The Role of Annealing Treatment on Crystallographic, Optical, and Electrical Features of Bi 2 O 3 Thin Films Prepared Using Reactive Plasma Sputtering Technology

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Introduction
Tere is a considerable number of Bi 2 O 3 flms which are very striking. Te same is true with nanostructures because they are optical and electrical, including their wide energy band gap, the dielectric permittivity refractive index, Bi 2 O 3 , impressive photoluminescence, photoconductivity optoelectronics, gas sensors, Schottky barrier optical coatings, and metal-insulator-semiconductor capacitors. Furthermore, solar cells and microwaves are also integrated into the circuits [1][2][3][4][5][6][7][8][9][10][11]. Generally, physical [12,13], and chemical [14][15][16], and electrodeposition methods [17] were developed for preparing diferent bismuth nanostructures like nanoparticles [16], triangular nanoplate [18], nanotubes [19,20], nanowires [21], and nanospheres [22]. Bi 2 O 3 contains the following vital polymorphic phases: α-, β-, c-, δ, and ε-Bi 2 O 3 . According to the crystal structures, the Bi 2 O 3 optical band gap ranges from 3.6 eV to 1.7 eV [3,23]. Te phases show various forms and diferent physical or electrical features. Tus, the use of Bi 2 O 3 's other main applications is practical. Te annealing temperature diferences afect the crystallite sizes, optical band gaps, and surface porosities [3,23]. Te phases show various forms and different physical or electrical features. Tus, the use of Bi 2 O 3 's other main applications is practical. Te annealing temperature diferences afect the crystallite sizes, optical band gaps, and surface porosities. Some studies [24,25] showed that these diferences (often in oxidation) caused the transition in Bi 2 O 3 structures. Yet, investigating reactive plasmaassisted annealing's ability to form Bi 2 O 3 flms or nanostructures has been a lonesome feld [26,27]. Some works discuss extensively, such as flm-based transistors using indium zinc tin oxide (IZTO) semiconducting thin flm by Bak et al. [28] and low-temperature growth of crystalline tin (II) monosulfde thin flms using atomic layer deposition (ALD) by Ansari et al. [29] Authors studied the infuence of postannealing on the structural optical and electrical properties of SnN x thin flms [30]. Moreover, researchers are investigating bismite nanoisland thin flms for optoelectronics, which have a narrow bandgap of 1.95 eV with suitable properties for nonlinear optoelectronics [31]. However, various flm deposits obtain specifc properties. For gas sensors, Rasheed et al. [32] developed (NiO : Zn) thin flms by easy chemical spray pyrolysis, and for optical properties, Najim [33] and Hassan [34]  Te current work examined the temperature of the substrate infuence on growing, phase compositing, and electrical and optical features of Bi 2 O 3 structures made through reactive plasma-assisted annealing treatment. Te approach is straightforward and economical with no additives, including surfactants, template agents, and more metal oxides with ions following the transition.

Experimental Setup
Te plasma sputtering devices are fully illustrated in Figure 1(a), which consists of a chamber of ionization linked to a chamber of sputtering by one aperture with a pressure decrease (see Figure 1(b)). A heated tantalum flament discharges the electrons into the ionization chambers and the argon gas emission onsets by the auxiliary anodes. An arc discharge is latched between the hot cathodes and anodes in the sputtering chamber. To hasten the arc plasma quickly to the Bi bulk aim to sputter the material, a negative voltage is applied on the target, driving the ions.
