Self-compliance-improved resistive switching using Ir/TaO x /W cross-point memory
© Prakash et al.; licensee Springer. 2013
Received: 5 November 2013
Accepted: 5 December 2013
Published: 17 December 2013
Resistive switching properties of a self-compliance resistive random access memory device in cross-point architecture with a simple stack structure of Ir/TaO x /W have been investigated. A transmission electron microscope and atomic force microscope were used to observe the film properties and morphology of the stack. The device has shown excellent switching cycle uniformity with a small operation of ±2.5 V and a resistance ratio of >100. The device requires neither any frorming-process nor current compliance limit for repeatable operation in contrast to conventional resistive random access memory devices. The effect of bottom electrode morphology and surface roughness is also studied. The improvement is due to the enhanced electric field at the nanotips in the bottom electrode and the defective TaO x switching layer which enable controlled filament formation/rupture. The device area dependence of the low resistance state indicates multifilament formation. The device has shown a robust alternating current endurance of >105 cycles and a data retention of >104 s.
Resistive random access memory (RRAM) is the most promising candidate for the next-generation nonvolatile memory technology due to its simple structure, excellent scalability potential (<10 nm), long endurance, high speed of operation, and complementary metal-oxide-semiconductor (CMOS) process compatibility[1–7]. RRAM in cross-point architecture, in which top and bottom electrodes are placed at right angle to each other, is very attractive as it offers high-density integration with 4 F2, F being the minimum feature size area; three-dimensional (3D) stacking; and cost-effective fabrication[8, 9]. Switching uniformity is one of the important properties which require practical realization of cross-point devices with large array size. So it is necessary to investigate the factors affecting switching uniformity. Various binary transition metal oxides such as HfO x [5, 6, 10–12], TiO x [13, 14], TaO x [2, 7, 15–18], AlO x [19–21], ZrO x [22–24], WO x , etc. as a switching material are reported for RRAM application. Among them, recently, TaO x has attracted much attention owing to its superior material and switching properties such as having two stable phases, high thermal stability, small difference between the free energies of low and high resistance states, CMOS compatibility, long endurance, and high switching speed. So far,a cross-point resistive switching memory device in an Ir/TaO x /W structure has not yet been reported.
In this study, self-compliance-limited and low-voltage-operated resistive switching behaviors with improved switching cycle uniformity in a simple resistive memory stack of Ir/TaO x /W in cross-point architecture are reported. The physical properties of switching stack and bottom electrode morphology have been observed by transmission electron microscope (TEM) and atomic force microscope (AFM) analyses. The improvement is due to the defective switching layer formation as well as the electric field enhancement at the nanotips observed in the bottom electrode surface which results in controlled and uniform filament formation/rupture. The self-compliance property shows the built-in capability of the device to minimize the current overshoot during switching in one resistance (1R) configuration. The device has shown an alternating current (ac) endurance of >105 cycles and a data retention of >104 s.
Results and discussion
Improvement in the resistive switching and self-compliance behaviors of a forming-free resistive memory stack of Ir/TaO x /W in a cross-point structure has been obtained. The cross-sectional TEM image confirms the amorphous TaO x /WO x film. The AFM image shows the presence of nanotips on the W bottom electrode surface. The device has shown excellent switching uniformity during 100 consecutive dc sweeps with set/reset voltages of ±2.5 V and a resistance ratio of >100. The self-compliance behavior which comes from the bulk resistance of the stack shows the built-in capability of the device to minimize current overshoot during switching. The improvement in the switching is attributed to the formation of a defective switching layer and bottom electrode surface morphology with nanoscale tips which can enhance the electric field resulting in the uniform formation/rupture of the oxygen vacancy conducting filament. The device has exhibited an ac cycle endurance of >105 cycles and a data retention of >104 s. It is expected that this self-compliance, low-voltage-operated cross-point resistive memory device could be useful for the development of future nanoscale nonvolatile memory devices.
This work was supported by the National Science Council (NSC), Taiwan, under contract number NSC-102-2221-E-182-057-MY2.
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