Electronic structure of semiconductor surfaces

We shall only attempt to introduce some of the more important physical concepts involved, since several excellent and comprehensive reviews already exist [65—71]. 

The electron states in an infinite periodic solid are described by Bloch functions, and are therefore non-localized, extending over all of real space. The introduction of a surface imposes a spatial restriction in one direction 

All electron states at a surface, comprising those bulk bands which extend to the surface plus localized states, are described by a local density of states (LDOS) which is given by 

The methods used to calculate surface states need not concern us here in any detail, but it will be instructive to give a brief indication of the two approaches currently employed (self-consistent calculations of the electronic energy and surface potential and realistic tight binding models), since this will provide some insight into semiconductor surface bonds and hence into chemisorption. 

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Lieske, N. P. (1984). The electronic structure of semiconductor surfaces. J. Phys. Chem. Solids 45, 821-870. 

The electronic structure of semiconductor surfaces close to the Fermi level is dominated by the dangbng bonds of its surface atoms. While the states originating from strong surface bonds such as dimers or adatom backbonds are almost completely hidden as broad resonances in the valence band, the dangbng bond orbitals from dimers, adatoms, or rest atoms form bands that are partially locabzed in the energy range forbidden for bulk electrons. 

Figure 7.29 The surface states for Si(lOO) in the 2x1 Feconstmction. Circles indicate experimental data and dashed curves show the calculated surface states. The bands indicated represent a range of energies covered by the surface component of the bulk band structure. Bulk band details are conventionally omitted in such diagrams to emphasize the surface states. Because the surface is reconstructed the surface Brillouin zone is rectangular. The results demonstrate why a maximum in the surface-state density of states occurs at an energy of -0.5 eV. After Surface Science 299/300, Himpsel F.J., Electronic structure of semiconductor surfaces and interfaces. , 525-540 (1994) with permission, copyright Elsevier 1994.

Himpsel F.J., Electronic structure of semiconductor surfaces and interfaces. Surface Science, 1994 299/300 525-540, and references therein. 

STM found one of its earliest applications as a tool for probing the atomic-level structure of semiconductors. In 1983, the 7x7 reconstructed surface of Si(l 11) was observed for the first time [17] in real space all previous observations had been carried out using diffraction methods, the 7x7 structure having, in fact, only been hypothesized. By capitalizing on the spectroscopic capabilities of the technique it was also proven [18] that STM could be used to probe the electronic structure of this surface (figure B1.19.3). 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

Charge transport through an array of semiconductor nanocrystals is strongly affected by the electronic structure of nanocrystal surfaces. It is possible to control the type of conductivity and doping level of quantum dot crystals by adsorbing/desorbing molecular species at the nanocrystal surface. As an... 

The first successful first-principle theoretical studies of the electronic structure of solid surfaces were conducted by Appelbaum and Hamann on Na (1972) and A1 (1973). Within a few years, first-principles calculations for a number of important materials, from nearly free-electron metals to f-band metals and semiconductors, were published, as summarized in the first review article by Appelbaum and Hamann (1976). Extensive reviews of the first-principles calculations for metal surfaces (Inglesfeld, 1982) and semiconductors (Lieske, 1984) are published. A current interest is the reconstruction of surfaces. Because of the refinement of the calculation of total energy of surfaces, tiny differences of the energies of different reconstructions can be assessed accurately. As examples, there are the study of bonding and reconstruction of the W(OOl) surface by Singh and Krakauer (1988), and the study of the surface reconstruction of Ag(llO) by Fu and Ho (1989). 

The structure and composition of a nanocrystalline surface may have a particular importance in terms of chemical and physical properties because of their small size. For instance, nanocrystal growth and manipulation relies heavily on surface chemistry [261]. The thermodynamic phase diagrams of nanocrystals are strongly modified from those of the bulk materials by the surface energies [262]. Moreover, the electronic structure of semiconductor nanocrystals is influenced by the surface states that He within the bandgap but are thought to be affected by the surface reconstruction process [263]. Thus, a picture of the physical properties of nanocrystals is complete only when the structure of the surface is determined. 

The first atomic resolution for metal surfaces came later because of the different surface electronic structure of semiconductors and metals. In the case of semiconductor, the energies... 

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. 

Semiconductor materials are used to manufacture discrete devices and integrated circuits for the electronics industry. Integrated circuits and discrete devices are made of metal-semiconductor, semicondutor-semiconductor, and metal-insulator-semiconductor junctions and layered thin film structures. In general, the electrical properties of a device (IC or discrete) are determined by one or more junctions. The electrical properties of the junction are dependent on the microchemistry at and near the interface. Consequently, it is very important for the device designer/physicist to understand the electronic properties which occur both at semiconductor surfaces and at semiconductor interfaces. The study of the electronic properties of semiconductor surfaces has been accomplished by either cleaving a solid sample in ultrahigh vacuum or... 

In this section, the application of APS to the study of surface phenomena will be discussed. The section is divided into three parts. In the first part, the elucidation of electronic structure of the surfaces of semiconductors and metals by APS is described with suitable examples. The second part deals with the phenomenon of adsorption of gases on metallic surfaces leading to the formation of compounds. The third and final part examines the determination of local structure of semiconductor surfaces from the fine structure observed on the high energy side of an appearance potential edge. 

After band structures of metal, insulator, and semiconductors are described and historical back-grotmd of semiconductor electrochemistry is presented, electronic structure of semiconductor/ electrolyte solution interface is discussed in relation to the unique electrochemical behavior of semiconductor electrode. Finally, effect of illumination as well as the surface modification on the electrochemical behavior of semiconductor electrode are described. Fundamental knowledge of semiconductor electrode presented here should be very important for the future development of photoelectrochemical and photocatalytic energy... 

Control and observation of electrochemical and photoelectrodiemical reactions at semiconductor electrodes are very important in establishing the electrochemical/ photoelectrochemical etching processes and stable photoelectrochemical cells (1,2). To understand the mechanism of the electrochemical and photoelectrochemical reactions, in situ information of morphological and electronic structures of semiconductor electrode surfaces with atomic resolution is essential. Although techniques such as electron microscopy and optical microscopy have been applied to examine the morphology of the surface of solid substrates, the former can be used only for ex situ examination and the latter has poor resolution (3,4),... 

To understand how a given surface (e.g., a metal, semiconductor, or insulator) interacts with an adsorbate, we first need to understand the electronic structure of the surface. We also need to understand how interactions between the different constituents in the surface affect its reactivity. 

In this contribution it is shown that local density functional (LDF) theory accurately predicts structural and electronic properties of metallic systems (such as W and its (001) surface) and covalently bonded systems (such as graphite and the ethylene and fluorine molecules). Furthermore, electron density related quantities such as the spin density compare excellently with experiment as illustrated for the di-phenyl-picryl-hydrazyl (DPPH) radical. Finally, the capabilities of this approach are demonstrated for the bonding of Cu and Ag on a Si(lll) surface as related to their catalytic activities. Thus, LDF theory provides a unified approach to the electronic structures of metals, covalendy bonded molecules, as well as semiconductor surfaces. 

It is well known that the flotation of sulphides is an electrochemical process, and the adsorption of collectors on the surface of mineral results from the electrons transfer between the mineral surface and the oxidation-reduction composition in the pulp. According to the electrochemical principles and the semiconductor energy band theories, we know that this kind of electron transfer process is decided by electronic structure of the mineral surface and oxidation-reduction activity of the reagent. In this chapter, the flotation mechanism and electron transferring mechanism between a mineral and a reagent will be discussed in the light of the quantum chemistry calculation and the density fimction theory (DFT) as tools. 

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