Nano suboxide layer generated in Ta2O5 by Ar+ ion irradiation

Nano suboxide layer generated in Ta2O5 by Ar+ ion irradiation W. D. Song, J. F. Ying, W. He, V. Y.-Q. Zhuo, R. Ji, H. Q. Xie, S. K. Ng, Serene L. G. Ng, and Y. Jiang Citation: Applied Physics Letters 106, 031602 (2015); doi: 10.1063/1.4906395 View online: http://dx.doi.org/10.1063/1.4906395 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Comparison of Ar+ Monoatomic and Cluster Ion Sputtering of Ta2O5 at Different Ion Energies, by XPS: Part 1 Monoatomic Ions Surf. Sci. Spectra 21, 50 (2014); 10.1116/11.20140701 Infrared optical properties of amorphous and nanocrystalline Ta2O5 thin films J. Appl. Phys. 114, 083515 (2013); 10.1063/1.4819325 Electronic structure of δ-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment J. Appl. Phys. 110, 024115 (2011); 10.1063/1.3606416 Electrochromic properties of large-area and high-density arrays of transparent one-dimensional β -Ta 2 O 5 nanorods on indium-tin-oxide thin-films Appl. Phys. Lett. 98, 133117 (2011); 10.1063/1.3568896 Vertically aligned liquid crystals on a γ -Al 2 O 3 alignment film using ion-beam irradiation Appl. Phys. Lett. 93, 233507 (2008); 10.1063/1.3046728

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APPLIED PHYSICS LETTERS 106, 031602 (2015)

Nano suboxide layer generated in Ta2O5 by Ar1 ion irradiation W. D. Song,a) J. F. Ying,a) W. He, V. Y.-Q. Zhuo, R. Ji, H. Q. Xie, S. K. Ng, Serene L. G. Ng, and Y. Jiang Data Storage Institute, Agency for Science, Technology and Research (A*STAR), DSI Building, 5 Engineering Drive 1, Singapore 117608

(Received 28 October 2014; accepted 4 January 2015; published online 21 January 2015) Ta2O5/TaOx heterostructure has become a leading oxide layer in memory cells and/or a bidirectional selector for resistive random access memory (RRAM). Although atomic layer deposition (ALD) was found to be uniquely suitable for depositing uniform and conformal films on complex topographies, it is hard to use ALD to grow suboxide TaOx layer. In this study, tantalum oxide films with a composition of Ta2O5 were grown by ALD. Using Arþ ion irradiation, the suboxide was formed in the top layer of Ta2O5 films by observing the Ta core level shift toward lower binding energy with angle-resolved X-ray photoelectron spectroscopy. By controlling the energy and irradiation time of an Arþ ion beam, Ta2O5/TaOx heterostructure can be reliably produced on ALD films, which proC 2015 AIP Publishing LLC. vides a way to fabricate the critical switching layers of RRAM. V [http://dx.doi.org/10.1063/1.4906395]

Resistive random access memory (RRAM), based on the filamentary resistance switching in an oxide layer sandwiched between two metal electrodes, is a promising candidate for next-generation non-volatile memory.1,2 Ta2O5/TaOx heterostructure has become a leading oxide layer in memory cells and/or a bidirectional selector for RRAM due to good performance in many aspects.3–11 In a memory cell, oxygen vacancies are able to exchange between TaOx and Ta2O5 layers upon the application of external voltages, which results in controllable resistive switching.3–9 In a bidirectional selector, Ta2O5 as a tunneling barrier affects both the selectivity and on-state tunneling current while TaOx controls the offstate current.10,11 Ta2O5/TaOx heterostructure has been fabricated by a few methods. Lu et al.4,9 prepared TaOx by direct current reactive sputtering of a Ta target in Ar/O2 gas mixture at high temperature and deposited Ta2O5 by radio frequency sputtering of a tantalum oxide (Ta2O5) target at room temperature. Lee et al.3 used oxygen plasma to form Ta2O5 on TaOx layer deposited by sputtering. Elliman et al.12,13 proposed a controllable means to fabricate Ta2O5/TaOx heterostructure by implantation of the oxygen ions into the Ta film. Three-dimensional (3D) RRAM is essential to achieve the highest memory densities.14 In order to realize 3D RRAM, especially for the vertical RRAM, atomic layer deposition (ALD) was selected to deposit the oxide layer since ALD is uniquely suitable for depositing uniform and conformal films on complex 3D topographies.14,15 Although Ta2O5 films can be grown by ALD,16–20 it is hard to use ALD to deposit the suboxide TaOx layer for the Ta2O5/TaOx heterostructure. We noticed that Woo et al.10,11 deposited TiO2 film by ALD and transferred the sample to a sputtering chamber to grow the Ta2O5/TaOx heterostructure on the ALD film. Therefore, it is valuable to explore a configuration for the ALD system to grow Ta2O5/TaOx heterostructure for a)

