Charge transport and the nature of traps in oxygen deficient tantalum

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Charge transport and the nature of traps in oxygen deficient tantalum oxide Vladimir A. Gritsenko, Timofey V. Perevalov, Vitaliy A. Voronkovskii, Andrei A. Gismatulin, Vladimir N. Kruchinin, Vladimir Sh. Aliev, Vladimir A. Pustovarov, Igor P. Prosvirin, and Yakov Roizin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16753 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Charge transport and the nature of traps in oxygen deficient tantalum oxide Vladimir A. Gritsenko1,2,3, Timofey V. Perevalov1,2, Vitaliy A. Voronkovskii1, Andrei A. Gismatulin1*, Vladimir N. Kruchinin1, Vladimir Sh. Aliev1, Vladimir A. Pustovarov4, Igor’ P. Prosvirin5, Yakov Roizin6 1

Rzhanov Institute of Semiconductor Physics SB RAS, 13 Lavrentiev ave., 630090, Novosibirsk,

Russia 2

Novosibirsk State University, 2 Pirogov str., 630090 Novosibirsk, Russia

3

Novosibirsk State Technical University, 20 Prospekt K. Marksa, 630073, Novosibirsk, Russia

4

Ural Federal University, Experimental Physics Department, 19 Mira str., 620002 Ekaterinburg,

Russia 5

Boreskov Institute of Catalysis SB RAS, 5 Lavrentiev ave., 630090, Novosibirsk, Russia

6

TowerJazz, P.O. Box 619, Migdal HaEmek 23105, Israel

ACS Paragon Plus Environment

1

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ABSTRACT

Optical and transport properties of nonstoichiometric tantalum oxide thin films grown by ion beam sputtering were investigated in order to understand the dominant charge transport mechanisms and reveal the nature of traps. The TaOx films content was analyzed by X-ray photoelectron spectroscopy and by quantum-chemistry simulation. From the optical absorption and photoluminescence measurements and density functional theory simulations, it was concluded that the 2.75 eV blue luminescence excited in a TaOx by 4.45 eV photons, originates from oxygen vacancies. These vacancies are also responsible for TaOx conductivity. The thermal trap energy of 0.85 eV determined from the transport experiments coincides with the half of the Stokes shift of the blue luminescence band. It is argued that the dominant charge transport mechanism in TaOx films is phonon-assisted tunneling between the traps.

Keywords: traps, oxygen vacancy, optic, XPS, ab initio simulation, photoluminescence, charge transport.

INTRODUCTION Stoichiometric tantalum oxide (Ta2O5) dielectric films find wide applications in microelectronics due to their high dielectric constant (κ=25-50), electrical strength and good thermal and chemical stability

1-3

. In the last years, the interest to tantalum oxide strongly

increased due to its successful use in non-volatile resistive random access memories (ReRAM) or/and memristors

4-14

, especially after nonstoichiometric oxygen deficient tantalum oxide

(TaOx) based ReRAM was moved into production

14-15

. ReRAM is considered both for stand-

alone data storage applications, as well as an embedded non-volatile memory. Neuromorphic


2

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applications, in particular delegating a part of data processing to memories in computer architectures are also considered

16-17

. Among various oxide-based resistive switching elements,

devices based on TaOx exhibit excellent memory retention performance endurance (number of switching cycles) tantalum oxide ReRAM

14

5, 20

18

, switching speed

and low power consumption

6, 21

19

,

. The industrial

is based on resistance changes in TaOx filaments formed in the

stacked dielectric structures comprising TaOx and a stoichiometric Ta2O5. Single layered TaOx memories were also considered ReRAM

23-25

22

. Various models were suggested to explain switching in

. A widely discussed model is oxygen vacancies migration in the TaOx filament in

the stoichiometric tantalum pentoxide matrix. Despite a large number of contributions focused on switching mechanisms, as well charge transport in high resistance state (HRS) and low resistance state (LRS) 12-13, 26 there were limited studies of the basic conduction mechanisms of the nonstoichiometric tantalum oxide before forming process and their connection with traps. In its turn, the precise conduction model of TaOx before filament forming is important for understanding the nature of the forming in ReRAM structures. The trap ionization via Frenkel mechanism is often suggested describing the charge transport in Ta2O5 27-29. However, the preexponential fitting factor is not analyzed in that studies, while it can be one of the criteria of the Frenkel model applicability. It is the nonphysical low value of preexponential fitting factor that allowed establishing that the Frenkel model is not suitable to describe the charge transport in Si3N4 and HfO2

30-31

. Thus, the charge transport

mechanism of tantalum oxide films is a matter that requires a detailed investigation. Thus, the correctness of using the Frenkel model using is in question. In Ref.

