Dissertation organization

Chapter 1 summarizes the introduction about novel NVM, resistive switching concepts of RRAM, and its related unipolar and bipolar switching mechanism.

Several effects to the switching characteristics, improvement methods, nonpolar properties, and integrated stacked structure are also mentioned. Finally, we provide the motivation of the whole experimental investigation.

Chapter 2 first introduces the basic concept of the fabrication process and measurement analyzer. Then, the detailed experimental procedure of the MIM stacked structure is presented.

In Chapter 3, the factors to the resistive switching characteristics of the atomic-layer-deposited HfOx thin film was investigated and discussed, including bulk HfOx thin films, top electrode, the inserted reactive metal at the electrode/oxide interface, rapid thermal annealing in Ar and O2 circumstance, process effect, and UV light irradiation. The electrode material, thickness, and fabrication process have to be well chosen for a more reliable switching behavior. For the HfOx film, when the stacked metal Pd/Al was used as top electrode, both the unipolar and bipolar resistive switching characteristics can be performed for more than 1000 switching cycles under the continuously sweeping curves. Rapid thermal annealing under a controlled

treatment time in oxygen ambient effectively improve the switching properties, while do no help on the switching characteristics in Ar circumstance. UV light direct irradiation on HfOx surface can cause the oxygen-related defects or oxygen vacancies inside HfOx film, which results in more random distribution on the conducting filaments. When ITO electrode was shielded above the HfOx surface, an oxygen-riched HfO2 layer could be formed at the M/O interface effectively modulate the switching properties and reduce the dispersion on switching parameters.

In Chapter 4, we discuss on the electrical characteristics of the Gd2O3 thin film, which were fabricated under different substrate temperature during the pulsed laser deposition (PLD). The Si-based and glass-based Gd2O3 devices were both investigated here. The resistance switching behaviors of Ti/Gd2O3/Pt devices are highly correlated with the anode electrode. The switching characteristics operating under positive bias exhibits stable switching characteristics of 100 switching cycles with low voltage and resistance dispersion, while the switching characteristics becomes unstable and turn into a large fluctuation on resistance and voltage values when the devices are operated under negative bias. The other is the highly transparent RRAM devices based on ITO/Gd2O3/ITO sandwiched structure. The Gd2O3-based T-RRAM device exhibits excellent bipolar resistance switching, good endurance of more than 1000 cycles, and multi-level states properties. A possible switching mechanism based on the migration of the oxygen ions between the interfacial layer and Gd2O3 film dominating the state to LRS or HRS under applied bias. Based on the results of XPS analysis, the different composition ratio on the metallic (Gd⁰) / oxidized (Gd³⁺) was calculated to be 65.5 % / 34.5 % and 49.3 % / 50.7 % for the 25

°C and 300 °C-deposited samples, respectively. The large amounts of defect in the Gd2O3 film, including oxygen vacancies or metal atoms, etc., may greatly dominate the switching characteristics.

In Chapter 5, we will first discuss the fabrication process effect on the thin film morphology, surface roughness, crystallinity, and material composition by varying oxygen pressure during PLD process of the amorphous LAO film. Then, the electrical characteristics of the three samples, such as leakage current, breakdown voltage, temperature effect to conducting current, and conducting behavior were also discussed.

We observed that all the samples in LAO films during the electroforming process follow the three conduction mechanism: Hopping conduction → Schottky emission

→ Frenkel-Poole emission, as the applied voltage increases to breakdown voltage (VBD). To explain why samples grown under different oxygen pressure during LAO films deposition will cause different electrical characteristics , voltage/resistance dispersion and its physical mechanism, the forming process is the key factor to be discussed. The residual electrons inside LAO contribute to the space-charge-limit conduction during the RS operation. LAO films grown at higher oxygen partial pressure is beneficial for a more reliable resistive switching performance, because the formation of the interfacial layer and lower oxygen vacancy concentration exist in the LAO thin film. The interfacial layer can serve as a good oxygen reservoir residence and the more oxygen ions involve can ensure the switching reliability. A blocking layer growth between LAO and ITO (BE) terminate the extended defects, which leads to a uniform distribution on the oxygen vacancy concentration from top to bottom insulator film.

Fig. 1-1 DRAM cell with 1T1C structure.

Fig. 1-2 SRAM CMOS cell with six-transistor (6T) configuration example

Fig. 1-3 Schematic cross section of a floating-gate Flash memory

Fig. 1-4(a) The ferroelectric compositional structure of a perovskite crystal unit cell.

Fig. 1-4(b) The schematic diagram of the typical hysteresis curve related to the moving atoms under the applied bias.

Fig. 1-4(c) Polarization versus temperature curve.

Fig. 1-5 Illustration of the low resistance state (parallel direction) and high resistance state (antiparallel direction) of the MRAM device.

