Homogenous semiconductor interfaces

This is about interfaces formed by two materials that ate initially identical and only differ through the nature and/or the concentration of introduced doping species. The most typical example is that of a jrmction formed by an n-type aird a p-type semiconductor material (see Figitre 4.11). 

During this study we will assume that all defects are ionized at woricing temperature. There is simply a charge transfer through the junction, involving electrons and electron holes. 

Influenced by the concentration gradient, the elections resulting from the presence of electron donors in the n-type material will migrate towards the p-type material. Conversely, the electron holes resulting from the presence of electron acceptors in the p-type material will move to the n-type side. At the junction, electrons and electron holes will react through the annihilation reaction  

This relation leads to n = p = 0 in the exchange region, if n and p are, respectively, the concentrations of electrons and electron holes. 

At equilibrium, the chemical forces created by the concentration gradients equal the electric forces. The physical and mathematical processing of this system is identical to that suggested for chemisorption in section 4.3.4.1. 

---

If a homogeneous semiconductor structure is in contact with the electrolyte, then the band bending at the liquid interface would bend in the same direction everywhere. This would create a potential well for electrons in n-type semiconductors and for holes in p-type semiconductors. The existence of these wells would inhibit majority carrier charge transfer from the semiconductor to the electrolyte depending upon the width and height of the potential well. 

Metal-Semiconductor or Metal—CP Contacts Up to now, metal-CP interfaces have in most cases been discussed within a very simplified model an abrupt separation, with no localized electronic states (interface states), between two media which are homogeneous right to the interface. The vast amount of work on semiconductors [236,237] has shown that this is not usually true. Nevertheless, some basic properties are already evidenced in that model, which we shall use to begin with, but with the caveat that one should not expect from it a detailed understanding this is considered further in Section V.C. 

Since photoexcitation induces significant enhancement of the reactivity of electron transfer, photochemical reactions via photoinduced electron transfer have been explored in homogeneous systems [43 52], On the other hand, the term photocatalysis has usually been used in heterogeneous systems involving photoinduced electron transfer across the gas-solid or liquid-solid interface [53-60], Photocatalysis has been extensively studied using a semiconductor particle as a photocatalyst [53-60], Photocatalysis is initiated by the absorption of a band gap photon... 

Although the lifetime of the reactive electron-hole pair is not known, the reasonable estimate of 10 -10 s leads to an electron transfer rate constant between 10 and 10 M s . In general, electron transfer reactions at the semiconductor-liquid interface are very fast [see Ref [33] and N. Serpone, E. Pellizetti (Eds.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986, p. 51]. 

Figure 9.10 Equipotential contours within a semiconductor as a function of a low-barrier height nanopattern scale. Calculated equilibrium potential contours for non-homogenous barrier height interfaces, based on the pinched-off model and the assumption of simple superposition of the electric potential perturbations. The model surfaces consist of metal disks of radius Ro, spaced 5Ro apart, where (a) Ro = 180...

If the whole semiconductor/electrolyte interface is illuminated uniformly, both conjugate reactions proceed at the same rate over the same areas on the interface. The stationary potential of an illuminated semiconductor is thus a mixed potential. If the surface of a semiconductor, homogeneous in its composition and properties, is illuminated nonuniformly, in the illuminated and nonillumi-nated areas conditions will not be identical for electrochemical reactions. Here the conjugate reactions appear to be spatially separated, so that we can speak about local anodes and cathodes. This situation is deliberately created, for example, for selective light-sensitive etching of semiconductors (see Section V.2). 

Interface control at the molecular level is important for the development of molecular electronic junctions [1,2] with potential impact in the fields of molecular memories, photovoltaic conversion and biochemical sensing. In order to achieve homogeneous and reliable long term electrical properties, the design of robust interfaces requires the selection of molecules bearing a chemical functionality reactive towards a solid surface, usually a metal (e.g., gold) or a semiconductor (e.g., hydrogen-passivated crystalline silicon). However, these materials may not be ideal substrates because thiol chemistry produces weak Au-S bonds (167 kJ/mol), while the unreacted Si-H interface bonds at Si(lll) H surfaces are prone to partial oxidation [3],... 

This section is divided into two parts, the first dealing with photogalvanic cells, in which a homogeneous photochemical reaction in solution forms one or more products which diffuse to and react at the electrodes. The second part deals with cells containing semiconductor-electrolyte interfaces. A review23 which covers the literature on these topics up to mid-1974 in a more or less comprehensive way is available. 

---