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RRAM characteristics using a new Cr/GdOx/TiN structure.

Jana D, Dutta M, Samanta S, Maikap S - Nanoscale Res Lett (2014)

Bottom Line: After measuring 50 RRAM devices randomly, the 8-μm devices exhibit superior resistive switching characteristics than those of the 0.4-μm devices owing to higher recombination rate of oxygen with remaining conducting filament in the GdOx film as well as larger interface area, even with a thinner GdOx film of 9 nm.The GdOx film thickness dependence RRAM characteristics have been discussed also.Memory device shows repeatable 100 switching cycles, good device-to-device uniformity with a switching yield of approximately 80%, long read endurance of >10(5) cycles, and good data retention of >3 × 10(4) s at a CC of 300 μA.

View Article: PubMed Central - PubMed

Affiliation: Thin Film Nano Technology Laboratory, Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan, Tao-Yuan, 333, Taiwan, debanjan.jana@gmail.com.

ABSTRACT
Resistive random access memory (RRAM) characteristics using a new Cr/GdOx/TiN structure with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm(2) have been reported in this study. Polycrystalline GdOx film with a thickness of 17 nm and a small via-hole size of 0.4 μm are observed by a transmission electron microscope (TEM) image. All elements and GdOx film are confirmed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. Repeatable resistive switching characteristics at a current compliance (CC) of 300 μA and low operating voltage of ±4 V are observed. The switching mechanism is based on the oxygen vacancy filament formation/rupture through GdOx grain boundaries under external bias. After measuring 50 RRAM devices randomly, the 8-μm devices exhibit superior resistive switching characteristics than those of the 0.4-μm devices owing to higher recombination rate of oxygen with remaining conducting filament in the GdOx film as well as larger interface area, even with a thinner GdOx film of 9 nm. The GdOx film thickness dependence RRAM characteristics have been discussed also. Memory device shows repeatable 100 switching cycles, good device-to-device uniformity with a switching yield of approximately 80%, long read endurance of >10(5) cycles, and good data retention of >3 × 10(4) s at a CC of 300 μA.

No MeSH data available.


Related in: MedlinePlus

Cumulative probability of leakage current, formation voltage, SET/RESET voltage, and RESET currents. (a) Leakage current distributions with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. The thicknesses of GdOx film are 17 and 9 nm. (b) Forming voltage, (c) SET/RESET voltage, and (d) RESET currents with different device sizes and a thickness of GdOx film of 17 nm. Fifty devices were measured randomly for each size. It is found that the 8-μm RRAM device shows best uniformity as compared to other sizes.
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Fig5: Cumulative probability of leakage current, formation voltage, SET/RESET voltage, and RESET currents. (a) Leakage current distributions with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. The thicknesses of GdOx film are 17 and 9 nm. (b) Forming voltage, (c) SET/RESET voltage, and (d) RESET currents with different device sizes and a thickness of GdOx film of 17 nm. Fifty devices were measured randomly for each size. It is found that the 8-μm RRAM device shows best uniformity as compared to other sizes.