Tere is a deposition of Bi 2 O 3 thin flms onto soda lime glass substrates through the reactive plasma helped by the annealing step with a 10°C min −1 heating over 2 hours in the air furnace and then cooled to ambient temperatures (see Figure 1(c)). Te glass substrates are fxed 50 mm from the target and with temperature substrate during sputtering at a range of 30-35°C and 1.5 × 10 −2 mbar pressure with a 5% O 2 fow rate. Te sputtering process would involve bombarding the Bi target with high-energy ions to sputter Bi atoms from the target to deposit Bi 2 O 3 thin flms. Te sputtered Bi atoms would then interact with an oxygen source, such as a reactive oxygen gas, in the vacuum chamber to form Bi 2 O 3 molecules. Tese Bi 2 O 3 molecules would then deposit onto the substrate to start a thin flm. It is important to note that the deposition conditions, such as the power input, gas pressure, and substrate temperature, would need to be optimized to obtain the desired Bi 2 O 3 flm properties, such as the composition, thickness, and microstructure. Additionally, the resulting Bi 2 O 3 thin flms are characterized using various analytical techniques to confrm their properties. Te samples applied to annealing treatment temperatures (200, 300, 400, and 500°C) were signed as S1, S2, S3, and S4. Metallic Bi bulk disc (Kurt J. Lesker Company-UK, 99.999% purity) of 50 gm was used as the target material. We cleaned the glass substrates ultrasonically for 20 minutes with acetone and deionized water before drying them in the nitrogen fow. Te creation of Ar-O plasma resisted the bismuth boat shield, and the substrate holders heated from room temperature to 35°C and negative during sputtering with the 2 kV bias voltage. Te annealing process formed whitish yellow-coloured glass slides, as shown in Figure 1(c). A quartz crystal thin flm monitor controls flm thickness during deposition, while a Tencor Alpha Step profler measures Bi 2 O 3 flm thickness following up the annealing process. SEM was utilized to examine the material's surface morphology (RAITH-e-LiNE, Raith GmbH) [35][36][37][38][39][40]. Te voltage for the SEM images was 10 kV, and the distances used were 5.6 mm and 10.6 mm, respectively. By applying (Bragg-Brentano) geometry and monochromatic Cu-Kα radiation in X-ray difraction (XRD) (Bruker D8 Advance) [36,37,[41][42][43][44][45][46][47], we examined the thin flm crystallographic structure. We calculated the flm absorption spectra with UV-Vis spectrophotometers (ocean Optics USB 4000) [48][49][50][51] and determined the optical band gap Eg.

Results and Discussion
Sputtering of Bi particles in the oxygen atmosphere formed the Bi 2 O 3 , as Bi + O 2 ⟶ Bi 2 O 3 [52,53]. Te sputtering rate of Bi 2 O 3 rises when the oxygen pressure increases to 1.5 × 10-2 mbar for 40 min. Te temperature of substrate annealing has a signifcant infuence on the Bi 2 O 3 flm surface morphology. Te surface structures in the SEM pictures are of Bi 2 O 3 at various temperatures, as shown in Figure 2. Te flm was grown at an annealing temperature of 200°C and showed identical and quite dense structures with various grain sizes of 100-200 nm (Figure 2(a)). When the annealing temperature reached 300°C (see Figure 2(b)), the grain size shrank to 11-20 nm, and the gaps in the grain boundaries disappeared with a rise in the density. At temperatures of 200°C and 500°C, the thickness of Bi 2 O 3 flms was 606 nm and 571 nm, respectively. When the deposition was completed at higher temperatures of 400°C (see Figure 2(c)) and 500°C (see Figure 2(d)), nanostructures were formed on the surface. At 400°C, 3D nanostructures covered the surface with an irregular, branching morphology. Bi 2 O 3 nanostructures have a length range of 100 to 500 nm and in diameter of 50-100 nm, as shown in Figure 2(d).Te results show that changing the annealing temperature during the formation of the Bi 2 O 3 structure changed the surface morphology of dense layers from one of low temperature to that of high temperature, leading to a lower nanostructure and its crystal.
Te data for the XRD were obtained over the range of 20°t o 70°with a 0.02°step and 0.2 s for data acquisition. Te 2 Journal of Nanotechnology phase identifcation process used the JCPDS PDF database 42. Figure 3 shows the amorphous Bi sample (S1 and S2) preannealing treatment, but S3 and S4 show some peaks in the transformation to crystallite phases. Annealing Bi 2 O 3 flms at 200°C showed the XRD pattern of low-intensity broad peak as shown in  . Te XRD data determine crystallite sizes by conventional Scherer's formula for the average crystallite sizes of nanostructures made at 400°C and 500°C temperatures, respectively, 50.5 nm and 37.7 nm. Yet, the Bi 2 O 3 stayed in an amorphous phase at a temperature less than 200°C; the peak narrowing of 2θ = 27°-28°showed atom rearrangement in bulk, and the energy stayed decreased for the flm crystallizations, which conforms with previous research [54,55]. Te evaporation of the electronically excited, ionized plasma produces the working gas atoms. In addition, the molecules are also produced. Te nanosized droplets agglomerate into bigger grains during the Bi and Bi 2 O 3 evaporation on the lowtemperature forms, creating the continuous amorphous flms. Te amorphous phase's transformation into the crystalline phase reduces grain sizes for the deposited flm at 200°C. Te frst nanoscale crystallite nucleation centres have begun to emerge.
