Wenfeng Xiang

Characteristics of LaAlO3/Si(100) deposited under various oxygen pressures

High-k dielectric LaAlO3 (LAO) thin films were deposited by laser molecular-beam epitaxy under various oxygen pressures. X-ray diffraction showed that the LAO films were amorphous. The chemical compositions and thickness of the LAO thin films were measured using Rutherford backscattering spectrometry. Al/LAO/Si metal–oxide–semiconductor capacitor structures were fabricated and measured. The leakage current density versus voltage curves showed that the leakage current of the LAO thin films decreased by increasing the oxygen pressure of the preparation. Specifically, when prepared under the active oxygen (containing atom oxygen), the leakage current of the LAO sample was lower than that prepared under the general oxygen. All of the capacitance–voltage curves have a positive shift along the voltage axis and the flatband voltage decreased with increasing oxygen pressures during the LAO thin-film preparation.

Positive colossal magnetoresistance in a multilayer p–n heterostructure of Sr-doped LaMnO3 and Nb-doped SrTiO3

A positive colossal magnetoresistance (CMR) has been discovered in an epitaxial multilayer p–n heterostructure fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by laser molecular-beam epitaxy. In contrast to the negative CMR of the LaMnO3 compound family, positive CMR is observed in the temperature range from 100 to 300 K. The largest value of the magnetoresistance (MR) ratio (ΔR/R0,ΔR=RH−R0), 517%, is one order of magnitude larger than that of simple p–n junctions of the same materials previously reported. A very large MR ratio, 297%, remains in a low field of 0.01 T. Even at a temperature as high as 300 K, a MR ratio as large as 17.3% is still observed.

Thermal stability of LaAlO3/Si deposited by laser molecular-beam epitaxy

High-k gate dielectric material LaAlO3 (LAO) films were deposited directly onto silicon substrates by laser molecular-beam epitaxy. The thermodynamic stability of LAO films deposited at different substrate temperatures and of LAO films postannealed at 1000u200a°C was studied by high-resolution transmission electron microscopy and capacitor–voltage measurements. These studies show that the interfacial reaction between the LAO film and silicon substrate is strongly correlated to the substrate temperature and ambient conditions. In oxygen containing ambient, the interfacial reaction often occurs not only during film deposition but also during the course of postannealing. LAO films annealed at 1000u200a°C in nitrogen ambient have better thermal stability with silicon than LAO films annealed in oxygen ambient do. Both kinds of films remain amorphous after 1000u200a°C annealing.

Field-effect transistors with LaAlO3 and LaAlOxNy gate dielectrics deposited by laser molecular-beam epitaxy

High permittivity LaAlO3 (LAO) and LaAlOxNy (LAON) thin films have been deposited directly on a Si(100) substrate using a laser molecular-beam epitaxy technique. Metal–oxide–silicon field-effect transistors (MOSFETs) are fabricated using such LAO and LAON thin films as gate dielectrics and well-behaved transistor characteristics have been observed. High-resolution transmission electron microscopy observations indicate that LAO thin films can remain amorphous structure even after annealing at 1000°C. The small equivalent oxide thickness (EOT) of 17A is achieved for 75A LAO film with an effective dielectric constant of 17.2±1 for the whole gate stack. Furthermore, a smaller EOT, larger drive current, and lower subthreshold slope have been observed for devices with the LAON thin film. For all the devices, the gate leakage currents are at least two orders of magnitude lower than that of the same electrical thickness SiO2. Reasonable subthreshold slopes of 248 and 181mV∕dec were obtained for MOSFETs with LAO a...

