Surface potential semiconductor interfaces

To obtain a relationship between the voltage VK applied to the metal and the surface potential Vv, we assume a continuity of the electric field at the insulator-semiconductor interface that implies... 

Surface recombination processes of charge carriers are mechanisms that cannot easily be separated from real semiconductor interfaces. Only a few semiconductor surfaces can be passivated to such an extent as to permit suppression of surface recombination (e.g., Si with optimized oxide or nitride layers). A pronounced dip is typically seen between the potential-dependent PMC curve in the accumulation region and the photocurrent potential curve (e.g., Fig. 29). This dip may be partially caused by a surface... 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

Any charge change occurring only between the reference electrode and the semiconductor is a candidate for a change of Ids. In particular one of the most important points is the surface potential at the oxide-solution interface (surface potentials between the CIM and the solution and the potential between the SiC>2 and the CIM, in the presence of a given CIM. The ISFET operation may be represented by the following changes-flow which may be considered as superimposed on the quiescent point determined by the reference electrode potential ... 

Fig. 96. Schematic illustration of a colloidal semiconductor. Band-gap excitation promotes electrons from the valence band (VB) to the conduction band (CB). In the absence of electron donors and/or acceptors of appropriate potential at the semiconductor surface or close to it, most of the charge-separated, conduction-band electrons (e CB) and valence-band holes (h+VB) non-pro-ductively recombine. Notice the band bending at the semiconductor interface [500]...

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

Figure 4.2(d) shows that an energy barrier forms at the semiconductor/redox electrolyte interface, similar to the Schottky barrier at a metal/semiconductor interface. The most important quantity is the barrier height (q ) or the flat band potential U, which essentially determines the surface band positions of the semiconductor with respect to the energy levels of solution species. The q B is given for an n-type semiconductor by... 

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

The Effect of Surface States on the Distribution of Potential in the Semiconductor Interface... 

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 ... 

Cao et al. reported an alternative Fermi-level pinning model to rationalize the potential distribution at negative applied potentials [147]. They suggested that the reduction of Ti surface states to TF , observed by low-temperature EPR spectroscopy, pins the Fermi level and, as the potential is raised further, the potential drops across the solution-semiconductor interface. 

Effect of Radius of Curvature. The bottom of all pores is curved and the curvature of a semiconductor surface affects the field at the surface. For an interface with a spherical shape as illustrated in Fig. 8.64, the potential and field in the semiconductor can be calculated by solving Poisson s equation ... 

The kinetic analysis [103] of electron transfer in colloidal semiconductor systems is often complex. Apart from the energetics of the conduction band of the semiconductor and the redox potential of the acceptor, factors such as the surface charges of the colloids, adsorption of the substrates, participation of surface states, and competition with charge recombination influence the rate of charge transfer at the semiconductor interface [102], This fact is evident from the widely differing rates of experimentally observed charge transfer rates, with time scales ranging from picoseconds to milliseconds for different experimental conditions and various semiconductor systems. 

It is remarkable that the surface potential fall toward its bulk value is a similar exponential function, (4.148) or (4.152), in the semiclassical Thomas-Fermi theory of electronic screening in the Debye-Huckel/Gouy-Chapman theory of ionic screening and at semiconductor interfaces. Here we consider the following issue When two such phases come into contact as in Fig. 4.9, and a potential bias is set between their interiors, how is the potential drop distributed at the interface ... 

Fig. 2.1 Schematic presentation of different surface potentials at the solid-vacuum interface (details in the text), a) Metal b) n-type c) p-type semiconductor. (After ref. [1])...

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