Accumulation of holes

Figure 11-14 shows the calculated hole density (upper panel) and the electric field (lower panel) as a function of position for the three structures. For the devices with a hole barrier there is a large accumulation of holes at the interface. The spike in the hole density at the interface causes a rapid change in the electric field at the interface. The field in the hole barrier layer is significantly larger than in the hole injection layer. For the 0.5 eV hole barrier structure, almost all of the... 

We discuss the dissolution of surface atoms from elemental semiconductor electrodes, which are covalent, such as silicon and germanium in aqueous solution. Generally, in covalent semiconductors, the bonding orbitals constitute the valence band and the antibonbing orbitals constitute the conduction band. The accumulation of holes in the valence band or the accumulation of electrons in the conduction band at the electrode interface, hence, partially breaks the covalent bonding of the surface atom, S, (subscript s denotes the surface site). 

Oskam et al. [66] have used IMPS to investigate the role of surface states at the n-Si(lll)/NH4F interface. In this case, the redox reaction is simpler, and appears not to involve holes trapped at surface states. This is probably due to the presence of a surface oxide layer. Electron transfer is evidently exceptionally slow in this case, since these authors observed a modulated photocurrent even at potentials far from the flatband potential where recombination is expected to be negligible. Accumulation of holes modifies the potential drop across the Helmholtz (and presumably also surface oxide region), leading to a capacitive charging current. This effect has also been treated in more detail by Peter et al. [71]. 

In further investigations Lehmann ° found that the pores propagate at similar rates at different applied current densities. It was then postulated that all pore tips are limited by mass transfer in the electrolyte defined by J (see Fig. 5.1) in the steady-state condition. It was further proposed that the relative rates of carrier transport in the silicon semiconductor and mass transport in the electrolyte determine the PS morphology of n-Si. At low current densities the reaction rate is limited by the transport of carrier to the pore tips and there is no accumulation of holes so that dissolution occurs only at pore tips while the pore walls do not dissolve because of the depletion of holes. At high current densities the reaction at pore tips is mass transport limited and holes accumulate at the pore tips and some of them move to the walls resulting in the dissolution of walls and larger pore diameters. When the concentration of holes in the walls is close to that at the pore tips, the condition for the preferential dissolution at pore tips disappears and PS ceases to form. 

The photocolouration of the metal-oxide specimen must achieve the limits of accumulation of hole and electron colour centres where the number of electrons trapped by the solid s defects equals the number of trapped holes. 

The practical use of these calculations is limited, however, because the kinetics of a reaction can play an important role. This becomes quite obvious for layer compounds such as M0S2. The kinetics may be controlled by adsorption, surface chemistry, surface structure and crystal orientation. According to Fig. 8.15, pEdecomp is close to the conduction band, i.e. M0S2 is rather easily oxidized. In the case of a flat basal surface, it has been observed with several transition metal chalcogenides that the photocurrent onset at n-electrodes occurs with high overvoltages accompanied by a shift of Gfb.(see Section 5.3). Since this is caused by an accumulation of holes at the surface the hole transfer is kinetically inhibited. 

In the opposite case, hv > g, photons produce electron-hole pairs. Accumulation of holes at the oxide surface increases the local potential drop which may cause a fast photocorrosion. Ion migration is enhanced in the thin film, corrosion is enhanced, and altogether a fast dissolution of metal takes place by a photoelectro-chemical process in the passive film. An example is given for Ti [160]. This technique can be used for microstructuring of Ti- or Al surfaces [104]. On the other hand, anodic metal ion dissolution competes with the opposite anodic film forming ITR of oxygen ions. Therefore, in dependence on the special conditions, laser induced oxide growth may overcome pit formation [160]. 

The accumulation of holes would cause an equally large cathodic transient when the light is turned off and electrons in the conduction band react with the accumulated holes. Only a small cathodic transient is observed in Fig. 3.10b, so accumulation of holes can be ruled out as the main recombination mechanism. A useful check for hole accumulation in a photoanode is by adding a hole scavenger, such as methanol [22] or hydrogen peroxide, or by catalyzing the water oxidation reaction with a co-catalyst such as IrO [9, 36], [21], Ru02 [41,... 

The fact that holes are generated and accumulated at the interface during the galena decomposition in regions C, D, and E implies the formation of a sulfur-rich layer, which is inconsistent with reaction (III). In the case of reactions (II) and (la), the increase in hole absorption would be accompanied from the very beginning by an increase of the absorption band of lead hydroxide, which is not observed in 0.01 M buffer. Hence, because the formation of lead hydroxide is preceded by the generation and accumulation of holes and possibly by... 

Additional information about the semiconductor can be obtained from the interface capacitance C , which arises because each interface state stores a charge. A surface potential C can be defined as the potential at the semiconductor-insulator interface which causes the center of the band gap of the semiconductor [the Fermi level of the intrinsic material, ( /),] to shift to a new value (Figure 4.3.5). This surface potential arises whenever the applied potential causes charge to build up at the interface. For example, for an -type material when E = Ef- ( /)ref is very much less than zero, ( /), will cross Ef as shown in Figure 4.3.5, leading to an accumulation of holes at the interface, that is, inversion as described above for the MOS devices. Now yZj can be calculated from the capacitance data described above by means of (NicoUian and Goetzberger [1967])... 

Fig. 3 and increases the dip, i.e., decreases the stationary photocurrent. The flat-band situation is reached at a bias of - 0.95 V as indicated by the disappearance of the initial photocurrent peak, labeled peak (filled squares in Fig. 5). Both the stationary photocurrent, labeled final (filled circles in Fig. 5), and the corresponding stationary recombination loss are linked to the stationary accumulation of holes at the interface. The stationary photocurrent disappears already at finite band bending of about 0.3 V with respect to the flat-band potential at - 0.95 V (Fig. 5).

In this way, a new chemical bond is formed with one of the atoms while the other remains in a radical state. The electron in this radical state will have a higher energy, and therefore this intermediate will either act as an efficient trap for holes or it might interact with another intermediate forming a new chemical bond. Fig. III.12 shows the first case in which now one atom is left with a much weaker bond to the surface which will further be easily attacked and in this way, the atoms are stepwise removed from the surface by an oxidation due to an accumulation of holes. 

According to these experimental results, the primary effect here is an accumulation of holes on the surface because the anodic dissolution seems to be a very slow reaction. Working at a potential between f/ft,(dark) and f/f, (light), the bands become flattened due to their shift. Since then the majority carrier density is increased near and on the surface, the recombination rate increases (the recombination rate is proportional to n and p, see Section 1.6). Accordingly, the high recombination is a consequence of the band-edge shift on the surface. In the case... 

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