Characterization of V₂O₃ Nanoscale Thin Films Prepared by DC Magnetron Sputtering Technique

Abstract

Vanadium sesquioxide V₂O₃, a transition metal oxide, is an important metal transition insulator due to its potential applications in novel electronic and memory devices. V₂O₃ thin films of thickness around 230 nm were grown on Si/SiO₂/Ti/Pt substrates at deposition temperature of 723 K in a controlled Ar:O₂ atmosphere of 35:2.5 sccm employing Direct Current (DC) magnetron sputtering. X-ray diffraction studies confirmed single phase of the material stabilized in corundum rhombohedral R$\overline{3}$C phase. X-ray photoelectron spectroscopic results revealed chemical oxidation states are of V³⁺ and O²⁻ and have nearly stochiometric elemental compositions in the films. Magnetization studies down to 10 K predicts a canted antiferromagnetic transition around 55 K. Out of 7 expected Raman active modes (2A_1g + 5Eg), two A_1g Raman active modes at 242 and 500 cm⁻¹ were observed at ambient R$\overline{3}$C phase. Temperature dependent Raman spectroscopic studies carried out from 80 to 300 K identified a monoclinic to rhombohedral phase transition at ~143 K.

1. Introduction

Studies on vanadium oxide, an important class of transition metal oxide materials, have received a special attention due to its involvement in several technological applications: catalysis compounds [1,2], lithium battery high-density cathode materials [3], chemical sensors [4], and several electronic and optical devices [5]. Mixed valence vanadium oxides are the subject of current research due to their Mott insulating behavior. Studies on vanadium sesquioxide V₂O₃ has been given special attention due to occurrence of magnetic and metal transition insulator (MTI)transition at the same temperature [6]. The interfacial electronic structure of the V₂O₃ nanoscale thin film heterostructure constitutes a fascinating area of research. The V₂O₃ crystal is considered a prototype in the physics of the Mott-Hubbard transition and the metal-insulator transition within the paramagnetic phase. The complex interactions of its various electronic, magnetic, phonon, and structural aspects, on the other hand, have posed significant challenges to their understanding over the years [6,7]. The Mott insulator phase is defined as an insulating state induced by electron correlations [8]. That phase can often be suppressed by quantum tuning of nonthermal parameters such as chemical composition or pressure, resulting in a zero-temperature quantum phase transition to a metallic state driven by quantum fluctuation [8,9]. At room temperature, V₂O₃ crystallizes in a rhombohedral corundum-type R$\overline{3}$C structure with a c/a ratio of 2.827 in a hexagonal unit cell setting and that has a trigonal primitive cell with two formula units (Z = 2) [10]. It shows paramagnetic and metallic behaviors at room temperature. Upon lowering temperature, a first-order structural phase transition from the room temperature rhombohedral phase (Sp. Gr. R$\overline{3}$C) to a monoclinic polymorph M₁ phase (Sp. Gr. B₂/_b) occurs at around 170 K [6]. This low temperature phase exhibits antiferromagnetic behavior and turns into an electric insulator [11] with a change in resistivity of seven orders of magnitude across the MTI. Above the critical temperature T_c, the metallic nature of the material results from the overlapping of an a_1g orbital along the c axis and an e_g(π) orbital on the basal plane of the corundum structure, which originated from the splitting of the t_2g orbital [12]. It can be mentioned that cation-anion and cation-cation interactions (in the basal plane and along the hexagonal c axis) provide a trigonal component to the octahedral crystal field which splits the t_2g orbital. On the other hand, in the low temperature insulating state, the a_1g band is completely filled while the e_g(π) band is entirely empty with some energy separation between these two bands [12]. At around 50 K, V₂O₃ powder reported a strong magnetic field dependence of the magnetic transition temperature, which indicated a canted antiferromagnetic transition and/or spin-glass-like behavior [11]. The MI and structural phase transition in V₂O₃ can be driven by temperature, pressure, light, or electric current [13]. Often, thin film materials [14] show distinct physical properties compared to their corresponding bulk counterparts due to their geometry reduction and have attracted a great deal of attention. High-quality thin films are known to have compact and sized miniaturization device applications. In V₂O₃ thin films, phase transition and resistivity behaviors are found to be related to the morphology of the films [15]. At room temperature, these films have shown large resistivities of the order of ~3.18 × 10⁻⁴ Ω cm [15], resulting from the difference in thermal expansion coefficients of the film and substrate [16,17]. Reports on phase transition behavior in V₂O₃ thin films are rare, and only a few studies on the thin films have been reported [15,18]. Phonon anomalies as a function of temperature are often useful to identify crystallographic phase transitions. Recently, sol-gel based V₂O₃ thin films with a thickness of around 90 nm showed a complex phase behavior from 135–165 K due to stress involved in the films. A large phase coexistence of 30 K and hysteresis of 10 K were reported and attributed to the effect of the film’s stress and defects [18]. Furthermore, the transition temperature Tc is influenced by the oxygen defects in the films [17,18], which increases monotonically when the oxygen pressure is systematically varied (oxygen defect variation) [19,20]. In other words, stress, oxygen defects, and temperature influence phase transition behavior in V₂O₃ thin films, which is still unknown. In this regard, V₂O₃ thin films of other thicknesses prepared by different deposition techniques with controlled stress and defects are expected to provide greater insight into the phase transition behavior. In this work, we have reported the growth of V₂O₃ thin films with a thickness of roughly 230 nm utilizing DC magnetron sputtering and their characterization results involving Raman spectroscopic and magnetization studies. The temperature dependence of phonons and magnetization curves has been examined to obtain information about the crystallographic and magnetic phase transition.

