Incident angle and energy dependences of low‐energy Ar+ ion sputtering of GaAs/AlAs multilayered system

Dependences of the depth resolution in Auger electron spectroscopy sputter‐depth profiling of a GaAs/AlAs superlattice reference material on the incident angle and energy of primary Ar⁺ ions were investigated. The results revealed that the depth resolution is improved for the lower primary energy as a square root of the primary energy of ions at both the incident angles of 50° and 70° , except for 100 eV at 50° , where the significant deterioration of the depth resolution is induced by the preferential sputtering of As in AlAs, and the difference in the etching rate between GaAs and AlAs. The deterioration of the depth resolution, i.e. the difference in the etching rate and the preferential sputtering, observed for 100 eV at 50° was suppressed by changing the incident angle of ions from 50° to 70° , resulting in the high‐depth resolution of ～1.3 nm. The present results revealed that the glancing incidence of primary ions is effective to not only reducing the atomic mixing but also suppressing the difference in the etching rates between GaAs and AlAs and the preferential sputtering in the GaAs/AlAs multilayered system. The results also suggest that careful attention is required for the optimization of conditions of sputter‐depth profiling using GaAs/AlAs superlattice materials under low‐energy ion irradiation. Copyright © 2009 John Wiley & Sons, Ltd.     

Electronic structure of some mesoscopic systems: I. Semiconductor superlattices

As an example of a mesoscopic system, a Kronig-Penney-type model of a semi-infinite GaAs/Ga_1−xAl_xAs superlattice is considered. The effect of the superlattice termination on the formation of a surface state as well as on the density-of-states distributions is discussed. Varying the position of the superlattice surface (i.e., superlattice/substrate interface) within a superlattice period influences strongly the appearance and position of surface states. They occur in particular minigaps for some ranges of the outermost layer thickness only.     

High-resolution electron-energy-loss spectroscopy of surface and interface phonons in multilayered materials

Long-wavelength surface and interface phonons have been investigated by high-resolution electron-energy-loss spectroscopy (HREELS) in two heterostructures grown by molecular-beam epitaxy. The first system is a CaF₂ insulating layer on Si(1 1 1), while the second consists of GaAs/AlAs superlattices grown on a thick GaAs(0 0 1) substrate. The HREELS experimental results are successfully explained by the dielectric theory, with some refinements brought about by lattice dynamics calculations.     

Preferential sputtering observed in Auger electron spectroscopy sputter depth profiling of AlAs/GaAs superlattice using low‐energy ions

Auger electron spectroscopy (AES) sputter depth profiling of an ISO reference material of the GaAs/AlAs superlattice was investigated using low‐energy Ar⁺ ions. Although a high depth resolution of ～1.0 nm was obtained at the GaAs/AlAs interface under 100 eV Ar⁺ ion irradiation, deterioration of the depth resolution was observed at the AlAs/GaAs interface. The Auger peak profile revealed that the enrichment of Al due to preferential sputtering occurred during sputter etching of the AlAs layer only under 100 eV Ar⁺ ion irradiation. In addition, a significant difference in the etching rates between the AlAs and GaAs layers was observed for low‐energy ion irradiation. Deterioration of the depth resolution under 100 eV Ar⁺ ion irradiation is attributed to the preferential sputtering and the difference in the etching rate. The present results suggest that the effects induced by the preferential sputtering and the significant difference in the etching rate should be taken into account to optimize ion etching conditions using the GaAs/AlAs reference material under low‐energy ion irradiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of the inelastic mean-free-path and mean inner potential for AlAs and GaAs using off-axis electron holography and convergent beam electron diffraction.

The mean-free-paths for inelastic scattering of high-energy electrons (200 keV) for AlAs and GaAs have been determined based on a comparison of thicknesses as measured by electron holography and convergent-beam electron diffraction. The measured values are 77 +/- 4 nm and 67 +/- 4 nm for AlAs and GaAs, respectively. Using these values, the mean inner potentials of AlAs and GaAs were then determined, from a total of 15 separate experimental measurements, to be 12.1 +/- 0.7 V and 14.0 +/- 0.6 V, respectively. These latter measurements show good agreement with recent theoretical calculations within experimental error.     

己烯在Ru(1010)表面价带电子特性研究

在200 K以下己烯（C6H12）可以在Ru()表面上以分子状态稳定吸附.偏振角分辨紫外光电子谱（ARUPS）结果表明,己烯分子在垂直于衬底表面并沿衬底表面<>晶向的平面内,己烯分子的轴向沿<>晶向倾斜.随着衬底温度的提高,到200 K以上,己烯分解生成新的碳氢化合物.己烯分解后,πCH分子轨道能级向高结合能方向移动了0.2 eV,同时己烯中C的1s能级向低结合能方向移动了 0.3 eV.     

