Quantum well and quantum wire states at metal surfaces

Novel effects in magnetic multilayer structures, such as oscillatory magnetic coupling and "giant" magnetoresistance, have created new materials that allow for an order of magnitude higher sensitivity in the detection of magnetically-recorded data. Determination of their electronic and magnetic structures with angle-resolved photoemission and inverse photoemission reveals quantized states in the noble metal spacer layers which are connected with oscillatory magnetic coupling and have implications on magnetoresistance. These states can be understood by a simple interferometer model, including the spin-dependent interface reflectivity that polarizes them and transmits the magnetic coupling through the noble metal spacer.Current efforts are discussed, which aim towards fabricating quantum wires and lateral superlattices on metals by decorating steps at vicinal surfaces. STM work on the growth mode of such structures is presented, which uses spectroscopic contrast to distinguish different metals. Specific electronic states at decorated step edges are found with inverse photoemission.     

Origin of the defect states at ZnS/Si interfaces

Electrical characterisation of silicon surfaces contaminated by a zinc-sulphide overlayer has been carried out by forming Schottky diodes on the silicon after the ZnS has been etched off. The techniques include current-voltage, capacitance-voltage, and deep-level transieni spectroscopy. The Schottky diodes show clear memory of the presence of the ZnS overlayer and the electrical characteristics are far from ideal. Five deep levels in the sub-surface region of the silicon are detected, corresponding to the Zn⁺, Zn⁺⁺, S^–, S^–– states and probably to a Zn–B complex (p-type). Diffusion of the zinc and sulphur into the silicon is therefore confirmed and this diffusion is thought to create a compensated layer at the interface. These impurity states control the electrical characteristics of the surface in these diodes.     

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO₂ growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He⁺ ions is demonstrated to be a powerful tool for interface and surface studies.     

Acceptor- and donor-like interfacial states at ZnSe/GaAs heterovalent interfaces

The properties of ZnSe/GaAs interface with acceptor-like or donor-like state have been studied. The electric field and potential profile at the interface with donor-like or acceptor-like state has been calculated by solving Poisson equation, and been detected by photo-voltage measurement. The photocurrent measurement shows different behaviors in these two kinds of samples.     

Electronic structure at rubrene metal interfaces

Many studies have been done on low energy (1–200 keV) and high dose (10¹⁶–10¹⁷) implantation of Mn in GaAs. This study is an attempt to incorporate Mn ions in GaAs through implantation of 1 MeV Mn⁺¹ ions in semi-insulating GaAs substrates at doses of 3×10¹⁵/cm² and subsequent annealing. This was done to find out if any alloy of Mn–Ga–As, or binary compounds of Mn–Ga or Mn–As form due to annealing of Mn⁺¹ ions implanted in GaAs substrates. High Resolution XRD (HRXRD) performed before annealing shows a possibility of Ga–Mn–As alloy formation, and after annealing at 800°C, except for GaAs main peaks no other phase peaks were detected. Scanning electron microscopy (SEM) shows nanostructures of various dimensions which are thought to be formed due to the defects generated due to implantation. Fourier Transform Infrared (FTIR) study shows the shift in bandgap due to Mn doping. Raman spectroscopy shows the red shift in LO and TO peak positions of GaAs after annealing, which indicates the presence of disorder and damage due to implantation. Resistivity measurement shows a thermally activated semiconductor character of charge conduction with an activation energy of 51 meV and this activation may have occurred through the transitions from impurity band to valence band. Large positive (∼25%) magnetoresistance and a mixture of ferromagnetic and paramagnetic behavior obtained in the magnetization measurement indicate the presence of ferromagnetic MnAs nanoclusters embedded in paramagnetic GaAs:Mn matrix.     

On surface states at superconductor oxide interfaces

Localized electron states in oxides on metal surfaces hybridize with conduction electrons leaking into the oxide and form so a metallic layer and a new type of surface state. For a superconductor such surface states weaken the superconductivity. Because these surface states extend into the oxide, they cause an enhanced tunnel conductivity, which is finite below the gap voltage due to resonance tunneling via the surface states below the gap.     

Existence of localized electronic states at interfaces

A simple self-consistent atomic model is used to show that both true localized interface states as well as interface resonances can occur at the junction between a metal and a non-metal. These results resolve a controversy of long standing over whether or not true localized states (Bardeen states) can exist when one of the electrodes is a metal. Two distinct physical situations are considered for the metal-non-metal interface showing that these interface states may or may not exist depending upon the band gap of the non-metal and the coupling strength across the interface. In addition, results are presented for the case when both electrodes are non-metals (heterojunctions).     

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

Electrode reactions at oxygen,noble metal / stabilized zirconia interfaces

The polarization behaviour of electrodes of the type “oxygen, noble metal / stabilized zirconia”, comprising different zirconia-based materials as electrolyte, platinum or gold as metal component and an oxygen containing gas atmosphere, was investigated at elevated temperatures under equilibrium and non-equilibrium conditions by means of impedance spectroscopy. Massive metal contacts were used as part of the working electrodes. Under non-polarized conditions, the experimental results for platinum indicate a basically uniform reaction mechanism in a vast range of temperature and oxygen partial pressure, involving the surface diffusion of dissociatively adsorbed oxygen on platinum towards the electrochemical reaction sites on the electrolyte surface as rate-determining step. The experimental findings for gold are consistent with the occurrence of two competing reaction mechanisms, namely a charge transfer controlled process and a surface diffusion controlled process, each of them prevailing in different regimes of temperature and oxygen partial pressure. Under polarized conditions, a significant decrease of the polarization resistance takes place, followed by the onset of low frequency loops in the impedance spectra. In the case of cathodic polarization, the onset voltage can be correlated with the partial electron conductivity of the electrolyte, thus confirming the hypothesis of direct participation of electronic species of the electrolyte in the electrode reaction under biased conditions. At moderate temperatures, the polarization induced changes in the electrode properties exhibit a slow relaxation behaviour. This can be attributed to the successive annihilation of additional metastable electrochemical reaction sites having been created during the preceding polarization treatment. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy     

