Shuwei Li

Highly uniform resistive switching properties of amorphous InGaZnO thin films prepared by a low temperature photochemical solution deposition method.

We report on highly uniform resistive switching properties of amorphous InGaZnO (a-IGZO) thin films. The thin films were fabricated by a low temperature photochemical solution deposition method, a simple process combining chemical solution deposition and ultraviolet (UV) irradiation treatment. The a-IGZO based resistive switching devices exhibit long retention, good endurance, uniform switching voltages, and stable distribution of low and high resistance states. Electrical conduction mechanisms were also discussed on the basis of the current-voltage characteristics and their temperature dependence. The excellent resistive switching properties can be attributed to the reduction of organic- and hydrogen-based elements and the formation of enhanced metal-oxide bonding and metal-hydroxide bonding networks by hydrogen bonding due to UV irradiation, based on Fourier-transform-infrared spectroscopy, X-ray photoelectron spectroscopy, and Field emission scanning electron microscopy analysis of the thin films. This study suggests that a-IGZO thin films have potential applications in resistive random access memory and the low temperature photochemical solution deposition method can find the opportunity for further achieving system on panel applications if the a-IGZO resistive switching cells were integrated with a-IGZO thin film transistors.

Resistive switching properties and physical mechanism of cobalt ferrite thin films

We report reproducible resistive switching performance and relevant physical mechanism of sandwiched Pt/CoFe2O4/Pt structures in which the CoFe2O4 thin films were fabricated by a chemical solution deposition method. Uniform switching voltages, good endurance, and long retention have been demonstrated in the Pt/CoFe2O4/Pt memory cells. On the basis of the analysis of current-voltage characteristic and its temperature dependence, we suggest that the carriers transport through the conducting filaments in low resistance state with Ohmic conduction behavior, and the Schottky emission and Poole-Frenkel emission dominate the conduction mechanism in high resistance state. From resistance-temperature dependence of resistance states, we believe that the physical origin of the resistive switching refers to the formation and rupture of the oxygen vacancies related filaments. The nanostructured CoFe2O4 thin films can find applications in resistive random access memory.

Anomalous photoelectric effect of a polycrystalline topological insulator film.

A topological insulator represents a new state of quantum matter that possesses an insulating bulk band gap as well as a spin-momentum-locked Dirac cone on the surface that is protected by time-reversal symmetry. Photon-dressed surface states and light-induced surface photocurrents have been observed in topological insulators. Here, we report experimental observations of an anomalous photoelectric effect in thin films of Bi2Te3, a polycrystalline topological insulator. Under illumination with non-polarised light, transport measurements reveal that the resistance of the topological surface states suddenly increases when the polycrystalline film is illuminated. The resistance variation is positively dependent on the light intensity but has no relation to the applied electric field; this finding can be attributed to the gap opening of the surface Dirac cone. This observation of an anomalous photoelectric effect in polycrystalline topological insulators offers exciting opportunities for the creation of photodetectors with an unusually broad spectral range. Moreover, polycrystalline topological insulator films provide an attractive material platform for exploring the nature and practical application of topological insulators.

Structures and magnetic properties of p-type Mn:TiO2 dilute magnetic semiconductor thin films

The behavior of p-type conductivity in Mn-doped TiO2 films grown on LaAlO3 substrates by plasma-assisted molecular beam epitaxy has been investigated. Raman scattering, x-ray photoelectron spectroscopy, and x-ray diffraction studies indicate that the films are single phase, and Mn is successfully doped into the TiO2 matrix. Semiconducting behavior with p-type carriers was confirmed by Hall-effect measurements. The structural and electrical investigations demonstrate that the ferromagnetism observed at room temperature is an intrinsic property of the Mn:TiO2 films, and does not originate from any secondary phase. The magnetic properties of Ti1−xMnxO2 might be related to the formation of acceptor bound magnetic polarons, in which the spins of the holes and manganese are aligned via exchange interaction.

Polarization dependent photocurrent in the Bi2Te3 topological insulator film for multifunctional photodetection

Three dimensional Z2 Topological insulator (TI) is an unconventional phase of quantum matter possessing insulating bulk state as well as time-reversal symmetry-protected Dirac-like surface state, which is demonstrated by extensive experiments based on surface sensitive detection techniques. This intriguing gapless surface state is theoretically predicted to exhibit many exotic phenomena when interacting with light, and some of them have been observed. Herein, we report the first experimental observation of novel polarization dependent photocurrent of photodetectors based on the TI Bi2Te3 film under irradiation of linearly polarized light. This photocurrent is linearly dependent on both the light intensity and the applied bias voltage. To pursue the physical origin of the polarization dependent photocurrent, we establish the basic TI surface state model to treat the light irradiation as a perturbation, and we adopt the Boltzmann equation to calculate the photocurrent. It turns out that the theoretical results are in nice qualitative agreement with the experiment. These findings show that the polycrystalline TI Bi2Te3 film working as a multifunctional photodetector can not only detect the light intensity, but also measure the polarization state of the incident light, which is remarkably different from conventional photodetectors that usually only detect the light intensity.

