Improved Endurance and Resistive Switching Stability in Ceria Thin

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Improved Endurance and Resistive Switching Stability in Ceria Thin Films Due to Charge Transfer Ability of Al Dopant Muhammad Ismail, Ejaz Ahmed, Anwar Manzoor Rana, Fayyaz Hussain, Ijaz Talib, Muhammad Younus Nadeem, Debashis Panda, and Nazar Abbas Shah ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11682 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 19, 2016

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ACS Applied Materials & Interfaces

Improved Endurance and Resistive Switching Stability in Ceria Thin Films Due to Charge Transfer Ability of Al Dopant M. Ismail1,*, E. Ahmed1, A. M. Rana1, F. Hussain1, I. Talib1, M. Y. Nadeem1, D. Panda2, N.A. Shah3 1

Department of Physics, Bahauddin Zakariya University, Multan-60800, Pakistan

2

Department of Electronics Engineering, National Institute of Science and Technology, Berhampur

Orrisa, India 3

Thin films Technology Research Laboratory, Department of Physics, COMSATS Institute of

Information Technology, Islamabad-45320, Pakistan

* Corresponding author: [email protected], Tel. +92619210091, Fax: +92619210098

Keywords: Al-doping, density functional theory, Ceria, RRAM, resistive switching,

ABSTRACT An improvement in resistive switching (RS) characteristics of CeO2 based devices has been reported by charge transfer through Al metal as a dopant. Moreover, density functional theory (DFT) calculations have been performed to investigate the role of Al-layer sandwiched between CeO2 layers by the Vienna ab-initio simulation package (VASP). Total density of states (TDOS) and partial electron density of states (PDOS) have been calculated and analyzed with respect to resistive switching. It is established that the oxygen vacancy based conductive filaments are formed and ruptured in the upper region of CeO2 layer, because of the fact that maximum transport of charge takes place in this region by Al and Ti (top electrode). While, the lower region revealed less capability to generate conductive filaments because minimum charge transfer takes place in this region by Al and/or Pt (bottom electrode). The effect of Al and Al2O3 on both the electronic charge transfer from valence to conduction bands and the formation

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stability of oxygen vacancies in conductive filament have been discussed in detail. Experimental results demonstrated that the Ti/CeO2:Al/Pt sandwich structure exhibits significantly better switching characteristics including lower forming voltage, improved and stable SET/RESET voltages, enhanced endurance of more than 104 repetitive switching cycles and large memory window (ROFF/RON > 102) as compared to undoped Ti/CeOx/Pt device. This improvement in memory switching behavior has been attributed to a significant decrease in the formation energy of oxygen vacancies and to the enhanced oxygen vacancies generation within the CeO2 layers owing to charge transferring and oxygen gettering ability of Al-dopant.

INTRODUCTION Rare-earth oxide (REO)-based resistance change memory devices are finding considerable interest as potential candidates for next generation nonvolatile memory applications.1-2 Structural simplicity, low power consumption, high density, fast switching and compatibility with conventional CMOS technology of the memory devices involving CeOx, Gd2O3, Lu2O3, Dy2O3, Eu2O3 and Yb2O3 make them attractive for future non-volatile memory device applications.3-8 However, because of its distinguished ability to store and release oxygen, non-stoichiometric ceria is of particular interest for exploring its resistive switching ability in emerging nonvolatile memory devices. Such capability of CeO2 originates from the fact that Ce exists in two valence states, as it can change its oxidation state from Ce4+ (in CeO2) to Ce3+ in reduced ceria (CeO2-x) by the formation of oxygen vacancies. During this process, two electrons transfer from oxygen to cerium reducing two Ce4+ ions to Ce3+.9 Among two resistive switching (RS) types; unipolar and bipolar, bipolar resistive switching (BRS) looks to be more encouraging due to its faster switching speed, lower set/reset voltages and higher endurance than unipolar resistive switching