32

the charge transport in

Ta2O5 is described in the framework of the model trap-assisted tunneling of electrons through


3


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traps. However, this model utilizes the strong assumption that there is a continuous spectrum of the defect density of states in the bandgap, and the shape of this spectrum is not defined. Thus, though the charge transport in oxygen deficient tantalum oxide was investigated previously, the details of the process and connection with high concentrations of vacancies were not definitively established. At the same time, understanding of the trap nature (connection with atomic and electronic properties) and the role of traps in conductivity is critical for the device operation and prediction of ReRAM reliability.

METHODS The amorphous tantalum oxide layers were deposited on bare silicon (100) and Si-SiO2 substrates with TaN films by ion beam sputtering (IBSD) of a Ta target by an Ar+ ion beam in the presence of oxygen. The necessary level of stoichiometry was defined by the amount of oxygen in the deposition chamber. The tantalum oxides samples synthesized at the oxygen pressure in the growth chamber of 9.34 mPa (Sample 1 (S1), it is expected as stoichiometric), 3.49 mPa (Sample 2 ( S2)) and 2.80 mPa (Sample 3 (S3)) were investigated. The thicknesses of the studied films were about 30nm. X-ray photoelectron spectra (XPS) were recorded on a SPECS photoelectron spectrometer using a hemispherical PHOIBOS-150-MCD-9 analyzer and FOCUS-500 (Al Kα radiation, hv = 1486.74 eV, 150 W) monochromator. The sampling depth is about 6.6 nm. The C1s peak at 284.8 eV was used as internal standard (related to hydrocarbons for the as-inserted surface). Atomic concentrations ratio [O]/[Ta] were found from the integrated intensities of photoelectron lines (Ta4f: Ta2O5 + Ta suboxides peaks and O1s lines)


4

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which were corrected with theoretical sensitivity factors based on Scofield's photoionization cross sections 33. The absorption coefficient spectral dependence α(hω) at hω range of 1.12 – 4.96 eV was determined by means of spectroscopic ellipsometry on the «SPECTROSCAN» equipment (produced by ISP SB RAS). The calculations of the α(hω) were carried out in the frames of onelayer film (an isotropic substrate – isotropic film – air) and semi-infinite reflecting medium (an isotropic substrate – air) optical models according to the basic equation of ellipsometry. The spectral dependences of polarization angles were fitted for each points of the spectrum. The photoluminescence (PL) emission spectra and photoluminescence excitation (PLE) spectra in the energy range of 1.5 – 5.9 eV were measured using a double-prism DMR-4 type monochromator and a R6358-10 (Hamamatsu) type photoelectron multiplier tube with multialkaline photocathode (laboratory of Solid State Physics of Ural Federal University). A 400W deuterium discharge lamp (DDS-400) and the primary double-prism monochromator DMR-4 type with inverse linear dispersion of 10 A/mm in region of 5 eV were used for PL excitation. PL excitation spectra are normalized to an equal number of photons incident on the sample using a yellow lumogen – luminophore with a unit quantum yield in the studied energy range. It was found that the PL signal of thin tantalum oxide films at room temperature is vanishingly small; so, the PL measurements were performed at 88 K. The sample was placed in a cryostat in which was provided an oil-free vacuum better than 10-4 Pa For charge transport measurements, MIM structures comprising TaN (50nm) / TaOx (30nm) / Ni (50nm) on the Si-SiO2 substrates were used. The current-voltage characteristics of MIM samples were measured at different temperatures in the range 20°C to 80°C using Keithley 6517a electrometer. Attempts to interpret the current-voltage characteristics (j-F) within Frenkel


5


model (2)

34-36

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and phonon-assisted tunneling between traps (PATT) (3)

37

were done. The

Frenkel mechanism is the thermal ionization of an isolated Coulomb trap, which involves reducing the energy barrier by applying an electric field:

j=

e P s2

(1).

 W −β F  P = ν exp  − . 2kT  

(2)

P is the probability of electron emission from traps per second, s = N-1/3 is the distance between traps, N is the trap concentration, W is the energy of thermal ionization of the trap, ν is the attempt-to-escape frequency (electron collisions frequency with the walls of the potential well, evaluated in Frenkel model as W/h), h is the Planck constant, β = e3 / πε ∞ε 0 , F is the electric field, k is the Boltzmann constant, T is the temperature, ε∞ is the dynamic permittivity and ε0 is the dielectric constant, e is the electron charge. In PATT model, the act of charge transfer occurs via the electron tunneling between the neighboring traps with multi-phonon excitation. In this model electrons tunnels between the deep traps without excitation in the conduction band, as is the case in Mott hopping. This mechanism was shown to be dominating for high trap densities. In this case, the emission probability in (1) is:  Wopt − Wt P = ν exp  − 2kT 

ν=

* 2

ms

 2 s 2m*W  t − exp   h  

2πhWt . kT (Wopt − Wt )

 eFs  sinh    2kT 

 , 

(3a)

(3b)

where Wopt is the trap optical excitation energy, Wt is the trap thermal excitation energy, m* is the effective tunneling mass, s is the distance between traps (evaluated as N-1/3).


