Electronic Properties of Interfaces

The surface between grains is usually a region of increased electrical potential compared to the bulk, so that a barrier to conductivity occurs across the boundary. These grain boundary potentials are often called Schottky barriers. The height and form of the barrier depends sensitively upon the materials in contact. 

The best understood barriers are formed between a semiconductor and a metal. When a metal and a semiconductor are brought into contact, electrons will flow from the metal to the semiconductor or vice versa, depending upon the relative energy levels across the boundary. Suppose, for example, that the Fermi level of 

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Interest in the electronic properties of interfaces centers around a-Si H/Si3N4, because this combination is used in multilayers (Section 9.4) and field effect transistors (Section 10.1.2). The electronic structure of the interface is illustrated in Fig. 9.18. Apart from the band offset which confines carriers to the a-Si H layer, the distribution of localized interface states and the band bending are the main factors which govern the electronic properties of the interface. The large bulk defect density of the SijN also has an effect on the electronic properties near the interface. Band bending near the interface may result from the different work functions of the two materials or from an extrinsic source of interface charge - for example, interface states. 

In this paper, we will discuss the structural and electronic properties of interfaces formed by Cu deposits on the (110) and the (101) surfaces of Sn02, and the subsequent formation of a CuO overlayer by oxidation. Due to the lattice mismatch, epitaxial growth of CuO on the Sn02 single crystal is not expected. We will therefore deposit nanoscale small particles of Cu in small steps, and oxidize these at each step to CuO, gradually increasing the size of... 

Kawamura F, Yasui I, Sunagawa I (2001a) Effects of supersaturation and impurity on step advancement on HOj (110) faces grown from high-temperature solution. J Cryst Growth 233 517-522 Kawamura F, Yasui I, Kamei M, Sunagawa I (2001b) Habit modifications of SnO crystals in SnOj-Cu O flux system in the presence of trivalent impurity cations. J Am Ceram Soc 84(6) 1341-1346 Klein A, Sauberlich F, Spath B, Schulmeyer T, Kraft D (2007) Non-stoichiometry and electronic properties of interfaces. J Mater Sci 42 1890-1900... 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

The late 1980s saw the introduction into electrochemistry of a major new technique, scanning tunnelling microscopy (STM), which allows real-space (atomic) imaging of the structural and electronic properties of both bare and adsorbate-covered surfaces. The technique had originally been exploited at the gas/so id interface, but it was later realised that it could be employed in liquids. As a result, it has rapidly found application in electrochemistry. 

The metal-solution interface as the locus of the deposition processes. This interface has two components a metal and an aqueous ionic solution. To understand this interface, it is necessary to have a basic knowledge of the structure and electronic properties of metals, the molecular structure of water, and the structure and properties of ionic solutions. The structure and electronic properties of metals are the subject matter of solid-state physics. The structure and properties of water and ionic solutions are (mainly) subjects related to chemical physics (and physical chemistry). Thus, to study and understand the structure of the metal-solution interface, it is necessary to have some knowledge of solid-state physics as well as of chemical physics. Relevant presentations of these subjects are given in Chapters 2 and 3. 

Kahn A, Koch N, Gao WY (2003) Electronic structure and electrical properties of interfaces between metals and pi-conjugated molecular films. J Polym Sci B Polym Phys 41 2529... 

Hirose Y, Kahn A, Aristov V, Soukiassian P, Bulovic V, Forrest SR (1996) Chemistry and electronic properties of metal-organic semiconductor interfaces Al, Ti, In, Sn, Ag, and Au on PTCDA. Phys Rev B 54 13748... 

The purpose of performing calculations of physical properties parallel to experimental studies is twofold. First, since calculations by necessity involve approximations, the results have to be compared with experimental data in order to test the validity of these approximations. If the comparison turns out to be favourable, the second step in the evaluation of the theoretical data is to make predictions of physical properties that are inaccessible to experimental investigations. This second step can result in new understanding of material properties and make it possible to tune these properties for specific purposes. In the context of this book, theoretical calculations are aimed at understanding of the basic interfacial chemistry of metal-conjugated polymer interfaces. This understanding should be related to structural properties such as stability of the interface and adhesion of the metallic overlayer to the polymer surface. Problems related to the electronic properties of the interface are also addressed. Such properties include, for instance, the formation of localized interfacial states, charge transfer between the metal and the polymer, and electron mobility across the interface. 

In the following discussion of the electronic properties of metal-semiconductor interfaces, the properties of the electron-injecting contact are taken as the example for contacts. Most of our studies of metals on polymers have involved low work function metals with the... 

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