Cs/p-GaAs(100)表面的变角XPS研究

在实验数据的基础上 ,采用变角XPS分析表面层状结构的计算程序 ,应用了新算法 ,使该程序能快速可靠地计算多层多种组分的含量和层厚度。计算了Cs吸附在清洁p GaAs(1 0 0 )表面上的Cs层覆盖率及弛豫层的厚度和组分。在Cs/GaAs达到峰值光电发射时 ,Cs覆盖率为 0 71个单层 ,Ga与As弛豫层厚度为 2 3个单层 ,Ga相对As轻微富集。     

GaAs和（Cs，O）／GaAs热处理工艺的XPS研究

用X射线光电子能谱(XPS)研究了GaAs(100)面基片的化学清洗、热净化、(Cs，O)激活后加热的处理效果。GaAs的氧化过程是：首先，As氧化为低价态，然后Ga被氧化，再就是As氧化为高价。确保样品从刻蚀清洗到超高真空环境中，表面Ga没有被氧化，就可在较低的温度热净化，获得As稳定的且原子级清洁的GaAs表面。净化后GaAs表面Ga含量越高，则(Cs，O)激活后形成氧化物越多，从而形成了界面势垒，使电子逸出概率降低。     

（Cs,O）／GaAs热退火的变角XPS定量研究

用变角X射线光电子能谱(XPS)技术分析了GaAs光阴极的表面及其在热退火后的效应,首次定量地计算了(Cs,O)/GaAs系统的表面(Cs,O)层和界面(Ga,As)弛豫层的厚度和组分.(Cs,O)/GaAs系统在625～650℃的热退火后,(Ga,As)驰豫层厚度减薄,Ga,As的原子浓度增大;且消除了O与As的化学连接.     

GaAs和(Cs,O)/GaAs热处理工艺的XPS研究

用X射线光电子能谱(XPS)研究了GaAs(100)面基片的化学清洗、热净化、(Cs,O)激活后加热的处理效果。GaAs的氧化过程是:首先,As氧化为低价态,然后Ga被氧化,再就是As氧化为高价。确保样品从刻蚀清洗到超高真空环境中,表面Ga没有被氧化,就可在较低的温度热净化,获得As稳定的且原子级清洁的GaAs表面。净化后GaAs表面Ga含量越高,则(Cs,O)激活后形成氧化物越多,从而形成了界面势垒,使电子逸出概率降低。     

用变角XPS定量分析研究GaAs光电阴极激活工艺

用变角X射线光电子能谱（XPS）技术分析了GaAs光电阴极的激活工艺，定量计算了阴极表面激活层和界面氧化层的厚度和组成。界面氧化物是由于O原子的穿过激活导，扩散到GaAs与（Cs，O）激活层的界面上而形成的，导入过量O会增加O－GaAs界面层的厚度，而对（Cs,O）激活层厚度影响较小，在激活过程中，严格控制和减少每次导入的O量是减少界面氧化层厚度，提高灵敏度的重要途径，在第一步激活后的阴极样品，通过较低温度的加热和再激活，能获得比第一步高出30％的光电灵敏度的原因是较低温度加热减少了界面氧化层的厚度和界面势垒。     

Fe／GaAs（100）界面构成和电子态的同步辐射研究

利用同步辐射光电子能谱技术详细研究了Fe/GaAs(100)的界面反应和电子结构，在界面处，Fe与As形成稳定的化学键，而Ga则溶解到Fe薄膜中形成合金，但反庆只能在界面处发生，形成窄的反应层，Fe沉积后改变了GaAs表面的电存在As和Ga的扩散现象，同步辐射价带变也证实了这一点。     

Na、O在n-GaAs(100)-(4×1)表面吸附以及共吸附的光电子谱研究

利用紫外光电子谱,对Na、O在n型GaAs(100)-(4×1)表面吸附及共吸咐作了详细研究。结果表明:随着GaAs(100)表面Na覆盖度的变化,其表面呈现不同的能带弯曲。当Na的覆盖度很低时(通常认为θ≤0.02ML).n-GaAs(100)表面能带向下弯曲量达0.3eV。但随着覆盖度的增加,最后费米能级钉扎在价带顶以上0.8eV处。另外,O在n-GaAs(100)-(4×1)表面吸附。暴露量达10~5L时,导致了向上0.2eV的能带弯曲。如果在吸氧的表面继续蒸Na,则其费米能级钉扎在价带顶以上1.0eV处。同时还可看到,由于Na存在,O_2在n型GaAs(100)-(4×1)表面有粘附系数将大大地提高。     

