Yanmei Liu

Magnetic properties and formation of Sr ferrite nanoparticle and Zn, Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization ss and coercive field strength Hc are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization ss ¼ 74 emu/g and coercive field strength Hc ¼ 6:4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 5001C for several hours . It can prevent the formation of intermediate g-Fe2O3 and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn 2+ ,Ti 4+ and Ir 4+ are discussed. For an amount of substitution 0oxp0:6; the (Zn, Ti)x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)x substituted phase. r 2001 Elsevier Science B.V. All rights reserved.

Interface control and modification of band alignment and electrical properties of HfTiO/GaAs gate stacks by nitrogen incorporation

Effects of nitrogen incorporation on the interface chemical bonding states, optical dielectric function, band alignment, and electrical properties of sputtering-derived HfTiO high-k gate dielectrics on GaAs substrates have been studied by angle resolved X-ray photoemission spectroscopy (ARXPS), spectroscopy ellipsometry (SE), and electrical measurements. XPS analysis has confirmed that the interfacial layer of a HfTiO/GaAs gate stack is suppressed effectively after nitrogen incorporation. Analysis by SE has confirmed that reduction in band gap and increase in refractive index are observed with the incorporation of nitrogen. Reduction in valence band offset and increase in conduction band offset have been observed for a HfTiON/GaAs gate stack. Electrical measurements based on metal-oxide-semiconductor (MOS) capacitors have shown that the MOS capacitor with a HfTiON/GaAs stacked gate dielectric annealed at 600 °C exhibits low interface-state density (2.8 × 1012 cm−2 eV−1), small gate leakage current (2.67 × 10−5 A cm−2 at Vg = Vfb + V), and large dielectric constant (25.8). The involved mechanisms may originate from the decrease in the interface state density and the increase in the conduction band offset. The appropriate band offset relative to GaAs and excellent interface properties render HfTiON/GaAs as promising gate stacks in future III–V-based devices.

CVD-derived Hf-based High-k Gate Dielectrics

Hf-based high-k gate dielectric has been recently highlighted as the most promising high-k dielectrics for the next-generation CMOS devices with high performance due to its excellent thermal stability and relatively high dielectric constant. This article provides a comprehensive view of the state-of-the-art research activities in advanced Hf-based high-k gate dielectrics grown by chemical-vapor-deposition-based method, including metal-organic-chemical-vapor-deposition (MOCVD), atomic-layer-chemical-vapor-deposition (ALCVD), and plasma-enhanced- chemical-vapor-deposition (PECVD), in CMOS device. We begin with a survey of methods developed for generating Hf-based high-k gate dielectrics. After that, most attention has been paid to the detailed discussion of the latest development of novel Hf-based high-k gate dielectrics grown by CVD. Finally, we conclude this review with the perspectives and outlook on the future developments in this area. This article explores the possible influences of research breakthroughs of Hf-based gate dielectrics on the current and future applications for nano-MOSFET devices.

Composition dependence of interface control and optimization on the performance of an HfTiON gate dielectric metal-oxide-semiconductor capacitor

Composition dependence of interface control, band alignment and electrical properties of HfTiON/Si grown by sputtering has been studied by spectroscopy ellipsometry (SE), x-ray photoelectron spectroscopy (XPS) and electrical measurement. Analysis from XPS has confirmed that the interfacial layer consisting of silicate and SiOx is formed unavoidably, irrespective of composition ratio. Meanwhile, reduction in band gap and asymmetric band alignment has been detected for HfTiON films with the increase in Ti composition. To meet the requirements of high-k dielectrics with the barrier height of over 1 eV, the incorporation composition ratio needs to be carefully optimized. As a result, improved C–V characteristics and reduced leakage current have been achieved from HfTiON gate dielectric MOS capacitors with optimized composition ratio of Hf:Ti = 1:1, which can be attributed to the reduction in oxygen-related traps and the obtained near-symmetric band alignment relative to Si.

