Structure of semiconductor surfaces

In the following section, we provide a brief review of the structures of the major semiconductor surfaces for which the adsorption and reaction chemistry will be covered in this chapter. This includes the (100) and (111) crystal faces of silicon and germanium. Chapter 1 of this book also provides a brief overview of the structure of the silicon surface. The surface structures of compound semiconductors, including GaAs and InP, can be quite complex and are not covered here. A number of reviews describe the structure of these surfaces much more extensively [5,6,25-29], and the reader is referred to those references for more detail. 

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Because of the minimization of the number of dangling bonds semiconductor surfaces often show large displacements of the surface atoms from their bulk lattice positions. As a consequence these surfaces are also very open and the agreement is more in the range of 7 p factor values of approximately 0.2. Determination of the structure of semiconductor surfaces is reviewed in a recent article by Kahn [2.275]. 

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

The structure of semiconductor surfaces under bulk solution can generally not be extrapolated from adsorption studies in UHV. For instance, Si(lOO) oxidizes in pure water with time and the surface is not simply saturated by Si-H and Si-OH... 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

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. 

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. 

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... 

In the case of a quasi-isolated surface, the bulk of the semiconductor (e.g., impurities inside the crystal) no longer influences its chemisorptive and catalytic properties, the latter depending only on the structure of the surface. This dependence is implicit in Equation (25), according to which the position of the Fermi level on the surface e.+ (and hence the chemisorptive and catalytic properties of the surface) depends on the concentration and nature of the chemisorbed particles and also on the concentration and nature of the structural defects on the surface. 

A real semiconductor surface, in contrast to an ideal plane surface, contains different kinds of imperfections perturbing the strictly periodic structure of the surface. Such a surface is distinguished by a number of peculiarities in its adsorptive and catalytic properties (95). 

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 study of semiconductor surface structure and surface chemistry was actually begun several decades ago together with the advent of surface analytical techniques. Many of the earliest surface science studies examined semiconductors. However, for... 

Several excellent reviews are available concerning both surface structure of semiconductors and surface chemistry of semiconductors, including Refs. [5-23]. Here, a comprehensive review is not attempted and the reader is referred instead to those references. The focus of this chapter is primarily on the surface chemistry of silicon and germanium, as these are the two most heavily studied systems. We strive to provide insight into the chemical reactivity of these two surfaces, and hence... 

A BRIEF REVIEW of the research in semiconductor surface physics is presented. Emphasis is placed on die limits of present theory and the importance of knowing the composition and structure of the surface of interest. The feasibility of new experimental approaches to the study of surfaces such as nuclear magnetic resonance and quadrupole res -onance is discussed. A review of recent developments in an understanding of the energy level diagram of the cleaned germanium surface is reviewed. 

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... 

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