Surface potential electron accumulation layer

To summarize, the semiconduetor/electrolyte interfaee presents two types of currents in the dark this is a current of majority carriers whereas the photocurrent is a current of minority carriers. The same reactions can be monitored at n- and p-type electrodes but under different conditions. Hole accumulation corresponds to corrosion, since holes are trapped in surface bonds. Electron accumulation is generally not destructive for the surface unless cathodic reduction leads to decomposition. The band diagrams of Fig. 5 indicate that a downward shift of the flat band potential is expected at an illuminated n-type electrode. At negative bias, conversely, the shift is upward since electrons are accumulated in a thin surface layer (metallic-like behavior). 

Fig. 3.18 Types of space-charge region in an n-type semiconductor, dependent on the potential applied relative to the flat band potential, Un,. U represents potential (V) and Ec sur the electronic energy corresponding to Ec close to the surface, (a) c,sur = E no space-charge region (b) c,sur> E (U < U ) formation of an accumulation layer (c) c,sur formation of a depletion layer (d) c,sur efb (U U ) formation of an inversion layer.

Fig. 1. Four possible states of an n-type semiconductor as the sign of the charge in the surface region changes from positive to negative (a) an n-type accumulation layer, (b) the flat band condition, (c) a depletion layer, (d) an inversion layer. Ec and Ev represent the edge of the conduction band and valence band respectively. Bp represents the Fermi energy or chemical potential of electrons in the solid. + represents ionized donor atoms, mobile electrons and mobile holes.

When the potential is made more negative than with an w-type material, electrons collect at the semiconductor surface to form an accumulation layer. Under these conditions, the semiconductor is said to become degenerate and behaves more like a metal. In... 

Si amounts to approximately 1 ML. Under accumulation conditions. Si oxidation by hole injection processes is difficult because of the large concentration of conduction band electrons in an accumulation layer that leads an increased surface recombination rate. Hole injection into occupied surface states would only be possible if the reaction rate of an oxidized surface defect with the surrounding water were faster than the recombination via conduction band electrons which is unlikely. It is therefore concluded that oxide formation is hkely to have occurred by the procedure where the sample was scanned from open circuit potential (about -0 V) to the peak C2 thus allowing for oxidation under transient depletion conditions. 

Several techniques can be used to determine the flatband potential of a semiconductor. The most straightforward method is to measure the photocurrent onset potential, ( onset- At potentials positive of (/>fb a depletion layer forms that enables the separation of photogenerated electrons and holes, so one would expect a photocurrent. However, the actual potential that needs to be applied before a photocurrent is observed is often several tenths of a volt more positive than ( fb- This can be due to recombination in the space charge layer [45], hole trapping at surface defects [46], or hole accumulation at the surface due to poor charge transfer kinetics [43]. A more reliable method for determining ( fb is electrolyte electroreflectance (EER), with which changes in the surface free electron concentration can be accurately detected [47]. The most often used method, however, is Mott- chottky analysis. Here, the 1/ Csc is plotted as a function of the applied potential and the value of the flatband... 

This is the regime of cathodic currents. The silicon atoms of the electrode do not participate in the chemical reaction in this regime. An n-type electrode is under forward bias and the current is caused by majority carriers (electrons). The fact that photogenerated minority carriers (holes) are detectable at the collector indicates that the front is under flat band or accumulation. A decrease of IBC with cathodization time is observed. As Fig. 3.2 shows, the minority carrier current at the collector after switching to a cathodic potential is identical to that at VQcp in the first moment, but then it decreases within seconds to lower values, as indicated by arrows in Fig. 3.2. This can be interpreted as an increase of the surface recombination velocity with time under cathodic potential. It can be speculated that protons, which rapidly diffuse into the bulk of the electrode, are responsible for the change of the electronic properties of the surface layer [A17]. However, any other effect sufficient to produce a surface recombination velocity in excess of 100 cm s 1 would produce similar results. 

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