Enhancement of Long-Term Memory Characteristics of Ion-Gated IGZO Synaptic Transistors Using UVO Interface Treatment

Neuromorphic architecture mimicking the signal transmission method of biological neurons has attracted much attention for next-generation computing with the advantages of fast response speed and high energy efficiency by parallel weight update processing. Especially, synaptic transistors have been extensively studied since they perform both calculation and memory functions and easily control channel conductance without channel damage by electrical pulses from a separated gate terminal. Among various types of transistors, electrolyte-gated transistors (EGTs) using electrolytes (e.g., solid polymers, gels, ionic liquids) as gate-insulator are a promising candidate for synaptic transistors due to the low operation voltage by electrical double layer (EDL) and high applicability to flexible electronic devices. However, conventional EGTs show volatile and irregular memory characteristics because of the rapid self-dissipation of ions in the electrolyte. Here, we have demonstrated the WiBS/polymer electrolyte-gated synaptic transistor (WEST) with ultra-violet ozone (UVO) surface treatment which highly improves the long-term memory characteristics and other synaptic functions as well. Super-concentrated WiBS electrolyte composed of excess lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium trifluoromethanesulfonate (LiOTf) in water enables the WEST to conduct synaptic functions in a voltage range of 1.23 V or higher, which is the electrochemical stability window of water. Also, the WiBS electrolyte can take advantage of the characteristics of pure water: high dielectric constant and biocompatibility. UVO treatment is one of the effective post-treatment that has been widely used to surface modification by controlling the oxygen vacancy concentration on the surface in various fields. Ultra-violet (UV) in the two different wavelengths of 185 and 254 nm decomposes metal-oxide bonds of InGaZnO (IGZO) thin film and the bonds are broken more intensively by the supplied ozone, which generates reactive oxygen radicals and they trigger the excessive ion bombardment process. We performed XPS analysis and measured hysteresis characteristics to calculate the memory window to verify the cause of the improvement in long-term memory performance of the WEST. As a result, the oxygen vacancy ratios of the UV-only and UVO-treated WEST calculated by XPS data are 0.35 and 0.44, respectively, 1.5 times and 2 times higher than the non-treated WEST, respectively. These increased oxygen vacancies act as trap sites at the interface, trapping lithium (Li) ions in the electrolyte and suppressing the self-diffusion of the Li ions. Also, the UVO-treated WEST performs long-lasting nonvolatile memory characteristics longer than 10,000 s, which is 1186 times higher than the non-treated WEST. Residual Li ions are detected only in the UVO-treated WEST and appeared as interstitial defects and Li-O bond peaks at 54.9 and 55.7 eV, respectively. After the UVO treatment, both the nonlinearity and asymmetricity of synaptic weight update characteristics are greatly improved from 5.91 to 0.32 for potentiation and from -6.11 to -0.55 for depression. By using the surface post-treatment, the WEST has shown great suitability as a neuromorphic device capable of long-lasting memory functions and elaborate weight controllability comparable to real human brains.