Houfang Liu

Resistive switching mechanisms relating to oxygen vacancies migration in both interfaces in Ti/HfOx/Pt memory devices

Resistive switching mechanism of Ti/HfOx/Pt memory devices was studied using X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy images. Spatial distributions of valence of Hf demonstrated that the fraction of Hf4+ increased from Ti/HfOx interface to HfOx/Pt interface in high resistance state (HRS), but it maintained a constant level in low resistance state (LRS). Rupture of oxygen vacancies formed conducting paths occurred near the HfOx/Pt interface. The cross sectional images of active switching region also varied with HRS and LRS. A dynamic model of interface processes was proposed to interpret interfaces migration of oxygen vacancies near both the top and bottom electrodes.

Resistive switching and conductance quantization in Ag/SiO2/indium tin oxide resistive memories

The Ag/SiO2/indium tin oxide (ITO) devices exhibit bipolar resistive switching with a large memory window of ∼102, satisfactory endurance of >500 cycles, good retention property of >2000 s, and fast operation speed of <100 ns, thus being a type of promising resistive memory. Under slow voltage sweep measurements, conductance plateaus with a conductance value of integer or half-integer multiples of single atomic point contact have been observed, which agree well with the physical phenomenon of conductance quantization. More importantly, the Ag/SiO2/ITO devices exhibit more distinct quantized conductance plateaus under pulse measurements, thereby showing the potential for realizing ultra-high storage density.

Resistive switching with self-rectifying behavior in Cu/SiOx/Si structure fabricated by plasma-oxidation

We report a resistive switching memory structure based on silicon wafers by employing both materials and processing fully compatible with complementary metal-oxide semiconductor technology. A SiOx nanolayer was fabricated by direct plasma-oxidation of silicon wafers at room-temperature. Resistive switching behaviors were investigated on both p- and n-Si wafers, whereas self-rectifying effect was obtained in the Cu/SiOx/n-Si structure at low-resistance state. The self-rectifying effect was explained by formation of the Schottky barrier between the as-formed Cu filament and the n-Si. These results suggest a convenient and cost-efficient technical-route to develop high-density resistive switching memory for nowadays Si-based semiconductor industry.

Enhanced tunnel magnetoresistance in fully epitaxial ZnO:Co-based magnetic tunnel junctions with Mg-doped ZnO barrier

The fully epitaxial ZnO-based ZnO:Co/ZnO:Mg/ZnO:Co magnetic tunnel junctions were grown on Al2O3(0001) substrate by oxygen plasma-assisted molecular beam epitaxy. The magnetoresistance behavior and spin injection through ZnO:Mg barrier were investigated. An enhanced positive tunnel magnetoresistance ratio of 85.6% is observed at 1.8 T at 5 K. The junction resistance at zero magnetic field is linear with respect to temperature power law T−4/3 between 5 K and 70 K, indicating that carriers tunnel through ZnO:Mg barrier via two localized states.

Epitaxial yttrium iron garnet film for fabrication of high frequency on-chip inductors

The application of epitaxial yttrium iron garnet (YIG) thin film on high frequency on-chip spiral inductors is investigated. The YIG thin film with the thickness of 3.6 μm was grown on GGG(111) substrate using the liquid phase method, which exhibits relatively high saturation magnetization 4πMs of 1615 Oe close to the bulk value of 1750 Oe and low initial coercivity Hc of 0.5 Oe that minimizes the hysteretic losses. Subsequently, the spiral inductors were directly fabricated on the YIG/GGG(111) substrate. The results show substantial improvement in the optimum operating frequency and self-resonance frequency of the on-chip spiral inductor with the YIG thin film with an increase of 50% up to ∼7.5 GHz and 14.2 GHz, respectively, implying that on-chip spiral inductors with the YIG thin film can be applied to much higher frequency RF circuits.

