Dissertation committee:
Athanasios Dimoulas, Research Director, Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research Demokritos,
Spiros Gardelis, Associate Professor, Physics Department, National and Kapodistrian University of Athens,
Vlassios Likodimos, Associate Professor, Physics Department, National and Kapodistrian University of Athens,
Kosmas L. Tsakmakidis, Assistant Professor, Physics Department, National and Kapodistrian University of Athens,
Anthony Papathanassiou, Assistant Professor, Physics Department, National and Kapodistrian University of Athens,
Dimitrios Tsoukalas, Professor, Department of Physics, National Technical University of Athens,
Evangelos K. Evangelou, Associate Professor, Department of Physics, University of Ioannina.
Summary:
The synapses of neurons in brain are vital for the processing, learning, and memorization of information. The ability of artificial electronic synapses to simulate the function of biological synapses, makes them promising in the field of neuromorphic circuits having high performance, energy efficiency, and small dimensions. Neuromorphic circuits are capable of efficiently managing the volume of data and overcoming the limitations of conventional computer architectures (von Neumann bottleneck) to keep up with today's requirements. Dielectric material HfO2, has been in industrial production since 2007 as a gate dielectric for CMOS transistor technology. Among other elements, ferroelectric tunnel junctions (FTJs) based on this dielectric material, are leading candidates for integration into neuromorphic circuits. HfO2 is ferroelectric when crystallized in the orthorhombic non-centrosymmetric phase Pca21, while stabilizes the ferroelectric phase or transforms into an antiferroelectric (tetragonal structure) depending on its stoichiometry when it is doped with Zr. The ability of ferroelectric tunnel junctions based on stoichiometric Hf0.5Zr0.5O2, to function as non-volatile memories, combined with the compatibility of HfO2 and ZrO2 with the Silicon technology, opens up avenues for the construction of circuits suitable for use in neuromorphic computers and other applications.
As part of this doctoral thesis, the conditions for the synthesis of thin films (<5nm) based on the ferroelectric Hf1-xZrxO2 (HZO) were investigated for optimization. Metal-ferroelectric-semiconductor (MFS) capacitor structures were fabricated using various thicknesses of the ferroelectric HZO. The fabrication of the films was carried out on different substrates such as Ge, n-doped SrTiO3, and differently thick (10nm, 20nm) n-doped SrTiO3 (STO) on single-crystal Si. On top of the ferroelectric layer, metal deposition with TiN or W was performed. The samples were prepared using the molecular beam epitaxy assisted by plasma atomic oxygen/nitrogen deposition in an ultra-high-vacuum chamber in the Molecular Epitaxy and Surface Science Laboratory of the NCSR "Demokritos". The prevalence of the orthorhombic Pca21 phase, essential for stabilizing ferroelectricity, was confirmed by XRD. Additionally, clean interfaces between HZO and Ge/STO were observed. The stoichiometry of Ti, the Zr/Hf ratio of ~1 in HZO (stoichiometric), and the thickness of the HZO films on STO substrates on single-crystal Si were verified.
In the following, the effect of thickness scaling of Hf0.5Zr0.5O2 (HZO) films on the ferroelectric characteristics was studied. It was proven that thinner films exhibit reduced remanent polarization, and in most cases, in films thinner than 5nm ferroelectricity is not feasible due to a strong depolarization field within the ferroelectric material. Using the Landau Ginzburg Devonshire (LGD) theory, it was demonstrated that for a 5nm thickness of HZO, due to the depolarization field, the system behaves as a first-order ferroelectric material above the critical temperature Tc, where stable paraelectric and metastable ferroelectric states coexist. The effect of this field was studied as a function of film thickness and the density of interfacial traps both experimentally and theoretically. Within the Landau Ginzburg Devonshire (LGD) theory, it was shown that as the thickness of the ferroelectric increases, the induced depolarization field decreases, resulting in well-defined hysteresis loops of the remanent polarization as a function of applied voltage. Finally, it was experimentally confirmed that the injection through the "wake-up" method and trapping of charges into interfacial states that exist due to imperfections, reduces the intensity of the depolarization field, converting metastable ferroelectric states into stable ones. It was established that a high density of energy states (Dit = 1.4×10^13 eV^-1cm^-2) is necessary for stabilizing ferroelectricity.
