Up to now we discussed shortly the principles of semiconductor-electrolyte interfaces. However, it was said nothing about the practical implementation of electrochemical techniques. Let us start by discussing the most simple experimental setup for performing electrochemical measurements. The following elements are necessary:
The electrode, which has to be studied or where the electrochemical reaction should take place (in our case it will be the electrode in which we intend to introduce pores), is called the working electrode (WE). The second electrode, which is closing the circuit, is called the counter electrode (CE). The third one, used to measure the voltage between the electrolyte and WE, is called the reference electrode (RE). A schematic representation of such a simple experimental set up is presented in Figure 2.14a.
If a current is flowing thought a solid-electrolyte interface, chemical reactions will occur at the interface. Electrons leaving the solid will reduce species in solution, whereas electrons moving into the solid lead to the oxidation of species in solution. Due to the chemical reactions at the interface the composition of the electrolyte near the surface of the solid will change continuously and thus the distribution of the voltage across the semiconductor-electrolyte interface will vary as well. This is especially true at high current densities. For this reason, in order to maintain a constant composition of the solution near the surface of the sample, i.e. the working electrode, continuous pumping of the electrolyte is necessary.
Figure 2.14: a) The simplest experimental set up for performing electrochemical experiments. b) A schematic representation of a three electrode potentiostat .
However, the system presented in Figure 2.20a does not contain only the WE/electrolyte junction but also the CE/electrolyte junction. Therefore, assuming that the voltage losses in the bulk of the solid and electrolyte are negligible, the potential controlled by the battery and applied to the system will be distributed as follows:
where UWE and UCE are the voltages accros WE/electrolyte and CE/electrolyte junctions respectively. This means that not all the voltage supplied by the battery will be applied to the junction of interest, i.e. WE/electrolyte . Therefore, in practice the setup presented in Figure 2.14a is not always satisfactory due to the fact that we wish to control exactly the voltage (UWE) between the sample and solution and to measure the resulting current flow.
Therefore, in practice the setup presented in Figure 2.20a is not always satisfactory. In nearly all case we wish to control, the voltage (Uwe) between the sample and solution, and consequently to measure the resulting current change.
Taking into account that the current will stimulate electrochemical reactions also at the CE/electrolyte interface, it can be expected that the state of the counter electrode, i.e. UCE, will vary in time. Consequently, the instability of UCE will influence the value of UWE due to the fact that their sum should be equal to the voltage supplied by the battery. Thus, controlling the voltage U supplied by the battery, e.g. setting it constant as indicated in Figure 2.14, does not assure us that the voltage of interest, i.e. measured between the sample and the reference electrode (UWE), will stay constant as well.
The first thing to do in order to avoid this problem is to decrease somehow the value of UCE in such a way that UCE << UWE. The logic behind this is that a negligible value of UCE will have a negligible influence on UWE. This can be done by choosing a counter electrode with a relatively high area ACE>>AWE. In this case, the resistance of CE will be much smaller than the resistance of WE and consequently the condition UCE << UWE can be satisfied. Thus, in Equation 25 it will be possible to neglect the value of UCE and the result will be
Eq. 26 states that the potential applied by the battery will drop entirely on the WE/electrolyte junction. This is actually what we want, i.e. to control UWE by controlling U.
Nevertheless, choosing a CE with a large area is not always the best solution. The influence of UCE on UWE will be again significant when appreciable current densities will flow through the system. As we will see later, for pore formation in III-V compounds very high current densities are sometimes required. Therefore, additional improvement of the set up presented in Figure 2.14a is needed.
To solve this problem one can use a device called potentiostat. A potentiostat allows to control the voltage between the working electrode and the reference electrode directly. The working principle of a potentiostat is shown in Figure 2.14b. A potentiostat can be a three or a four electrode device (see Figure 2.15).
The voltage intended to be applied between the reference electrode and the sample is provided as the input voltage for the potentiostat. The actual WE-RE voltage is measured by a differential amplifier (Electrometer in Figure 2.14b) and is compared with the input voltage by a second differential amplifier (Control Amplifier). In case there is a difference between the input voltage and the WE-RE voltage the controlling amplifier will increase/decrease the voltage to the cell until the actual and the input voltages will be equal.
