First-principles investigation on diffusion behaviours of H isotopes: From W(110) surface into bulk and in bulk W

The diffusion behaviours of hydrogen (H), deuterium (D), and tritium (T) from W(110) surface into bulk and in bulk W are investigated using a first-principles calculations combined with simplified models. The diffusion energy barrier is shown to be 1.87 eV from W(110) surface to the subsurface, along with a much reduced barrier of 0.06 eV for the reverse diffusion process. After H enters into the bulk, its diffusion energy barrier with quantum correction is 0.19 eV. In terms of the diffusion theory presented by Wert and Zener, the diffusion pre-exponential factor of H is calculated to be 1.57×10^-7 m²·s^-1, and it is quantitatively in agreement with experimental value of 4.1×10^-7 m²·s^-1. Subsequently, according to mass dependence (√1/m ) of H isotope effect, the diffusion pre-exponential factors of D and T are estimated to be 1.11×10^-7 m²·s^-1 and 0.91×10^-7 m²·s^-1, respectively.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Diffusion of titanium atoms in Ni-Si-B metallic glass

The diffusion ot Ti atoms in the Ni₇₇Si₃B₂₀ metallic glass was studied by the Auger electron spectroscopy. The temperature dependence of the diffusion coefficient obeys the Arrhenius relation with the activation energyE=1·7 eV and the pre-exponential factorD ₀=0·86 × ×10^–4 m² s^–1. Possible mechanisms of the atomic transport in metallic glasses are discussedThe authors wish to thank Dr. P. Duhaj for providing the metallic glass samples.     

Diffusion of nitrogen in ion-implanted chromium and tungsten

The diffusion of N in the group VI B metals Cr and W has been studied in the previously uninvestigated temperature ranges 300°–550 °C (Cr) and 600°–800 °C (W) using ion-beam techniques. Diffusion couples were created by ion-implantation. The timedependent diffusion profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plots extracted from the measured diffusivities indicate that the diffusivity of implanted N in Cr and W can be described by the activation energyQ=1.39±0.06 eV and 2.32±0.16 eV and the pre-exponential factorD ₀=(7.0±7.2)×10^–4cm^2/s and 4.3±8.3cm^2/s, respectively. The solubilities of N in Cr and W from the implanted distributions were found to deviate from those obtained using conventional metallographical methods.     

注入Mo 在Zr57Nb5Cu15.4Ni12.6Al10非晶合金中的扩散

通过二次离子质谱仪（SIMS）研究了Mo在Zr₅₇Nb₅Cu_15.4Ni_12.6Al₁₀非晶合金中的扩散，并计算出其扩散激活能Q和前置系数D₀分别为1.95 eV和1.13×10^-5m²s^-1.根据Stokes-Einstern关系式研究了玻璃转变温度以下593—673 K之间Zr₅₇Nb 关键词： 扩散 离子注入 二次离子质谱（SIMS） 黏滞特性     

Study of W bulk bands with normal (001) photoemission using synchrotron radiation

Photoemission normal to the (001) face of W was measured using polarized synchrotron radiation at normal incidence for 10 eV ?hv? 30 eV. All the major features observed agree very well with the predictions of three-dimensional, bulk band calculations for direct transitions along the line Δ, even for final energies well above E_F. No surface resonances were observed with s-polarized radiation.     

Sodium beta-alumina thin films as gate dielectrics for A1GaN/GaN metal-insulator-semiconductor high-electron-mobility transistors

