Tingao Tang

Reproducible unipolar resistance switching in stoichiometric ZrO2 films

The resistance switching characteristics of stoichiometric ZrO2 film were investigated for nonvolatile memory. The Al∕ZrO2∕Al device presents reliable and reproducible switching behaviors. The on/off ratio of two stable states is larger than 2×103. It is suggested that the current-voltage characteristics are governed by the Schottky conduction mechanism in high voltage region, while the filament conduction is suggested in low voltage region. The switching process is explained in terms of the spontaneous reversible reaction between electrode and ZrO2 films with the contribution of Joule heating effect by the external current. It provides a possible solution for low device yield of nonstoichiometric oxides.

Endurance enhancement of Cu-oxide based resistive switching memory with Al top electrode

We investigated the switching performance of Cu-oxide films with Al, Pt, and Ti electrodes. Compared with Pt electrode, the Al electrode shows better stability, preferable endurance, and larger resistance ratio. An interface AlOx layer is detected by transmission electron microscopy and Auger electron spectroscopy. This layer can strongly affect the movement of oxygen vacancies. However, the sample with pure Ti electrode almost has no switching characteristics. Ti/TiN electrode with thin Ti exhibits good switching behavior. The thickness control of Ti layer is quite critical. So we suggest that the oxygen diffusion in electrode is another important factor for switching performance.We investigated the switching performance of Cu-oxide films with Al, Pt, and Ti electrodes. Compared with Pt electrode, the Al electrode shows better stability, preferable endurance, and larger resistance ratio. An interface AlOx layer is detected by transmission electron microscopy and Auger electron spectroscopy. This layer can strongly affect the movement of oxygen vacancies. However, the sample with pure Ti electrode almost has no switching characteristics. Ti/TiN electrode with thin Ti exhibits good switching behavior. The thickness control of Ti layer is quite critical. So we suggest that the oxygen diffusion in electrode is another important factor for switching performance.

Resistive Memory Switching of

Poly crystalline CuxO films produced by plasma oxidation are investigated for nonvolatile memory applications. Reversible bistable resistive switching from a high-resistance state to a low-resistance state, and vice versa, is observed in an integrated Al/CuxO/Cu structure under voltage sweeping. More than 3000 repetitive cycles are observed in 180-mum memory devices with an on/off ratio of ten times. Data testing shows that the devices meet the ten-year retention requirement for the storage of programmed logic signals.

\hbox{Cu}_{x}\hbox{O}

The compliance-current dependence of the resistive-switching behaviors is investigated in TaN/Cu<i>x</i>O/Cu memory devices with 1R architecture and 1T1R architecture, respectively. The correlation of reset current <i>I</i> _reset and on-state resistance <i>R</i> _on can be verified by adjusting the compliance current <i>I</i> _comp. Meanwhile, <i>I</i> _reset and <i>R</i> _on become independent on <i>I</i> _comp in the 1R architecture when <i>I</i> _comp is below 1 mA. A serious compliance-current overshoot phenomenon is <i>in situ</i> observed in 1R-architecture device, and it remarkably affects the resistive-switching characteristics because the compliance current dominates the memory behaviors. Therefore, resistive-switching investigation based on 1T1R architecture is much more reliable.

Films for a Nonvolatile Memory Application

An analytical model for channel potential and subthreshold swing of the symmetric and asymmetric double-gate Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is presented. Two-dimensional Poisson equation is solved analytically using series method and channel potential is obtained. The analytical expression for subthreshold swing is achieved. Model results are compared with Medici simulation results, both of them turn out to agree very well. The results show the variation of channel potential and subthreshold swing with channel length, gate bias, and oxide thickness, which will provide some guidance for the integrated circuit designs.

In Situ Observation of Compliance-Current Overshoot and Its Effect on Resistive Switching

For the embedded application of the resistive switching memory using Cu x O films, the controllable switching polarity and optimized low resistance state was investigated. Stable bipolar switching behavior was demonstrated for the TaN / Cu x O / Cu device. TaON was observed at the anode interface by x-ray photoelectron spectroscopy and transmission electron microscopy, which is believed to play a key role in the resistance transition. The physics behind this phenomenon of reactive top-electrode is revealed. A filament/charges trapped combined model is proposed to clarify the electrical characteristics of the top-electrode/oxide reactions. This observation presents a unique opportunity to elucidate a universal mechanism for the resistive switching of transitional metal oxides.

An analytic model for channel potential and subthreshold swing of the symmetric and asymmetric double-gate MOSFETs

Analytical models for electric potential, threshold voltage, and subthreshold swing of the junctionless surrounding-gate field-effect transistors are presented. Poisson equation is solved and the electric potential is obtained. With the potential model, explicit expressions for threshold voltage and subthreshold swing are obtained. The analytical results are compared with those from simulations and excellent agreements are observed. The analytical models are useful not only for fast circuit simulations, but also for device design and optimization.

Role of TaON interface for CuxO resistive switching memory based on a combined model

Broad dispersions of operation parameters are generally observed with continuous resistive switching. In this letter, we explore a prototype resistive random access memory (RRAM) device for switching uniformity improvement. Compared with Al/CuxO/Cu structure, by introducing a thin phase-change Ge2Sb2Te5 (GST) film between CuxO and Al top electrode, the device exhibits much better resistive hysteresis and switching uniformity. A combined filamentary conduction model is proposed to clarify the role of GST layer on the resistive switching stabilization.

Analytical Models for Electric Potential, Threshold Voltage, and Subthreshold Swing of Junctionless Surrounding-Gate Transistors

Crystallization process and amorphous state stability of Si-Sb-Te films with different Si concentration (10, 20 at. %) and Sb/Te ratio (2:3 and 1:1) have been studied and compared with Ge2Sb2Te5 (GST) film by in situ film resistance measurements. The effects of Si concentration and Sb content on crystalline resistivity, crystallization temperature, activation energy of crystallization, and amorphous state stability of films have been studied. The activation energy Ea of crystallization of GST film was confirmed to be 2.34 eV, while the activation energy Ea of Si10Sb36Te54 film increased to 2.99 eV and further reached to 3.65 eV for Si20Sb32Te48 film when the Si content increased to 20 at. %. Si addition increased the crystallization temperature and crystalline resistivity of Si-Sb-Te films largely, and enhanced the amorphous state stability of the films, while Sb revealed contrary effects to Si. The microstructures of Si-Sb-Te films were analyzed through x-ray diffraction and high resolution transmission ...

Improvement of Resistive Switching Uniformity by Introducing a Thin GST Interface Layer

It is found that the voltage drop across a 170-nm-thick Pb(Zr0.4Ti0.6)O3 film keeps constantly at a well-defined coercive voltage during domain switching, irrespective of the applied voltage and frequency, and that the switching current of domains is reversely proportional to the resistance of loading resistors in the circuit. A simple formalism is derived for the speed of polarization reversal short into a few nanoseconds. The maximum speed of domain switching is limited by the time of compensation charge dissipation via loading resistors in the circuit, instead of reverse domain nucleation and growth. However, in most cases, the switching current decays with time and is thus peaked under different applied voltages, as observed in an 87-nm-thick film. This phenomenon is understood from our work due to the presence of interfacial passive layers that modulate switching current transient through the circuit RC-time constant, besides the consideration of a broad coercive-voltage distribution in a genuine thi...