Metal-semiconductor junction

The forward current at a semiconductor-metal junction is mainly determined by a majority carrier transfer i.e. electrons for n-type, as illustrated in Fig. 1 d. In this majority carrier device the socalled thermionic emission model is applied according to which all electrons reaching the surface are transferred to the metal. In this case we have ... 

Semiconductor - Metal Junctions Besides the semiconductor-liquid interface, electron-hole separation can be attained also when the couple is generated in the space charge layer of a homo/heterojunction or semiconductor-metal junction. The metal can also act as electrocatalyst (e.g., for reduction of 02, H+ or C02). The development of the proper structure, including arrays of multiple junctions in series to enhance photovoltages and efficiently harvest radiation [53] and/ or the inclusion of suitable electrocatalysts, is crucial. 

In addition to the illumination of the catalyst surface, another simple method is used for the alteration of the electron concentration and the occupation of the bond orbitals in the semiconductor surface. This method is a modification of the inverse mixed catalysts introduced by Schwab 89 9 . The electron concentration and distribution upon the bond states is achieved 1. by putting the surface bonds into the potential of a boundary layer of a metal-semiconductor junction and 2. by illumination of the semiconductor-metal junction with ultraviolet light (photovoltaic effect). 

It seems possible that the same approach will suffice for Junctions between semiconductors and insulators, since the latter will be described by the same LCAO parameters. However, the treatment of semiconductor metal junctions is better handled with pscudopotentials, so will be discussed in Chapter 18. 

Figure 3 The spatial dependence of the charge density, the electric field, and the electric potential in the semiconductor at equihbrium for an n-type semiconductor/metal junction, where all of the voltage is dropped in the semiconductor space-charge region. The origin of the x-axis is chosen for convenience as the point closest to the interface where the net charge in the semiconductor equals zero, (a) The distance dependence of the charge density under the depletion approximation, (b) The electric field as a function of distance. Note that the maximum electric field occurs at the semiconductor interface, (c) The distance dependence of the electric potential. The electric potential in the bulk of the semiconductor has been defined as zero. Because the sign of the electric field strength is positive, the electric potential at the interface is more negative than in the bulk...

As given in equation (1), since the depletion approximation ensures that the charge density in the depletion region is independent of distance, the electric field in this region increases linearly as it approaches the semiconductor/metal junction. The maximum electric field in the semiconductor (Emax) is obtained at the position x = W ... 

Thus, the maximum electric field in a semiconductor/metal junction is located at the interface. This is an important feature for constructing numerous devices using semiconductor junctions, because the maximum ability for charge separation by the electric field occurs at the junction. 

In the literatme, the work function of a metal, p (in eV), is often used to estimate the degree of charge transfer at semiconductor/metal junctions. The work function of a metal is defined as the minimum potential experienced by an electron as it is removed from the metal into a vacuum. The work function ip is often nsed in lieu of the electrochemical potential of a metal, because the electrochemical potential of a metal is difficult to determine experimentally, whereas tp is readily accessible from vacuum photoemission data. Additionally, the original model of semiconductor/metal contacts, advanced by Schottky, utilized differences in work functions, as opposed to differences in electrochemical potentials, to describe the electrical properties of semiconductor/metal interfaces. A more positive work function for a metal (or more rigorously, a more positive Fermi level for a metal) would therefore be expected to produce a greater amount of charge transfer for an n-type semiconductor/metal contact. Therefore, use of metals with a range of tp (or fip.m) values should, in principle, allow control over the electrical properties of semiconductor/metal contacts. 

Current-voltage Behavior of a Semiconductor/metal Junction... 

Similarly, when a voltage V is applied across a semiconductor/metal junction, the total voltage drop in the semiconductor depletion region is Vbi -F V, so we obtain an analogous Boltzmann relationship away from eqnihbrinm ... 

