Surfaces of Elemental Semiconductors

The potential distribution at the surface of the semiconductor is such that the bulk of the potential change is accommodated within the depletion layer. It follows, as discussed in Sect. 4, that ns will be a strong function of the applied potential. However, the corollary of this is that the matrix element V and the thermal distribution parameters ox(Ec) and Qrei(Ec) will be much weaker functions of potential. Although, therefore, we would expect to find an exponential or Tafel-like variation of current with potential for a faradaic reaction on a semiconductor, the underlying situation is quite different from that of a metal. In the latter case, the exponential behaviour arises from the nature of the thermal distribution function Q and the concentration of carriers at the surface of the metal varies little with potential. To see this more clearly, we may expand eqn. (179) assuming that the reverse process of electron injection into the CB can be neglected eqn. (179) then reduces to... 

An attempt was made in this paper to sketch the behavior of elemental semiconductors (with the diamond-type structure) and of the IH-V compounds (with the zinc blende strut ture) in aqueous solutions. These covalent materials, in contrast to metals, exhibit properties which sharply reflect their crystalline structure. Although they have already contributed heavily to the understanding of surfaces in general, semiconductors with their extremely high purity, crystalline perfection, and well-defined surfaces are the most promising of materials for surface studies in liquid and in gaseous ambients. 

This section will begin with a discussion of the fundamental concepts of the electronic and crystallographic structure of semiconductor surfaces, followed by a description of the methods used to prepare surfaces in as ideal a state as possible experimentally. The emphasis will be on Si and GaAs as typical examples of elemental and compound semiconductor, respectively, and with which the great majority of published work has been carried out. We will conclude with some examples of the determination, experimentally and theoretically, of the electronic and crystallographic structure of specific surfaces of elemental and compound semiconductors. 

The surfaces of metal oxides and their H2 chemisorption characteristics have been far less studied than the surfaces of elemental metals and semiconductors [113,133]. Cation surface states are formed on ideal oxide surfaces at about 2 eV below the bottom ofthe conduction band. The charge of the surface ions is found to be reduced compared with that of the bulk ions and this leads to an enhanced co valency at the surface. The reduction amounts to less than 10 % for oxides of simple metals such as MgO and to 20-30% for transition metal oxides. Cluster and slab calculations reveal that special surface state bands with metallic character can be formed on polar surfaces by charge compensation effects. To what extent the metallic band accounts for special catalytic activity is not yet known [114]. 

When starting this project, the first volume was platmed to describe Bare surfaces and Methods , that is, all the physical properties of clean surfaces of elemental and composite sohds as well as the most relevant analytical methods. It soon turned out that an adequate treatment of aU these subjects was far beyond any reasonable size of a single volume, and the material now easily fills the first three of the eight volumes as they stand now Volume 1 Concepts and Methods, Volume 2 Properties of Elemental Surfaces, Volume 3 Properties of Composite Surfaces Alloys, Compounds, Semiconductors, Volume 4 Solid/Solid Interfaces and Thin Films, Volumes 5 and 6 SoHd/Gas Interfaces, Volume 7 Solid/Liquid and Biological Interfaces, and Volume 8 Applications of Surface Science. 

We have noted in the case of elemental semiconductors that the saturation of the dangling bonds at the surface within the framework of a (1x1) unit ceU is to a large extent facilitated by the existence of a (nontrivial) atomic basis in this unit ceU (Si bilayers, basis of the Te crystal structure). In the case of a compound semiconductor, such a basis exists by definition, so that one may expect surface relaxations in a (1x1) periodicity. Nevertheless, since the preservation of bond lengths is an equally important factor in the energy minimization, in most cases reconstructed surfaces with superstmctures are observed, as we see in Section 4.4. 

Another example of epitaxy is tin growdi on the (100) surfaces of InSb or CdTe a = 6.49 A) [14]. At room temperature, elemental tin is metallic and adopts a bet crystal structure ( white tin ) with a lattice constant of 5.83 A. However, upon deposition on either of the two above-mentioned surfaces, tin is transfonned into the diamond structure ( grey tin ) with a = 6.49 A and essentially no misfit at the interface. Furtliennore, since grey tin is a semiconductor, then a novel heterojunction material can be fabricated. It is evident that epitaxial growth can be exploited to synthesize materials with novel physical and chemical properties. 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

There has been an increasing number of studies of the UPD of main group elements, including S, Se, Te, I, Br, Cl, and As, on metal substrates, whereas studies of UPD processes on the surface of semiconductors and semimetal substrates are significantly less. Presently, most interesting in this connection is the combined use of photoexcitation of a semiconductor substrate and/or an immobilized precursor, and electrodeposition, as will be discussed in a subsequent paragraph. 

In Chapter 3 we briefly outline the methods of manufacturing of sensitive elements of semiconductor sensors in order to proceed with the studies of several physical and chemical processes in gases, liquids as well as on the surface of solids. Here we show the peculiarity of preparation of these elements depending on objective pursued and operation conditions. We outline the detection methods (kinetic and stationary), their peculiarities and advantages of their application in various physical and chemical systems. 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

Let us start with a definition. Semiconductor chemical sensor is an electronic device designed to monitor the content of particles of a certain gas in surrounding medium. The operational principle of this device is based on transformation of the value of adsorption directly into electrical signal. This signal corresponds to amount of particles adsorbed from surrounding medium or deposited on the surface of operational element of the sensor due to heterogeneous diemical reaction. 

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