Metal semiconductor ideal contact

When a contact is made between a semiconductor and a metal, the Fermi levels in the two materials must coincide at thermal equilibrium. We will consider here two limiting cases as shown in Fig. 2.3. In the first, we have a somewhat ideal situation (Fig. 2.3a). The semiconductor is assumed to be free from surface states so that the energy bands are flat as far as the surface before contact. In addition, any dipole layer on the metal and on the semiconductor surface is neglected. In the example shown in Fig. 2.3a, the Fermi level of the semiconductor occurs at a higher energy than that of the metal. Accordingly, some electrons are transferred from the semiconductor to the metal after close contact is made between them. This leads to a positive space charge layer after... 

It should be mentioned that for p-type materials, usually, a negative space charge is formed because the work function of the semiconductor is below that of the metal. Again assuming an ideal contact, the energy barrier is given by... 

The type (1) interface is an ideal Schottky barrier contact in which the barrier height varies directly with the metal work function in accordance with Equation [3.18], The type (2) interface approximates to a Bardeen barrier, provided that the surface states are assumed to be spaced inside the semiconductor so as to allow a potential drop across this region. In the clean contacts of this type, one would expect the barrier height to show a weak dependence on 0 ,.The type (3) interface represents a case of strong chemical bonding between the metal and the semiconductor and, hence, we would expect the barrier height to depend on some quantity related to chemical or metallurgical reactions at the interface. The type (4) contact is the one most frequently encountered in actual metal-semiconductor devices. 

The principal object of electrochemical interest is given by another type of electrified interface, contacts of an electronic (liquid or solid metal, semiconductor) and an ionic (liquid solution, SEs, membranes, etc) conductor. For numerous contacts of this kind, one can ensure such ionic composition of the latter that there is practically no dc current across the interface within a certain interval of the externally apphed potential. Within this potential interval the system is close to the model of an ideally polarizable interface, the change of the potential is accompanied by the relaxation current across the external circuit and the bulk media that vanishes after a certain period. For sufficiently small potential changes, d , the ratio of the integrated relaxation current, dQ, to dE is independent of the amplitude and it determines the principal electrochemical characteristics of the interface, its differential capacitance per unit surface area, C ... 

There are various driving forces that result in the formation of space charge across surfaces or interfaces. Perhaps the simplest example is the case of a Schottky diode that is formed from a metal-semiconductor heterojunction. For an ideal metal-semiconductor contact, the height of the potential barrier is given by... 

So far we only considered ideal heterojunctions. Often times in real heterojunction, the lattice constants of the contacting materials are never precisely the same. The lattice mismatch between the crystals gives rise to a network of so-called misfit dislocations, which can extend several nanometers into the bulk lattice. Although the details of such defects are beyond the scope of this book, the interface states introduced by dislocations and defects, similar to the surfaces states at the metal-semiconductor junction, induce additional band bending at the surface. 

Semiconductor surfaces can be modified by electrochemical deposition of metals. This is of great technological importance because metal-semiconductor contacts such as ohmic contacts and Mott-Schottky diodes are required for many semiconductor devices. For instance, an ideal Mott—Schottky junction was formed by electrochemical deposition of Cu on n-GaAs as discussed in Section 2.2. The electrochemical method is very attractive because the adjustment of the electrode potential offers a unique tool of controlling and structure of the interface. 

When a metal and a semiconductor (ideally with no surface states) are brought in contact and equilibrium is maintained, their Fermi levels will align. If the Fermi levels of the metal and the semiconductor were the same before contact, then there would be no change in the band structure after contact. Usually, matching the work functions is nearly impossible in part because the Fermi level in the semiconductor, and thus the work function, depends on the carrier concentration. 

Etch Mechanisms. Most wet etches for the compound semiconductors employ oxidation of the semiconductor followed by dissolution of the oxide. For this reason, many wet etches contain the oxidant hydrogen peroxide, although nitric acid can also be used. One advantage of wet etching over dry is the absence of subsurface damage that is common with dry etching. Metal contacts placed on wet-etched surfaces exhibit more ideal characteristics than dry-etched surfaces. 

Figure 18. C(V) curves for a metal-Ba stearate semiconductor structure (multilayer thickness, 1000 A). Capacitance levels are indicated the max/min ratio depends on the parameters of the structure, and the absolute values of the capacitance depend of course on the area of the metal contact (a mercury probe). Different areas of the same sample were used to obtain the curves in the top and bottom figures.------------- an ideal, theoretical C(V) curve.

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