脉冲光电子枪中光电子束性能研究

利用新设计的脉冲光电子枪,研究了光电子束的能量分布,光电子数密度与激光强度的关系和光电子在电离区内的滞留时间。大部分电子的能量为光发射电子的剩余能量,但是由于电子的空间电荷效应,电子能量分布具有加宽现象·每个激光脉冲发射到电离区内的光电子数密度在109/cm3以上。     

Analysis of AlGaAs/GaAs superlattices by means of sputter-assisted AES,SEM and TEM

This paper reports the developments of evaluation methods on epitaxially grown superlattices by means of sputterassisted Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). AlGaAs/GaAs semiconductor superlattics were grown epitaxially by metal–organic chemical vapour deposition (MOCVD). The layer thickness of the superlattices ranged from a few to ten nanometers. Firstly, developments of Auger depth profiling were tried by using: (1) a differential pumping-type ion gun instead of a static pressure type to reduce the oxygen adsorbates on the AlGaAs layer; (2) low-energy Auger signals instead of high-energy ones to shorten the escape depth; and (3) the lowest ion etching energy of 0.2 keV instead of 1 keV to reduce the surface roughening effects. It is shown that the depth resolution of sputter-assisted AES is attainable to 1.5 nm. Secondly, high-resolution SEM can be used as an easy evaluation method by observing the cleaved surface of superlattices, since the layers can be distinguished by signal contrast. Also, TEM can be used as an evaluation method by observing the (110) cross-section thinned sample. The dark field image has a high contrast between AlGaAs and GaAs using the (002) diffraction. It is confirmed from these AES, SEM and TEM evaluations that the hetero-interface abruptness of AlGaAs/GaAs superlattices grown by MOCVD is of the order of one monoatomic layer.     

电子转移溶剂重组能计算的自洽反应场新方法

基于非平衡溶剂化理论, 推导了用于非平衡溶剂化能数值计算的类导体屏蔽模型(COSMO)的相关公式. 在此基础上, 修改了HONDO99中COSMO模块, 并用以估算了[(CH₂)₂C]+—(CH₂)_n—C(CH₂)₂(n＝1～13)体系中的电子转移溶剂重组能. 结果表明, 溶剂重组能值与电子转移距离的倒数有很好的线性关系. 根据溶剂重组能数值解结果, 用新的双球模型给出了合理的给受体球半径.     

Rigid POSS Nanofiller Regulated the Interfacial Self-assembly of Particles into 2D Monolayer Superlattice with Fixed Interparticle Spacing†

Plasmonic superlattices of nanoparticles (NPs) possess unique “surface lattice resonances” properties that facilitates their wide applications in plasmonic sensing, photocatalysis, and nanoscale light manipulation. However, it is still challenging to manufacture superlattices with precisely controllable NPs distance and break the size limitation of NPs. Herein, we provided an effective strategy to construct NPs superlattices via shape-persistent polyhedral oligosilsesquioxane (POSS) molecular nanoparticles govern interfacial assembly. As a nanoscale molecule with diameter of 1.5 nm, the POSS-SH molecule provides sufficient rigid steric hindrance and hydrophobic effect for tailoring the uniformity and controllable distance between NPs in superlattices. Interestingly, synergistically with hydrophilic ligands of polyethylene glycol (PEG-SH) with optimized ratio, the rigid POSS ligands can effectively regulate the distance between NPs in a fixed range of 2.3—2.8 nm, which is independent of ligands molecular weight and particle size. Furthermore, the effective approach can be universal to anisotropic NPs for manufacturing monolayer films with high NPs density. We believe this nanoscale molecule tailored interfacial self-assembly strategy can effectively break the size of NPs and assembly obstacles for superlattice monolayer film. Additionally, the definite distance between NPs in superlattices can minimize optical energy attenuation and facilitates the applications such as surface-enhanced Raman spectroscopy and photocatalysis.      

eaq− hydration energy and photoemission threshold potentials for mercury in contact with aqueous electrolytes

The meaning of the “red limit” potential in photoemission experiments is discussed. For mercury in contact with aqueous electrolytes, the energy of a photoemitted electron at the “red limit” is 0.6 eV higher than the solvation energy of e_aq^?. This difference is attributed to the solvent reorganization energy contribution to the hydration energy of e_aq^?.     

Photoemission of benzene on Os (0001 ): A synchrotron radiation study

The molecular structure and orientation of benzene on Os (0001 ) has been investigated by angle-resolved UV photoemission spectroscopy (ARUPS) using synchrotron radiation. At temperatures below 285 K benzene adsorbs in a molecular state and ARUPS indicates C_3v symmetry. This implies that the adsorbed benzene molecules are oriented parallel to the metal surface with a trigonal distortion of their aromatic rings. Bonding to the surface occurs through the benzene π system.     