The next step involved studying the behavior of structures as ferroelectric tunnel junctions. It was observed that for both polarization states, the leakage current ratio (TER) was approximately 2-3 for structures with metallic TiN as top electrode, while the maximum TER was around 4 for structures with W as top electrode. In addition to the two polarization states, intermediate states due to partial polarization switching were observed, from which the devices exhibited memristive behavior. Leakage current measurements at different temperatures were conducted at devices with W top electrode, and the change of the leakage current was attributed to a Schottky barrier potential within the semiconductor whose height was affected by the direction of the ferroelectric polarization of the HZO. A comparison was made between samples with a W top electrode on Nb:SrTiO3 (100) substrates and samples on n-type epitaxial SrTiO3-δ (STO) substrates to study the influence of the substrate. The change in current density for the epitaxial STO substrates for the two polarization states was approximately 3. XRD measurements confirmed the epitaxial growth of HZO on epitaxial STO, showing a 4/3 lattice plane match between HZO and 5/4 lattice planes of STO (domain matching epitaxy). Furthermore, the stability of multiple resistance states over time was studied, revealing that they remain unchanged for a long period (10^4 - 10^5 sec) both on
the bulk Nb:SrTiO3 substrate and the thin epitaxial SrTiO3-δ (STO) on Si, except for the high-resistance states in the latter. Those states showed an unexpected increase in resistance, attributed to the migration of oxygen ions from the ferroelectric into the semiconductor, resulting in an increased width of the Schottky barrier potential at the interface.
This dissertation also describes the study of the behavior of W/HZO/SrTiO3/Si structures as memristors under different sequences of voltage pulses (varying amplitude, varying width, and identical pulses). Between the pulses, the leakage current through the ferroelectric films was measured. The study effectively demonstrated a gradual change in the leakage current between multiple resistance states (20 ~ 4 bits) for series of variable voltage pulses, confirming the ratio of currents (TER ~ 3-4) mentioned with a bias voltage of less than 2V. This makes these structures suitable for low-power operation circuits. This research goes on to describe the behavior of these structures under the influence of sequences of voltage pulses with varying pulse widths (50 ns – 1 ms). It is observed that the required voltage to transition from low-resistance state (LRS) to high-resistance state (HRS) increases as the width of the applied pulses decreases. This voltage-time trade-off is described by Mertz's law and is in agreement with the P-V hysteresis loops obtained from electrical measurements. Furthermore, the samples were subjected to a sequence of voltage pulses with varying pulse widths (1 μs – 1 ms) to access more than 20 intermediate resistance states (4-bit memory) and (50 ns – 1 ms) to improve symmetry in transitions between the minimum and maximum resistance states. These measurements confirm the hypothesis of the movement of oxygen ions mentioned previously.
In the end, the behavior of W/HZO/SrTiO3/Si structures under a sequence of identical voltage pulses was studied, with an emphasis on the time intervals between them. A gradual change in the leakage current between various resistance states (20 ~ 4 bits) was achieved, and the potential of identical pulses to replace variable width voltage pulses was recognized, which is important for integration into electronic circuits that operate with fixed-amplitude voltage pulses. It was also observed that by increasing the time gap between the voltage pulses, the change in resistance becomes smaller as the pulses are not correlated with each other. Based on these findings, the behavior of these structures regarding synaptic plasticity was studied. These structures function as both short-term memory synapses (Short Term Potentiation/Depression) based on Pair Pulse Facilitation (PPF) measurements and long-term memory synapses (Long Term Potentiation/Depression) based on Spike Timing Dependent Plasticity (STDP) measurements. With the appropriate choice of pulses, response times of the same order (~ms) as those of biological synapses can be achieved. However, there is room for improvement in the performance of these structures as synapses through the use of identical voltage pulses. The goal is to incorporate these structures into neuromorphic systems for the construction of hardware spiking neural networks.
