X-Ray Diffraction Analysis
In order to assess the structure and crystallinity of the Nb2O5 films that were deposited at powers ranging from 25 W up to 75 W with a step of 25 W, an X-ray diffraction (XRD) investigation was carried out. The results of this analysis are depicted in Figure (1). According to the XRD patterns, all films have polycrystalline crystal structures. It evinced peaks at 2θ = 22.431°, 28.819°, 36.805°, and 46.095° which correspond to the (001), (100), (101), and (002) planes of Nb2O5 with a hexagonal phase, respectively. Each and every one of these peaks may be indexed to the JCPDS card using the number 00-028-0317. Additionally, at a diffraction angle of 2θ = 22.42°, all films had a very strong preference for one orientation over another of crystallites in the (001) diffraction peaks. As the DC power of sputtering has risen from 25 to 75 W, the preferred orientation manifested a gradual intensification of its preferred orientation, especially for (001) and (100) planes, revealing that the crystallinity of Nb2O5 films became better. Increasing the DC power enhanced the sample crystallinity, and the three diffraction lines appeared for the (110), (102), and (004) lattice planes at the diffraction angles of 50.674°, 55.187°, and 58.971°, respectively. Furthermore, the broadening of peaks in the diffraction lines indicated the increasing of the crystalline size directly with the DC power. This result is in agreement with a previous study [19].
Optical Properties
The fundamental absorption edge is a significant feature seen in the absorption spectrum of both crystalline and amorphous materials.
$${\alpha }=2.303\frac{A}{t}$$
A crucial characteristic of materials' absorption spectra is the fundamental absorption edge. The fundamental absorption, or transition from the valence band to the conduction band, may be used to compute the material band gap. The energy band gap (Eg) has been identified [20].
$$\alpha h\nu = B(h\nu -{E}_{g}{)}^{r}$$
Where, the value of r is an index that describes the procedure known as an optical absorption and may be either 1/2, 3/2, 2, or 3 based on the kind of electronic transition that causes the absorption, the optical energy gap is denoted by Eg, and \(B\) is a constant that is dependent on structure [21][22]. UV-Vis spectra were used to compute the values of the electromagnetic radiation absorption coefficient at each of the spectrum's wavelengths. The experimental findings are more matched to the r-coefficient's value of 0.5 than the other exponents, demonstrating the presence of direct optical transitions [23]. Figures (2) reveals the optical absorbance curves for the Nb2O5 thin films prepared at the RT by different DC powers within the range (200–800 nm) of wavelength. The films are highly transparent in the visible region, and when the DC power was increased, there was also an increase in the absorbance as a result of increasing both the thin film thickness and sputtering yield. It can also be seen that the absorption edge converts from stepwise into sharper with increasing the DC power indicated by reducing the lattice defects and removing the tail states near the band edges [24].
Figure (3) illustrates the plot of (αhʋ)2 versus the photon energy (hʋ) for the Nb2O5 thin films prepared at the RT by different DC powers. The band gap was an extrapolation from the crossing of the tangential linear with the x-axis. It was observed that the direct optical energy gap decreases with the increase of DC power. This decrement is a consequence of increasing the crystallite size, as evinced by the X-ray diffraction, due to reducing the quantum confinement effect and the high influence within the nano-size. A previous research has manifested that an increase in the DC power results in a reduction in the optical energy gap; the values that have been determined for the band gap in in the current study are consistent with those findings in [19][25].
Surface Morphology
Figure (4) displays the top-view images of Nb2O5 thin films formed at the room temperature using varying amounts of DC sputtering power onto quartz slide substrates employing a field-emission scanning electron microscope a at magnification power of 100 kx. The thin film sample deposited by 25 W DC power is uniformly deposited on the entire surface with a good coverage. Increasing the DC power to 50 W resulted in an increase in the diameter of the particles due to the merge of adjacent particle and the formation of particles aggregations as clusters. Raising the DC power to 75 W onto the substrate at the room temperature caused the presence of some bulges with some cracks indicating the low sticking degree of the deposited film as a result of the high difference between the temperature of grown material and the substrate[19][26].
EDX Analysis
For the purpose of conducting a quantitative examination of the elements, energy-dispersive x-ray spectroscopy (EDX) was used. Following the x-ray excitation, the EDX operating theory is based on the premise that each element possesses a unique atomic set of emission spectra for electromagnetic radiation, and that these spectra can be distinguished from one another. Figures (5) elucidates the Nb to O atomic percentages in Nb2O5 thin films deposited at the indicated powers. The variation in weight ratios provides interesting information regarding how the deposition power affects the oxygen-to-metal content. And, according to this figure, increasing the DC sputtering power resulted in an increase in the sputtering yield, which led to a rise in the Nb content from 12.2 atm% to 20.1 atm% [27].
I-V Characteristics
The I-V curve of the photoconductive detector was analyzed both while it was dark and when the light was shining on it. Figures (6) portrays the typical current-voltage properties of the Nb2O5 thin films that were deposited at 25 W, 50 W, and 75 W, respectively. The measurement was carried out under various light conditions, and the photocurrent grows as the light intensity rises. Sweep voltage was used for the experiments, ranging from − 1 V to + 1 V.
The applied voltage causes a drop in the built-in potential and widens the depletion layer by injecting majority carriers. The forward dark current results from the passage of majority carriers. The recombination current is produced at the low voltage zone (0-0.35 volt), because the intrinsic carrier concentration is lower than both the majority carrier concentration and the minority carrier concentration (ni2 < np). This occurs as a direct consequence of electrons being stimulated from the valence band (V.B.) into the conduction band (C.B.), where they will recombine with the holes that are already present in the valence band. This is shown by the fact that there is only a little increase in the recombination current in the low voltage zone. This is the result of electrons that have been stimulated all the way up to the conduction band from the valence band, where they will recombine with the holes that are already present in the valence band. The fact that there is only a slight increase in the recombination current in the low-voltage zone demonstrates this.
Diffusion current, which predominates, is the next phenomenon, which can be identified by a current magnitude that quickly and exponentially increases in response to an increase in voltage [28]. However, the reverse bias current value falls with increasing Nb2O5 sputtering power, which is related to the development of defects and dislocations that affect the mobility of charge carriers. These defects may function as active recombination sites, which reduce the current flow across the junction and allow the energy levels to be inside the energy gap.
Photosensitivity
When the light of a certain wavelength is shone onto a photodetector, this phenomenon takes place. The photodetector will react (turn on and off) for a certain amount of time. In the time domain, the rising time (tr) and fall time (tf) of the photodetector's response to the square wave pulse signal are specified. This length of time, which is referred to as the rise time (tr), is the amount of time that must pass before the output of the detector may increase from 10–90% of the peak value under fast illumination. As it can be seen in figure (7), a good rule of thumb for dealing with anything is to choose an amount that is 1/10 of the shortest pulse length found by the detector's rise time. When the light is shut off, the detector does not immediately return to its usual luminance; rather, it requires some time to do so. This is what is referred to as the time of fall (tf), also known as the dark condition. In other words, it is the amount of time that passes between when a reaction is at 90% of its maximum and when it is at 10% of its maximum[29]. Onto a substrate made of p-type silicon (100), Nb2O5 was deposited by shining the light at a wavelength of 350 nm and an intensity of 104.98 mW/cm2. According to the data, there was a rise in photo-responsiveness when the sputtering power was raised from 25 W to 50 W, reaching a maximum of 514.89%, but subsequently, it reduced to 371.11% when the power was increased to 75 W.