Surface states in semiconductor

Formation of Electronic Surface States in Semiconductor Band Gap as a Result of Deposition of Metal Particles on Semiconductor Surface... 

The importance of surface states in semiconductor photoelectrochemical cells has become increasingly apparent as materials with narrow band-gaps have been investigated. Bard et have discussed Fermi-level pinning by surface... 

A.J. Bard, F.-R.F. Fan, A.S. Gioda, G. Nagasubramanian, H.S. White, On the role of surface states in semiconductor electrode photoelectrochemical cells. Faraday Discuss. Chem. Soc. 

The bulk is characterized by a band gap and discrete electronic levels due to point defects, some of them being mobile the surface, by specific,intrinsic and extrinsic, electronic levels called surface states. In semiconductor physics, numerous experimental results have conclusively shown that the surface states frequently have a dominant influence in locally fixing the position of the Fermi level . With solid ionic conductors, at the present time, this is still questionable. However, two extreme possible situations can be reasonably delineated. 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

We consider the existing models of adsorption response of electrophysical characteristics of ideal monocrystalline adsorbent, monocrystal with inhomogeneous surface as well as polycrystal adsorbent characterized by an a priori barrier disorder. The role of rechar g of biographic surface states in the process of adsorption charging of the surface of semiconductor is analyzed. 

Degeneracy can be introduced not only by heavy doping, but also by high density of surface states in a semiconductor electrode (pinning of the Fermi level by surface states) or by polarizing a semiconductor electrode to extreme potentials, when the bands are bent into the Fermi level region. 

Even for ideal (surface-state free) semiconductors the behavior with respect to Ey vs. Ere(jox can be confusing. For the ideal p- or n-type semiconductor sufficiently negative or positive Ere(jox, respectively, will result in carrier inversion at the surface of the semiconductor, Schemes II and 111.(14 19)... 

Manipulating surface states of semiconductors for energy conversion applications is one problem area common to electronic devices as well. The problem of Fermi level pinning by surface states with GaAs, for example, raises difficulties in the development of field effect transistors that depend on the... 

The dangling and the surface ion-induced states are intrinsic surface states that are characteristic of individual semiconductors. In addition, there are extrinsic surface states produced by adsorbed particles and siuface films that depend on the enviromnent in which the siuface is exposed. In general, adsorbed particles in the covalently bonded state on the semiconductor surface introduce the danglinglike surface states and those in the ionically bonded state introduce the adsorption ion-induced surface states. In electrochemistiy, the adsorption-induced surface states are important. 

Simple calculation gives a comparable distribution of the electrode potential in the two layers, (64< >h/64( sc) = 1 at the surface state density of about 10cm" that is about one percent of the smface atoms of semiconductors. Figure 5—40 shows the distribution of the electrode potential in the two layers as a function of the surface state density. At a surface state density greater than one percent of the surface atom density, almost all the change of electrode potential occurs in the compact layer, (6A /5d )>l, in the same way as occurs with metal electrodes. Such a state of the semiconductor electrode is called the quasi-metallic state or quasi-metallization of the interface of semiconductor electrodes, which is described in Sec. 5.9 as Fermi level pinning at the surface state of semiconductor electrodes. 

Fig. 5-57. Surface states in the band gap of semiconductors Z> = surface state density.

Fig. 6.130. The analogy between the role of surface states in the semiconductor and contact-adsorbed ions in the solution.

The situation described so far with semiconductor electrode kinetics is the simplest case The semiconductor has no states for electrons or holes at the surface. More frequently met—particularly for semiconductors that evolve H2 or 02, are semiconductors with surface states. In such a case, the potential-distance relation inside the semiconductor becomes flatter, and the behavior of the semiconductor becomes more like that of a metal. Thus (see Fig. 7.27), for the high surface-state case for a /5-type semiconductor anode, there reappears a substantial p.d. in the solution the p.d. inside the semiconductor is reduced toward a small value. 

What are surface states In an ideal semiconductor, the electron distribution in the conduction band follows Fermi s distribution law and the assumptions behind the deduction is that the conduction electrons are mobile ( free ). In this model, electrons may come to the surface and overlap or underlap a bit, but there are no traps to spoil the sample distribution. 

The existence of surface states in general can lead to a variety of nonidealities in the output parameters associated with semiconductor-electrolyte junctions. Figure 28.6 provides the current-potential response for a photo-electrochemical cell containing a cadmium ferrocyanide-modified n-CdS electrode in an aqueous ferri/ferrocyanide electrolyte. Although open-circuit and... 

Ions that are not chemisorbed do not affect the performance of semiconductor liquid junction solar cells.32 Weakly chemisorbed ions produce inadequate splitting of surface states between the edges of the conduction and valence band and increase rather than decrease the density of the surface states in the band gap and thus the recombination velocity. Bi3+ is an example of such an ion. As seen in Figure 5, it decreases the efficiency of the n—GaAs 0.8M K2Se-0.1M K2Se2-lM KOH c cell.30 Since the chemisorption of Bi3+ is weak, the deterioration in performance is temporary. The ion is desorbed in 10 min. and the cell recovers. 

In the next section factors that affect the reaction cross section are discussed. It is argued that electrolyte species on the surface of the semiconductor can qualify as surface states. In the subsequent section several examples of such surface states will be discussed. 

There is a growing tendency to invoke surface states to explain electron transfer at semiconductor-electrolyte interfaces. Too frequently the discussion of surface states is qualitative with no attempt to make quantitative estimates of the rate of surface state reactions or to measure any of the properties of these surface states. This article summarizes earlier work in which charge transfer at the semiconductor-electrolyte interface is analyzed as inelastic capture by surface states of charge carriers in the semiconductor bands at the surface. This approach is shown to be capable of explaining the experimental results within the context of established semiconductor behavior without tunneling or impurity conduction in the bandgap. Methods for measuring the density and cross section of surface states in different circumstances are discussed. 

While the ability to treat capture cross sections theoretically is very primitive and the experimental data on capture cross sections are very limited this phenomenological parameter seems to be an appropriate meeting place for experiment and theory. More work in both of these areas is needed to characterize and understand the important role of surface states in electron transfer at semiconductor-electrolyte interfaces. 

Nishida, M., Charge Transfer by Surface States in the Photoelectrolysis of Water Using a Semiconductor Electrode, Nature, 277, 202, 1979. 

The formation of electronic surface states in a semiconductor band gap by metal nanoparticles is the major factor that determine the efficiency of electron exchange between metal particles and a semiconductor matrix. It also influences the efficiency of electro-... 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

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