Doppler程序对金红石型TiO_2的空位缺陷研究

金红石型Ti O2是一种非常好的稀磁半导体材料,其自身的本征缺陷与室温铁磁性起源密切相关.本文利用Doppler程序在广义梯度理论(GGA)的基础上,计算了正电子在金红石型Ti O2块材中不同缺陷处的湮没寿命.主要包括自由态正电子的湮没寿命,单空位和双空位处束缚态正电子的湮没寿命.并从理论上给出了含有空位缺陷时金红石型Ti O2的符合多普勒展宽能谱.     

高温氧气热处理对TiO_2晶体结构和铁磁性的影响

金红石TiO_2晶体先在真空中进行退火处理,随后在1173 K的氧气中进行不同时间(2 h、5 h、8 h)的热处理.理论上,通过Doppler程序计算了晶体中存在单空位、双空位和间隙O原子时的正电子湮没寿命.实验上,利用正电子湮没寿命谱仪、符合多普勒能谱仪和超导量子干涉仪分别表征了氧气退火后晶体内部的缺陷结构和常温铁磁性.分析结果得出:真空退火晶体的常温铁磁性主要与O空位的存在相关联;而经过氧气退火后,虽然极大地减少了氧空位,但晶体中却产生了大量的Ti双空位,这使得晶体的常温铁磁性有所增加.     

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：     

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：     

化合物半导体材料的正电子寿命计算

在局域密度理论（LDA）和广义梯度理论（GGA）的基础上计算了ZnO,GaN,GaAs,SiC和InP五种化合物半导体材料中的正电子湮没信息,包括化合物半导体材料中的自由态正电子的湮没寿命;还有不同类型空位（单空位,双空位）附近俘获的束缚态正电子密度分布和湮没率分布,以及束缚态正电子的湮没寿命. 关键词： 半导体 正电子寿命     

金红石型TiO2点缺陷性质的第一性原理研究

本文运用基于局域密度泛函和赝势的第一性原理方法研究了金红石相TiO2点缺陷的电子性质,结果表明氧空位缺陷使晶体的费米能量升高,在能隙中没有产生杂质能级.钛空位缺陷使晶体的费米能量降低,并在价带顶部产生了一个杂质能级,与价带顶能量相差约0.4 eV.本文还计算了金红石相TiO2在具有氧空位和钛空位点缺陷情况下的键长变化、态密度和电荷布居状况.     

利用正电子湮没技术研究钾掺杂钨合金中的缺陷

钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在.     

用正电子湮没技术研究纯镍多晶体疲劳过程中的晶体缺陷

对充分退火纯镍多晶体样品分组进行了恒应变幅拉-压疲劳试验和冷轧形变后,测量了正电子湮没参数;并选少数疲劳试样进行了电子显微镜薄膜衍射象观察。用多指数拟合方法从疲劳试样的正电子湮没寿命谱中分解出与正电子在小空位团中湮没相应的成份。这些寿命值在不同疲劳阶段分别平均为209,255和>300ps。其相对强度的变化与小空位团浓度增加然后尺寸增大的趋势相符合。从而,本文根据正电子湮没技术提供了纯镍多晶体疲劳过程中除大量位错外空位聚集的实验证据。利用简单三态正电子捕获模型估算了上述缺陷浓度。本工作还提供了用多指数拟合方法分解复杂寿命谱和用于简单三态捕获模型的例证。 关键词：     

掺镧PbWO4闪烁晶体的缺陷研究

利用正电子湮没寿命谱(PAT)和X射线电子能谱(XPS)研究了掺镧所引起的PbWO₄ 晶体缺陷的变化.结果表明：掺镧后,PbWO₄晶体中的正电子捕获中心铅空位(V< sub>Pb)浓度增加,并进一步诱导低价氧浓度的增加.讨论了掺La的作用机制,认为掺 La将抑制晶体中的氧空位,增加铅空位浓度. 关键词： 掺镧钨酸铅晶体 正电子湮没寿命谱 X射线电子能谱 缺陷     

HgCdTe中特征正电子湮没寿命值的确定

提出了一种新的在热平衡状态下实时测量碲镉汞(MCT)中特征正电子湮没寿命的方法,用该方法测得HgCdTe中基体正电子湮没寿命τ_b为277±1Ps,汞空位缺陷捕获态寿命τ_d为306±2ps,并与用其它方法测量的结果进行了比较与讨论.     

