Resistive switching in MIM structure based on overstoichiometric tantalum oxide

Accepted Manuscript Resistive switching in MIM structure based on overstoichiometric tantalum oxide

D.S. Kuzmichev, Yu.Yu. Lebedinskii PII: DOI: Reference:

S0167-9317(17)30187-9 doi: 10.1016/j.mee.2017.04.041 MEE 10546

To appear in:

Microelectronic Engineering

Received date: Revised date: Accepted date:

27 February 2017 26 April 2017 27 April 2017

Please cite this article as: D.S. Kuzmichev, Yu.Yu. Lebedinskii , Resistive switching in MIM structure based on overstoichiometric tantalum oxide, Microelectronic Engineering (2017), doi: 10.1016/j.mee.2017.04.041

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ACCEPTED MANUSCRIPT Resistive switching in MIM structure based on overstoichiometric tantalum oxide D. S. Kuzmicheva,*, Yu. Yu. Lebedinskiia,b Moscow Institute of Physics and Technology, 9 Institutskii per., Dolgoprudny, 141700

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National Research Nuclear University MEPhI (Moscow Engineering Physics Institute),

Kashirskoye Shosse 31, 115409 Moscow, Russia

Corresponding author: [email protected], +7 903-174-96-47

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Moscow region, Russia

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Abstract

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We report on the resistive switching effect in metal-insulator-metal (MIM) structures Pt/Ta2O5+x/Ta with over-stoichiometric tantalum oxide film. These devices

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exhibit forming-free behavior, Roff/Ron ratio >10 and high endurance (108 cycles). Based on the experimentally observed correlation between the applied voltage and the

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resistance changes (bias memory effect - BME), the resistive switching model for this

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kind of devices is proposed.

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Key words: resistive switching (RS), tantalum oxide

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ACCEPTED MANUSCRIPT 1. Introduction Among the binary metal oxides, tantalum oxide based ReRAM devices demonstrate good electrical performance, particularly, high endurance, high switching speed and low power operation [1-5]. However, similar devices exhibit diverse characteristics [1,5]. The possible reason is that the oxide properties strongly depend on the deposition technique,

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and, particularly for magnetron sputtering, critically depends on O/Ar ratio and

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pressure [6].

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The detailed understanding of the switching mechanism, which explains the variety of experimental results on tantalum oxide, is the prerequisite for using this material in

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resistive memory devices. According to numerous studies, charged oxygen vacancies migration is the driving mechanism of the resistive switching in the transition metal oxide

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layers [4]. The alternative models suggest oxygen-ion-migration under the external electric field responsible for resistive switching [7,8]. This work presents the results on the “overstoichiometric” Ta2O5+x (x ≈ 1) thin films grown by magnetron sputtering and

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their integration in Pt/Ta2O5+x/Ta forming free devices with initial state is between low

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(LRS) and high resistance state (HRS). The model explaining the peculiarities of reversible resistive switching is proposed.

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2. Materials and methods

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In our experiments, p-type Si wafers with 150-nm-thick Pt layer was used as substrate. 20 nm thick TaOx film was grown by DC sputtering of Ta target in pure O2

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atmosphere (flow 4,4 sccm, P= 2.4 mTorr). Top Ta electrodes ~300 μm in diameter were formed by DC sputtering of Ta in Ar through a shadow mask. The thickness and refractive index of TaOx films deposited on Si substrate were measured by laser ellipsometry with the wavelength 632.8 nm. The stoichiometry of TaOx film (O/Ta ratio) was determined by Rutherford Backscattering Spectrometry (RBS) for films deposited on highly oriented pyrolitic graphite (HOPG), since it allows to use clearly resolved O and Ta peaks to precisely derive the composition. X-ray photoemission spectroscopy (XPS) was used for determine the chemical composition and electronic structure of interfaces. XPS measurements were performed using ThetaProbe (Thermo Scientific) spectrometer 2

ACCEPTED MANUSCRIPT with a monochromatic Al Kα (1486.6 eV) X-ray source. Agilent B1500A source-meter unit with the probe station was used for I-V measurements.

3. Results and discussion

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The composition of TaOx films was measured with RBS, since RBS measurements

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of thick 20 nm (on HOPG) films allow reducing the surface layers contribution into the composition measurements. The composition of Ta2O5+x films used in this work as grown

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0.5

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on HOPG substrate is revealed by RBS and yields O/Ta ratio ~3.1±0.2 (Fig.1) . 2.0

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Normalized Yield

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Fig. 1 – RBS spectrum of TaOx film grown on HOPG by DC sputtering of Ta target in pure O2 and modeled with the structure TaO3.1 (20 nm). The refractive index of Ta2O5+x films measured on Si substarte is ~1.92, in

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agreement with the results for high oxygen flow, which leads to the decrease of refractive index [6].

