Flexible and transparent ReRAM with GZO memory layer and GZO-electrodes on large PEN sheet

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Abstract

Fabrication of flexible transparent resistive random access memory (FT-ReRAM) which consists of Ga doped ZnO (GZO) film not only as a memory layer but also as electrodes on the large Poly Ethylene Naphthalate sheet was attained by introducing RF plasma assist DC magnetron sputtering method. The averaged transmittance in the visible region (400–800 nm) was 66%. The memory effect was studied by using conducting atomic force microscope. It was suggested that the increase of Joule heating and oxygen vacancy density enhances memory effect, which is consistent with the redox model which has been proposed as the switching mechanism for conventional ReRAM. Stable and repeatable bi-polar resistive switching by application of the low voltage less than 2 V and low current less than 100 μA was confirmed in the FT-GZO-ReRAM. Reset switching, which is a switching from the low to the high resistance states, in GZO-ReRAM was confirmed to be smooth and continuous, which will enable a multilevel application. It was suggested that the smooth and continuous reset was brought about by Ga-doping.

Research highlights

► Low temperature formation of GZO films with arbitrary resistivity. ► Fabrication of all-GZO-based flexible and transparent ReRAM on a large plastic film. ► High memory performance such as compatibility for multilevel application was confirmed.

Introduction

Flexible transparent resistive random access memory (FT-ReRAM) can be fabricated by sandwiching a wide band gap oxide such as ZnO between transparent electrodes such as tin-doped indium oxide (ITO) on polymer flexible sheets [1]. However, an alternative transparent conducting oxide (TCO) to ITO is desired because of the high cost and scarcity of the indium element. Although one of the best candidates is Ga doped ZnO (GZO) [2], low glass transition temperature, Tg, of flexible substrates prevents fabrication of GZO films with low resistivity ρ [2]. Moreover, bombardment by energetic oxygen to the position of the substrate facing the erosion area of the target introduces the nonuniformity of the sputtered film [2], [3]. On the other hand, large dispersion of the resistance in the high resistance state, RHRS, during cycling is a serious problem in ReRAMs which show steep and discontinuous reset switching, where reset means resistance switching from the low resistance state (LRS) to the high resistance state (HRS). This is due to the fact that limiting the rapidly increasing current is relatively easy, whereas limiting the rapidly decreasing current is difficult. That is, RLRS is controllable simply by limiting the current which flows at the moment of the set switching to a target value using, for example, a transistor as a current limiter [4], whereas there is no method to limit the rapidly decreasing current which flows at the moment of the reset switching. Therefore, RHRS is uncontrollable and dispersive in ReRAMs with steep and discontinuous reset switching.

In this paper, we have solved all the above mentioned issues by introducing RF plasma assist for DC magnetron sputtering, which has not been practically applied to GZO–TCO films, and fabrication of FT-ReRAM consisting of GZO memory layer and GZO-electrodes (all-GZO-FT-ReRAM) was attained. It has been elucidated that memory effect is enhanced in moderately oxygen deficient GZO films, which is consistent with a resistance switching mechanism based on a redox reaction [5], [6], [7], [8], [9], [10]. GZO films deposited in the atmosphere of Ar + H2 (H2 5%) showed smooth and continuous reset switching. Therefore, RHRS of these films is controllable, which enables verify operation [11], [12], [13] and the multilevel application.

Section snippets

Experimental

Fig. 1 shows schematic of RF plasma assist DC magnetron sputtering system. For the optimization of the deposition conditions for fabrication of electrodes with low ρ and a memory layer with high ρ, GZO films were sputtered onto glass (Corning Eagle2000) or Poly Ethylene Naphthalate (PEN; Teijin DuPont Films Q-65FA) substrates in either Ar, Ar + H2, or Ar + O2 atmosphere at room temperature, where the strain point of the glass substrates and Tg of the PEN substrates are 593 °C and 155 °C,

Optimization of sputtering conditions for fabrication of FT-ReRAM

Fig. 2a and b represent the dependence of ρ and carrier concentration, n, measured at the center (x = 0) of the substrate on the H2-ratio as a function of the dT−S. WRF’s were set to 200 W for dT−S = 70 and 120 nm, whereas 0 W for dT−S = 50 nm. This is due to the structural problem of our sputtering system that dT−S cannot be reduced to less than 50 mm because the target interferes with the coil unless the coil is removed. However, ρ in the center of the substrate (i.e. x = 0) is almost independent of WRF

Conclusions

Fabrication of flexible transparent ReRAM consisting of the GZO memory layer and GZO-electrodes on the PEN sheet with large area was attained by introducing the RF plasma assist DC magnetron sputtering method. Resistive switching mechanism of all-GZO-FT-ReRAM can be explained by redox model as well as that of conventional binary transition metal oxides. Reset switching of all-GZO-FT-ReRAM which memory layer is GZO(H2-ratio 5%) is smooth and continuous, which enables the verify operation and the

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