A Threshold Switching Selector Based on Anti-Ferroelectric Tunnel Junction (AFTJ) in Ultra-Thin ZrO₂ for Resistive Memory Applications

The semiconductor technology currently faces explosive demand due to advances in artificial intelligence (AI), driving the need for non-volatile memory that offers both large capacity and fast processing speeds. This demand represents a new area that existing mass-produced memory solutions cannot adequately address. Resistive memory based on a cross-point structure is a potential candidate that meets these requirements, but it necessitates selective reading of specific devices within the array without disturbing other cells. Incorporating a selector device in series with each cross-point memory device can achieve the desired operational selectivity, and many studies have explored new materials and mechanisms for this purpose. However, even when the electrical requirements for the selector are met, some materials fail to provide back-end-of-line (BEOL) compatibility or silicon compatibility, which poses significant challenges for process integration.
One class of materials that shows promise to meet these needs for selector device application are anti-ferroelectric (AFE) type materials. Non-linear current characteristics, which is a requirement of the selector device, are implemented through rapid current switching in a tunnel junction architecture. These operating characteristics are derived from the AFE characteristics represented by double polarization hysteresis loop. ZrO₂, one of the well-known dielectrics, exhibits these AFE characteristics with fast switching and high fatigue resistance at ultra-thin thicknesses and is BEOL-compatible and is the subject of our current work.
Zirconia tunnel junctions (AFE-TJ) with various physical thicknesses are fabricated at different atomic layer deposition (ALD) temperatures with different, asymmetric electrodes (TiN, Mo and Pt). The double polarization hysteresis and the current-voltage response of the AFE-TJs are recorded. Different combinations of electrodes are tested to optimize the double polarization hysteresis. This double polarization hysteresis, in turn, changes the tunneling current by lowering the barrier of the tunnel junction. An important performance parameter of a selector device is the tunneling current (I_on/I_off) ratio. To improve the performance of the AFTJ device, technology computer-aided design (TCAD) tunneling current simulations with metal-ZrO₂-metal structure are conducted. To obtain high I_on, the work functions of the top electrodes (TE) and the bottom electrodes (BE) are engineered. The effect of interlayers (IL) on the metal-ZrO₂ interface is also examined using TCAD simulations and by optimizing the electric field applied to the bandgap, dielectric constant and thickness of the IL, to inform design and experimental demonstration of high performance AFE tunnel junctions.