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]

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RRAM. In this study, we demonstrate a nano-suboxide layer generated in Ta2O5 by using an Arþ ion beam irradiation to form a Ta2O5/TaOx heterostructure. Tantalum oxide films were grown on Si wafers and Pt substrates by an ALD system (Cambridge NanoTech, Inc.). Argon was used as both the carrier and purging gas. Prior to deposition, the substrates are inserted into the ALD chamber and heated to 250 C. A precursor of Ta(OC2H5)5 is pulsed into the reaction chamber. The chemisorption reaction of the precursor takes place on the substrate surface. After the unreacted precursor and gaseous by-products are pumped away, a reactant of H2O is pulsed into the reaction chamber. The chemisorption reaction of the reactant was carried out to form an atomic layer of tantalum oxide on the substrate surface. The unreacted reactant and gaseous by-products are removed before a new cycle starts. Figure 1(a) shows the ALD process and pulses of precursor and reactant. Argon purge time was set to 10 s. Ta(OC2H5)5 and H2O pulse times were fixed at 1 s. The number of cycles was varied from 30 to 300 cycles. The growth rate is 0.086 nm/cycle. An Arþ ion beam was used to irradiate Ta2O5 films at different energy values and time durations in the same chamber with X-ray Photoelectron Spectroscopy (XPS). The compositions of tantalum oxide thin films and the chemical states of Ta and oxygen were characterized by angle-resolved XPS. The sampling depth in angle-resolved XPS can be varied by changing the photoelectron take-off angle as shown in Fig. 1(b). The sampling depth, d, is given by the equation d ¼ L sin h, where L is the escape depth of the photoelectron and h is the photoelectron take-off angle.21 As h is reduced, the sampling depth decreases. At very small take-off angle such as h ¼ 10 , most of the collected electrons originate from atoms near the surface while more electrons will be collected from atoms deeper in the sample at a high take-off angle such as h ¼ 80 . Figure 2 shows Ta4f, O1s, and Si2p spectra collected at different take-off angles for a 9 nm thick tantalum oxide film on Si wafers deposited by ALD. The peaks at 26.6 eV and

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FIG. 1. (a) ALD process and pulses of precursor and reactant. The pulse times were fixed at 1 s both for a precursor of Ta(OC2H5)5 and a reactant of H2O. The purge time is 10 s. (b) Angle-resolved XPS diagram. The sampling depth, d, is given by the equation d ¼ L sin h, where L is the escape depth of the photoelectron and h is the photoelectron take-off angle.

28.4 eV belong to Ta 4f7/2 and Ta 4f5/2, respectively. These binding energy positions correspond to the þ5 oxidation state of Ta. The normalized Ta4f spectra is shown in the inset of Fig. 2(a), which exhibits the same binding energy positions for Ta 4f7/2 and Ta 4f5/2 at different take-off angles from 5 to 80 . This means that the signal coming from the electrons originating from atoms near the top surface is the same as that from atoms deeper in the film. The similar shape and binding energy position of O1s were found at different take-off angles as shown in Fig. 2(b). In the inset of Fig. 2(b), we observed the peak attributed to SiOx (at 102 eV) in the Si2p spectra at a take-off angle of 45 and even a weak peak Si(0) (at 99 eV) at a take-off angle of 80 . This indicates that a signal from the substrate has been detected by Angle-resolved XPS. Therefore, we can conclude that the whole 9 nm thick tantalum oxide film contributed to the Ta4f spectra. The composition for this 9 nm tantalum oxide film is uniform across the depth, and is Ta2O5. In addition, the native oxide on silicon substrate is about 1.5 nm. Thus, the detection thickness of Angle-resolved XPS at a take-off angle of 80 is around 10 nm. The Ta4f spectra of Ta2O5 films at a take-off angle of 45 , measured before and after Arþ ion beam irradiation at energy of 1 keV with different times are shown in Fig. 3. In order to clearly see different oxidation states, curve fitting based on Gaussian and Lorentzian line shapes using a Shirley-type background subtraction was carried out.22

Appl. Phys. Lett. 106, 031602 (2015)

FIG. 2. Ta4f, O1s, and Si2p spectra collected at different take-off angles from 9 nm thick tantalum oxide films on Si wafers deposited by ALD.

