Synthesis, structure, I–V characteristics, and optical properties of chromium oxide thin films for optoelectronic applications

2.1 Precursors and preparation

Chromium(ii) acetate (CrA) [Cr₂(CH₃CO₂)₄·2(H₂O), 376.2 g mol⁻¹, LOBA Chemie] and chromium nitrate (CrN) [Cr(NO₃)₃·9H₂O, 400.15 g mol⁻¹, LOBA Chemie] were used as the Cr source. 2-Methoxyethanol (M) [C₃H₈O₂, 76.1 g mol⁻¹, Merck] and absolute (Et) [C₂H₅O, 46.1 g mol⁻¹, Merck] were used as solvents. Monoethanolamine (M) [C₂H₇NO, 61.1 g mol⁻¹, Merck] and AA [C₂H₄O₂, 60.05 g mol⁻¹, Merck] were used as stabilizing agents. These precursors were used as received. Three different solutions with a 0.35 M concentration were prepared as follows: solution (1) was prepared by dissolving a certain amount of CrA in 10 mL of 2-methoxy ethanol. After 10 min of magnetic stirring at a speed of 1,200 rpm and heating at 60°C, a few drops of monoethanolamine were added to make the solution clear. The stirring was continued for 2 h at RT. Solution (2) was prepared by dissolving a similar amount of CrA in 10 mL of ethanol. For the solutions (1) and (2), the required mass of CrA is calculated using the equation: Mass_(CrA) = 0.35 × 10 × 10^–3 × 376.2 = 1.317 g. The solution (3) was prepared by dissolving the required mass CrN (Mass_(CrN) = 0.35 × 10 × 10^–3 × 400.15 = 1.401 g) in 10 mL of ethanol, and after 10 min of stirring, a few drops of AA were added to make the solution clear. The speed, time, and temperature during the magnetic stirring process were the same for the three solutions. The final volume of the three solutions was the same. The solution was stirred continuously for 2.0 h at RT. Next, the solutions were aged at RT for 24 h. For simplicity, the solutions (1), (2), and (3) and the resultant films were named CrA/M/M, CrA/Et, and CrN/Et/AA, respectively.

Before the deposition process, the glass substrates (1 × 2 × 0.15 cm³) were cleaned in an ultrasonic bath using a detergent, acetone, and ultraclean water for 5 min for each step. An air gun was used to dry the substrates. The solutions (1), (2), and (3) were deposited using a spin coater operated at 2,500 rpm for 45 s. After each layer, the formed film was pre-heated on a hotplate at 200°C for 5 min. The deposition and drying procedure was repeated five times to increase the film thickness. Finally, the films deposited on the substrates were annealed at 500°C for 2.0 h in a ceramic furnace. After returning to RT, the films were put in plastic Zipper bags to avoid moisture.

2.2 Characterization and devices

A PANalytical X’Pert Pro (high-resolution), Holland, XRD system was used to study the influence of precursors on the structure and crystallite size of spin-coated Cr₂O₃ films. The device was operated in the 2θ range of 5°–80°; applying a Cu source provides a K _α radiation of wavelength λ ∼1.541 Å. The Cr₂O₃ film’s surface morphology and cross-sectional investigations (thickness evaluation) were performed using FE-SEM (ZEISS-SUPRA-55-VP). FTIR spectra were gathered using a spectrophotometer of model Bruker-vertex 70 in the wavenumber range of 400–4,000 cm⁻¹. Direct current–voltage (IV) characteristics were determined using the two-point probe technique using Keithley 2400. Ultraviolet-visible-near infrared (UV-Vis-NIR) optical spectra (transmittance and reflection) were recorded using the spectrophotometer (Shimadzu; UV-3600/UV-Vis-NIR) in the λ range of 200–2,000 nm, in normal incidence mode.

