Tan Tingting

Influence of Radio-Frequecy Power on Structural and Electrical Properties of Sputtered Hafnium Dioxide Thin Films

Hafnium dioxide (HfO2) thin films are prepared by rf magnetron sputtering. The influences of rf power on the structure, chemical states and electrical properties of the thin films were investigated through x-ray diffraction, x-ray photoelectron spectroscopy, capacitance-voltage and leakage current density-voltage measurement and UV-VIS spectrophotometry. The results show that the HfO2 thin films have a mixed structure of amorphous and poly crystalline phases. With increasing rf power, the crystallinity is enhanced and the crystallite size of the thin films is increased. The oxidation of Hf atoms is improved with increasing rf power for the HfO2 thin films. The flat band shift, oxide charge density and leakage current density of the thin films all decrease as the rf power increases from 50 to 110 W, and then increase as the rf power is increased to 140 W. The band gap energy is smaller for the thin film deposited at 110 W.

Bipolar Resistive Switching Characteristics of TiN/HfOx/ITO Devices for Resistive Random Access Memory Applications

Resistive switching characteristics of hafnium oxide are studied for possible nonvolatile memory device applications. The HfOx films with different oxygen contents are deposited by rf magnetron sputtering under different O2 flow rates. The films are amorphous, and the stoichiometric of the film is improved by increasing the O2 flow rate. Current-voltage characteristics of the TiN/HfOx/ITO device are investigated with 1 mA compliance. The bipolar resistive switching behavior is observed for the TiN/HfOx/ITO structure, and the resistive switching mechanism of the TiN/HfOx/ITO structure is explained by trap-controlled space charge limit current conduction.

Electrical, Structural and Interfacial Characterization of HfO2 Films on Si Substrates

Hafnium oxide films are deposited on Si (100) substrates by means of rf magnetron sputtering. The interfacial structure is studied using high-resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS), and the electrical properties of the Au/HfO2/Si stack are analyzed by frequency-dependent capacitance-voltage (C—V—f) measurements. The amorphous interfacial layer between HfO2 and the Si substrate is observed by the HRTEM method. From the results of XPS, the interfacial layer comprises hafnium silicate and silicon oxide. For C—V—f measurements, the C—V plots show a peak at a low frequency and the change in frequency has effects on the intensity of the peak. As expected, rapid thermal annealing can passivate the interface states of the HfO2/Si stack.

Chemical Structure of HfO2/Si Interface with Angle-Resolved Synchrotron Radiation Photoemission Spectroscopy

Interfacial chemical structure of HfO2/Si (100) is investigated using angle-resolved synchrotron radiation photoemission spectroscopy (ARPES). The chemical states of Hf show that the Hf 4f binding energy changes with the probing depth and confirms the existence of Hf-Si-O and Hf-Si bonds. The Si 2p spectra are taken to make sure that the interfacial structure includes the Hf silicates, Hf silicides and SiOx. The metallic characteristic of the Hf-Si bonds is confirmed by the valence band spectra. The depth distribution model of this interface is established.

Effects of Film Thickness and Ar/O2 Ratio on Resistive Switching Characteristics of HfOx-Based Resistive-Switching Random Access Memories

Cu/HfOx/n+Si devices are fabricated to investigate the influence of technological parameters including film thickness and Ar/O2 ratio on the resistive switching (RS) characteristics of HfOx films, in terms of switch ratio, endurance properties, retention time and multilevel storage. It is revealed that the RS characteristics show strong dependence on technological parameters mainly by altering the defects (oxygen vacancies) in the film. The sample with thickness of 20 nm and Ar/O2 ratio of 12:3 exhibits the best RS behavior with the potential of multilevel storage. The conduction mechanism of all the films is interpreted based on the filamentary model.

Resistive Switching Behavior of Hafnium Oxide Thin Film Grown by Magnetron Sputtering

Abstract The HfO2 thin films with resistive switching behaviors were grown on indium tin oxide (ITO) /Glass substrates by radio-frequency (RF) magnetron sputtering method. The results of electrical tests indicate that HfO2 thin films show a bipolar resistive switching behavior. Using Ti as a buffer layer on the cathodal side of the memory cell, relatively reliable endurance (>2×102 cycles at 20 °C) and long data retention time (>104s at 20 °C) have been demonstrated, and the high-resistance to low-resistance ratio can reach 104. Based on the electrical property test results, the phenomena can be correlated with oxygen-vacancy-related traps. Space charge limited current (SCLC) mechanism is believed to be the reason for the resistive switching from the OFF state to the ON state.

Electronic structure and optical properties of monoclinic HfO2 with oxygen vacancy

Abstract The electronic structure and optical properties of monoclinic HfO 2 ( m -HfO 2 ) with the consideration of oxygen vacancy were calculated using the plane-wave ultrasoft pseudopotential method based on the first-principles density functional theory (DFT). The results reveal that a new defect level at the conduction band edge is introduced for the defective m -HfO 2 , and the 5d states of Hf atoms nearest to the oxygen vacancy make significant contribution to it. Furthermore, the dielectric functions and absorption coefficients of the perfect and defective m -HfO 2 were calculated and analyzed by means of the electronic structures, which were related to the defect levels. The calculated absorption coefficient shows that m -HfO 2 with oxygen vacancy exhibits an absorption band in the ultraviolet region.

Influence of Pressure on the Structural, Electronic and Mechanical Properties of Cubic SrHfO3: A First-Principles Study

The structural, electronic and mechanical properties of cubic SrHfO3 under hydrostatic pressure up to 70 GPa are investigated using the first-principles density functional theory (DFT). The calculated lattice parameter, elastic constants and mechanical properties of cubic SrHfO3 at zero pressure are in good agreement with the available experimental data and other calculational values. As pressure increases, cubic SrHfO3 will change from an indirect band gap (Γ–R) compound to a direct band gap (Γ–Γ) compound. Charge densities reveal the coexistence of covalent bonding and ionic bonding in cubic SrHfO3. With the increase of pressure, both the covalent bonding (HfO) and ionic bonding (SrO) are strengthened. Cubic SrHfO3 is mechanically stable when pressure is lower than 55.1 GPa, whereas that is instable when pressure is higher than 55.1 GPa. With the increasing pressure, enthalpy, bulk modulus, shear modulus and Youngs modulus increase, whereas the lattice parameter decreases. Moreover, cubic SrHfO3 under pressure has higher hardness and better ductility than that at zero pressure.

Effects of Electrodes on the Filament Formation in HfO2- Based Resistive Random Access Memory

The resistance switching characteristics of HfO2-based resistive random access memory (RRAM) with Cu/HfO2/ITO and TiN/HfO2/ITO structures were investigated. Results show that both devices exhibit a stable and reproducible bipolar switching behavior during successive cycles. The formation of Cu conducting filaments is believed to be the reason for the resistive switching of Cu/HfO2/ITO device. For TiN top electrode, the interfacial layer between TiN and HfO2 film is formed and acts as an oxygen reservoir therefore, oxygen vacancy filaments are responsible for the resistive switching of TiN/HfO2/ITO device.