Steep-slope NbO2-based threshold switch enabled by pulsed-laser-induced phase transformation

Abstract

The correlated oxides, including NbO2, show promising potential for threshold switching device in that it prevents sneak current in 3D cross-point array architecture and overcomes a limitation of subthreshold swing (SS < 60mV/dec) owing to high theoretical Ion/Io ratio (107), fast switching speed (22 ns) and thermal stability at device-working temperature[1􀀀2]. However, to realize voltage-induced insulatorto-metal transition with a NbO2, high-temperature processes, such as growth or post-annealing at high temperature, were required for crystallization (>573K) to observe voltage-induced insulator-to-metal transition (IMT), this high-temperature processing needs to be avoided due to degradation by unwanted layers and high energy consumption[3]. Moreover, due to the multivalency of Nb cation, there was very limited oxygen partial pressure (pO2) window to stabilized Nb4+ valence states during the NbO2 lm growth[3]. In contrast, pulsed laser annealing, which generates temperature eld at conned area, is powerful tool for rapid crystal growth and a reduction reaction is also possible by changing process environment. In this research, we report new strategy for fabricating high performance threshold switch with correlated oxides NbO2 using pulsed laser annealing, showing steep voltage-induced insulator-to-metal transition. As the number of pulses and process environment were accurately controlled during laser annealing, the as-grown Nb2O5 lms were transformed into NbO2 by forming stable Nb4+ valence states. The fabricated selector device using laser-annealed NbO2 lms shows low o-current (665 nA) and high Ion=Ioff ratio (>230) without high temperature process. A comprehensive study with transmission electron microscopy and synchrotron X-ray photoemission spectroscopy reveals that the crystallization combined with oxygen loss stimulates the formation of stable NbO2 crystallites in the lms during pulsed laser annealing. Our approach provides novel solution for facile fabrication of high-performance correlated oxide-based selector device with steep transition.