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(URS). Moreover, necessity of high electroforming voltage before showing resistance switching behavior might lead to unpredictable resistance states and/or non-uniform switching properties. To conquer such troubles/problems various techniques such as embedding nano-particles10, bilayer structure11, process optimization12, metal doping13 and interface control14 have been adopted. Among them doping technology is one of the most effective methods to modulate and improve the performance of RRAM devices. Active metal doping is found helpful in enhancing the local electric field within the resistive layer which consequently lowers the operating voltages and improved switching characteristics.15 Recently ZrO2, TiO2 and HfO2 films doped with bi/trivalent ions (Gd, Cu, Al etc.) have demonstrated adequate improvements in the device uniformity. A significant improvement in the performance of Gd-doped HfO2 devices have been reported by Zhang et al.16 Zeng et al.17 demonstrated that doping of Al, Cr, and Cu into TiO2 films caused better switching uniformity and relatively lower set voltages by enhancing oxygen vacancy generation. Lee et al.18 also showed that doping of Gd and Dy into ZrO2 films significantly reduced the ON/OFF resistance ratio with more uniform distribution of endurance characteristics. Al possesses low work function (4.28 eV) as compared to ceria (4.69 eV) and is recognized as one of the most oxidizable metals. Therefore, it is expected that Al can extract oxygen from cerium oxide to form an Al oxide (AlOx) layer, which can help to form conductive filamentary paths.19 In addition, Al can attract/repel charge carriers from/to the bottom Pt (work function is 5.69 eV) electrode due to a large work function difference of 1.41 eV, which may assist in generating a high local electric field for charge transfer. This is the reason Al was selected to behave as a dopant in cerium oxide based RRAM devices. Therefore, this study was aimed for the demonstration of reproducible bipolar RS characteristics in Al-doped CeO2 memory devices.

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As anticipated, significant improvement in the performance and reliability, including reduction in forming as well as set and reset voltages along with good endurance characteristics is noticed in the Al-doped devices. Experimental results have been explained on the basis of calculations involving electronic charge distributions by applying density function theory (DFT) using the Vienna ab-initio simulation package (VASP).

EXPERIMENTAL PROCEDURE Three sequential thin layers of CeO2 (3 nm)/Al (1 nm)/CeO2 (3 nm) were deposited using radio frequency (RF) sputtering at room temperature on Pt/Ti/SiO2/Si substrate. The working pressure was maintained at 10 mTorr by Ar:O2 gas mixture (12:6) during CeO2 sputtering using RF power of 100 W. The 1 nm Al layer deposited at 10 mTorr working pressure of Ar (flow rate 20 sccm) and the RF power of 50 W. The substrate to target distance was maintained at 4 cm. Deposition of the 1 nm thick Al layer was realized by employing better vacuum conditions and small deposition time (in the order of 50 s). Finally, Ti top electrodes (50 nm) and Pt capping layer (20 nm)

were

deposited

by

e-beam

evaporation

at

room

temperature

to

form

the

Pt/Ti/CeO2/Al/CeO2/Pt memory structure. A Ti/CeO2/Pt device was also fabricated under the same conditions as control for comparative studies. Electrical measurements were performed using Agilent B1500A semiconductor analyzer by grounding the bottom Pt electrode. Structural variations and any new phase formation were confirmed through x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). Schematic structures of the undoped (Ti/CeO2/Pt) and Al-doped (Ti/CeO2/Al/CeO2/Pt) devices are shown in Figure 1 (a). For theoretical verification of charge transfer mechanism, electronic charge density calculations have been carried out within the framework of plane-wave

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density functional theory by employing VASP codes.20,21 The generalized gradient approximation along with Perdew, Burke and Ernzerhof functionals and projector augmented wave potentials were selected for these calculations.22-24

