Chen-Hsi Lin

Resistive Switching Mechanisms ofV-Doped

The resistive switching behaviors of sputtered V-doped SrZrO3 (V:SZO) memory films were investigated in this letter. The current states of the memory films were switched between high current state (H-state) and low current state (L-state). The resistance ratio of the two current states was over 1000 at a read voltage. The switching mechanism from L- to H-state corresponds to the formation of current paths. However, this mechanism from Hto L-state is thought to be due to the fact that the defects present in the V:SZO film randomly trap electrons, and hence, the current paths are ruptured. The conduction mechanism of the H-state is dominated by ohmic conduction, whereas the L-state conduction is dominated by Frenkel-Poole emission. The polarity direction of the resistive switching is an intrinsic property of the SrZrO3 oxides. The V:SZO films with high uniformity and good stability are expected to be used in nonvolatile memory

hboxSrZrO_3

In this paper, nonpolar resistive switching behavior is reported for the first time in a SZO-based memory device. The electrode materials used which have different conductivities affect the resistive switching properties of the device. The Al/V:SZO-LNO/Pt device shows nonpolar switching behavior, whereas the Al/V:SZO/LNO device has bipolar switching property. The resistance ratios of these two devices are quite distinct owing to the difference between the resistance of low resistance states. The Al/V:SZO-LNO/Pt device with lower resistive switching voltages (mnplus7 V turn on and mnplus2 V turn off) and higher resistance ratio is more suitable for practical applications compared to the Al/V:SZO/LNO device. The switching speed of the Al/V:SZO-LNO/Pt device is 10 ns, which is the fastest speed that has ever been reported. The conduction mechanisms, nondestructive readout property, retention time, and endurance of this device are also reported in this paper.

Memory Films

The Ti/ZrO2/Pt resistive memory devices with one transistor and one resistor (1T1R) architecture are successfully fabricated in this letter. The tested devices show low operation current (20 μA), low switching voltage (set/reset, 0.8/-1 V), and reliable data retention for low-resistance state (LRS) with a 20-μA set current at 80°C (over ten years) via an excellent current limiter, namely, a metal-oxide-semiconductor field-effect transistor (MOSFET). In addition, multilevel storage characteristics are also demonstrated by modulating the amplitude of the MOSFET gate voltage. The various LRS levels obtained are possibly attributed to the formation of different numbers and sizes of conducting filaments consisting of oxygen vacancies caused by an external electric field. Moreover, reproducible resistive switching characteristics up to 2000 switching cycles are achieved in the same device. Our 1T1R ZrO2-based resistive switching access memory with low-power and highly reliable multilevel operation has high potential for practical applications.

Voltage-Polarity-Independent and High-Speed Resistive Switching Properties of V-Doped

Low-power, bipolar resistive switching (RS) characteristics in the Ti/ZrO2/Pt nonvolatile memory with one transistor and one resistor (1T1R) architecture were reported. Multilevel storage behavior was observed by modulating the amplitude of the MOSFET gate voltage, in which the transistor functions as a current limiter. Furthermore, multilevel storage was also executed by controlling the reset voltage, leading the resistive random access memory (RRAM) to the multiple metastable low resistance state (LRS). The experimental results on the measured electrical properties of the various sized devices confirm that the RS mechanism of the Ti/ZrO2/Pt structure obeys the conducting filaments model. In application, the devices exhibit high-speed switching performances (250 ns) with suitable high/low resistance state ratio (HRS/LRS > 10). The LRS of the devices with 10 year retention ability at 80 °C, based on the Arrhenius equation, is also demonstrated in the thermal accelerating test. Furthermore, the ramping gate voltage method with fixed drain voltage is used to switch the 1T1R memory cells for upgrading the memory performances. Our experimental results suggest that the ZrO2-based RRAM is a prospective alternative for nonvolatile multilevel memory device applications.

\hbox{SrZrO}_{3}

The fabrication of SrZrO3 (SZO) memory devices with oxygen-rich (OR) and oxygen-deficient (OD) double layers, their resistive switching (RS) characteristics and mechanisms are investigated in this study. Due to the difference in oxygen content between the OR and OD layers formed by an oxygen flow control (OFC) process during SZO deposition, the RS region is effectively reduced and localized within the OR layer, which leads to a low operation voltage and stable RS behaviours. Furthermore, the OFC SZO device exhibits high-speed switching (10 ns) over 400 times and long retention (>106 s), showing promising potential for next-generation nonvolatile memory applications.

