Commutació resistiva en Hf0.5Zr0.5O2 unions túnel ferroelèctriques
- Autores: Milena Cervo Sulzbach
- Directores de la Tesis: Josep Fontcuberta i Griñó (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2021
- Idioma: catalán
- Tribunal Calificador de la Tesis: Gustau Catalán Bernabé (presid.), Ana M. Palau Masoliver (secret.), Christian Rinaldi (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
The requirement for high-performance data storage and computing systems in the Internet of Things (IoT) era reaches the limits of the current technology. DRAM and NAND flash memories show significant drawbacks as data volatility and limitations of speed. On the other hand, Von Neumann bottleneck limits computing performance by imposing a physical limitation for communication between parts of the computer. New memory technologies, like resistive random-access memory (ReRAM), have been proposed as alternative devices that combine high performance, low price, and high density.
One promising type of ReRAM is a ferroelectric tunnel junction (FTJ), which is composed of an ultrathin layer of ferroelectric material sandwiched between two metallic electrodes. The ferroelectric polarization (P) direction modulates the barrier properties at the interface with the electrodes, changing electrons' conductivity. A high and low resistance states (HRS and LRS, respectively) can be stabilized in a device by reversing P. Therefore, information can be written by applying an external voltage pulse to polarize the sample in one particular direction and store in its resistance. Nevertheless, other electric field-driven effects can also cause resistive switching in FTJ. This thesis aims to explore the resistive switching phenomena in Hf0.5Zr0.5O2 ferroelectric tunnel junctions that could be used in highly efficient data storage.
HfO2-based oxides have been explored as ReRAM elements due to their resistance change caused by redox reactions. However, the discovery of ferroelectricity in doped-HfO2 opens doors to use polarization reversal as a phenomenon to control the resistance and, therefore, to writing information. Here, epitaxial HZO films with a thickness smaller than 5 nm are used. Electrical and structural analyses have allowed identifying the coexistence of genuine ferroelectric switching and ionic-like motion as mechanisms to induce resistance change in the same junction. It was found that ionic-motion-driven resistive switching takes advantage of incoherent grain boundaries between orthorhombic (ferroelectric) phase and monoclinic (paraelectric) phase grains. By engineering the film's microstructure, using a substrate with a different in-plane lattice parameter, the ferroelectric switching was optimized, and the ionic motion was suppressed. Also, sealing the grain boundaries with an extra dielectric layer increased the writing voltage window for purely ferroelectric switching and significantly impacted device performance.
Ultrathin (2 nm) HZO films grown on scandate substrates were used to demonstrate robust memristive properties associated with polarization reversal. Reproducible potentiation/depression cycles and spike-timing-dependent plasticity (STDP) measurements were demonstrated. These results, combined with HZO films' compatibility with current CMOS technology, environmentally friendly production, and good performance, show HZO tunnel junctions are feasible alternatives for application in non-volatile memories. Also, memristive properties of modulation of conduction show they can be used in neuromorphic inspired devices.
