Annealing effects on the optical and electrochemical properties of tantalum pentoxide films

Tantalum pentoxide (Ta2O5) has attracted intensive attention due to their excellent physicochemical properties. Ta2O5 films were synthesized via electron beam evaporation (EBE) and subsequently annealed at different temperatures ranging from 300 to 900 °C. X-ray diffraction (XRD) results show that amorphous Ta2O5 thin films form from 300 to 700 °C and then a phase transition to polycrystalline β-Ta2O5 films occurs since 900 °C. The surface morphology of the Ta2O5 films is uniform and smooth. The resulted Ta2O5 films exhibit excellent transmittance properties for wavelengths ranging from 300 to 1100 nm. The bandgap of the Ta2O5 films is broadened from 4.32 to 4.46 eV by annealing. The 900 °C polycrystalline film electrode has improved electrochemical stability, compared to the other amorphous counterparts.

1 Introduction  Ta 2 O 5 (tantalum pentoxide), which forms at either tetragonal (α-Ta 2 O 5 ) or orthogonal (β-Ta 2 O 5 ) crystalline phase, is one of the important transition-metal oxides with high refractive index (n = 2.02-2.16), high dielectric constant ( = 20-46), wide bandgap (4.0-4.5 eV), and wide transmission spectrum [1]. Ta 2 O 5 thin films have been widely used in new types of optoelectronic devices, electroluminescent devices [2,3], optical waveguides [4], optical coatings [5,6], and optical filters [7], owing its excellent thermal and chemical stability, high mechanical strength, and good resistance to friction. For examples, a protective layer of Ta 2 O 5 is often coated on thermal sensors utilizing its higher heat resistance and mechanical strength in satellites. Ta 2 O 5 and its ternary alloys have been widely used as passivation layer in MOS-based devices [8]. Electrode materials in modern batteries often need a Ta 2 O 5 cover layer to obtain long-term electrochemical energy storage and charge-and-discharge cycle toughness [9]. The crystalline structure obtained by annealing and the reduction of oxygen vacancies can increase the laser damage threshold. Thin-film saturable absorber materials typically need a coating of Ta 2 O 5 on its top surface to maintain a high laser damage threshold and sustain higher output-power in ultrafast lasers [10,11]. Because of its compatibility with commercial integrated circuit technology, Ta 2 O 5 is a promising non-silicon dielectric material for microelectronic devices and the ideal capacitive material for next generation dynamic random-access memory [12] and multi-chip modules [13,14]. With its high acidity, Ta 2 O 5 can serve as a catalyst for important processes such as the oxidation of alkanes and the pollution reduction [15]. In addition, Ta 2 O 5 is an important dielectric material for thin-film capacitors, and has been successfully used for anode oxidization of electrodes in lithium batteries [16].
