Fabrication and Characterization of Metastable Anatase Tantalum Oxynitride (Taon) Thin Films by Sputter Deposition

Fabrication and Characterization of Metastable Anatase Tantalum Oxynitride (TaON) Thin Films by Sputter Deposition

Gil Hwan Lim*,1, Yannan Wang2, Takuto Wakasugi2, Yasushi Hirose2,3, Tetsuya Hasegawa2,3

1Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States 2Department of Chemistry, The University of Tokyo, Tokyo 113-0033, Japan 3Kanagawa Academy of Science and Technology, Kawasaki 213-0012, Japan

ABSTRACT: We report on the attempt to synthesize metastable anatase tantalum oxynitride (TaON) in thin film form on lattice-matched LSAT and LAO single crystals by using sputter deposition. Sputter deposition is known to be a better method for large-scale fabrication in industrial purposes as opposed to pulsed laser deposition, which is used in conventional thin film synthesis of anatase TaON. Several different experimental conditions were used to determine optimal growth temperature and supply of oxygen and nitrogen gases for obtaining high-quality anatase TaON thin films. Two-dimensional X-ray diffraction (2D-XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy analysis (SEM-EDS) measurements were used to determine the crystallinity and chemical composition of the deposited thin films. A growth temperature of 650ºC (instead of 700ºC) yielded thinner films and better chemical composition, but varying oxygen and nitrogen flow rates yielded mixed results. Epitaxial single crystalline growth was only partially successful, as seven out of nine samples were amorphous while the other two samples showed hints of crystallinity. After annealing the samples at higher temperature (750ºC), however, better crystallinity was obtained in samples grown at 0.4 sccm O2 flow.

KEYWORDS: transparent conductor, tantalum oxynitride, anatase, thin film, sputter deposition

9-11 I. INTRODUCTION In 2005, anatase Nb-doped TiO2 (TNO) was developed as an alternative for ITOs. TNOs For the past decade, transparent conductors1 exhibit electron mass anisotropy12, indicative of have drawn considerable interest in many the material’s anisotropic crystal structure and industries due to their novel properties. These spatially-localized d-electron based conduction materials are in high demand because optical band (not s-electron based). These are unique transparency and electrical conductivity are features of the TNO compared to other two qualities crucial to a wide range of conventional TCOs that lead to novel properties applications, such as flat-panel displays such as high Hall mobility, wide controllability (FPDs)2, light-emitting diodes (LEDs)3, and of carrier density by doping13, and high solar cells4. However, high optical transparency refractive index. Such properties are the reason and low electrical resistivity are often difficult why TNOs are suitable electrodes in LEDs – to achieve due to the challenge of finding a the high refractive index will allow for material that has a bandgap (Eg) wide enough refractive index matching, which suppresses for visible light to transmit without absorbing5, reflection and hence increases efficiency. but narrow enough for valence electrons to Therefore, TNOs have many applications in freely jump to the conduction band. Currently, transparent conductors, ferro-magnetic Sn-doped In2O3 (ITO) is one of the most semiconductors, thin film transistors, and commonly used transparent conducting oxides resistive random-access memories. (TCO)6-8, but indium is rare and expensive, which is why there is a need to look for The success of TNO as a widely applicable substitutes. TCO incentivizes the search for other transparent conducting materials with further novel properties. One promising candidate is

