Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V2O5

Article 2

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Facile phase control of multi-valent vanadium oxide thin films (VO and VO) by atomic layer deposition and post-deposition annealing 2

Gwang Yeom Song, Chadol Oh, Soumyadeep Sinha, Junwoo Son, and Jaeyeong Heo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Facile phase control of multi-valent vanadium oxide thin films (V2O5 and VO2) by atomic layer deposition and post-deposition annealing Gwang Yeom Song†,∥, Chadol Oh‡,∥, Soumyadeep Sinha†, Junwoo Son‡*, and Jaeyeong Heo†*

†Department of Materials Science and Engineering, and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 61186, Republic of Korea ‡Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 37673, Republic of Korea

∥These

authors contributed equally to this work

KEYWORDS:

atomic

layer

deposition,

vanadium

oxide,

post-deposition

annealing,

electrochromism, insulator-to-metal transition

ACS Paragon Plus Environment

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ABSTRACT: Atomic layer deposition was adopted to deposit VOx thin films using vanadyl triisopropoxide (VO[O(C3H7)]3) and water (H2O) at 135 °C. The self-limiting and purge-timedependent growth behaviors were studied by ex-situ ellipsometry to determine the saturated growth condition for atomic-layer-deposited VOx. The as-deposited films were found to be amorphous. The structural, chemical, and optical properties of the crystalline thin films with controlled phase formation were investigated after post-deposition annealing at various atmospheres and temperatures. Reducing and oxidizing atmospheres enabled the formation of pure VO2 and V2O5 phases, respectively. The possible band structures of the crystalline VO2 and V2O5 thin films were established. Furthermore, an electrochemical response and the voltageinduced insulator-to-metal transition in the vertical metal-vanadium oxides-metal device structure were observed for V2O5 and VO2 films, respectively.


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1. INTRODUCTION Transition metal oxides with partially filled d electron shells have drawn significant attention because of the reversible phase transition between the metal and insulator states, which makes them distinct from conventional insulators or semiconductors. Vanadium, with an electronic configuration of [Ar]3d34s2, forms multivalent oxides with ionic bonding, e.g., VO, V2O3, V3O7, VO2, and V2O5, because of the switchable valence of vanadium ions.1 Among vanadium oxides with different cation/anion ratios, VO2 and V2O5 are the most extensively studied because of their various applications. Vanadium pentoxide (V2O5), a band insulator with a 3d0 electronic configuration, is the most stable compound and is receiving attention for broad application in smart windows and gas sensors because of its electrochemical properties.2-5 On the other hand, vanadium dioxide (VO2), a correlated insulator with a 3d1 electron configuration, has the intriguing feature of a sharp change in resistivity and near-infrared (NIR) transmission.6-7 VO2 can be applied as a selector for resistive random-access memories (RRAMs),8-9 switches and neuromorphic devices in microelectronics,10-11 and a thermochromic smart window.12-14 As vanadium oxides have many applications, it is essential to deposit vanadium oxide thin films with a suitable stoichiometry by controlling the process parameters during deposition or by postdeposition processing. Structural change from V2O5 to V2O3 was demonstrated by controlled reduction reaction in H2 atmosphere by by Rampelberg et al.15 Huang et al. demonstrated the phase evolution from V2O3 to V2O5 by changing oxygen partial pressure during annealing.16 Liu et al. proposed a sandwich-structure (V2O5/metal (V or W)/V2O5) for the control on VO2 polymorphs.17 To date, several studies have reported thin film growth of vanadium oxides with various phases using electrodeposition,18-19 electron beam evaporation,20-21 sputtering,6, 22-23 pulsed laser


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deposition,11, 24 chemical vapor deposition (CVD),25-26 and atomic layer deposition (ALD).3-5, 7, 27-31

