Xiangao Zhang

Enhanced electrical properties of c-axis epitaxial Nd-substituted Bi4Ti3O12 thin films

High-quality c-axis epitaxial ferroelectric thin films of Bi4Ti3O12 (BTO) and Nd-substituted BTO, Bi3.15Nd0.85Ti3O12 (BNT), were prepared on (001)-LaNiO3-coated (001) LaAlO3 substrates by pulsed-laser deposition. The epitaxial alignments were established by the x-ray diffraction, including θ–2θ and φ scans. Compared to the BTO films, the BNT films have significantly improved electrical properties with about 2 times larger remanent polarization, 0.6 times lower coercive field, better fatigue-resisting characteristics, and 1.7 times larger dielectric constant. These results showed experimentally that Nd substitution could enhance the c-axis electrical properties of BTO. The reason for the improved properties of BNT films was discussed.

Energy transfer and enhanced luminescence in metal oxide nanoparticle and rare earth codoped silica

A significant enhancement of photoluminescence from Eu3+ embedded in SiO2 matrix is observed by codoping with wide band-gap In2O3 nanoparticles. The enhanced photoluminescence characteristics are strongly influenced by the postannealing temperature and the In3+ concentration. Synchronous scanning photoluminescence technique was used to understand the excitation and luminescence behavior in codoped silica films. Based on the experimental results, we argue that the enhancement of photoluminescence is associated with the effective energy transfer process from In2O3 nanoparticles to the charge transfer band of O2+–Eu3+ instead of the direct transfer to the rare-earth energy levels.

Field emission from a periodic amorphous silicon pillar array fabricated by modified nanosphere lithography

We prepare an array of amorphous silicon nanopillars by using a modified nanosphere lithography method. The fabrication process includes three steps: (1) 70 nm thick a-Si film was deposited on a crystalline silicon substrate; (2) the substrate was coated with a monolayer of polystyrene (PS) spheres to form an ordered structure on the a-Si thin film surface; (3) the sample was etched by reactive ion etching to produce the amorphous silicon pillar array. The results of field emission measurements show a low turn-on electrical field of about 4.5 V microm(-1) at a current density of 10 microA cm(-2). A relatively high current density exceeding 0.2 mA cm(-2) at 9 V microm(-1) was also obtained. The field enhancement factor is calculated to be about 1240 according to the Fowler-Nordheim (FN) relationship. The good field emission characteristics are attributed to the geometrical morphology, crystal structure and the high density of the field emitter of the silicon nanopillar.

Defect states and charge trapping characteristics of HfO2 films for high performance nonvolatile memory applications

In this work, we present significant charge trapping memory effects of the metal-hafnium oxide-SiO2-Si (MHOS) structure. The devices based on 800 °C annealed HfO2 film exhibit a large memory window of ∼5.1 V under ±10 V sweeping voltages and excellent charge retention properties with only small charge loss of ∼2.6% after more than 104 s retention. The outstanding memory characteristics are attributed to the high density of deep defect states in HfO2 films. We investigated the defect states in the HfO2 films by photoluminescence and photoluminescence excitation measurements and found that the defect states distributed in deep energy levels ranging from 1.1 eV to 2.9 eV below the conduction band. Our work provides further insights for the charge trapping mechanisms of the HfO2 based MHOS devices.

In situ electrical-field-induced growth and properties of Bi3TiNbO9 ferroelectric thin films

With Bi3TiNbO9 (BTN) ferroelectric samples, in situ electrical-field-induced growth of ferroelectric thin films was demonstrated to control the films’ microstructure and manipulate their ferroelectric properties. BTN films on Ti/SiO2/Si substrates were grown at a relatively low temperature (650 °C) with a biased electrical field during pulsed-laser deposition. The (001) orientation of the films, which makes no contribution to their polarization, was effectively reduced by the in situ electrical field of strength of 70 V/cm. This results in a large increase of remnant polarization from 1.1 to 6.2 μC/cm2, and reduction of the coercive field from 70 to 50 kV/cm, comparing the films grown freely under the same condition. Furthermore, the films showed an excellent fatigue-free property of up to 1010 switching cycles.

Size dependence of optical eigenmodes in photonic quantum dots prepared by conformal deposition method

An approach was proposed to achieve Si-based three-dimensional optical microcavities, in which photons were confined by Bragg reflectors in all dimensions instead of the lateral confinement by using total internal reflection. Size-dependent photonic energy modes were observed when the lateral size is changed from 4.5to1.5μm. With decreasing lateral size the modes shift to higher energies, and the splitting between the modes increases which indicates that the three-dimensional optical microcavity looks like photonic quantum dots. The observed discrete optical eigenmodes show a quantitative agreement with numerical calculations of quantized photon states in photonic quantum dots.

Fabricating a silicon nanowire by using the proximity effect in electron beam lithography for investigation of the Coulomb blockade effect.

We present an approach to fabricate a silicon nanowire relying on the proximity effect in electron beam lithography with a low acceleration voltage system by designing the exposure patterns with a rhombus sandwiched between two symmetric wedges. The reproducibility is investigated by changing the number of rhombuses. A device with a silicon nanowire is constructed on a highly doped silicon-on-insulator wafer to measure the electronic transport characteristics. Significant nonlinear behavior of current-voltage curves is observed at up to 150 K. The dependence of current on the drain voltage and back-gate voltage shows Coulomb blockade oscillations at 5.4 K, revealing a Coulomb island naturally formed in the nanowire. The mechanism of formation of the Coulomb island is discussed.

On-chip silicon-based active photonic molecules by complete photonic bandgap light confinement

We demonstrate an on-chip silicon-based active photonic molecule (PM) structures formed by two coupled photonic quantum dots with complete photonic bandgap (PBG) light confinement. The photonic quantum dots are grown by conformal deposition of amorphous silicon nitride multilayers on patterned substrates. A fine structure of the coupled optical modes in PMs has been observed which shows similarity to the electronic bonding (BN) and antibonding (ABN) states in a molecule.

Fabrication and characterization of Si nanotip arrays for Si-based nano-devices

A simple and efficient technique for fabricating two-dimensional arrays of silicon nanotips by using electron beam lithography (EBL) and reactive ion etching (RIE) was reported. For the RIE processes, two kind of reactive gases, CHF3 and SF6, were used as plasma etching source for Si. The experiment results indicate that the reactive ion etching mechanism is different: the isotropic process for SF6 and anisotropic for CHF3. It is found that the mixed O2/SF6 plasma etching can improve the properties of profile and surface of Si nanotips. Under the condition of ratio ~15%, the 10 nm top size of Si nanotips was obtained.

Bismuth-layered-ferroelectric BI3TINBO9thin films grown by pulsed laser deposition

Abstract The polycrystalline ferroelectric films Bi3TiNbO9 (BTN) were deposited on Pt/Ti/SiO2/Si (001) substrates. The ferroelectric capacitors Pt/BTN/Pt show good hysteresis loop with 2Pr = 4.0 μC/cm2 under an applied voltage 2.5 V. This ferroelectric capacitor remained about 90% of its initial remnant polarization after 1×1010 reversing cycles at 100 kV/cm and 1 MHz. These ferroelectric capacitors also show an excellent retention property.