A Study on In2Se3 Nanowires and Their Applications: PCRAM and UV-Visible Light Photodetector

DoctorRecently, nanowires from group IV, III-V, II-VI, and other materials have received considerable attention. They show an one-dimensional quantum confinement, a bandgap engineering by simply varying their radius below the Bohr radius and an exceptional surface quality for as-grown nanowires by techniques such as vapor-liquid-sold (VLS) technique. These desirable attributes are coveted for fabrication of nanoscale electronic and optoelectronic devices due to their excellent performances depending on surface quality. In addition, nanowire based devices can have smaller footprint (size, mass) than conventional thin film devices. In this study, In3Se3 nanowires have been synthesized by VLS technique which is one of bottom approaches. The bottom-up approach can provide high-quality surface and easy-control of material ratio of nanowires, compared to top-down approach. Two-terminal In2Se3 nanowire devices for phase change random access memory (PCRAM) and ultraviolet (UV)-visible light photodetector applications have been successfully fabricated. The decomposition of In2Se3 nanowire for PCRAM applications during current-driving operation and photoresponse characteristics of In2Se3 nanowire for photodetector applications with different phase structures over a wide range of wavelengths, have been demonstrated. First, the In2Se3 nanowires have been successfully synthesized by VLS technique. The synthesis mechanism of In2Se3 nanowires and effects of synthesis conditions are well described. The morphologies and chemical compositions of the as-synthesized In2Se3 nanowires are analyzed. In2Se3 nanowires present single crystalline nature with diameters in the range of 50-200 nm and lengths up to several tens of micrometers. They have a smooth surface and uniform thickness along the growth direction. The dependence of nanowire synthesis conditions such as reaction temperature, time and pressure are also investigated. Second, with In2Se3 nanowire devices for PCRAM applications, the resistive switching behavior has been investigated. A pulse of 200 ns was applied for the amorphization. A large increase in resistance (8.7 GΩ) was observed upon applying the pulse above a threshold value of 86 μA due to the amorphization by joule heating and a rapid quenching. A pulse of 100 μs was applied for the crystallization. In contrast to the amorphization, a significant decrease in resistance (85 kΩ) was observed upon applying the pulse above a threshold value of 0.51 nA for the crystallization. The morphology and composition analyses of the In2Se3 nanowire during the operation for PCRAM applications are important to understand the nanowire decomposition. The devices were subjected to thermal/electrical stress with current density and electric field during the reset operation at 0.24-0.38 MA/cm2 and 5.3-6.4 kV/cm, respectively. After multiple operation cycles, a change in morphology and composition of the In2Se3 nanowire was observed and led to the device failure. The electromigration causes the catastrophic failure by void formation. In the active region, In atoms migrate toward the cathode and Se atoms migrate toward the anode depending on their electronegativities in liquid phase under influence of thermal/electrical stress. Third, the photoresponse characteristics of the κ-phase and α-phase In2Se3 nanowire structures with In2Se3 nanowire devices for UV-visible light photodetector applications have been examined. The as-grown κ-phase In2Se3 nanowires by the VLS technique were phase-transformed to the α-phase nanowires by thermal annealing. The photoresponse performances of the κ-phase and α-phase In2Se3 nanowire photodetectors were characterized over a wide range of wavelengths (300-900 nm). The electrical conductivity and photoresponse characteristics were significantly enhanced in the α-phase due to smaller bandgap structure compared to the κ-phase nanowires. The spectral responsivities of the α-phase devices were 200 times larger than those of the κ-phase devices. The superior performance of the thermally phase-transformed In2Se3 nanowire devices offers an avenue to develop highly sensitive photodetector applications