Light-Sensing Properties of Amorphous Vanadium Oxide Films Prepared by RF Sputtering

In this study we analyzed the structure and light-sensing properties of as-deposited vanadium oxide thin films, prepared by RF sputtering in different Ar:O2 flow rate conditions, at low temperature (e.g., 65 °C). X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDX), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to analyze the film microstructure, composition and the oxidation states of vanadium ions. The SEM micrographs evidence VxOy films with smooth surfaces, whereas the XRD patterns show their amorphous structure. Raman spectra indicate an increased structural disorder in the films deposited in Ar:O2 flow comparatively with those deposited solely in Ar flow. The XPS data suggest the modification of the oxidation state from V4+ to V5+, thus proving the formation of the V2O5 phase when increasing the oxygen content, which further affects the films’ optical properties. We observed a good stability of the photogenerated current in Si/SiO2/VxOy/TiN heterostructures upon excitation with pulses of UV (360 nm), VIS (white light) and NIR (860 nm) light. The responsivity, detectivity and linear dynamic range parameters increase with the O/V ratio in the VxOy films, reaching comparable values with photodetectors based on crystalline V2O5 or VO2.


Introduction
Vanadium oxides have gathered a constantly growing interest in different fields of applications, due to their ability to tune material properties according to the vanadium oxidation state. Vanadium can occur in numerous oxidation states, e.g., V 5+ , V 4+ or V 3+ , which is advantageous for the preparation of thin films with different electrical and chemical properties for a wide range of applications [1]. Several studies have been conducted, and a rich phenomenology has been revealed in crystalline compounds, such as VO, VO 2 , V 2 O 3 or V 2 O 5 oxides. For instance, V x O y compounds can change their optical properties due to external stimuli in the form of photon radiation (photochromic) [2], change in temperature (thermochromic) [3] and voltage pulse (electrochromic) [4]. More specifically, thin films of VO 2 and V 2 O 3 have been found to show good thermochromism in the infrared region, while maintaining transparency to visible light, e.g., a smart window modulates infrared irradiation from a low-temperature transparent state to a high-temperature opaque state. Vanadium sesquioxide (V 2 O 3 ) is a member of vanadium oxides (V x O y ) called Magneli phases, defined by the general stoichiometric formula V n O 2n−1 [5]. Their property of undergoing a metal-insulator transition (MIT) with the change in temperature has attracted thermo-and electrochromic applications, such as smart windows [6], ultra-fast nanoelectronic switches [7], transistors [8], thermoelectric devices [9]. Moreover, vanadium pentoxide (V 2 O 5 ) has proved its ability for electrochromic and charge storage devices, due to its ability for Li-ion intercalation in the layered crystal structure [10][11][12][13]. On the other hand, amorphous vanadium oxides (a-V x O y ) were relatively unexplored in comparison with the crystalline counterparts. These films became popular mostly for energy-related applications. To date, a-V x O y has been obtained by electrochemical oxidation [14], reactive sputtering [15], using a combination of sol-gel processing paired with electrochemical deposition [16], atomic layer chemical vapor deposition [17], atomic layer deposition [18], or recently, gas impulse magnetron sputtering [19].
Magnetron sputtering seems to be one of the most interesting processing routes of V x O y thin films, because of the possibility of producing large area thin films with repeatable properties, a good uniformity of deposition, as well as a good adherence to various substrates. Also, the possibility to control the process parameters allows the preselection of the films' physical properties at the stage of synthesis, enabling a rational design of the future opto-electronic devices. From the applications point of view, it was shown that the a-V x O y films with a thickness of around 650 nm can be used for Li-and Na-ion batteries [17], while other studies presented the applications of composites, such as vanadium oxide on graphene nanosheets [18] for stable, high energy lithium-ion anodes, a-V x O y /C composites for positive electrodes in rechargeable aluminum batteries [19] or a-VO x /MXene nanohybrid architecture for high-performance Na-ion batteries [20]. Recently, the applicability of a-VO x for hydrogen gas sensing applications has been demonstrated, with the films' conductivity controlled from p-to n-type via increasing the amount of the oxygen in the gas impulse magnetron sputtering process [21]. The investigations of the VO x films deposited onto quartz substrate by RF sputtering deposition from a vanadium target at 400 • C substrate temperature, with 8.5% O:Ar ratio, showed that amorphous films have flat and stable optical transmittance curves. Films with different thicknesses could be used to control the intensity of the light [22]. Moreover, it was shown that the amorphous V 2 O 5 films provided a quick response time (4.9 ms) with high detectivity (1.45 × 10 12 Jones) for broadband transparent photodetectors [23].
In this study we investigate the relationship between the deposition Ar:O 2 flow rate and the thin films' structure and stoichiometry, pointing out that the increased oxygen content in the deposition process leads to a transition from the VO 2 to the V 2 O 5 phase, which further affects the light-sensing properties of V x O y thin films. Also, we show that the responsivity, detectivity and linear dynamic range parameters of the heterostructures with different V x O y amorphous films increase with the O/V ratio, reaching comparable values with recently published results for photodetectors based on crystalline VO 2 [24] or V 2 O 5 [25].