RESULTS AND DISCUSSION Figure 1(b) depicts typical grazing incidence (3º) XRD patterns of Al-doped and undoped ceria based RRAM devices. Both Al-doped and undoped devices show three broad XRD reflections indicating weak polycrystalline structure of CeO2 (JCPDS #: 34-0394) involving (111), (220) and (400) planes centered at 28.5o, 47.5o and 69.4o, respectively. In addition, the presence of (111) reflection of TiO (JCPDS #: 82-0803) in the XRD pattern of the undoped device might lead to the formation of an interfacial layer between Ti and CeO2. While XRD pattern of Aldoped devices illustrates some strong reflections related to Al (JCPDS #: 04-0787) i.e. Al(220), Al(311) as well as Al2O3 (JCPDS #: 62–0703) i.e. Al2O3(110), Al2O3(113), Al2O3(202), Al2O3(214) indicating the presence of Al as element and as a compound by forming a thin Al2O3 interlayer sandwiched between top and bottom CeO2 films, instead of displaying any reflection related to TiO. Being more oxidizable as compared to Ti, Al extracts more oxygens to form oxide Al2O3 layer, even during deposition process.25 It is expected that both Al and Al2O3 layers might play crucial roles in the improvement of resistive switching characteristics. Moreover, it is probable that doping of trivalent Al might reduce the formation energy of oxygen vacancies as well as it can localize the conduction filaments inside the oxide layer.17,26,27 It is, however, difficult to state here that a sufficiently high number of oxygen vacancies are present in the two CeO2 layers as metallic Al is still found between these two layers. The cross-sectional view of the Al-doped CeO2 devices through HRTEM image (Figure 1(c)) clearly illustrates the

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formation of Al2O3 layer as recognized by its slightly brighter contrast than Al and its corresponding EDS plot shown in Figure 1(d), clearly displays the presence of Al metal in between the two CeO2 layers. Therefore, the formation of conducting filaments may or may not be expected in virgin state of the device.

Figure 1. (a) Schematics and (b) XRD patterns of undoped and Al-doped CeO2-based devices, (c) Cross-sectional HRTEM micrograph and (d) EDS spectrum of Al-doped CeO2 devices.

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All the fresh devices were found in very high resistance state, and RS behavior was triggered only after the application of a large voltage, so called “electroforming” or “forming” process. This phenomenon leads to a decrease in the stability of ON- and OFF-states28, as a result RS characteristics become much difficult to modulate, so it is undesirable in RRAM devices. A typical forming process for present undoped and Al-doped devices occurring at the same current compliance of 10 mA is shown in Figure 2(a). It is notable that forming voltage of the Al-doped devices is considerably smaller than that of the undoped ones. This result appears fulfilling the expectations of Al-doping to decrease the electroforming voltage from ~ 10 V (for undoped devices) to 5 V (for Al-doped devices). Such a decrease in the electroforming voltage is desirable for applications of RRAM devices to initiate the RS behavior. Such behavior is analogous to those reported for ZrO2 and HfO2 based devices, which demonstrate that doping can reduce the variability and switching/forming voltages.16,29 For statistical analysis of forming voltages, electroforming of ~50 fresh cells at different locations of the same Si wafer (substrate) was performed on both types of the devices. Figure 2(b) compares the distribution of electroforming voltages in both undoped and Al-doped devices. Much narrower dispersion in electroforming voltages of Al-doped devices was noticed than that of the undoped devices. A reduction from 3.47 to 1.45 in the standard deviation of electroforming voltages was observed, which is surely an evidence of more uniform distribution of electroforming voltages. This elucidates an indirect improvement in the stability of Al-doped devices during switching cycling. This seems to be happened due to the formation of Al2O3 interlayer [as illustrated by XRD and HRTEM (Figure 1)] by extracting oxygen ions from both top and bottom ceria layers, which has created/added some more oxygen vacancies to those already existing in CeO2 layers by reducing their formation energy.30 That is why lower voltage of ~5 V was observed to be sufficient enough to