Bipolar Resistance Switching in Transparent ITO/LaAlO3/SrTiO3 Memristors

We report reversible bipolar resistance switching behaviors in transparent indium-tin oxide (ITO)/LaAlO3/SrTiO3 memristors at room temperature. The memristors exhibit high optical transparency, long retention, and excellent antifatigue characteristics. The high performances are promising for employing ITO/LaAlO3/SrTiO3 memristors in nonvolatile transparent memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the migration of positively charged oxygen vacancies from the SrTiO3 substrate to the LaAlO3 film, resulting in Poole-Frenkel emission for the low resistance state and thermionic emission for the high resistance state.

Uniform bipolar resistive switching properties with self-compliance effect of Pt/TiO2/p-Si devices

We report uniform bipolar resistive switching characteristic with self-compliance effect of Pt/TiO2/p-Si devices in which TiO2 thin films were prepared directly on p-Si substrates by chemical solution deposition method. The resistive switching parameters of the Pt/TiO2/p-Si cell obtained, such as distribution of threshold voltages, retention time, as well as resistance variation of high resistance state (HRS) and low resistance state (LRS), were investigated, and the conduction mechanisms of HRS and LRS were analyzed. The conductive mechanism at LRS and low voltage region of HRS was dominated by Ohmic law. At the high voltage region of HRS, the conductive mechanism followed the space charge limited current theory. The resistive switching phenomenon can be explained by electron trapping and de-trapping process, in which the defects (most probably oxygen vacancies) act as electron traps. Our study suggests that using p-type silicon as bottom electrode can provide a simple method for fabricating a resistive ...

Conducting nanofilaments formed by oxygen vacancy migration in Ti/TiO2/TiN/MgO memristive device

High-quality TiN and TiO2thin films were grown on MgO(100) substrate by plasma-assisted molecule beam epitaxy.X-ray photoelectron spectroscopy and x-ray diffraction studies indicate that the films are single phase. The memory cell composed of Ti/TiO2/TiN/MgO showed a bistable bipolar resistive switching behavior with either electrode grounded. The nature of the bipolar resistive switching phenomenon could derive from the formation and annihilation of filaments induced by oxygen vacancy and oxygen ion migration near/at the bias-applied electrode. In addition, a simple calculation of the filamentary resistivity in model might indicate that different devices made by TiO2thin films with different phases might share the similar resistive switching mechanism.

High-Performance Bi2Te3-Based Topological Insulator Film Magnetic Field Detector

Topological insulators with the nanoscaled metallic surface state (3-5 nm) are actually of typical functional nanostructures. Significant efforts have been devoted to study new families of topological insulators and identifications of topological surface state, as well as fundamental physics issues relating to spin-polarized surface electronic states in the past few years. However, transport investigations that can provide direct experimental evidence for potentially practical applications of topological insulators are limited, and realization of functional devices based on topological insulators is still under exploration. Here, using the Sn-doping Bi2Te3 polycrystalline topological insulator films, we fabricated high-performance current-controlled magnetic field detectors. When a parallel magnetic field is applied, the as-fabricated device exhibits a stable and reproducible magneto-resistance (MR) switching behavior, and the corresponding MR ratio can be modulated by the applied current. Even under such a low magnetic field (0.5 kG), the device still shows a distinguishable MR switching performance, suggesting that topological insulator devices are very sensitive to external stimulation and potentially applicable to weak magnetic field detection.

Magnetic properties of Mn3O4 film under compressive stress grown on MgAl2O4 (001) by molecular beam epitaxy

High quality single-crystalline Mn3O4 thin films were grown on MgAl2O4 (001) substrates by plasma-assisted molecular beam epitaxy. It is found that the films are compressed in the (001) plane and elongated in the perpendicular direction via in-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction, which is confirmed by frequency hardening of relevant Raman bands. Different from the bulk, the epitaxial film with a thickness of 65 nm shows more obvious magnetic anisotropy and higher magnetic phase transition temperatures (TN = 50 K, T1 = 40.5 K, and T2 = 36 K) than that of the bulk (TN = 42 K, T1 = 39 K, and T2 = 33 K). The variation of magnetic properties could be attributed to the changes of interplay among spin, orbital, and lattice degrees of freedom owing to the residual strain in the epitaxial film.