6

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The quantum-chemical calculations were performed in terms of the density functional theory in the periodic supercell model in the Quantum-ESPRESSO code 38. The orthorhombic λ-Ta2O5 was used in the simulations

39

because its electronic structure was consistent with the electronic

structure of the amorphous tantalum oxide 39-40. The 2-fold coordinated «intralayer» vacancy, as the most stable and lowest energy cost vacancy, was investigated

40

. Hybrid functional B3LYP

was used to reproduce the correct tantalum oxide bandgap of 4.2 eV

41

. Norm-conserving

pseudopotentials with the configurations of 5d3 6s2 for Ta and 2s22p4 for O were used with the plane wave cutoff of 950 eV. By projecting the Kohn-Sham states onto the atomic orbitals, we decompose the density of electronic states into the contributions of different elements (projected density of states, PDOS) 42. The ratio [O]/[Ta] varies by changing the supercell size, from which oxygen atom is removed. The supercell is constructed by translating the 14-atom unit cell along the crystallographic axes. The valence band XPS was calculated by summing up the partial density of filled states with the weight factors corresponding to photoionization cross-sections 33 and by smoothing using a Gauss function with σ = 0.6 eV.

RESULTS For tantalum oxide, grown at the highest oxygen pressures, XPS Ta 4f5/2,7/2 peak is observed at an energy corresponding to the stoichiometric Ta2O5 and consists of the spin-orbit doublet Ta 4f5/2,7/2 with 1.89 eV splitting (Fig. 1a). Decreasing the oxygen pressure during the tantalum oxide synthesis results in the appearance of new features in the Ta 4f spectra at the lower binding energy side, which corresponds to a partially reduced Ta. XPS spectra for such a film are well described with three doublets having Ta 4f7/2 binding energies of 22.41 eV and 24.54 eV (related to TaOx), as well as 25.95 eV (related to Ta2O5). This can be attributed to Ta2+, Ta3+/Ta4+, and


7


Ta5+, respectively

43

Page 8 of 21

and indicates the presence of oxygen vacancies in the studied films. The

atomic ratio [O]/[Ta] ratios, calculated from XPS data, are 2.45, 2.4 and 2.3 for the tantalum oxides samples Sample 1, 2 and 3, respectively. This result confirms the oxygen depletion of samples, although the numerical accuracy in determining the stoichiometry is not allow for quantitative conclusions. The oxygen depletion increase leads to raising the XPS peak located about 2 eV above the tantalum oxide valence band top (Fig. 1b). This peak can be detected even for the Sample 1, which is expected to be stoichiometric. The experimental valence band XPS spectra of all samples are in a good agreement with the simulated ones for the λ-Ta2O5 with oxygen vacancies. The calculated bandgap peak into the XPS spectra varies at the proportion to the O vacancies concentration, i.e.by the ratio [O]/[Ta]. Thus, we can propose a method for a determining of the stoichiometry from the comparison of experimental and theoretical valence band photoelectron spectra. The [O]/[Ta] ratio in the theoretical model, at which the calculated bandgap peak describes the experimental one, corresponds to the atomic ratio [O]/[Ta] in the studied film. The valence band XPS spectra for Samples 1, 2, 3 is described by the theoretical spectra for one O vacancy in the supercell with 840, 168 and 42 аtoms, respectively. Thus, the atomic ratio of [O]/[Ta] = 479/192≈2.495 for Sample 1 (almost stoichiometric), 119/48≈2.48 - for Sample 2 and 29/12≈2.42 - for Sample 3. Hereinafter, we use these values to differentiate our samples. The optical bandgap value of 4.2 eV derived from the absorption threshold for the almost stoichiometric Ta2O5 (Fig. 2a) agrees to the quantum-chemical simulation data. The oxygen deficiency increase leads to the optical absorption edge shifting towards lower energies. The low-energy absorption edge shift for TaO2.48 and TaO2.42 to the value of 3.6 eV and 2.2 eV, respectively, is, most likely, due to the presence of tantalum suboxides. The spectrum for TaO2.42