Materials and Methods
The V x O y films were deposited on Si (100) n-type (1-10 Ω·cm)/SiO 2 and glass substrates by RF sputtering, using a V 2 O 3 target (99.99% purity, Testbourne Ltd., Basingstoke Hampshire, UK). The deposition was performed in Ar or Ar:O 2 plasma with the temperature of the substrate maintained at 65 • C. The pressure inside the deposition chamber was 5 mTorr and the RF power was 300 W. The V x O y films were prepared by changing the Ar:O 2 ratio in the deposition plasma. We used three working conditions: (A) Ar (30 sccm), (B) Ar (30 sccm): O 2 (0.5 sccm) and (C) Ar (30 sccm): O 2 (1 sccm) mass flow rate. Electrical contacts on the film surface were obtained by deposition of titanium nitride (TiN) square pads, with different areas, S (1) = 9 mm 2 , S (2) = 1 mm 2 , S (3) = 0.25 mm 2 and S (4) = 0.09 mm 2 , through a mechanical mask. The deposition process was performed by RF sputtering from a Ti target, in atmosphere of Ar (20 sccm): N 2 (1 sccm), at a pressure of 5 mTorr. All the deposition processes were performed by using a PlasmaLab System 400 (Oxford Instruments Plasma Technology, North End Yatton Bristol, UK) equipment. The structure, morphology and composition of the films were analyzed by XRD and SEM-EDX, using a SmartLab/X-ray Thin Film Diffraction System (Rigaku Corporation, Tokyo, Japan) and a Nova Nano SEM 630 Field Emission Gun Scanning Electron Microscope (FEG-SEM), equipped with EDX spectroscopy (FEI Company, Hillsboro, OR, USA), respectively. Raman spectra of the as-deposited films were acquired by using a High-Resolution Raman Spectrometer-LabRAM HR 800 with a 633 nm HeNe Laser (Horiba, Jobin Yvon, France). X-ray photoemission spectroscopy (XPS) spectra were recorded on a Sigma Surface Science photoelectron spectrometer (Scienta Omicron, Taunusstein, Germany) equipped with a 160 mm hemispherical energy analyzer with a 1D detector (ASPECT) and using an Al Kα X-ray source at 13 kV at a power of 200 W. The analysis area was 1.3 × 1.3 mm 2 . The transmission and absorption analysis were performed at room temperature using Cary 5000 UV-Vis-NIR Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). The electrical characteristics of the Si/SiO 2 /V x O y /TiN heterostructures were measured using SCS 4200 Keithely (Tektronix Inc. Beaverton, Oregon, USA)-Suss Microtech system (Cascade Microtech, Beaverton, OR, USA), at 15 V, in the dark and under illumination with white light (10 mW/cm 2 ) and monocromatic wavelengths in the UV (360 nm) and NIR (860 nm) ranges, using a set-up with laser diode sources with intensity of 140 mW/cm 2 .