Mentions: Figure 4 exhibits typical bipolar current-voltage (I-V) characteristics of the Cr/GdOx/TiN RRAM device with a size of 8 × 8 μm2. The thickness of the GdOx film is 17 nm. The sweeping voltage is shown, as indicated by 1 to 5 inside the figure. This RRAM device is operated with a CC of 300 μA. First switching cycle of the memory device shows low formation voltage (Vform) +1.5 V. Initially, memory devices show low leakage current, which is controlled by the size of the device, and defects and thickness of the GdOx film. Figure 5a represents cumulative probability of the leakage currents of randomly measured more than 50 RRAM devices with sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. It is observed that the leakage current increases with increasing device sizes from 2 to 8 μm. A large size device has more defects than that of a smaller device. That is why the 8-sμm devices have the highest leakage current. On the other hand, the leakage currents are the same for the 1- and 0.4-μm devices, which is due to the current measurement limitation by our probe station. The leakage current is increased by decreasing the thickness of the switching layer of 9 nm, as shown in Figure 5a. Basically, both the smaller device size and the thicker GdOx film of 17 nm have smaller leakage current. As similar to the device size dependent leakage current, the Vform also decreases with increasing the device sizes. Figure 5b represents the distribution of the formation voltages of more than 50 RRAM devices. The average values of Vform are found to be 3.5 and 1.9 V for the 2- and 8-μm devices, respectively. However, the average SET voltage (VSET) has little changes from 1.27 to 1.12 V for the 2- to 8-μm devices (Figure 5c). Therefore, the VSET is independent of the device sizes from 2 to 8 μm. This indicates that all 50 devices with size of 8 μm can be operated at a low voltage of <4 V, which would be very useful for practical realization. It is also observed that all 8-μm devices show formation (yield of 100%) whereas the 2-μm devices have only 72% yield. Even after formation, the clear SET is observed only 40% of 2-μm devices. Therefore, some devices do not show RESET. However, the clear SET is observed 78% of the 8-μm devices. The 8-μm device shows a typical VSET (1.2 V) from the second cycle, as shown in Figure 4. After that, the memory device shows good bipolar resistive switching phenomena under small RESET voltage (VRESET) of -1.2 V. The average VRESET value of 50 devices is found to be -1.5 V (Figure 5c). The value of average VRESET is similar or higher than the value of VSET, which is useful for better read operation of these RRAM devices. Even this RRAM device can read at negative voltage because of the higher VRESET values. In Figure 4, the RESET current (IRESET) is found to be 320 μA. This suggests that both SET and RESET currents (300 vs. 320 μA) are almost the same which signifies good current clamping between two electrodes and GdOx switching material. Considering 50 RRAM devices with a size of 8 μm (Figure 5d), the average value of IRESET is higher for the first cycle as compared to the second cycle (320 vs. 390 μA), which is owing to a current overshoot effect during the formation or the first cycle of the pristine device at a CC of 300 μA. However, most of the devices show slightly higher IRESET for the first cycle. The current conduction is understood by fitting an I-V curve in a log-log scale, as shown in Figure 6. Slope value of current at a low resistance state (LRS) is 1.1 (IαV1.1) whereas slope values of current at a high resistance state (HRS) are 1.1 (IαV1.1), 1.8 (IαV1.8), 2.8 (IαV2.8), and 3.6 (IαV3.6) at low to high voltage regions, respectively. The slope values of HRS are reported 1, 2, 4, and 6 by Shang et al. [39], 1.1, 1.3, and 8.5 by Rubi et al. [40], and 1.2, 2.2, and 3.9 by us [41]. This represents that the current transport of LRS is dominating by Ohmic whereas HRS follows by trap controlled space charge limited current conduction (TC-SCLC) of our RRAM device. The resistive switching mechanism is based on the formation and rupture of oxygen vacancy conducting filament in the GdOx material depending upon electrical stimulus. When positive bias is applied on the TE, the weak Gd-O bonds on the grain boundaries break and oxygen ions (O2-) migrate towards the TE and leaving behind oxygen vacancy as well as conducting path formed through polycrystalline grain boundary. Then, the memory device triggers from HRS to LRS. Considering the Gibbs free energy of Cr2O3 and Gd2O3, the Cr TE is not oxidized and a part of GdOx is shown to be oxygen-rich (Figure 2). Basically, the oxygen vacancy filament is formed in between the O-rich GdOx and TiOxNy layers. The oxygen vacancy filaments in different switching materials are also reported by other groups [1, 5, 7]. Both O-rich GdOx and TiOxNy interfacial layers will behave as a series of the conducting filaments, and a current overshoot effect is not observed (Figure 5d). When a negative voltage is applied on the TE, O2- ions are driven out from TE/GdOx interface and re-oxidize the conductive path and memory device switch back from LRS to HRS. Therefore, the O2- ions migrate through crystal grain boundaries and will control the SET/RESET of both the resistance states.Figure 4


RRAM characteristics using a new Cr/GdOx/TiN structure.

