Yongdan Zhu

Ferroelectric memristor based on Pt/BiFeO3/Nb-doped SrTiO3 heterostructure

We report a continuously tunable resistive switching behavior in Pt/BiFeO3/Nb-doped SrTiO3 heterostructure for ferroelectric memristor application. The resistance of this memristor can be tuned up to 5 × 105% by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The observed memristive behavior is attributed to the modulation effect of the ferroelectric polarization reversal on the width of depletion region and the height of potential barrier of the p-n junction formed at the BiFeO3/Nb-doped SrTiO3 interface.

Enhancement of properties of dye-sensitized solar cells by surface plasmon resonance of Ag nanowire core–shell structure in TiO2 films

A series of TiO2 nanocomposite photoanodes with different amounts of Ag nanowire (AgNW) coated with SiO2 in a AgNW@SiO2 core–shell structure are prepared by a ball milling method and applied to assemble a series of dye-sensitized solar cells (DSSCs). The influence of AgNW@SiO2 on the performance of the TiO2–AgNW photoanodes and DSSCs is investigated. Studies indicates that the range and strength of light absorption of the TiO2 photoanodes, the photon capture ability of the dye molecules, and the short-circuit current density (Jsc) are significantly increased while the dark current is decreased due to the incorporation of AgNW@SiO2. The optimal properties are obtained in a DSSC with a Jsc of 11.83 mA cm−2, and an overall photoelectric conversion efficiency (η) of 6.26%, significantly superior to those of the DSSC with a pure TiO2 photoanode. The enhancements of Jsc and η are attributed to the increase of light coupling and thus the light absorption of the dye due to the localized surface plasmon resonance and the possible enhanced light scattering of AgNW@SiO2 in the photoanode.

Flexible resistive switching memory based on Mn0.20Zn0.80O/HfO2 bilayer structure

We have investigated Mn0.20Zn0.80O/HfO2 bilayer structure resistance memory fabricated on flexible Kapton substrates. The Mn0.20Zn0.80O/HfO2 bilayer structure shows a steady and bipolar resistive switching characteristic with an on/off ratio of ~70 at 0.5 V. Through the bending investigation, our flexible memory exhibits no degradation in switching property, even when the substrate is bent up to 11 mm radius. The resistive switching mechanism of the Mn0.20Zn0.80O/HfO2 bilayer structure can be attributed to the control of oxygen vacancies in HfO2 through the forward or reverse bias.

Middle-ultraviolet-enhanced photodetectors based on Mg0.4Zn0.6O/ZnO homojunction with a high selectivity for 300 nm around light

High-quality Mg0.4Zn0.6O film with a ZnO buffer layer was prepared on glass by the reactive magnetron cosputtering method, and the MgZnO/ZnO homojunctions were fabricated into metal?semiconductor?metal middle ultraviolet (UV) photodetectors (PDs). The MgZnO-based PD with a ZnO buffer layer showed a high selectivity for 300 nm around middle UV light and its responsivity was larger than that of the PD without a buffer layer. Its peak responsivity was 0.45 A W?1?at 300 nm under??5 V bias, which was comparable to the highest value ever reported in MgZnO-based PDs. Also, we observed that the PDs with a smaller spacing of the finger electrodes showed a larger responsivity, indicating that the PD with a small spacing shows a large internal gain. The results may provide a simple route to gain low-cost and high-performance middle UV PDs.

Epitaxial growth and capacitance-voltage characteristics of BiFeO3/CeO2/yttria-stabilized zirconia/Si(001) heterostructure

We report the epitaxial growth of multiferroic BiFeO3 (BFO) film on Si(001) substrate by pulsed laser deposition using CeO2/yttria-stabilized zirconia (YSZ) as buffer layers. The epitaxial relationships of the films were BFO(001)/CeO2(001)/YSZ(001)/Si(001) for out-of-plane and [110]BFO‖[100]CeO2‖[100]YSZ‖[100]Si for in-plane, respectively. Capacitance-voltage characteristics of a Pt/BFO/CeO2/YSZ/p-Si capacitor exhibited clockwise hysteresis loops with a large memory window of 2.5 V at sweeping voltages of ±16 V. Both the high and low capacitance values showed no obvious degradation after 104 s. The improved retention property was attributed to the use of high-k CeO2/YSZ insulating layers that effectively eliminated the charge trapping in the heterostructure.

Nanointerlayer Induced Electroluminescence Transition from Ultraviolet- to Red-Dominant Mode for n-Mn:ZnO/N-GaN Heterojunction

High-quality Mn:ZnO (MZO) film had been prepared on N-GaN coated sapphire substrates followed by postdeposition thermal annealing treatment at 700 °C. For the annealed MZO/GaN heterojunction, a 15 nm cubic structural ZnGa(2)O(4) layer was observed at the MZO/GaN interface through transmission electron microscope analysis. Through electroluminescence (EL) measurement, the formation of the nanointerface results in an EL transition from ultraviolet- to red-dominant mode for n-Mn:ZnO/N-GaN heterojunction light-emitting diodes (LEDs). The heterojunction LED showed a rectification ratio of ∼2.0 × 10(5) at ±2 V, a dark current of 3.5 nA at -2 V and a quite strong red EL with a low turn-on voltage of 3 V. On the basis of the energy band diagram, we think the EL transition from ultraviolet- to red-dominant mode is mainly due to the formation of a thin oxide blocking nanolayer at the MZO/GaN interface during the annealing process.

A device with two kinds of functions —Ultraviolet photodetector and electroluminescence: Fabrication and carrier transport mechanism

We reported an n-ZnO/n-GaN heterojunction device in which both ultraviolet (UV) detecting and electroluminescence performances of the device are controlled by the applied forward-bias voltage. For ZnO-based UV photodetectors, our devices showed excellent photoresponse characteristics with detectivity of ~2.80×1013 cm Hz1/2/W and responsivity of ~276 A/W at 2 V. UV and visible electroluminescences of the device were also observed. Also, a Mn:ZnO/GaN heterojunction had been prepared and it also possessed the two functions. Furthermore, the Mn:ZnO/GaN device showed better UV detectivity and enlarged the visible emission. The reason for our devices possessing two functions had been explored through the carrier transport mechanism and the channel current formation diagram.

Electroluminescence Transition From Visible- to Ultraviolet-Dominant Mode in

We report an n-Mn_0.04Zn_0.96O(MZO) /i-ZnGa₂O₄/ n-GaN structure both as a light-emitting diode and as an ultraviolet (UV) photodetector at forward and reverse biases, respectively. The n-MZO films were prepared on n-GaN coated sapphire substrates, followed by postdeposition thermal annealing treatment at 700 ^°C, and an i-ZnGa₂O₄ interface layer was formed at the MZO/GaN interface after the annealing treatment. We found that the electroluminescence characteristics showed a transition from visible- to UV-dominant mode when the deposition temperature of the MZO film is from 100 ^°C to 300 ^°C. For the UV detection performance, the devices based on the low-temperature deposition (100 ^°C) of the MZO film showed the lowest dark current and the biggest ratio of photocurrent to dark current with a selectivity detectivity for 365 nm around light.