Thin Films

The effects of vanadium doping on resistive switching (RS) characteristics and mechanisms of RF-sputtered SrZrO3 (SZO)-based thin films are investigated in this paper. The physical and electrical properties of SZO-based thin films are modulated by vanadium doping due to the Zr4+ ion replaced by V5+, further affecting the RS parameters of SZO-based thin films. The conduction mechanisms of SZO-based thin films are dominated by ohmic conduction (hoping conduction) and Frenkel-Poole emission for the low resistance state (LRS) and the high resistance state (HRS), respectively. The turn-on process might be attributed to the formation of conducting filaments consisting of oxygen vacancies with the effective barrier height (φB,eff) in the range of 0.10-0.13 eV, whereas the turn-off process might result from thermally assisted oxidation of oxygen vacancies by the Joule heating effect. Furthermore, the introduction of the high valence cation (V5+) in a Zr4+ site of SZO crystalline structure can suppress the formation of oxygen vacancies due to the charge neutrality restriction. Such suppression leads to the changes in the forming voltage, turn-on voltage, HRS resistance, dielectric constant, and φB,eff with vanadium doping concentration up to 0.2 mol%, which is within the solid solubility limit based on our measured lattice constants and Vegards law.

Low-Power and Highly Reliable Multilevel Operation in

The effects of embedded Pt (E-Pt) metal layer on the resistive switching characteristics and mechanisms of SrZrO3 (SZO) memory devices are investigated in this study. The E-Pt is shown by transmission electron microscopy observation to thermally diffuse into SZO thin film to form E-Pt clusters and no chemical reaction occurs between Pt and SZO during 600 °C postannealing process. The carrier transport of high resistance state current of 600 °C E-Pt devices is dominated by Ohmic conduction and Frenkel–Poole (F–P) emission in the low- and high-voltage region, respectively, which is quite different from that of without E-Pt memory devices being principally dominated by F–P emission. Furthermore, the forming voltage and turn-on voltage of E-Pt devices are significantly lowered to −3.5 V and |2.3| V, respectively, due to the reduction in effective thickness of SZO thin films caused by E-Pt clusters formed, which benefit the future development of resistive random access memory devices in practical application.

\hbox{ZrO}_{2}

The stabilization of the resistive switching properties is necessary to realize the memory application of the SrZrO3(SZO)-based resistive switching devices. During continuous resistive switching cycle, broad variations of the resistive switching parameters of the SZO-based memory devices can be improved by a thin embedded Cr layer. The Cr metal layer is proposed to diffuse into and dope the SZO thin film to produce the space charge region, further reducing the effective resistive switching region. Hence, the good stabilization of the resistive switching properties can be obtained in the SZO films with embedded Cr layer.

1T1R RRAM

Based on the phenomenon of endurance degradation problem caused by no sufficient oxygen ions for resistive switching, we use the oxygen plasma treatment in HfO2 layer to increase the extra available oxygen ions in resistive random access memory devices. To avoid the Ti top electrode directly absorbing the additional oxygen ions from HfO2 layer with oxygen plasma treatment, a thin HfO2 film is inserted to separate them. Therefore, the endurance degradation can be suppressed in the present structure. High speed (30 ns) and large endurance cycles (up to 1010 cycles) are achieved in this device structure for next generation nonvolatile memory application.

A study on low-power, nanosecond operation and multilevel bipolar resistance switching in Ti/ZrO2/Pt nonvolatile memory with 1T1R architecture

The effect of the annealing treatment of a HfO2 resistive switching layer and the memory performance of a HfO2-based resistive random access memory (cross-bar structure) device were investigated. Oxygen is released from HfO2 resistive switching layers during vacuum annealing, leading to unstable resistive switching properties. This oxygen release problem can be suppressed by inserting an Al2O3 thin film, which has a lower Gibbs free energy, between the HfO2 layer and top electrode to form a Ti/Al2O3/HfO2/TiN structure. This device structure exhibited good reliability after high temperature vacuum annealing and post metal annealing (PMA) treatments. Moreover, the endurance and retention properties of the device were also improved after the PMA treatment.