3.1 XRD, FTIR, and FE-SEM analyses

Figure 1(a) illustrates the influence of precursor materials on the crystal structure (XRD patterns) of the deposited films. The charts of all films contain peaks at 2 θ = 24.48 ° , 33.57 ° , 35.97 ° , 41.49 ° , 50.05 ° , 55.07 ° , 63.49 ° , 65.25 ° , and 73.23 ° . The corresponding Miller indices are (012), (104), (110), (113), (024), (116), (214), (300), and (119), respectively. The charts represent the fingerprints of the crystalline Cr₂O₃ of the rhombohedral structure, which belongs to the R-3c space group. The lattice parameters a (=b) = 4.959 Å and c = 13.594 Å are of pure Cr₂O₃ (Eskolaite phase) without any traceable impurities. The obtained data are consistent with the data in JCPDS card no. 01-072-4555 for crystalline Cr₂O₃. The CrA/M/M film displays higher (intense) peaks, and the XRD peaks of CrN/Et/AA have a lower intensity. The main reasons for this difference are the used precursors, solvents, and stabilizing agents. These change the reaction environment, the viscosity of the solution, and the size of the grains [21]. Saadi et al. [24] found that the crystallinity of the spray-deposited Cr₂O₃ on glass slides using chromium chloride is higher than that prepared from chromium nitrate. In addition, the CrA/M/M film may have a reasonable thickness compared to the other films. Tsegay et al. [3] reported that the deposited films at 10³ and 1.2 × 10³rpm (which certainly have a lower thickness) displayed diffraction peaks of less intensity compared to those prepared at 600 and 800 rpm or normal casting. The crystallite size ( C s ) of the films was derived using the well-known Scherrer’s equation [23]:

where β and θ are the full-width of the XRD peaks at their half-maximum intensity and the Bragg’s angle, respectively. The average C s values, given in Table 1, were calculated taking into account the three most intense peaks: (104), (110), and (116).

Figure 1

(a) XRD patterns and (b) FTIR spectra of Cr₂O₃ thin films.

For further structural and compositional investigations of the spin-coated Cr₂O₃ films, FTIR spectroscopy was applied to obtain the transmittance spectra as shown in Figure 1(b). The two bands at 490 cm − 1 and 765 cm − 1 are attributed to Cr – O bending and stretching modes of vibration, respectively, confirming the presence of Cr₂O₃ [25,26]. Similarly, the spectrum of Cr₂O₃ NPs derived by dissolving CrO₃ in an alcoholic medium showed two bands at 510 and 720 cm − 1 arising from the vibrations in Cr₂O₃ of the rhombohedral structure [15]. The wide and intense band at 889 cm − 1 could be assigned to Cr ═ O vibration [27]. Yasmeen et al. [18] detected the vibration of Cr═O at 836–901 cm − 1 as a band or a shoulder to Cr – O at 610 cm − 1 in the spectra of bio-synthesized Cr₂O₃ NPs. According to R-Nasarabadi et al. [28], the thermal decomposition of chromium carbonate at 550°C to chromium oxide was verified through FTIR spectra, where intense bands at 556 and 622 cm⁻¹ were attributed to the vibration of octahedral CrO₆ of Cr₂O₆ and CrO₂, respectively. Also, they ascribed the two bands at 880 and 946 cm⁻¹ to the Cr₂O₃ phase. These results agree well with the published data, where it is well known that metal (Cr)–oxygen (O) vibration bands occurred below 1,000 cm − 1 [16,29].

In addition, the weak band at 1,030 cm − 1 may arise from Cr – O – Cr vibration [27]. The mall bands at 1,540–1,640 cm − 1 could be attributed to water (adsorbed moisture) at the surface of the film from the surrounding area [26]. Ashika et al. [16] detected a peak at about 1,120 cm − 1 , assigned to Cr − O stretching vibration, and some minor peaks in the range of 1,450–4,000 cm − 1 , which were attributed to the adsorption of water and some carbonates at the surfaces of Cr₂O₃ NPs. The CrA/M/M film displays more intense (deep) bands, whereas the CrN/Et/AA film has a less intense peak. This may be due to the amount of Cr₂O₃ material (film thickness) and the films’ crystallinity. These results indicate the purity of Cr₂O₃ thin films and are consistent with XRD results.