Voltage tunable photodetecting properties of La0.4Ca0.6MnO3 films grown on miscut LaSrAlO4 substrates

We report the voltage tunable photodetecting properties of a La0.4Ca0.6MnO3 film grown on miscut LaSrAlO4 (001) substrates under ultraviolet pulsed laser irradiation at ambient temperature. The photovoltage and photocurrent peak sensitivities can be tuned in the range of 0.295–0.786 V/mJ and 0.172–0.314 A/mJ, respectively, when the applied bias is changed from −20 to +20u2002V, indicating that the vicinal manganite film can be used as an electric tunable ultraviolet photodetector. A possible mechanism based on the high resolution transmission electron microscopy is introduced to explain the experiment results.

Optical properties of p-type In-doped SrTiO3 thin films

We report the optical properties of p-type SrInxTi1−xO3 (x=0.1 and 0.2) films prepared by laser molecular beam epitaxy under different oxygen pressures. The results of x-ray diffraction and near-field scanning optical microscopy indicate that the films have high crystallinity, smoothness, and uniformity. Hall measurement confirms that the films are p-type semiconductors. The optical transmittance spectra reveal that the films are highly transparent in most of the visible region. A blueshift of the absorption edge with an increase in oxygen pressure during deposition has been observed and discussed. Moreover, the first-order Raman scattering peaks of the optical phonons in these In-doped SrTiO3 films are presented, and they are Raman inactive in bulk SrTiO3 single crystal.

Magnetical and electrical tuning of transient photovoltaic effects in manganite-based heterojunctions

Magnetically and bias current tunable transient photovoltaic (TPV) responses have been investigated in a manganite-based heterojunction composed of a La2/3Ca1/3MnO3 film and an n-type Si substrate at ambient temperature. Under irradiation of 248 nm pulsed laser with 20 ns duration the TPV peak values can be modulated in a range of -125 to 138 mV when the applied magnetic field perpendicular to the interface changes from -6.4 to + 6.4 kOe, and the relative variations (TPV(H) - TPV(0))/TPV(0) reach up to about 1000%. In addition, TPV responses can be also affected by bias current, and the photoresponse peaks change from positive to negative with the currents from -350 to 350 μA. These results indicate that the manganite-based heterojunction can be used for magnetically and electrically tunable ultraviolet photodetectors.

Nano-domain orientation modulation of photoresponse based on anisotropic transport in manganite films

Anisotropic carrier transports have been observed in La0.67Ca0.33MnO3 and La0.4Ca0.6MnO3 films grown on miscut MgO (001) substrates under pulsed-laser irradiations without external bias. We discussed the mechanism of the transport process under ultrafast laser irradiation in tilted manganite films. A model of anisotropic diffusion in transient photoresponse is presented. From the microstructure of La0.4Ca0.6MnO3 and its photoresponse property in different directions, we can see that the nanometer size domain orientation can modulate the photoresponse signals, indicating a new-type nanometer scale photosensitive source. Copyright (C) EPLA, 2012

Near Infrared Lateral Photovoltaic Effect in LaTiO3 Films

We have reported on the lateral photovoltaic effect of LaTiO3 films epitaxially grown on (100) SrTiO3 substrates. Under illumination of continuous 1064u2009nm laser beam on the LaTiO3 film through SrTiO3 substrate, the open-circuit photovoltage depended linearly on the illuminated position. The photosensitivity can be modified by bias current. These results indicated that the LaTiO3 films give rise to a potentially photoelectronic device for near infrared position-sensitive detection.

Enhancing mid-infrared spectral response at the LaAlO3/SrTiO3 interface by magnetic field

Many unexpected properties have been found in the LaAlO3/SrTiO3 heterostructure, but the interaction of the many ground states at its interface remains unclear. Here, we demonstrate an optical property of this n-type heterostructure where the mid-infrared spectral responsivity at the interface is enhanced by an external magnetic field. The field intensity ranged from 0.8 to 6 kOe at a low temperature (19u2009K) as measured with our spectral response measurement system. Two spectral peaks related to the spin-orbit coupling effect were also observed at wavelengths 2400u2009nm and 3700u2009nm. The intriguing phenomena relate to changes in the crystallographic structure and subband structure at the interface.