The I/U converter measures the cell current and forces it to flow through the resistor Rk. The voltage drop across Rk is a measure of the cell current. The cell current during an anodization experiment can often vary by several orders of magnitude. In practice it is impossible to measure currents over wide ranges using a single resistor. Therefore, a number of different Rk resistors should be used. Measurement of small currents requires large Rk values and vice versa.
Figure 2.15: a) A schematic representation of a setup with a three electrode potentiostat. b) A schematic representation of a set up with a four electrode potentiostat.
A three electrode potentiostat solves the problems discussed above (see Figure 2.15a) as long as the contact between the sample and the working electrode is good enough. However, if the ohmic resistance between the WE and the sample is not sufficiently small, e.g. when using In/Ga alloy, a four electrode potentiostat must be used (Figure 2.15b). The fourth electrode is called the sense electrode (SE ) and is connected to the sample. Now, the potentiostat can be regarded as being composed of two 'independent' subsystems. One containing the WE and CE electrodes through which the current is flowing. Correspondingly, the second subsystem contains SE and RE electrodes and measures the potential. A feedback interaction between the two subsystems results in an ideal tool for controlling electrochemical processes. In this configuration the contact between the sense electrode and the sample is not critical because no current is flowing through it. Now the desired potential will be exactly applied on the sample/electrolyte junction.
In spite of the fact that a four electrode potentiostat diminishes the importance of the contact quality between WE and the sample, during the pore formation process the quality of the contact between the sample and the working electrode is still very important. It determines how uniform the current is distributed across the whole surface of the sample. If the contact is not uniform the distribution of the current and consequently the porous layer will not be uniform as well.
A significant improvement of the uniformity of the backside contact can be achieved by a liquid contact, i.e. the sample has two electrolyte junctions. The first junction (the front side) will be the one of interest and where the pores will grow. The second junction will play the role of an uniform backside contact. On both junctions electrochemical reactions will take place. If at the front junction an anodic reaction takes place, then at the back contact a cathodic reaction will occur. This idea can be realized by the so called double electrochemical cell . A schematic description of such a double cell is presented in Figure 2.16.
Figure 2.16: Schematic representation of the electrochemical double cell used for anodization of III-V compounds. a) The cell consists of two Teflon rooms (Cell 1 and Cell 2) which are nearly identical. The only difference between them is that Cell 1 contains two Pt electrodes (CE and RE) whereas Cell 2 only one electrode (WE). Two windows in the cells provide the possibility for front and back side illumination. The cells slide easily on four horizontal metallic rods and can be fixed face to face by eight clamping bolts. The sample is placed between the cells and a mild mechanical contact with the Teflon cells is supplied by two rubber O-rings. The internal and external diameters of the O-rings are 7 and 5 mm respectively. On the sample, additionally, the fourth electrode is placed (sense electrode (SE)). The electrolyte can flow in and out of the cells through four rubber tubes. The tubes also connect the chambers to two different electrolyte containers; b) The cross section of the double cell along the cutting plane A ( see a). It shows the inside of the cell and how the Pt electrodes are arranged. The electrodes are designed to provide a short distance between the sample and the reference electrode. Each Pt electrode is quite long and has an external socket for connection to the potentiostat.
Additional to the cell and potentiostat, an electrochemical setup requires pumps and containers for electrolyte, temperature control etc. A schematic overview of a complete experimental set up is presented in Figure 2.17.
Figure 2.17: The overview of the experimental set up used for anodization of III-V compounds in this work. The set up has six important components: the electrochemical double cell, the electrolyte containers, the peristaltic pumps, the potentiostat, the thermostat and the computer. The containers and the cells are made of Teflon in order to withstand all kinds of electrolytes. The electrolyte from Container 1 is pumped by Pump 1 through the front cell, whereas the electrolyte from Container 2 is pumped by Pump 2 through the back cell. The thermostat is pumping the oil through both electrolyte containers. The temperature is measured only in Container 1, which supplies the front cell with electrolyte. The potentiostat has a four electrode configuration: Working electrode (WE), Counter electrode (CE), Reference electrode (RE), Sense electrode (SE). Additionally, the potentiostat is collecting the information from the thermocouple, which is transmitted to the computer. The computer controls the thermostat and the potentiostat. The rotation velocity of the peristaltic pumps is fixed manually at the beginning of each experiment (normally 100 rotations per minute).