La estandarización de la lógica de programación

4.3

Characterization of the Dielectric Layer

One of the main issues for realizing a sensor device based on OTFTs was the change of the electrical properties of the dielectric layer SiO2 under the influence of ionic solutions. As

the operation in ionic solutions restricts the magnitude of the drain-voltages to very low values, the resulting drain currents are generally very low. Therefore, already very small leakage currents would destroy the OTFT performance. Besides the high demands on the insulating properties of the SiO2 layer, also the influence of trap-states on the threshold

voltage and the hysteresis (see section 2.2.3) plays a crucial role. Additionally, it turned out that especially the penetration of sodium ions (N a+_{) change the capacitance of the}

SiO2 layer or even cause high leakage currents to the gate electrode. On the one hand,

this observation is astonishing as N a+ _{is a small ion, causing a high electric field and}

therefore have a very high hydration energy of 440kJ/mol [90]. On the other hand the small size ofN a+ _{allows for a more easy penetration into the SiO}

2 lattice. We confirmed

the penetration of N a+ _{ions into SiO}

2 by sweeping a voltage VSi between a Pt-electrode,

introduced in a 1mM NaCl solution, and the highly doped Si layer of a wafer oxidized with 300nmdry oxide (Active Business Company GmbH) and measured the current ISi across

theSiO2 layer (see inset Fig. 4.18). The measurement was performed with a Keithley 2612

Sourcemeter. The contact area to the NaCl solution was 0.04cm2 _{and the Pt-electrode}

was set to ground potential.

Si V SiO2 NaCl Si ISi 7

Figure 4.18: Sweep of the voltage across the SiO2 layer and measurement of the corre-

sponding current for different sweep velocities. The shift at VSi = 0V origins from the

penetration of sodium ions into SiO2.

When a negative voltage is applied the N a+ _{ions are pulled into the SiO}

2 layer and in-

crease the permittivity and therefore the capacitance of the SiO2 layer. The increased

current at VSi = 0V after one complete cycle in Fig. 4.18 confirms this assumption. It

is easy to see that this change in current ∆I depends on the sweep velocity vs by the

at different sweep velocities. It turned out that the increase of the current is not direct proportional to the sweep velocity, what can be explained by the shorter time the N a+

ions have to penetrate in theSiO2 layer for higher sweep velocities and by the influence of

leakage currents. Note that the sourcemeter can only apply potential steps and that after every potential step the current originating from redox reactions decrease with the square root of time (see Fig. 3.13 b), while the charging current of the capacitor decreases expo- nentially. Therefore, in order to estimate the change in the capacitance we choose a very low sweep velocity (0.2V /s) and obtain a change in the capacitance ofδC = 4.1nF cm−2_,

what is quite high compared to the total capacitance of SiO2 (about 11.5nF cm−2). Due

to the simplifications mentioned above the estimated value for the change in capacitance is probably too high. However, the measurement prove that the penetration of N a+ _ions

occurs and has an impact on the capacitance of SiO2.

We also observed that it is possible to regain the initial state, i.e. pushing the N a+ _ions

out of the SiO2 layer, by applying a very positive voltage. A more serious problem than

the change in the capacitance is the drastic decrease of the breakdown voltage.

Besides the substrates with 300nm SiO2 dielectric layers, we also used substrates with a

stack consisting of 100nmSiO2and 100nmSi3N4, as they have a very positive effect on the

performance of our pentacene TFTs. However, this double dielectric layer shows higher leakage currents to the gate. We also observed that a thin COC layer can additionally prevent the penetration of N a+ _{ions (see Fig. 4.19). Again, one can see a shift of the}

current due to the penetration of N a+ _{ions. The irregular shape of the curve may be}

explained by the influence of the SiO2/Si3N4 interface. A similar effect of the COC layer

on the current was observed for SiO2 dielectrics. However, here the current shows a much

more abrupt increase at a certain voltage.

Si V SiO + Si N2 3 4 NaCl Si Si

Figure 4.19: Sweep of the voltage across the SiO2 / Si3N4 layer and measurement of the

corresponding current. The leakage currents across the dielectric layer can be significantly reduced by spin coating a thin COC layer on top.

Chapter 5

Electrochemical Characterization of

Organic Semiconductor Interfaces

In this chapter the basic electrochemical experiments are described we used to characterize the interfaces which are from decisive importance for the realization of a sensing device based on TTC capped pentacene TFTs.

5.1

Setup

For the electrochemical characterization an electrochemical cell made of polytetrafluo- rethylen (PTFE) was fabricated, see Fig. 5.1 a. The basic principle is that a chip can be placed on a socket and electrically contacted from above by spring contacts (see Fig. 5.1 b). For this purpose usually the whole chip was covered by a thin Au-layer by e-beam deposition, which acted as the working electrode. Depending on the demands of the elec- trochemical experiment a small area in the center of the chip was covered by the organic material of interest. Next, the chip was sealed by a thin PDMS mat to prevent currents from the electrolyte to the bare Au region. After closing the setup by pressing the electro- chemical cell on top of the socket and the sealed substrate, the working electrode can be connected to a potentiostat (Ivium Compactstat) by the electric connectors of the spring contacts. The electrochemical cell allows for the introduction of several electrodes, e.g. counter- and reference electrode. In order to suppress noise in the current measurements caused by electromagnetic fields, we fabricated a shielding box made of aluminum, see Fig. 5.2. The box has two compartments, one for the potentiostat and one for the electrochem- ical cell, which are separated by an aluminum plate which shields the electrochemical cell from the electromagnetic field of the potentiostat. The cables to the electrodes of the cell are guided via BNC-feedthroughs from one compartment to to the other. A stand was used to introduce reference-electrodes with a high diameter in the electrolyte from the top. For more complex measurements, e.g. the characterization of the influence of the electrolyte potential on TFT performance, additional feedthroughs allow to apply external voltages. The whole box was grounded externally and connected to the chassis of the potentiostat

by the intended plug (green). sample socket PDMS seal electrode electro- chemical cell electric connectors spring contacts (a) (b)

Figure 5.1: Design drawing of the self-made electrochemical cell: (a) Components of the complete setup. (b) View of the inner build-up of the electrochemical cell.