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create some more oxygen vacancies and their rearrangement to form conducting filaments. Another factor that might have contributed in reducing the electroforming voltage is the strengthening of local electric field within the CeOx layers due to Al-doping.31,32 These results are corroborating with the findings reported by Chen et al.33 (local Al doped HfO2) and Peng et al.34 (Al-doped HfO2). Both of the above mentioned electroformed devices started switching from ON-state (i.e. the electroformed low resistance state, LRS) to OFF-state (the high resistance state, HRS) leading to RESET process on applying negative biasing. A subsequent positive biasing again returned both the devices from OFF-state back to ON-state (phenomenon called SET process) as shown in Figure 2(c). A current compliance of 10 mA was applied to prevent hard breakdown during all such SET processes. It is notable that data related to bipolar resistive switching characteristics of undoped devices presented here is only for comparison purposes, which indicates that undoped devices gradually switch from ON- to OFF-states, while Al-doped devices show abrupt RESET characteristics. Such abrupt transitions can be attributed to easier filling of oxygen vacancies in the Al-doped devices.17 Electroforming and switching behavior in Al-doped devices was also performed with negative bias using compliance current of 10 mA as shown in Figure 2(d). It was noticed that with the rise of negative bias field, initially current increased in a gradual way to mA range, then there was a sudden fall in current of about one to two orders of magnitude transferring the device back to HRS (the unipolar like behavior). With further rise of applied negative field, device abruptly switched from this HRS to LRS (ON-state), this leads to the forming process based on negative biasing as obvious from Figure 2(d, left inset). After this electroforming process, negative SET and positive RESET processes occurred successfully representing BRS behavior of Al-doped

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devices without any unipolar like behavior as shown in Figure 2(d). Right inset in Figure 2(d) demonstrates endurance performance of the negatively formed (NF) devices depicting resistance evolution of HRS and LRS for more than 40 continuous switching cycles. The HRS and LRS show large-resistance dispersion, so there is quite a small memory window. Such a small resistance ratio and dispersion does not fulfill the requirements of an efficient memory cell. That is why no more characterizations were performed on NF devices.

Figure 2. (a) Typical dc sweep I-V plots for the pristine cells showing electroforming process, (b) statistical distribution of forming voltages (Vf) in ~50 cells, (c) bipolar I-V characteristics of undoped and Al-doped devices and (d) Al-doped device in the negative forming mode, Left inset shows the negative forming and Right inset depicts the dispersion of endurance characteristics.

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To compare the memory performance of both undoped and Al-doped devices, their cycling endurance tests were performed as illustrated in Figure 3(a-b). In Al-doped devices, HRS/LRS resistance ratio is ~102 and the resistance states are relatively more stable for >104 dc switching cycles (Fig. 3b). The observed resistance ratio (~102) can fulfill the requirements of a highly efficient memory cell.35 Both devices display slight decrease in HRS resistance with increasing switching cycle number. In addition, many such set/reset operations were noticed, which could not complete themselves, after about 220 switching cycles in the undoped devices as clear from Figure 3(a). It looks noteworthy to point out that durability of the RS behavior in high-k materials is strongly linked to the movement of oxygen vacancies or, in other words, to the absence of non-mobile impurities. With each RS cycle performed, such impurities from the adjacent layers may penetrate in the insulating stack, thus increasing the current in the HRS of the device. Some incomplete set/reset operations were noticed to occur in the undoped device, because filament thickness was expected to increase with repetitive switching cycles as the top Ti electrode keeps on extracting more and more oxygen ions from the adjacent cerium oxide layer to form TiO interlayer thereby creating more vacancies which can further extend or increase the size of CFs.

36,37

At this stage current compliance and set/reset voltages become

insufficient to provide adequate number of oxygen ions from the top Ti electrode to fill the vacancies and rupture the thicker filaments. So it becomes necessary to increase either the current compliance or set/reset voltages for continuing the successful repetitive switching cycles.18 At some higher voltages, switching starts again. But after about 300 switching cycles, the density of impurities and/or the size of CFs can be so high that the CF could not be completely re-oxidized (filled with oxygen ions to break the filaments) anymore (Figure 3(a)), leading to the failure of switching transitions because HRS and LRS cannot be distinguished.