ALD, a modified version of CVD, is an excellent tool for the formation of thin films owing

to its advantages over other growth processes. Its ability to achieve self-limiting growth offers precise control of the film thickness down to the angstrom level, perfect conformality over a large area, good repeatability, and step coverage for substrates with three-dimensional nanostructures.32 ALD of vanadium oxide (VOx) thin films using several vanadium metal precursors with water (H2O),3, 7, 27-28, 31, 33 ozone (O3),4, 7 molecular oxygen (O2),3, 34 or oxygen (O2) plasma3, 27, 29 as the oxidant has been reported. Vanadium oxytrichloride (VOCl3) was used as the metal halide precursor for ALD of VOx thin films with H2O as the oxygen source.35 The introduction of metal–organic ALD precursors instead of an inorganic halide salt is always preferable to avoid the production of corrosive byproducts; it also often reduces the deposition temperature.36 Several metal–organic compounds have been used for VOx thin film deposition by ALD with different oxidants, such as bis[2,4-pentanedionato]vanadyl(II) (VO(acac)2) with O2,34 vanadium n-propoxide with CH3COOH,37 tetrakis(dimethylamino)vanadium [V(NMe2)4] with H2O or O3,38 and tetrakis[ethylmethylamino]vanadium [V(NEtMe)4] with H2O7, 28, 30-31 and O37, 39

. Direct growth of amorphous VO2 film by ALD was recently reported by Peter et al. using

V(NEtMe)4 and H2O, and careful annealing was required to crystallize VO2 films.31 VO(thd)2 (thd = 2,2,6,6-tetramethylhepta-3,5-dione) was also used as the vanadium source with O3.40 VOx thin films with different phases have reportedly been obtained by ALD using vanadyl triisopropoxide [VO[O(C3H7)]3, VTIP] as the metal precursor and either H2O, O3, O2, H2O2 + H2O (thermal ALD), or H2O or O2 plasma (plasma-enhanced ALD) as the oxygen source.4-5, 27, 33 Badot et al. presented a detailed study on the electrical properties of V2O5 thin films grown by ALD on a titanium substrate at 105 °C using VTIP and H2O and treated with post-deposition


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annealing at 500 °C in air.41 A comparative study of thermal ALD using the same metal precursor with H2O and O2 plasma for crystalline V2O5 deposition was reported by Musschoot et al.27 The electrochemical behavior of VOx thin films grown by ALD and post-annealed was studied by Badot et al.42 Le Van et al. also showed the electrochemical characteristics of asdeposited amorphous thin films from VTIP with H2O, where the deposited films were applied as a cathode material in Li-ion batteries.43 In recent years, crystalline V2O5 thin films with stable performance were demonstrated as a potential candidate for electrochemical energy storage by Chen et al. using O3-based ALD.4 ALD-grown VOx thin films obtained using a similar VTIP and H2O ALD reaction were also used for switching in a memory device.33 However, direct deposition of VO2 using VTIP as the metal precursor has not been reported because it is difficult to deposit VO2 in the +4 oxidation state from a precursor having the +5 oxidation state such asVTIP.28, 44 Thus, it is possible to grow crystalline VO2 thin films by only post-deposition heat treatment under inert ambient.33 In this work, VOx thin films were grown by ALD using VTIP and water at 135 °C. The selflimiting nature and purge-time-dependent growth behavior were studied by ex-situ ellipsometry. Under saturated condition, the average growth per cycle (GPC) was found to be ~0.03 nm/cycle. The as-deposited films were found to be amorphous according to structural characterizations, with a combination of the V5+ and V4+ states as determined by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses. By adopting post-deposition annealing at different temperatures and ambient conditions (air and a forming gas), we selectively obtained the phasepure VO2 and V2O5 phases. Detailed structural, electrical, and optical analyses again verified their different chemical natures. Finally, an electrochemical response was observed for the V2O5 phase. An abrupt and stable voltage-induced insulator-to-metal transition (MIT) was observed in


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ultrathin ALD-VO2 phase with vertical metal-VO2-metal structures, which will be potentially applicable for 3D cross-point memory integration.45 The advantage of uniform coating of ALDVO2 in complex features can be employed to enhance the thermochromic properties using core/shell 2D photonic crystal, i.e. SiO2/VO2.14 These two results of electrochemical response and MIT support the possibility of phase control of ALD-VOx via post-deposition annealing.