2. Experimental Details

Thin films of vanadium sesquioxide V₂O₃ were deposited on 10 mm × 10 mm Si/SiO₂/Ti/Pt substrates using DC magnetron sputtering. The substrates were ultrasonically cleaned in acetone for 10 min, followed by isopropanol cleaning reagents for another 10 min to remove any organic impurities from the substrate surface. A vanadium metal plate (purity 99.5%) was used as a target. The deposition conditions were optimized. Those are: background pressure of 1.7 × 10⁻⁵ Torr, deposition pressure of 20 mTorr, distance between target and substrate of 150 mm, DC power of 300 Watts, deposition temperature of 723 K, and the processing gases of Ar:O₂ were 35:2.5 sccm. The material was deposited for 10 min in an oxygen atmosphere and slowly cooled down to room temperature at the rate of 1 K/min in vacuum after deposition. The schematic diagram for growth of V₂O₃ thin film is shown below (Scheme 1). The diffraction pattern was recorded using a Rigaku Smartlab X-ray diffractometer (Rigaku, Tokyo, Japan) in the Bragg-Brentano (Ɵ−2Ɵ) geometry at an operating voltage of 40 kV and a current of 40 mA using a Cu-Kα radiation (λ = 1.5405 Å) source. The thickness of the thin films was obtained by employing an Ambios Tech XP-200 profilometer (Ambios Technology, Santa Cruz, CA, USA). The surface topography of the thin films was studied using a Veeco Nanoscope atomic force microscope (AFM, Nanoscope, Veeco, Camarillo CA, USA). The compositional elemental mapping of the constituent elements was carried out using a scanning electron microscope (SEM) (model: JEOL JSM-6480LV, JEOL, Austin, TX, USA) based energy dispersive X-ray spectra analysis (EDS) (model: SD Dry 30, JEOL, Austin, TX, USA) recorded at a magnification of 4000× operated with an accelerating voltage of 20 kV. The number of frames of 64, and dwell time of 200 µs, and a matrix resolution of 256 × 200 were used for the EDS measurements and mapping. The composition and valence states of the chemical elements were studied using a using a Physical Electronics PHI 5600 ESCA high-resolution X-ray photoemission spectrometer system (Perkin-Elmer Corporation, Waltham, MA, USA) using a monochromatic Al Kα X-ray radiation source equipped with a single Mg anode operated at 400.5 Watts with an operating anode voltage of 15 kV and current of 26.7 mA. The pass energy of 58.7 eV for survey, and pass energy of 23.5 eV for high-resolution spectra were used in X-ray photoelectron spectroscopy (XPS) experiment. The XPS spectra were analyzed using Voigt function employing Multipak software (v 9.0). Temperature-dependent Raman spectroscopic studies were carried out using a HORIBA Jobin Yvon micro-Raman spectrometer (model: T64000, HORIBA, HORIBA, Piscataway, NJ, USA) equipped with a 50× long-working distance objective. Raman spectra were recorded in the back-scattering geometry (180°) using the 514.5 nm radiation of a coherent Ar⁺ ion Laser (Innova 70-C) in the temperature range from 82 to 300 K using a Linkam temperature controller, ensuring a temperature stability of ±1 K. The laser power density on the film was less than 1 mWatt to avoid any local heating effect. The spectra were analyzed using the Lorentzian function employing Jandel peakfit software (v 4.12) to obtain their Raman band frequencies, line widths, and intensities. Temperature-dependent magnetic ordering in the thin films was studied using a Physical Property Measurement System (PPMS) (Quantum design DynaCool) operated in the VSM module from 10–300 K. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were carried out at several applied field (H) from 100–1000 Oe in the same temperature range.