8

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clearly exhibits the 4.55 eV peak. It is reasonable to assume that this peak is related to the high oxygen vacancy concentration in the film, whereas, for almost stoichiometric Ta2O5 and TaO2.48, the defect concentration is too low for this method to distinguish that feature. That assumption is confirmed by the results of quantum-chemical calculations that predict the possibility of optical transitions in tantalum oxide with the energy values of 4.50 eV and 4.65 eV caused by the oxygen vacancy (Fig. 2b). These values were obtained by calculating of PDOS spectra at the tantalum and oxygen atoms in the vicinity of the oxygen vacancy. One can see, the defect level near the middle of the band gap is almost completely formed by the Ta 5d states with the admixtures of 6s and 6p states of Ta

39

. The calculated peaks in the PDOS spectra predict the

4.65 eV transitions from the defect level to the conduction band Ta 5d states and the 4.5 eV transitions to the conduction band of Ta 6p, Ta 6s states. The photoluminescence of the almost stoichiometric Ta2O5 and TaO2.42 films exhibits a single wide band PL with its maximum at 2.75 eV (Fig. 2c). Since the spectra of all samples are qualitatively similar, the spectra for TaO2.48 are not depicted. A maximum intensity of the 2.75 eV blue luminescence band was observed under the excitation with 4.45 eV quanta. A similar result (2.8 eV PL peak and the 4.5 eV PLE peak) was previously obtained for the tantalum oxide synthesized by the molecular layer deposition method by using the Xe lamp as the photon excitation source 44. Within the experimental accuracy, the blue luminescence excitation peak for our samples has the same energy as the 4.55 eV optical absorption peak (Fig. 2a) and it is close to the possible optical transitions with energies of 4.50 eV and 4.65 eV that are predicted from the quantum-chemical calculations (Fig. 2b). Thus, it is argued that the 4.45 eV PLE band and the blue luminescence band at 2.75 eV are due to the oxygen vacancies. It is interesting to note that the DFT calculations also predict the transitions with the energies of 3.65 eV and 5.27 eV


9


Page 10 of 21

caused by the oxygen vacancy, which agree to the experimental 3.6 eV and 5.3 eV PLE spectrum peaks, respectively. The intensity of these PLE peaks is small, probably, because of the small matrix elements of the transitions. The applied method of luminescence spectroscopy does not allow identifying a correlation between the PL output and the oxygen vacancy concentration in our samples because the spectra are in arbitrary units. In addition, it is reasonable to expect that there are different kinds of defects along with the oxygen vacancies, which provide a competing channel drain of charge carriers and reduce the PL signal from the oxygen vacancy. This reasoning is explained the blue PL band broadening for TaO2.42 The difference between the positions of the band maxima of the PLE spectra 4.45 eV and the PL spectra 2.75 eV for the same electronic transition on the oxygen vacancy (Stokes shift) in TaOx is equal to 1.7 eV. The Stokes shift is much higher than the phonon energy, which has its typical value of 0.1 eV. Hence, a large Stokes shift value in TaOx is indicative of a strong electron-phonon interaction

45

. The energy diagram in the simple single-band model of the

optical transitions for the neutral oxygen vacancy in TaOx are shown in Fig. 2d. The PLE maximum at 4.45 eV corresponds to the Frank–Condon transition from the occupied ground state to the excited empty state. From this exited state, the defect changes its configuration and converts into another (lower energy) excited state by emitting phonons. The electron energy decreases by the relaxation energy. The subsequent vertical transition into the nonrelaxed ground state corresponds to the radiative (PL) transition with the energy of 2.75 eV. It was found that the experimental current-voltage (j-F) characteristics of TiN/TaOx/Ni structure, with almost stoichiometric Ta2O5 measured at different temperatures, can be formally described both by the Frenkel model (Fig. 3b) and by the PATT model (Fig. 3c). The fit by the


10

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Frenkel model allows to obtain reasonable values of N = 1.9·1019 cm-3, W = 0.55 eV, ε∞ = 5. However, the necessary for fitting value of the attempt-to-escape (103 sec−1) is many orders of magnitude lower than the characteristic value W/h ~ 1014 sec−1. Phonon-assisted tunneling between traps adequately represents the experimental j-F characteristics, both qualitatively and numerically. The fitting parameters: ν ≈ 1014 sec−1, N = 1.9·1019 cm-3, Wt = 0.85 eV, Wopt = 1.7 eV, m* = 0.42 me have clear physical meanings. The Wt= 0.85 eV value is in a good agreement with the reported trap energies in Ta2O5: 0.8 eV

