Germanium surface structure

In its crystalline state, germanium, similar to silicon, is a covalent solid that crystallizes into a diamond cubic lattice structure. Like for Si, both the (100) and (111) 

Although Si(100) and Ge(100) undergo similar dimer reconstructions, the Ge(l 11) surface reconstructions differ from those of Si(lll). As described above, Si(lll) reconstructs into a (7 x 7) structure that contains 49 surface atoms in the new unit cell. Ge(lll) is found in various reconstructed forms depending on surface preparation, but the most common reconstruction under vacuum is Ge(lll)-c(2 x 8) [51-53]. This structure involves charge transfer from adatoms to restatoms [5]. On the other hand, most of the passivation and functionalization studies reviewed here lead to the Ge(lll)-1 x 1 surface structure. This structure, in which the surface Ge atoms retain their bulk positions, can be achieved by hydrogen, chlorine, or alkyl termination of the surface (discussed below). The structure is analogous to that for H-terminated Si(lll). 

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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... 

We begin by reviewing the surface structure of the most commonly studied crystal faces of these materials. We will then briefly discuss the oxidation of both silicon and germanium, then review the passivation of silicon and germanium surfaces. In addition, we explore the reactivity of Si, Ge, and to a lesser extent GaAs and InP, toward various organic molecules that can be used to functionalize the surface and impart new properties to the semiconductor. 

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. 

Figure 5.3. Models of the Si(100) and Ge(100) surface (Left) (2 x 1) dimer reconstruction involving symmetric dimers (Middle) c(4 x 2) dimer reconstruction with buckled dimers These two structures are observed for silicon at room temperature and lower temperature, respectively. For germanium, the structure at (Right), the p(2 x 2) dimer reconstruction with buckled dimers, is also observed at lower temperatures. In the top view model, the open circles represent the top layer atoms, with the larger and smaller circles designating the up and down atoms of the dimer, respectively. The filled circles represent the next layer of atoms.

The adsorption of albumin from aqueous solution onto copper and nickel films and the adsorption of B-lactoglobulin, gum arabic, and alginic acid onto germanium were studied. Thin metallic films (3-4 nm) were deposited onto germanium internal reflection elements by physical vapor deposition. Transmission electron microscopy studies indicated that the deposits were full density. Substrate temperature strongly Influenced the surface structure of the metal deposits. Protein and/or polysaccharide were adsorbed onto the solid substrates from flowing... 

The spectrum of energy levels for electrons at the germanium surface usually differs from that in the interior of the crystal. This situation arises because of the unsaturated energy levels of the surface atoms, misfits in the structures of germanium and the oxide, and other impurities like chemisorbed gases. All these factors, either independently or combined together, result in the formation of the... 

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 effect of gases on the oxide-covered germanium surface has been extensively studied over the past few years. The oxide-covered surface is usually produced by means of an aqueous etch. It is not stable in air, but changes rapidly during the first day and somewhat more slowly thereafter. Its structure and chemical composition are unknown and its thickness may vary from a few angstroms to as many as a hundred. 

In the present paper the reaction of some elemental and compound semiconductors with aqueous solutions will be considered in relation to their surface structure. The influence of their semiconducting nature will be discussed where it is of significance. This paper is based primarily on work on germanium and HI-V compounds performed at die Lincoln Laboratory of M. I. T. over the last few years. 

The kinetics of the reaction of germanium with dissolved oxygen can be related to the surface structure. Thus it was found (2) that the dissolution rates for the three principal low-index planes vary in the order of the densities of dangling bonds as shown in Fig. 4 and Table I. It was speculated that the intermediates Ge-OH and Ge-C H are formed on the surface during dissolution. Orientation effects have also been observed in fast etching media (4), although high rates usually mask relatively small differences due to orientation. 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

Gerischer H., Mauerer A. and Mindt W. (1966), Proof of an intermediate radical structure of the germanium surface at the transition between hydride and hydroxide coverage . Surf. Sci. 4, 431 39. 

In the case of semiconductors, it was first shown in this laboratory that the arrangements of the atoms in the surface monolayers of (100) and (111) germanium and silicon are not the same as those for these planes in the bulk (25). The altered arrangements were revealed by the presence of fractional order beams for the surface gratings in certain azimuths. This was later found to be the case for all crystals tested which have a diamond-type lattice, including semiconducting diamond and several of the intermetallic compounds. The surface structure of silicon was observed to be much more complex than that of germanium. In some azimuths, several fractional orders less than one-half were observed. 

The size of each tube in Fig. 3 can also be examined by means of a dye. If we put a dye in tank R, it will become colored rapidly. So will tank G. But tanks L, E, and F will be colored after some delay, as tube (3) is very narrow. If the dye is added to tank G, a similar phenomenon will take place, but if it is in F, tank L as well as E will be colored rapidly, while G and R will be colored at a much slower rate. In this way, by measuring at what rate each tank gets colored, the size of each tube may be estimated. Isotope tracers have been used to study the kinetic structure of the decomposition of germane on a germanium surface and of formic acid on a gold catalyst as will be described later. 

The atomic relaxations visualized here are small repositionings of surface atoms. This is shown by the ease with which such relaxation transformations can take place. For example, Palmberg 346) has made the observation that the Ge(lll)-(2 x 8) and Ge(lll)-(1 x 2) clean surface structures relax to (1 X 1) when sodium is deposited with the germanium crystal held at the very low temperature of — 195°C. 

Thus, our very preliminary results would indicate that adsorption on a germanium surface produces a protein structure similar to the structure in solution, i.e. it is the solution structure that first adsorbs. However, this initial adsorbed structure may not be the most stable structure and thus, rearrangement to a more stable structure may occur for some proteins. It is important to note that these results apply to germanium surfaces. It has already been noted (7) that the structure of adsorbed albumin and adsorbed IgG changes with the nature of the adsorption surface. 

The reactions of germanium with oxygen and with nitrogen are reviewed. Particular emphasis has been placed on the relationship between the structure and electronic properties of the oxide and nitride films formed on single crystal germanium surfaces under different processing conditions. A summary of the electronic properties reported is presented and some conclusions are offered based on the recent literature. 

The surface structure of melt-mixed blends of bisphenol A polycarbonate and PETP was investigated by FTIR attenuated total reflectance spectroscopy. Based on the peak intensity of the aromatic carbonate band for polycarbonate and the aliphatic ester band for PETP by using germanium and KRS-5 ATR crystals, the enrichment of the PETP component in the surface layer of the polycarbonate/PETP blend films was observed. 27 refs. 

Using a different Zintl anion as building block, we and others prepared mesoporous germanium with well-ordered pore structure and high internal surface area [43,44],... 

Interface states played a key role in the development of transistors. The initial experiments at Bell Laboratories were on metal/insulator/semiconductor (MIS) structures in which the intent was to modulate the conductance of a germanium layer by applying a voltage to the metal plate. However, only 10% of the induced charges were effective in charging the conductance (3). It was proposed (2) that the ineffective induced charges were trapped in surface states. Subsequent experiments on surface states led to the discovery of the point-contact transistor in 1948 (4). 

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