Tip Current Voltammetry TCV

This technique, thus named by Carlsson et al. [78], consists in recording the tip current as a function of the potential of the sample. TCV can be applied with the regulation loop of the tunnel current active or not. Though the terminology tip current voltammetry is rather confusing, because the tip potential is fixed, we use it in the following to conform with published work. 

This approach has been applied to GaAs [78-81], GaP [78, 82] and CdS [80]. Examples of TCV obtained with various n-type GaAs electrodes are presented in Fig. 12. Results are redrawn from [80]. According to the principle of TCV, imaging is possible in regions of potential where the tip current is equal to its pre-set value as indicated by horizontal arrows. A negative tip current means that electrons are flowing out of 

The discussion above shows that TCV can be reasonably interpreted in the framework of known electronic states. The direct determination of interface states from TCV seems difficult because the dependence of (7° with the tip potential [78, 81] finds no simple explanation within a simple one-dimensional energy diagram. Tip-induced local modifications of the band diagram of the semiconductor may exist (see Sec. 4.2.3) which complicates the determination of energy levels. The experimental dependence of [7° on the pre-history of the electrode [78, 81] stem probably from 

If one assumes, as in Fig. 15 a, a one-dimensional band diagram and that tunneling electrons are collected from the sample within a disk of diameter 100 A, which is already larger than the radius of the tip, the current /jh is several orders of magnitude smaller than typical tunnel currents in the nanoamp range. To reach a thermionic current of 1 nA locally, the surface 9 must be a disk of diameter in the micrometer range. An explanation in terms of surface diffusion of carriers seems insufficient. 

The simple evaluation above suggests that the surface barrier is not the ideal one shown in Fig. 15 a and we speculate that the tip induces a potential well at the surface. The modified band diagram is presented in Fig. 15 b. Away from the tip, the po- 

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