The XRD patterns shown in Fig. 1 correspond to the IO, CIO, and BIO films. Identified peaks match with the (211), (222), (400), (411), (440), (600), and (611) planes, consistent with the cubic In₂O₃ structure (JCPDS 06-0416). All samples display a dominant (400) diffraction peak. XRD does not reveal any secondary crystalline phases across all studied samples. Additionally, a suppression of non-primary peaks is observed, particularly in the CIO sample.

Fig. 1.

XRD patterns of the synthesized thin films. The plot displays the diffraction intensity versus 2$\theta$ for undoped IO, CIO, and BIO samples. All indexed peaks correspond to the bixbyite cubic phase of In₂O₃ according to JCPDS card no. 06-0416. A prominent (400) orientation is observed across all samples, indicating a preferred growth direction. IO, In₂O₃; CIO, Cu-doped In₂O₃; BIO, Br-doped In₂O₃; XRD, X-ray diffraction.

The lattice constant (a) of the cubic In₂O₃ was determined using the following relation:

(1)$$\frac{1}{d_{hkl}^2}=\frac{h^2+k^2+l^2}{a^2}$$

Where d denotes the interplanar spacing and h, k, l are the Miller indices. The calculated d values, lattice constants, and the positions of the (400) diffraction peak are summarized in Table 1. For the IO sample, the peak is located at 35.5018°. In the BIO sample, the peak position is found at 35.4694°, while for the CIO sample, it is recorded at 35.5336°. The determined lattice constants (a) are 10.106 Å for IO, 10.115 Å for BIO, and 10.097 Å for CIO.

The crystallite size (D) and strain ($\epsilon$) values, derived from the (400) reflection, are provided in Table 1. The crystallite size was determined using the Debye–Scherrer equation [26]:

(2)$$D=\frac{0.9\lambda }{\beta \cos \theta }$$

Where D represents the crystallite size, $\lambda$ is the X-ray wavelength, $\beta$ is the FWHM in radians, and $\theta$ is the Bragg angle. The calculated crystallite sizes are 44.269 nm for IO, 42.173 nm for CIO, and 44.220 nm for BIO.

According to the biaxial strain model [27, 28], the strain is defined as:

(3)$$\epsilon =\frac{a\mathrm{–}{a}_0}{a_0}$$

Where a is the calculated lattice constant and a₀ is the standard value (10.118 Å) [29]. All prepared films exhibit negative strain values, as summarized in Table 1.

Fig. 2 presents representative SEM micrographs of the prepared In₂O₃-based thin films (Fig. 2a–c). Incorporation of Cu and Br dopants noticeably modifies the surface topography, resulting in distinct changes in grain morphology characterized by pyramidal and granular features. The BIO sample (Fig. 2c) exhibits densely packed grains. Furthermore, based on the SEM micrographs, the IO and BIO films appear more uniform, with a smoother surface texture than the CIO sample.

Fig. 2.

SEM micrographs illustrating the surface morphology of the prepared thin films. (a) IO, (b) CIO, and (c) BIO. The images reveal a significant influence of the dopant type on the grain size and surface topography. The IO film exhibits a dense distribution of crystalline grains, whereas Cu doping induces a modification in grain boundaries and surface roughness. In contrast, Br incorporation results in a comparatively smoother surface with a more uniform grain distribution, which correlates with the improved optical transparency discussed in the text. All images were captured at the same magnification to facilitate a direct comparison of the structural features. SEM, scanning electron microscope. Scale bar = 3 μm.

Fig. 3 presents the transmittance spectra of the IO, CIO, and BIO thin films. All films show high transparency across the visible region, with the BIO sample achieving the highest average transmittance of about 85%. A slight shift in the absorption edge toward the ultraviolet region is observed for the BIO film, while a shift toward the visible region is noted for the CIO film.

Fig. 3.

Optical transmittance spectra of IO, CIO, and BIO thin films recorded in the wavelength range of 300–1100 nm. The spectra reveal high transparency for all samples within the visible region, with the BIO film exhibiting the maximum average transmittance of approximately 85%. The variations in the absorption edge positions reflect the influence of copper and bromine incorporation on the electronic band structure. The enhanced transparency in the BIO sample is attributed to its improved crystalline quality and smoother surface morphology, which minimizes light scattering effects.

The optical absorption coefficient ($\alpha$) was determined from the transmittance (T) data using the following relation:

(4)$$\alpha =\frac{1}{t}\ln \left(\frac{1}{T}\right)$$

Where t represents the thickness of the thin film. To determine the optical bandgap (Eg), Tauc’s plot was utilized by plotting ($\alpha$h$\nu$)² as a function of photon energy (h$\nu$), as shown in Fig. 4.

Fig. 4.