2. EXPERIMENTAL SECTION Vanadium oxide (VOx) thin films were deposited in a laminar-flow-type thermal ALD reactor (CN1, Atomic Classic, Korea) at 135 °C on Si and glass substrates. A continuous flow of N2 was used as the purge and carrier gas throughout the deposition process. VTIP (V5+) (EG Chemical, Korea) and deionized water were the ALD precursors acting as V and O sources, respectively. The metal precursor was kept at 40 °C during deposition. The precursor doses and purging sequence can be represented as VTIP pulse (t1)-purge (t2)-H2O pulse (t3)-purge (t4) in one ALD cycle, where t1 and t3 are the dose times, and t2 and t4 are the purge times. The thicknesses of the as-deposited films were obtained by ellipsometry measurement (J.A. Woollam, ESM 300ellipsometer). To study selective formation of the crystalline phase of the VOx thin films, as-deposited films were annealed at 300, 400, and 500 °C for 1 h in air or a forming gas atmosphere (FGA, 95% N2 + 5% H2, 40 sccm) in a box-type furnace (Han Tech, CA14P). The crystallinity of the as-deposited and annealed films was examined by XRD (PANalytical, X'Pert Pro MPD) analysis, which was supplemented by cross-sectional highresolution transmittance electron microscopy (HR-TEM, JEOL, JEM-2100F). Scanning electron microscopy (SEM, Hitachi, S-4800) and atomic force microscopy (AFM, PSIA, XE150) were used to investigate the surface morphology of the as-deposited and annealed films. XPS (Thermo


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Fisher Scientific, ESCALAB 250, U.K.) measurements were performed to determine the chemical nature of the VOx thin films before and after annealing. The work function of the annealed materials was estimated from valence-band ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Scientific, ESCALAB 250, U.K.) using the He I (21.22 eV) line. The optical properties were studied using a UV–vis–NIR spectrophotometer (Cary5000, Varian), and the bandgaps of the annealed films were calculated from the spectra. To verify the selective formation of the phase-pure V2O5 and VO2 phases for possible use in applications, the electrochemical and voltage-induced insulator-to-metal transition properties were investigated. To explore the electrochemical properties of the V2O5 thin films, a 30-nmthick V2O5 thin film was obtained on an indium tin oxide (ITO)-coated glass substrate and used as the working electrode in cyclic voltammetry (CV) measurements (PGSTAT204, Autolab). Pt and Ag/AgCl were used as the counter and reference electrodes, respectively, and a 1.0 M LiCl solution was used as the electrolyte. A CV profile was observed by measuring the current– voltage response at a 50 mV/s scan rate within the voltage range of −0.2 to +0.8 V vs. Ag/AgCl with respect to only the ITO substrate. To characterize the voltage-induced insulator-to-metal transition, a 10-nm-thick VOx thin film was deposited using ALD on a nanopatterned substrate (Pt/TiN/SiO2/Si) with a 200-nm-diameter device size. The experimental procedure for nanoholepatterned 3D substrates, which provide uniform control of the current–voltage (I–V) characteristics of selectors by reducing the current flow area, is described in detail elsewhere.46-47 Then, the films were annealed in air or the FGA at 500 °C to adjust the valence of the vanadium cations, followed by deposition of Pt top electrodes through a shadow mask. The I–V characteristics of the air-annealed V2O5 and FGA-annealed VO2 thin films were measured using a semiconductor parameter analyzer (AgilentB1500). The measurements were performed in air


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ambient at room temperature by sweeping the voltage from 0 to 5 V and then back to 0 V in 0.025 V steps. The current compliance was set to 10 mA to protect the sample during the measurements.

