Surface characterisation and interface studies of high-k materials by XPS and TOF-SIMS

High-k dielectric LaAlO₃ (LAO) films on Si(100) were studied by TOF-SIMS and XPS to look for diffusion processes during deposition and additional thermal treatment and for the formation and composition of possible interfacial layers. The measurements reveal the existence of SiO₂ at the LAO/Si interface. Thermal treatment strengthens this effect indicating a segregation of Si. However, thin LAO layers show no interfacial SiO₂ but the formation of a La-Al-Si-O compound. In addition, Pt diffusion from the top coating into the LAO layers occurs. Within the LAO layer C is the most abundant contamination (10²¹ at/cm³). Its relatively high concentration could influence electric characteristics. XPS shows that CO₃²⁻ is intrinsic to the LAO layer and is due to the adsorption of CO₂ of the residual gas in the deposition chamber.     

Surface and interface studies of GaN epitaxy on Si(111) via buffer layers

Gallium nitride films, epitaxially grown on Si(111) via a lattice-matched ZrB(2) buffer layer by plasma-assisted molecular beam epitaxy, have been studied in situ by noncontact atomic force microscopy and also in real time by reflection high-energy electron diffraction. The grown films were determined to be always N-polar. First-principles theoretical calculations modeling the interface structure between GaN(0001) and ZrB(2)(0001) clarify the origin of the N polarity.     

Surface studies with hyperfine probes

The current status of surface hyperfine fields measured with NMR, Mössbauer spectroscopy and PAC is reviewed. Emphasis is put on experiments with well-characterized, free surfaces and on results on surface electric field gradients. Applications to detection of probe sites and diffusion phenomena are discussed.This article is dedicated to the memory of our late colleague Dr. Wolfgang Keppner     

Surface and interface phonons and related topics

We analyze the state of the art of surface and interface phonons with an emphasis on their relevance to many aspects of surface and interface physics.     

Surface studies with an imaging atom-probe

The imaging atom-probe (TAP) is an atom-probe field-ion microscope in which the conventional time-of-flight mass spectrometer is replaced by a nanosecond-gated channel-plate image intensifier which is used to record the arrival positions of ions of preselected mass-to-charge ratio which have been field-evaporated from the surface of a field-ion emitter. The evaporated metal ions have the same radial projection that is seen in the normal field-ion image and the recorded image displays the points of origin of the ions with a spatial resolution of 1 nm or better. The field of view covers a selected area of about 0.3 steradian at the curved specimen surface. The mass resolution, limited by the channel-plate gating time, is of order $\text{Δm}\text{m}\text{=}\text{1}\text{15}\text{to}\text{1}\text{25}$. The wide field of view allows the rapid determination of the crystallographic distribution of differently-charged ions from those elements which evaporate as a mixture of charge species, and shows that this distribution is frequently a sensitive function both of the local surface geometry and of the presence or absence of field-adsorbed helium or neon at the specimen surface. The wide field of view of the IAP gives it a considerable advantage over the conventional atom-probe in the detection of segregation at grain boundaries.     

Surface states at the metal-electrolyte interface

A survey is given of the study of surface states at metal-electrolyte interfaces, which comprises both crystal-induced and image-potential-induced states. Because the energetic position varies with electrode potential, surface states can be conveniently investigated in an electrochemical cell by electroreflectance spectroscopy. It is demonstrated that crystal-induced surface states can act as a probe for the potential gradient inside the electric double layer and hence can yield valuable information on interfacial properties. Furthermore, the influence of anion adsorption on surface states, the appearance of surface states in thin metal overlayers with increasing film thickness and the monitoring of surface reconstruction by surface states are briefly discussed.     

Surface plasma waves over bismuth-vacuum interface

A surface plasma wave (SPW) over bismuth-vacuum interface has a signature of mass anisotropy of free electrons. For SPW propagation along the trigonal axis there is no birefringence. The frequency cutoff of SPW lies in the far infrared region and can be accessed using free electron laser. The damping rate of waves at low temperatures is low. The surface plasma wave may be excited by an electron beam of current ∼100 mA propagating parallel to the interface in its close proximity.     

