New electronic memory device concepts based on metal
oxide-polymer nanostructures planer diodes
Asal Kiazadeh, Paulo R. Rocha, Qian Chen and Henrique L. Gomes
Center of Electronic, Optoelectronic and Telecommunications, Faculdade de Ciências e Tecnologia, Universidade do Algarve, Campus de Gambelas,
8005-139 Faro, Portugal
Abstract:
Nanostructure silver oxide thin films diodes can exhibit resistive switching effects. After an electroforming process the device can be programmed between a low conductance (off-state) and high conductance (on- state) with a voltage pulse and they are already being considered for non-volatile memory applications. However, the origin of programmable resistivity changes in a network of nanostructure silver oxide embedded in polymer is still a matter of debate. This work provides some results on a planer diode which may help to elucidate resistive switching phenomena in nanostructure metal oxide diodes. The XRD pattern after switching appears with different crystalline planes, plus temperature dependent studies reveal that conduction of both on and off states is weak thermal activated. Intriguing the carrier transport is the same for both on and off-states. Difference between states comes from the dramatic changes in the carrier density. The main mechanism of charge transport for on-state is tunneling. The charge transport leads to SCLC in higher voltages pulse for the off state. The mechanism will be explained based on percolation concepts.
Keywords: Nanoclusters, resistive switching, planer device
1
Introduction
Assembly of metal oxide nanostructured material in a polymeric matrix exhibits different resistivity states at room temperature. Distinctly, in order to discuss mechanism is better to address the charge transport based on temperature measurements and voltage-current relation. In general, electrical switching is a significant phenomenon in all composite devices which includes of nanoparticles embedded in an insulating or semi conducting host. There are lots of reports showing different mechanism for resistance switching observation [1-9]. Up to now, all switching devices prepared with nanoparticles have some functional components in common: i. Need a polymer network with the nanostructure material which can be metal sulfide, oxide or halide. This kind of structure is so-called percolating system which means an intermediate matrix between insulating and conducting. ii. The initial
state of device before switching is a Pristine-state. The pristine device is usually needs an Electroforming process to reach to a breakdown voltage, the electric field needs to switch for each device is different due to various matrixes chemically in material, work function and physically such as difference in layer thickness. After an electroforming the reproducible on/off system often obtains with a voltage pulse at low resistance state (LRS) voltage region or high resistance (HRS) voltage domain. Lack of an organic matrix, a destructive electrical switching is observed. In fact optimizing a percolating matrix is obviously important to observe a reliable switching memory. This report implies some interesting features, which may help better understand the mechanism of electrical switching in metal nanostructure polymeric network. In previous work we have shown the device structure which is planer. it was shown that An abrupt switching event is occurred after electroforming process under an electrical field of 5.5 Mv/m for a planar device with a 10 µm electrode separated. Conceivable mechanism for the resistive switching in this system is following with temperature measurement. In this work it is shown that the XRD pattern after switching appears with different crystalline planes, plus temperature dependent studies reveal that conduction of both on and off states is week thermal activated. Intriguing the carrier transport is the same for both on and off-states. Difference between states comes from the dramatic changes in the carrier density. The main mechanism of charge transport for on-state is tunneling. The charge transport leads to SCLC in higher voltages pulse for the off state. The mechanism will be explained based on percolation concepts. A myriad of conducting percolative paths are created after a soft breakdown in oxide. Some of these paths they switch on-and off leading to the observation of RTS noise.
2 Contribution to Value Creation
In this work, we utilized the nanostructure silver oxide thin films. Despite of having a sandwich device, the diode is planer in order to explore charge carrier transport within switching phenomena. We reported a memory device, which makes use of thin polymer film hosting a matrix of silver oxide nano structure. With on/off ratio as high as 103, a large retention time and good cycle endurance, the nanoparticles based device is a serious candidate to replace currently available non-volatile memories, and is able to improve all the relevant components as a reliable memory device.
In addition, these planar structures have the active layer directly exposed and can be probed by a number of surface analytical techniques like XRD to identify and characterize structural changes that occur in the devices upon resistive switching.
3 System Description
The colloidal solution of silver oxide nano structure in PVP was deposited on top of preformed gold microelectrode arrays fabricated on thermal oxidized silicon wafers. The interdigitated gold microelectrode arrays were separated apart 10 µm and 10.000 µm long. Prior to electrical measurements the samples were also pumped in vaccum to remove further any residual solvent. Electrical measurements were carried
out using a Keithley 487 picoammeter/voltage source in dark conditions, high vacuum. During all the measurement the conductive silicon substrate is kept grounded to prevent charging of the SiO2 layer.
3.1 Current-Voltage characteristics
Before the devices show resistive switching behavior, they have to undergo a forming process that is induced by applying a high bias voltage. In practice, forming voltages are typically ~50V. After forming the device exhibits the bistable current-voltage characteristics represented in previous work. (see fig.1)
Figure1.(a) Electroforming process. (b). Current-voltage characteristics showing the high conductance on-state and low conductance off-state.