Morpho-Structural Investigations
In order to assess the impact of the Ar:O 2 gas flow rate on the crystalline structure of the V x O y, thin films, the XRD patterns were recorded at the grazing-incidence. The diffraction patterns of the Si/SiO 2 /V x O y /TiN heterostructures are shown in Figure 1a.  The XRD patterns also reveal the presence of the cubic TiN film with a = 0.423 nm, confirmed by the TiN (111) and TiN (002) reflections at 36.72 and 42.27 • (ICDD card no. 031-1403). In addition, a sharp diffraction peak from the Si substrate occurred at 51.21 • . Moreover, the broad diffraction feature located at smaller angles (e.g., between 20 and 32 • ) shows the amorphous nature of the V x O y films, regardless of the oxygen flow in the deposition process. That indicates that, at the low temperature of the substrate (~65 • C), the deposited material may be just sticking to the Si/SiO 2 substrate surface at its place of hitting, with almost no surface diffusion. The substrate temperature is not high enough to provide sufficient thermal energy to the ad-atoms to find the locations for further bonding, resulting in an amorphous structure [26].
Previous XRD studies showed that the deposition of the crystalline V 2 O 5 phase occurs at around 200 • C in the case of d.c. reactive magnetron sputtering [27]. Recent investigations on V 2 O 5 RF sputtered films also showed that sputtered V 2 O 5 films grown at room temperature have an amorphous structure [28]. Other investigations on the V x O y films deposited by pulsed laser deposition (PLD), showed that the amorphous-crystalline transition temperature of V 2 O 5 films is around 200 • C [26,29], also suggesting that the fundamental thermodynamic parameter, deposition temperature, governs the crystallinity of the samples. However, in our experiment, it could be observed that the XRD broad halo is shifted to smaller angles with increasing oxygen flow, which can be attributed to an additional interplanar spacing, induced in the presence of the oxygen.
The XPS investigation was carried out on as-grown samples on the glass substrates, in order to obtain more information on the chemical state of the vanadium species and for additional phase identification. The O 1s and V 2p signals were recorded together in one energy window. These spectra reveal the presence of V 2p doublets, as well as the O 1s core level peak, and the fitting was performed using a mixed Lorentzian-Gaussian function, see Figure 1c. Whereas the peak of O 1s was found at 530.4 eV, the V2 p 3/2 and V2 p 1/2 present shift to higher binding energy (BE), when the oxygen flow increases, namely from 523.8 eV to 524.7 eV and from 516.4 eV to 517.6 eV, respectively. The BE values for vanadium oxides are in a good agreement with the previous work on VO x [30,31]. The slight shifts in the peak position were further ascribed to the modification of the oxidation, evolving from a V 4+ oxidation state to V 5+ (black arrow was used to show this trend). Interestingly, it is important to observe a similar trend in the XRD patterns (see blue dashed arrow), indicating a different crystal packing and an additional strain in the presence of oxygen. The latter observation could give the explanation of the different band gaps of the films, as will be shown later. According to the XPS data, this trend implies a transition from a VO 2 phase to V 2 O 5 phase, as the oxygen flow increases. Additional confirmation of the V 2 O 5 presence at high oxygen flow is given by the spin-orbit splitting of approximately 7.5 eV between V2 p 3/2 and V2 p 1/2 orbitals, as well as by the energy difference of 12.9 eV between the binding energy of V2 p 3/2 and the O 1s orbital [16,32].
Moreover, the Raman spectra of the films illustrate the significant influence of the oxygen flow in the Raman modes, see Figure 2. The peaks are more intense and well resolved in the spectrum of the film deposited in the Ar flow and become broader in the spectra of the films where an oxygen flow is added in the process.   [33].
The Raman spectra are very sensitive to vanadium oxygen coordination; the analysis of the specific position of the Raman bands in comparison with the reference compounds could give an insight towards their structural assignment. As such, the Raman data could reveal the effect of the oxygen flow in the deposition process on the structure of the VxOy films. The Raman spectra of vanadium oxide thin films deposited on Si by reactive sputtering, by varying the O2/Ar gas flow ratio, e.g., 1.5/100 sccm, 1.5/50 sccm, show main peaks at ~225 cm −1 and 504 cm −1 , corresponding to the V2O3 phase, while a 1.3/100 sccm gas flow ratio leads to the formation of a VO2 phase. The further increase in the oxygen content in the deposition process determines a transition to the V2O5 phase, with the characteristic Raman peaks at 145 cm −1 , 195 cm −1 , 284 cm −1 , 303 cm −1 , 405 cm −1 , 483 cm −1 , 701 cm −1 and 992 cm −1 [34].
In reference [15], the maxima observed in the Raman spectrum of a V2O5 film deposited by RF sputtering in an amorphous state are attributed to the stretching modes of the V3-O bonds (520 cm −1 ), V2-O (650 cm −1 ) and to the stretching mode of terminal oxygen atoms V 5+ =O (1027 cm −1 ). Also, a maximum situated at 932 cm −1 is attributed to V 4+ =O bonds. Shiver et al. reported Raman peaks in the Raman spectra of vanadium oxides without long range order, situated at 950 cm −1 related to V 4+ =O bonds and, respectively, at 1020 cm −1 related to V 5+ =O [35]. The dominant bands in films B) and C) suggest the V 4+ and, respectively, V 5+ valence states in agreement with the XRD and XPS data. The broad and Lattice vibrations of crystalline compounds may also be present below 150 cm −1 [33].
The Raman spectra are very sensitive to vanadium oxygen coordination; the analysis of the specific position of the Raman bands in comparison with the reference compounds could give an insight towards their structural assignment. As such, the Raman data could reveal the effect of the oxygen flow in the deposition process on the structure of the V x O y films. The Raman spectra of vanadium oxide thin films deposited on Si by reactive sputtering, by varying the O 2 /Ar gas flow ratio, e.g., 1.5/100 sccm, 1.5/50 sccm, show main peaks at 225 cm −1 and 504 cm −1 , corresponding to the V 2 O 3 phase, while a 1.3/100 sccm gas flow ratio leads to the formation of a VO 2 phase. The further increase in the oxygen content in the deposition process determines a transition to the V 2 O 5 phase, with the characteristic Raman peaks at 145 cm −1 , 195 cm −1 , 284 cm −1 , 303 cm −1 , 405 cm −1 , 483 cm −1 , 701 cm −1 and 992 cm −1 [34].
In reference [15], the maxima observed in the Raman spectrum of a V 2 O 5 film deposited by RF sputtering in an amorphous state are attributed to the stretching modes of the V3-O bonds (520 cm −1 ), V2-O (650 cm −1 ) and to the stretching mode of terminal oxygen atoms V 5+ =O (1027 cm −1 ). Also, a maximum situated at 932 cm −1 is attributed to V 4+ =O bonds. Shiver et al. reported Raman peaks in the Raman spectra of vanadium oxides without long range order, situated at 950 cm −1 related to V 4+ =O bonds and, respectively, at 1020 cm −1 related to V 5+ =O [35]. The dominant bands in films B) and C) suggest the V 4+ and, respectively, V 5+ valence states in agreement with the XRD and XPS data. The broad and weak Raman bands corresponding to films B) and C) indicate the presence of a high degree of amorphous phase.
The morphology and structural investigations show that RF sputtering from a V 2 O 3 target led to a more crystalline structure of the films deposited in process (A) comparatively with the (B) and (C) processes. Different oxygen flow added to the deposition atmosphere beside Ar flow led to different microstructural characteristics. The effect on optical properties is further investigated by absorption and transmission measurements.

