Woo-Sik Nam

Multilevel nonvolatile small-molecule memory cell embedded with Ni nanocrystals surrounded by a NiO tunneling barrier.

Four-level nonvolatile small-molecule 4F(2) memory cells were developed with a sandwiched device structure consisting of an upper Al electrode, upper small-molecule layer (Alq(3), aluminum tris(8-hydroxyquinoline)), Ni nanocrystals surrounded by NiO tunneling barrier, lower small-molecule layer, and bottom Al electrode. In particular, an in situ O(2)-plasma oxidation process following Ni evaporation was developed to produce uniformly stable 10 nm Ni nanocrystals surrounded by a NiO tunneling barrier embedded in the small-molecule layer. They presented a memory margin (I(on)/I(off) ratio) of approximately 1 x 10(3), a retention time of more than 10(5) s, an endurance of more than 5 x 10(2) erase-and-program cycles, and multilevel cell (MLC) operation, being a terabit nonvolatile memory-cell. A vertically double-stacked 4F(2) multilevel nonvolatile memory cell was also developed, showing a memory margin of approximately 1 x 10(3) in both the top and bottom memory cells and eight-level cell operation.

Hysteretic characteristics of low-field microwave absorption of a Co thin film

We have investigated the spin dynamics of a sputtered Co thin film using our broadband ferromagnetic resonance (FMR) spectrometer. From FMR spectra taken at frequencies of 4–20 GHz, we found that our Co film has a g-factor of 2.25 and a Gilbert damping factor of 4.5×108 s−1, indicating an enhanced spin-orbit interaction compared to bulk material or epitaxial films. Besides the normal FMR mode in the saturated state, we also observed the evolution of the low-field hysteretic behavior in the unsaturated state, which affects the FMR mode as the excitation frequency is lowered from 5.000 to 1.636 GHz. We found that the microwave absorption process persists in the unsaturated state for frequencies higher than 1.868 GHz such that the absorption minima occur at −12 Oe on a down-field sweep and at +12 Oe on a up-field sweep, respectively.

Erratum: “Small-Molecule Nonvolatile Memory Cells Embedded with Ti Nanocrystals Surrounded by TiO2 Tunneling Barrier”

We developed cross-bar 4F2 small-molecule nonvolatile memory-cells embedded with Ti nanocrystals surrounded by a TiO2 tunneling barrier. The Ti nanocrystals size and distribution were primarily determined by the Ti layer evaporation rate followed by in-situ O2-plasma oxidation; i.e., the size increased with the evaporation rate and the size uniformity of Ti nanocrystals became better as the evaporation rate increased from 0.1–2.0 A/s, then became worse as the evaporation rate increased above 2.0 A/s. At a specific evaporation rate, small-molecule memory-cells embedded with Ti nanocrystals demonstrated typical nonvolatile memory characteristics, such as symmetrical current vs voltage (I–V) characteristics, Ion/Ioff ratio of 4.2×102, stable retention time, and program/erase cycles.

Small Molecular Organic Nonvolatile Memory Fabricated with Ni Nanocrystals Embedded in Alq 3

Recently, organic nonvolatile memory has attracted much interest as a candidate device for next generation nonvolatile memory because of its simple process, small device area, and high speed. To investigate electrical characteristics of small molecular organic nonvolatile memory with Ni as a middle metal layer, we developed a small molecular organic nonvolatile memory with the device structure of Aluminum tris (8-hydroxyquinolate) (Al/Alq 3 ), Ni nanocrystals, and Alq 3 /Al. A high vacuum thermal deposition method was used for the device fabrication. It is critical that the fabrication process condition for Ni nanocrystals be optimized, including ∼100 A thickness, 0.1 A/sec-evaporation rate, and in-situ plasma oxidation for effective oxidation. The reasons we chose Ni for the middle metal layer are that Ni has a smaller grain boundary, which is beneficial for scaling down and has a larger work function (∼5.15 eV) that can make a deep quantum well in an energy band diagram, compared with that of Al. Our device showed an electrical nonvolatile memory behavior including V th of ∼2 V, V w (write) of ∼3.5 V, negative differential region (NDR) of 3.5∼7 V, V e (erase) of 8 V, and symmetrical electrical behavior at reverse bias. In addition, an interesting behavior of electrical properties was that, although retention and endurance characteristics were similar to the Al device, the I on /I off ratio was greater than 10 4 at V r (read) of 1 V. This value of the Ni device was higher than 10 2 compared to that of the Al device. Also, small molecular organic nonvolatile memory with a Ni middle layer with α-NPD at same fabrication condition showed more unstable characteristics than Alq 3 . We can speculate that there is a relationship in fabrication condition between the middle metal material and the organic material. Finally, we conclude that our device with a Ni nanocrystals middle layer is more reliable and useful for small molecular organic nonvolatile memory.