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However, Al-doped devices exhibit 100% successful set and reset operations (Fig. 3b), which are caused by enhanced oxygen vacancy generation due to Al-ions and subsequent suppression in the randomness of oxygen vacancy filaments formation as described by Lee et al.18 The Al2O3 interlayer also plays a key role in controlling the size of the CFs and hindering the migration of oxygen vacancies (and hence the thicker filaments extending) through upper CeO2 layer to top electrode Ti. To further evaluate the memory performance, temperature dependent retention characteristics of Al-doped devices were also investigated. Figure 3(c) portrays the retention test performed at 25 ºC and 90 ºC for Al-doped devices. It is evident that both HRS and LRS do not depict any degradation in data retention for time duration of more than 105 s. Such improved retention of these devices might be caused by an active role of Al interlayer and Ti top electrode to act as an oxygen diffusion barrier.38 Basically improvement in the retention characteristics means a perfection in the reliability of Al-doped devices. Figure 3(d) shows the dependence of resistances in the OFF- (RHRS) and ON-states (RLRS) on the cell area of Al-doped devices, both the resistances demonstrate decreasing trend with increasing cell area. This behavior attributes that it is not a single filamentary path which is responsible for low resistance in the ON- state of Al-doped devices; rather there is more than one filamentary path which cooperatively contributes to the high conductance of these devices in the ON-state. A similar correlation between resistance and device area has been reported in a number of other metal doped devices16-18 on the basis of same reasoning as discussed above.

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Figure 3. Endurance characteristics of the (a) undoped and (b) Al-doped devices, (c) Data retention properties of the HRS and LRS of Al-doped devices at room temperature and at 90 ºC, (d) cell area dependence of the RLRS and RHRS for Al-doped devices.

To realize the switching stability in Al-doped and undoped devices Vset/Vreset distributions for first 100 switching cycles are shown in Figure 4(a). Relatively much smaller fluctuations are noted in Vset and Vreset of Al-doped devices leading to the better uniformity in their resistive switching characteristics as compared to those of undoped devices. Undoped devices show wider variations in Vset (1.95 – 3.60 V) and Vreset (-1.05 – -1.85 V). However, Al-doping has reduced these voltage dispersions i.e. Vset (1.0 – 2.2 V) and Vreset (-0.6 – -1.0 V) as obvious from Figure 4(a). It is expected that Al-doping has probably enhanced the local electric field within the CeO2 12


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layers (behaving as a defect center), which leads to a reduction in the set and reset voltages.39,40 Moreover, CFs are preferentially formed in the large electric field regions which become an additional source of reduction in switching dispersion. However, it is known that metal dopant decreases the average formation energy of oxygen vacancies41 due to which density of preexisting oxygen vacancies becomes higher, consequently there is a reduction in the electroforming voltage as well as in the set and reset voltages. Figure 4(b) compares the statistical distribution of resistances in HRS (RHRS) and LRS (RLRS) during continuous sweep cycles. Resistances in the two states were read out at biasing voltage of 200 mV. Al-doped devices display much better dispersion of both RHRS and RLRS. Doping of trivalent element like Al may be capable to reduce the electrical insulation of oxide layer, as a result operation current may rise decreasing RHRS and RLRS.39,40 However, it is noted that cycle-to-cycle uniformity and reliability performance have significantly been improved by sandwiching Al into CeO2 layers.

Figure 4.(a) Statistical distribution of Vset and Vreset, (b) RHRS and RLRS for first 100 switching cycles. Resistances are read out at a voltage of 200 mV.

In order to understand the distribution of electronic charge in Al-doped (Ti/CeO2/Al/CeO2/Pt) devices, the valence electronic charge density has been calculated through VASP using a 13



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supercell of layered structure along (111) crystallographic plane. Because of complications in the complete Ti/CeO2/Al/CeO2/Pt structure, it has been divided into three parts (Ti/CeO2/Al, CeO2/Al/CeO2 and Al/CeO2/Pt) to clearly understand the charge transformation mechanism. The transfer of charge from Al to CeO2 layers is shown by arrows in Figure 5(a-c). It is noted from Figure 5(a-c) that electrons are injected by Al atoms which seem to be accumulated mostly in the upper layer of the dielectric CeO2 and somewhat in the lower layer. It can also be noted from Figure 5(c) that some of the charge has also accumulated around Al atoms consequently Al atoms attract oxygen ions from the bottom CeO2 layer to form Al2O3 as illustrated by XRD pattern and HRTEM image (Figure 1). Movement of the charged oxygen’s in the upward direction is clearly evident from Figure 5(c). Accordingly an electric field directed from the positively charged Al ions to electrons in the conduction band is built, which attracts the oxygen ions. As a result, oxygen vacancies may be created in the bottom CeO2 layer. This is reasonably in agreement with the conclusion that the charge transfer occurs mainly from Al to CeO2 layer. The energy required to form multilayered structure termed as formation energy (Ef) in Ti/CeO2/Al/CeO2/Pt structure has been calculated from the energies of metals or metal oxide films (EA, EB, EC) in the individual three basic structures as well as from the total energy (Etotal) of complete structure using following relation:42