27, 46

, 0.85

29

eV and 0.7 eV

32

. The

m* is the range of the (0.3–0.7)me predicted from the first-principles simulations of various Ta2O5 allotropic modifications 41, 43, 47. The Wopt = 1.7 eV value is close to the optical trap energy 1.5 eV extracted from the photoconductivity experiments 48. It turns out that, for tantalum oxide the optical trap energy is equal to two thermal trap energy values. This is relation was earlier observed by us in Si3N4 49, Al2O3 50, HfO2 51, Hf0.5Zr0.502 52. The j-F curves of TaOx samples, with different levels of stoichiometry, are also well described by the PATT model with the same values of Wt, Wopt and m* (Fig. 4a). The decrease of x leads to eight orders of magnitude current increase at 0.4 MV/cm. The simulation within the PATT model shows that this effect can be explained by the increase of the trap density in the TaOx films from 1.9·1019 cm-3 for almost stoichiometric Ta2O5, to 1.9·1021 cm-3 for TaO2.42. The calculated and experimental I-V curves can diverge at small electric fields due to the current limiting by charges contact injection from electrodes into traps in the dielectric. The previous experiments on the injection of the minority carriers from Si into Ta2O5 demonstrated that the amorphous Ta2O5 conductivity is unipolar: the current is dominated by the electron injection, whereas the flow of holes is negligible

41

. The simulations of the negatively

charged oxygen vacancy in λ-Ta2O5 revealed that the oxygen vacancy forms an additional


11


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localized single-occupied state ~ 0.5 eV below the conduction band edge (Fig. 4b). This is an additional evidence that oxygen vacancies in oxygen deficient tantalum oxide capture electrons and the conductivity is electronic. The simulations show that electron density is distributed evenly between the two Ta atoms nearest to the vacancy (inset in Fig. 4b), which reflects the bonding character of the Ta atomic orbitals. Thus, in current investigation, we shown the direct current density dependence on stoichiometry of tantalum oxide. Oxygen vacancies acting as traps allow to explain the low values of resistances for LRS and HRS states for TaOx ReRAM 11, 14-15. However, in ReRAM the typical values of LRS resistance are 6-8 kOhm, while HRS values are of the 200k-300 kOhm. For the discussed TaOx films with maximum non-stoichiometry (Fig. 4a), for the readout field ~1 MV/cm, the current densities are of the order of 102A/cm2 and this corresponds to the resistance ~1 MOhm, for a memory dot of ~1 µm2 area. Thus, the oxygen deficiency in the low resistance state of ReRAM device’s filament is sufficiently higher than 1.9·1021 cm-3 12-13. CONCLUSION The optical and transport properties of tantalum oxide films with different oxygen deficiency were investigated experimentally and simulated. It was found that the presence of oxygen vacancies in nonstoichiometric tantalum oxide results in the optical absorption peak at 4.55 eV, corresponding to the photoluminescence excitation peak (at 4.45 eV) for maximum of 2.75 eV photoluminescence emission. It was shown that charge transport in oxygen deficient tantalum oxide is can be consistent described by the phonon-assisted tunneling between the traps. The thermal energy of the trap 0.85 eV is equal to the half of the Stokes shift of blue photoluminescence. The exponential increase of the leakage current in TaOx with the decreasing x is explained by the increasing oxygen vacancy corresponding to the smaller distance between


12

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traps. The DFT simulation showed that oxygen vacancies can localize electrons, confirming that the TaOx conductivity is electronic. Chains of closely located oxygen vacancies in TaOx can act as hopping paths for electrons in formed memory cells. ACKNOWLEDGMENT This work is supported by the Russian Science Foundation under grant 16-19-00002 (XPS, photoluminescence, transport experiments) and the Russian Science Foundation under grant 1772-10103 (quantum-chemical simulations). The computations were carried out at the Novosibirsk State University Supercomputer Center. The authors thank Dr D.R. Islamov for useful discussions.

