Taek Joon Lee

Electrically bistable nonvolatile switching devices fabricated with a high performance polyimide bearing diphenylcarbamyl moieties

In this study, novel nonvolatile memory devices, based on a high performance polyimide, poly(3,3′-bis(diphenylcarbamyloxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-DPC PI), were fabricated with a simple conventional solution coating process. The devices were found to exhibit programmable, rewritable nonvolatile memory characteristics with a high ON/OFF current ratio of up to 109, a long retention time in both ON and OFF states, and low power consumption. Moreover, the active 6F-HAB-DPC PI layer is thermally and dimensionally stable and thus hybridization with a complementary metal-oxide-semiconductor platform is feasible. The advantageous properties and ease of fabrication of the 6F-HAB-DPC PI based devices open up the possibility of the mass production of high performance digital nonvolatile polymer memory devices at low cost.

Electrical Memory Characteristics of a Nondoped π-Conjugated Polymer Bearing Carbazole Moieties

Poly[bis(9H-carbazole-9-ethyl)dipropargylmalonate] (PCzDPM) is a novel pi-conjugated polymer bearing carbazole moieties that has been synthesized by polymerization of bis(9H-carbazole-9-ethyl)dipropargylmalonate with the aid of molybdenum chloride solution as the catalyst. This polymer is thermally stable up to 255 degrees C under a nitrogen atmosphere and 230 degrees C in air ambient; its glass-transition temperature is 147 or 128 degrees C, depending on the polymer chain conformation (helical or planar structure). The charge-transport characteristics of PCzDPM in nanometer-scaled thin films were studied as a function of temperature and film thickness. PCzDPM films with a thickness of 15-30 nm were found to exhibit very stable dynamic random access memory (DRAM) characteristics without polarity. Furthermore, the polymer films retain DRAM characteristics up to 180 degrees C. The ON-state current is dominated by Ohmic conduction, and the OFF-state current appears to undergo a transition from Ohmic to space-charge-limited conduction with a shallow-trap distribution. The ON/OFF switching of the devices is mainly governed by filament formation. The filament formation mechanism for the switching process is supported by the metallic properties of the PCzDPM film, which result in the temperature dependence of the ON-state current. In addition, the structure of this pi-conjugated polymer was found to vary with its thermal history; this change in structure can affect filament formation in the polymer film.

Programmable digital nonvolatile memory behaviors of donor-acceptor polyimides bearing triphenylamine derivatives: Effects of substituents

Two aromatic polyimides bearing triphenylamine (TPA) derivatives with reasonably high molecular weights were synthesized: poly(N-(2,4,6-trimethylphenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-Me3 PI) and poly(N-(4-dimethylaminophenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-NMe2 PI). These polymers were thermally and dimensionally very stable, providing high-quality nanoscale thin films using a conventional solution coating process. The film densities, optical properties, and electrochemical properties were determined. The polymers displayed a different nonvolatile memory behavior that depended on the substituents of the TPA unit. The 6F-TPA-Me3 PI film showed a unipolar write-once-read-many-times (WORM) memory behavior, whereas the 6F-TPA-NMe2 PI film revealed unipolar and bipolar switching memory behavior. All PI films displayed excellent retention in both the OFF- and ON-states, even under ambient conditions. The ON/OFF current ratio was high, up to 108–109. All memory behaviors were governed by a mechanism that involved trap-limited space charge limited conduction and local filament formation. The memory characteristics may originate from the electron-donating TPA and substituents and from the electron-accepting hexafluoroisopropylidenyl and imide units in the polymer backbone, which acted as effective charge-trapping sites. The film density was found to significantly influence the memory behavior. This study demonstrated that the thermally and dimensionally stable 6F-TPA-Me3 and 6F-TPA-NMe2 PIs are suitable active materials for the low-cost mass production of high-performance programmable memory devices that can be operated with very low power consumption. Moreover, the memory mode and its polarity may be tuned by changing the substituent on the TPA unit.

Structural analysis of thin films of novel polynorbornene derivatives by grazing incidence X-ray scattering and specular X-ray reflectivity along with ellipsometry

In the present study, structural analyses using synchrotron grazing incidence X-ray scattering, specular reflectivity and ellipsometry were performed on thin films of two novel polynorbornene derivatives, chiral poly(norbornene acid methyl ester) and racemic poly(norbornene acid n-butyl ester), which are potential low dielectric constant materials for advanced microelectronic and display applications. These analyses provided important information on the structure, electron density gradient across the film thickness, chain orientation, refractive index and thermal expansion characteristics of the polymers in substrate-supported thin films. The structural characteristics and properties of the thin films depended on the tacticity of the polymer chain and were further influenced by the film thickness and thermal annealing history.