Absorption and Transmission Measurements on VxOy Films
The transmission and absorption graphs of the V x O y films deposited on glass substrates are shown in Figure 3a,b. (αhν) ∝ hν − E gopt n where hν is the incident photon energy, α the absorption coefficient and the exponent n = 1/2, 2 for direct allowed transition and indirect allowed transitions, respectively. Both direct and indirect Eg optic decrease with the increase in the oxygen content in the films, as one may observe in Figure 3a,b. The allowed direct and indirect Eg optic values are listed in Table 1. One may observe that the optical band gap energy, Eg opt , in both direct and indirect cases, shifts towards lower energy with the increase in oxygen concentration. According to reference [37], the presence of the tetravalent vanadium ion (V 4+ ) enhances the UV light absorption in the oxides. Since the XPS data indicated V 4+ is suppressed at high oxygen flow, this could be the explanation for the band gap energy decreasing.

Current−Time Characteristics
The photogeneration processes under illumination with various wavelengths were investigated aiming to evaluate the sensing properties of amorphous vanadium oxide thin films obtained by (A), (B) and (C) conditions. The current-time, (I−t), characteristics presented in Figure 6a−c show the currents photogenerated in Si/SiO2/VxOy/TiN heterostructures, under excitation with different wavelengths.

Current−Time Characteristics
The photogeneration processes under illumination with various wavelengths were investigated aiming to evaluate the sensing properties of amorphous vanadium oxide thin films obtained by (A), (B) and (C) conditions. The current-time, (I−t), characteristics presented in Figure 6a