A Ferroelectric Thin Film Transistor Based on Annealing-Free HfZrO Film

A ferroelectric thin film transistor (Fe-TFT) based on annealing-free hafnium zirconium oxide (HfZrO) is demonstrated in this paper. Indium zinc oxide was used as channel semiconductor. The as-deposited 30-nm HfZrO film implemented as gate dielectric was proved to be crystallized with a mixture of monoclinic, tetragonal, and orthorhombic phases and showed ferroelectricity naturally. Thus, high temperature annealing process was avoided. The transfer characteristic of this Fe-TFT was demonstrated with operating voltage that was smaller than 3 V, memory window about 1 V, and small subthreshold slope (SS) about 82 mV/dec. The charge trapping phenomenon in this device was explored by characterizing the transfer curves with different ranges of gate voltages. This HfZrO-based device with low processing thermal budget and small SS has high potential for Fe-TFT memory which can be used in oxide semiconductor-based systems and applications.

Direct laser-patterned ultra-wideband antennae with carbon nanotubes

Ultra-wideband (UWB), a radio transmission technology with wide bandwidth exceeding the minimum of 500 MHz or at least 20% of the center frequency, is a revolutionary approach for short-range high-bandwidth wireless communication. In this study, carbon nanotube (CNT) UWB antennas by direct laser-patterning technology have been successfully designed, fabricated and characterized. In contrast with traditional fabrication methods, the direct laser-patterning technology offers an exceptional potential for custom-designed, high-complexity and accuracy device fabrication. The “engraving” process on CNTs exposed to laser can be attributed to the bond breaking of C–C, evaporation of carbon atoms, and oxidation of CNTs by the oxygen molecules. Numerical analysis and experimental studies provide characteristics of CNT slot antennas with a wide impedance bandwidth (from 3.4 GHz to 14 GHz for S11 ≤ −10 dB), high average radiation efficiency (76%) and fractional bandwidth (121%) with small size of 30 × 30 mm2. The results indicate the advantages of laser-patterned UWB antennas based on carbon nanotubes, which paves the way for industrial applications, particularly in the world of consumer electronics.

Tailoring perpendicular magnetic anisotropy with graphene oxide membranes

Graphene oxide (GO) membranes have been widely explored for their excellent physical and chemical properties, and abundant functional groups. In this work, we report the improvement of the perpendicular magnetic anisotropy (PMA) of CoFeB thin films by applying a coating of GO membranes. We observe that the PMA of the CoFeB/MgAl–O stacks is strongly enhanced by the coating of GO membranes and even reaches 0.6 mJ m−2 at room temperature after an annealing process. The critical thickness of the membrane-coated CoFeB for switching the magnetization from the out-of-plane to the in-plane axis exceeds 1.6 nm. First-principle calculations are performed to investigate the contribution of the GO membranes to the magnetic anisotropy energy (MAE). Due to changes in the hybridization of 3d orbitals, varying the location of the C atomic layer with Co changes the contribution of the Co–C stacks to PMA. Thus, the large PMA achieved with GO membranes can be attributed to the orbital hybridization of the C and O atoms with the Co orbitals. These results provide a comprehensive understanding of the PMA and point towards opportunities to achieve multifunctional graphene-composite spintronic devices.

Magneto-Seebeck effect in magnetic tunnel junctions with perpendicular anisotropy

As one invigorated filed of spin caloritronics combining with spin, charge and heat current, the magneto-Seebeck effect has been experimentally and theoretically studied in spin tunneling thin films and nanostructures. Here we analyze the tunnel magneto-Seebeck effect in magnetic tunnel junctions with perpendicular anisotropy (p-MTJs) under various measurement temperatures. The large tunnel magneto-Seebeck (TMS) ratio up to −838.8% for p-MTJs at 200 K is achieved, with Seebeck coefficient S in parallel and antiparallel states of 6.7 mV/K and 62.9 mV/K, respectively. The temperature dependence of the tunnel magneto-Seebeck can be attributed to the contributing transmission function and electron states at the interface between CoFeB electrode and MgO barrier.

A novel MEMS-based 13.56 MHz micro antenna for RFID application

Radio frequency identification (RFID) technology [1] is a wireless communication technology of automatic identifying the target objects using radio frequency signal. In recent years, RFID technology has been widely used in many areas, such as access control, public transportation systems, libraries and food safety traceability [2]-[5]. RFID system consists of three parts: reader, transponder and antenna [6]. The reader is a particular radio frequency (RF) data signal generation system, which is usually made as fixed or portable communication device. The transponder is a sensing device that senses the particular RF data signal and transmits the corresponding data signal back to the reader. The antenna is an energy transmission device, which is responsible for the transmission and reception of the data signal in the communication.