Phase formation polycrystalline vanadium oxide via thermal annealing process under controlled nitrogen pressure

This article provides an approach to improve and control crystal phases of the sputtering vanadium oxide (VxOy) thin films by post-thermal annealing process. Usually, as-deposited VxOy thin films at room temperature are amorphous phase: post-thermal annealing processes (400 °C, 2 hrs) under the various nitrogen (N2) pressures are applied to improve and control the crystal phase of VxOy thin films. The crystallinity of VxOy thin films changes from amorphous to α-V2O5 phase or V9O17 polycrystalline, which depend on the pressure of N2 carrier during annealing process. Moreover, the electrical resistivity of the VxOy thin films decrease from 105 Ω cm (amorphous) to 6×10-1 Ω cm (V9O17). Base on the results, our study show a simply method to improve and control phase formation of VxOy thin films.


Introduction
Metal-insulator transitions (MIT) are reversible changes in the conductivity of materials when the temperature above or below the transition point, which are the smart transitions in advanced material [1]. Vanadium oxide (VxOy) demonstrated excellent MIT characteristics at the transition temperature (Tc) due to the lattice distortion such as VO2 (68 º C), V4O7 (-23 º C), V6O11 (-103 º C), V9O17 (-194 º C) [2]. Several techniques have been applied to deposit vanadium oxide films such as sputtering, pulsed laser deposition and thermal evaporation. The sputtering technique demonstrated an excellent uniformity with high deposition rate, consequently the sputtering technique is promising to prepare VxOy thin film [3]. However, the sputtering VxOy thin films, which prepared at room temperature have amorphous phase. To improve the MIT or the crystallinity of VxOy films, the post-annealing controlled ambient processes were applied to recrystallize and control phase formation of the material.

Experiment and methods
Vanadium oxide thin films were deposited on glass slides from 2-inch vanadium sputtering target (Kurt J. Lesker) by pulsed DC magnetron sputtering (ATC 2000-F, AJA International, Inc.) at room temperature. High purity (99.999%) of Argon (Ar) gas and Oxygen (O2) gas were used as the sputtering gas and the reactive gas, respectively, both of them were controlled by mass flow controller (1179A, MKS). VxOy thin films have been growth at base pressure 2×10 -6 mbar. To find out the oxide mode in reactive sputtering process, the flow rate of Ar was fixed at 45 sccm and various flow rates of O2 were controlled at 0, 5, 10 and 15 sccm before introduced into the chamber [4]. Then, the operating pressure was set at 5×10 -3 mbar by automatic gate valve. The pulsed DC power was set at 300 W and the deposited time was 60 minutes, respectively. To improve crystallinity of VxOy, as-deposited thin films with O2 flow rate 15 sccm were annealed at 400 ºC for 120 minutes by using a contact heater under the various controlled nitrogen pressure in vacuum chamber. Annealing treatment was not performed higher than 400 ºC to avoid the deformation of thin films at the high annealing temperature [5]. The electrical transport characteristics were evaluated by a four-point probe technique using precision DC source (6621, Keithley) and nano-voltmeter (2182A, Keithley). The temperature was controlled between 30-120 °C in the air by precision hot plate (1000-1, Electronic micro systems). After that, the crystalline phases of VxOy thin films were investigated by X-ray diffraction (XRD) (Smartlab, Rigaku) using Cu-Kα (λ=1.54 Å) radiation. All measurements were taken by generator voltage of 40 kV and a current of 30 mA. Then, the chemical bondings of VxOy were confirmed by confocal Raman spectroscopy using 532 nm Ar laser (NTEGRA Spectra, NT-MDT).