Synthesis of 1D and heavily doped Zn1−xCoxO six-prism nanorods: improvement of blue–green emission and room temperature ferromagnetism

In this work, one-dimensional (1D) and heavily doped Zn1−xCoxO (x = 0.05, 0.1, 0.15 and 0.2) nanorods (NRs) were successfully fabricated by a new solvothermal method. Such a Zn1−xCoxO nanorod exhibits a hexagonal prism-like microcrystal with a pyramidal top. Analyses from transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) indicated that Zn1−xCoxO NRs possess perfect single crystal wurtzite structures. The influence of Co doping on the structural, optical and magnetic properties of NRs was investigated in detail. It was verified that Co was successfully doped into the ZnO wurtzite lattice structure by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The Zn1−xCoxO NRs show an obvious blue–green emission except for the weak UV emission. The corresponding luminescence mechanism was discussed, and the environment of cobalt in the ZnO wurtzite lattice was also identified through UV-vis spectroscopy. In addition, the magnetic hysteresis (M–H) curves demonstrated that the Zn1−xCoxO NRs possess obvious ferromagnetic characteristics at room temperature. These Zn1−xCoxO NRs are suggested to have potential applications in spintronic, field emission and optoelectronic devices, etc.

Synthesis of highly-transparent Al-doped ZnO porous network thin films

Highly-transparent Al-doped ZnO (ZAO) porous network thin films are prepared via solvothermal method for the first time. It is found that the morphology of the nanostructures evolved from nanosheets to porous network structures because of the different annealing temperature. The incorporation of Al and the contents of NaOH play important roles in the morphology of porous network nanostructures. A growth mechanism is proposed to explain these findings. Al is successfully doped into the ZnO wurtzite lattice structure as revealed by X-ray diffraction (XRD) and verified by the energy dispersive X-ray detector (EDX). The unique nanostructure thin films exhibit a typical wurtzite structure and strong violet emission. Ultraviolet-visible spectroscopy (UV-vis) shows the films have a good transparency, and the absorption edges are obviously blue shifted with a temperature increase. By optimizing the reaction parameters, the morphology, and size of the network thin films could be tailored, which will be very significant for fabricating novel and surface morphology controlled photocatalysis, optoelectronics, solar energy and self-cleaning devices.

Optical and Magnetic Properties of ZnO-based Semiconductors Regulated by Cu Ions

Cu doped ZnCoAlO (ZCAO) films deposited on single-crystal Si (100) substrates were prepared by pulsed laser deposition (PLD). The microstructures of the ZCAO films with different Cu concentrations were studied by X-ray diffraction (XRD); the findings show that the quality of the crystal structure increases with Cu ion doping. The near-band gap and deep-level-defect emissions at room temperature (RT) are observed in the Cu-doped samples via photoluminescence (PL) measurement, revealing its effects on the short-wavelength emission for the Cu-doped film compared with the ZCAO film. The magnetism and electron transport in the Cu doped ZCAO films were also investigated. The results show that interstitial Cu ions affect the carrier concentration. The magnetic measurement shows that the Cu ions occupying adjacent cation positions breaks the magnetic double exchange interaction and weakens the magnetism of the samples.

Synthesis, Photoluminescence, and Magnetic Properties of Arrayed Al Doped Zn 0.95Co 0.05O Nanorods

Arrayed Zn(subscript 0.95-x)AlxCo0.05O (ZnAlCoO for x=0.02-0.07 and ZnCoO for x=0) nanorods have been synthesized by a low temperature hydrothermal method and characterized as ZnO hexagonal structures by their X-ray diffraction (XRD) spectra, Raman spectra, and scanning electron microscopy (SEM), which show that Al doping can modulate the density and the cross section shape of the ZnCoO nanorods. Al and Co atoms substituting for Zn atoms in ZnAlCoO samples were successfully doped into a ZnO lattice, which has been characterized by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectra. PL spectra exhibit that Al doping in ZnCoO markedly enhanced the visible light emission (VL) while it depressed the ultraviolet emission (UV). The positions of the UV peak first redshifted from 386 to 387 nm, then blueshifted to a shorter wavelength side (374 nm) with increasing Al doping concentration. Moreover, VL appears with redshifts and a maximum of intensity for the middle Al doping (x=0.04) sample. The high Al (x=0.07) doping sample exhibits room temperature ferromagnetism (RTFM) with saturation magnetization 20 times larger than that of the middle Al (x=0.04) doping sample while the low Al doping (x=0.02) sample exhibits no RTFM. The above structural, optical and magnetic phenomena are all discussed.