to two different EAM function sets and validated for both bulk and non-bulk environments in elemental platinum. The obtained material properties, including the binding energies of Pt particles and the Pt adatom diffusion barrier on the Pt(1 1 1) surface, show a significant improvement over the conventional EAM formalism.     

Ozone oxidation of 4H-SiC and flat-band voltage stability of SiC MOS capacitors

We investigate the effect of ozone (O₃) oxidation of silicon carbide (SiC) on the flat-band voltage (V_fb) stability of SiC metal-oxide-semiconductor (MOS) capacitors. The SiC MOS capacitors are produced by O₃ oxidation, and their V_fb stability under frequency variation, temperature variation, and bias temperature stress are evaluated. Secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) indicate that O₃ oxidation can adjust the element distribution near SiC/SiO₂ interface, improve SiC/SiO₂ interface morphology, and inhibit the formation of near-interface defects, respectively. In addition, we elaborate the underlying mechanism through which O₃ oxidation improves the V_fb stability of SiC MOS capacitors by using the measurement results and O₃ oxidation kinetics.     

基于铂电阻的机载测温系统校准方法改进

为了解决机载铂电阻测温系统机上校准工作量大、实施难等问题,根据机载铂电阻测温原理和校准方法,分析了对于单个测温通道在实验室与机载环境下测量结果之间的差异,提出了一种对实验室校准曲线进行修正而替代机上校准曲线的方法,并通过实验对该方法进行验证;设计了根据机载测试系统的数据文件、系统配置文件,批量修正实验室校准曲线的软件;任意选择的8个参数进行工程验证,采用原方法和本文方法进行校准得到的两组校准曲线,分别对测试数据进行处理,得到测量结果的最大偏差与量程比不超过0.27％,证明修正后实验室校准曲线与机上校准曲线基本一致,可以用作数据处理;结果证明,改进后校准方法方便、高效,校准结果满足机载测试要求,为机载参数校准提供了新思路。     

XPS and STM study of carbon deposits at the surface of platinum (110)

The deposition of carbon due to high temperature ethylene decomposition at Pt(110) was studied by XPS and STM techniques. In the temperature range 700–1400 K no graphite species were observed. Instead, two carbon states were distinguished by XPS. At temperatures 700–850 K chemisorbed carbon layer is formed with BE(C1s) = 284.2 eV, this carbon state reacting readily with both oxygen and hydrogen. At T> 850 K carbon layer with BE(C1s) = 284.6–284.9 eV is formed. Further study showed this carbon species to be stable up to 1150 K and to be inert towards both hydrogen and oxygen up to 1000 K. This state was attributed to diamond-like carbon (DLC). STM study of DLC on Pt(110) revealed the patched pseudo-C-(3 × 1) structure. This reconstruction is believed to account for the DLC formation at platinum (110) surface.     

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlO_x high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔV_FB-T) measurements. A 6.5 V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 10¹³ cm⁻² and about 88% stored electron reserved after apply ±8 V program or erase voltage for 10⁵ s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlO_x based MOS structure has a promising application in non-volatile flash memory devices.     

Sonochemical enantioselective hydrogenation of ethyl pyruvate over platinum catalysts

Chiral sonochemical hydrogenation of an aliphatic -ketoester, ethyl pyruvate to ethyl lactate was carried out over various platinum catalysts in different solvents under atmospheric hydrogen pressure. The reaction rates and the enantiomeric excesses were determined over Pt/C, Pt/SiO₂ and Pt/K-10 catalysts both under conventional and sonochemical conditions. The effect of ultrasounds on the catalytic activity and enantioselectivity was tested applying sonochemical pretreatment before the reaction. The ultrasonic irradiation was found to be highly advantageous in these hydrogenations. After insonation of the catalysts, the enantioselectivity was highly improved over Pt/SiO₂ and Pt/K-10 catalysts. In addition, the reactions took place in quantitative yield and with complete chemoselectivity and the hydrogenation rates increased with one order of magnitude despite the very mild (atmospheric hydrogen pressure, room temperature) experimental conditions.     

Intracavity laser absorption spectroscopy of platinum sulfide in the near infrared

A new electronic transition of PtS with a red degraded band head at 12 460 cm⁻¹ was recorded and analyzed. Gas phase PtS was produced in a platinum-lined hollow cathode with a trace of SF₆ gas, and the spectrum was recorded at near Doppler resolution using intracavity laser absorption spectroscopy. Molecular constants for the newly identified excited state are presented.     

Interface characterization and current conduction in HfO2-gated MOS capacitors

Metal-oxide-semiconductor (MOS) capacitors incorporating hafnium dioxide (HfO₂) dielectrics were fabricated and investigated. In this work, the electrical and interfacial properties were characterized based on capacitance-voltage (C-V) and current-voltage (I-V) measurements. Thereafter the current conduction mechanism, electron effective mass (m*), mean density of interface traps per unit area and energy (), energy distribution of interface traps density and near-interface oxide traps density (N_NIOT) were studied in details. The characterization reveals that the dominant conduction mechanism in the region of high temperature and high field is Schottky emission. The mean density of interface traps per unit area and energy is about 6.3 × 10¹² cm⁻² eV⁻¹ by using high-low frequency capacitance method. The maximum D_it is about 7.76 × 10¹² cm⁻² eV⁻¹ located at 0.27 eV above the valence band.     