Doppler程序对C、N掺杂金红石型TiO_2的理论研究

本文在密度泛函理论和广义梯度近似(GGA)的基础上,应用Doppler程序计算了C掺杂、N掺杂、C-N共掺杂TiO_2体系的正电子湮没寿命,并且从理论上给出TiO_2体系不同缺陷处的符合多普勒展宽能谱.     

Chemical analysis using coincidence Doppler broadening and supporting first-principles theory: Applications to vacancy defects in compound semiconductors

The Doppler broadening of the positron annihilation radiation contains information on the chemical environment of vacancy defects trapping positrons in solids. The measured signal can, for instance, reveal impurity atoms situated next to vacancies. As compared to integrated quantities such as the positron annihilation rate or the annihilation line shape parameters, the full Doppler spectrum measured in the coincidence mode contains much more useful information for defect identification. This information, however, is indirect and complementary understanding is needed to fully interpret the results. First-principles calculations are a valuable tool in the analysis of measured spectra. One can construct an atomic-scale model for a given candidate defect, calculate from first principles the corresponding Doppler spectrum, and directly compare results between experiment and theory. In this paper we discuss recent examples of successful combinations of coincidence Doppler broadening measurements and supporting first-principles calculations. These demonstrate the predictive power of state-of-the-art calculations and the usefulness of such an approach in the chemical analysis of vacancy defects.     

氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.     

Vacancy-solute complexes in aluminum

Several vacancy-solute complexes in the Al matrix are examined theoretically. In particular, these are V-Cu, V-Cd, V-In, V-Sn, V-Si and V-Fe. We concentrate on coincidence Doppler broadening (high momentum) profiles and positron lifetimes that bring complementary information about these defects. Positron calculations are carried out utilizing the atomic superposition method employing realistic atomic configurations obtained using an ab initio pseudopotential method. In this study we inspect to what extent such defects are detectable and differentiable using positron annihilation techniques. The influence of lattice relaxations around defects on the positron properties turns out to be important and is also debated. The obtained results are discussed in connection with experimental data published in literature.     

金红石结构TiO2晶体点缺陷形成能的经验途径计算

基于经验参数化途径 ,通过对晶体结构、晶格形成能、介电性质和弹性实验数据拟合确定金红石结构氧化物晶体TiO2 的电子壳模型参数和非Coulomb互作用势参数 .计算点缺陷形成能 .论证Schottky缺陷是金红石结构TiO2中的本征缺陷 .     

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol(PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.     

The use of positron annihilation Doppler broadening spectroscopy in the characterization of radiochromic dosimetry films

The positron annihilation Doppler broadening (PADB) spectroscopy has been employed to probe the defects and structural changes of various types of materials. In this work, an investigation was carried out for the first time to use PADB spectroscopy in probing the γ irradiated radiochromic films. The GafChromic MD–55 and HD–V2 radiochromic films irradiated with absorbed doses ranges of 0–80 and 0–2000 Gy, respectively, were subjected to Doppler broadening measurements employing a HPGe γ–ray spectrometer. The Doppler broadening line–shape parameters (S and W) of 511 keV annihilation radiations were discussed in terms of the polymerization degree of the active components of these dosimetry films. The S- and W-parameters were found to be dependent on the active component structures of both films. A reasonable correlation was also found between the values of the S–parameter and the optical density of these radiochromic films. In addition, the Doppler broadening line–shape parameters were successfully used to provide explanation of the observed nonlinearity of these films at the end of their dynamic dose ranges. The results demonstrate the applicability of the PADB technique to probe the physical and chemical changes occurred in the active layer of the studied radiochromic films during the solid–state polymerization reaction caused by γ irradiation of different absorbed doses.     

Vacancy-solute complexes and their clusters in iron

In this contribution, several vacancy-solute complexes in iron are investigated theoretically from the viewpoint of positron annihilation. In particular, V-Si, V-P, V-Cr, V-Mn, V-Ni, V-Cu and V-Mo complexes are examined. In addition, nano-sized vacancy-Cu clusters in the Fe matrix are also studied. We concentrate on positron lifetimes and coincidence Doppler broadening profiles that bring complementary information about the studied complexes and their clusters. Positron calculations are carried out using the atomic superposition method employing realistic atomic configurations obtained recently using an ab initio pseudopotential method (vacancy-solute complexes) and Monte Carlo/molecular dynamics methods (vacancy-Cu clusters). The main aim of this study is to predict as to what extent such defects are detectable and differentiable using positron annihilation techniques. The results obtained are discussed in the context of experimental data available in the literature.