Substituting equations (15) and (16) into eqnation (14), we obtain the desired relationship between the cnrrent and the voltage of a semiconductor/metal junction ... 

Figure 7 The current-voltage (I-V) behavior of an n-type semiconductor/metal junction in the dark. The shape of the I V curve is described by the diode equation (equation 17) thus, such a curve is referred to as a diode curve. The difference between curves 1 and 2 is that the equilibrium exchange current, 7o, is greater for curve 2...

Bard et al. have emphasized that there may be exceptions in so far as Fermi level-pinning by surface states may occur similarly as at semiconductor-metal junctions [62]. Such an effect would lead to an unpinning of bands. There are some examples in the literature, such as FeS2 in aqueous solutions [63,64] and Si in CH3OH [65], for which an unpinning of bands has been reported. In some cases, the interpretation is based on investigations of photocurrents, which will be discussed in the next section. 

It should be mentioned here that Eq. (41) had already been derived by Gartner [86] for the photocurrent of a semiconductor-metal junction in reverse bias, assuming P = 0. In the derivation of Reichman [85], however, P was obtained in a manner consistent with the interface boundary conditions. Although the derivation of Reichman is much more general, most scientists only applied the Gartner-equation (see e.g. [87]). Later on, Wilson extended the Gartner model by including recombination via surface states [88]. 

This section is of special interest because at first sight there are certain similarities between semiconductor-metal junctions and semiconductor-liquid interfaces. This will be discussed in more detail in Chapter 7. 

A large number of semiconductor metal junctions have been studied. There are various experimental techniques for measuring barrier heights, such as photoelectric and capacity measurements and current-voltage investigations. The first-mentioned technique seems to be the most accurate. These methods are not described here some of them are discussed in Chapters 4 and 5 (see also refs. [7, 12, 16]). 

Many experimental values for barrier heights at semiconductor-metal junctions have been obtained. Many researchers have also measured the barrier height as a function of the work function of the metal, and have mostly obtained a straight line, as expected from Eq. (2.3). However, in many cases the slope, d(/)b/dbarrier height to different metals increases with the ionicity of the semiconductor [17]. In order to obtain a better characterization of the experimental data, they defined an index of interface behavior, 5, which they introduced into Eq. (2.3) as... 

A system in which only majority carriers (electrons in n-type) carry the current, is frequently called a majority carrier device . On the other hand, if the barrier height at a semiconductor-metal junction reaches values close to the bandgap then, in principle, an electron transfer via the valence band is also possible, as illustrated in Fig. 2.8a. In this case holes are injected under forward bias which diffuse towards the bulk of the semiconductor where they recombine with electrons ( minority carrier device ). It is further assumed that the quasi-Fermi levels are constant across the space charge region i.e. the recombination within the space charge layer is negligible. In addition Boltzmann equilibrium exists so that we have according to Eqs. (1.57) and (1.58)... 

In solid state devices such as p-n junctions, luminescence is created by forward polarization in the dark. In such a minority carrier device, electrons move across the p-n interface into the p-type and holes into the n-type regions, where they recombine with the corresponding majority carriers. As already pointed out in Section 2.3, this kind of luminescence has not been found with semiconductor-metal junctions (Schottky junctions), as nobody has succeeded in producing a minority carrier device because of Fermi level pinning. Since the latter problem usually does not occur with semi-... 

Several other attempts have been made by various authors to avoid anodic corrosion at n-type electrodes and surface recombination at p-type electrodes, by modifying the surface or by depositing a metal film on the electrode in order to catalyse a reaction. It has been frequently overlooked that the latter procedure leads to a semiconductor-metal junction (Schottky junction) which by itself is a photovoltaic cell (see Section 2.2) [14, 27]. In the extreme case, then only the metal is contacting the redox solution. We have then a pure solid state photovoltaic system which is contacting the solution via a metal. Accordingly, catalysis at the semiconductor electrode plays a minor role under these circumstances. 

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