<正>Sodium beta-alumina（SBA） is deposited on AlGaN/GaN by using a co-deposition process with sodium and Al₂O₃ as the precursors.The X-ray diffraction（XRD） spectrum reveals that the deposited thin film is amorphous.The binding energy and composition of the deposited thin film,obtained from the X-ray photoelectron spectroscopy（XPS） measurement,are consistent with those of SBA.The dielectric constant of the SBA thin film is about 50.Each of the capacitance-voltage characteristics obtained at five different frequencies shows a high-quality interface between SBA and AlGaN.The interface trap density of metal-insulator-semiconductor high-electron-mobility transistor（MISHEMT） is measured to be（3.5～9.5）×10¹⁰ cm^-2·eV^-1 by the conductance method.The fixed charge density of SBA dielectric is on the order of 2.7×10¹² cm^-2.Compared with the AlGaN/GaN metal-semiconductor heterostructure high-electronmobility transistor（MESHEMT）,the AlGaN/GaN MISHEMT usually has a threshold voltage that shifts negatively. However,the threshold voltage of the AlGaN/GaN MISHEMT using SBA as the gate dielectric shifts positively from—5.5 V to—3.5 V.From XPS results,the surface valence-band maximum（VBM-E_F） of AlGaN is found to decrease from 2.56 eV to 2.25 eV after the SBA thin film deposition.The possible reasons why the threshold voltage of AlGaN/GaN MISHEMT with the SBA gate dielectric shifts positively are the influence of SBA on surface valence-band maximum （VBM-E_F）,the reduction of interface traps and the effects of sodium ions,and/or the fixed charges in SBA on the two-dimensional electron gas（2DEG）.     

On viscous mechanism for surface diffusion at high temperatures (T/Tm > 0.75) due to formation of a 2D dense fluid on metallic surfaces

Experimental determinations of temperature dependence of surface self-diffusion coefficient of several metals exhibit a strong increase in D_s values and in activation energy for temperatures near the melting point T_m. This variation is illustrated by a bending of the Arrhenius plot of surface self-diffusion coefficients of tungsten, which are obtained experimentally by tip profile variation technique. For T/T_m < 0.75 the apparent activation energy for W is 2.85 eV and the pre-exponential term is equal to 0.24 cm²/s, while for T/T_m > 0.75 we have respectively 5.57 eV and 1.08 × 10⁴ cm²/s. To account for these unexpected variations in the activation energy and diffusivities, the hypothesis that the surface mass transport mechanism changes from individual atomic jumps at low temperatures towards a cooperative motion at temperatures near the bulk melting point, namely a viscous mechanism, is proposed. This model is based on the postulation of the formation of a 2D dense fluid on the metallic surfaces about 75% of the bulk melting temperature. Discussions of existing models on surface diffusion proposed by Rhead, by Bonzel, or by Tsong are given, and a technique to characterize surface viscosity of a 2D dense fluid is suggested.     

Optical transmission of bulk plastic material and plastic lightguides at high optical powers

Optical transmission measurements were performed with bulk plastic materials and plastic fibres regarding the possible application as a transmission media for high optical powers. Transmission losses of PMMA bulk material and PS/PMMA core-cladding combination optical fibres were determined to be 0.757 dB m^-1 and 1.41 dB m^-1 at 514.5 nm, respectively. The core material of the investigated fibres withstands a power density of 3.8 x 10³ W mm^-2 when the light is suitably launched. With the light source available a transmitted cw output power of 2.1 W could be attained with a 1.5 m long fibre.     

An auger electron spectroscopy study of low temperature segregation of silicon at aluminum surface

Low temperature surface segregation of silicon out of refined aluminum samples is studied by AES. The analysis of kinetics data gives an Arrhenius type diffusivity with an activation energy equal to 65 kJ mol^?1 (0.68 eV atom^?1). The pre-exponential factor is of the order of 3 × 10^?6 m² s^?1. These results fit well within the limits of grain boundaries and dislocation diffusivities, as compiled from the literature by Hwang and Balluffi, and show that the segregation proceeds from distorted sites in grain boundaries and dislocations, rather than from bulk undistorted sites. The analysis of data obtained from a slow-cooling-rate experiment gives a segregation energy of ?60 kJ mol^?1 (?0.63 eV atom^?1) and shows that the silicon surface concentration limit is not a monolayer, but rather unexpectedly about 8%. This may be related to LEED observation of silicon adsorption on various faces of aluminum by Jona and suggests that long range repulsive forces are effective to saturate silicon condensation on aluminum at a low coverage value.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Field distribution and criterion for bulk-limited and injection-limited current conduction in single layer organic light-emitting devices