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10. Rahaman, S. Z.; Maikap, S.; Tien, T. C.; Lee, H. Y.; Chen, W. S.; Chen, F. T.; Kao, M. J.; Tsai, M. J., Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaOx interface. Nanoscale Res Lett 2012, 7, 345. 11. Kim, B. Y.; Lee, K. J.; Chung, S. O.; Kim, S. G.; Ko, Y. S.; Kim, H. S., Low power switching of Si-doped Ta2O5 resistive random access memory for high density memory application. Japanese Journal of Applied Physics 2016, 55 (4), 04EE09. 12. Park, G. S.; Kim, Y. B.; Park, S. Y.; Li, X. S.; Heo, S.; Lee, M. J.; Chang, M.; Kwon, J. H.; Kim, M.; Chung, U. I.; Dittmann, R.; Waser, R.; Kim, K., In situ observation of filamentary conducting channels in an asymmetric Ta2O5-x/TaO2-x bilayer structure. Nat Commun 2013, 4, 2382. 13. Yang, M. K.; Ju, H.; Kim, G. H.; Lee, J. K.; Ryu, H. C., Direct evidence on Ta-Metal Phases Igniting Resistive Switching in TaOx Thin Film. Sci Rep-Uk 2015, 5, 14053. 14. Panasonic Starts World's First Mass Production of ReRAM Mounted Microcomputers. http://news.panasonic.com/global/press/data/2013/07/en130730-2/en130730-2.html, 2013. 15. Wei, Z. Q.; Ninomiya, T.; Muraoka, S.; Katayama, K.; Yasuhara, R.; Mikawa, T., Switching and Reliability Mechanisms for ReRAM. 2014 Ieee International Interconnect Technology Conference / Advanced Metallization Conference (Iitc/Amc) 2014, 349-351. 16. Prezioso, M.; Merrikh-Bayat, F.; Hoskins, B. D.; Adam, G. C.; Likharev, K. K.; Strukov, D. B., Training and operation of an integrated neuromorphic network based on metal-oxide memristors. Nature 2015, 521 (7550), 61-64. 17. Yang, J. J. S.; Strukov, D. B.; Stewart, D. R., Memristive devices for computing. Nature nanotechnology 2013, 8 (1), 13-24. 18. Goux, L.; Fantini, A.; Chen, Y. Y.; Redolfi, A.; Degraeve, R.; Jurczak, M., Evidences of Electrode-Controlled Retention Properties in Ta2O5-Based Resistive-Switching Memory Cells. Ecs Solid State Lett 2014, 3 (11), Q79-Q81. 19. Torrezan, A. C.; Strachan, J. P.; Medeiros-Ribeiro, G.; Williams, R. S., Sub-nanosecond switching of a tantalum oxide memristor. Nanotechnology 2011, 22 (48), 485203. 20. Miao, F.; Strachan, J. P.; Yang, J. J.; Zhang, M. X.; Goldfarb, I.; Torrezan, A. C.; Eschbach, P.; Kelley, R. D.; Medeiros-Ribeiro, G.; Williams, R. S., Anatomy of a Nanoscale Conduction Channel Reveals the Mechanism of a High-Performance Memristor. Adv Mater 2011, 23 (47), 5633–5640. 21. Kim, W.; Rosgen, B.; Breuer, T.; Menzel, S.; Wouters, D.; Waser, R.; Rana, V., Nonlinearity analysis of TaOx redox-based RRAM. Microelectronic Engineering 2016, 154, 3841. 22. Sharath, S. U.; Joseph, M. J.; Vogel, S.; Hildebrandt, E.; Komissinskiy, P.; Kurian, J.; Schroeder, T.; Alff, L., Impact of oxygen stoichiometry on electroforming and multiple switching modes in TiN/TaOx/Pt based ReRAM. Applied Physics Letters 2016, 109 (17), 173503. 23. Chang, K. C.; Chang, T. C.; Tsai, T. M.; Zhang, R.; Hung, Y. C.; Syu, Y. E.; Chang, Y. F.; Chen, M. C.; Chu, T. J.; Chen, H. L.; Pan, C. H.; Shih, C. C.; Zheng, J. C.; Sze, S. M., Physical and chemical mechanisms in oxide-based resistance random access memory. Nanoscale Res Lett 2015, 10, 120. 24. Seok, J. Y.; Song, S. J.; Yoon, J. H.; Yoon, K. J.; Park, T. H.; Kwon, D. E.; Lim, H.; Kim, G. H.; Jeong, D. S.; Hwang, C. S., A Review of Three-Dimensional Resistive Switching Cross-Bar Array Memories from the Integration and Materials Property Points of View. Adv Funct Mater 2014, 24 (34), 5316-5339.