Determinants of thermal conductivity and diffusivity in nanostructural semiconductors

The origin of size effects in the thermal conductivity and diffusivity of nanostructural semiconductors was investigated through the establishment of a unified nanothermodynamic model. The contributions of size-dependent heat capacity and cohesive energy as well as the interface scattering effects were considered during the modeling. The results indicate the following: (1) both the thermal conductivity and diffusivity decrease with decreasing nanocrystal sizes (x) of Si and Si/SiGe nanowires, Si thin films and Si/Ge(SiGe) superlattices, and GaAs/AlAs superlattices when x > 20 nm; (2) the heat transport in semiconductor nanocrystals is determined largely by the increase of the surface (interface)/volume ratio; (3) the interface scattering effect predominates in the reduction of thermal conductivity and diffusivity while the intrinsic size effects on average phonon velocity and phonon mean free path are also critical; (4) the quantum size effect plays a crucial role in the enhancement of the thermal conductivity with a decreasing x (<20 nm). These findings provide new insights into the fundamental understanding of high-performance nanostructural semiconductors toward application in optoelectronic and thermoelectric devices.     

银(110)表面苝有序薄膜电子态的研究

运用紫外光电子能谱(UPS)和低能电子衍射(LEED)技术,对银(110)表面上有机分子苝（perylene）的生长进行了研究.有机分子价带的4个特征峰分别位于费米能级以下3.5、4.8、6.4和8.5 eV处.当有机薄膜约为单分子层(厚度为0.3 nm)时, 苝在银(110)表面上形成C(6×2)的有序结构.角分辨紫外光电子能谱(ARUSP)的测量显示:在界面处的苝分子平面平行于衬底.苝在银(110)表面稳定性很高,随着对衬底加热，有机材料发生脱附,在140 ℃以下没有观察到分解现象.     

Structures and conformations of trifluoromethanesulfonic anhydride, (CF3SO2)2O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF3SO2)2CF2

The geometric structures and conformational properties of trifluoromethanesulfonic anhydride, (CF₃SO₂)₂O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF₃SO₂)₂CF₂ have been studied by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*). The calculations predict for both systems two stable conformers with C₂ symmetry and one with C₁ symmetry. In both compounds structures with C₂ symmetry and dihedral angles SOSC ≈ 100° ((CF₃SO₂)₂O) and SCSC≈ 150° ((CF₃SO₂)₂CF₂ are lowest in energy. According to the GED analyses the dominant conformer of (CF₃SO₂)₂O₂ possesses C₂ symmetry with SOSC dihedral angles of 99.1(14)°. The presence of up to 30% of the two other conformers cannot be excluded; for (CF₃SO₂)₂CF₂ only one conformer with C₂ symmetry and SCSC dihedral angles of 143(2)° is observed. A complete set of geometric parameters is given.     

Probing occupied states of the molecular layer in Au-alkanedithiol-GaAs diodes

Internal photoemission (IPE) studies were performed on molecular diodes in which the alkanedithiol [HS(CH(2))(n)SH, n = 8, 10] molecular layer is sandwiched between Au and GaAs electrodes. The results are compared to those from Au-GaAs Schottky diodes. An exponential energy dependence in the IPE yield was observed for the molecular diodes, in contrast to the quadratic energy dependence characteristic of metal-semiconductor Schottky diodes, indicating that Au is not the source of electrons in the IPE process in the molecular diodes. From the GaAs dopant density dependence, we also can rule out GaAs being the source of these electrons. Compared with the results of cluster electronic structure calculations, we suggest that IPE is probing the occupied levels of GaAs-molecular interfacial states.     

Structures and conformations of trifluoromethanesulfonic anhydride, (CF3SO2)2O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF3SO2)2CF2

The geometric structures and conformational properties of trifluoromethanesulfonic anhydride, (CF₃SO₂)₂O, and bis(trifluoromethylsulfonyl)difluoromethane, (CF₃SO₂)₂CF₂ have been studied by gas electron diffraction (GED) and ab initio calculations (HF/3–21G*). The calculations predict for both systems two stable conformers with C₂ symmetry and one with C₁ symmetry. In both compounds structures with C₂ symmetry and dihedral angles SOSC ≈ 100° ((CF₃SO₂)₂O) and SCSC ≈ 150° ((CF₃SO₂)₂CF₂) are lowest in energy. According to the GED analyses the dominant conformer of (CF₃SO₂)₂O possesses C₂ symmetry with SOSC dihedral angles of 99.1(14)°. The presence of up to 30% of the two other conformers cannot be excluded; for (CF₃SO₂)₂CF₂ only one conformer with C₂ symmetry and SCSC dihedral angles of 143(2)° is observed. A complete set of geometric parameters is given.     

Self-Organization of PbS into Quantum Dots Superlattices

Orderedcolloidsor"colloidalcrystals",whichhavelong-rangespatialordef,havebeeninvestigatedextensively,andstillremainanactiveareaofresearchI.Selforganizationisageneralphenomenonofcolloidaldispersionswherecontroloverparticlesizeandstabilizationhasbeenachieved.Selforganizationrequiresonlyahard-sphererepulsion,acontro1ledsizedistribution,theinherentVanderWaaIsattractionbetWeenparticles,andameansofgentlydestabilizingthedispersion.Theinherenttendencyformonodisperselyophobiccolloidstoselforganizepro…