3. RESULTS AND DISCUSSION 3.1 ALD Growth of VOx Thin Film The average GPC of the ALD-grown VOx thin films was monitored by ellipsometry. The selflimiting growth behavior of the ALD-grown VOx was studied at 135 °C using a variable precursor dose time. Figure 1(a) and (b) show the GPC of the VOx with increasing precursor dose time for VTIP or H2O while the dose time of the other precursor was kept constant. Thus, the sequential precursor doses can be represented as t1-30s-3s-30s and 2s-30s-t3-30s in Figure 1(a) and (b), respectively, where t1 and t3 are the variable dose times of VTIP and H2O, respectively. The purge time was fixed at 30s throughout deposition. Figure 1(a) clearly shows a linear initial increase in the GPC as the dose time of the VTIP pulse increases up to 1.8s, beyond which it becomes saturated at a GPC of ~0.02 nm/cycle. On the other hand, a much larger change in the GPC is found with increasing dose time of the H2O pulse, where the saturated GPC is reached at dose times greater than~5s, as shown in Figure 1(b). Here a saturated GPC of ~0.03 nm/cycle was obtained. No significant change in the GPC was observed beyond this limiting value even when the dose time was increased further owing to complete coverage of available surface species on the substrate surface. Therefore, the GPC exhibited the self-saturation characteristics of ALD. Further, the saturation was found to be slower at a higher precursor dose time for the reaction with H2O. This suggests that the oxidant determines the rate of VOx ALD, which has also been observed in other H2O-based ALD processes.27, 42


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To obtain the optimized ALD reaction condition, another important parameter is the purge time after each precursor dose. An insufficient purge time results in a significantly high GPC. Figure 1(c) and (d) show the variation in the GPC as a function of purge time after VTIP and H2O pulses, respectively, at 135 °C. In this study, the VTIP and H2O pulse times were fixed at 2 and 5s, respectively, as determined from the saturated growth condition. In both cases, the GPC first decreased slightly with increasing purge time, and then saturation occurred. Thus, the figures show a saturated GPC of ~0.028–0.029 nm/cycle for 15s (t2) and 20s (t4), which are similar to the GPC values measured during the pulse time study. Hereafter, all the VOx thin films for other characterizations were deposited using a t1-t2-t3-t4 sequence of 2s-15s-5s-20s at 135 °C.

(a) 0.03 0.02 0.01

saturation: 2 s 0.00

0

0.04

t1-30-3-30

1

2

3

4

GPC (nm/cycle)

GPC (nm/cycle)

0.04

0.03 0.02 0.01 0.00

5

0.04

0.02

saturation: 15 s 0.01 0

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20

25

30

VTIP purge time (s,t2)

0 1 2 3 4 5 6 7 8 9 10

0.04

GPC (nm/cycle)

0.03

saturation: 5 s

H2O pulse time (s,t3)

2-t2-5-20

(c)

2-30-t3-30

(b)

VTIP pulse time (s,t1) GPC (nm/cycle)

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2-30-5-t4

(d) 0.03

0.02

saturation: 20 s 0.01 0

5

10

15

20

25

30

H2O purge time (s,t4)

Figure 1. Average GPC as a function of pulse time of (a) VTIP and (b) H2O and as function of purge time after (c) VTIP and (d) H2O for ALD of VOx at 135 °C. Lines are guiding to the eye. 3.2 Structural Characterizations and Effect of Post-Deposition Annealing


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The crystallinity of the as-grown VOx thin films and the effect of annealing at different atmospheres and temperatures on the crystal structure were investigated using the XRD patterns, as shown in Figure 2(a) and (b). For this study, 25-nm-thick films were deposited on Si substrates. The as-deposited VOx films were amorphous in nature; thus, no signature peak appears except for the Si substrate peak.

300 ℃ As-dep. #00-041-1426 (o-V2O5) #00-054-0513 (m-V2O5)

10

20

30

40

50 o

60

70

Si

Si

VO 2 (003)

Si

VO 2 (002)

VO 2 (001)

400 ℃

Intensity (a.u.)

