Solution processed laterally grown Zinc Oxide microstructures for next generation computing devices
Introduction
Traditional computing memories such as Flash memories are quickly approaching to their fundamental physical and technological scaling limits [1]. To tackle modern computing challenges, continued technological innovation is a pre-requisite to catalyze further advancements for the development of next generation computing devices [2]. Resistive random-access memories (RRAM) have gained considerable attention as an alternative computing memory due its simple structure, high operational speed, low power consumption, high density and importantly great potential to replace conventional Flash memories [[3], [4], [5], [6], [7], [8]]. Usually, RRAM devices are comprised of a layered architecture, where a solid electrolyte (as a switching layer) is sandwiched between two metal contacts which stores data in multiple electrical resistance states, namely, low resistance states (LRS) and high resistance states (HRS). The electrical conduction states of the RRAM are switched by imposing potential in continuous sweeping and/or in pulse modes [9,10].
Up to now, a vast range of materials have been reported to exhibit resistive switching (RS) characteristics. Among many, Zinc oxide (ZnO) is of significant importance due to its wide bandgap (~3.4 eV), high excitation binding energy (60 meV), low cost, inherent multi-functionality easy fabrication and availability in a wide range of low-dimensional nanostructures [11,12]. One-dimensional (1-D) semiconducting nanoarchitectures exhibit high surface-to-volume ratios and have great tendency for novel device applications. Moreover, 1-D ZnO nano-microstructure-based RS memories have expressed tremendous potential with ultralow leakage current at low conductive state, low operating potentials and ultra-high memory windows [[13], [14], [15], [16]].
Most of the reported ZnO RS devices are constructed by vertically stacked structure considering the short channel lengths between two electrodes [[17], [18], [19], [20], [21]]. Usually, the device operational voltage is proportional to the channel length and therefore, by controlling the thickness of switching layer (channel lengths), operational voltages can be finely tuned [22]. On the other hand, if the packing density of switching material in the vertically stacked device is not high enough, then it causes unembellished uncertainties in the device performance due to short circuiting between two electrodes. A laterally configured device structure not only avoid short circuiting problems, but also have a great potential to integrate on a planar substrate at sub-micro or nano meter scale. Decent efforts have been made to fabricate 1-D nanostructured lateral RS devices such as by randomly dispersing 1-D nanostructures on pre-fabricated electrodes using electron beam lithography and focused-ion-beam techniques [23,24]. But these techniques demand complex processing and are not time efficient. Therefore, mass production and large-scale manufacturing of lateral devices through these methods are almost scarce.
In this work, a simple, stable and reliable method to fabricate few tens of micrometre long, one-dimensional laterally oriented ZnO microstructures (micro-bushes) is reported. The ZnO micro-bushes were directly grown within a gap of about 100 μm between two Cu-islands (created by grooving copper tape) through electrochemical deposition. The laterally grown ZnO micro-bushes-based RS device exhibited highly repeatable, stable and reliable “write-once-read-many-times” (WORM) behavior. The WORM memories can store information only once that cannot be modified. Thus, WORM memories are very useful to store permanent data as required for archiving and non-editable storage equipment. Furthermore, for the first time, the investigations on intrinsic randomness of RS behavior in laterally bridged ZnO micro-bushes were carried out in this work. Thus, laterally bridged ZnO microstructures have proven an excellent candidate for future non-volatile, noneditable and neuromorphic computing applications.
Section snippets
Results and discussion
In this study, a cost-effective, solution processed and simple fabrication route to synthesize laterally grown ZnO micro-bushes is reported. For a typical synthesis of ZnO micro-bushes, about 2 cm long piece of copper tape was pasted on the glass slide and a cut was made on it using sharp medical grade blade. As a result, two distinct portions of Cu tape with a gap of about 100 μm were created and a significant loss in conductivity was verified using a multi-meter (a very high resistance (~106
Conclusions
In summary, a simple and unique strategy to grow laterally bridged ZnO micro-bushes architecture across a ~100 μm cavity between two distinct copper regions is developed. A direct integration of ZnO lateral microstructures into solution processible resistive memory device with excellent memory characteristics is achieved. The memory device exhibited WORM behavior with exceptional stability, non-volatile nature, on−off current ratio higher than ~106 with long data detention abilities. Moreover,
Materials
The common available materials such as standard glass slides, conductive copper tape (with 7.8 mm width, 0.10 mm thickness) and single edge razor blade was used. Before using (Cu tape, it was treated with diluted Nitric acid followed by rinsed with de-ionized water and then dried with N2 gas. All chemicals were purchased from Sigma-Aldrich and used without further purification.
Synthesis of ZnO micro-bushes
First a strip of copper tape (~2 cm long) was adhered to the glass slide and then a cut (grove) of about 100 μm was
CRediT author statement
Adnan Younis: Conceptualization, Methodology, Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing.
Prime novelty statement
This work is a first attempt to fabricate a new type of laterally bridged Zinc Oxide (ZnO) nano/micro-bushes-based resistive memory by employing simple Cut and Grow strategy using routinely available materials. The laterally bridged RS device demonstrated (i) write once read many times (WORM) behavior and (ii) a great potential to tackle large variations in stochastic computing schemes. The memory device was also found exceptionally stable, reliable with an on-off ratio higher than 106. The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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