Band bending surface states

M. Green, Semiconductor Electrochemistry, in Modern Aspects of Electrochemistry, J. O M. Bockris and B. E. Conway, eds., Vol. 2, Ch. 2, Plenum, New York (1959). First formulation of semiconductor electrode kinetics in terms of equations band bending surface states limiting currents. 

Fig. XVIII-19. Band bending with a negative charge on the surface states Eu, E/, and Ec are the energies of the valance band, the Fermi level, and the conduction level, respectively. (From Ref. 186.)...

K. Metder. AppL Phys. 12,75, 1977. PL measurements of surface state densides and band bending in GaAs. 

Although the observations for PPV photodiodes of different groups are quite similar, there are still discussions on the nature of the polymer-metal contacts and especially on the formation of space charge layers on the Al interface. According to Nguyen et al. [70, 711 band bending in melal/PPV interfaces is either caused by surface states or by chemical reactions between the polymer and the metal and... 

Assume, that there are adsorption particles with concentration Nt on the surface of semiconductor which is in adsorption equilibrium with a certain gas. A fraction of adsorption particles is charged with concentration designated as w<. Apart from them, on the surface there are various biographic surface states with concentration of the charged particles ng controlling the degree of an a priori band bending qUso-... 

The presence of surface states results in a certain compression of the space charge region and leads to a decrease in the band bending. 

The photocurrent density (/ph) is proportional to the light intensity, but almost independent of the electrode potential, provided that the band bending is sufficiently large to prevent recombination. At potentials close to the flatband potential, the photocurrent density again drops to zero. A typical current density-voltage characteristics of an n-semiconductor electrode in the dark and upon illumination is shown in Fig. 5.61. If the electrode reactions are slow, and/or if the e /h+ recombination via impurities or surface states takes place, more complicated curves for /light result. 

The surface Fermi level, Cp, which depends on the surface state, is not the same as the interior Fermi level, ep, which is determined by the bulk impurity and its concentration. As electron transfer equilibrium is established, the two Fermi levels are equilibrated each other (ep = ep) and the band level bends downward or upward near the surface forming a space charge layer as shown in Fig. 2-31. 

The semiconductor surface where the Fermi level is pinned at a surface state of high density (Fig. 2-31) is in the state of degeneracy of electron levels, because of the high electron state density at the surface Fermi level. Similarly, the surface degeneracy is also established when the band bending becomes so great that the Fermi level is pinned either in the conduction band or in the valence band as shown in Fig. 2-32. 

Let us consider in more detail, using the above concepts, how a photocorrosion process occurs under the illumination of a semiconductor. Suppose that electron transitions at the interface between the semiconductor and solution do not take place in darkness in a certain potential range (the semiconductor behaves like an ideally polarizable electrode). This range is confined to the potentials of decomposition of the semiconductor and/or solution. The steady state potential of a semiconductor is usually determined in this case by chemisorption processes (e.g., of oxygen) or, which is the same in the language of the physics of semiconductor surface, by charging of slow surface states. It is these processes that determine the steady state band bending. 

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