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25. Pan, F.; Gao, S.; Chen, C.; Song, C.; Zeng, F., Recent progress in resistive random access memories: Materials, switching mechanisms, and performance. Mat Sci Eng R 2014, 83, 1-59. 26. Graves, C. E.; Davila, N.; Merced-Grafals, E. J.; Lam, S. T.; Strachan, J. P.; Williams, R. S., Temperature and field-dependent transport measurements in continuously tunable tantalum oxide memristors expose the dominant state variable. Applied Physics Letters 2017, 110 (12), 123501. 27. Egorov, K. V.; Kuzmichev, D. S.; Chizhov, P. S.; Lebedinskii, Y. Y.; Hwang, C. S.; Markeev, A. M., In situ Control of Oxygen Vacancies in TaOx Thin Films via Plasma-Enhanced Atomic Layer Deposition for Resistive Switching Memory Applications. Acs Appl Mater Inter 2017, 9 (15), 13286–13292. 28. Novkovski, N.; Atanassova, E., Origin of the stress-induced leakage currents in AlTa2O5/SiO2-Si structures. Applied Physics Letters 2005, 86 (15), 152104. 29. Houssa, M.; Degraeve, R.; Mertens, P. W.; Heyns, M. M.; Jeon, J. S.; Halliyal, A.; Ogle, B., Electrical properties of thin SiON/Ta2O5 gate dielectric stacks. Journal of Applied Physics 1999, 86 (11), 6462-6467. 30. Vishnyakov, A. V.; Novikov, Y. N.; Gritsenko, V. A.; Nasyrov, K. A., The charge transport mechanism in silicon nitride: Multi-phonon trap ionization. Solid-State Electronics 2009, 53 (3), 251-255. 31. Gritsenko, V. A.; Perevalov, T. V.; Islamov, D. R., Electronic properties of hafnium oxide: A contribution from defects and traps. Phys Rep 2016, 613, 1-20. 32. Houssa, M.; Tuominen, M.; Naili, M.; Afanas'ev, V. V.; Stesmans, A.; Haukka, S.; Heyns, M. M., Trap-assisted tunneling in high permittivity gate dielectric stacks. Journal of Applied Physics 2000, 87 (12), 8615-8620. 33. Scofield, J. H., Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV. Journal of Electron Spectroscopy and Related Phenomena 1976, 8 (2), 129-137. 34. Frenkel, J., On the theory of electric breakdown of dielectrics and electronic semiconductors. Technical Physics of the USSR 1938, 5 (8), 685-695 (in Russian). 35. Frenkel, J., On Pre-Breakdown Phenomena in Insulators and Electronic SemiConductors. Phys. Rev. 1938, 54, 647. 36. Hill, R. M., Poole-Frenkel Conduction in Amorphous Solids. The Philosophical Magazine 1971, 23 (181), 39. 37. Nasyrov, K. A.; Gritsenko, V. A., Charge transport in dielectrics via tunneling between traps. Journal of Applied Physics 2011, 109 (9), 093705. 38. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M., QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. Journal of physics: Condensed matter : an Institute of Physics journal 2009, 21 (39), 395502. 39. Lee, S. H.; Kim, J.; Kim, S. J.; Kim, S.; Park, G. S., Hidden Structural Order in Orthorhombic Ta2O5. Phys Rev Lett 2013, 110 (23), 235502. 40. Guo, Y. Z.; Robertson, J., Comparison of oxygen vacancy defects in crystalline and amorphous Ta2O5. Microelectronic Engineering 2015, 147, 254-259.


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41. Shvets, V. A.; Aliev, V. S.; Gritsenko, D. V.; Shaimeev, S. S.; Fedosenko, E. V.; Rykhlitski, S. V.; Atuchin, V. V.; Gritsenko, V. A.; Tapilin, V. M.; Wong, H., Electronic structure and charge transport properties of amorphous Ta2O5 films. Journal of Non-Crystalline Solids 2008, 354 (26), 3025-3033. 42. Giustino, F., Modeling materials using density functional theory. Oxford university press: 2014. 43. Ivanov, M. V.; Perevalov, T. V.; Aliev, V. S.; Gritsenko, V. A.; Kaichev, V. V., Electronic structure of δ-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment. Journal of Applied Physics 2011, 110 (2), 024115. 44. Baraban, A. P.; Dmitriev, V. A.; Prokof'ev, V. A.; Drozd, V. E.; Filatova, E. O., Photoluminescence of Ta2O5 films formed by the molecular layer deposition method. Tech Phys Lett+ 2016, 42 (4), 341-343. 45. Struck, C. W.; Fonger, W. H., Understanding Luminescence Spectra and Efficiency Using Wp and Related Functions. Springer: Berlin, 1991; p 253. 46. Lau, W. S.; Leong, L. L.; Han, T. J.; Sandler, N. P., Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy. Applied Physics Letters 2003, 83 (14), 2835-2837. 47. Perevalov, T. V.; Shaposhnikov, A. V., Ab initio simulation of the electronic structure of Ta2O5 crystal modifications. J Exp Theor Phys+ 2013, 116 (6), 995-1001. 48. Thomas, J. H., Defect photoconductivity of anodic Ta2O5 films. Applied Physics Letters 1973, 22 (8), 406-408. 49. Gritsenko, V. A.; Perevalov, T. V.; Orlov, O. M.; Krasnikov, G. Y., Nature of traps responsible for the memory effect in silicon nitride. Applied Physics Letters 2016, 109 (6), 062904. 50. Novikov, Y. N.; Gritsenko, V. A.; Nasyrov, K. A., Charge transport mechanism in amorphous alumina. Applied Physics Letters 2009, 94 (22), 222904. 51. Gritsenko, V. A.; Islamov, D. R.; Perevalov, T. V.; Aliev, V. S.; Yelisseyev, A. P.; Lomonova, E. E.; Pustovarov, V. A.; Chin, A., Oxygen Vacancy in Hafnia as a Blue Luminescence Center and a Trap of Charge Carriers. J Phys Chem C 2016, 120 (36), 1998019986. 52. Islamov, D. R.; Perevalov, T. V.; Gritsenko, V. A.; Cheng, C. H.; Chin, A., Charge transport in amorphous Hf0.5Zr0.5O2. Applied Physics Letters 2015, 106 (10), 102906.


