Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：     

铝中氢-空位复合体的正电子湮没特性

本文从第一性原理出发,应用离散变分法、嵌入集团模型和三维数值弛豫技术计算了铝中一氢-空位复合体和多氢-空位复合体的正电子湮没特性,讨论了氢在铝单空位中可能的占据位置,表明正电子湮没技术是研究固体中缺陷的微观结构的有力工具。 关键词：     

Co掺杂纳米ZnO微结构的正电子湮没研究

利用正电子湮没技术研究了10 at.% Co掺杂的Co₃O₄/ZnO纳米复合物中退火对缺陷的影响. 利用X射线衍射(XRD)测量了Co₃O₄/ZnO纳米复合物的结构和晶粒尺寸. 随着退火温度升高,Co₃O₄相逐步消失,ZnO晶粒尺寸也有显著增加. 经过1000 ℃以上退火后,Co₃O₄相完全消失,并出现了CoO的岩盐结构. 正电子湮没寿命测量显示出Co₃O₄ /ZnO纳米复合物中存在大量的Zn空位和空位团. 这些空位缺陷可能存在于纳米复合物的界面区域. 当退火温度达到700 ℃后Zn空位开始恢复,空位团也开始收缩. 900 ℃以上退火后,所有空位缺陷基本消失,正电子寿命接近ZnO完整晶格中的体态寿命值. 符合多普勒展宽谱测量也显示Co₃O₄ /ZnO纳米复合物经过900 ℃以上退火后电子动量分布与单晶ZnO基本一致,表明界面缺陷经过退火后得到消除. 关键词： ZnO 界面缺陷 正电子湮没     

充氚不锈钢中氦行为的实验研究

对充氚和未充氚的抗氢-2不锈钢(HR-2)样品进行退火处理,利用正电子湮没寿命谱技术以及金相检验技术探讨不锈钢中氦和微缺陷的相互作用行为.未充氚样品中,影响正电子寿命值的主要因素为杂质元素在晶界的析出.充氚样品实验中,退火温度小于300℃时,正电子寿命值的增加说明了氦泡的形成过程为非热形成,通过“冲出位错环”机制形成及长大;退火温度在300~600℃之间,充氚样品正电子寿命值的降低以及He的跃迁概率的计算结果,说明He原子通过热迁移至晶界;退火温度大于600℃时,热平衡空位浓度的计算结果以及正电子寿命值的增加说明热平衡空位开始发挥作用.     

金属中点缺陷的电子结构和缺陷谱

本文以胶体模型为基础,利用密度冷函理论和局域密度近似研究了过渡金属Cr,Fe,Ni和贵金属Cu中空位型缺陷、氢杂质、空位-杂质复合体的电子结构以及以正电子湮没寿命表征的缺陷谱。表明了用正电子湮没寿命谱研究金属点缺陷电子结构和缺陷尺度大小的可行性。 关键词：     

Fe-V-N稀合金中空位复合体特性的研究(Ⅱ)

本文利用离散变分法,集团模型和三维数值弛豫技术,研究了Fe-V-N稀合金中V-N-空位复合体的几何构型,正电子湮没特性和电子结构。根据正电子湮没寿命实验的结果和总能量极小原理,确定了V和N在V-N-空位复合体中的最佳占据位置。分析了V-N-空位复合体的成键特性和电荷转移情况。 关键词：     

H,He,Kr杂质在过渡金属和贵金属中的行为

本文以胶体模型为基础,利用密度泛函理论,局域密度近似研究了H,He和Kr在过渡金属Ni,Fe,Cr和贵金属Cu中的空位团复合体的电子结构、正电子湮没寿命。结果表明:随着复合体尺寸的增加,杂质的束缚态电子能级变浅,散射态电子在复合体内的平均密度变小,正电子在复合体内的几率增大,正电子湮没寿命增加。 关键词：     

Fe-Ni合金马氏体相变缺陷的正电子湮没研究

本文中用正电子湮没技术研究了Fe-Ni系合金的马氏体相变。实验结果表明,含镍量28.22—31.30wt％的六种退火态合金,正相变后,产生了大量的缺陷,使正电子湮没平均寿命及多普勒加宽线型参数S值分别约增加30％和20％。对28.93wt％Ni和31.30wt％Ni两种马氏体亚结构不同的合金,观测了正电子湮没参数与处理温度之间的关系,发现:缺陷主要产生在爆发马氏体形成阶段;在等时退火曲线上形成两个明显的台阶,是由于空位和位错恢复引起的。 关键词：     

氦在球磨贮氢合金中的存在行为研究

通过正电子寿命测量及结构分析，研究了氦气氛球磨LaNi4.75Al0.25和Zr50Co50贮氢合金中氦的存在行为.研究结果表明：随着气氛压力增加，氦的溶入量增加，相对而言，LaNi4.75Al0.25更有利于氦的溶入；球磨样品正电子寿命的特征参数发生变化，这表明氦在材料中存在行为的差异；对于Zr50Co50合金，氦首先填充在超细晶粒的三叉晶界或空位团处，随着气氛压力增加，有小部分氦填入单空位的自由体积处，形成空位-氦复合体；对于LaNi4.75Al0.25合金，氦首先进入单空位大小的自由体积中，形成空位 关键词： 正电子寿命 氦引入 高压球磨 贮氢合金     

