Current Voltage Characteristics

Current (I)-voltage (V) curve measurement, i.e. the I-V curve,
is a simple characterization method for any type of junctions.

3.2.4 Current Voltage Characteristics

Current (I)-voltage (V) curve measurement, i.e. the I-V curve, is a simple characterization method for any type of junctions. The I-V curves of solid state devices reflect purely the electronic properties of the junctions. I-V measurements of the semiconductor-electrolyte junction, while still containing information about the electronic states of the semiconductor, in addition reflect the chemical reactions which determine the kinetics of the anodic dissolution. This makes the I-V characteristics of semiconductor-electrolyte junctions far more difficult to interpret.

Under anodic conditions p- and n-type III-V materials behave quite differently. They show a diode behavior. The forward current direction needs positive (anodic) bias in the case of p-type doping, and negative (cathodic) bias in the case of n-type doping. For instance, in the case of p-GaAs substrates in acidic solutions, the current in the anodic direction rises rapidly to very high values (in excess of 100 mA/cm2 at 1 V), causing electropolishing of the material. In the cathodic regime the current remains relatively low for a quite large interval of applied potentials [49]. In what follows only anodically biased n-type III-V/electrolyte junctions will be considered, due to the fact that only in this regime the pores can be formed.


A word of warning is necessary: The current through cathodically biased III-V compounds does not always produce just H2 (as in the case of Si). In the case of InP, for example, extremely poisonous PH3 might be generated. If an electrochemical double cell is used, the sample backside is necessarily in the cathodic regime and extreme care is necessary in conducting experiments.



A schematic representation of the IV curves for a)  GaAs (in H2SO4 solutions), b)  GaP (in H2SO4 solutions) and c)  InP (in HCl solutions).

Figure 3.2: A schematic representation of the IV curves for a) GaAs (in H2SO4 solutions), b) GaP (in H2SO4 solutions) and c) InP (in HCl solutions).


The reverse current in n-type III-V compounds increases steeply when increasing the voltage, and pits are formed on the surface as soon as a critical potential, the so-called pore formation potential (PFP), is reached (see Figure 3.8a). The pore formation potential is also called the breakdown potential. It occurs usually at 2-3 V for moderately doped samples. Schmuki et al measured the current-voltage curves of samples with intact and diamond-scribe scratched surfaces. The scratched samples showed a PFP significantly lower than the PFP of the intact sample. These results demonstrated that the PFP depends on the number of defects on the surface of the sample, namely it decreases as the number of defects increases. The PFP thus can be viewed as the defect-triggered onset of some kind of junction breakdown in areas of locally large electrical field strengths.


In contrast to n-GaAs, Tjerkstra et al. [50] reported for n-GaP (in H2SO4 aqueous electrolytes) that the current does not increase strongly after the PFP has been reached, but goes through a maximum at more positive potentials. After the peak the current decreases rapidly to lower values, which show only a weak potential dependence. Such peaks are usually related to oxide formation, making the current flow difficult. Since the generated oxide has to be dissolved before the current can flow again, the I-V curves show a hysteresis effect, see Figure 3.8b. The region on the I-V curves before and after the peak are usually called active and passive anodization regions respectively. Similar results, i.e. the hysteresis and a pronounced peak on the IV-curve, have been reported by Kaneshiro et al for InP in HCl electrolytes (Figure 3.8b), which were also attributed to oxide formation.

Oxide formation is supposed to play a very important role in the formation of the so-called Current-line Oriented Pores. Such kind of pores can be intuitively considered to be obtained in the regime of passive anodization. On the other hand, the so-called crystallographically oriented pores show very strong crystallographic features and, according to the current burst model, direct dissolution should be dominant (see next section). Thus, the crystallographically oriented pores can be considered to be obtained in the active anodization regime.



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