Te substrate temperatures provide the crystallization of the flm using the energy supplied by the modifcation applied by annealing treatment. Te oxygen concentration       increases in droplets with annealing in air, completely oxidizes, and functions as the centre nucleation of Bi 2 O 3 nanostructures [56]. However, the under-study plasma sputtering system and conditions still need to be improved to develop crystalline Bi 2 O 3 flms in one step.
Te bismuth thin flm Ultraviolet-V's absorbance spectrum forms camel-like fgures at 280 and 320 nm, with a little shift to the red region at 540 nm for the surface plasm on resonance and the light scatters (see Figure 5). Te absorption coefcient with the annealing temperature is the postannealed thin flm optical absorbance spectra at various annealing temperatures at the wavelength range of 300 to 1100 nm. Tere are many studies on high annealing temperature infuence on Bi x O x flms [57,58]. According to the data, the annealing of the flms at 300°C maximizes absorbance when there is extra annealing to 400°C, absorbance decreases in the annealed flms at 500°C because of the rise of the flm's roughness, and the following formula has been used to evaluate the bandgap [44,[59][60][61][62][63][64]: Here, α is the absorbance coefcient, h] is the beam energy, B is the proportion constant, and Eg refers to energy gaps. Regarding Bi 2 O 3 flms, n equals 2 for direct allowed transition [65][66][67]. Te Tauc method was used for the Bi 2 O 3 optical band gaps to extrapolate the linear curve portions in the plot (αhυ)2 versus hυ [67]. Te annealing temperature of the glass substrate changes the band gap of Bi 2 O 3 formations. Te flm gap deposited at 30°C is 3 eV. As the Bi 2 O 3 thin-flms glass slide annealing temperature rose to 200°C, the band gap shrunk to 2.70 eV. Te band gap and temperature reached 3.05 eV and 500°C, respectively. Te crystal structure, flm thickness, and substrate temperature signifcantly impact the band gap values of Bi 2 O 3 nanostructures and flms confrmed by Salih et al. [68].
Te exact mechanism by which the bandgap decreases during annealing can depend on the specifc material and the annealing conditions, such as the annealing temperature and duration. However, it is essential to note that the relationship between band gap and grain size is only sometimes straightforward and can depend on several factors, including the specifc material and the processing conditions used. Terefore, while a decrease in band gap and an increase in grain size coincide during annealing, it is not necessarily a universal rule that applies to all materials based on the current study data and, other studies [58,59,68]. In addition, upon flm annealing at 500°C, the bandgap drops because of the nonstoichiometric Bi 2 O 3 with a smaller bandgap [69,70]. Te rise in substrate temperature from 400°C to 500°C modifes the morphology of the surface, lowers the crystallite size, and transitions from β-Bi 2 O 3 with a little amount of δ-Bi 2 O 3 phase to pure δ-Bi 2 O 3 , which is the fundamental reason of Bi 2 O 3 structures having excellent band gap values [67].

Electrical Characterization.
Te use of the D.C. twoprobe method is to examine electrical resistivity as the sample temperature from (30-500)°C as Table 1 shows, and flm preannealing at room temperature showed a resistivity of 1.  that under our annealing conditions, changing the transport properties was not wholly possible (temperature and duration). Nevertheless, the electrical characteristics of the bismuth flms exhibit an abrupt change when the temperature exceeds 280°C. Te annealing was in the air atmosphere to prevent the oxidation and reaction of bismuth elements with oxygen adsorption on the flm surface or in glass substrates; according to Leontie et al. [69], the thermal oxidation of bismuth forms amorphous oxide layers at the substrate-flm interfaces on glass. Yet, the optical and electrical features were approximately between them, often determined by the structure nanometric sizes, in which intergrain faws ofer electrons to the optical transitions and the electrical conductivity. Nanowires formed and exhibited a 3.09 eV band gap made entirely of the δ-Bi 2 O 3 phase, whose mean crystallite size is 38 nm. Te electrical resistivity was boosted by 53% with elevated annealing temperatures up to 500°C. However, the absorbance also increased by annealing, which refers to an increase in the band gap and the crystal size at the maximum at 400°C.