Figure 2(a)–(c) depicts the FE-SEM surface pictures of the Cr₂O₃ films prepared using different precursors. In addition, Figure 2(a′)–(c′) shows the cross-section of the films, and the measured thickness is listed in Table 1. The CrA/M/M film is crack-free and consists of closely spaced spherical particles of approximately the same size. Using ethanol as a solvent without a stabilizing agent resulted in smaller particles, as shown in Figure 2(b), compared to the particle size, as shown in Figure 2(a). This note is consistent with the C s values deduced from XRD data. The particles look like combinations separated by a large number of voids. The absence of a stabilizing agent may lead to less viscous materials. According to Tsegay et al. [3], low-viscous solutions are subject to a high rate of ejection and evaporation from the substrate surface during the spin-coating procedures. This results in films with less thickness, as shown in Figure 2(a′) and (b′).

Figure 2

FE-SEM images and the cross-section for thickness determination for (a, a′) CrA/M/M, (b, b′) CrA/Et, and (c, c′) CrN/Et/AA.

The acetate salt (CrA), used in the preparation of Cr/M/M and Cr/Et films, hydrolyzes easily in the solvent medium and gives soluble products that can be decomposed to volatile materials under upon heating [23]. The CrN/Et/AA film appears rough and has a different morphology in which its particles agglomerate and appear with sizes in the range of 15 − 75 nm. In the same way, Saidani et al. [30] found that sol–gel dip-coated ZnO using zinc acetate, nitrate, and chloride had sizes of 20.5 nm (180 nm), 29.6 nm (275 nm), and 47.1 nm (350 nm), respectively. XRD results showed that the dip-coated sodium–copper chlorophyllin film had a C _s value of 13.28 nm, and FE-SEM results showed that the grains were 44 nm in size [31]. The discrepancy between the size obtained from XRD and that given by FE-SEM is due to the distinct nature of the measurements. XRD is an indirect characterization technique that gives the size of the crystal, while FE-SEM is a direct characterization technique that gives the observable size or the distance between grain boundaries. Each grain, or particle, may consist of several grains [32]. The same thing was found for Cr₂O₃ NPs prepared using a sol–gel route; the C _s was about 46 nm, and the TEM and FE-SEM measurements showed that the size was between 10 and 110 nm [33].

3.2 IV curves

The I–V characteristic curves of the Cr₂O₃ thin films prepared by sol–gel route and spin-deposition using different sources are shown in Figure 3. The linear behavior indicates a good ohmic feature. The sheet resistance ( R sh ) was determined from the curve’s slope using the following equation [11]:

Figure 3

IV characteristic curves for the Cr₂O₃ thin films spin-coated using different precursors.

The R sh values are shown in Table 1. The films exhibit R sh values in the range of 10.66–22.7 k Ω . The lower R sh value of the CrN/Et/AA film is owing to the larger grain size, as revealed in Figure 2(c), where the charge carrier mobility increases with increasing grain size [34]. A lower R sh value of 2.02 kΩ was reported by Saadi et al. [12] for a 298 nm-thick Cr₂O₃ film spray-deposited on a glass substrate.

3.3 Optical analyses

The optical transmittance (T%) in the λ range of 200–1,900 nm is shown in Figure 5(a). The films are highly transparent in the Vis–NIR regions. The M/M/Cr-acetate film has 76–88% transmittance, while the two other films have a relatively lower transmittance (65–86%) at λ ≥ 400 nm. Similarly, T% of the spray-deposited Cr₂O₃ film prepared from chloride salt was about 75% in the visible range of the spectra; however, T% of the film prepared from CrN was lower (∼60%) [24]. In the UV range, T% increases sharply and reaches 70% for CrA/M/M film and about 50% for the other two films at the beginning of the visible region ( λ = 370 nm), as shown in the inset of this figure. Two small humps centered around 0.45 and 0.6 µm arising from the d–d electronic transition of the 3d orbital of Cr and owing to ⁴ A 2 g → ⁴ T 1 g and ⁴ A 2 g → ⁴ T 2 g , respectively [10,29].

