Kyooho Jung

Influence of oxygen content on electrical properties of NiO films grown by rf reactive sputtering for resistive random-access memory applications

NiO films were prepared on Pt∕Ti∕SiO2∕Si substrates by rf reactive sputtering. The voltage-current characteristics of the Pt∕NiO∕Pt structures showed reproducible resistive switching behaviors at room temperature. The high- and low-resistance states were retained without applying an external bias voltage; the high- to low-resistance ratio was greater than 10. To investigate the influence of the oxygen content on the electrical properties, voltage-current characteristics of NiO films grown at various oxygen contents were investigated. As oxygen content increased from 5% to 10%, the resistance value of the NiO film drastically increased, and a resistive switching behavior was observed. However, as the oxygen content increased to 20%, the resistive switching behavior disappeared. The change in switching behavior was discussed in terms of Ni vacancies and compensating holes inside the NiO film. In addition, the memory properties of NiO-based resistive random-access memory were also investigated.

Effects of Ultrathin Al Layer Insertion on Resistive Switching Performance in an Amorphous Aluminum Oxide Resistive Memory

We prepared resistive switching Al–AlOx multilayered junctions and observed considerably improved endurance properties. The mechanism of the observed resistance switching basically reflects the filament model. The temperature dependence of the transport in each resistance state revealed additional features, that is a well-defined thermal activation behavior in the high-resistance state is not observed in the layered device and the metallic conduction in the low-resistance state is not affected. The improved endurance properties are discussed in terms of the increased effective number of active regions, where the Reset and Set processes probably occur before a permanent dielectric breakdown.

Resistive switching characteristics and set-voltage dependence of low-resistance state in sputter-deposited SrZrO3:Cr memory films

60-nm-thick Cr-doped SrZrO3 thin films with polycrystalline structure were fabricated on SrRuO3∕SrTiO3 (100) substrates at 450°C by off-axis radio-frequency sputtering. From room temperature current-voltage measurements of Pt∕Cr-doped SrZrO3∕SrRuO3 structures, reproducible bistable resistive switching behavior was observed. The dominant conduction mechanisms of the high-resistance state were Ohmic conduction in the low-electric-field region and Frenkel-Poole emission in the high-electric-field region, while the low-resistance state fully followed Ohmic conduction. These bulk-limited conduction mechanisms imply that resistive switching phenomena may be related to the conducting path in the SrZrO3:Cr film matrix. In addition, the initial-resistance value of as-deposited SrZrO3:Cr films was much higher than that of the high-resistance state, indicating that the first soft-breakdown behavior (defined as the resistance change from the initial-resistance state of as-deposited film to the low-resistance state) i...

Resistance switching mode transformation in SrRuO3/Cr-doped SrZrO3/Pt frameworks via a thermally activated Ti out-diffusion process

This work reports on a mechanism for irreversible resistive switching (RS) transformation from bipolar to unipolar RS behavior in SrRuO3 (SRO)/Cr-doped SrZrO3 (SZO:Cr)/Pt capacitor structures prepared on a Ti/SiO2/Si substrate. Counter-clockwise bipolar RS memory current-voltage (I–V) characteristics are observed within the RS voltage window of −2.5 to +1.9 V, with good endurance and retention properties. As the bias voltage increases further beyond 4 V under a forward bias, a forming process occurs resulting in irreversible RS mode transformation from bipolar to unipolar mode. This switching mode transformation is a direct consequence of thermally activated Ti out-diffusion from a Ti adhesion layer. Transition metal Ti effectively out-diffuses through the loose Pt electrode layer at high substrate temperatures, leading to the unintended formation of a thin titanium oxide (TiOx where x < 2) layer between the Pt electrode and the SZO:Cr layer as well as additional Ti atoms in the SZO:Cr layer. Cross-sectional scanning electron microscopy, transmission electron microscopy and Auger electron spectroscopy depth-profile measurements provided apparent evidence of the Ti out-diffusion phenomenon. We propose that the out-diffusion-induced additional Ti atoms in the SZO:Cr layer contributes to the creation of the metallic filamentary channels.

Low-voltage resistive switching of polycrystalline SrZrO3:Cr thin films grown on Si substrates by off-axis rf sputtering

