Dongmin Chen

Photoconductivity of single-crystalline selenium nanotubes

The photoconductivity of single-crystalline selenium nanotubes (SCSNTs) under a range of illumination intensities of a 633 nm laser is examined using a novel two-terminal device arrangement at room temperature. It is found that SCSNTs forms Schottky barriers with W and Au contacts, and the barrier height is a function of the light intensity. In the low-illumination regime below 1.46 × 10−4 µW µm−2, the Au–Se–W hybrid structure exhibits sharp on/off switching behaviour, and the turn-on voltages decrease with increasing illuminating intensities. In the high-illumination regime above 7 × 10−4 µW µm−2, the device exhibits ohmic conductance with a photoconductivity as high as 0.59 Ω cm−1, which is significantly higher than the reported values for carbon and GaN nanotubes. This finding suggests that a SCSNT is potentially a good photo-sensor material as well as a very effective solar cell material.

Observation of persistent photoconductance in single ZnO nanotube

Vertically aligned ZnO nanotube fabricated on an indium tin oxide substrate is found to exhibit strong persistent photoconductivity (PPC). Excitation wavelength-dependent conductance measurement on individual ZnO nanotube reveals the presence of defect states at 240 meV above the valence band edge, which are directly associated with the PPC effect. Our observations are consistent with the hypothesis that double ionization of defect-localized states is responsible for the PPC effect.

Evidence for electric-field-driven migration and diffusion of oxygen vacancies in Pr0.7Ca0.3MnO3

Low temperature growth Pr0.7Ca0.3MnO3 (PCMO) thin film showed high performance in electric field induced resistance switching (RS). To understand the micro-mechanism of RS in Metal/PCMO/Metal devices, structure evolution of PCMO under external electric field monitored inside transmission electron microscope (TEM) were performed. Evolution of the modulation stripe in as-grown PCMO sample was investigated when applying electric field. The new-generated modulation stripe gradually disappeared. These results indicate that oxygen ion migration plays a key role in RS.Combined in situ scanning probe microscopy with transmission electron microscope (TEM) has been used to study the field-induced migration of oxygen vacancies in the thin films of Pr0.7Ca0.3MnO3. Local structural stripes which are associated with the existing oxygen vacancies in the material have been imaged in situ in real time with TEM and are found to migrate under external electric field. The stripes can also be induced by an electric field and relax as the field is dismissed. The characteristic decay time of field-induced stripes are found to be in the similar order of magnitude as the measured resistance relaxation time in the materials. These results confirm microscopically that oxygen migration plays a key role in the bipolar resistance switching behaviors in this class of oxide materials.

Optical modulation of persistent photoconductivity in ZnO nanowires

In this study, ZnO nanowires (ZNWs)-based optoelectric devices are found to exhibit strong persistent photoconductivity (PPC) effect. An optical modulation on the PPC effect of the ZNWs with 980 nm infrared (IR) laser has been investigated. It was found that the decay time for the PPC can be significantly shortened by IR irradiation. The modulation mechanism related with the oxygen vacancies and the subband gap excitation is proposed. Based on this mechanism, the modulation behavior of the IR can be well explained. The present optical modulation on the PPC is suggested to have potential applications in enhancing the performance of ZnO-based photodetectors.

Electron density dependence of in-plane spin relaxation anisotropy in GaAs/AlGaAs two-dimensional electron gas

The authors investigated the spin dynamics of two-dimensional electrons in (001) GaAs∕AlGaAs heterostructure using the time-resolved Kerr rotation technique under a transverse magnetic field. The in-plane spin lifetime is found to be anisotropic below 150K due to the interference of Rashba [J. Phys. C 17, 6039 (1984)] and Dresselhaus [Phys. Rev. 100, 580 (1955)] spin-orbit coupling and D’yakonov-Perel [Sov. Phys. Solid State 13, 3023 (1972)] spin relaxation. The ratio of in-plane spin lifetimes is measured directly as a function of temperature and pump power, showing that the electron density in two-dimensional electron gas channel strongly affects the Rashba spin-orbit coupling.

Electroforming and endurance behavior of Al/Pr0.7Ca0.3MnO3/Pt devices

We have investigated the electroforming (EF) and resistive switching (RS) of Al/Pr0.7Ca0.3MnO3 (PCMO)/Pt devices by using high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy combined with transport measurement. The device prefers EF with positive bias with respect to Pt electrode and their endurance is enhanced with the chemically reactive Al electrode. The presence of an Al2O3−δ layer in Al/PCMO junction indicates that the oxidization and reduction near the Al/PCMO interface play a key role in the RS.

Effect of TaOx thickness on the resistive switching of Ta/Pr0.7Ca0.3MnO3/Pt films

The influence of interfacial structure on the resistance switching behavior of Ta/Pr0.7Ca0.3MnO3/Pt films was investigated by varying the reactive Ta electrode thickness. Structure and component analyses revealed that a TaOx layer formed at the interface and its thickness increased with the Ta thickness in the thin region while staying the same in the thick region. The similar thickness dependences of the negative differential resistance and resistance switching characteristics were observed and interpreted by the TaOx thickness dependent oxidization and reduction reaction across the interfacial region. This study demonstrates that the resistance switching characteristics could be improved by suitable interfacial engineering.

Impact of interfacial resistance switching on thermoelectric effect of Nb-doped SrTiO3 single crystalline

The thermoelectric properties of the bistable resistance states in Nb doped SrTiO3 single crystal have been investigated. The Seebeck coefficients for both low and high resistance states change linearly with temperature. The three-terminals contrast measurement demonstrates that a large fraction of the voltage drop is applied at the tiny volume near the bottom interface between the electrode and the oxide bulk. Therefore, the metallic oxide bulk plays a dominant role in the temperature dependence of Seebeck coefficients. The thermoelectric properties of new resistance switching (RS) devices with minimized non-RS volume could be exploited for the RS mechanism and novel applications.

Experimental observation of isotropic in-plane spin splitting in GaN/AlGaN two-dimensional electron gas

The circular photogalvanic effect (CPGE) was used to study the in-plane-orientation dependent spin splitting in the C(0001)-oriented GaN∕AlGaN two-dimensional electron gas (2DEG). The CPGE current induced by the interband transition shows an isotropic in-plane spin splitting in this system at room temperature. The spin relaxation time is found to be 14ps using the time resolved Kerr rotation technique, which is another evidence of the spin splitting in this 2DEG system.

Dependence of Band Alignment and Interfacial Suboxide GeOx Thickness of Thermal GeO2/Ge Stacks on GeO2 Thickness by X-ray Photoelectron Spectroscopy

The band discontinuity and interfacial suboxide GeOx of thermal GeO2/Ge stacks are investigated by X-ray photoelectron spectroscopy (XPS) on various thicknesses of GeO2 on Ge substrate. The band structure of GeO2/Ge stacks is found to be GeO2 thickness dependent with the variation range of band offset being ~0.8 eV, while the thickness of GeOx is GeO2 thickness independent. The physical origin of this dependence is attributed to the distribution of the gap states and fixed charges across the whole stacks.