Chemical and physical interactions at metal/self-assembled organic monolayer interfaces

The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between metal (M) and eventually metal oxide overlayers on well-ordered organic substrates. Application of M/SAM and inorganic/SAM research results to the understanding of real inorganic/ organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The M/SAM approach to interface research is delineated as a new subfield in surface science in the context of other approaches to inorganic/organic interface research. Current issues in M/SAM research are outlined, including chemical compound formation, the morphology (spreading, clustering, or penetration) of the metal species, the kinetics of the metal morphology, the effect of the metal on the degree of order in the SAM, and the rate of metal penetration into the SAM. Probes are recommended that are suitable for M/SAM research. The results of M/SAM studies to date are reviewed, and M/SAM combinations are ranked according to reactivity and penetration. Key probes for addressing gaps in the research results are identified. The effects of defects, disordering, air exposure, and X-ray and electron beam exposure on the experimental results to date are evaluated. Thus far, the results have successfully revealed qualitative relationships of M/SAM chemistry, temperature, and penetration. The chemical interactions that have been found are applicable to real M/polymer interfaces as formed in vacuum. It has yet to be shown that M/SAM research will yield quantitative understanding of interface formation or that M/SAM interfaces are entirely analogous to M/polymer interfaces in the details of interface formation. The future of this subfield of surface science lies in its expansion from M/SAM interfaces in vacuum to other inorganic/SAM interfaces in vacuum and, eventually, under environmental conditions.     

Photoemission and x-ray studies of metal hydrides and hydride formation at metal/hydride interfaces

Synchrotron radiation photoemission was used to investigate the concentration dependent intensity of hydrogen-induced states for a series of niobium hydrides. These intensities were found to be linearly related to the fraction of β phase hydride present in the sample, as deduced from the bulk phase diagram, and we conclude that fracturing did not produce a self-sustaining surface hydride layer. Photoemission studies of V overlayers on NbH_0.799 showed strong hydrogen outdiffusion and the formation of the interface hydride phase which is more stable than VH prepared in bulk form. X-ray diffraction studies of thick V-H overlayers subsequently showed the absence of long-range order. These results demonstrate that interfacial diffusion of hydrogen from NbH_0.799 into the V overlayer produced a stable hydride with electronic properties analogous to those of crystalline VH but with dominant local interactions.     

A method of determining the charge trapped at the interfaces of a metal/ferroelectric/metal thin-film structure

A method is developed for determining the trap density at the metal/ferroelectric interfaces in a completely depleted ferroelectric film with two Schottky barriers. The method is based on the recharging of traps induced by an external pulsed bias. The ranges of the bias fields and of the parameters of the metal/ferro-electric/metal structure for which analytical solution of the Poisson equation is possible are found. Using this method and measurements of the transient current, the density of the charge trapped at the upper and lower interfaces of Pt(Ir)/PZT/Ir(Ti/SiO₂/Si) capacitors is determined. The interface charge as estimated from the trap density proved to be much smaller than the residual polarization of the PZT film. The observed correlation between the symmetry of the interface trap charges and the symmetry of the hysteresis loops and switching currents indicates the reliability of the estimation of the trap density determined using the proposed method.     

Perpendicular-current studies of electron transport across metal/metal interfaces

We review what we have learned about the scattering of electrons by the interfaces between two different metals (M1/M2) in the current-perpendicular-to-plane (CPP) geometry. In this geometry, the intrinsic quantity is the specific resistance, AR, the product of the area through which the CPP current flows times the CPP resistance. We describe results for both non-magnetic/non-magnetic (N1/N2) and ferromagnetic/non-magnetic (F/N) pairs. We focus especially upon cases where M1/M2 are lattice matched (i.e., have the same crystal structure and the same lattice parameters to within ∼1%), because in these cases no-free-parameter calculations of 2AR agree surprisingly well with measured values. But we also list and briefly discuss cases where M1/M2 are not lattice matched, either having different crystal structures, or lattice parameters that differ by several percent. The published calculations of 2AR in these latter cases do not agree so well with measured values.     

Annealing effects on the plasmonic excitations of metal/metal interfaces

The effects of the annealing procedure at 400-450 K on the electronic properties of nanoscale thin films of Ca, Au and Ag grown on Cu(1 1 1) at room temperature were probed by high-resolution electron energy loss spectroscopy measurements. Ca surface plasmon underwent to a significant red-shift upon annealing, due to the oxidation of the topmost Ca layer. Water strongly interacted with the CaO interface at room temperature. Au surface plasmon disappeared upon annealing the gold film, as a consequence of the formation of an Au-Cu alloy. Ag surface plasmon red-shifted both in the annealed adlayer and with increasing temperature compared with the frequency recorded for the as-deposited silver film.     

X-ray scattering: Liquid metal/vapor interfaces

Dilute aqueous solutions of alcohols with a high number of carbon atoms can be considered as self-rewetting fluids due to their properties associated to an anomalous dependency of the surface tension with temperature in some ranges of concentrations. In this paper research activities focused on numerical simulations and laboratory experiments of the behaviour of a thin layer of liquid subject to a horizontal thermal gradient. The investigated liquids include ordinary liquids and water/alcohols mixtures. Physical properties measurements, in particular surface tension and refractive index, are also presented. Flow visualization and interferometric analysis have been carried out using optical diagnostic systems.