E f = Etotal − (E A + EB + EC ) It is found that the formation energy is minimum (12 eV) for CeO2/Al/CeO2 structure shown in Figure 5(b). This simply means that this layered format is energetically most favorable as compared to other structures. The other structures (Ti/CeO2/Al and Al/CeO2/Pt) are definitely less likely to be stable due to their considerably higher formation energies (i.e. 78 eV and 94 eV respectively). In these structures, Ti is relatively more active electrode as compared to Pt 14


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(relatively inactive) due to its higher oxygen gettering ability and lower work function (Ti: 4.33 eV and Pt: 5.69 eV). Due to the accumulation of maximum charge in the upper CeO2 layer (Fig. 5(a,b)) and better chemical affinity of Ti electrodes, the lowering of formation energy of oxygen vacancies takes place in this region. Such a decrease in formation energy of oxygen vacancies can be attributed to the coulomb interactions between the dipoles formed by Al-dopant (behaving as negatively charged acceptor) and oxygen vacancy (acting as a positively charged donor).17,43 Because of the lowest formation energy of oxygen related defects it looks the most probable that this CeO2 layer will contain a large concentration of oxygen vacancies, since maximum charge transfer occurs in the CeO2/Al/CeO2 structure as compared to other stacks i.e. Ti/CeO2/Al and Al/CeO2/Pt. In other words, formation and rupture of conductive filaments is more likely to take place in the upper CeO2 layer instead of the lower one.

Figure 5. Total charge density distribution in: (a) Pt/CeO2/Al, (b) CeO2/Al/CeO2 and (c) Al/CeO2/Pt structures.

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To elucidate the role of charge transfer and/or distribution in the switching mechanism, total density of states (TDOS) of the Ti/CeO2/Al/CeO2/Pt stack have been calculated theoretically as shown in Figure 6(a). It is clearly evident from Figure 6(a) that sufficient number of defects states have been created in the band gap region, which transfer energy states from valance (V.B) to conduction bands (C.B) leading to their overlap at the Fermi level (EF) in all these three structures. So the Ti/CeO2/Al stack provides relatively easy path for charge transport as compared to other stacks. Sufficient number of energy states is found close to the Fermi level in the C.B. region with a few defects states in the V.B. region for CeO2/Al/CeO2 structure, but in Al/CeO2/Pt stack, relatively more defects states have been noted in the V.B. region with almost zero band gap. Consequently, it can be said that the charge transfer mechanism is strongly preferable in the region of Ti/CeO2/Al structure and weekly towards Pt electrode as evident from Figure 5(c). Figure 6(b-d) shows partial density of states (PDOS) calculated for a deep understanding in all the Ti/CeO2/Al, CeO2/Al/CeO2 and Al/CeO2/Pt structures. Conduction band of the Ti/CeO2/Al stack is mainly composed of Ti and O2 states, whereas only small number of Al and Ce states is found in the C.B. region with sufficient states for Ti, Ce and O2 in the V.B., from which adequate number of charge carriers are available to be transferred to conduction band. In addition, as an oxygen vacancy is created in the lattice, two excess electrons from the vacancy try to compensate the dopant; consequently the defect states in the bandgap are not fully occupied. These unoccupied states provide easy path for charge conduction. Thus, due to the presence of such states in the C.B. of Ti, the transport of oxygen ions is possible to and out of Ti, which is helpful and responsible for switching mechanism and hence main cause for the enhancement in conductivity of the device. Moreover, such accumulation of charge also helps to reduce the