金属夹层对Ge/GaAs(100)能带偏离影响的研究

用光电发射的方法研究了碱金属Na和碱土金属Mg对Ge／GaAs（100）异质结形成及能带偏离的影响。实验结果表明，Na和Mg的薄夹层可使Ge／GaAs（100）的价带偏离分别增加0．19和0．18eV。通过Na，Mg和Al夹层的对比研究，可认为金属夹层对异质结能带偏离的影响与金属的电负性及其与衬底的相互作用有关。     

ZnSe光学薄膜对GaAs表面特性的影响

GaAs基半导体激光器芯片在空气中解理后,解理腔面会被空气氧化形成腔面缺陷,在腔面形成的缺陷严重影响了器件的寿命.用GaAs衬底表面模拟半导体激光器的解理腔面,研究了不同的光学薄膜对GaAs表面特性的影响.研究结果表明暴露在大气中的GaAs表面会形成Ga2O3、As2O3和As2O5缺陷.在表面镀含氧光学膜的GaAs表面上会形成少量Ga2O3缺陷,不形成As2O3和As2O5缺陷,在表面镀ZnSe光学薄膜的GaAs表面没有形成Ga2O3缺陷,也没有形成As2O3和As2O5缺陷.在GaAs表面上蒸镀ZnSe光学薄膜能有效地抑制GaAs表面缺陷的形成,提高半导体激光器的寿命.     

MBE生长GaAs薄膜表面形貌的RHEED图样研究

本文通过在GaAs(100)单晶衬底上MBE生长GaAs过程中形成的RHEED衍射图样,对GaAs薄膜的表面形貌进行研究。分析GaAs表面粗糙和生长时不发生RHEED强度振荡的原因。讨论在生长GaAs时出现In(Ga)As/GaAs(100)体系的RHEED衍射图样这种异常现象的原因。     

Unusual diffusivity and clustering of alkali metals on the Si(111)-7x7 surface

In this paper I review our recent experimental and theoretical works on the behaviors of submonolayer alkali metals (Li, Na, K, Cs) on the Si(111)-7x7 surface. The important aspects in the experimental preparation are discussed. While Li forms clusters even at the initial coverage, all other alkali metals (Na, K, Cs) form two-dimensional (2D) gases at room temperature and low coverage, due to their unusual high diffusivity on the surface. At higher coverage, cluster formation is observed, but with very different behaviors for Na, K, and Cs. Issues relative to the phase transition with increasing coverage are discussed: (1) the mechanism for the 2D gas formation; (2) the effect of 2D gas formation on STM observations; (3) The effect of structure on the charge transfer from alkali metals to the surface; (4) the exchange of alkali metal atoms with the Si adatoms during the cluster formation.     

Viscosity and thermal conductivity of binary eutectics of alkali metals in the vapor phase

Values calculated for the dynamic viscosity and thermal conductivity are presented for vapors of binary eutectics of the alkali metals at temperatures from 800 to 1500 K and at pressures from 100 to 8×10⁵ Pa. Data are presented for the vapors of the systems Li + Na, Na + Rb, Na + Cs, K + Rb, K + Cs, Na + K, and Rb + Cs. The values of the concentrations of the five components in the vapor phase of each binary eutectic are also presented. The accuracy of the calculated viscosities is estimated to be within 4–5% and the accuracy of the calculated thermal conductivities is estimated to be within 8–10%.     

Behavior of Alkali Metal and Ammonium Salts in Nanofiltration of Their Aqueous Solutions

Radiochemistry - The behavior of salts (chlorides, nitrates, sulfates, phosphates, borates, ferri- and ferrocyanides) of various alkali metals (Li, Na, K, Cs) and ammonium in the course of...     