3. Results and Discussion

The single-phase formation of the as grown thin films was examined by X-ray diffraction (XRD) measurement.

Figure 1 shows the XRD pattern of the thin films measured in the 2θ range from 20–65°. The diffraction peaks were found to match with the V₂O₃ rhombohedral R$\overline{3}$C structure (JCPDF # 340187) along with the reflection peaks of the Pt substrate located at 39.88° and 46.41°. The main X-ray reflection peaks from the compound were observed at 2θ positions of 24.44°, 33.34°, 36.43°, 41.32°, 50.09°, 54.21°, and 58.72°, those were corresponded to the lattice planes (012), (104), (110), (113), (024), (116) and (122), respectively, of the R$\overline{3}$C phase, indicating the formation of the polycrystalline V₂O₃ thin film. In addition, the absence of any impurity peak in the diffraction pattern, suggesting the formation of a phase pure V₂O₃ thin film. The lattice mismatch between the grown thin film and the Pt-substrate may introduce a lattice strain (η) on the thin film. The strain can be calculated using the relation [21,22], η = (a_bulk−a_film/a_bulk) × 100. The XRD patterns from the substrate (2θ = 39.825°) and the film (2θ = 39.877°) have Pt (111) peaks. The tensile lattice strain can be obtained by using these peaks as a reference. Pt stabilize in cubic phase with their estimated lattice parameters turn out to be a_bulk = 3.920(1) Å, a_film = 3.915(2) Å, that yields a negligible strain of 0.13% in the thin film. The profilometer curve, scanned across bottom platinum step to the surface of the thin films, indicates that the thickness of the films is about 230 nm (Figure 1b).

Figure 2a shows the surface morphology of the thin films measured on a surface area of 5 μm × 5 μm using AFM microscopy. The surface of the thin film is found to be smooth with uniformly distributed grains with a surface roughness R_Q of around 8.73 nm. The SEM image (Figure 2b) recorded from the film surface indicated a crack free smooth surface of the film. The homogeneous distribution of constituent chemical elements was evident from the EDS-elemental mapping measured on the thin film’s surface (Figure 2c). The EDS spectrum analysis revealed that the thin film is made up of multiple components of the crystal V₂O₃ phase and the amorphous carbon phase. The chemical oxidation states of the constituent elements V and O of the thin films were further examined by XPS. The XPS survey spectrum of the films grown on Si/SiO₂/Ti/Pt substrates is depicted in Figure 3a. The peaks identified in the spectrum correspond to elements V, O, and C, and no impurity peaks were seen. The adventitious carbon C1s peak is observed due to the exposure of the sample to the environment. The binding energy (BE) 284.8 eV of this carbon C 1s peak was used as the BE scale reference. The high resolution XPS spectrum for the core level V2p and O1s states is shown in Figure 3b, in which the binding energies of the peaks of V2p_3/2, V2p_1/2, and O1s are basic representative of the oxidation state of VO_x. The XPS spectrum was analyzed using a Shirley background to obtain the oxidation state of V and O ions (

Table 1. XPS spectral parameters of the deconvoluted peaks for sesquioxide vanadium.

