Quasi-one-dimensional quantized states in an epitaxial Ag film on a one-dimensional surface superstructure

The in-plane energy dispersion of quantized states in an ultrathin Ag film formed on the one-dimensional (1D) surface superstructure Si(111)-(4 x 1)-In shows clear 1D anisotropy instead of the isotropic two-dimensional free-electron-like behavior expected for an isolated metal film. The present photoemission results demonstrate that an atomic layer at the film-substrate interface can regulate the dimensionality of electron motion in quantum films.     

稳定In/Si表面的LEED研究

利用LEED图形拟合的方法对大量不同取向In/Si表面的稳定性和小面化进行了研究，新发现了In覆盖度在1/2单层原子以下的三个稳定表面：Si(214)-In，Si(317)-In和Si(436)-In，以及In覆盖度在1单层原子左右的两个稳定表面Si(101)-In和Si(313)-In.此外还确定了In覆盖度在1单层原子左右的6个稳定In/Si表面的家族领地以及In覆盖度在1/2单层原子以下的4个稳定In/Si表面的家族领地.特别值得注意的是 Si(103)-In的家族领地相当大，甚至比最稳定的Si(1 关键词： 硅表面 铟 稳定表面 家族领地     

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis-low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3-In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1-In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb-Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.     

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

IR absorption enhancement for physisorbed methanol on Ag island films deposited on the oxidized and H-terminated Si(111) surfaces: effect of the metal surface morphology

Infrared absorption measurements using a multiple internal reflection geometry are reported for condensed methanol at 90 K on Ag island films deposited on the oxidized and hydrogen-terminated surfaces of Si(111). The attenuated total reflection (ATR) spectra obtained as a function of methanol exposure (up to 14 L) show that a 1-nm mass thickness of Ag island film on the oxidized Si(111) surface yields an absorption intensity 2–3 times larger than the intensity in the absence of Ag on the oxidized surface. Deposition of the same thickness of Ag on the hydrogen-terminated Si(111) surface results in approximately twice the enhancement. The different magnitudes of the enhancement are discussed based on SEM micrographs for Ag island films formed on the oxidized and H-terminated Si(111) surfaces. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Thickness dependent stripe structure stability of Ag films on Si(1 1 1)-(4 × 1)-In substrate

The thickness dependent stripe structure stabilization of Ag films on Si(1 1 1)-(4 × 1)-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for n_c of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar n_c value of Ag.     

Band structure and oscillatory electron-phonon coupling of Pb thin films determined by atomic-layer-resolved quantum-well states

Using a low temperature growth method, we have prepared atomically flat Pb thin films over a wide range of film thickness on a Si-(111)-7 x 7 surface. The Pb film morphology and electronic structure are investigated in situ by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Well-defined and atomic-layer-resolved quantum-well states of the Pb films are used to determine the band structure and the electron-phonon coupling constant (lambda) of the films. We found an oscillatory behavior of lambda with an oscillation periodicity of two atomic layers. Almost all essential features in the Pb/Si(111) system, such as the growth mode, the oscillatory film stability, and the 9 monolayer envelope beating pattern, can be explained by our results in terms of the electron confinement in Pb films.     

Visualization of submonolayers and surface topography by biassed secondary electron imaging: Application to Ag layers on Si and W surfaces

An ultra high vacuum-scanning electron microscope (UHV-SEM), equipped with micro-Auger electron spectroscopy, RHEED, and a deposition source has been used to study submpnolayers of Ag on Si(111), Si(100) and W(110) surfaces. A new SEM technique has been developed to visualize these submonolayers directly by applying a high negative bias (?500 V) to the sample. The dependence of the visibility on bias voltage, layer thickness and angle of incidence of the electron beam has been studied. The sensitivity was found to be better than 0.1 ML in the case of Ag on Si surfaces. Some applications to surface diffusion have been made and preliminary results for Ag on Si(111) are noted. Contrast at the 1 ML level can be > 10% in certain cases. The understanding of this contrast in terms of work function and ionization energy (band-bending) changes is discussed.     

