Study On Organic Ferroelectric Field Effect Transistor For Memory Application

Ferroelectric was firstly discovered in 1920s, but it was used only in few areas before 1980s. Novel ferroelectric materials and fabrication technique made the ferro-electric properties widely used in various areas afterward, from fundamental memories to cosmos explorers. Organic ferroelectric materials, especially PVDF (Polyvinylidene Fluoride) and its derivative PVDF-TrFE [Poly(vinylidenefluoride-trifluoroethylene)], have various advantages including low-cost Jiigh physical and chemical stability and compatibility with solution processes. These advantages made them promising in large-scale and low-cost fabrication of organic ferroelectric memories. In various kinds of organic ferroelectric memories, organic ferroelectric transistors are the most attractive. Benefited form their transistor-like structure, organic ferroelectric transistors show ad-vantages like high on/off ratio, and non-destructive read-out. However, the low mobil-ity limits their memory performance in organic circuits.In this thesis, we realized organic ferroelectric transistors with high mobility. Fur-ther investigations showed that ferroelectric fluctuation influence the charge transport in the conducting channel of organic ferroelectric transistors, and there is a correla-tion between ferroelectric dielectric layer and contact resistance. Our results showed that buffering ferroelectric layer is effective in optimization of all kinds of ferroelec-tric transistors. These result will be a great help for the development of future organic memories. The achievements in our work including:1. We fabricated high quality PVDF-TrFE films with satisfying flatness and high fer-roelectric polarization. First, we studied the relation between film morphology and process parameters (solvent type, solution concentration and ambient humidity) during spin coating, and spin coated PVDF-TrFE with good flatness. Second, we studied the annealing process of PVDF-TrFE, and successfully reduced the surface roughness (RMS; Root Mean Square) of the annealed PVDF-TrFE thin film to 2.36 nm. By applying high molecular weight Polymethylmethacrylate (PMMA) as a buffering layer on the annealed PVDF-TrFE thin film, we further modified the organic ferroelectric layer. Meanwhile, the polarization of the buffered layer slightly improved after the spin-coating of an ultra-thin PMMA layer. It indicated that the buffering is not a kind of screening effect. The high quality PVDF-TrFE film is also an important part of high performance organic ferroelectric transistor.2. We successfully realized high mobility organic ferroelectric field effect transistors with the help of buffered PVDF-TrFE thin film and C8-BTBT (2,7-dioctyl [1]ben-zothieno [3,2-b][1]benzothiophene) which is a kind of novel organic semiconductor material. The mobility of our organic ferroelectric transistors is 3.46 cm2V-1s-1 in average while the highest value is higher than 4 cm2V-1s-1. It was several times higher than those in previous reports, exceeded the record value in organic ferro-electric transistors. Meanwhile, the improvement of mobility increased the mem-ory performances of the ferroelectric field effect transistors, including data writing speed (faster than 30 ms), and data retention (<10005, on/off ratio>104).3. We confirmed the correlation between ferroelectric fluctuation and carrier mobil-ity in organic ferroelectric transistors. Analysis of the carrier transport in organic ferroelectric transistor showed that the improvement of mobility came from the sup-pression of the ferroelectric fluctuation at the organic semiconductor/ferroelectric insulator interface. At the same time, we find the suppression effect came from the molecular interaction between PMMA and PVDF-TrFE. These results can help us further improve the performance of organic ferroelectric transistors.4. We analyzed the correlation between ferroelectric dielectric layer and carrier injec-tion in organic ferroelectric transistors. We confirmed that the ferroelectric dielec-tric layer influence the energy distribution of carriers in the organic semiconductor in accessing region. By buffering the ferroelectric fluctuation, the contact resis-tance in organic ferroelectric transistor was reduced from 260 kQ · cm to 55 kΩ · cm. The optimization in contact resistance can help us reduce the power consumption in organic ferroelectric transistors.