A polycrystalline Cr-doped SrZrO3∕SrRuO3 layered structure showing a reproducible resistive switching behavior with a resistive switching voltage of ±2.5V was successfully fabricated on commercial Si (100) substrate by off-axis rf sputtering. Typical resistance values of high- and low-resistance states were about 100 and 5kΩ, respectively, so that the ratio of high- to low-resistance value is about 20. These values are appropriate for memory applications, and the resistive switching voltage of ±2.5V is the lowest value among the switching voltages of Cr-doped SrZrO3 films on Si substrates reported in recent literature. We suggest that the low-voltage resistive switching of the polycrystalline Cr-doped SrZrO3 thin film is attributed to the reduction of resputtering effects and the formation of a clean interface between the Cr-doped SrZrO3 thin film and the SrRuO3 bottom electrode layer by the use of 90° off-axis sputtering.A polycrystalline Cr-doped SrZrO3∕SrRuO3 layered structure showing a reproducible resistive switching behavior with a resistive switching voltage of ±2.5V was successfully fabricated on commercial Si (100) substrate by off-axis rf sputtering. Typical resistance values of high- and low-resistance states were about 100 and 5kΩ, respectively, so that the ratio of high- to low-resistance value is about 20. These values are appropriate for memory applications, and the resistive switching voltage of ±2.5V is the lowest value among the switching voltages of Cr-doped SrZrO3 films on Si substrates reported in recent literature. We suggest that the low-voltage resistive switching of the polycrystalline Cr-doped SrZrO3 thin film is attributed to the reduction of resputtering effects and the formation of a clean interface between the Cr-doped SrZrO3 thin film and the SrRuO3 bottom electrode layer by the use of 90° off-axis sputtering.

Effects of Switching Parameters on Resistive Switching Behaviors of Polycrystalline

Polycrystalline SrZrO3:Cr-based metal-oxide-metal (MOM) structures were fabricated on Si substrates by RF sputtering for commercial memory applications. From current-voltage measurements of the MOM structures, reproducible and bistable resistive switching behaviors were observed. In this paper, the effects of switching parameters, such as set power and voltage sweep rate, on the resistive switching characteristics of SrZrO3:Cr-based MOM structures were investigated. With increasing set power (= int I(V)dV, set voltage les V les 0) during set process [resistance change from a high-resistance state to a low-resistance state (LRS)], the LRS current was increased, and the shape of the LRS curve was changed from nonlinear to linear. Also, as the voltage sweep rate was decreased from 50 to 0.5 V/s, the current level of the LRS was increased, and the resistive switching behavior was more clearly observed. The results suggest that the change in switching behaviors is attributed to the variation of local conduction paths and that resistive switching behaviors are energy dependent.

\hbox{SrZrO}_{3}

We investigate the resistive switching power from unipolar resistive switching current-voltage characteristics in various binary metal oxide films sandwiched by different metal electrodes, and find a universal feature (the so-called universality) in the switching power among these devices. To experimentally derive the switching power universality, systematic measurements of the switching voltage and current are performed, and neither of these correlate with one another. As the switching resistance (R) increases, the switching power (P) decreases following a power law P ∝ R−β, regardless of the device configurations. The observed switching power universality is indicative of the existence of a commonly applicable switching mechanism. The origin of the power universality is discussed based on a metallic filament model and thermo-chemical reaction.

:Cr-Based Metal–Oxide–Metal Structures

We have investigated growth of CdTe thin films by using (As, GaAs) buffer layers for application of large scale IR focal plane arrays(IFPAs). Buffer layers were grown by molecular beam epitaxy(MBE), which reduced the lattice mismatch of CdTe/Si and prevented native oxide on Si substrates. CdTe thin films were grown by metal organic chemical deposition system(MOCVD). As a result, polycrystalline CdTe films were grown on Si(100) and arsenic coated-Si(100) substrate. In other case, single crystalline CdTe(400) thin film was grown on GaAs coated–Si(100) substrate. Moreover, we observed hillock structure and mirror like surface on the (400) orientated epitaxial CdTe thin film.

Switching Power Universality in Unipolar Resistive Switching Memories.

Nano structure films were deposited on (100) GaAs substrates using a modified MOCVD system and the effect of growth parameters on the structural properties were investigated. Different from conventional MOCVD systems, our reactor consist of pressure control unit and two heating zones ; one for formation of nano-sized particles and the other for the growth of nano particles on substrates. By using this instrument we successfully grow films with nano-grain size. The film grown at high reactor pressure has large grain size. On the contrast, the grain size decreases with a decrease in pressure of the reactor. Here, we introduce new growth methods of nano-grain structured films for high thermoelectric figure of merit.

Growth of Large Scale CdTe(400) Thin Films by MOCVD

In order to design nano structured materials with enhanced thermoelectric properties, the alloys in the pseudo-binary Bi2Te3-PbTe system are investigated for their micro structure properties. For this synthesis, the liquid alloys are cooled by the water quenching method. Micro structure images are obtained by using an electron probe micro analyzer(EPMA). Dendritic and lamellar structures are clearly observed with the variation in the composition ratio between Bi2Te3 and PbTe. The increase in the Bi2Te3 composition ratio causes to change of the structure from dendritic to lamellar. The Seebeck coefficient of sample 5, in which the mixture rate of Bi2Te3 is 83%, is measured as the highest value. In contrast, the others decrease with the increase of the Bi2Te3 composition ratio. Meanwhile, p-type characteristics are observed in sample 6, at 91%-Bi2Te3 mixture rate. The power factors of the all samples are calculated with the Seebeck coefficient and resistivity.