Phonon dispersion in alkali metals and their equiatomic sodium-based binary alloys

In the present article, the theoretical calculations of the phonon dispersion curves (PDCs) of five alkali metals viz. Li, Na, K, Rb, Cs and their four equiatomic sodium-based binary alloys viz. Na_0.5Li_0.5, Na_0.5K_0.5, Na_0.5Rb_0.5 and Na_0.5Cs_0.5 to second order in a local model potential is discussed in terms of the real-space sum of the Born von Karman central force constants. Instead of the concentration average of the force constants of pure alkali metals, the pseudo-alloy-atom (PAA) is adopted to directly compute the force constants of the four equiatomic sodium based binary alloys and was successfully applied. The exchange and correlation functions due to the Hartree (H) and Ichimaru-Utsumi (IU) are used to investigate the influence of the screening effects. The phonon frequencies of alkali metals and their four equiatomic sodium-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. The PDCs of pure alkali metals are found in qualitative agreement with the available experimental data. The frequencies in the longitudinal branch are suppressed rather due to IU-screening function than those due to static H-screening function.     

A comparative study of electrical resistivity of liquid alkali metals

The resistivities of liquid alkali metals are investigated using Ziman formula and the alternative expression based on Kubo formula suggested by Ünal and Alkan [B. Ünal, B. Alkan, J. Phys. Soc. Jpn. 62 (1993) 2425]. As the input pseudopotential, the individual version of the electron-ion potential proposed by Fioalhais and coworkers which was originally developed for solid state is used and static structure factors are derived from the solution of Ornstein–Zernike integral equation with Rogers–Young closure. Good agreement with experiment is found for liquid Na and K using both formulas. For Li, Rb and Cs the agreement between the calculated values using Ziman formula and experimental data is less satisfactory. From present investigations, it is shown that the formula based on Kubo formula suggested by Ünal and Alkan is the better chose for the resistivity calculations of liquid Li, Rb and Cs metals.     

Self-oscillatory effects accompanying the field-induced desorption of alkali metals

Self-oscillatory effects were observed during continuous field-induced desorption of alkali metals (K, Cs) from a gold-coated tungsten surface. The substrate surface images in alkali metal ions, reflecting various stages of the desorption process, were obtained using a field-ion microscope.     

A comparative study of DFT and XPS with reference to the adsorption of caesium ions in smectites

Chemical states of Cs ion adsorbed on dioctahedral smectites were studied using X-ray photoelectron spectroscopy (XPS) and molecular orbital calculations based on the density functional theory (DFT). Smectites investigated were montmorillonite, montmorillonite–beidellite, Fe–montmorillonite, and nontronite. It was found that montmorillonite samples kept approximately 50% of adsorbed Cs ions after a treatment by BaCl₂ aqueous solution, which were the highest values among the smectite samples. Cs(4d) binding energy was slightly shifted towards a lower energy by 0.2–0.8 eV after a BaCl₂ treatment, which suggests that plural adsorption sites exist on the smectite surface. Stable Cs adsorption sites and the adsorption energies were determined by DFT calculations. As a result, Cs ion is adsorbed strongly on a basal oxygen hexagonal hole. The chemical shift of the calculated Cs(4d) peak gave the same tendency as shown in the XPS measurements.     

Surface energy calculation of bcc metals using the analytical equivalent crystal theory method

We have used the newly formulated analytical equivalent crystal theory method which is a modification of the ECT method to calculate the surface energies for three low-index surfaces of bcc alkali metals Li, Na, K, Rb, Cs and the bcc transition metals V, Nb, Ta, Cr, Mo, W and Fe. The data may be used as a starting point for models of surface science phenomena. The accuracy of the surface energies is established from a comparison with other theoretical results and experiment. The surface energy of each (h k l) plane in alkali metals is much lower than that in transition metals. For all bcc metals, the order of the three low-index surface energies is σ₁₁₀ < σ₁₁₁ < σ₁₀₀ in good agreement with the MEAM of Baskes [Phys. Rev. B 46 (1992) 2727] except for Cr and Mo where the ordering obtained in this work is different from that of Baskes. The calculated results show that the surface energy is anisotropic and that the closed-packed plane (1 1 0) is the lowest of the three low-index surfaces.