High-Resistance-State Tunneling in 25 nm TiOx/Y-Doped HfO2/Pt Nanocrossbar Ferroelectric Tunnel Junctions

Abstract

We report nanocrossbar-type ferroelectric tunnel junctions (FTJs) with a Ti/TiO_x/7% yttrium-doped HfO₂ (YHO₇)/Pt structure integrated on thermally oxidized Si substrates, which exhibit clear direct tunneling conduction even in the high-resistance state (HRS) and a tunneling electroresistance ratio exceeding 10³. The nanocrossbar FTJs were fabricated using a double-exposure electron-beam lithography (EBL) process with lateral dimensions scaled down to 25 nm. The temperature dependence of the TER effect measurements at 9 and 300 K confirms that both low-and high-resistance states are dominated by the direct tunneling conduction. A maximum TER ratio of 2.2 × 10³ was obtained in a 3 nm-thick YHO₇ nanocrossbar FTJ with an effective area of 26 × 24 nm². The FTJ area was reduced from 42,000 to 255 nm², and the scaling behavior of the TER effect in 3 nm-thick YHO₇ devices closely resemble that in 2 nm-thick devices. The OFF-state current decreased with a slope of 1.1 between 42,000 and 2,600 nm², followed by a steeper reduction below 2,600 nm², whereas the ON-state current decreased more gradually with a slope of 0.30. These contrasting area dependences are attributed to the suppression of leakage pathways along grain boundaries in the OFF state and uniformly aligned remanent polarization in a small number of grains in the ON state. The demonstrated nanocrossbar FTJs highlight a promising route toward high-density, energy-efficient, and CMOS-compatible integration of ferroelectric memory.