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electroforming voltage. PDOS of CeO2/Al/CeO2 layers shows very weak overlapping of defect energy states at the Fermi level with a few states in the C.B region. However, most of the defects states created deep in the band gap region caused by Al and O2 can allow transfer of charge from V.B to C.B as noted from Figure 6(c). PDOS of Al/CeO2/Pt stack shown in Figure 6(d) reveals the creation of more O2 defect states in the V.B and C.B regions with an overlap at the Fermi level as compared to CeO2/Al/CeO2 stack. As observed from Figure 5(c), Al transfers more charge to oxygen in comparison to Pt as the oxygen states are localized deep in the V.B region. In addition, due to oxygen gettering ability of Ti, more oxygen vacancies are expected at the Ti/CeO2 interface rather than CeO2/Pt interface as obvious from Figures 5(c) and 6(d). Based on the theoretical analysis of charge distributions to cope with the resistive switching of present RRAM devices, it can be concluded that Al layer sandwiched between two CeO2 films poses a significant impact on enhancing the oxygen vacancy concentration near the dopants. This phenomenon might lead to the controllable formation of conducting filaments based on oxygen vacancies and possibly lowering the forming/set/reset voltages.

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Figure 6. (a) Total density of sates (TDOS) of the complete Ti/CeO2/Al/CeO2/Pt structure. Partial density of states (PDOS) for (b) Ti/CeO2/Al, (c) CeO2/Al /CeO2 and (d) Al/CeO2/Pt stacks. Fermi level (EF) in each plot is indicated by the red dashed lines.

It has already been reported that the main contribution in charge carrier injection is not from the work function of the active metallic electrode, rather it is the electronegativity and ionic size of the metal that plays the dominant role.44-45 Clearly, the smaller the size of the metal cation, the smaller is its electronegativity, consequently the greater will be the ease with which a cation can 18


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diffuse into the active oxide layer. No doubt, the electronegativity of Al is about 1.4 times greater than that of Ce but the size of Al cation (53 pm) is far smaller (2.3 times) than that of Ce cation. So due to the dominant smaller size effect of Al cations, they can be expected to diffuse with more ease in the CeO2 layer and create oxygen vacancies. Diffusion of Al ions can be proposed to be from one Ce vacancy to another Ce vacancy in the polycrystalline CeO2 layer because this will need the least energy for diffusion. In an oxygen deficient CeO2 layer, Al ions may exist as substitutional impurities, at the sites of missing oxygen ions. Due to smaller size of Al cations as compared to oxygen ions (140 pm), presence of these substitutional impurities causes distortion in the lattice and degradation in the local symmetry. This might result in a greater density of oxygen vacancies, which not only decrease the forming, set and reset voltages but also improves the device stability against repetitive resistive switching cycling. The observed less charge transfer from Pt to CeO2 can be explained on the basis of electronegativities of Ce (1.12) and Pt (2.82), due to this large difference (1.70) Pt cannot be expected to diffuse into CeO2 layer. Usually, most of the oxygen vacancies in resistive switching oxides are created during forming process at high voltage. However, the doping of CeO2 with Al enhances the availability of oxygen vacancies even before the electroforming process as shown in Figure 7(a). These preexisting oxygen vacancies assist to create conductive paths even at lower forming voltages. As described earlier, most of the oxygen vacancies are created close to Al in the adjacent CeO2 layers leading to better control of creation (SET) and rupture (RESET) of CFs. As illustrated in Figure 7(b), on applying positive voltage to Ti top electrode, oxygen vacancies are pushed down by the applied field into the lower half of doped CeO2 layers leading to the formation of some conductive paths near the Pt cathode region. These conductive filaments then begin to grow with