Tauc plots of ($\alpha$h$\mathrm{\nu }$)² versus h$\mathrm{\nu }$ for IO, CIO, and BIO thin films. The direct E_g for each sample is estimated by extrapolating the linear portion of the curves to the photon energy axis where ($\alpha$h$\nu$)² = 0. The plots demonstrate a noticeable modulation in the bandgap energy, with a narrowing effect observed for the Cu-doped film and a broadening (blue shift) for the Br-doped film. This widening in the BIO sample is consistent with the Burstein–Moss effect resulting from increased carrier concentration. The high linearity of the plots in the high-energy region confirms the direct nature of the electronic transitions in these In₂O₃-based films. $\alpha$, absorption coefficient; h$\nu$, photon energy; E_g, optical bandgap.

The values of E_g were extracted by extrapolating the linear portion of the curves to the photon energy axis where ($\alpha$h$\nu$)², according to the following equation [30]:

(5)$${\left(\alpha hv\right)}^2=A(hv\mathrm{–}{E}_g)$$

Where A is a material-dependent constant and h is Planck’s constant. The calculated optical bandgap values for the IO, CIO, and BIO thin films are summarized in Table 2. The IO film exhibits an optical bandgap of approximately 3.69 eV, while doping with copper results in a slight reduction to 3.66 eV, and bromine incorporation increases the bandgap to around 3.73 eV.

The Urbach energy (E_u) was determined by following the empirical Urbach rule [31], which describes the exponential dependence of the absorption coefficient on photon energy near the band edge.

(6)$$\alpha ={\alpha }_0\exp (\frac{hv}{E_u})$$

Where $\alpha$₀ is a pre-exponential constant and E_u is the Urbach energy. The values of E_u were calculated from the reciprocal of the slope of the linear plot of ln($\alpha$) versus h$\nu$. As shown in Table 2, the estimated E_u values for the IO, CIO, and BIO thin films are 0.332 eV, 0.343 eV, and 0.329 eV, respectively.

Sheet resistance (R_s⁢h) is commonly determined using the linear four-point probe method, in which a constant current (I) is passed through the two outer probes while the voltage drop (V) is measured between the inner probes. This setup significantly reduces the influence of contact and spreading resistance, enabling a more precise evaluation of sheet resistance. For thin films with a thickness (t) much smaller than the probe spacing, a geometric correction factor is applied, and the sheet resistance is estimated as follows:

(7)$$R_{sh}=\mathrm{\hspace{0.25em}4.532}(\frac{V}{I})$$

The electrical resistivity ($\rho$) of the film is then obtained by multiplying the sheet resistance by the film t:

(8)$$\rho =R_{sh}\times t$$

As summarized in Table 2, the IO film exhibits a resistivity of 0.0089 $\mathrm{\Omega }$$\cdot$cm. Upon doping with 4% copper, the resistivity increases to 0.013 $\mathrm{\Omega }$$\cdot$cm, whereas bromine doping leads to a significant reduction in the film’s resistivity to 0.006 $\mathrm{\Omega }$$\cdot$cm.

To compare the performance of such films, the study uses the figure of merit ($\mathrm{\Phi }$) defined by Haacke [32], which balances optical transmittance with sheet resistance.

(9)$$\mathrm{\Phi }=\frac{T^{10}}{R_{sh}}$$

Where $T$ is the optical transmittance at 550 nm (T₅₅₀) and $R_{sh}$ is the sheet resistance. The figure-of-merit values of the IO, CIO, and BIO films in the present study are calculated using Eqn. 9, and the results are shown in Table 2.

Regarding the structural analysis, the dominance of the (400) diffraction peak observed in the XRD patterns (Fig. 1) indicates that this plane is the favorable growth direction due to its low surface energy and the high degree of crystallinity supported by it. This trend aligns with prior results reported by Bagheri Khatibani et al. [33] for films produced via spray pyrolysis. The preferred orientation remains unaffected by the incorporation of copper or bromine dopants. Ionic radius considerations suggest that Br^– (1.96 Å), being larger than O^2- (1.40 Å), may substitute anion sites, while Cu²⁺ (0.73 Å), slightly smaller than In³⁺ (0.80 Å), may replace cationic positions in the lattice.

Although no secondary phases were detected, amorphous phases or nanoscale Cu/Br-related precipitates below the technique’s detection limit cannot be fully ruled out. The suppression of non-primary peaks, especially in the CIO sample, indicates a reinforced preferential orientation in all films. Nevertheless, direct confirmation of dopant site occupancy and chemical state requires complementary techniques beyond XRD.

Regarding the lattice parameters, the shift of the (400) diffraction peak toward lower diffraction angles in the BIO sample suggests a small expansion in the interplanar distance. This change may be attributed to the possible substitution of O^2- in the lattice by Br^– of a larger ionic radius. In contrast, the shift of the (400) peak toward higher diffraction angles for the CIO sample is indicative of a contracted interplanar spacing, which is consistent with the possible replacement of In³⁺ by smaller Cu²⁺ ions [34]. These trends are supported by ionic radius considerations and align with findings in the literature [35].

The observed reduction in crystallite size upon doping is particularly pronounced with copper incorporation compared to the almost negligible decrease induced by bromine (see Table 1). This behavior is consistent with reports regarding the influence of dopants on the nucleation and growth of In₂O₃ thin films [36]. Furthermore, the negative $ϵ$values indicate a net compressive strain associated with a reduction in the interplanar spacing of the cubic In₂O₃ lattice. For the IO film, the slight compressive stress is most likely due to the lattice mismatch between the film and the substrate.