Si

Si

V2O 5 (002)

Si

V2O 5 (001)

500 ℃

V3O7 (600)

(b)

(a)

Intensity (a.u.)

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500 ℃ 400 ℃ 300 ℃ As-dep. #01-081-2392 (m-VO2) #01-071-0454 (m-V3O7)

10

20

2 Theta ( )

30

40

50 o

2 Theta ( )

60

70

Figure 2. XRD patterns of VOx thin films as-deposited and after post-deposition annealing at 300, 400, and 500 °C for 1 h in (a) air and (b) the FGA.


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Figure 3. Surface SEM images of films annealed in air at (a) 300 °C, (b) 400 °C, and (c) 500 °C and annealed in the FGA at (d) 300 °C, (e) 400 °C, and (f) 500 °C. Inset shows as-deposited VOx film. In contrast, the VOx films heat-treated under air and the FGA show clear crystalline characteristics. Figure 2(a) shows the crystallinity of VOx thin films annealed at 300, 400, and 500 °C in air for 1 h. The amorphous VOx thin film becomes crystalline after annealing at 300 °C, where the peaks for the (001) and (002) planes correspond to orthorhombic V2O5, which is consistent with the JCPDS file (JCPDS card no.00-041-1426). The film also shows a strong caxis-preferred orientation along the [001] direction with increasing annealing temperature from 400 to 500 °C, which was also observed in previous reports.7, 26 Similarly, the amorphous as-deposited VOx thin films annealed in the FGA showed a spontaneous change in the crystal structure to polycrystalline monoclinic VO2 (JCPDS card no. 01-081-2392) at 500 °C via monoclinic V3O7 (JCPDS card no. 01-071-0454) at 400 °C, as shown in Figure 2(b). The film annealed at 300 °C remained amorphous. Thus, these observations can be related to the results obtained by Premkumaret al.39 on the crystallization of phases with higher and lower valence during annealing under oxygen-rich and relatively oxygenpoor ambient conditions, respectively. This controlled phase formation of VOx thin films depending on the annealing atmosphere is advantageous, as phase formation does not depend much on the deposition parameters. Therefore, it is possible to tune the required phases of VOx thin films deposited at low temperature by controlling the post-annealing process. Figure 3 shows surface SEM images of the annealed VOx thin films, where the inset of Figure 3(a) shows an image of an as-deposited film. The image shows uniform film deposition on the entire substrate surface, which ensures pinhole-free film formation by ALD. The SEM images


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also confirm the distinctive effect of annealing under air and the FGA observed in the XRD patterns in Figure 2. Figure 3(a), (b), and (c) show the change in the surface morphology with increasing crystallization of the V2O5 films at different annealing temperatures in air. On the other hand, the VOx thin film also appears amorphous after annealing at 300 °C in the FGA [Figure 3(d)], finally crystallizing at 400 and 500 °C, as shown in Figure 3(e) and (f), respectively. Thus, these SEM images also verified the XRD results obtained earlier. Note, however, that larger, elongated grains appear in the film annealed at 500°C under the FGA [Figure 3(f)], and this observation is inconsistent with the XRD data in Figure 2(b). These results suggest that precise temperature control is needed, especially to form the VO2 phase.

Figure 4. Surface morphologies of films annealed at (a) 300 °C, (b) 400 °C, and (c) 500 °C in air and at (d) 300 °C, (e) 400 °C, and (f) 500 °C in the FGA and (g) as-deposited VOx film. (h) Variation in root-mean-square surface roughness (Rq) with annealing temperature in each atmosphere. The surface roughness of the as-grown and annealed VOx thin films on Si substrates was investigated using AFM. The surface morphology of all the films is shown in Figure 4(a)–(g).


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The as-deposited film [Figure 4(g)] and the film annealed at 300 °C in the FGA [Figure 4(d)] have similar morphologies; both are amorphous, with a very low root-mean-square surface roughness (Rq) of

Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V2O5

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