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Page 17 of 21

(a) TaOx

(b)

Ta4f7/2 Ta4f5/2

S2

×10

S3

x=2.147

S2

x=2.479

S1

x=2.495

×10

S1 20

expt.

EV

Intensity, arb. units

S3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


×10

22 24 26 28 Binding energy, eV

30

-4

-2

0 2 4 Energy, eV

6

8

Figure 1. (a) Ta 4f photoelectron spectra of three samples of tantalum oxide films (black) and their deconvolution (colored); the curves with the filled area under them refer to Ta+2/3/4. (b) Valence band XPS of three samples of tantalum oxide films (black), in comparison with the calculated ones for the λ-Ta2O5 with different atomic ratio x=[O]/[Ta]: single O vacancy in supercell of 840 atom (red), 168 atom (blue) and 42 atom (green). The zero energy is taken as the valence band top EV.


17


(b)

0.8

5.27 eV 4.65 eV 4.50 eV 3.65 eV

4.55 eV S1 - Ta2O5

0.6

PDOS, arb. units

(a)

Absorption α, cm-1 (×10-6)

S2 - TaO2.48 S3 - TaO2.42

0.4

Ta 5d Ta 6p Ta 6s

0.2

4.2 eV 3.6 eV

2.2 eV

EC

EV

0.0 2

3

4

5

0

Energy, eV (c)

2.75


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

TaO2.42

PL Eexc=4.5eV

Ta2O5

2

4

6

8

Energy, eV

PLE Eemis=2.7eV 5.3

(d)

4.45 3.6

2

3

4

5

Energy, eV Figure 2. (a) Optical absorption spectra of almost stoichiometric Ta2O5 (black), TaO2.48 (violet) and TaO2.42 (red). (b) Calculated density of states (PDOS) of the Ta atoms nearest to the O vacancies: Ta 5d (blue), Ta 6p (red) and Ta 6s (green). Ev is the valence band top, Ec is the conduction band bottom. (c) The PL spectra with 4.5 eV excitation (blue), and the PLE spectra of the 2.7 eV emission band (green) for almost stoichiometric Ta2O5 (lower curves) and TaO2.42 (upper curves). (d) Energy diagram of the optical transitions at the neutral oxygen vacancy in TaOx in single-band model.


18

Page 19 of 21

(a)

10-5

10-5

(b) W=0.55 eV 19

77°C

-3

N=1.9⋅10 cm ν ≈103 sec-1 ε∞=5

N=1.9⋅1019 cm-3 ν ≈1014 sec-1 m*=0.42 m0

j, A/cm2

27°C

10-6

10-7 0.6

(c) W =0.85 eV W =1.7 eV t opt

52°C

j, A/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


77°C 52°C 27°C

10-6

10-7 0.7

0.8

0.9

1.0

F, MV/cm

0.6

0.7

0.8

0.9

1.0

F, MV/cm

Figure 3. (a) Schematic illustration of TaN/Ta2O5/Ni structure used in charge transport measurements. (b,c) Experimental (symbols) and simulated (dashed) j-F characteristics of almost stoichiometric Ta2O5 at different temperatures. Simulated curves are in Frankel model (b) and PATT model (c). Lowering of the trap barrier in strong electric fields (Frankel mechanism) is shown in inset (b). The PATT traps model scheme is shown in inset (c). The parameters used for fitting in both model are presented in the graphs.


19


102 (a) 101

(b)

TaO2.42 N=19⋅1020cm-3

EV

EС

10-1 10

TaO2.48 N=3⋅1020cm-3

-2

10-3 10-4 10-5 10

-6

10-7

Wt=0.85 eV

20

Ta2O5 N=0.19⋅10 cm-3

Wopt=1.7 eV

TDOS, arb. units

100

j, A/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

m*=0.42 m0

T=27 °C 10-8 0.0 0.2

0.4

0.6

0.8

1.0

-2

0

Electric field, MV/cm

2

4

6

Energy, eV

Figure 4. (a) Experimental (symbols) and simulated in frame of PATT model (dashed lines) j-F characteristics of TaOx of different composition at room temperature. The parameters used for fitting are presented in the graphs. (b) Total density of states for the single-negatively charged O vacancy in λ-Ta2O5. Localized gap state electron charge density isosurface is shown in the inset.


20

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Charge transport and the nature of traps in oxygen deficient tantalum

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