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electric field extending to the anode. This leads to the formation of conductive filaments between top and bottom electrodes, and a sudden rise in current, which is an indication that the device has switched to its ON-state. A negative voltage applied to the top electrode (Fig.7(c)) pulls off some oxygen vacancies towards the top electrode, which ruptures the conductive filament leading the device to OFF-state (HRS). A positive biasing then brings back the device into its ON-state by reconnecting the filaments (Fig. 7(d)) and so on. The crucial role in this switching process is that of the Al dopant which ensures the availability of oxygen vacancies in the sandwiched layers. This is made possible by extracting oxygen ions from the upper and lower CeO2 layers and thereby forming a thin Al2O3 layer, as illustrated by XRD and HRTEM results. Often a single conductive filament is highly desirable to suppress the switching fluctuations, so efficient control of conductive filaments growth during forming process is crucial. The formation energy of an oxygen vacancy in the bulk CeO2 is 4.5 eV.38 Large density of pre-existing oxygen vacancies in Al-doped devices indicate that Al-doping lowers the formation energy of oxygen vacancies in the CeO2 layers. Although conducting filaments are more easily formed in a doped CeO2 layer, the set/reset voltage fluctuations can only be suppressed by increasing the uniformity of doping.45 During negative forming process, as negative voltage is applied to the top electrode, oxygen ions are extracted from the upper CeO2 layer, which drift towards the Al layer (Fig. 7(e)) thereby creating oxygen vacancies. On further increasing the applied bias voltage, these charged oxygen vacancies form percolation paths as filaments within the CeO2 layers consequently device enters into the ON-state (forming process). On applying a positive bias to the top electrode, oxygen ions are pulled back and reoccupy the oxygen vacancies as indicated in Figure 7(f). This process disconnects the conductive filaments resulting in the resistance transition from ON- to OFF-state

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as the applied bias is higher than the VRESET. In the following SET transition, a negative bias is again applied to top electrode and the oxygen deficient filaments in the CeO2/Al interface are regenerated (Fig. 7(g)), which complete the SET process as the applied negative voltage on the device exceeds VSET (ON-state). In this state, concentration of oxygen ions in the proximity of Ti/CeO2/Al region is high as confirmed by total charge density calculations. But the charge transfer from Al towards the lower portion (Al/CeO2/Pt region) is comparatively smaller (Figure 5(c)). In addition, the migration of oxygen vacancies depends on the non-uniform distribution of electric field as well as their concentration in the oxide layer. This might cause fluctuations in the ON-/OFF-states resistance ratio and switching voltages. During the successive RESET phenomenon, applied negative bias to top electrode causes oxygen ions near the CeO2/Al interface to recombine with the charged oxygen vacancies in the conductive paths resulting in disconnection of the filaments. This process switches the device from LRS to HRS. Due to the poor nature of oxide formation and rupture of CFs close to the CeO2/Al interface during the SET/RESET processes, the distribution of resistance switching parameters in the ON- and OFFstates is relatively wider. Thus, the NF devices do not demonstrate successful repetitively operated BRS modes. Hence, from the above cited facts, it can be concluded that Al-doped device is more reliable and repeatable in performance as it is electroformed in PF mode.

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Figure 7. Illustration of the proposed RS mechanism based on the I–V characteristics. In each part of the illustration, voltage is applied to the top contact after the depicted oxygen vacancy configuration is achieved within the Al-doped CeO2 sandwiched structure. Oxygen vacancy configuration (a) in the pristine state, (b) positive forming, (c) negative RESET (HRS), (d) positive SET (LRS) processes and in the negative forming mode: (e) negative forming, (f) positive RESET (HRS) and (g) negative SET (LRS) processes. 22


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CONCLUSIONS The influence of distribution and transfer of charge from Al on the electronic properties of Ti/CeO2/Al/CeO2/Pt structure has been investigated experimentally and theoretically using density function theory. The behavior of TDOS and PDOS in the Al-doped devices depicts that the conductive filaments are formed and ruptured in the upper region of CeO2 layer, because maximum charge transfer occurs in this region by Al and Ti (top electrode). Moreover, Al dopant enhances local electrical field during the forming process consequently decreasing the electroforming as well as SET and RESET voltages. The interaction between the dipoles formed between the Al dopant and oxygen vacancies seems to be responsible for decreasing formation energy of oxygen vacancies. This ultimately helps in making the switching behavior stable and overall performance of the Al-doped devices.

Acknowledgements Authors acknowledge the financial support by Higher Education Commission (HEC), Islamabad Pakistan under the International Research Support Initiative Program (IRSIP). Authors are very grateful to Prof. Tseung-Yuen Tseng, Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu, Taiwan for providing experimental facilities and useful suggestions. Authors are also thankful to the National University of Singapore (NUS) for the utilization of their computational facilities. REFERENCES (1)

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Improved Endurance and Resistive Switching Stability in Ceria Thin

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