Surface states at the metal-electrolyte interface

Normal incidence electroreflectance spectra of Ag and Au single crystal electrodes in aqueous electrolytes reveal pronounced structural features, which can be assigned to optical transitions involving empty surface states. The transition energies show a marked dependence on the electrode potential as predicted by the calculation of Ho, Harmon and Liu. It is demonstrated that electroreflectance of metal electrodes in the double layer charging region is a sensitive tool for studying metallic surface states.     

Surface and interface dipoles on catalytic metal films

The work function changes introduced by hydrogen on the surface and at the metal-support interface of a thin Pd-film were studied by simultaneous Kelvin probe and C(V)-measurements. It is demonstrated how these techniques can be used at atmospheric pressures to yield information about catalytic metals and on chemical kinetics on the metal surface. The main purpose of the communication is to point out the correlation between the surface and interface dipoles on catalytic metal films. Furthermore since the interface dipole is only created by hydrogen atoms it is shown how this can be used in a more complex situation to independently monitor the hydrogen content of the metal films.     

Surface andinterface studies with perturbed angular correlations

The present status of surface, interface andthin film studies with perturbed -angular correlation (PAC) spectroscopy is reviewed. Applications include adsorbate diffusion andstructural phase transitions on stepped metal surfaces, surface andinterface magnetism as well as compound formation at reactive metal/metal interfaces. First applications to semiconductor surfaces andmetal/semiconductor interfaces are discussed.     

Surface electromagnetic waves at a superconductor-dielectric interface

This paper reports on the results of investigating the frequency dependence, the structure, and the polarization-and energy-related characteristics of surface electromagnetic waves propagating along a superconductor-dielectric interface. An expression for the complex permittivity of a superconductor is derived in the approximation of a two-component plasma containing “normal” and “superconducting” electrons. Basic relations are obtained in the general case at temperatures T≤T _c (where T _c is the critical temperature) and in the limiting case at T?T _c, when the contribution from normal electrons to the permittivity of the superconductor can be disregarded.     

Surface acoustic waves at a free-metallized interface

Light scattering was used to study surface acoustic waves at a free-metalized interface on piezoelectric lithium niobate. An interaction region of several acoustic wavelengths was observed in which a reflected surface wave and a continuous spectrum of bulk waves are generated.     

Surface polaritons at the magnetized semiconductor-dielectric interface

Conditions for the existence of a surface polariton at the semiconductor-dielectric interface have been analyzed. With allowance for the gyrotropy and the resonance frequency dispersion of the semiconductor, which manifests itself in an external magnetic field, the dispersion relation, the characteristic frequencies determining the existence region of a surface polariton, the wave fields, and the energy flux distribution in each of the interfacing media have been obtained. A numerical analysis of the influence of the external field and absorption, which lead to the manifestation of nonreciprocal properties of the polariton and to a substantial reconstruction of the spectrum, has been performed.     

Surface/interface issues in THz electronics

Basic THz elements are produced by standard semiconductor science and technology. Therefore, three main material systems are used. These are first of all semiconductors, for active and passive layer formation; metals, for interconnect and contact formation; and insulators, for passivation and isolation purposes. Additionally, these materials are structured in order to produce a device with desired dimensions and characteristics. Semiconductor surfaces, in particular suffer considerable changes during technological processing. Thus, surface and interface issues are essential here to be considered. Semiconductor–dielectric, semiconductor–metal, and semiconductor–semiconductor interfaces as well as surface effects for the case of GaAs are discussed in detail from the point of view of Schottky diodes and heterostructure-based devices for THz applications.     

Surface state scattering at a buried interface

The free-electron-like surface state of Mg(0001) is strongly modified in thin films grown on W(110). The long bulk penetration length of its wave function makes it sensitive to the reflective properties of the buried interface, and hence to the complex electronic structure of the substrate. In particular we find a many-fold splitting of the Mg surface band by entering a wide projected band gap of W(110). There is a strong thickness-dependent two-band splitting, which is a clear signature of the formation of a surface-interface resonant state. An additional split-off from these two surface bands is explained by the substrate induced spin-orbit interaction.