A numerical model for current conduction in single layer OLEDs including both injection and bulk effect is proposed. Based upon this model, a nearly linear distribution of the electric field was found, and the slope of the distribution, or the field at the injection electrode (F₀) is dependent on the energy barrier, mobility, trap density and trap depth. F₀ equals the half of the mean field of the device (F_m), which equals the quotient of the bias to the thickness of organic layer, is proposed as the limit for bulk-limited (BL) and injection-limited (IL) conduction. OLEDs with F₀ greater than F_m/2 are considered as IL-conducting, while those with F₀ less than F_m/2 are considered as BL-conducting. It was found that, the state of current conduction is not only determined by the energy barrier at the injection electrode, but also by the mobility, trap density and trap depth of the organic semiconductor. OLEDs with high injection barrier (>0.7 eV), trap density less than 10¹⁹ cm^-3, and reduced trap depth shallower than 5, will be IL-conducting, while those with low energy barrier (<0.2 eV), low carrier mobility (<10^-6 cm²V^-1s^-1), and trap density higher than 10¹⁷ cm^-3, will be BL-conducting. PACS 78.60.Fi; 75.40.Mg; 73.21.Ac     

一种侧链共轭聚噻吩衍生物薄膜的三阶非线性光学响应

合成了一种侧链共轭的聚噻吩衍生物聚3-（5′辛基-噻吩乙烯基）噻吩（POTVTh）,并通过溶液旋涂制备了聚合物薄膜.吸收光谱显示该聚合物薄膜具有较小的禁带宽度和宽的光谱响应.采用Z扫描技术在800 nm下用飞秒激光器研究了该聚合物薄膜的三阶非线性光学特性,非线性吸收系数为5.63×10^-7cm/W,非线性折射率为-6.38×10^-11 cm²/W,三阶非线性极化率为4.21×10^-9esu,比侧链未共轭的 关键词： 聚噻吩衍生物 侧链共轭 三阶非线性极化率 非线性折射率     

Ion bombardment induced phase transformations and inert gas mobility in the semiconductors Ge,Si, and GaAs

Abstract

The present study contributes some new aspects to the general understanding of the ion implantation behaviour of 3 common semiconductor materials, and of diffusion processes in these materials. Single crystals of Si, Ge, and GaAs were bombarded with Kr- or Xe-ions at energies of 40 or 500 keV and doses between 10¹¹ and 2 × 10¹⁶ ions/cm². Gas release measurements and Rutherford scattering of 1 MeV He⁺-ions combined with channeling were used to study bombardment damage (amorphization) and inert gas diffusion. At low bombardment doses (10¹¹ ions/cm²) and energy (40 keV), no damage was observed and the gas release was compatible with volume diffusion resembling Group I and VIII behaviour. Hence, the pre-exponential terms, D ₀, were low (range 10-^5±1 cm² sec^?1) and the activation enthalpies, Δ H, were much lower than those of self-diffusion or of diffusion of Group III and V elements. The Δ H's for gas diffusion followed the relation Δ H = (1.05±0.1) × 10^?3 T_m eV with the melting point, T_m , in °K. The mechanism of gas mobility might be the Turnbull dissociative mechanism. Rutherford scattering and channeling data indicated that part of the gas occupied lattice sites.

At higher doses, the bombarded layers turned amorphous. Channeling experiments showed a coincidence in temperatures for a gas release process different from the above one of volume diffusion, and recrystallization of the disordered layer to the single crystalline state. Both processes occurred in the temperature range 0.60 to 0.65 T_m . The gas release indicated a (partial) single jump character with implied Δ H's following the relation Δ H = (2.1±0.1) × 10^?3 T_m eV. Contrary to previous results on oxides, this new gas release occurred at temperatures near to those or even above those of volume diffusion of the gas.