3.2 Morphological and structural characterization
Silver oxide is very interesting class of metal oxides. Silver being multivalent forms various phases like Ag2o, AgO, Ag3O4 and Ag2O3 by interacting with oxygen The sample was analyzed for its structural and microstructural details using X-ray diffraction (XRD). Figure 2 shows three different XRD patterns of device: 1. The silicon wafers substrate, 2. pristine state (i.e. right after thin film deposition) and 3. Electroformed state (i.e. after voltage sweeping application). The first obvious observation from XRD results indicates that there are different crystal structures for pristine device and electroformed state. The best possible match of the XRD shown in figure 2, in terms of peak positions, is found to be with Ag/Ag2O/AgO nanocomposite though the relative intensities are not matching well possibly due to strong effects of Au electrodes and SiO2. It can be concluded here that with voltage application different crystals can be appeared which might induce trap levels, and also silver nanoclusters (Agn, n< 10 atoms) will be reduced across the reduction of Ag2O to AgO in the matrix. -50 0 50 10-10 10-8 10-6 10-4 Applied Voltage (V) C ur re nt ( A ) OFF ON -50 0 50 10-12 10-10 10-8 10-6 10-4 Applied Voltage (V) C ur re nt ( A ) -3 -2 -1 0 10-14 10-12 10-10 10-8 Applied Voltage (V) C ur re nt ( A )
Figure 2.
XRD patterns
for the substrate
(black), Pristine
state (blue),
Electroformed state (Red).
3.3 Voltage dependence
To further explore the mechanism, the correlation of the observed switching in I-V characteristics of device were also investigated, as shown in a log I-log V in figure 3. At the on-state the current increases linearly with the voltage ramp up to 25V, and the slope tends to 2 for the small region of 25V to 30V, where the NDR occurs. The observed current-voltage (I-V) characteristic for the off-state also goes linearly but then appears to be the voltage-square dependence for voltage bias more than 20 V. Therefore, in the off-state the trap-filled SCLC (space charge limited current) transport is proposed. The mobility in the off-state is ~ 0.75* 10 -9 cm2/V-s. The low effective mobility is probably due to a large trap density or charge injection caused by the silver clusters into PVP.
Figure 3. Log (I)- Log (V) for the on-state (red) and the off-state (black).
3.4 Temperature dependence
To get insight into the charge transport mechanism for both on and off states we measure the temperature dependence of the current. The device is weak thermal activated. The temperature measurement has been done at the read voltage (3V) for
-2 0 2 4 -22 -20 -18 -16 -14 -12 -10 -8 Log(V) Lo g( I) off-state on-state Slope~1 S lope~2
the on/off states. Usually with the pulse at the bottom of NDR, here is 50V, the off-state occurs entirely (super off). But also we can see the multi off-off-states with different pulse length. The activation energy for different states obtained from the Arrhenius plot has shown in Figure 8. The activation energy for all states is almost the same approximately 72 meV (Ea for on-state: 70.7 meV, off-state: 72 meV and super off: 74.2meV). Therefore, the identical activation energy implies the different number of charge carriers within various conductance states. It also proves the same mechanism of charge transport for both states (see fig.4).
Figure 4. Arrhenius plot for the on, off and the super off states with almost the same activation energy.
The breakdown (BD) can be considered as a phase-transition from a non-conducting (insulating) to a non-conducting state. After soft BD a percolation network of paths is established. The high conductance state is as a result of a myriad of conducting paths within the matrix. For both states the charge transport via tunneling between nanoclusters, but at the off-state the amount of percolative paths are less than on-state. In fact, due to high electric field some of charge carriers being injected into insulator from nanoclusters where there is no compensating charge present in the insulator. Therefore, they cannot participate in conduction nor have a very small contribution in the conduction. In real systems, trapped charges near the silver oxide might lead to a random offset charge disorder. In the presence of such disorder, charge carriers do not flow uniformly through the nanocomposite film. Because of such kind of disorder, it can exist both off and super off states.
4 Conclusion
As mentioned previously, the charge transport mechanism can help to improve memory properties in terms of scaling down, cost, data retention time. Therefore, in this work we tried to make the charge transport more clear…
3.5 4 4.5 5 5.5 -22 -20 -18 -16 -14 -12 1000/T (K-1) Lo g( I) ( A ) super off-state off-state on-state
Acknowledgement. We gratefully acknowledge the financial support received from
the Dutch Polymer Institute (DPI), project n.º 703, from Fundação para Ciência e Tecnologia (FCT) through the research Unit, Center of Electronics Optoelectronics and Telecommunications (CEOT), REEQ/601/EEI/2005 and the POCI 2010, FEDER and the organic chemistry laboratories in Algarve University
.
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