Schottky-Mott behavior

Moderately doped diamond demonstrates almost ideal semiconductor behavior in inert background electrolytes (linear Mott -Schottky plots, photoelectrochemical properties (see below), etc.), which provides evidence for band edge pinning at the semiconductor surface. By comparison in redox electrolytes, a metal-like behavior is observed with the band edges unpinned at the surface. This phenomenon, although not yet fully understood, has been observed with numerous semiconductor electrodes (e.g. silicon, gallium arsenide, and others) [113], It must be associated with chemical interaction between semiconductor material and redox system, which results in a large and variable Helmholtz potential drop. 

The effect of illumination seen in the current/potential behavior is reflected also in capacity measurements as evaluated in the form of Mott/ Schottky-plots (Fig. 2). Illumination leads to a parallel shift of this plot in the same direction and by about the same amount as in I/E curves. The plot is shifted back to its dark position if the appropriate redox couple is added. Other minority carrier acceptors on the other hand are not able to shift the light-plot back onto the plot obtained in the dark. 

A similar cathodic limiting current has also been observed for the electroreduction of peroxide on LaNi03 (Fig. 18) [48] and this behavior occurs at potentials where the reduction of the solid surface takes place changing the potential distribution at the oxide-electrolyte interface. This change of surface properties is quite similar to the behavior of NiO [347] under cathodic polarization and is also reflected in the inhibition of electron transfer to or from redox couples in solution [81] and capacitance Mott-Schottky type plots [87, 290, 291] of these interfaces. 

A continuous metal deposit layer may behave as an ohmic contact or a Schottky barrier. For a relatively thick metal film the silicon can still behave like a semiconductor before the onset of current. For example, for n-Si deposited electrochemically with 150nm An, the electrode behavior is similar to that of bare silicon electrode At positive potentials the anodic current is small whereas at cathodic potentials current from hydrogen evolution increases with increasing polarization. " In the potential region before the onset potential for the cathodic current a linear Mott-Schottky plot is obtained giving a flatband potential similar to that of bare silicon sample. In the potential region where hydrogen evolution occurs, it behaves like a metal with potential drops mostly in the Helmholtz layer. 

A certain relationship, which exists between the bulk and surface properties of semiconducting materials and their electrochemical behavior, enables, in principle, electrochemical measurements to be used to characterize these materials. Since 1960, when Dewald was the first to determine the donor concentration in a zinc oxide electrode using Mott-Schottky plots, differential capacity measurements have frequently been used for this purpose in several materials. If possible sources of errors that were discussed in Section III.3 are taken into account correctly, the capacity method enables one to determine the distribution of the doping impurity concentration over the surface" and, in combination with the layer-by-layer etching method, also into the specimen depth. The impurity concentration profile can be constructed by this method. It has recently been developed in greatest detail as applied to gallium arsenide crystals and multilayer structures. 

Obviously, such so-called frequency dispersion hampers a proper determination of Vfb although reliable values are generally obtained at high measuring frequencies (>10kHz). The origin of this nonideal behavior is not well understood [37, 41]. However, to check the rehabihty of Mott-Schottky measurements, the capacitance should be measured in a broad frequency range [42-44]. 

For various illumination intensities, the diameter of the semicircle fitting the data at high frequencies equals approximately kT/ely pHl [45-47, 49]. In addition, it was shown that upon illumination, a capacitive peak appears in the C versus V plot of the n-GaAsjO.l M H2SO4 interface [45,46, 51], The peak value proved to be a function of the frequency and the photocurrent density as measured in region G [51]. This behavior is markedly different from the purely capacitive impedance (vertical line in the Nyquist plane and straight Mott-Schottky plot) expected for a blocking s/e interface (see Sect. 2.1.3.1). 

If the majority carriers of the semiconductor are accumulated at the interface, the electrode behavior approaches that one of a metal because now the excess charge remains very concentrated at the interface. Fig. II.3 gives a picture of the capacity behavior of an n-type semiconductor against the potential drop across the interface in a linear representation and also in a Mott-Schottky plot. 

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