Current−Time Characteristics
The photogeneration processes under illumination with various wavelengths were investigated aiming to evaluate the sensing properties of amorphous vanadium oxide thin films obtained by (A), (B) and (C) conditions. The current-time, (I−t), characteristics presented in Figure 6a  The films demonstrate the good stability of the photogenerated currents upon excitation with light pulses. The intensity of the photogenerated current by applying pulses lasting 10 s varies depending on the wavelength. The characteristics indicate that the films are responsive in UV, VIS and NIR spectral domains. However, the selectivity changes with the O/V ratio and the film stoichiometry. The I−t characteristics presented in Figure 6b indicate increased sensitivity in the NIR as well as UV range for the V x O y films prepared in process (B), whereas the I−t characteristics presented in Figure 6c indicate increased sensitivity only in the NIR range for the films prepared in process (C), for which the photogenerated current has double intensity. All the V x O y films exhibit increased responsivity to NIR light, with the photogenerated current values of 53 µA, 80 µA and 110 µA, respectively. Previous studies attributed the near-infrared absorption in V 2 O 5 films to small polaron effects [39]. Kim et al. [23] reported that amorphous V 2 O 5 thin film transparent photodetector yields an excellent performance with quick response times of 4.9 ms for blue, 12.7 ms for green and 16.1 ms for red light, respectively.
The main parameters used for photodetection characterization are Responsivity (R), specific detectivity (D*) and the linear dynamic range (LDR). Responsivity can be described as: which is the ratio of the output current density (J ph ) to the power of input illumination (P in ) and indicates how sensitive a sensor is to the light. Specific detectivity (D*) indicates the ability to detect a weak light signal and can be calculated with the following equation: where R is the responsivity, q is the electron charge and J d is the dark current density. Finally, linear dynamic range (LDR) describes the response range in which the photodetector sensitivity is linear with the input light: Figure 7 shows the values of R, D* and LDR parameters calculated for Si/SiO 2 /V x O y / TiN heterostructures exposed to NIR illumination where the highest sensitivity was previously demonstrated. All the parameters increase proportionally with the phase transitioning triggered by increased oxygen flow during the deposition of V x O y thin film. The heterostructure based on the film obtained in the richest O 2 atmosphere achieved the maximum values of R~13 mA/W, D*~1.7 × 10 6 Jones and LDR~20 dB, respectively. The responsivity range is similar with other reports on V x O y materials [24,25,40] and makes the heterostructures compelling for NIR photodetection applications [41]; however, high voltage bias is also required. The observed dark current reached a value of 1 mA leading to decreased detectivity. The level of defect-related recombination is presumably high in the as−deposited amorphous vanadium oxide structures; however, the further optimization of Ar:O 2 flow deposition could enable reduced charge recombination followed by charge transport improvement. Figure 7. Responsivity, detectivity and linear dynamic range calculated for VxOy film-based heterostructures exposed to NIR wavelength (860 nm).

Conclusions
We investigated the effect of Ar:O2 mass flow rate in the RF magnetron sputtering process of VxOy film deposition at low temperatures, by means of spectroscopic XRD, XPS and Raman techniques. It was evidenced that the increase in oxygen content determines a vanadium oxidation state transition from V 4+ (VO2) to V 5+ (V2O5). According to the absorption and transmission measurements on VxOy films, different microstructural features as resulted in the deposition process significantly affect the optical band gap energy and the films' light-sensing properties. The Si/SiO2/VxOy/TiN heterostructures demonstrated a broad spectral sensitivity. The responsivity, detectivity and the dynamic linear response of amorphous VxOy thin film photodetectors were evaluated for NIR monochromatic light. The highest responsivity was achieved for the films deposited in the Ar (30 sccm): O2 (1 sccm) mass flow rate condition. The main parameters of photodetectors based on amorphous vanadium thin films could be comparable to the previously reported results for crystalline VOx−based photodetectors.

Conclusions
We investigated the effect of Ar:O 2 mass flow rate in the RF magnetron sputtering process of V x O y film deposition at low temperatures, by means of spectroscopic XRD, XPS and Raman techniques. It was evidenced that the increase in oxygen content determines a vanadium oxidation state transition from V 4+ (VO 2 ) to V 5+ (V 2 O 5 ). According to the absorption and transmission measurements on V x O y films, different microstructural features as resulted in the deposition process significantly affect the optical band gap energy and the films' light-sensing properties. The Si/SiO 2 /V x O y /TiN heterostructures demonstrated a broad spectral sensitivity. The responsivity, detectivity and the dynamic linear response of amorphous V x O y thin film photodetectors were evaluated for NIR monochromatic light. The highest responsivity was achieved for the films deposited in the Ar (30 sccm): O 2 (1 sccm) mass flow rate condition. The main parameters of photodetectors based on amorphous vanadium thin films could be comparable to the previously reported results for crystalline VO x −based photodetectors.

Conflicts of Interest:
The authors declare no conflict of interest.