Element	Chemical State	Peak Position (eV)
O	O1s	531.677
V	2p1/2	523.075
	2p3/2	515.863

), and that indicated that the fitted peak position energy values of the characteristic XPS peaks match well with the literature [23,24]. It can be mentioned that a relatively large intensity of Carbon C1s peak is observed in the XPS survey spectrum, that could be probably due to contributions from two sources: a detectable quantity of adventitious carbon contamination due to exposer of the sample to the atmosphere, and from the double-sided carbon-tape normally that was used to fix a tiny thin film on the sample holder. The oxygen binding energy of the core level O1s is 531.677 eV [23], which is the standard energy reference for vanadium oxide as the ratio between oxygen (N_O) and vanadium (N_V) concentration N_O/N_V does not depict the differentiation between different oxide state [23]. The doublet of V2p corresponds to 2p_3/2 and 2p_1/2, resulted due to spin orbit coupling effect, are found to center around 515.863 and 523.075 eV. The energy difference Δ [= BE (V2p_1/2) – BE (V2p_3/2)] turns out to be 7.21 eV, a value close to the standard reference [23,25]. The difference in binding energy (ΔE) between V2p_3/2 and O1s (531.67 eV) is 15.863 eV, confirming the presence of V on the surface of the film in V³⁺ oxidation state. Therefore, the oxidation states of vanadium and oxygen are inferred as V³⁺ and O²⁻, respectively [23,26], and that corroborates the growth of the V₂O₃ thin films.

DC magnetic hysteresis loop measurements were carried out at several temperature from 10–300 K in the applied magnetic field range of ±2 kOe (Figure 4). As can be seen, the magnetic order is found to change with increasing temperature. At a low temperature of 10 K (Figure 4a), the hysteresis loop is largely bifurcated with remnant magnetization M_R = 5.23 emu/cm³ and coercive filed H_C = 103.62 Oe. The bifurcation gap of the branches of the hysteresis curve becomes narrow upon increasing temperature and does not show any paramagnetic linear curve up to 300 K. For instance: at 100 K (Figure 4c), M_R = 3.02 emu/cm³ and H_C = 72.98 Oe, and at 300 K (Figure 4d), M_R = 1.74 emu/cm³ and H_C = 58.46 Oe were obtained. This evolution of magnetic ordering can be attributed to the slight ferrimagnetic polarization of mainly antiferromagnetic spins.

To identify the magnetic ordering phenomena, ZFC-FC magnetizations measurements were carried out at different magnetic fields. Figure 5 shows the ZFC and FC magnetization curves measured from the thin films at 100–1000 Oe. The ZFC and FC magnetization curves followed different paths up to applied field of 1000 Oe, that suggests that magnetization is related to the sample cooling history, which is typically observed in other magnetic systems [27,28]. These magnetization curves showed a small kink around 55 K at 100 Oe (Figure 5a) and that shifted to lower temperature at higher applied fields. The strong field dependence of that AFM transition is suggestive for a canted AFM transition and/or spin-glass-like magnetic ordering, which is similar to the magnetic ordering that reported in the bixbyite-type V₂O₃ bulk sample [11].

The temperature dependent behavior of the remanent M_R and coercive field H_C of the hysteresis loops measured at several temperatures is depicted in Figure 6. Both the remanent M_R and the coercive field H_C were found to decrease with increasing temperature as expected due to thermal spin disorder [27], and they exhibited an anomaly near 55 K. This anomaly can be attributed to a canted antiferromagnetic phase transition, in accord with the field dependence of kink anomaly temperature observed in the ZFC and FC magnetization curves.