超声引起的SrAl_2O_4:Eu,Dy薄膜的应力发光

为了实现超声探伤和应力发光探伤二者结合,研究了SrAl_2O_4∶Eu,Dy(SAOED)/硅橡胶在超声振动下的应力发光性质。薄膜的微观结构表明,SrAl_2O_4∶Eu,Dy应力发光颗粒被高弹性的硅胶包裹。当超声波作用到薄膜表面时,超声振动可以促使应力发光颗粒周围的硅橡胶发生各种形变,从而使被包裹的应力发光颗粒发生有效形变,产生高效的应力发光。薄膜厚度以及超声频率对SAOED发光薄膜的应力发光有明显的影响。薄膜的超声应力发光强度随着薄膜厚度的增加先增大,当薄膜的厚度为1 mm时达到最大,之后随着薄膜厚度的增加而降低。薄膜的超声应力发光强度与超声频率大小成正比,即使最低频率仅为500 Hz时仍能检测到信号,说明SrAl_2O_4∶Eu,Dy/硅橡胶发光薄膜是一种很有应用前途的无损检测传感器。     

The growth mode of aluminium on silver (111)

The geometric and electronic structures occuring during the growth of Al on a single crystal Ag(111) surface have been studied using a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), energy loss spectroscopy (ELS) and work function measurements. The Auger signal versus deposition time plots, which were used to monitor the growth mode, are shown to behave in an identical fashion to that expected for layer-by-layer (Frank-van der Merwe) growth. LEED was used to determine the lateral periodicity of thin Al films and shows that Al forms, at very small coverages, 2D islands which have the same structure as the Ag(111) substrate and which grow together to form the first monolayer. At substrate temperatures of 150 K a well defined (1 × 1) structure with the same orientation as the underlying Ag(111) can be seen up to at least 12 ML. After completion of the third monolayer the ELS spectrum approached that observed for bulk aluminium. At a coverage of 3 ML the work function decreases by 0.4 eV from the clean silver value.     

Quantum interference of Rashba-type spin-split surface state electrons

We studied the quantum interference of electrons in the Bi (p(x), p(y)) orbital-derived j = 1/2 spin-split surface states at Bi/Ag(111)√3 × √3 surfaces of 10 monolayer thick Ag(111) films on Si(111) substrates. Surface electron standing waves were observed clearly at the energy (E) below the intersection of the two spin-split downward dispersing parabola bands (E(x)). The E dependence of the standing wave pattern reveals the dispersion as the average of the two spin-split surface bands due to the interference between |(k + Δ), ↑> and |-(k - Δ), ↑> [or (|(k - Δ), ↓>) and |-(k + Δ), ↓>] states. In contrast, it was impossible to deduce the dispersion from the standing wave pattern at E ≥ E(x) because the surface electron cannot find its backscattered state with the same spin polarization.     

Growth of atomically flat ultra-thin Ag films on Si surfaces

A two-step growth with deposition at low temperature and subsequent slow annealing to room temperature (RT) has been widely used to obtain atomically flat metal thin films on semiconductor substrates. In the case of Ag films, almost atomically flat films were obtained at a thickness of six monolayers (ML) on Si and GaAs. The flatness then gradually degraded with an increase in thickness. The existence of the critical thickness has been well explained by the “electronic growth theory”, which considers thickness-dependent change of vertically quantized electrons and their spilling out at the interfaces. However, several questions remain unanswered. How does the electronically grown Ag flat film accommodate the large misfit energy between the film and substrate? Up to what deposition temperature is the two-step growth effective to obtain atomically flat films? In this article, we review previous studies on the electronic growth of Ag films on Si(1 1 1) substrates, paying special attention to these two points. In addition, we also discuss the possibility of engineering electronic growth for artificial control of the critical thickness.     

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si(111)√3-Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si(111)√3-Ag surface.     

Ag(111)和Au(111)上铋的初始生长行为

半金属铋(Bi)的表面合金具有的Rashba效应,和其具体结构性质有重要关联.本文结合扫描隧道显微镜(STM)和密度泛函理论(DFT),系统地研究了Bi原子在Ag(111)和Au(111)上的不同初始生长行为.在室温Ag(111)上,连续的Ag₂Bi合金薄膜会优先在Ag台阶边缘形成;在570 K Ag(111)上,随着覆盖度增加到0.33分子层(ML),Bi优先取代配位数低的台阶边原子并从单原子随机分布转变为长程有序的Ag₂Bi合金相;随着覆盖度增加,Ag₂Bi通过退合金过程转变成p×3^1/2结构的Bi膜.Bi在室温和570 K的Au(111)上的生长行为一致:在覆盖度低于0.40 ML时,Bi会优先吸附在配位为5的Au原子上,并以单原子和团簇的形式分别分散在Au(111)的密堆积区域和鱼骨纹重构的拐角处;随着覆盖度增加到0.60 ML,无序的Bi会逐渐转变成长程有序的((37)^1/2×(37)^1/2)相;Bi的吸附会导致Au(111)表面应力逐步释放.Bi在Ag(111)和Au(111)上的不同生长行为表明,Bi原子与衬底之间的相互作用起着关键作用.     