Figure 2 shows Ta4f and O1s XPS spectra measured from a 20 nm thick Ta2O5+x film formed by DC sputtering of Ta target in pure O2 on Pt underlayer. Spin–orbit split Ta4f line is the single doublet with Ta4f7/2 component centered at Eb=25.4 eV. O1s is a single line with Eb=529.7eV. The small shoulder with the higher binding energy can be attributed to OH and CO groups absorbed on the surface while transferring the sample from the growth chamber to the spectrometer. Alternatively, it can be an 3

ACCEPTED MANUSCRIPT «overstoichiometric» interstitial O. The presence of the surface contamination hinders the use of XPS for accurate evaluation of the stoichiometry and bulk chemical state of O. The binding energy of Ta4f is significantly smaller compared to those reported in other works (e.g. [9], 26.2 – 26.4 eV). It should be noted that the binding energy in core-level XPS lines is defined not only by the chemical state of the atom in the oxide, but also by the

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Intensity (a. u.)

Intensity (a. u.)

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effective work function (Weff.) of the metal which is in contact with the oxide layer [10].

28 27 26 25 24 Binding Energy (eV)

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532 530 528 Binding Energy (eV)

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Fig. 2 – XPS spectra of Ta2O5+x on Pt bottom electrode.

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The detailed XPS study of Pt/TaOx interfaces performed previously [11] has revealed band offsets of TaOx with respect to EF of Pt. In particular, it has been found that

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the conduction and valence band offsets at Ta2O5+x /Pt interface are 2.1 eV and 2.3 eV, respectively. It can therefore be concluded that the low value of Ta4f binding energy is

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defined by the high WPteff. in contact with Ta2O5+x. The TaOx/Ta interface is also important: The migration of excess oxygen from Ta2O5+x to Ta leads to the formation of a

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layer of high-resistivity Ta2O5 whose thickness determines the resistance of the structure.

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Fig. 3 – The initial I–V curves measured on Pt/Ta2O5+x/Ta

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Basing on XPS analysis, we suggest that the magnetron sputtering of Ta in oxygen plasma results in the growth of the chemically homogeneous oxide film with the

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stoichiometry close to TaO3, where all Ta atoms are coordinated in the same chemical state. Recently, the lattice stability of TaO3 has been studied using density functional

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theory calculations. The calculated free energy of formation shows that ReO3-type TaO3 is thermodynamically stable [12]. TaO3 trioxide is listed as a layered compound by

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Nicolosi et al. [13].

The initial resistance state of devices is between low (LRS) and high resistance

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state (HRS), therefore the “forming step” is not clearly observed (Fig.3). During the test, Roff/Ron ratio was found >10 (at -0.1V). The endurance test at +0.9/-1.2 V and the pulse width 1µs has revealed 108 switching cycles (see Supplementary. Fig.1). Also the retention data shows low stability of HRS (see Supplementary. Fig.2).

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ACCEPTED MANUSCRIPT Fig. 4а shows the experimental I-V curves taken from Pt/Ta2O5+x/Ta devices at different temperatures up to T=200 С. The scheme of the applied voltage is 0 → Unegative → 0 → Upositive → 0→ temperature increasing. The conductivity in 50

-3 -3.5

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Current, mA

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Fig. 4 – Pt/Ta2O5+x/Ta LRS exhibits Ohmic behavior (J=AU). The temperature dependence of the current

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shown in Fig. 4b is fitted with ln(J)=BТ-1. The derived activation energy is Ea= 0.2 eV. The conductivity in HRS does not strongly depend on the temperature, which indicates

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that the electron transport in these devices takes place via tunneling process such as TrapAssisted Tunneling (TAT) or Fowler-Nordheim (F-N) [14].

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In addition, the bias memory effect was found, i.e. the increase of Upos (Uneg is fixed) leads to the increase of Uoff and the return to the lower positive voltages causes the in

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decreasing Uoff (Fig.5). This effect can be attributed to the redistribution of oxygen atoms TaOx

layer.

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0 → Upositive → 0 → Unegative → 0.

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of

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Fig. 5 – Bias memory effect - Pt/Ta2O5+x/Ta.

