Semiconductor interfaces, composition, surface structure

Schottky barrier height modulation and stabilization as a "by product" of the original studies. Moreover, since these results are thought to follow from replacement reactions in the vicinity of the interface, they reflect the importance of surface structure as well as composition at semiconductor interfaces. 

Experimental determinations of barrier heights on oxide semiconductor interfaces using photoelectron spectroscopy are rarely found in literature and no systematic data on interface chemistry and barrier formation on any oxide are available. So far, most of the semiconductor interface studies by photoelectron spectroscopy deal with interfaces with well-defined substrate surfaces and film structures. Mostly single crystal substrates and, in the case of semiconductor heterojunctions, lattice matched interfaces are investigated. Furthermore, highly controllable deposition techniques (typically molecular beam epitaxy) are applied, which lead to films and interfaces with well-known structure and composition. The results described in the following therefore, for the first time, provide information about interfaces with oxide semiconductors and about interfaces with sputter-deposited materials. Despite the rather complex situation, photoelectron spectroscopy studies of sputter-deposited... 

One frequently examined interface is the solid-liquid interface, where the solid phase may be a dielectric, a semiconductor, or a metal. Species located at these interfaces are of primary importance in electtochemisny and in chemistry of surface-active substances (surfactants). Another common type of interface is the solid-solid interface, specifically dielectric-dielectric, dielectric-semiconductor, dielectric-metal, semiconductor-semiconductor, semiconductor-metal, and metal-metal interfaces. These structures have an extremely important role in such areas as microelectronics and the chemistry of composites. Furthermore, positioning an ultrathin film at the interface of two media, one can substantially increase surface sensitivity of all IR spectroscopic methods. 

Films of InP on SiO are used as substrates for the fabrication of many semiconductor devices. Although there are in general no particular problems associated w ith the in-.service performance of the substrates, it is instructive to examine their surface chemistries, since during fabrication of devices both InP and SiO I lid their interfaces, become exposed. The various phases that appear are to be found distributed among a series of well-defined surface structures of various shapes and dimensions. Here it is clear that the material is laterally inhomogeneous. and if the local composition and chemistry are to be studied, then spatially resolved information, probably on the scale of a few pm. is necessary. 

Here, Ws is the work function of electrons in the semiconductor, q is the elementary charge (1.6 X 1CT19 C), Qt and Qss are charges located in the oxide and the surface and interface states, respectively, Ere is the potential of the reference electrode, and Xso is the surface-dipole potential of the solution. Because in expression (2) for the flat-band voltage of the EIS system all terms can be considered as constant except for tp (which is analyte concentration dependent), the response of the EIS structure with respect to the electrolyte composition depends on its flat-band voltage shift, which can be accurately determined from the C-V curves. 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

The atomic geometry of a surface or interface is, in certain respects, its most fundamental property. Since most surfaces and interfaces are metastable, especially those of technological interest, their composition and structure depends on their process history. Their structures determine, moreover, the "interesting" interfacial properties which are utilized in specific applications, e.g., reactivity and specificity in catalysis or Schottky barrier height in metal-semiconductor contacts. In addition, the interface structure is measurable by one or more of the techniques noted earlier. Therefore the structure of an interface is a measurable link between the process used to prepare it and the electronic and chemical properties which determine its utility. 

As a probe of lattice vibrations, Raman spectroscopy is very sensitive to intrinsic crystal properties and extrinsic stimuli, especially in semiconductors. It may be employed to study crystal structure and quality, crystal orientation, optical interactions, chemical composition, phases, dopant concentration, surface and interface chemistry, and local temperatme or strain. As an optical technique, important sample information may be obtained rapidly and nondestructively with minimal sample preparation. Submicron lateral resolution is possible with the use of confo-cal lenses. These features have made it a vital tool for research labs studying semiconductor-based technologies. They also are increasingly important for the study of semiconductor NWs fabricated by both top-down and bottom-up approaches since many of the common characterization methods used with bulk crystals or thin films cannot be applied to NWs in a direct manner. 

Rapid advances in semiconductor techrwlogy, including thin film formation by deposition, interface preparation or microstructuring, demand characterization techniques that provide understanding of the fundamental processes involved, including information on structural order—disorder and spatial inhomogeneity. Raman spectroscopy is used both in process control and quality assessment [34]. Typical examples of semiconductor applications are composition determination, analysis of crystal structure, surface and interface analysis, phase determination, doping, point defects, temperature influence and mechanical stress. 

That there are differences between the properties of materials in bulk and close to interfaces is well known for many materials. The density and composition of liquids is different close to surfaces or interfaces (this can be thought of as the effect of lost molecules ), whereas in solids there may also be qualitative differences in structure the surfaces of semi-conductors may be reconstructed and have a different crystal structure from the bulk. For all these situations, the most important question to be clarified is that of what actually constitutes the interfacial region - how far does one need to be from a surface or interface before essentially bulk properties are recovered In many cases -including the examples of liquid density and semiconductor surface reconstruction just mentioned - this distance is set by atomic or molecular length scales. Hence in a macroscopic sample, even of a polycrystalline solid, the fiaction of molecules that have properties characteristic of the interface rather than the... 

This article describes the current capabilities for predicting materials properties using atomistic computational approaches. The focus is on inorganic materials including metals, semiconductors, and insulators in the form of bulk solids, surfaces, and interfaces. Properties of isolated molecules, liquids. and organic polymers are treated as separate entries. Besides a computational approach based on physical laws, materials properties can also be predicted by empirical rules and statistical correlations between chemical composition, bonding topology, and macroscopic properties. These very useful and quick approaches, which include so-called quantitative structure-property relationship (QSPR) methods, are covered in other entries of this encyclopedia (see Quantitative Structure-Property Relationships (QSPR)). 

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