Fig. 1-6(a) Schematic diagram for the phase change memory cell consisting of Ge2Sb2Te5 material. [13]

Fig. 1-6(b) According to the Monte Carlo model simulation for crystallization, the phase change in resistance values as a function of time can be shown. Crystal grains are in blue, amorphous phase in red. [14]

Fig. 1-6(c) Temperature-time relationship in SET and RESET pulse operation. [13]

Fig. 1-7 Two types of differential negative resistance for (a) voltage controlled mode (N shape) and (b) current controlled mode (S shpae).

Fig. 1-8 The typical Unipolar I-V curves. The arrows indicate the sweep current loop.

Fig. 1-9 The typical bipolar I-V curves at (a) clockwise current loop and (b) counterclockwise current loop.

Fig. 1-10 Schematic illustration of conduction processes in the high-field region of a metal-oxide-metal structure. [15]

Fig. 1-11 Energy diagram showing the position of the stored charge relative to the Fermi level: (a) at a voltage bias of V volts, (b) immediately after voltage has been rapidly reduced to zero, (c) after a longer time interval. [16]

Fig. 1-12 (a) Temperature of filament as a function of filament radius with different resistivity values. (b) The relation between temperature of filament and measured time for 40 nm radius and resistivity = 100 μΩ. [19]

Material Model Equation Condition Heating

Table. 1 Comparison on the estimated heating temperature of different material.

Fig. 1-13 Simulation test on two reset/set cycles of C-RRAM. (a) the applied voltage as a function of temperature, (b) distribution of the atomic structure in the simulated filaments, and (c) temperature distribution among the entire TMO film. [21]

Fig. 1-14 Surface morphology (a),(c),(e) and its corresponding current images (b),(d),(f) are shown by AFM and C-AFM measurement, respectively. Operation condition of the (a)(b) SET scan under 2.3 V bias, (c)(d) RESET scan under 1.5 V bias, and (e)(f) SET scan under 2.3 V bias. [32]

Fig. 1-15 (a) A schematic diagram showing the sample geometry. (b) Simulation on temperature as a function of measured time at the filament center under applied bias.

Temperature distribution for the case with (c) 200 nm and (d) 10 nm bottom Pt electrode. [33]

Fig. 1-16 (a) Typical I-V curve of NiO film. Cross-sectional TEM images of the Pt/NiO/Pt structure of (b) pristine, (c) LRS, and (d) HRS. [18]

Fig. 1-17 Schematic diagram of the MIM stacked structure.

Fig. 1-18 (a) Resistive switching test of NiO films with Au, Pt, or Al as top electrode. (b) Al/NiO/Pt and (c) Al/NiO/Pt structures. The arrows indicates the sweeping directions of applied bias. [35]

Fig. 1-19 Ellingham diagram reveals the free energies for oxidation of the various metals. [37]

Fig. 1-20 Cross-sectional TEM image and corresponding EDX analysis of NiPt/NiO/NiPt thin film (a) beform forming and (b) after forming. Ni penetration into the NiO film after forming process was clearly observed. [57]

Fig. 1-21 The EDX analysis in STEM mode for detection on the atomic materials.

Four places were detected for investigation the Ag bridge involving inside the TMO films. (a) out side the Ag bridge, (b) in the middle of the Ag bridge, (c) near the Pt electrode, and (d) a line profile for showing the intensity of Ag atoms along the bridge.

[94]

Fig. 1-22 A schematic diagram for the resistive switching mechanism of the electrometallization memory. (a) Oxidation of Ag atoms at anode, (b) migration of Ag cations toward the cathode and their reduction process, (c) The precipitations of Ag atoms connect the anode and cathode and form a metallic Ag conducting bridge. And (d) the electrochemical dissolution of Ag bridge takes place. [94]

Fig. 1-23 SEM images showing the Ag dendrite growth under the applied bias of -1 V for about (a) 0 s, (b) 1 s, (c) 2 s, and (d) 4 s, respectively. [98]

Fig. 1-24 Schematic diagram of Pt/Nb:STO Schottky junction for indicating the Schottky barrier height modulated by the trapped electrons. [105]

Fig. 1-25 Schematic diagram of the three domains model. The insulator film can be regarded as top, middle, and bottom domains, respectively. The tunneling amplitudes between two domains regulate the injected carriers. [119]

Fig. 1-26 (a) virgin resistance vs. WF, and (b) virgin resistance vs. free energy. [142]

Fig. 1-27 I-V characteristics for (a) Ag, Cu, Au, Pt/PCMO/Pt, (b) Al/PCMO/Pt, (c) Ti/PCMO/Pt, and (d) Ta/PCMO/Pt. The switching loop is followed by 0 V → 3 V

→ -3 V → 0 V. The cross-sectional HRTEM images at right side are the corresponding microstructure. [142]

Fig. 1-28 Bubble gas observation at (b),(c) negative bias, then at (d)-(h) positive bias. (g) The eruption features observed by atomic force micrograph after bias voltage was removed. [191]

Fig. 1-29 The schematic diagram is shown for (a) ohmic conduction, (b) Schottky emission, (c) Frenkel emission, and (d) Fowler-Nordheim Tunneling.