To correlate the optical and electrical features of the films, the new figure of merit ( ∅ HR ) was introduced to determine the quality of the transparent and conductive films to evaluate their performance for solar cells and photovoltaic applications and can be determined using the following equation [35]:

where HR stands for high resolution, and s can take 10, 12, and other values, and R st is the sheet resistance (Section 3.2), and the transmittance T is taken at λ = 550 nm. The calculated ϕ HR values, for s = 10, are listed in Table 1. The CrN/Et/AA exhibits the highest ϕ HR of 0.305 Ω − 1 . This value is smaller than that obtained for Al-doped ZnO films [35].

The optical band gap ( E g ) of the films was determined using Tauc’s equation:

where α is the absorption coefficient α = 1 d ln ⁡ 1 T , d is the film thickness, h υ is the photons’ energy ( h υ = 1,242 λ ( nm ) ( eV ) ) , D is a constant, and m = 1 2 and 2 for direct and indirect transition, respectively. Figure 5(b) shows the ( α h υ ) 2 vs h υ plots for the Cr₂O₃ sol–gel prepared using different precursors. The E g of the films was obtained by extrapolating the linear region of these plots at ( α h υ ) 2 = 0, which we have included in Table 1. The Cr₂O₃ films have E g in the range of 2.6–2.9 eV. The improved crystallinity of the CrA/M/M film, as revealed from XRD data, results in higher E g . Crystallinity deterioration means a greater defect and a higher concentration of the charge carriers. The obtained E g and R sh are consistent with each other.

A similar result was found by Fan et al. [22] for the PLD Cr₂O₃ thin films, which exhibited T% in the order of 53–90% in the UV and visible regions and E g value of 2.90 eV. The spray-deposited Cr₂O₃ film exhibited E g of 2.92 eV [11]. Also, Saidani et al. [30] found that the sol–gel dip-coated ZnO thin films prepared using zinc acetate, nitrate, and chloride salts had E _g values of 3.35, 3.28, and 3.36 eV, respectively. Ashika et al. [16] reported E g values in the range of 1.82–2.3 eV for Cr₂O₃ prepared by co-precipitation, ball milling, hydrothermal, and thermal heating at different synthetic conditions. Furthermore, Bhardwaj et al. [29] discovered that the co-precipitated Cr₂O₃ NPs, measuring C s = 29 nm, had an E g value of approximately 2.96 eV. Moreover, the RF-sputtered Cr₂O₃ exhibited E g of 3.02 eV after annealing at 550°C [6]. In contradiction to these results, the PLD Cr₂O₃ film, C s ∼38.2 nm, thickness of 108 nm, T% of about 72%, showed E g of 3.61 eV [10]. Another research group found that the ALD Cr₂O₃ film on Si substrate has indirect and direct E g of 3.0 and 3.49 eV, respectively [5]. In addition, the spray-deposited Cr₂O₃ film, with a thickness of 298 nm, exhibited an E g of 3.52 eV [12]. Moreover, the bio-synthesized Cr₂O₃ NPs, which were produced using either ethanolic or aqueous leaf extract, demonstrated E g values in the range of 3.02–3.03 eV or 3.24–3.29 eV, depending on the pH value of the solution [18]. These results illustrate that the T% and E g values of the Cr₂O₃ films are sensitive to the preparation condition and the used precursor materials.

3.4 Films’ optical constants and conductivity

The interaction of photons with the films involves absorption, dispersion, scattering, and attenuation. Considering the complex f-dependent function n * = n + ik , where n (the real part) is the refractive index. The film’s n value provides information about photon interaction and film composition. It is a fundamental design parameter for various optoelectronic devices. The imaginary part (k) is the absorption index, which is associated with α , and the n values can be determined using the following formula [11,36]:

where R is the reflectance. Figure 4(a) and (b) shows the dependence of k and n on the applied λ . Along with the small bands at 450 and 600 nm that come from the Cr³⁺ species in octahedral symmetry, Figure 4(a) shows a sharp and strong band around 280 nm in the k curves. Moreover, the shoulder at about 375 nm suggests the presence of a Cr^4+/Cr⁶⁺ oxidation state with tetrahedral coordination [16,37]. The k values begin to increase with λ at λ > 550 nm, and the spectra of CrA/M/M and CrA/Et coincide, whereas the CrN/Et/AA film exhibits relatively higher values. This is due to the film’s morphology, decreased film crystallinity, and the existence of a large number of charge carriers that absorb the coming photons. These results illustrate the possibility of developing such a composition of sensing applications in the infrared (IR) regions.