Before irradiation, Ta4f7/2 and Ta4f5/2 peaks corresponding to Ta2O5 are observed at 26.6 eV and 28.4 eV, respectively. After an irradiation of 20 s, the peaks of suboxide TaOx emerge at lower binding energies (24.6 eV and 26.2 eV) as shown in Fig. 3(b). The percentage of Ta2O5 is 77.7% with respect to the TaOx percentage of 22.3%. As the ion beam irradiation duration increases to 40 s, two additional fitting peaks at binding energies of 22.3 and 24.1 eV are shown in Fig. 3(c), which correspond to the Ta metal. The calculated Ta2O5, TaOx, and Ta fractions are 59.7%, 37.0%, and 3.3%, respectively. By increasing the duration of the ion beam irradiation, the intensity of suboxide increases while the intensity of the Ta2O5 component decreases, which indicates that TaOx has been generated in the Ta2O5 films by an Arþ ion beam irradiation. This is due to the exposure of Ta2O5 films to energetic radiation sources, resulting in decomposition and the formation of different compounds within the radiation layer.23–26 In order to perform a more complete and systematic study of TaOx generated in Ta2O5 surface layer by an Arþ ion beam irradiation, we use Angle-resolved XPS for in-situ characterization of compositions and chemical states of the tantalum oxide layer. Figure 4 shows the Ta4f spectra collected at different take-off angles from the Ta2O5 films with an Arþ ion beam irradiation at the energy of 2 keV and the irradiation time varying from 10 to 40 s. It was found that higher percentage of TaOx is created with increasing


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FIG. 3. Ta4f spectra of Ta2O5 films at a take-off angle of 45 before and after Arþ ion beam irradiation at energy of 1 keV with different time durations of (a) 0 s, (b) 20 s, and (c) 40 s.

irradiation time. Comparing the Ta4f spectra at different take-off angles, the intensity of TaOx is much stronger at smaller take-off angles, which exhibits that TaOx exists in the top surface layer of Ta2O5 films. Furthermore, the curves of take-off angle 10 and 20 coincide each other, which indicate that the compositions at different detecting thickness are the same. Based on the information from Fig. 2, the detection thickness at a take-off angle of 20 is estimated to be 3.5 nm. Thus, the suboxide of TaOx mainly locates from the top surface to the thickness below 3.5 nm at the irradiation energy of 2 keV. Figure 5 investigates the impact of irradiation energy with fixing irradiation time to 60 s. At the energy of 0.5 keV, Arþ ion beam irradiation induced TaOx in the top surface layer of Ta2O5 films. With an increase of the ion beam energy, thicker TaOx layer was generated in the Ta2O5 films. Therefore, the thickness of TaOx can be controlled by the energy and irradiation time of the Arþ ion beam. Since the thickness of Ta2O5 films is around 10 nm,

FIG. 4. Ta4f spectra collected at different take-off angles from the Ta2O5 films with an Arþ ion beam irradiation at the energy of 2 keV and the irradiation time of (a) 10 s, (b) 20 s, and (c) 40 s.

we infer a few nanometers of TaOx are generated in the top layer of Ta2O5 films to form Ta2O5/TaOx heterostructure. A resistive switching device with the Pt/Ta2O5/TaOx/Pt structure as shown in the inset of Fig. 5(d) was fabricated for initial demonstration. A Pt pad with a radius of 500 lm was grown by pulsed laser deposition for the top electrode. The Ta2O5 layer (6.5 nm) with slightly brighter contrast than the TaOx layer (4 nm) was observed in a high-resolution TEM image (the inset of Fig. 5(d)). Bias voltage was applied to the top electrode with the bottom electrode grounded during measurements. For the SET process, a compliance current of 10 mA was used to protect the structure against permanent breakdown. The forming voltage is about 2.8 V.


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FIG. 5. Ta4f spectra collected at different take-off angles from the Ta2O5 films with an Arþ ion beam irradiation at the irradiation time of 60 s and different energy of (a) 0.5 keV, (b) 1 keV, and (c) 2 keV. (d) Typical currentvoltage (I-V) curves of a resistive switching device with the Pt/Ta2O5/ TaOx/Pt structure.

After the forming process, the current-voltage (I-V) curves are shown in Fig. 5(d), which exhibits bipolar resistanceswitching behaviour between the low-resistance (SET) and high-resistance (RESET) states. This bipolar resistanceswitching behaviour is induced by oxygen vacancies and ions through nanoionic transport and a redox process.3,4,8 In summary, tantalum oxide films have been grown by ALD. The composition is Ta2O5 and is uniform for the entire tantalum oxide thin films. A nano suboxide layer can be generated in the top layer of the Ta2O5 films by an Arþ ion beam irradiation. This phenomenon is induced by the exposure of Ta2O5 films to energetic radiation sources resulting in decomposition and the formation of different compounds within the radiated layer. The thickness of TaOx is controlled by the energy and irradiation time of the Arþ ion beam. According to this study, a Ta2O5/TaOx heterostructure can be controllably grown by installing an Arþ ion beam gun in the ALD chamber. This will be valuable for 3D RRAM fabrication. 1

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