Zhemi Xu,
Peiyuan Guan,
Adnan Younis,
Dewei Chu* and
Sean Li
School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. E-mail: d.chu@unsw.edu.au
First published on 15th December 2017
In this work, multilevel switching was achieved by a strategically designed alternative multi-layer structure with pure and Mn-doped SnO2. In this multilayer structure, by utilizing the pure SnO2 layer as an ionic defect diffusion barrier, the migration of ionic defects from the doped layers can be controlled and the intermediate resistance states were stabilized. The multilevel devices exhibit superior performances with a high ON/OFF ratio, low operation voltage and excellent retention. Such an alternative multi-layer structure could be a potential strategy for achieving high-density memories.
To date, the multilevel resistance states have been achieved by manipulating the applied voltage, such as pulse sequences.16–19 More often, these techniques are too complex or require sophisticated instrumentations that hinder their practical applications. On the other hand, engineering the device structures is another alternative approach to manipulate ionic defects and their kinetics in oxide based films.20 However, to explore engineered design memories for achieving multi-level resistance states in metal oxides is still in its infancy. Controlling the cationic or anionic defects distribution can regulate the ON/OFF ratio and multilevel resistive switching.
In this work, we strategically engineer alternative layered structures to manipulate the defect migration, thus enhancing the ON/OFF ratio and achieving multilevel resistance states. The alternative layer structure has been built by the pristine and doped metal oxides. In the structure, the pristine layer which has less defects acts as diffusion barriers to control the ionic migration in the doped layer. The abundant nature of SnO2 makes this material to be a good representative of wide band-gap metal oxides for the applications of transparent devices, including RRAMs.21,22 The nature defects in SnO2 is known as oxygen vacancies,22,23 which claims as the main factor that contributing the switching behaviour in SnO2. However, due to the limited concentration of oxygen defects, a further improvement in ON/OFF ratio of SnO2 is always desired.24 According to our calculations,25 Mn interstitials (Mnint) with Mn substitutions (Mnsub) is a favourable cation defect in Mn-doped SnO2. Its concentration strongly affects mobility in SnO2. Thus, in this work, pure and Mn-doped SnO2 were utilized as ionic migration barriers and ionic defect providers, respectively.
Fig. 2 (a) XRD patterns and enlarged (110) peaks of pure and Mn-doped SnO2 thin films; TEM and HRTEM images of (b) pure and (c) self-assembled Mn-doped SnO2 nanoparticles. |
The I–V curves of pure-layer is provided in Fig. 3(a). Pure-layer has been switched to ON state. In the testing cycle, the cell was switched from low resistance state (LRS) to high resistance state (HRS) at 4.4 V and then switched back to ON state at a reverse polarity of −2.3 V. It has been reported that oxygen vacancies are the intrinsic defects in SnO2 nanocrystals.22,23 This resistance switching behaviour could be attributed to the migration of oxygen vacancies within two electrodes in the presence of electric potential.28 However, it is hard to distinguish between two states (HRS/LRS = 3.8) during IV curves.
To compare the IV characteristics of Mn-doped SnO2, a device was fabricated with single Mn-doped layer (1-layer) by keeping all other parameters as same. The current–voltage characteristics of the 1-layer device expressed superior resistive switching characteristics over pure layer device as shown in Fig. 3(b). The separation between both resistance states was quite high (HRS/LRS = 243.4) than that of pure-layer. Moreover, the device switches itself to LRS at 1.9 V and then RESET back to HRS at −4.0 V. For 1-layer device, an additional intermediate state was also detected between the ON and OFF states as in Fig. 3(b).
The switching mechanism of the as-fabricated 1-layer device was illustrated in Fig. 3(d). As reported, Mn interstitials with Mn substitutions pair are the most favourite point defects in Mn-doped SnO2, and 6.25% Mn-doping concentration is enough to forming Mnint.25 Thus, Mnint should be the main factor contributing the switching behaviour. Initially, the defects – Mnint were distributed in the film, and the device is in the HRS state. When a voltage applied to the top electrode, the positive charged Mnint generally migrated towards the cathode electrodes, leaving the original interstitial sites as negative charged traps, forming the filament-like conductive paths and switched the thin film to ON state. While the negative voltage is applied, the Mnint ions were pushed back to the surrounding traps, rupturing the migration path ways, which switched the film to OFF state. The intermediate states are associated with lags in formation and blockage process of the Mnint filaments between being switched ON or OFF. The Mnint migration driven by the applied electric field forms filaments to conduct the charge throughout the film. Due to the low Mnint concentration, the energy barrier for the cation migration is relatively high, thus introducing the intermediate states between ON and OFF states. To further investigate the retention property of this intermediate state, data retention test was conducted, and surprisingly, the intermediate state failed to retain its state for longer time, and within 100 seconds, it merged with the more stable LRS as shown in Fig. S-1.† The unstable intermediate states indicate that the device can be switched between ON or OFF within relatively small voltages, therefore, the energy barrier of resistive switching is insufficient to maintain an intermediate state. To increase the ionic migration barriers, pure SnO2 was introduced as a diffusion barrier to control the ionic migration.