Jana D, Dutta M, Samanta S, Maikap S - Nanoscale Res Lett (2014)

Cumulative probability of leakage current, formation voltage, SET/RESET voltage, and RESET currents. (a) Leakage current distributions with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. The thicknesses of GdOx film are 17 and 9 nm. (b) Forming voltage, (c) SET/RESET voltage, and (d) RESET currents with different device sizes and a thickness of GdOx film of 17 nm. Fifty devices were measured randomly for each size. It is found that the 8-μm RRAM device shows best uniformity as compared to other sizes.
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Fig5: Cumulative probability of leakage current, formation voltage, SET/RESET voltage, and RESET currents. (a) Leakage current distributions with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. The thicknesses of GdOx film are 17 and 9 nm. (b) Forming voltage, (c) SET/RESET voltage, and (d) RESET currents with different device sizes and a thickness of GdOx film of 17 nm. Fifty devices were measured randomly for each size. It is found that the 8-μm RRAM device shows best uniformity as compared to other sizes.
Mentions: Figure 4 exhibits typical bipolar current-voltage (I-V) characteristics of the Cr/GdOx/TiN RRAM device with a size of 8 × 8 μm2. The thickness of the GdOx film is 17 nm. The sweeping voltage is shown, as indicated by 1 to 5 inside the figure. This RRAM device is operated with a CC of 300 μA. First switching cycle of the memory device shows low formation voltage (Vform) +1.5 V. Initially, memory devices show low leakage current, which is controlled by the size of the device, and defects and thickness of the GdOx film. Figure 5a represents cumulative probability of the leakage currents of randomly measured more than 50 RRAM devices with sizes ranging from 0.4 × 0.4 to 8 × 8 μm2. It is observed that the leakage current increases with increasing device sizes from 2 to 8 μm. A large size device has more defects than that of a smaller device. That is why the 8-sμm devices have the highest leakage current. On the other hand, the leakage currents are the same for the 1- and 0.4-μm devices, which is due to the current measurement limitation by our probe station. The leakage current is increased by decreasing the thickness of the switching layer of 9 nm, as shown in Figure 5a. Basically, both the smaller device size and the thicker GdOx film of 17 nm have smaller leakage current. As similar to the device size dependent leakage current, the Vform also decreases with increasing the device sizes. Figure 5b represents the distribution of the formation voltages of more than 50 RRAM devices. The average values of Vform are found to be 3.5 and 1.9 V for the 2- and 8-μm devices, respectively. However, the average SET voltage (VSET) has little changes from 1.27 to 1.12 V for the 2- to 8-μm devices (Figure 5c). Therefore, the VSET is independent of the device sizes from 2 to 8 μm. This indicates that all 50 devices with size of 8 μm can be operated at a low voltage of <4 V, which would be very useful for practical realization. It is also observed that all 8-μm devices show formation (yield of 100%) whereas the 2-μm devices have only 72% yield. Even after formation, the clear SET is observed only 40% of 2-μm devices. Therefore, some devices do not show RESET. However, the clear SET is observed 78% of the 8-μm devices. The 8-μm device shows a typical VSET (1.2 V) from the second cycle, as shown in Figure 4. After that, the memory device shows good bipolar resistive switching phenomena under small RESET voltage (VRESET) of -1.2 V. The average VRESET value of 50 devices is found to be -1.5 V (Figure 5c). The value of average VRESET is similar or higher than the value of VSET, which is useful for better read operation of these RRAM devices. Even this RRAM device can read at negative voltage because of the higher VRESET values. In Figure 4, the RESET current (IRESET) is found to be 320 μA. This suggests that both SET and RESET currents (300 vs. 320 μA) are almost the same which signifies good current clamping between two electrodes and GdOx switching material. Considering 50 RRAM devices with a size of 8 μm (Figure 5d), the average value of IRESET is higher for the first cycle as compared to the second cycle (320 vs. 390 μA), which is owing to a current overshoot effect during the formation or the first cycle of the pristine device at a CC of 300 μA. However, most of the devices show slightly higher IRESET for the first cycle. The current conduction is understood by fitting an I-V curve in a log-log scale, as shown in Figure 6. Slope value of current at a low resistance state (LRS) is 1.1 (IαV1.1) whereas slope values of current at a high resistance state (HRS) are 1.1 (IαV1.1), 1.8 (IαV1.8), 2.8 (IαV2.8), and 3.6 (IαV3.6) at low to high voltage regions, respectively. The slope values of HRS are reported 1, 2, 4, and 6 by Shang et al. [39], 1.1, 1.3, and 8.5 by Rubi et al. [40], and 1.2, 2.2, and 3.9 by us [41]. This represents that the current transport of LRS is dominating by Ohmic whereas HRS follows by trap controlled space charge limited current conduction (TC-SCLC) of our RRAM device. The resistive switching mechanism is based on the formation and rupture of oxygen vacancy conducting filament in the GdOx material depending upon electrical stimulus. When positive bias is applied on the TE, the weak Gd-O bonds on the grain boundaries break and oxygen ions (O2-) migrate towards the TE and leaving behind oxygen vacancy as well as conducting path formed through polycrystalline grain boundary. Then, the memory device triggers from HRS to LRS. Considering the Gibbs free energy of Cr2O3 and Gd2O3, the Cr TE is not oxidized and a part of GdOx is shown to be oxygen-rich (Figure 2). Basically, the oxygen vacancy filament is formed in between the O-rich GdOx and TiOxNy layers. The oxygen vacancy filaments in different switching materials are also reported by other groups [1, 5, 7]. Both O-rich GdOx and TiOxNy interfacial layers will behave as a series of the conducting filaments, and a current overshoot effect is not observed (Figure 5d). When a negative voltage is applied on the TE, O2- ions are driven out from TE/GdOx interface and re-oxidize the conductive path and memory device switch back from LRS to HRS. Therefore, the O2- ions migrate through crystal grain boundaries and will control the SET/RESET of both the resistance states.Figure 4