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In order to explain the observed resistive switching and bias memory effects, the model describing the resistive switching in terms of oxygen migration is proposed (also

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see Supplementary). The high initial conductivity indicates the high concentration of traps and acceptor dopants, originating from excessive oxygen. Ta2O5+x layer is placed

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between two electrodes and is divided into cells with various oxygen concentration. During negative voltage to Ta electrode process oxygen ions migrate to Pt electrode. In this way, the stoichiometric Ta2O5 layer with tunneling thickness and high resistance is

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formed in the vicinity of Ta electrode and stack switches to HRS. The formation of this dielectric layer gives rise to the redistribution of the potential across the whole film. Subsequently, since the main potential drop occurs across Ta2O5 layer, the oxygen drift in Ta2O5+x is stopping. The applied critical positive voltage gives rise to the negative oxygen ion migration to Ta electrode and the redistribution of oxygen between cells. The total oxygen distribution across the structure defines the resistance of the stack and its RS parameters [3]. Also, non-linear film resistivity changing with oxygen concentration

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ACCEPTED MANUSCRIPT growth should be taken into account [1]. The ion speed migration can be evaluated as [15]:

υ=A·sinh(B·E)·exp(-ΔG/kT) where A,B – constants depending on the jump distance, vibration frequency, charge and temperature, E – the electrical field in the cell defined by the current across the structure and depending on the oxygen concentration, which changes from cell to cell. The biasing

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time also affects to the degree of the oxygen ions migration. The modelling of bias

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memory effect is in good agreement with the experimental results (Fig. 6).

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Fig. 6 – 1-D model of RS and the bias memory effect

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According to the model, the oxygen ion migration can be controlled by the integral time of applied voltage. The intermediate current and resistance values can be achieved by applying lower voltages during longer times. To check the validity of the model, the device in HRS was tested under constant voltages U +0.48, +0.55, +0.57 V for 300 sec. (Fig. 7). The current I= 0.8 mA was achieved in ~200 sec. upon applying U= +0.48V, in ~2 sec. upon U= +0.55 V and in lower then 0.1 sec. upon U= +0.57 V. The device resistance measured after 300 sec time tests constitute 88%, 68%, 51% of HRS, respectively. 8

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Fig. 7 – I-t relaxation curves

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4. Conclusions

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The forming free resistance switching effect in Pt/Ta2O5+x/Ta devices with overstoichiometric tantalum oxide functional layer with promising characteristics

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(Roff/Ron >10, 108 cycles) is demonstrated. The conductivity in HRS does not strongly depend on the temperature. The correlation between the applied voltage and the changes

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in the device (bias memory effect - BME) is observed and modeled. I-t relaxation curves show that equal device resistance state can be achieved by different combinations of time and voltage amplitude.

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ACCEPTED MANUSCRIPT 5.Acknoledgements This work was particularly supported by the Russian Science Foundation (grant No. 14-19-01645) (preparing samples, XPS, modelling of resistive switching) and by the Russian Foundation for Basic Research (grant No. 15-08-08014A) (performing transport

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measurements, calculations and modelling).

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ACCEPTED MANUSCRIPT References [1] Z. Wei, et al, Proceedings of the IEEE International Electron Devices Meeting, San Francisco, CA, USA, 15–17 December 2008 [2] J. Joshua Yang, et al, Appl. Phys. Lett. 97, 232102, 2010 [3] M.-J. Lee, et al, Nat. Mater.10, 625, (2011) [4] Kumar et al, Adv. Mater., 28, 2772–2776, (2016)

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[5] J. Joshua Yang et al, Appl. Phys. Lett. 100, 113501 (2012)

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[6] J. M. Ngaruiya et al, Phys. Stat. Sol. (a) 198, No. 1, 99– 110 (2003)

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[7] S.Larentis et al, IEEE Transactions on Electron Devices 59(9):2468-2475 (2012)

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[8] K. M. Kim, T. H. Park, and C. S. Hwang, Sci. Rep. 5(2015) [9] E. Atanassova, D. Spassov, Applied Surface Science 135 1998. 71–82

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[10] Yuri Lebedinskii, Andrei Zenkevich, Evgeni P. Gusev J. Appl. Phys. 101, 074504 (2007)

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[11] Yu. Yu. Lebedinskii, A. G. Chernikova, A. M. Markeev, and D. S. Kuzmichev, Appl Phys Lett., 107, 142904 (2015)

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[12] C. Ravi, Gurpreet Kaur, A. Bharathi, Computational Materials Science 90 (2014) 177–181

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[13] V. Nicolosi, M. Chhowalla, M.G. Kanatzidis, M.S. Strano, J.N. Coleman, Science 340 (2013) 1226419.

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[14] Ee Wah Lim and Razali Ismail, Electronics 2015, 4, 586-613 [15] W.D. Kingery, H.K. Bowen and D.R. Uhlman, Introduction

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to Ceramics (Wiley, New York, 1976) pp. 852-854

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Graphical Abstract

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ACCEPTED MANUSCRIPT Highlights

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Process of overstoichiometric tantalum oxide deposition is presented Structural properties of oxide were studied by RBS and XPS Electrical properties of oxide were analyzed in Pt\oxide\Ta forming-free structure with initial state is between low (LRS) and high resistance state (HRS) Resistive switching effect in Pt\oxide\Ta structure with promising characteristics (Roff/Ron~10, high endurance ~108 cycles) is detected Bias memory effect in Pt\oxide\Ta structure and its explanation by developed model of resistive switching effect is proposed

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