To further examine the role of defects and doped layer, another sample was made, in which pure and Mn doped SnO2 nanoparticles were physically mixed together to form a suspension solution. This solution was further used to fabricate a single layer device by following similar method as for previous two devices, attempting to increase the Mnint diffusion barrier by introducing pure SnO2 nanoparticles. The I–V curves are shown in Fig. 3(c). The resistive switching performance of the mixed-layer sample did not show much improvement than both samples. A smaller ON/OFF ratio, and higher SET/RESET voltages were observed (−4.82 V and 4.83 V respectively). In addition, the intermediate states were not observed any more. This may be because by mixing the doped and pure SnO2 nanoparticles, the defect concentration is not high enough to trigger multilevel switching, as illustrated in Fig. 3(e). Therefore, by simply mixing these two nanoparticles, the dispersion of pure SnO2 nanoparticles cannot obstruct the defect diffusion effectively. It indicates that reducing the Mnint concentration cannot effectively control the transportation behaviour and an optimal concentration of Mnint is critical to introduce the intermediate states.
Interestingly, by depositing another Mn-doped SnO2 layer on 2-layer thin film, the 3-layer sample demonstrates much better performance, which shows very large ON/OFF ratio (∼103), small SET/RESET voltage (1.93 V and −3.42 V respectively). Meanwhile, significant intermediate states were also observed. To better analyse the intermediate states, the I–V curves of 1-layer and 3-layer structure were replotted in log–log scale as compared in Fig. S-2.† Both I–V curves have linear relationship in the low voltage range. The slopes of linear curves were calculated and found close to 1 (marked as I ∝ V region), which represents ohmic behaviour at low voltages. At higher potentials, The IV curves were found to obey the Child's law (marked as I ∝ V2 region). The intermediate states are the regions marked as I ∝ Vα, known as the steep current increase region. It can be well explained by the trap-controlled space charge limited conduction (SCLC).15
For the 3-layer device, significantly high intermediate states were found as compared to the 1-layer as shown in Fig. S-2.† To explain such positive transition, a purposed model for the switching in 3-layer is illustrated in Fig. 4(e). In the pristine state, the Mnint defects are dispersed randomly in the bottom and top Mn-doped SnO2 layers. When a small negative voltage is applied, the Mnint starts to migrate towards top electrodes, attempting to form filaments. However, due to the diffusion barrier from pure SnO2 in between, the Mnint from the bottom layer cannot completely break through the middle layer. In this stage, the 3-layer thin film is not fully switched ON, thus, achieving an intermediate state. When increasing the applied voltage, Mnint from bottom layer may further diffuse to the middle layer, and finally connect with the top layer. At this stage, the 3-layer device is fully switched to ON state. Similarly, as in Fig. 4(c), when another pristine SnO2 layer added on top of the 3-layer structure, the intermediate state was also detected. This additional layer can further increase the Mnint diffusion barrier that results in even higher SET/RESET voltages and higher ON/OFF ratio. Thus, design of alternate layered device structures can lead to superior resistive switching performance with sustainable intermediate states.
Fig. 5 Retention tests of (a) sandwiched layered and (b) alternative 4 layered thin film; endurance tests of the 4-layer between (c) ON/intermediate States and (d) ON/OFF States. |
In addition, to understand the mechanism of resistive switching in the multi-layer structure, the cell size effect has been analyzed on the 4-layer sample with different top electrode diameters of 250 μm and 150 μm. The retention tests are provided in ESI Fig. S-3.† The resistance levels of 150 μm are only slightly higher than that of 250 μm, suggesting that the resistive switching is mainly because of local phenomenon.29 The temperature dependence of resistance level were tested by heating up the 4-layer on a hot plate from 20 °C to 100 °C. Compared to the change in ON state, there is an obvious decrease in the resistance of OFF states and a slight decrease in that of intermediate states as shown in ESI Fig. S-4.† Such results agrees with previous report, which was ascribed to a typical semiconductor behavior with temperature dependence.30
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra11681k |
This journal is © The Royal Society of Chemistry 2017 |