In addition to the structural changes, the surface morphology of the films was also significantly affected by the dopant type. These morphological observations are consistent with earlier reports showing that grain structure in In₂O₃ films is highly dependent on deposition conditions [37]. Although the present SEM discussion is qualitative, such variations in grain geometry and packing density are significant as they may influence the optical and electrical properties of the films by affecting light scattering, charge transport pathways, and surface state density [38].

Consistent with the transmittance spectra shown in Fig. 3, the improvement in optical transmission observed in the BIO sample is possibly linked to a reduction in light scattering and enhancement in crystalline quality. In particular, the greater transparency observed in the IO and BIO films may be associated with their comparatively smoother surface morphology, which helps to suppress scattering effects at the film surface. This relationship is consistent with the SEM observations discussed in the previous section. It is well established that increasing the surface roughness leads to greater light diffusion and consequently lowers transmittance. Previous work confirms that rougher surfaces increase light diffusion, thereby degrading transmittance [39].

As evidenced by the optical data presented in Section 3.3, the optical bandgap of the IO film aligns well with previously reported values for In₂O₃ prepared via different synthesis methods [40, 41]. The observed trends of bandgap narrowing in Cu-doped In₂O₃ and widening in Br-doped In₂O₃ are consistent with comparable results found in the literature for Cu and F doping [22, 42, 43]. Among the studied samples, the CIO film exhibits the lowest bandgap (3.66 eV) accompanied by an increase in Urbach energy (0.343 eV), which may be attributed to structural disorder and localized states that reduce the effective bandgap [44]. This is consistent with findings reported in the literature [45].

In contrast, bromine incorporation into indium oxide thin films leads to an optical bandgap enlargement and a reduction in Urbach energy. The shift of the absorption edge toward shorter wavelengths is consistent with a Burstein–Moss effect, whereby an increase in carrier concentration raises the Fermi level and shifts the onset of interband transitions. The lower Urbach energy indicates a reduction in disorder-induced tail states near the band edges. Collectively, these trends suggest that Br doping enhances the optical transparency window of In₂O₃ films [23]. The tunable bandgap indicates that doped In₂O₃ films can offer tailored optical properties for transparent conductive applications.

Consistent with the data presented in Section 3.4, the electrical resistivity of In₂O₃ thin films is strongly influenced by the dopant species and the resulting defect chemistry. The intrinsic n-type behavior of the IO film is primarily due to the presence of oxygen vacancies (V₀) and interstitial indium atoms (Inᵢ), both of which act as shallow donor states [46, 47]. The increase in resistivity observed in the CIO film may be related to the substitution of indium sites by copper atoms, where they act as acceptor impurities. These acceptor levels introduce holes that compensate conduction-band electrons, thereby reducing the free carrier concentration and increasing the film’s resistivity [48].

In contrast, the reduction in resistivity for the BIO film is consistent with the possible substitution of O^2- in the lattice by Br^–, which may introduce shallow donor levels. These defects may increase the electron density in the conduction band without a substantial increase in ionized impurity scattering, resulting in enhanced conductivity.

The observed enhancement in electrical conductivity for the BIO sample, combined with its high optical transparency, directly influences its overall performance as a transparent conductor. Consequently, the BIO sample achieves the highest figure of merit ($\mathrm{\Phi }$ $\approx$ 1.45 $\times$ 10^-3 $\mathrm{\Omega }$^-1) among all tested films, suggesting that incorporating 4 wt% Br enhances the balance between transparency and conductivity. Although this value remains below those of optimized TCO benchmarks such as indium tin oxide (typically $\mathrm{\Phi }$ $\mathrm{>}$ 3 $\times$ 10^-3 $\mathrm{\Omega }$^-1), it highlights the potential of BIO and underscores the need for further optimization of deposition and doping parameters. It is worth noting that ITO coatings reported in the literature can exhibit sheet resistances as high as $\sim$500 $\mathrm{\Omega }$ while maintaining high optical transparency, depending on the intended use case [49]. In this context, our BIO films are promising transparent conductive coatings and potential candidates for higher-resistance-range transparent electrodes at the materials screening level.

This work compares Cu- and Br-doped In₂O₃ thin films deposited by spray pyrolysis at a single dopant loading (4 wt.%) and one deposition temperature; thus, broader optimization is still required. Future studies should include dopant concentration series and Hall effect measurements to establish concentration–property trends and to separate carrier concentration from mobility effects. Dopant incorporation is inferred mainly from XRD; compositional/chemical-state analysis (e.g., EDS mapping, XPS, and/or SIMS) is needed for direct confirmation, while crystallite sizes from Debye–Scherrer remain approximate without microstrain/instrumental-broadening deconvolution (e.g., Williamson–Hall or Rietveld refinement). Finally, long-term stability and device-level validation were beyond the scope of this study and should be addressed using simple prototype demonstrators and stability testing.