Structure of the Ba-induced Si(111)- (3 x 2) reconstruction

The Ba/Si(111) surface, previously known as a 3 x 1 phase, is found to have a 3 x 2 periodicity and a semiconducting band gap. The substrate reconstructs into the honeycomb chain-channel (HCC) structure with Ba atoms in the channel, as in the alkali-metal-induced Si(111)-(3 x 1). However, the metal coverage is determined to be 1/6 monolayers, half the alkali-metal coverage. We propose that the structure and the metal coverage determined for the Ba adsorbate is universal for other alkaline-earth-metal adsorbates. With the alkali-metal-induced 3 x 1 case, our results lead to a rule that one donated electron per 3 x 1 surface unit is necessary to stabilize the HCC reconstruction of Si.     

Si衬底上分子束外延Ge,Si时的反射式高能电子衍射强度振荡观察

本文观察了在Si(100)和Si(111)衬底上分子束外延Si,Ge时的反射式高能电子衍射(RHEED)强度振荡现象。其振荡特性表明,外延一定厚度的缓冲层可以改善表面的平整性,较慢的生长速率或中断生长一段时间有利于外延膜晶体质量的提高。Si(100)上外延Si或Ge时,沿[100]和[110]方位观测到的振荡特性均为单原子模式,起因于表面存在双畴(2×1)再构;而Si(111)上外延Ge时,[112]方位观测到的振荡为双原子层模式,但在[110]方位观察到不均匀周期的强度振荡行为。两种衬底上保持RHEED     

常压化学气相沉积法在Ag/Si(111)模板上生长ZnO薄膜及其性能研究

在Si衬底上利用磁控溅射的方法沉积1.5 nm厚度的Ag膜用以阻挡Si衬底被氧化。采用常压金属有机化学气相沉积法(MOVCD),在Ag/Si(111)衬底上成功地生长出马赛克结构的ZnO薄膜。用光学显微镜观察表面形貌,结果显示有带晶向特征的微裂纹,裂纹密度为100 cm-1。依据X射线晶体衍射的结果,薄膜结晶质量良好,呈C轴高度择优取向。用双晶X射线衍射得到(002)面的ω扫描半峰宽为1.37°。温度10 K时光致发光谱(PL)观察到自由激子、束缚激子发射及它们的声子伴线。结果表明,金属有机化学气相沉积法方法在Si(111)衬底上制备ZnO薄膜时,Ag是一种有效的缓冲层。     

Surface electromigration of metals on Si(001): In/Si(001)

The possibility of surface electromigration (SE) of metals of In, Ga, Sb and Ag on a very flat Si(001)2×1 substrate (single domain 2×1) was examined by SEM, μ-RHEED and μ-AES under UHV conditions. It was found that Ga, Sb and Ag show no SE on Si(001) surface even at DC annealing temperatures for the desorption of these metals. For In on Si(001), a very fast SE (8000 μm/min) towards the cathode side was found that suddenly sets in at 450°C DC annealing, which was related to a surface phase transition. μ-RHEED and μ-AES observation showed that the SE is related to an ordered 4×3-In phase together with two-dimensional In gas phase over the 4×3-In phase and an In-disordered phase at the front end of SE. Single domain 4×3-In phases were found to occur under sequences of In deposition and DC annealing which involve the In SE on Si(001).     

Low-temperature structure of indium quantum chains on silicon

The array of quasi-one-dimensional indium chains in the Si(111)- (4x1)-In surface reconstruction exhibits a phase transition to a low-temperature (8x2) phase. It has been suggested that this phase transition is related to a charge density wave (CDW) formation. The x-ray diffraction results presented here demonstrate that at 20 K the CDW has not yet condensed into a superstructure even though good transverse coupling was established. This indicates that CDW formation cannot be the driving force for the phase transition. Furthermore we elucidate the subtle highly anisotropic interchain correlations and reveal the detailed atomic structure of the low-temperature (8x2) phase.     

Anisotropy in conductance of a quasi-one-dimensional metallic surface state measured by a square micro-four-point probe method

We have devised a "square micro-four-point probe method" using an independently driven ultrahigh-vacuum four-tip scanning tunneling microscope, and succeeded for the first time to directly measure anisotropic electrical conductance of a single-atomic layer on a solid surface. A quasi-one-dimensional metal of a single-domain Si(111)4 x 1-In had a surface-state conductance along the metallic atom chains (sigma(axially)) to be 7.2(+/-0.6) x 10(-4) S/square at room temperature, which was larger than that in the perpendicular direction (sigma(radially)) by approximately 60 times. The sigma(axially) was consistently interpreted by a Boltzmann equation with the anisotropic surface-state band dispersion, while the sigma(radially) was dominated by a surface-space-charge-layer conductance.