Surface States and Band Bending

Worl nction, Surface States, and Band Bending 879... 

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

Figure 2.97 Energy scheme for interpretation of the SRPES data obtained at C and open circuit potential in Eigure Z9. The left-hand side shows the semiconductor band bending at open circuit potential, the Rj" surface states, and the Pt deposition levels also shown is the inversion layer extension the...

Semiconductor Junctions, Solid-Solid Junctions, Fig. 3 Effect of surface states on band structure, (a) Semiconductor without any surface states and hence no Fermi-level pinning, (b) n-type semiconductor with a depletion region and upward band bending, (c) p-type semiconductor with a hole depletion layer and resulting downward band bending, (d) p-type semiconductor with a hole accumulation region and upward band bending... 

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.)...

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... 

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. 

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