Bottom Line: After measuring 50 RRAM devices randomly, the 8-μm devices exhibit superior resistive switching characteristics than those of the 0.4-μm devices owing to higher recombination rate of oxygen with remaining conducting filament in the GdOx film as well as larger interface area, even with a thinner GdOx film of 9 nm.The GdOx film thickness dependence RRAM characteristics have been discussed also.Memory device shows repeatable 100 switching cycles, good device-to-device uniformity with a switching yield of approximately 80%, long read endurance of >10(5) cycles, and good data retention of >3 × 10(4) s at a CC of 300 μA.

View Article: PubMed Central - PubMed

Affiliation: Thin Film Nano Technology Laboratory, Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan, Tao-Yuan, 333, Taiwan, debanjan.jana@gmail.com.

ABSTRACT
Resistive random access memory (RRAM) characteristics using a new Cr/GdOx/TiN structure with different device sizes ranging from 0.4 × 0.4 to 8 × 8 μm(2) have been reported in this study. Polycrystalline GdOx film with a thickness of 17 nm and a small via-hole size of 0.4 μm are observed by a transmission electron microscope (TEM) image. All elements and GdOx film are confirmed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. Repeatable resistive switching characteristics at a current compliance (CC) of 300 μA and low operating voltage of ±4 V are observed. The switching mechanism is based on the oxygen vacancy filament formation/rupture through GdOx grain boundaries under external bias. After measuring 50 RRAM devices randomly, the 8-μm devices exhibit superior resistive switching characteristics than those of the 0.4-μm devices owing to higher recombination rate of oxygen with remaining conducting filament in the GdOx film as well as larger interface area, even with a thinner GdOx film of 9 nm. The GdOx film thickness dependence RRAM characteristics have been discussed also. Memory device shows repeatable 100 switching cycles, good device-to-device uniformity with a switching yield of approximately 80%, long read endurance of >10(5) cycles, and good data retention of >3 × 10(4) s at a CC of 300 μA.

No MeSH data available.


Related in: MedlinePlus