Liu Zheng-Tang

Effects of applied bias voltage on the properties of a-C:H films

Abstract The effects of the applied negative bias voltage on the hydrogen content, refractive index, extinction coefficient, internal stress, Vickers microhardness, adhesion and growth rate of a-C:H films deposited from a C 2 H 2 + Ar mixture by a d.c.-r.f. plasma-enhanced chemical vapour deposition process have been investigated. The results showed that the properties of a-C:H films strongly depend on the applied bias voltage. In the investigated bias range (−400 to −1000 V), it was found that, apart from the growth rate and extinction coefficient (which were found to increase with increasing bias), all the other properties studied showed a maximum at around −800 to −900 V. Compared with those studies reported previously on self-bias voltage effects, these results indicated a slight difference between the effects of the self-bias voltage and applied bias voltage. The experimental results are discussed.

Structural, electronic, optical, elastic properties and Born effective charges of monoclinic HfO2 from first-principles calculations

First-principles calculations of structural, electronic, optical, elastic, mechanical properties, and Born effective charges of monoclinic HfO2 are performed with the plane-wave pseudopotential technique based on the density-functional theory. The calculated structural properties are consistent with the previous theoretical and experimental results. The electronic structure reveals that monoclinic HfO2 has an indirect band gap. The analyses of density of states and Mulliken charges show mainly covalent nature in Hf-O bonds. Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function, and optical conductivity each as a function of photon energy are calculated and show an optical anisotropy. Moreover, the independent elastic constants, bulk modulus, shear modulus, Youngs modulus, Poissons ratio, compressibility, Lame constant, sound velocity, Debye temperature, and Born effective charges of monoclinic HfO2 are obtained, which may help to understand monoclinic HfO2 for future work.

Antireflective Characteristics of Sub-Wavelength Periodic Structure with Square Hole

In order to enhance transmittance in the long-wavelength infrared range, the major parameters of periodic sub-wavelength structures (SWS) with square holes on zinc sulfide (ZnS) substrates are designed and analyzed for applications in normal incidence by using rigorous coupled-wave analysis (RCWA) and thin film theory (TFT). Finally the optimal antireflective parameters are achieved. The structures on ZnS substrates are fabricated on the basis of the simulation results by photolithography technology and reactive ion etching technology. A substantial antireflection effect is observed over the wavelength band 8–12 μm by a factor greater than 4.5.

Electronic and Optical Properties of Cubic SrHfO3

In order to clarify the mechanism of optical transitions for cubic SrHfO3, we have investigated the electronic structure and optical properties of cubic SrHfO3 using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The ground-state properties, obtained by minimizing the total energy, are in favorable agreement with the previous work. From the band structure and charge densities as well as the theory of crystal-field and molecular-orbital bonding, we have systematically studied how the optical transitions are affected by the electronic structure and molecular orbitals. Our calculated complex dielectric function is in good agreement with the experimental data and the optical transitions are in accord with the electronic structure.

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.

First-Principles Calculations of Structural, Elastic and Electronic Properties of Tetragonal HfO2 under Pressure

Structural, elastic and electronic properties of tetragonal HfO2 at applied hydrostatic pressure up to 50 GPa have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The calculated ground-state properties are in good agreement with previous theoretical and experimental data. Six independent elastic constants of tetragonal HfO2 have been calculated at zero pressure and high pressure. From the obtained elastic constants, the bulk, shear and Youngs modulus, Poissons coefficients, acoustic velocity and Debye temperature have been calculated at the applied pressure. Band structure shows that tetragonal HfO2 is an indirect band gap. The variation of the gap versus pressure is well fitted to a quadratic function.

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.