Due to the easy formation of an amorphous layer it was difficult to observe the retarded release (trapping of gas) that has been found in many materials at high gas and damage concentrations. However, in a separate series of experiments with 500 keV Kr-ions, a release retarded with respect to volume diffusion of the gas was observed in Si and Ge.     

YBCO熔融织构准单晶中的进氧和脱氧扩散研究

采用热重法研究了YBCO熔融织构准单晶中氧的化学扩散过程. 与以往的实验不同，采用固定 氧偏压变化温度的方法获得氧浓度梯度. 拟合实验所得重量等温弛豫曲线可知，在375—600 ℃温区内，熔融织构准单晶的氧化学扩散系数比单晶体高出约50％，但都约为10^{-10c m2s-1，激活能为～1eV. 对YBCO熔融织构准单晶中的进氧和脱氧研 究表 明两者具有相同的速率，证实实验过程中环境氧分压的改变会导致进氧和脱氧过程不一致. 关键词： 高温超导 氧扩散 熔融织构 热重法}     

Ni-15%Mo合金熔体热物理性质的Monte Carlo模拟

采用EAM势对6×6×6的Ni-15%Mo合金熔体进行Monte Carlo模拟,通过对不同温度下获得的NVT系统的平衡态统计分析得出Ni-15%Mo合金熔体在过冷态和过热态时的热物理性质.通过构造系统生成新表面,表面张力做功使系统能量发生改变,从而得到液态表面张力的模拟结果.Ni-15%Mo合金熔体的表面张力在1500—2000 K的温度范围内,随温度的变化规律为σ=1.918-1.130×10^-3（T-T_m） N/m 关键词： Monte Carlo模拟 表面张力 比热 Ni-15%Mo合金     

Structure and dielectric behavior of TlSbS2

A comparison of structure and dielectric properties of TlSbS₂ thin films, deposited in different thicknesses (400–4100 Å) by thermal evaporation of TlSbS₂ crystals that were grown by the Stockbarger–Bridgman technique and the bulk material properties of TlSbS₂ are presented. Dielectric constant ε ₁ and dielectric loss ε ₂ have been calculated by measuring capacitance and dielectric loss factor in the frequency range 20 Hz–10 KHz and in the temperature range 273–433 K. It is observed that at 1 kHz frequency and 293 K temperature the dielectric constant of TlSbS₂ thin films is ε ₁=1.8–6 and the dielectric loss of TlSbS₂ thin films is ε ₂=0.5–3 depending on film thickness. In the given intervals, both of dielectric constant and dielectric loss decrease with frequency, but increase with temperature. The maximum barrier height W _m is calculated from the dielectric measurements. The values of W _m for TlSbS₂ films and bulk are obtained as 0.56 eV and 0.62 eV at room temperature, respectively. The obtained values agree with those proposed by the theory of hopping over the potential barrier. The temperature variation of ac conductivity can be reasonably interpreted in terms of the correlated barrier hopping model since it obeys the ω ^s law with a temperature dependent s (s<1) and going down as the temperature is increased. The temperature coefficient of capacitance (TCC) and permittivity (TCP) are evaluated for both thin films and bulk material of TlSbS₂.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Mass transport of W atoms and variation of stoichiometry in Al-Ni coating as a result of electron beam irradiation

Variation of the effective diffusion coefficient from 2.0 × 10^?7 to 1.1 × 10^?5 cm²/s is detected for the first time using back scattering (BS) of ions, scanning electron microscopy (SEM) with microanalysis (wave-length dispersive spectrometry, WDS) and nanohardness. Three stages are discovered in the motion of W ions: (i) from the coating surface to the bulk (without melting), (ii) accelerated mass transfer to the bulk with melting, and (iii) sharp decreases in the peak concentration of W ions and reversal of the direction (from the bulk to the surface).