The statistics of dielectric breakdown in MOS capacitors under static and dynamic voltage stress

The statistics of dielectric breakdown at fields 8 MV/cm has been studied under both static (constant voltage) and dynamic (ramped voltage) conditions. MOS capacitors of the same size and from the same wafer were used for all measurements. Data from both types of test conformed to a Weibull distribution. The Weibull parameters a and b, which determine the time and field dependence of breakdown, were calculated from static tests at various applied fields and independently from dynamic tests at various ramp-rates. The values of a and b were: a 0.24; b 6.89 and a 0.34; b 6.55, from static and dynamic tests respectively. The similarity of the values for the two pairs of Weibull parameters suggests that breakdown proceeds by the same mechanism in both types of test.     

Memory function for dressed particles

This paper is concerned with the calculation of the memory function and derivation of a kinetic equation for one-body phase space correlation functions. The theory uses a one-body additive projection operator and a division of the Liouville operator with an unperturbed part that describes dressed particles. Binary collisions are neglected, for the theory aims at describing the screening and backflow effects of a type contained in the plasma kinetic theory of Balescu and Lenard. We obtain an explicit kinetic equation which is an improvement of these theories for the plasma case, and involves the exact equilibrium pair and triplet distributions. The equation also describes systems with strong short-range forces and shows how the screening effects occur in this case as well. The unifying function is the direct correlation function. The theory is meant to provide understanding for a more complete theory of fluids where a proper account is given of close collisions.Work supported by National Science Foundation, Grant No. GH 35691.     

The dynamics of partial oxidation of ethane over platinum

Catalysts are increasingly finding application in fuel reformation leading to more reactive blends, micro-combustors and nano-catalysts aimed at improving application performance. In the current work, the interactions of surface and gas phase reactions in C₂H₆/O₂/N₂ mixtures over a supported platinum catalyst are reported. Experiments were combined with computations featuring comprehensive detailed chemistry for both phases in order to explore the dynamics of the conversion processes using high superficial gas velocities, extended Al₂O₃ foam monoliths up to 60 mm in length and with variable Pt loading of 1% and 3% by weight. The residence time was varied between 1 and 3 ms at different oxygen to carbon weight (0.5 < O/C < 0.8) ratios and a constant molar hydrogen to oxygen (H₂/O₂ = 2) ratio. The applied gas phase chemistry features a detailed C₁–C₂ mechanism including low-temperature chemistry. The heterogeneous chemistry is analysed using two independently developed detailed heterogeneous reaction mechanisms [1] and [2]. The study confirms that the heterogeneous chemistry is essential in providing reaction stability and initially contributes to the formation of C₂H₄. It is also shown that increased residence times can result in a consistent heterogeneous consumption of ethylene principally leading to methane, carbon monoxide and carbon deposition. The surface chemistry is analysed in detail and it is shown that residence times can be used to mitigate C₂H₄ loss with the need to balance the stability of the process.     

金属化膜脉冲电容器寿命特性

从热处理工艺、电极结构设计及工作场强等方面对电容器寿命性能加以分析与试验研究。结果表明:金属化膜电容器热处理温度的选择需要综合考虑膜的收缩以及电极厚度;对应不同方阻,热处理温度存在一个最优值,在该值下电容器可获得最佳的自愈性能,进而达到较长的工作寿命;在不同工作场强下,需权衡电极边缘局部放电以及膜中自愈产生的容量损失比例,优化电极结构。在电极结构、热处理工艺等参数优化设计的基础上,研制出的1.0 kJ/L电容器达到10 000次的大电流充放电寿命。     

金属化膜脉冲电容器寿命特性

从热处理工艺、电极结构设计及工作场强等方面对电容器寿命性能加以分析与试验研究。结果表明：金属化膜电容器热处理温度的选择需要综合考虑膜的收缩以及电极厚度;对应不同方阻,热处理温度存在一个最优值,在该值下电容器可获得最佳的自愈性能,进而达到较长的工作寿命;在不同工作场强下,需权衡电极边缘局部放电以及膜中自愈产生的容量损失比例,优化电极结构。在电极结构、热处理工艺等参数优化设计的基础上,研制出的1.0 kJ/L电容器达到10 000次的大电流充放电寿命。     

Anatase nanotubes as support for platinum nanocrystals

Anatase nanotubes were successfully produced via the sol–gel process involving organic titanium precursors and multi-walled carbon nanotubes as template. Controlled heat treatments were carried out in order to remove any solvents and to crystallise the initial amorphous titania coating into anatase. In order to use these structures for catalyst support, platinum particles were formed by the impregnation with hexachloroplatinic acid and subsequent calcination and reduction to obtain a final loading of 4 wt% platinum. This impregnation step was carried out either with the carbon nanotube former still present with subsequent heat treatment to remove the carbonaceous template (sample A) or with the carbon nanotube former already removed (sample B). The materials were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.     

γ射线作用下氧化铪基MOS结构总剂量效应研究

使用原子层淀积方法得到了7.8 nm厚度的HfO₂薄膜并通过直接溅射金属铝电极得到了Al/HfO₂/Si MOS电容结构,测量得到了HfO₂基MOS结构在⁶⁰Co γ射线辐照前后的电容-电压特性,使用原子力显微镜得到了HfO₂薄膜在辐照前后的表面微观形貌,使用X射线光电子能谱方法测量得到了HfO₂薄膜在辐照前后的化学结构变化。研究发现,使用原子层淀积方法制备的HfO₂薄膜表面质量较高;γ射线辐照在HfO₂栅介质中产生了数量级为10¹² cm^-2的负的氧化层陷阱电荷;HfO₂薄膜符合化学计量比,介质内部主要的缺陷为氧空位且随着辐照剂量的增加而增加,说明辐照在介质中引入了陷阱从而导致MOS结构性能的退化。