Figure 4

Optical constant of Cr₂O₃ thin films: (a) absorption index and (b) refractive index distribution; the inset show the reflection (R%) of the films.

The inset of Figure 4(b) displays the measured R (%) values. Table 1 lists the n values for the films at 550 nm. The n values decrease with increasing λ . This takes place at λ > 525 nm for CrA/M/M and CrA/Et films, and at > 675 nm for CrN/Et/AA film, and this is attributed to the normal dispersion. The incident photon’s equating with the plasma frequency may have caused the highest n values at 675 and 525 nm [11]. The higher n values for CrA/Et and CrN/Et/AA films, in comparison with CrA/M/M film, may be attributed to the increased numbers of traps and disorders that are acting as scattering centers. This resulted in a higher level of photon scattering [11] and an observed decrease in T (%), as shown in Figure 5(a). This also agrees with the XRD results. In addition, the n values are inversely proportional to the E _g values; for example, the CrA/M/M film has a higher E _g value and a lower n value. Different composite materials showed this behavior [36,38].

Figure 5

(a) UV–Vis–NIR transmittance and (b) Tauc’s plots for band gap determination of Cr₂O₃ thin films spin-deposited on glass substrates.

The polarizability of the Cr₂O₃ films is in relation to the density of states inside the forbidden E _g and can be described in terms of the function: ε * = n * 2 = ( n 2 − k 2 ) + i ( 2 nk ) . The real part ( n 2 − k 2 ) is termed the real dielectric constant and represents the impedance to light speed in Cr₂O₃ films. The fictional dielectric constant (2nk) indicates the energy absorbed through dipole motion in the material [23]. Figure 6(a) and (b) displays the λ -dependent plots of ( n 2 − k 2 ) and (2nk) for the spin-coated Cr₂O₃ thin films. The values of ( n 2 − k 2 ) decrease with λ , while the (2nk) increase with λ for all thin films, which mirrors the decrease of n and the increase of the absorbed energy by the dipoles. Compared with other films, CrA/M/M exhibits the lowest values of both ( n 2 − k 2 ) and 2nk. Each film exhibits ( n 2 − k 2 ) values that are higher than the 2nk levels. This leads to lower values of tan δ , where ( tan δ = 2 nk ( n 2 − k 2 ) ) , Figure 6(c), which indicates the low-loss dielectric feature or lower energy dissipation in the prepared films [11]. The low tan δ value of the CrA/Et film signifies the film’s favorable optical response. This film can be developed for energy storage applications.

Figure 6

Optical constant of Cr₂O₃ thin films obtained from different precursors: (a) real dielectric and (b) fictional dielectric constants and (c) tan δ = fictional part real part .

Finally, the optical conductivity σ op was calculated using the following equation [39]:

where c is the light speed. The obtained σ op values are shown in Figure 7. The σ op is very low (<1 × 10¹⁴ S) at the lower photon energies, h υ < 3.2 eV. Similar findings were reported for NiO and SnO₂ thin films [39]. The sharp increase in σ op at higher energies ( h υ ≥ 3.6 eV), is owing to the ability of the UV photons to excite the electrons in the Cr₂O₃ films to contribute to the conduction process. The CrA/M/M film exhibits a low σ op till h υ ∼4.0 eV; then, it displays relatively higher σ op as shown in the inset of Figure 7. Therefore, σ op can be tuned by the Cr₂O₃ film’s composition.

Figure 7

Optical conductivity of sol–gel spin-deposited Cr₂O₃ films on glass substrates.