RESISTANCE SWITCHING BEHAVIOR OF ZnO THIN FILMS FOR RANDOM ACCESS MEMORY APPLICATIONS

We investigated resistance switching behavior of the Ag/ZnO/Ti structures for random access memory devices. These films were prepared on glass substrate by dc sputtering technique at room temperature. The resistance switching follows unipolar switching mode with small switching voltages (0.4 V – 0.6 V). Our results figured out that the Ag/ZnO/Ti/Glass structure is a candidate structure for nonvolatile data storage applications.


1.INTRODUCTION
Resistance random access memory

RESULTS AND DISCUSSION
As-prepared Ag/ZnO/Ti/Glass structures were initially in a high resistance state (HRS).
To find out the switching voltages, we increased sequently the largest sweeping voltage V maz (V max = 0.1 V, 0.15 V, 0.2 V…) to observe the set process.Compliance current of 100 mA was applied to prevent permanent structure breakdown.Figure 2 shows the I-V characteristics of the set and the reset process.
In the 0 to 0.45 V sweeping, corresponding to the black line (V max = 0.45 V), a sudden current increase (resistance decrease) from HRS to low resistance state (LRS), was observed at about V SET ~ 0.42 V, the LRS remained until the end of the sweeping.This sweeping switched the structure into LRS, corresponding to "set" process.
In order to increase the ratio, the effect of the depositing oxygen pressure of ZnO thin films on the ratio switching is underdoing.

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ReRAM) has attracted extensive attention for their applications to nonvolatile data storage technologies due to its simple structure, low power consumption, low cost, nonvolality and high speed performance [1-12].The resistance switching was observed from various materials, such as perovskite oxides (Pr 0.7 Ca 0.3 MnO 3 , La 0.7 Ca 0.3 MnO 3 , Cr-doped SrZrO 3 , Cr-doped SrTiO 3 …) [1-9], transition metal oxides (NiO, Ti, CuO, ZrO 2 , ZnO…)…[10-12].The resistance switching effect is regarded as memory effect.The memory effect can be observed from the current -voltage characteristics of metal -insulator -metal structure (MIM), in which resistance of the MIM structure reversely changes between the high resistance state (HRS) and the low resistance state (LRS), corresponding to logic signal (off and on states or 0 and 1 states).Two switching modes, unipolar switching and bipolar switching, have been observed to describe the switching between HRS and LRS.Although the resistance switching effect can be obtained in various materials, the origin of resistance switching is controversial.The clear explanation of resistance switching in various materials is the challenge and motivation for current research.We have already published our research on the resistance switching effect in Pt/Cr-doped SrTiO 3 /La 0.5 Sr 0.5 CoO 3 structure [5-9].In this study, we reported resistance switching behavior in the Ag/ZnO/Ti Science & Technology Development, Vol 16, No.K1-2013 The Ag/ZnO/Ti/Glass structure was fabricated by sputtering technique at room temperature, ZnO thin film was sandwiched between top (Ag) and bottom (Ti) electrode materials.A 150 nm-thick metallic Ti layer was deposited on a commercial glass substrate in Ar gas ambient of 6x10 -3 Torr, depositing current I Ti = 1 A, while the 100 nm-thick ZnO layer was deposited in Ar + O 2 (Ar/O 2 = 1) mixed gas ambient of 6x10 -3 Torr, depositing current I ZnO = 0.3 A. The Ag top electrode of 75 nm-thick was deposited on the ZnO thin film in Ar gas ambient of 6x10 -3 Torr, depositing current I Ag = 0.15 A. During the deposition of Ag layer, a mask was used for top electrode patterning.The size of Ag top electrode is 1 mm in diameter.Current-voltage (I-V) measurements were carried out using a Keithley 2400 source meter.During the electrical measurement, the positive sweep voltage is applied to the Ag top electrode, while the Ti bottom electrode is grounded.The thickness of these films was determined by Dektak 6M Stylus Surface Profilometer.The crystalline phases of the thin films were characterized in -2 mode by D8 Advance (Bruker) x-ray diffractometer (XRD) with CuKα radiation (λ = 0.154 nm).The depositing process and crystalline analysis will be presented elsewhere.

Figure 1 .
Figure 1.Schematic diagram of the Ag/ZnO/Ti structure

Figure 2 .
Figure 2. I-V characteristics of the set and Reset process.The Ag/ZnO/Ti/Glass structure now is in LRS.The next sweeping process from 0 to V max (0.1 V, 0.15 V, 0.2 V….) was applied to the structure to observe the reset process, corresponding to a red line.The I-V characteristic in this process is different from that in the set process above (0 to 0.45 V).The value of current of the reset process is higher than the current value of the set process measured at the same voltage.In the 0 to 0.6 V sweeping process, current increased linearly with sweeping voltage until 0.51 V, then current gradually decreased.An abrupt drop of current could be observed at voltage of 0.55 V (V RESET ~ 0.55 V), the structure switched back to HRS.The set and reset process correspond to the "write" and "erase" data in storaging device.We can observe the memory effect or memory window (two current values at a certain voltage) in the voltages below V SET .The I-V curve with a triangle shape is visible, similar to the model of memristive system analyzed by Srtukov et al[13].The resistance switching is in unipolar mode, the switching between HRS and LRS only depends on magnitude of

Figure 3 .
Figure 3. I-V characteristics of the Ag/ZnO/Ti structure in the 0 to 0.6 V sweeping Figure 4 show the current evolution of the HRS and LRS within 10 switching cycles.The current values were read out at 0.25 V in each

Figure 4 .
Figure 4. Memory window of the Ag/ZnO/Ti structure