University of Twente Student Theses


Fabrication of a magnetic O-FET based on a semiconducting polymer for spin accumulation measurements

Wijngaarden, Frans van (2014) Fabrication of a magnetic O-FET based on a semiconducting polymer for spin accumulation measurements.

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Abstract:Presented in this work are the results of a masters assignment at the NanoElectronics research chair of the University of Twente, the Netherlands. The goal of this assignment was to develop a magnetic organic field-effect transistor, which can be used for spin accumulation measurements. The organic layer is the polymer poly(3-hexylthiophene). In such a device, the injection of a spin polarised current is achieved by using a ferromagnetic material for both the source and the drain contacts. Both a back-gated and a top-gated geometry was developed. For a back-gated magnetic O-FET device, the fabrication process is short and uncomplicated. However, this geometry has disadvantages when compared to a top-gated magnetic O-FET device. Experiments have been designed and realised to develop the fabrication recipe, focussing on obtaining high-quality magnetic electrodes without ridges at the edge of the electrodes. This is achieved by using an image reversal photoresist with a pronounced undercut, in combination with lift-off photolithography. Furthermore, the process of spin-coating the organic polymer was tested. For the spin-coating process, different organic solvents were tested to ensure no negative reactions occurred with the magnetic injection electrodes. As a result of these tests, 1,2- dichlorobenzene was selected as the solvent for poly(3-hexylthiophene). The spin-coating process was calibrated, resulting in a range of available thicknesses of approximately 10-40 nm, selectable by adjusting the spin-coating rotational speed. To achieve high-quality magnetic electrodes, Ti35 image reversal photoresist was used to obtain a pronounced undercut. The specific recipe for applying this photoresist was tested, and the resulting photoresist profiles were imaged using scanning electron microscopy. These measurements show that a good undercut is achieved. Both back-gated and top-gated O-FET devices have been fabricated, and preliminary electrical measurements were performed on these devices. Difficulties with etching of the magnetic electrodes were traced to the organic solvent used for lift-off, and were solved by changing the organic solvent to a more suitable one. A combination of AFM and dark field microscopy images indicate that this mostly prevents the etching of magnetic electrodes. AFM measurements also indicate that lift-off is achieved without ridges on the edge of the electrodes, although some surface roughness is still observed in between the electrodes. In this work, major progress is reported on the fabrication recipe for magnetic O-FET devices. The optimisation of a method to obtain high-quality magnetic electrodes is a crucial step, since any imperfections in these electrodes will negatively influence the spin polarisation of the injected current. However, the fabrication recipe is not yet completely tested and optimised. Furthermore, no spin accumulation measurements have yet been done. In addition to many small improvements, three critical steps remain to be optimised in the fabrication recipe. The first step is finding a solution for the etching that is observed on the magnetic electrodes. Although the major etching problem is solved, minor etching pits are still visible. The second step is the organic etching step, using reactive ion beam etching. The third step is the integration of electron-beam lithography into the recipe, since it is suspected that a four-terminal device may be necessary for spin accumulation measurements. In such a device, two of the four terminals need to be closer together than is achievable using photolithography.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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