1 anatase TaON14 because its crystal structure II. EXPERIMENTAL SECTION and cation electron configuration (d0) are 15 identical to those of anatase TiO2 . This Anatase TaON thin films were grown by sputter material has already been extensively studied as deposition on four different substrates: 16 17,18 nontoxic pigments or photocatalysts due (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) (100), to their Eg in the visible light region and their LaAlO3 (LAO) (110), silicon (Si) (111), and band alignment, making them suitable for glass. LSAT and LAO, which are lattice- photocatalytic water splitting. Furthermore, matched perovskite single crystalline substrates, previous studies have revealed that oxynitrides were used with the intention to synthesize exhibit novel electrical functionalities such as epitaxial single crystalline films. Si and glass high electron mobility19,20, carrier type were used to verify that the deposition tunability and colossal magnetoresistance21, technique can successfully synthesize suggesting that anatase TaON is another very polycrystalline films, which would be useful promising candidate. for large area deposition in future industrial purposes. Deposition was conducted nine The problem with anatase TaON is that it is different times, each with slightly different difficult to fabricate. Prior to 2014, TaON has experimental conditions, for a total of 36 only been synthesized as polycrystalline fine samples. The deposition target was TaN. The powder form via nitridation of oxides by substrate temperature (TS), serving as one of the ammonolysis22,23. With this method, however, main growth parameters, was varied to be 650 acceptor impurities were doped to stabilize the or 700°C. The other main growth parameter anatase polymorph, which hindered the was flow rate of N2 and O2 gases – N2 gas flow investigation of electrical properties. In 2014, was adjusted between 8 and 10 sccm, while O2 Suzuki et al. successfully synthesized gas flow rate was adjusted between 0.4 and 0.5 metastable anatase TaON via epitaxial growth sccm. The total gas flow rate of Ar, N2, and O2 on a lattice-matched single crystalline substrate was kept constant at 20 sccm, so the Ar flow using pulsed laser deposition (PLD)24,25. Single rate was tuned accordingly based on the other crystals were obtained using this method, two gas flow rates. Deposition was always enabling evaluation of transport properties. conducted under total gas pressure of 0.75 Pa Specifically, good electrical conductivity and (with experimental uncertainty of ~0.03 Pa) for temperature-independent n-type carrier density 1 hour. The Ar gas was activated into radicals were observed, characteristic of degenerated by a radio frequency wave (RF) plasma source semiconductors. with an output power of 100 W (with experimental uncertainty of ~5 W). These In this paper, we report on the synthesis of parameters were adjusted to control the anatase TaON thin films by sputter deposition, deposition rate to 60~80 nm/h. Typical film an alternative technique to PLD. PLD is useful thicknesses were 95−115 nm, evaluated by a for small scale fabrication of high quality, Dektak 6M stylus profiler (with experimental epitaxial, single crystalline thin films on lattice- uncertainty of ∼5 nm). Some of the films were matched single crystal substrates, so it is successively annealed at 740°C for 10 minutes generally used for research purposes, such as under base pressure in the main chamber. the investigation of intrinsic properties26. Meanwhile, sputter deposition is optimal for In-plane and out-of-plane crystal structures of large scale fabrication27 of polycrystalline the films were examined by X-ray diffraction films28-31 on cheap commercial substrates such (XRD) measurements using Cu Kα radiation as glass28,32, making it more applicable for and a four-axis diffractometer with a two- practical, industrial purposes. We further report dimensional area detector (Bruker AXS, d8 on the characterization of the crystallinities and discover with GADDS). The topographies and chemical compositions of these thin films, chemical compositions were evaluated by a comparing them with previous results from scanning electron microscope with energy Suzuki et al24. dispersive X-ray spectroscopy analysis (SEM- EDS), within experimental errors of ∼10%. The electron accelerating voltage was set at 3 kV in order to reduce the background signal from the substrate. The oxygen and nitrogen

2 content data from the EDS machine was cross- identical measurement frames on two different checked by using the elastic recoil detection samples: TaON with polycrystalline rings (blue) and analysis (ERDA) data of Suzuki et al.’s TaON without polycrystalline rings (red). The three sample24. small peaks near the substrate peak on the blue spectrum indicates polycrystallinity.

III. RESULTS AND DISCUSSION For one of nine samples, a bright spot was

observed at 2θ = 51.5º in-plane, previously Crystallinity of TaON films 2D-XRD was unseen in any of the other samples measured in used to observe the in-plane and out-of-plane the same frame. This spot may indicate crystal structures of fabricated TaON films. formation of monocrystalline films. The frame The films deposited on LSAT substrate were and spectra, along with the sample’s growth used for these measurements, which means that conditions, are shown in Figure 2 below. ideally monocrystalline peaks should be observed. The measurement sequence included six frames; each frame had a unique (a) combination of 2-theta, omega, phi, and chi values. Two frames measured out-of-plane (chi = 90º) crystallinity, while the other four frames measured in-plane (chi = 45º) crystallinity.

For seven of nine samples, no peaks were observed for any of the six frames measured, indicating that the structure was amorphous.

