Semiconductors contact potential

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

Semiconductor structures that develop space charge layers and contact potentials, like films of proper thickness, films with applied external bias, homo- and hetero-(nano)junctions, permit significant suppression of bulk recombination processes and, potentially, allow high quantum yields. Spatial separation of electron and holes also allows the separation of cathodic and anodic processes in a photoelec-trochemical cell (eventually at the micro and nano level), minimizing surface re-... 

Space charge layers and contact potential for efficient charge carrier separation can be achieved with proper semiconductor structure in several ways. When possible semiconductor structures are considered, the charge separation can be attained in an active mode, i.e., by the use of a potential bias in a photoelectrochem-ical cell, or in a passive mode, i.e., with the use of proper contact between different phases. 

Solid-solid contact (inc. solid breakup) Metal to metal Metal to semiconductor Semiconductor to semiconductor Volta potential (equalization of Fermi levels) Electrolytic potential (where adsorbed water films may be present)... 

The first term on the right-hand side of (6.61) is the contact potential between the semiconductor and the metal of the reference electrode (1) ... 

Fig. 2 Hel photoelectron spectra of n-type a) and p-type (b) (0001) for increasing coverages by H20. energy correlation at the semiconductor/ adsorbate interface is shown in Fig. 1. It corresponds to the diagram of the semiconductor/electrolyte interface as is suggested by a comparison of contact potential differences and photopotentials obtained for the different halogens in UHV and in electrochemical junctions (organic electrolytes) (compare Table 1).

Coming closer to a case which may be more relevant to the situation with insulators, let us now consider contact between a metal and a semiconductor. The energy-level scheme for the simple model usually adopted to explain this is shown in Fig. 7.14. In the particular case shown, the Fermi level of the -type semiconductor is higher than that of the metal, so that when contact is made, electrons flow to the metal until the equilibrium contact potential difference is established ... 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

The values of Vm and are key experimental quantities that are used to characterize the physical properties of semiconductor/metal interfaces. If Vbi or b can be determined, then W, Q, E(x), and most of the other important thermodynamic quantities that are relevant to the electrical properties of the semiconductor contact can be readily calculated using the simple equations that have been presented above. Methods to determine these important parameters can be found in the literature. However, it would be useful at this point in the discussion to consider what values of and Vbi are expected theoretically for a given semiconductor/metal interface. By definition, = (/ip.m - at the electrode surface (Figure 4b). Thus, in principle, the barrier height can be predicted if the energies of the semiconductor band edges and the electrochemical potential of the metal can be determined with respect to a common reference energy. 

The condition of thermodynamic equilibrium in a metal-semiconductor contact states that the electrochemical potential should be uniform throughout the system. If, before contact, the metal and the semiconductor have different electrochemical potential, then upon contact, charge will flow to the material with the smaller potential, until the potentials are equalized. When the two materials have no net charge, Jlf = Om and Ji = where M and S denote the metal and the semiconductor, respectively. If Om > the electron flux will be toward the metal. At equilibrium, the common electrochemical potential will be... 

Chapter 17 of this text focuses on the interface between molecular systems and metals or semiconductors and in particular on electron exchange processes at such interfaces. Electron injection or removal processes into/from metals and semiconductors underline many other important phenomena such as contact potentials (the... 

This cannot be measured because there are other metal-metal and metal-semiconductor contacts in the measuring circuit, including those of the voltmeter, and the sum of all contact potentials is zero. As can easily be deduced from Fig. 2.3, the barrier height e5 at the metal-semiconductor contact is given by... 

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