X. H. Li

Flexible Resistive Switching Memory Device Based on Amorphous InGaZnO Film With Excellent Mechanical Endurance

Semitransparent flexible resistive-switching memory devices, using amorphous InGaZnO as the switching layer, are fabricated on plastic substrates at room temperature. The device shows high performance, excellent flexibility, and mechanical endurance in bending tests. No performance degradation occurs, and the stored information is not lost after bending the device to different angles and up to 105 times. Studies on the temperature-dependent electrical properties reveal that the conducting channels of the low-resistance state are composed of oxygen-deficient defects, and partial oxidation of these defects switches the device to the high-resistance state. The unique electronic structure and flexible mechanical properties of amorphous InGaZnO ensure stable device performance in flexible applications.

Performance improvement of resistive switching memory achieved by enhancing local-electric-field near electromigrated Ag-nanoclusters

By introducing Ag nanoclusters (NCs), ZnO-based resistive switching memory devices offer improved performance, including improved uniformity of switching parameters, and increased switching speed with excellent reliability. These Ag NCs are formed between the top-electrode (cathode) and the switching layer by an electromigration process in the initial several switching cycles. The electric field can be enhanced around Ag NCs due to their high surface curvature. The enhanced local-electric-field (LEF) results in (1) the localization of the switching site near Ag NCs, where oxygen-vacancy-based conducting filaments have a simple structure, and tend to connect Ag NCs along the LEF direction; (2) an increase in migration and recombination rates of oxygen ions and oxygen vacancies. These factors are responsible for the improvement in device performance.

Local chemical states and thermal stabilities of nitrogen dopants in ZnO film studied by temperature-dependent x-ray photoelectron spectroscopy

Local chemical states and thermal stabilities of N dopants in ZnO:N film are investigated by temperature-dependent x-ray photoelectron spectroscopy. Different types of N local states are detected, including N2 molecules occupying O sites [(N2)O], –NO species, substitutional N atoms in O- and N-rich local environments (α- and β-NO). Compared to the β-NO, the α-NO shows a better thermal stability up to 723 K. However, the transformation from α-NO acceptor to undesirable (N2)O donor occurs above 743 K. The variation of N local states also affects Zn and O binding energies. Photoluminescence studies indicate the shallow acceptor nature of α-NO.

Pulsed laser deposition of high Mg-content MgZnO films: Effects of substrate temperature and oxygen pressure

High Mg-content, wurtzite MgZnO films were grown on sapphire (0001) substrates by pulsed laser deposition. It was found that the compositions, defect states, optical, and electrical properties of MgZnO films depend critically on the substrate temperature and oxygen pressure. As the substrate temperature increases, Mg content in single-phase wurtzite MgZnO films increases significantly from 27 close to 46 at.u2009%, and corresponding band gap is widened from 3.49 to 3.88 eV. X-ray photoelectron spectroscopy studies show that plenty of Zn interstitials are present in MgZnO films grown at lower oxygen pressures, which lead to the lattice expansion and the high electron concentration as native donor defects; while the films grown at higher oxygen pressure exhibit high resistivity with indeterminate conductivity type and the relatively wide band gap.

Effects of compliance currents on the formation and rupture of conducting filaments in unipolar resistive switching of CoO film

A study on the unipolar resistive switching of a CoO thin film shows that the compliance current (CC) has strong effects on the local structure of conducting filaments (CFs). Lower CCs produce CFs with simple connectivity and good controllability, resulting in a narrow distribution of switching voltages (SVs) and a high ratio of high-to-low resistance states. In contrast, the stronger net-like CFs are formed at higher CCs, and their complete rupture is difficult. Thus, the lower high-resistance states and a wide distribution of SVs appear in the reversible switching processes. Our results suggest that the generation of CFs with simple local structures is as important as the rupture process.

Size-controlled growth of ZnO nanowires by catalyst-free high-pressure pulsed laser deposition and their optical properties

Single crystalline ZnO nanowires were fabricated on Si (100) substrates by catalyst-free high-pressure pulsed laser deposition. It is found that the nanowires start to form when the substrate temperature and growth pressure exceed the critical values of 700 oC and 700 Pa, and their size strongly depends on these growth conditions. That is, the aspect ratio of the nanowires decreases with increasing temperature or decreasing pressure. Such a size dependence on growth conditions was discussed in terms of surface migration and scattering of ablated atoms. Room-temperature photoluminescence spectrum of ZnO nanowires shows a dominant near-band-edge emission peak at 3.28 eV and a visible emission band centered at 2.39 eV. Temperature-dependent photoluminescence studies reveal that the former consists of the acceptor-bound exciton and free exciton emissions; while the latter varies in intensity with the aspect ratio of the nanowires and is attributed to the surface-mediated deep level emission.

Improved resistive switching reliability by using dual-layer nanoporous carbon structure

We optimized the diameter and microgeometry of preformed conductive filaments (CFs) to improve the switching reliability of copper/nanoporous amorphous carbon (a-C)/platinum memory devices. Forming-free devices were obtained because of the introduction of preformed CFs into the nanoporous layer during the copper electrode evaporation process. The switching fluctuation decreased with the increasing preformed CF size in a certain range; however, the device with stronger preformed CFs suffered from high current in the first RESET process. Furthermore, to achieve both high switching uniformity and low power consumption, a dual-layer structure was proposed to regulate the microgeometry of preformed CFs. Compared with those of a pristine device and single-layer nanoporous device, the fluctuation of high/low resistance values was further suppressed to 26% and 21%, respectively. In addition, Resistive random access memory (RRAM) devices exhibited a fast switching speed ( 105 cycles),...