For one of nine samples, several faint rings (b) were observed in both in-plane and out-of- plane measurements. Such rings are usually a sign of polycrystallinity. The spectra verified that these rings were from the TaON and not from the LSAT. The frame and spectra, along with the sample’s growth conditions, are shown in Figure 1 below.

(a) Figure 2. (a) In-plane 2D-XRD frame of TaON sample grown on LSAT. Growth conditions were Ts = 650ºC, O2 = 0.4 sccm, and N2 = 10 sccm. The bright (but fainter than the one on the left) spot on the right is what may be a monocrystalline peak. (b) θ–2θ XRD patterns of identical measurement frames on two different samples: TaON with extra bright spot (blue) and TaON without the bright spot (red). The peak at 2θ = 51.5º (not the substrate peaks at 2θ (b) = 69º) on the blue spectrum may be a monocrystalline peak.

It is worth noting that the two films shown in Figures 1 and 2 – the ones that showed any sort of peaks – were grown at O2 = 0.4 sccm. This hinted at the possibility of improving crystallinity via post-annealing. Therefore, the Figure 1. (a) Out-of-plane (left) and in-plane (right) films grown at O2 = 0.4 sccm were successively 2D-XRD frames of TaON sample grown on LSAT. annealed at 740ºC for 10 minutes under base Growth conditions were T = 650ºC, O = 0.4 sccm, s 2 pressure in the sputtering main chamber. The and N2 = 8 sccm. Two rings on the out-of-plane and three rings on the in-plane frames indicate formation XRD spectra of two of these samples are shown of polycrystalline films. (b) θ–2θ XRD patterns of in Figure 3 below.

3 data gives more accurate values. This was a workaround for directly using ERDA to make measurements on the fabricated samples due to time constraints. Figure 5 below outlines the chemical composition results.

(a) O rate dependence (N : 8 sccm, 650ºC) Figure 3. θ–2θ XRD patterns of identical 2 2 measurement frames on the same sample, before 2 annealing (red) and after annealing (blue). Spectra 1.5 on the left is of the sample grown at Ts = 650ºC, O2 1 = 0.4 sccm, and N2 = 8 sccm, while spectra on the 0.5 right is of the sample grown at T = 650ºC, O = 0.4 0

s 2 CompositionalRatio 0.35 0.4 0.45 0.5 0.55 sccm, and N = 8 sccm. It is observed that for both 2 O2 rate (sccm) cases, there are new peaks that appear after annealing the samples, indicating an improvement O:Ta N:Ta in crystallinity.

(b) N2 rate dependence (650ºC) Chemical Composition of TaON films SEM-

EDS was used to observe the topographies and 1.8 calculate the chemical compositions of 1.4 fabricated TaON films. The films deposited on 1 0.6

Si substrate were used for these measurements. 0.2 The SEM images of all nine samples are shown CompositionalRatio 7.5 8 8.5 9 9.5 10 10.5 N rate (sccm) in Figure 4 below. 2

O:Ta N:Ta

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2

CompositionalRatio 640 650 660 670 680 690 700 710 Substrate Temperature (ºC)

O:Ta N:Ta

Figure 5. ERDA-calibrated EDS results of TaON chemical composition, O:Ta (blue) and N:Ta (orange). (a) O2 flow rate dependence on samples grown at N2 = 8 sccm and Ts = 650ºC. (b) N2 flow rate dependence on samples grown at Ts = 650ºC. (c) Substrate temperature Ts dependence on samples grown on Si substrate. Figure 4. SEM images of all nine samples with their growth conditions labelled below. It is clear from the The ideal chemical composition for anatase images that samples grown at O2 = 0.4 sccm are TaON is 1:1 O:Ta and 1:1 N:Ta. Thus, the smoother than those grown at O2 = 0.5 sccm. The closer to 1 (bolded in black) the compositional influence of temperature is difficult to determine. ratios are, the more ideal the TaON growth is.

