Recently, organic ferroelectric memory thin-film transistors have been increasing more interest to use in flexible devices. Due to the rise of the flexible substrate that has been limited to the use of inorganic ferroelectric material at low temperature. Therefore, a solution process of organic materials in a non-volatile memory device has intensively studied as one of the exemplary organic ferroelectric material, a ferroelectric polymer such as P(VDF-TrFE) copolymer of fluorine -containing organic ferroelectric PVDF(poly vinylidene fluoride) and TrFE(trifluoroethylene).
In this work, MFS & MFIS capacitor were fabricated by using the organic ferroelectric material of P(VDF-TrFE) and OFMT(organic ferroelectric memory thin-film transistor) was fabricated on the glass substrate using a material of the organic semiconductor F8T2 (poly(9,9- dioctylfluorene-co-bithiophene), a low temperature process was confirmed the applicability of the ferroelectric memory device using a possible low-cost solution process.
The detailed research items are fabrication technology and investigation of 1) MFS (metal-ferroelectric-semiconductor) capacitors using organic ferroelectric P(VDF-TrFE) copolymer film, 2) MIS (metal-insulator-semiconductor) capacitors using high dielectric Al2O3 (aluminum oxide) insulating material, and 3) MFIS (metal-ferroelectric -insulator-semiconductor) capacitors using ferroelectric P(VDF-TrFE) copolymer film and high dielectric Al2O3 buffer layer, and 4) fabrication and characteristics of Organic ferroelectric memory thin-film transistors.
Firstly, MFS capacitors by using rapid thermal annealed P(VDF-TrFE)/Si(100) structures were successfully fabricated and demonstrated nonvolatile memory operations of the MFS capacitors. The C-V characteristics of MFS capacitors showed a hysteresis loop due to the ferroelectric nature of the P(VDF-TrFE) thin film. The dielectric constant of the P(VDF-TrFE) film calculated from the capacitance in the accumulation region in the capacitance?voltage (C-V) curve was about 9.5. The leakage current density of the film at room temperature was on the order of 9.8×10-7 A/㎠ at the applied electric field of 1 MV/㎝. The typical measured remnant polarization (Pr) and coercive field (Ec) were estimated to be approximately 7.5 μC/㎠ and 750 ㎸/㎝, respectively. The ferroelectric capacitors showed 30% polarization degradation of the initial polarization up to about 1010 switching cycles when subjected to symmetric bipolar voltage pulse in the 500 ㎑.
Secondly, MIS capacitors by using Al2O3/Si (100) structure were fabricated. Al2O3 thin films were deposited on Si(100) by RPALD (remote plasma atomic layer deposition) technique using TMA (trimethyl aluminum) precursor and oxygen radicals. The oxygen radicals were produced by dissociating oxygen gas in remote plasma discharge region. High-purity Ar(99.999 %) was used as a purging gas. To complete the process within a self-limited region RPALD pulse time was maintained at 2/4/4/4 (TMA/Ar/O2/Ar). Because growth rate saturated at 4s oxygen radical pulse time and 2s TMA pulse time.
The C-V properties of MIS capacitors using high dielectric Al2O3 thin films showed no hysteresis and good interface properties. The dielectric constant of the Al2O3 film was about 7. The gate leakage current density of the Al2O3 film was the order of low 1×10-10 A/㎠ at the applied electric field of ?2 MV/㎝.
Thirdly, Ferroelectric P(VDF-TrFE) films on Si(100) wafer with a Al2O3 buffer layer have been fabricated to form a MFIS structure for nonvolatile memory operations. Low-temperature deposited Al2O3 films were deposited on Si(100) substrates by RPALD technique using alternative TMA precursor and oxygen radicals. The ferroelectric P(VDF-TrFE) films were deposited on Al2O3/Si(100) by using spin coating Process. A 2~3 wt% diluted solution of purified vinylidenefluoride?trifluoroethylene(VF2:TrFE = 70:30) in a DMF (dimethyl-formamide) solvent were prepared an deposited on Al2O3/Si(100) substrates at a spin rate of 2,500 ~ 3,500 rpm. After deposition of the P(VDF-TrFE) film, annealing was performed for 60 min at 150 ℃ in a vacuum ambient.
The C-V characteristics of Al/P(VDF-TrFE)/Al2O3/Si(100) structures showed clockwise hysteresis, and the memory window width was about 1.4 V. The clockwise hysteresis means that ferroelectric hysteresis controls the Si surface potential, and suggests that this could be applied to NVM device applications. The typical dielectric constant value of P(VDF-TrFE) film in the MFIS device was about 9. The gate leakage current density of the MFIS capacitor was 3×10-9 A/㎠ at the electric field of 1 MV/cm. The typical measured remnant polarization (2Pr) and coercive filed (EC) values were about 5.2 μC/㎠ and 750 kV/㎝, respectively.
Finally, The organic ferroelectric non-volatile thin-film transistors were fabricated on glass substrate for the organic ferroelectric materials of P(VDF-TrFE) as gate insulation layers, and for the organic semiconductor materials of F8T2 as semiconductor channel layers. The organic semiconductor layers were annealed at 140 ℃ for 60 min and a 3 ~ 5 wt% diluted solution of P(VDF-TrFE) in a DMF solvent were deposited on organic semiconductor layer by spin coating and then annealed at 100 ~ 150 ℃ for 60 min. The carrier mobility, on/off ratio, and subthreshold swing of the memory transistors fabricated on glass showed 4.53×10-2 ㎠V-1s-1, 104, and 2 V/decade, respectively. Results confirm the change of the drain current over time, on/off ratio after the lapse of 1000 seconds was 40. From the result, the organic ferroelectric memory transistors (OFMTs) fabricated on a glass substrate, it was confirmed that sufficient applicability as non-volatile memory devices.