Reconstruction dependence of the etching and passivation of the GaAs(001) surface

The microscopic nature of the selective interaction of iodine with an As- and Ga-stabilized GaAs(001) surface has been investigated by the photoelectron emission and ab initio calculations. The adsorption of iodine on the Ga-stabilized (4 × 2)/c(8 × 2) surface leads to the formation of the prevailing chemical bond with gallium atoms; to a significant redistribution of the electron density between the surface Ga and As atoms; and, as a result, to a decrease in their binding energy. Iodine on the As-stabilized (2 × 4)/c(2 × 8) surface forms a bond predominantly with surface arsenic atoms. Such a selective interaction of iodine with the reconstructed surfaces gives rise to the etching of the Ga-stabilized surface and the passivation of the As-stabilized surface; this explains the layer-by-layer (“digital”) etching of GaAs(001) controlled by the reconstruction transitions on this surface.     

Cesium effect on ion-induced secondary photon emission from a GaAs surface

The ion-induced photon emission from excited Ga atoms sputtered from a GaAs single crystal was measured as a function of cesium coverage on the specimen surface. The results showed a remarkable non-linearity in photon yield against cesium coverage. This implies that the photon enhancement effect due to cesium adsorption is nonlocal and relates to the nonradiative deexcitation process of sputtered atoms.     

Interaction of hydrogen with GaAs (0 0 1)-2×4

The absorption of hydrogen at a GaAs (0 0 1)-2×4 surface has been studied with angular resolved photoemission, core level photoemission and RHEED. We find a prominent hydrogen-induced peak in the heteropolar gap at -7.7 eV. Our results indicate, furthermore, that the (atomic) hydrogen interacts with both As and Ga surface atoms. The interaction is complex, leading to an As-depleted and disordered surface. Simple tight-binding considerations for the heteropolar gap state indicate that it could be caused by bonding of hydrogen to both As and Ga dangling bonds.     

碱金属在GaAs(110)表面上的吸附

本文讨论Li,Na,K,Cs在GaAs(110)表面上的吸附,考虑理想表面和弛豫表面两种情况。计算采用集团模型,用电荷自洽的ExtendedHucheltheory(缩写为EHT)方法进行。结果表明,吸附后表面原子趋向于理想位置,碱金属原子位于垂直于表面沿[001]方向横跨表面Ga原子的对称平面上。碱金属吸附后的费密能级在价带顶以上约0.7eV处,是由表面Ga原子与碱金属原子间的相互作用决定的。而在价带中碱金属原子主要与表面As原子成键。 关键词：     

Photoemission study of Gd metal overlayers on a GaAs(110) surface

We have studied the chemical reactions of Gd metal on an in situ cleaved GaAs(110) surface by photoemission spectroscopy of Ga 3d and As 3d core-levels as well as the Gd 4f level on- and off-resonance valence band using synchrotron radiation. We find that the Fermi-level pinning is completed before 0.13 ML coverage, and the deposited Gd atoms start to react with the GaAs substrate at a very low coverage (critical coverage < 0.067 ML). As more Gd atoms are deposited, they form stable compounds with As atoms which are then trapped in the relatively narrow interfacial layer of thickness less than about 3.3 ML, while Ga atoms diffuse out towards the surface and eventually become metallic. The thickness of the GdGa intermixed layers is estimated to be about 6.7 ML, which is somewhat greater than that for a interface.     

Selective Reflection of Laser Radiation from Ultrathin Layers of Cesium Atomic Vapors Confined in a Nanocell

The effect of the van der Waals interaction of cesium atoms with the sapphire windows of a nanocell was experimentally investigated using the selective reflection process. The distance L between the windows varied in the range of 50–2000 nm and the nanocell was filled with the vapors of cesium atoms. For the Cs atoms (the transition 6S_1/2 → 6P_1/2), the C₃ coefficient of the van der Waals interaction with the sapphire windows of the nanocell is measured. It is shown that it is possible to determine the magnetic fields with the spatial resolution of 70 nm using the selective reflection spectrum and, consequently, both the homogeneous and highly gradient magnetic fields can be measured.     

The oxidation of the GaAs (110) surface

It is demonstrated that, contrary to previous proposals, Ga surface atoms are already involved in the oxidation process for the lowest observable oxygen coverages (0.01 monolayer). A similar involvement of As atoms could not be readily ascertained experimentally, although it is to be expected from energetic considerations. An oxidation model consisting of multiple bridge bonds to both Ga and As surface atoms is proposed, which is consistent with diverse experimental data for the GaAs(110) surface.     

(Ga,Mn)As 体系中Mn自补偿效应的第一性原理研究

基于密度泛函理论的赝势平面波方法计算了处于填隙位置磁性原子(MnI)对(Ga,Mn) As体系电子结构和磁性的影响. 计算结果表明MnI在GaAs中是施主；代替Ga位的MnGa 与MnI的自旋按反铁磁序排列；静电相互作用使MnGa，MnI倾向于形成MnGa_MnI对. MnI的存在一方面补偿了(Ga,Mn)As中的空穴，降低了空穴浓度；同时还使邻近的MnG a失活. MnI的存在对获得高居里温度的(Ga,Mn)As是极为不利的. 关键词： (Ga Mn)As 稀磁半导体 密度泛函理论     

Formation of Gallium-induced nanostructures on single crystal HOPG surface

The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed. The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10⁻¹⁰ torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms. The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition are found to be stable and could be used as a template for the growth of metallic nanostructures.     

Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

Magnetic Resonance Frequency Shifts of Spin-Polarized Cesium Atoms in a Cs–Rb Mixture

The interaction of spin-polarized cesium atoms with cesium and rubidium atoms under conditions of optical orientation of atoms is considered. On the basis of data on spin-exchange complex cross sections in the Cs–Cs and Cs–Rb systems, a calculation of a magnetic resonance frequency shift of cesium atoms in a Cs–Rb mixture, as well as comparison of the calculated values of the frequency shift with experimental data, is performed.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1&#x0304;1&#x0304;1&#x0304;)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

Dynamic van der Waals interaction of a moving atom with the walls of a flat slit

A general expression was obtained for the dynamic energy of the van der Waals interaction of a neutral atom with a flat slit whose walls are characterized by a frequency-dependent dielectric permittivity. The interaction of cesium atoms with the walls of metallic (Au) and dielectric (SiC) slits is analyzed numerically at speeds of 10⁴ to 10⁷ m/s. As the speed of atoms grows, the dynamic potentials near the walls become substantially smaller in magnitude than static potentials, but, in the intermediate region, the former exceed the latter by a factor of 1.5–2.0 in a specific range of speeds.     

Surfactant properties of cesium in molecular beam epitaxy of GaAs(100)

The experimental and ab initio investigations of the effect of a decrease in the binding energy of surface arsenic atoms under the cesium adsorption on an As-stabilized GaAs(001)-(2 × 4) surface have been performed. The cesium-induced redistribution of the charge on the surface atoms reduces the electron density in the As-Ga bond of the upper layer of the GaAs(001) surface; thus, the As-Ga binding energy decreases and, as a result, the diffusion activation energy, as well as the arsenic atom desorption, decreases. An increase in the diffusion coefficient of surface atoms, along with the property of Cs to segregate on the surface of a growing semiconductor film, makes it possible to use cesium as a surfactant in the low-temperature growth of GaAs by molecular beam epitaxy.     

Ultrahigh-vacuum STM studies of the structure of a graphene layer on the Ir(111) surface

The atomic structure of a two-dimensional graphite film formed on the Ir(111) surface is studied. In order to weaken the interaction of the graphite film with the metal surface, cesium atoms are intercalated beneath the film. The studies by scanning tunneling microscopy in ultrahigh vacuum provide support for the formation of a continuous graphite layer, with the regular arrangement of carbon atoms at extended surface regions. From comparison of the roughness of the film and that of the substrate, it follows that the spacing between the film and metal surface varies within the limits of 1 nm. Characteristic structural defects of configuration (5, 7) are detected in the film.     

Cesium adsorption on the β2-GaAs(001) surface

The results of first-principles calculations of the cesium adsorption energy on the β2-GaAs(001) surface performed within approaches of the density functional theory are presented for two possible terminations of the surface. It is shown that, among the considered high-symmetry positions, the energy-preferred position for cesium is position T ₃ when the surface layer contains arsenic and position T ₄ for gallium terminated surface. Cesium introduces insignificant perturbations in the positions of surface-layer atoms, and surface dimers do not break even in the case of adsorption at the dimer bridge and top positions. It is shown that cesium bonding to the GaAs (001) substrate can be explained by sp hybridization of arsenic and gallium orbitals as well as by formation of cesium states mixed with delocalized states of a clean surface. At low coverage, more preferable adsorbate sites are those with nearest neighbor arsenic atoms for both surface terminations.     

GaP低阻欧姆接触层的AES和SIMS分析

本文用AES和SIMS分析讨论了p-GaP与三层金属膜Pd/Zn/Pd形成良好欧姆接触层的性质.     

Self-consistent tight-binding total energy calculations: Application to GaAs/Si and ZnSe/GaAs (100) interfaces

Self-consistent tight-binding total energy calculations are performed for various models of GaAs/Si and ZnSe/GaAs (100) interfaces. A graded GaAs/Si interface with the first monolayer on substrate having 1 As atom per 3 Si atoms followed by a second monolayer with 3 Ga atoms per 1 Si atom and continued with 2 bulk-like As and Ga monolayers is found to be structurally more stable than other interfaces. The instability of the abrupt interface is driven by elastic rather than electrostatic forces. Similar results are obtained for the ZnSe/GaAs (100) interface. The graded interface with Ga atoms exchanged against Zn atoms is found to be energetically most stable. Strong macroscopic electric fields are found in the surface and interface regions for both the GaAs/Si and ZnSe/GaAs interfaces.     

Cold-atom physics using ultrathin optical fibers: light-induced dipole forces and surface interactions

The strong evanescent field around ultrathin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold-atom cloud, we investigate the interaction of a small number of cold cesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.     

用低能电子衍射研究GaAs(110)表面的弛豫

用低能电子衍射研究了GaAs(110)表面的弛豫。发现当理论与实验之间符合得最好时,得到的结构是,保持表面上As—Ga键长不变用一个27.32°±0.24°的旋转角(ω),使As原子向外移动0.10±0.02?,Ga原子向内移动0.55±0.02?,而从Ga到第二层时空间为d₂=1.45±0.01?,从第二层Ga到第三层的空间为d₃=2.01±0.01?。对此结构As的背键长l_As=2.43±0.01?(收缩0.56％),而Ga的背键长l_Ga=2.253±0.004?(收缩8.0％)。 关键词：