Figure 5a represents the O flow rate The EDS results gave nominal values for the 2 dependence. Since both the blue and orange relative ratio of Ta, O, and N in each of the dots are closer to 1 for O = 0.5 sccm, it is clear samples. However, the EDS data was not 2 that O = 0.5 sccm yields both better O:Ta and properly calibrated, so the reference sample 2 N:Ta compared to O = 0.4 sccm. Figure 5b from Suzuki et al.24 was used to calibrate the 2 represents the N flow rate dependence. In this EDS data with ERDA data, since the ERDA 2 case, the blue dots are closer to 1 at N2 = 8 sccm,

4 but the orange dots are closer to 1 at N2 = 10 I am a third-year undergraduate student at sccm. In other words, 10 sccm N2 yields better Northwestern University, majoring in physics N:Ta ratio, but 8 sccm N2 yields better O:Ta and minoring in materials science. My research ratio, so there is a tradeoff here. Figure 5c is focused on understanding the fundamental represents the temperature dependence, and it is quantum interactions and collective behavior quite clear that the composition is more ideal at between photons and low-dimensional Ts = 650ºC instead of Ts = 700ºC. materials through exploring novel optical, spin, and magnetic properties of integrated nanoscale IV. CONCLUSION and hybrid photonic systems.

Anatase TaON thin films were synthesized on VI. ACKNOWLEDGEMENTS lattice-matched single crystalline substrates by using sputter deposition. The effects of I would like to express my fullest gratitude substrate temperature and gas flow rates on film towards Ms. Yannan Wang, Mr. Takuto thickness and chemical composition were Wakasugi, Prof. Yasushi Hirose, and Prof. identified. In terms of substrate temperature, Tetsuya Hasegawa from the Solid State lower temperature (650 instead of 700ºC) Chemistry Group of the University of Tokyo yielded both thinner films and better chemical for their unconditional support and guidance in composition. For flow rates, 0.5 sccm O2 conducting this project. I would also like to yielded better overall chemical composition, thank the other members of the group for but 0.4 sccm O2 yielded thinner films, while 10 welcoming me to their lab for six weeks. sccm N2 yielded better N:Ta ratio, but 8 sccm Special thanks is reserved for Mr. Gou Jin and N2 yielded better O:Ta. Epitaxial single Mr. Zhen Chen from the group for their crystalline growth on lattice-matched substrate assistance in 2D-XRD, SEM-EDX, and Dektak was partially successful. Out of nine samples, measurements. This study was supported by the seven samples were amorphous with no XRD Graduate School of Science Scholarship for peaks. One sample may be epitaxially single UTRIP-Related Expenses (GSS-UTRIP) as crystalline, and one other sample was part of the University of Tokyo Research polycrystalline. After annealing at 740ºC, better Internship Program (UTRIP) hosted by the crystallinity was obtained in samples grown at International Liaison Office (ILO) of UTokyo. 0.4 sccm O2. Therefore, an optimal growth temperature of 650ºC and a balanced supply of VII. REFERENCES oxygen and nitrogen are crucial for obtaining high-quality anatase TaON thin films. 1. D. S. Ginley and C. Bright, MRS Bull. 25, 15 (2000). As next steps for this research, further 2. E. Fortunato, D. Ginley, H. Hosono, and D. investigation is needed to determine optimal C. Paine, MRS Bull. 32, 242 (2007). conditions for ideal crystallinity and chemical 3. E. Fortunato, L. Raniero, L. Silva, A. composition. To improve crystallinity, a seed Gonçalves, A. Pimentel, P. Barquinha, H. layer with similar crystal structure32 (i.e. Águas, L. Pereira, G. Gonçalves, I. Ferreira, anatase TiO2) can be used on top of the E. Elangovan, and R. Martins, Sol. Energy perovskite LSAT layer. Finer adjustments of Mater. Sol. Cells 92, 1605 (2008). substrate temperature and gas flow rates will 4. H. L. Hartnagel, A. L. Dawar, A. K. Jain, improve chemical composition. After and C. Jagadish, Semiconducting determining these conditions, electrical Transparent Thin Films (Institute of transport properties and optical properties can Physics, Bristol, UK, 1995). be investigated using Hall measurements and 5. T. Hitosugi, H. Kamisaka, K. Yamashita, H. spectroscopic ellipsometry, respectively. Nogawa, Y. Furubayashi, S. Nakao, N. Yamada, A. Chikamatsu, H. Kumigashira, V. AUTHOR INFORMATION M. Oshima, Y. Hirose, T. Shimada, and T. Hasegawa, Appl. Phys. Express 1, 1112031 Corresponding Author (2008). *E-mail: [email protected] 6. C. A. Pan and T. P. Ma, Appl. Phys. Lett. Notes 37, 163 (1980).

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