Comparison of resistive switching characteristics using copper and aluminum electrodes on GeOx/W cross-point memories
© Rahaman and Maikap; licensee Springer. 2013
Received: 30 September 2013
Accepted: 20 November 2013
Published: 5 December 2013
Comparison of resistive switching memory characteristics using copper (Cu) and aluminum (Al) electrodes on GeO x /W cross-points has been reported under low current compliances (CCs) of 1 nA to 50 μA. The cross-point memory devices are observed by high-resolution transmission electron microscopy (HRTEM). Improved memory characteristics are observed for the Cu/GeO x /W structures as compared to the Al/GeO x /W cross-points owing to AlO x formation at the Al/GeO x interface. The RESET current increases with the increase of the CCs varying from 1 nA to 50 μA for the Cu electrode devices, while the RESET current is high (>1 mA) and independent of CCs varying from 1 nA to 500 μA for the Al electrode devices. An extra formation voltage is needed for the Al/GeO x /W devices, while a low operation voltage of ±2 V is needed for the Cu/GeO x /W cross-point devices. Repeatable bipolar resistive switching characteristics of the Cu/GeO x /W cross-point memory devices are observed with CC varying from 1 nA to 50 μA, and unipolar resistive switching is observed with CC >100 μA. High resistance ratios of 102 to 104 for the bipolar mode (CCs of 1 nA to 50 μA) and approximately 108 for the unipolar mode are obtained for the Cu/GeO x /W cross-points. In addition, repeatable switching cycles and data retention of 103 s are observed under a low current of 1 nA for future low-power, high-density, nonvolatile, nanoscale memory applications.
KeywordsMemory Resistive switches GeO x Copper Aluminum Solid electrolyte
Recently, resistive switching memory devices involving different materials such as Pr0.7Ca0.3MnO3 (PCMO), NiO x , SrTiO3[3, 4], TaO x [5–8], HfO x [9, 10], TiO2, ZrO2, Na0.5Bi0.5TiO3, and AlO x [14–16] are widely reported to replace conventional flash memory. On the other hand, conductive bridging resistive random access memory (CBRAM) involving the migration of cations (Ag+ or Cuz+, z = 1, 2) in solid electrolytes such as Ge x Se1-x[17–20], GeS2, Ta2O5, ZrO2[23–25], TiO x /ZrO2, GeSe x /TaO x , HfO2, CuTe/Al2O3, Ti/TaO x , ZnO, SiO2, and GeO x  is also reported. In this case, the mobile Ag+ or Cuz+ ions play an important role in the formation and dissolution of metallic filament in the solid electrolytes. Although memory characteristics using different solid electrolytes have been reported, GeO x -based CBRAM devices in the cross-point structure are also a beneficial choice. Memory characteristics using GeO x film in a Cu/GeO x /Al structure were first reported by Beynon and El-Samanoudy in 1987. Their extended work was published in 1991 using a Cu/GeO x /Au structure. Resistive switching memory using GeO x material in different structures such as Ni/GeO x /SrTiO x /TaN and Pt/SiGeO x /SiGeON/TiN has also been reported for future nonvolatile memory applications. On one hand, Schindler et al. has reported a GeO x layer for the Cu (Ag) diffusion barrier layer in a Cu (Ag)/GeSe/Pt structure. On the other hand, cross-point structures using different switching materials have been reported by several groups[6, 39–42] to have a high-density memory for future applications. It is known that resistive switching memories in cross-point architecture possess several attractive features and have attracted considerable attention in recent years because of the multilayer stacking of three-dimensional (3D) architecture, simplicity of their manufacturing, and the simplest interconnection configuration. Furthermore, resistive switching memory devices with low-current operation (<100 μA) are also an important issue. To mitigate those specifications, a cross-point memory using a Cu/GeO x /W structure has been compared with that using an Al/GeO x /W structure for the first time.
In this study, the memory characteristics using Cu and Al top electrodes (TEs) on GeO x /W cross-points have been compared. The cross-point structures were observed by high-resolution transmission electron microscopy (HRTEM). The Cu/GeO x /W cross-point memory devices have shown improved bipolar resistive switching characteristics as compared to the Al/GeO x /W cross-points, owing to the AlO x layer formation at the Al/GeO x interface. The RESET current deceases with the decrease of current compliances (CCs) from 50 μA to 1 nA for the Cu/GeO x /W devices, while the RESET current was independent (>1 mA) of CC in the range of 500 μA to 1 nA for the Al/GeO x /W cross-point memories. High resistance ratios of 102 to 104 under bipolar and approximately 108 under unipolar modes are observed for the Cu/GeO x /W cross-point memory devices. Repeatable switching cycles and data retention of approximately 103 s under a low CC of 1 nA were obtained for the Cu TE devices, which are very useful for low-power operation of high-density nonvolatile nanoscale memory applications.
Deposition parameters of different materials
Ar gas (SCCM)
1 × 10-5
2 × 10-5
8 × 10-6
8 × 10-6
Structures of the cross-point resistive switching memory devices
BE ~ 200 nm
Switching layer (10 nm)
Cu ~ 40 nm
Al ~ 160 nm
The cross-point structure and thicknesses of all materials were evaluated from a HRTEM image. HRTEM was carried out using a FEI Tecnai (Hillsboro, OR, USA) G2 F-20 field emission system. Memory characteristics were measured using an HP4156C semiconductor parameter analyzer (Agilent Technologies, Santa Clara, CA, USA). For electrical measurements, the bias was applied to the TE while the W BE was grounded.
Results and discussion
From Equation 1, the average LRS is 0.251/CC, which is close to the reported value of 0.250/CC for metallic filament[33, 48]. Therefore, the CBRAM device can be designed easily for low-power MLC operation.
Resistive switching memory characteristics using Cu and Al TEs on the GeO x /W cross-point memory devices have been compared. Improved memory characteristics of the Cu/GeO x /W structures under low current varying from 1 nA to 50 μA and a low voltage operation of ±2 V are observed as compared to those of the Al/GeO x /W structures. These cross-point memory structures are observed by HRTEM. The formation of AlO x layer with a thickness of approximately 5 nm at the Al/GeO x interface is observed, which is unstable to control the resistive switching phenomena. The RESET current scalability is observed for Cu TE, while it is high (>1 mA) and independent for the Al TE with CCs varying from 1 nA to 500 μA. Superior resistive switching memory performances in terms of high resistance ratio (102 to 104 under bipolar and approximately 108 under unipolar modes), long pulse endurance of >105 cycles under a CC of 50 μA, and good scalability potential are observed for the Cu/GeO x /W cross-point memory devices. Repeatable switching cycles and data retention of 103 s are also observed under a low CC of 1 nA. This study is important for high-density low-power 3D architecture in the future.
This work was supported by the National Science Council (NSC), Taiwan, under contract numbers NSC-101-2221-E-182-061 and NSC-102-2221-E-182-057-MY2.
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