The fabrication of Ta 2 O 5 films using various methods has been investigated intensively and the potential application in industry has been developed during the past two decades. Hudner et al. [17] reported the growth of β-phase Ta 2 O 5 films on Si(100) via electron beam evaporation in an atomic oxygen plasma at a substrate temperature of 650 ℃. Mannequin et al. [18] discussed the difference in the structural and chemical properties of amorphous Ta 2 O 5 thin films grown via EBE and RF sputtering. Using chemical vapor deposition (CVD), Devine et al. [19] developed a new plasma-enhanced process to obtain high-quality amorphous Ta 2 O 5 thin films, which were used as gate insulators to make p-channel MOS transistors. Using atomic layer deposition (ALD) method, Chaneliere et al. [3] and Lukosius et al. [20] characterized the structural and electrical properties of Ta 2 O 5 films and explored the possibility of applying Ta 2 O 5 thin films in microelectronic devices. Pulsed laser deposition (PLD) was used by Krishnan et al. [21] to obtain crystalline SnO 2 -doped Ta 2 O 5 thin films with good optical properties at relatively low temperatures. Byeon et al. [22] and Reddy et al. [23] used magnetron sputtering method to synthesize Ta 2 O 5 films and investigated the possibility of Ta 2 O 5 films as dielectric materials in capacitors of integrated circuits. Xia et al. [16] chose anodizing method to fabricate a self-supporting porous Ta 2 O 5 film for a battery electrode material with a large number of charge and discharge cycles. Ta 2 O 5 films were also produced via the sol-gel method [24] and polycrystalline Ta 2 O 5 film formed after heat treatment above 400 ℃, which decreased optical absorption. The performance adjustment of Ta 2 O 5 films was made at the same time. Xu et al. [10] improved the oxygen vacancy problem of evaporated Ta 2 O 5 film via annealing, and increased the bandgap and the laser damage resistance threshold of the film. Zhang et al. [25] used spectroscopic ellipsometry (SE) to characterize the refractive index of evaporated Ta 2 O 5 films with different thicknesses and studied the optical properties of ultrathin film as a function of thickness. Masse et al. [26] studied Ta 2 O 5 thin films at different annealing temperatures ranging from room temperature up to 900 ℃ and found mutual diffusion of oxygen, silicon, and Ta at high temperatures. He et al. [27] studied the effects of oxygen pressure on film roughness and optical transmission of PLD-grown Ta 2 O 5 films.
Technically, the synthesis of Ta 2 O 5 film is feasible using any of the above methods. Annealing of the deposited Ta 2 O 5 films is necessary to improve film crystallinity as well as the refractive index, absorption coefficient, and the electrical properties of the films [26,[28][29][30][31][32]. In this work, EBE, a simple film-growth method, had been employed to deposit amorphous Ta 2 O 5 films on quartz and Si substrates to produce optical coatings with uniform, smooth surfaces and a high film density [33][34][35]. However, when evaporated Ta 2 O 5 films undergo high temperature annealing, the effects on the film's structural, morphological, optical, and electrochemical properties are still not clear. Therefore, our purpose is to investigate the properties of Ta 2 O 5 samples at different annealing temperatures, and the research work should bear the significance to improve the long-term stability of the photocatalyst/ electrocatalyst electrodes.

Experimental
Ta 2 O 5 films were synthesized with the following procedure. The source materials were small Ta 2 O 5 pieces (purity 99.99%), which were put into a pure copper crucible in a commercial high-vacuum EBE system (ZZS700, Xing Nan Technology Co., China). The experimental device is shown in Fig. 1. Opticallypolished Si(100) substrates with 4 kΩ·cm 2 and the thickness of 650 μm (for the structural and morphological characterization) and quartz with the thickness of 500 μm (for transmittance studies) were cut into pieces (10 mm × 10 mm) and cleaned (ultrasonic cleaning in acetone, alcohol, and DI water, respectively, for 15 min). They were placed on a rotating workpiece which was heated with a contact heater in the upper vacuum chamber to maintain a constant substrate temperature of 300 ℃. The substrate temperature was monitored by a thermocouple with an accuracy of ±1 ℃ in real time. The evaporation source-to-substrate distance was ~130 cm, and the evaporation source-to-electrongun-filament distance was ~25 mm. Before the evaporation, the substrate surface was cleaned for 120 s using an ion source. The vacuum system consists of a low-vacuum mechanical pump and a high-vacuum molecular pump. The pump and vacuum monitor can (in real time) display the vacuum level in the system pipeline and the evaporation chamber. When the evaporation chamber was pumped down below 6×10 -4 Pa, the materials started to be evaporated.
