Surface space charge

Fig. 3 -13. (a) A ion levels at the surface and in the interior of ionic compound AB, and (b) concentration profile of lattice defects in a surface space charge layer since the energy scales of occupied and vacant ion levels are opposite to each other, ion vacancies accumulate and interstitial ions deplete in the space charge layer giving excess A ions on the surface. 

The formation of space-charge layers at contacts or at the surface of a semiconductor may lead to the generation of photovoltages. This type of photo-emf results from the separation of electron-hole pairs under the influence of the electric field in the contact or surface space-charge region. 

If, on the other hand, the field forces the mobile holes away from the surface, a space charge region consisting of the ionized acceptor atoms, which are fixed in the lattice, forms over an appreciable distance into the semiconductor. The thickness of the surface space charge region is a function of the strength of the field at the surface and the semiconductor doping profile, as is the difference between the surface potential and the bulk potential of the semiconductor. If the surface potential deviates sufficiently far from the bulk potential, the surface will invert that is, it will contain an excess of mobile electrons. In this case, an -type conductive channel... 

In the discussion of the surface space-charge region, the idea of electrons spilling out or tunneling out of the solid into vacuum was presented. While the phenomenon has been understood within the solid state physics community for many years, it has recently received a much broader exposure in the context of scanning tunneling microscopy (STM). In STM, a very sharp tip is brought within close proximity (a... 

Min L. and Miller R. J. D. (1990), Subpicosecond reflective electro-optic sampling of electron-hole vertical transport in surface-space-charge fields , Appl. Phys. Lett. 56, 524-526. 

Figure 5.36 Model of a solid plate of photocatalyst of thickness 2d irradiated uniformly from both sides (1). The graph also defines the surface potential U, and the width of the near-surface space charge region 6. Reprinted with permission from Emeline et al. (2003). Copyright (2003) American Chemical Society.

Although the intuitive model of disordered interfaces between nanocrystals has provided an explanation of the reported high diffusion coefficients in these samples, it will be seen later that this is unrealistic. The nature of the interfaces depends on the method of sample preparation, and these can be highly ordered, akin to grain boundaries in bulk samples. Thus, other explanations must be sought and, in the case of ionic materials, these have been based on surface space-charge and surface texture approaches. 

The Surface Space Charge at the Solid-Vacuum Interface... 

Perhaps the two most frequently measured electrical properties in surface science are the surface space charge potential Pdipoie the related work function . 

A more realistic model will take full account of the atomic nature of the surface and yield charge densities and electronic potentials similar to those obtained by the jellium model. In this circumstance, however, the charge density on the solid side of the surface exhibits fluctuations that are often called Friedel oscillations and which are due to the screening by the free electrons (Figure 5.1). The amplitude of this oscillation is a sensitive function of the electron density, as are the height and extent of the surface space-charge potential. 

So far we have only considered the properties of the surface space charge at the solid-vacuum interface. Let us now immerse the solid into a liquid. The molecules in the liquid adsorb onto the solid surface and become polarized as they respond to the electrical field at the interface to produce an electrochemical double layer. They may also line up in preferential bonding directions if they possess a permanent dipole... 

The opposite situation from weak interaction of inert gases with the surface space charge is surface ionization, when the adsorbate is ionized by the substrate. This typically occurs in alkali-metal adsorption on transition-metal surfaces. In the more usual situation with chemisorbed molecules, only partial charge transfer occurs to or from the substrate to the molecule. If the negative pole of the molecule points toward the vacuum, the induced electric fields cause an increase in the work function. Table 5.4 lists the work-function changes obtained by the chemisorption of several molecules on rhodium. 

Gas adsorption on insulator or semiconductor surfaces can cause very large changes in the height of the surface space-charge potential and its Debye length. As... 

When two different metal surfaces are brought into contact, the surface space charges that were present at their interfaces with a vacuum will be modified. The electrons from the metal of lower work function will flow into the other metal until an interface potential develops that opposes further electron flow. This is called the contact potential and is related to the work-function difference of the two metals. The contact potential depends not only on the materials that make up the solid-solid interface but also on the temperature. This temperature dependence is used in thermocouple applications, where the reference junction is held at one temperature while the other Junction is in contact with the sample. The temperature difference induces a potential (called the Seebeck effect), because of electron flow from the hot to the cold Junction, that can be calibrated to measure the temperature. Conversely, the application of an external potential between the two Junctions can give rise to a temperature difference (Peltier effect) that can be used for heat removal (refrigeration). 

The discontinuity and change of dimensionality (from three to two dimensions) at solid-vacuum, solid-gas, and solid-liquid interfaces gives rise to electron redistribution. These effects result in surface space charges, surface electronic states, and work functions that are altered by changes of surface stmcture and adsorption. 

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