As mentioned earlier, at room temperature, V₂O₃ crystallizes into a rhombohedral corundum-type phase, that belongs to R$\overline{3}$C space group symmetry with the D_3d point group [10]. Seven Raman active phonons (2A_1g + 5E_g) are expected [10] in the room temperature phase. Theoretically, the Raman modes of monoclinic C_2h phase at low temperature are [10]: 7A_g + 8B_g. Hence, 15 Raman active optical phonons are expected. The Raman spectrum measured at room temperature in the frequency range of 50–980 cm⁻¹ is shown in Figure 7, that matches well with the earlier report [10]. Following the standard curve fitting procedure [29], the spectrum was analyzed using the Lorentzian function to obtain their Raman band frequencies (Figure 7). A minimum number of peaks were considered to fit the experimental data. The Raman mode at 220 cm⁻¹ and 500 cm⁻¹ can be assigned to the A_1g mode, and the mode at 681 cm⁻¹ can be assigned to the B_1g mode of the rhombohedral phase [10,18].

A few modes are observed than the actual predicted one, probably due to insufficient intensities arising from the less polarizability of several phonons.

Table 2. Here, one can see the Raman mode frequencies and temperature coefficients in various phases of the V₂O₃ thin films. The number in the parenthesis indicates the standard error in the least significant digit.

Monoclinic Phase (B2/b)			$\mathbf{Rhombohedral} \mathbf{Phase} \left(\mathit{R}\overline{3}\mathit{C}\right)$
Mode	ω (cm−1) T = 82 K	dω/dT (cm−1/K)	Mode	ω (cm−1) T = 143 K	dω/dT (cm−1/K)
Ag	203	−0.009(6)	-	-	-
Ag	250	−0.038(5)	A1g	247	−0.040(1)
Ag	502	−0.051(6)	A1g	499	−0.009(5)

shows the frequency of observed Raman modes in the rhombohedral and monoclinic phases. The temperature dependent Raman spectra are shown in Figure 8. The Raman spectra exhibit changes upon passing through the monoclinic-rhombohedral phase transition in the temperature range from 82 to 300 K. At 82 K, the phonon modes were observed at 200 cm⁻¹, 249 cm⁻¹, and 500 cm⁻¹, those are attributed to A_g phonon of the low temperature monoclinic phase [10]. The intensity of the A_g mode at 203 cm⁻¹ reduces monotonically and disappears around 143 K. This observation suggests that the phase transition from mono-rhombohedral phase at 143 K, in accord with the earlier report on bulk V₂O₃ [10]. However, no signature of phase coexistence was observed in our study in contrast to that reported in V₂O₃ thin films [18]. This could be due to relaxation of stress in the present thin films of thickness around 230 nm as compared to reported thin films of 90 nm thickness that associated with significant stress and microstructural defects. The dependencies of mode frequency on temperature, obtained from curve fittings, are shown in Figure 9. The mode at 250 and 502 cm⁻¹ continue to exist up to 300 K, while the 203 cm⁻¹ mode disappeared at 143 K, the transition temperature. The temperature coefficient (dω/dT) of the modes in monoclinic phase is found larger than that in the rhombohedral phase (Table 2). This indicates that the monoclinic phase has higher anharmonicity. It can be mentioned that MIT of the V₂O₃ thin films occurs above 143 K, as reported in the temperature-dependent studies on electrical conductivity [15,30].

In the recent studies of the thickness dependence of V₂O₃ thin films [19,31], it was found that the electronic phase transition behaviors are strongly dependent on the film thickness and could also induce a metal-insulator transition, around the temperature-induced phase transition temperature.

4. Conclusions

V₂O₃ thin films were synthesized using a DC magnetic sputtering technique. At room temperature, the films were stabilized at a rhombohedral R$\overline{3}$C phase. EDX and XPS studies indicated presence of nearly stochiometric chemical elements in the thin films. High resolution XPS studies indicated the binding energy of O1s is 531.677 eV, and that for 2p_1/2 and 2p_3/2 chemical states of vanadium are 523.075 eV and 515.863 eV, respectively. In phonon spectroscopic studies, A_1g phonon modes at 220, 500 cm⁻¹, and B_1g mode at 681 cm⁻¹ were observed at ambient temperature. Temperature-dependent Raman studies indicated a low temperature monoclinic to room temperature rhombohedral phase transition around ~143 K. Temperature dependent magnetization studies suggested a canted antiferromagnetic phase transition around ~55 K in low temperature.