A quartz crystal monitor (Inficon G10) was used to display the real-time film deposition rate. The target thickness value of the Ta 2 O 5 film was pre-set to 50 nm twice in control panel of the EBE system (100 nm thick in total). The filament current of the electron gun was 310 mA. The film growth rate was set to be very slow (0.2-0.4 nm/s) so as to allow sufficient duration for film growth with uniform grain distribution. After the film growth was done and the chamber naturally cooled down, the samples were taken out of the chamber and subsequently annealed at 300, 500, 700, and 900 ℃ for 30 min in a tube furnace (with the temperature rising at 5 ℃/min). After film deposition, the samples in the tube furnace are naturally cooling down in air to the room temperature with the cooling rate estimated to be about 2 ℃/min.
The surface morphology of the samples was characterized using atomic force microscopy (AFM, Dimension ICON) and field-emission scanning electron microscopy (FE-SEM, Nova Nanosem 450). Phase identification was performed via XRD with the Cu Kα radiation line (λ = 1.5418 Å, Shimadzu 7000SAS). The optical transmittance spectra of the Ta 2 O 5 films with different annealing temperatures were measured via a UV-visible spectrophotometer (UV3300) for a wavelength range of 200-1100 nm. Using an electrochemical workstation (CHI760E), the capacitive test was performed on the sample by cyclic voltammetry. The electrochemical test uses a three-electrode system for a scan rate range of 0.1-0.5 V/s, using a thin film sample as the working electrode, metal platinum as the counter electrode, and calomel electrode (SCE) as the reference electrode. The electrolyte uses 1 M NaCl electrolyte. The surface valence state is characterized by X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd.), the binding energy was calibrated using C 1s (284.8 eV) spectrum of hydrocarbon that always remained in the XPS analysis chamber. Figure 2 shows the XRD diffraction patterns of the prepared Ta 2 O 5 thin films on Si substrates at different annealing temperatures. When comparing these four curves (300-900 ℃) with the Standard PDF Card #25-0922 [16,18,36], only 900 ℃ curve exhibits (001),   [36], the crystallization of Ta 2 O 5 films may start at less than 800 ℃ . All the above results are consistent with previous reports [17,18,33,36,37]. Figure 3 illustrates AFM images of the planar morphology of Ta 2 O 5 /Si(100) sample at 300-900 ℃ anneal. The Ra (the maximum roughness) values of these samples at 300, 500, 700, and 900 ℃ are 0.402, 0.865, 0.780, and 2.263 nm, respectively; and the Rq (the root mean square roughness) values of these samples at 300-900 ℃ are 0.520, 1.151, 0.989, and 3.132 nm, respectively. The overall color of the diagram for the 300-900 ℃ is dark brown, which indicates that the film surface is quite uniform and very dense [38]. Specifically, the light brown and white dots, representing the raised structures, are relatively small. The height errors are even within the scale of a single molecular layer, indicating the film surface has atomic flatness [39]. Figure 4 shows planar and cross-sectional SEM images and EDS mapping of Ta 2 O 5 thin films at different annealing temperatures. Table 1 shows the atomic weight percentage of oxygen and tantalum in EDS mapping energy spectra. In Fig. 4(a), it exhibits that the particle size of Ta 2 O 5 films decreases greatly with increasing of the temperatures (from 300 to 700 ℃ ). Significant changes occurred at 900 ℃ , where the film surface morphology becomes texturized structure in spite of the film surface keeping flat. This phenomenon is consistent with XRD results of the Ta 2 O 5 film transition from amorphous phase to crystalline phase at high temperature. Figure 4(b) measures the sample thickness values of 157.5, 105.7, 113.5, and 116.0 nm for 300-900 ℃ films. According to Ref. [25], the film thickness below 40 nm could affect the optical properties of Ta 2 O 5 thin film dramatically. When the film thickness exceeds 40 nm, the refractive index of the film will be close to the value of bulk Ta 2 O 5 . Thus, the optical properties of the obtained films are thickness-independent. In Fig. 4(c) and Table 1, it shows the elemental distributions of Ta and O on the film surface of four films, confirming high elemental distribution uniformity of all film surfaces.

