charge transport in nanoscale lateral and vertical organic semiconductor devices
Bojian Xu is a PhD student in the MESA+ research group NanoElectronics. His promoter is Wilfred van der Wiel.
Organic semiconductors have been drawing more and more attention due to their huge potential for low-cost, flexible, printable electronics and spintronics. N, N’-bis(2,6-dimethylphenyl)-perylene-3,4,9,10-tetracarboxylic diimide (DXP) is an organic semiconductor. DXP-loaded zeolite L crystals is expected to have unique spin-dependent transport property due to the strictly one-dimensional constrain. To realize various electrical transport measurements on the DXP-loaded zeolite L crystals, we design a fabrication method based on nanoindentation to embed the crystals into devices. Besides the DXP-loaded zeolite L crystals, we also plan to explore the spin-dependent transport property of the DXP thin films. Hence, we fabricate and investigate DXP lateral field-effect transistors. For an organic semiconductor device, short junction/channel length is beneficial to scaling down the integrated circuits based on the organic semiconductor device, high cut-off frequency, low operation voltage, and so on. In lateral devices, nanolithography techniques are usually necessary to fabricate short spacing electrodes. However, in a vertical configuration where an organic film is sandwiched between two metallic contacts, the channel length is defined by the film thickness which is well controllable down to a few nm. Nevertheless, fabricating top contacts on thin organic films and patterning organic layers are not straightforward. So, we demonstrate a fabrication method to realize vertical organic devices with short junction/channel length. Poly(3-hexylthiophene) (P3HT) is a widely-investigated organic semiconductor. It has good electrical conductivity. It is a polymeric material and can form continuous and flat thin films by spin-coating. Hence, we fabricate vertical P3HT devices and investigate the charge transport of the devices.
We start in Chapter 1 by putting the experimental and numerical results in context and provide a motivation for our work.
Chapter 2 concisely discusses the theoretical background related to the experimental and numerical simulation research reported in this thesis.
In Chapter 3, a fabrication method based on nanoindentation using atomic force microscope (AFM) is introduced for electron transport measurements on one-dimensional DXP-loaded zeolite L crystals and two-dimensional electron systems in LaAlO3/SrTiO3 heterostructures. Parylene-C is used as the indent material. Three common methods using the Veeco Dimension 3100 AFM (Veeco AFM), and the so-called “point-and-shoot” mode of the Bruker Dimension Icon AFM (Icon AFM) have been used to perform nanoindentation on DXP-loaded zeolite L crystals with ~150-300 nm diameter. The nanoindentation results demonstrate that the common methods do not meet the positioning requirement. Nanoindentation using the “point-and-shoot” function is able to create indents on top of the sub-micron zeolites. Au-parylene-Au test devices have been fabricated using the fabrication method. IV measurements of the test devices with the nanoindentation show high conduction (about 50 Ω). Test devices without nanoindentation show good insulation (about 70 GΩ). The IV measurement results demonstrate that the fabrication method based on the nanoindentation using the “point-and-shoot” mode of the Icon AFM is suitable for the fabrication of nanocontacts on the DXP-loaded zeolites and LaAlO3/SrTiO3 samples.
Chapter 4 discusses the results on DXP lateral field-effect transistors. For the final purpose of investigating magnetic field effects in the electrical transport properties of DXP, DXP lateral field-effect transistors have been fabricated by drop-casting DXP onto interdigitated Au electrodes pre-fabricated on SiO2/Si substrates. Electro-migration of the electrodes after IV measurements has also been observed. The electrical transport property of the DXP devices are light sensitive and much more stable when measuring in vacuum (~10-5 mbar) than in ambient atmosphere. Based on the output and transfer characteristics, the DXP devices exhibit n-type channel behavior. The output characteristics do not show saturation, and the threshold voltage of the transfer characteristics is drain-bias dependent. We propose that this is probably due to the short-channel effect. Large hysteresis in the transfer characteristics is also observed. We ascribe this to the bias stress effect caused by the untreated organic/dielectric interface. Further research is needed to confirm and improve the channel behavior of the DXP lateral devices. Spin-dependent transport measurements will also be executed.
In Chapter 5, two-terminal vertical P3HT pillar devices have been fabricated by wedging transfer and utilizing the wedge-transferred Au top contacts as etch masks for directional dry reactive ion etching (RIE) of the P3HT thin film. The diameters of the P3HT pillars are 2 µm to 200 nm and the thicknesses of the P3HT layers are 5 nm to 100 nm. SEM images show that the P3HT is well protected by the top contacts, and that there is a distinctive interface between the metal and the organic layer, suggesting that the metal does not penetrate the P3HT. The devices exhibit good electrical reproducibility when measuring in vacuum (< 10-4 mbar). The relatively high working device yield of 40-85% indicates that the top-contacting is very soft, allowing for charge transport through very thin organic films. The 5 nm thick P3HT junctions carry very high current density, up to 106 A/m2. The temperature dependence of the electrical transport has been measured between room temperature and 150 K. The results reveal thermally assisted hopping transport.
Simulation of the temperature dependence has been performed based on the drift-diffusion model with a Gaussian density of states. Experimental results of the devices with P3HT thicknesses of 40 and 100 nm can be explained by the model very well. The simulated results indicate a low injection barrier (less than 0.1 eV), which can explain the weaker temperature dependence of the devices with thinner P3HT. However, for devices with P3HT thicknesses of 5 and 10 nm, the simulated current densities for the P3HT thicknesses of 10 nm and 5 nm are higher than the measured values. Nonetheless, the measured temperature dependence is still well described by the simulations. We attribute the discrepancy between experiment and simulations to a different orientation of the P3HT chains close to the Au electrodes as compared to the bulk. We conclude that carrier injection in our devices is very good, yielding the prospect of new types of very thin and highly conducting organic devices.
Follow up on this work, we have investigated gated vertical P3HT pillar devices in Chapter 6. Output and transfer properties of the gated devices have been measured without finding a distinct gate effect. Device simulations have been performed as well, using the commercial software package Silvaco ATLAS. Simulation results of ideal devices show not only a distinct gate effect, but also a larger drain current than in the experiment. Assuming a damaged layer at the edge of the P3HT pillars due to the RIE, both the gate effect and the drain current are dramatically reduced. This suggests that a damaged layer could be the reason for the reduction of the gate effect and conductivity. To obtain reasonable agreement between simulated and measured results, the damaged layer has to be assumed very thick. Therefore, we believe that there are also other reasons for the small gate effect and conductivity. We suspect that the devices were probably severely affected by the oxygen and water in the air due to the long fabrication and storage time before the electrical characterization. The influence of several device parameters on the gate effect has also been investigated. A smaller P3HT pillar diameter, a larger overlap of the gate dielectric and the gate electrode around the pillars, and a thinner gate dielectric all lead to better gate control. In the future, the most critical issue is to keep the organic/dielectric interface as intact as possible during the fabrication of organic field-effect transistors. In addition, regions with a large bulk current should be avoided as much as possible in the device design to enhance the gate control.
In Chapter 7 we provide a general discussion of the experimental and numerical results obtained in this thesis, and give an outlook for future research.