Results and Discussion
X-ray diffraction pattern by general θ-2θ scanning mode shows in figure 1(a) According to database number (01-088-2322), a diffraction peak of a thin film without O2 (0 sccm) shown at 2θ = 42.14°, which matched with the (110) plane of vanadium. The broad area between 20º-35º indicates the diffraction from the glass substrate. When we increased the O2 flow rate (5, 10 and 15 sccm), the diffraction peaks of thin films were disappeared. Therefore, all of them were amorphous phase.  Figure 1(b) illustrated the increasing of voltage bias of target when the reactive O2 flow rate increased during the sputtering process. The voltage bias abruptly changed from 388 V to 481 V in case of O2 flow rate 5 sccm and slightly increasing for 10 sccm and 15 sccm. The difference in voltage biases revealed the oxide mode or the poisoning effect on target surface during the reactive sputtering [4]. The as-deposited thin films by pulsed DC magnetron sputtering with various O2 flow rate shown as inset in figure 1(b). The sputtered thin films at the O2 flow rate 5 sccm and 10 sccm were sub-oxide thin films, while the thin films at the O2 flow rate 15 sccm has a yellow colour and transparent, which was possible to be the VxOy. Consequently, the O2 flow rate 15 sccm was used to optimize for the post annealing process, and were used to characterize the electrical transport property, respectively. After performed post annealing treatment (400ºC, 120 minutes), figure 2(a) shown the decreasing of the VxOy thin films resistivity as a function of temperature measured by four-point probe technique with the various controlled nitrogen pressure. The resistivity at 30 °C by post annealing process decreased when the operating pressure increased from 2×10 -6 to 1 mbar, whereas the as-deposited thin film has the resistivity higher than 10 5 Ω cm, as shown in figure 2(b). According to above results, it can be implied that the crystal structure of the VxOy thin films has changed from amorphous to polycrystalline phase. The intensity ratios between the (002) plane of V9O17 and the (101) plane of α-V2O5 as a function of nitrogen operating pressure during annealing process.
After post annealing process under various nitrogen operating pressures, the crystal structures of the VxOy thin films could observe as figure 3(a). At the operating pressure 2×10 -6 mbar, the peaks located at 15.44º, 26.16º, 31.16º, 34.38º, 47.52º and 51.22º represent for the orthorhombic structure of α-V2O5 from the crystal plane (200), (101), (310), (301), (020) and (501), respectively. However, we found a small diffraction peak at 2θ = 12.8º which designated to be a beta-phase polymorph vanadium pentoxide (β-V2O5) [6]. That means the thin film at the operating pressure 2×10 -6 mbar has mixed phase between alpha and beta phase. When the operating pressure increased from 2×10 -6 to 1 mbar, the component peaks of mixed-phase V2O5 decreased, while the component peaks of mix phase of V6O11 and V9O17 were appeared and increased dramatically. At the nitrogen operating pressure 1 mbar, the diffraction pattern of the VxOy film preferred to be V9O17 dominant rather than V6O11. The intensity ratios between (002) of V9O17 and (101) of α-V2O5 were calculated, which were plotted as a function of nitrogen operating pressure, as presented in figure 3(b). Further analysis was performed to confirm the chemical bonding by using Raman spectroscopy. Figure 4 illustrated nitrogen pressure-dependent Raman spectra of the VxOy thin films as a function of the operating pressure. The intensity of Raman shifted spectra from the V2O5 thin film decreased; on the other hand the intensity of Raman shifted spectra from the V9O17 slightly increased when the operating pressure increased.
According to above results, the applied thermal energy under the ambient of nitrogen contributed to deform grains and recrystallization of VxOy thin films. Since, the oxygen atoms in thin films were carried out by thermal annealing process under controlled nitrogen pressure a vacuum chamber. The residual oxygen gas in annealing ambient decreased with increasing pressure of nitrogen gas, which suppressed higher valence state of VxOy thin films [7]. Consequently, the crystallinity of the VxOy thin films has changed from amorphous to polycrystalline structures, which leading to the reduced of resistivity when the operating pressure increased. Based on our strategy, we could improve the crystallinity and control VxOy phase by the post-annealing process from as-deposited thin film growth. This method is simple and flexible for crystal phase controlling of the VxOy, which is an alternative method to prepared VxOy materials for various applications, which depend on the transition temperature (Tc).

Conclusions
We offered a strategy to improve crystallinity and phase controlling of as-deposited sputtering VxOy thin films via thermal annealing controlled nitrogen pressure. After annealing process, the crystallinity of VxOy has changed from amorphous to polycrystalline phase. Consequently, we could measure the decreasing of resistivity from VxOy thin film when the nitrogen operating pressure increased. Our research work has shown the flexible and simply method to improve crystallinity and phase controlling of VxOy thin films for further development in advanced functional device.