Results and discussion
Since the optical properties of Ta 2 O 5 films can reflect the variation in the crystalline quality and energy bandgap of the films, the transmittance spectra of four films on the quartz substrate are plotted in the wavelength range of 200-1100 nm in Fig. 5. All the Ta 2 O 5 films exhibit relatively good optical transmittance over the wavelength range of 310-1100 nm. For 300 ℃ curve, the transmittance rate of the film is around 0.6 to 0.77 (60% to 77%) in the wavelength range of 310-660 nm. For 500 and 700 ℃ curves, the transmittance rates of the two samples increase to 0.75-0.82 (75%-82%) in the wavelength range of 310-660 nm. This phenomenon occurs probably because the optical properties of the samples were  relatively not so good with low density and there are many defects in the Ta 2 O 5 film at low temperature anneal [10,36]. Another reason may be that 300 ℃ film is thicker (157.5 nm) than the other three films (around 110 nm). Unlike Ref. [26], the transmittance spectra of 500 and 700 ℃ are very similar probably because the hexagonal structured Ta 2 O 5 (δ-Ta 2 O 5 ) forms at 700 ℃ in Ref. [26] while the film crystallization of 500 and 700 ℃ does not occur yet. In addition, the optical transmittance continues to substantially increase in the wavelength ranges of 300-550 nm and 700-1100 nm, from 700 to 900 ℃ since crystalline Ta 2 O 5 phase starts to form at 900 ℃ and thus there are fewer defects in 900 ℃ film. Finally, contrary to the results in Ref. [36], where the transmittance of the prepared films decreased when the annealing temperatures increased from 600 to 1000 ℃, the transmittance in Fig. 5 The bandgap of the samples is ~4.54 eV, which is close to the 4.2 eV of bulk Ta 2 O 5 [21]. In addition, the Ta 2 O 5 film coverage reduces the substrate transmittance slightly and the film color of 300-900 ℃ turns to light blue from dark blue in Fig. 5. As the temperature increases, the optical transmittance of the Ta 2 O 5 film generally increases. The transmittance of 900 ℃ film remains above 90% from 374 to 1100 nm (slightly below 90% at 500-600 nm), and reaches a maximum of 93 % at 400 nm. Thus, annealing facilitates removal of the voids and holes of thin films and the consequent improvement of the film optical quality as well as the film color.
Further, Fig. 5 shows the bandgap (E g ) variation of the Ta 2 O 5 films with annealing temperatures. The film E g value can be derived using Tauc [40] formula: where α is the absorption coefficient, K is a constant, and hv is the photon energy. α can be obtained using [41]: Here, d is the film thickness and T is the transmittance. The insets in Fig. 5 [10,21]. Annealing processing also affects the electrochemical properties of Ta 2 O 5 films. In a typical cyclic voltammetry test, the redox peak current (i p ) and the scan rate (ν) satisfy the following relationship [42]: where a and b are adjustable constants. When b = 0.5, the charge/discharge is controlled by diffusion-controlled electrode process; when b = 1.0, the charge/discharge is adsorption-controlled electrode process; if 0.5 < b < 1, the electrode reaction of NaCl electrolyte is an irreversible process driven by both adsorption and diffusion [42][43][44]. Take the logarithm of Eq. (4) at both sides of equal sign, we get the following equation: Figure 6 plots the cyclic voltammograms (CVs) of 300 and 900 ℃ samples dipped into 1 M NaCl neutral electrolyte collected at potential scan rates from 0.1 to 0.5 V/s. In Fig. 6(a), redox peaks appearing at (-0.5)-0.7 V represent the oxidation/reduction of Ta ion species on the anode surface. For the scan rate of 0.5 V/s anodic peak is broad, about 500 mV and cathodic peak is relatively narrow, about 375 mV. The maximum peak currents of anodic and cathodic peaks are 3.0 and -3.7 μA, respectively. When the scan rate decreases from 0.4 till 0.1 V/s, the cathodic peak potentials shift from -0.5 to 0.2 V in a positive direction (the negative shift of the peak potentials signifies a greater ohmic resistance in the thicker oxide coating) and the peak-to-peak potential separation decreases. While both the anodic/cathodic current peaks decrease in intensity, the cathodic peaks decrease faster. In Fig. 6(b), previous redox peaks are not present. When the scan rate increases from 0.1 to 0.5 V/s, the peak current gradually increases, and their shapes remain unchanged, reflecting good electrochemical stability of 900 ℃ electrode. The maximum peak currents are -0.8 and 0.8 μA, respectively. This phenomenon occurs probably because the film defects or vacancies are enormously eliminated with 900 ℃ annealing, and the capacity of the film surface to capture carriers decreases, leading to performance degradation of the film conductivity and peak current. The linear relationship between logi p and logν and the slope b values were obtained from the insets of Fig.  6. The b values of ~0.625 for 300 ℃ film and ~0.877 for 900 ℃ film indicate that Ta 2 O 5 film electrode behaves are driven by both adsorption and diffusion. Further, the b value of Ta 2 O 5 film electrode increases with annealing temperatures. The possible reason is that annealing reduces the oxygen vacancies, improves the flatness and uniformity of the film, and weakens the ability of the electrode surface to adsorb ions. Figure 7 shows the XPS results of 300 and 900 ℃ electrodes before and after the CV tests and their area ratio graph after Ta 5+ /Ta 4+ peak position fitting. In Fig.  7(a), the characteristic peak positions of Ta 5+ of Ta 4f 7/2 and 4f 5/2 of the samples are 26.2 and 28.0 eV [45,46]. The presence of surface states or oxygen vacancies may cause partial Ta 5+ to lower chemical valence state in the formula of Ta 2 O 5-δ [47]. After fitting the Ta 4f spectral line of (a), a small amount of Ta 4+ peaks at 25.7 and 27.9 eV [46] are obtained. In Fig. 7(b), the two pairs of peaks in the sample after the CV test are located at 25.4, 25.9, 27.7, and 27.9 eV, respectively. Among them, the 25.9 and 27.9 eV peaks represent Ta 5+ in the Ta 4f spectrum. Further, the Ta 4+ content of the sample has increased from 5.27% to 7.73% for 4f 7/2 and from 3.95% to 5.80% for 4f 5/2 , which infers that a certain amount of Ta 4+ in the 300 ℃ sample was produced after the electrochemical test. Thus, the anodic/cathodic peaks in CV test (Fig. 6) may be attributed to the redox transitions of Ta 5+ /Ta 4+ [45,48]. How the specific reactions take place still needs further  investigation. With larger reduction peak in Fig. 6(a) than oxidation peak, it might be concluded that the reduction process proceeds faster than the oxidation process and thus, more Ta 4+ was generated. In Figs. 7(c) and 7(d), the peak position and content of Ta 4f do not change much. Due to the symmetrical CV curve of Fig.  6(b), the Ta 5+ /Ta 4+ redox transition seems to be reversible, which suggests better chemical stability and cyclical performance of the electrode material after 900 ℃ heat treatment.

Conclusions
Ta 2 O 5 films were synthesized on Si(100) and quartz substrates via EBE method, followed by annealing at different temperatures in air. Amorphous dense films formed at temperatures between 300 and 700 ℃ with enlarged band gap values from 4.32 to 4.46 eV. As the annealing temperature increased, the film transmittance was improved due to the higher film density and less defects in the film. Film crystallization of β-Ta 2 O 5 occurred at 900 ℃, which produced the highest transmittance rate among these Ta 2 O 5 films. The smooth and flat film-surface, high film-density, less defects in the film, and crystallization may play vital roles in maintaining the excellent transmittance properties of Ta 2 O 5 films. The crystallization of amorphous Ta 2 O 5 film by annealing may lead to the reversible electrochemical reaction inside electrolyte.