直拉单晶硅中洁净区形成后铜沉淀行为的研究

本文研究了直拉单晶硅中形成洁净区后过渡族金属杂质铜的沉淀行为. 样品经过高低高三步常规热处理形成洁净区后, 在不同温度下引入杂质铜, 然后对样品分别进行普通热处理和快速热处理, 通过腐蚀和光学显微镜研究发现, 在700 ℃引入铜杂质后经过普通热处理和快速热处理都不会破坏洁净区, 在900 ℃和1100 ℃引入铜杂质后经过普通热处理不会破坏洁净区, 而经过快速热处理会破坏洁净区. 研究表明, 快速热处理可以使硅片体内产生大量的空位, 空位的外扩散是破坏洁净区的主要原因. 关键词： 直拉单晶硅 铜沉淀 洁净区     

Evolution of native point defects in ZnO bulk probed bypositron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

水蒸气退火多孔硅发光性能的正电子谱学研究

使用正电子湮没寿命谱和正电子寿命-动量关联谱对水蒸气和真空条件下退火的多孔硅样品的微观缺陷结构进行表征,结合发射光谱测量结果,对影响多孔硅发光性能的因素进行了讨论.实验结果表明,水蒸气退火后样品孔壁表面的悬挂键减少,并出现新的E′γ和EX类缺陷.水蒸气退火后样品中两种缺陷数量发生变化是导致多孔硅样品发光增强的直接原因;真空退火未使样品中发光相关缺陷发生变化,样品的发光性能没有显著改变.     

Atomic fraction around defects associated with nanoparticles in AlCuMg alloys

A positron annihilation spectroscopy analysis method to obtain a quantitative determination of the chemical composition around defects inside nanoparticles is presented here. This methodology is applied to AlCuMg alloys to study the rapid hardening phenomena associated with solute-vacancy aggregation. Coincidence Doppler Broadening (CDB) and lifetime spectroscopy measurements of reference samples of pure elements with and without defects were analyzed to give quantitative information of the average chemical environment around vacancies, i.e. the atomic fraction of the first neighbors of these defects, in the alloys studied. The accuracy and reproducibility of the methodology is confirmed not only by good fits to the experimental data but, in most cases, by the consistency between the mean lifetime values predicted, using the CDB estimation, and the mean lifetime values independently measured. Discrepancies in the methodology are expected when there is poor CDB contrast between elements, i.e. having similar electronic structure (for example, Al and Mg). The criterion for establishing the statistical accuracy of the separation of elements in these special cases is discussed. The methodology can be applied not only to study homogeneous materials as metallic alloys, but also to study the depth profile in thin films.     

The role of quenched‐in vacancies for the decomposition of aluminium alloys

The very early stages of decomposition during room temperature storage, i.e. just a few minutes after quenching, are investigated by positron annihilation lifetime spectroscopy for both an AlMgSi alloy and an AlCuMg alloy. It turns out that by freezing the decomposition kinetics during measurements we can detect vacancy–solute atom pairs. The formation of larger solute clusters with structural vacancies is seen by an increase of the mean positron lifetime in the course of storage at room temperature (RT). Earlier findings concerning aging at RT were unable to discover this effect. The detected changes are interpreted in terms of cluster formation. Thus we show that positron annihilation spectroscopy (PAS) is one of the very few methods to access early stages of decomposition in metallic alloys. Moreover, the lower limit of the concentration of quenched‐in vacancy‐like defects is calculated to be at least 2 × 10^–5 per atom. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Positrons and electron-irradiation induced defects in the layered semiconductor InSe

The positron annihilation characteristics of the layered semiconductor InSe have been investigated. No evidence for low temperature positron trapping is found in as-grown and heavily deformed InSe. The temperature dependence of the S-parameter in these sample exhibits an increase rate in good agreement with the linear expansion coefficient along the c-axis. The positron lifetime spectra of electron-irradiated 0.01% Sn-doped InSe show a long-lifetime component of 336 ps which is tentatively attributed to positrons trapped at isolated In vacancies. Isochronal annealing experiments performed on these samples show that the recovery of the positron lifetime measured at 77K is accomplished in two stages. The first, starting after annealing at 150K, could be induced by the formation of complexes (V_In-Sn_In). The second stage, observed at temperatures T375K, is attributed to the dissociation of these complexes and subsequent annealing of the In vacancies.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Reply to ‘On substructure in titanium alloy martensite’

Thermal stability of nanocrystalline Cu prepared by compacting nanoparticles (mean grain size about 50?nm) under high pressure has been studied by means of positron annihilation lifetime spectroscopy and X-ray diffraction. A gradual increase of mean grain size in the sample is observed with an increase in ageing time at 180°C, indicating an increase of volume fraction of the ordered regions. Furthermore, during the ageing, the increase in average size of the vacancy clusters in grain boundaries is confirmed by the positron lifetime results. The recrystallization is observed at the temperature of about 180°C, and becomes significant above 650°C. Three annealing stages, which are at the intervals 180–400°C, 400–650°C and 650–900°C have been characterized by positron average lifetime. The average volume of the defects almost remains constant in the interval 400–650°C but becomes considerably smaller in the interval 650–900°C.     

Thermal expansion effects in positron lifetimes

The precision of relative positron lifetime measurements is now sufficient to observe directly changes due to lattice thermal expansion. This information is necessary for the determination of accurate vacancy formation energies from positron annihilation data obtained over a higher temperature range. Measurements have been made of the change in positron lifetime in the metals Au, Ag, Cu, and Al over the temperature range 100–300 K where there is no contribution from positron trapping at vacancies. The results are compared with theoretical calculations which take account of positron annihilations with both valence and core electrons.     

Investigation of nanocrystalline CoFe 2 O 4 by positron annihilation lifetime spectroscopy

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail.