Tuning the Oxygen Content in the Nb2O5 Thin Films Deposited on Si by DC Magnetron Sputtering for Energy Storage Devices Applications

Nb2O5 thin films were deposited on silicon substrate using DC magnetron sputtering by applying variable deposition power on a Nb target and fixing the oxygen flow rate during the deposition process. This technique helped the control of thin film thickness and oxygen-to-metal ratio. Successive deposition powers between 200 W up to 300 W were applied. The minimum thin film thickness was about 25 nm, and the corresponding energy band gap was 3.59 eV. The thin films showed high transmittance and refractive index in the visible region. These characteristics match the high-k materials requirements as replacement of SiO2 layer in energy storage devices and DRAM applications.


Introduction
The complementary metal oxide semiconductor silicon-based field effect transistor has been the most important electronic device for about 40 years. Its stability and low power consumption encouraged engineers to increase the number of CMOS SiO 2 -based FET in microprocessors and integrated circuits. However, the need for the thin-sized memory storage devices forced the reduction of the SiO 2 layer size. Accordingly, the SiO 2 layer used as a gate had its size reduced down to about 1.2 nm. At this size, direct tunneling of electrons becomes inadequately very high, which lead to undesirable power dissipation [1,2]. It is also challenging to deposit such unreliable thin film. The solution to such  5 has the most chemical stability. It is considered as good optical material as it has good resistance to corrosion in any media, it is thermodynamically stable to be formed [5], it has a high refractive index in the UV-VIS region and of high transparency with a relatively wide energy bandgap. Since it is a transparent dielectric material, Nb 2 O 5 is ideal for super capacitor technology applications such as energy storage devices and DRAM modules.
In this paper, we focused on developing Nb 2 O 5 thin films of variable metal-to-oxygen content by employing sputtering technique with constant Argon-to-Oxygen ratio but applying different deposition powers on the Nb target. Successive Nb 2 O 5 thin films were deposited on Si and glass substrates by applying deposition powers in the range of 200 W up to 300 W, step 50 W. The energy band gap, crystal structure, surface morphology, and metal -to-oxygen content, were studied VS deposition power. Refractive index, extinction coefficient, transmittance, E g , thin film thicknesses, and roughness were also determined using Variable Angle Spectroscopic Ellipsometry (VASE) which has been a successful technique that helped exact determination of such optical parameters [6,7].

Synthesis of Nb 2 O 5 thin films
Nb target of diameter 3", thickness 0.6", and purity 99.99%, was used in DC magnetron sputtering operated at the 200 up to 300 W, step 50 W. Nb target was obtained from China Rare Material Co. Ltd. The vacuum pressure was initiated at 8.4 × 10 -6 mbar. Argon and oxygen mixture by Ar-to-O ratio 95:5 was used as an inert gas and oxidation agent, respectively. The flow rate was adjusted and fixed at 90 sccm. The silicon (111) and glass substrates of thickness 2 mm were held at 10 cm from the Nb target. SiO 2 layer was removed using plasma pre-deposition for 30 seconds. Working pressure reached 4.8 × 10 -2 mbar during the deposition process. The deposition time was adjusted at 60 minutes in each run. The substrate holder was kept rotating at a fixed rate.

Characterization
The crystal structure of Nb The grain size D was calculated using the Scherrer's formula [9] 0.9 = cos D λ β θ where β is the full width at half maximum of the diffraction peak. The lattice constants a and c were estimated using the tetragonal crystal identity The lattice constants, the grain size and the lattice parameters h, k, and l, are listed in Table 1 (Figure 1).

FESEM morphology of Nb 2 O 5 thin films
The morphology, thickness, and deposited particles shape were examined using FESEM. Figure 2 provides valuable information about the surface roughness, the thickness, the grain shape and size, the uniformity and homogeneity of the 300 W Nb . Thin film elemental composition was studied using energy-dispersive x-ray spectroscopy (EDX). VASE parameters, Ψ and Δ, were measured for the thin films using M-2000 Variable Angle Spectroscopic Ellipsometer (J. A. Woollam Co., Inc.).

Results and Discussion
XRD characterization of Nb 2 O 5 thin films 5 thin film showed a sharp peak at the 2θ = 31.26°. All other thin films exhibited an amorphous structure. Such variation in crystal structure indicated the influence of the oxygen content variation with deposition power applied to Nb target. The interplanar spacing d hkl is conveniently written in terms of diffraction angle θ as follows [8].  Where t inclined is the thickness of the inclined view, which is in this case is 46.6 nm and t is the calculated thickness which is 72.6 nm. It is noticed in Figure 2 that the thin film is uniformly distributed on the substrate and the thin film composition is homogeneous ( Figure   2).

EDX analysis
The atomic percentage of Nb to O in Nb 2 O 5 thin films deposited at the indicated powers are listed in Table 2. The variation in the weight ratios provides valuable information about the effect of deposition power on the oxygen-to-metal content ( Table 2).

Optical characterization
Transmission intensity for all thin films was mea-sured by aligning the VASE ellipsometer at 0° angle of incidence on the thin film deposited on a glass substrate. The thin films showed transmission intensity of about 0.85 at the visible region of the spectrum. Sharp absorption occurred at about 350 nm, which approximately corresponds to an energy gap of about 3.54 eV. The inset graph on Figure 3 shows the variation of the transmission intensities for thin films deposited at the indicated powers (Figure 3). . The measured spectrum of the ellipsometry parameters Ψ and Δ was then fitted to a Tauc-Lorentz oscillator model [10] which is a successful fitting model in case of amorphous materials, , and E g are the oscillator amplitude, the peak energy, the peak broadening, and the energy band gap, respectively. ɛ 2 (E) is the imaginary part of the refractive index which is a function of photon energy. Ψ and Δ were measured at different angles of incidence; though, we selected the proper angle of incidence which corresponds to a maximum change of the plane of polarization and abrupt phase change. The fitted ellipsometry parameters are shown in Figure 4 and  Fitting ellipsometry parameters revealed the extinction coefficient K and the thin film thicknesses, which were The refractive index VS. incident photon wavelength shown in Figure 6 was extracted after fitting Ψ and Δ.

Conclusion
Thin films of Nb 2 O 5 were successfully grown on silicon and glass substrates using DC magnetron sputtering using new technique by applying variable deposition power on a Nb target while keeping the oxygen flow constant. XRD verified that the crystal structure varied according to the deposition power. EDX revealed an alternative Nb-to-O ratio against deposition power. FESEM proved uniform thickness and homogeneous Nb 2 O 5 distribution on the substrate surface. By employing this technique, it is possible to tailor the energy gap of Nb 2 O 5 with thickness down to about 25 nm which has a potential application in energy storage devices and SiO 2 replacement in CMOS integrated circuits.