CoIn₂S₄, one of the type–AB ₂ x ₄ (A = Co, Mn, Fe; B = Al, Ga, In; X = O, S, Se), is a ternary compound semiconductor with potential application in optoelectronic devices [1–3]. CoIn₂S₄ is also expected as an interesting material with magnetic properties due to the cobalt constituent elements. It has been reported that the cobalt constituent elements of CoGa₂S₄ has an influence on the magnetic behavior [4]. In addition, the constituent transition metals or the doped transition metals in semiconductors introduce localized states such as sensitizing and luminescent centers in the forbidden band gap. These desirable properties make this compound useful materials for more applications in optoelectronic devices, such as the II_1-x M_x (M = Co, Fe, Mn, etc.) diluted magnetic semiconductors [5–7]. A few studies of CoIn₂S₄ have been made on the structural and magnetic properties of this material [8, 9]. Also, a few information of CoIn₂S₄ has been appeared in the systems of CoxCd_1−x In₂S₄ [10], Co _x Zn_1-x In₂S₄ [11–14] CoGaInS4 [15,16], CoIn₂S₄ – CoIn₂S₄ [17], and CoIn_2-2x Cr_2x S₄ [18]. However, there has been no information on band gap and optical properties of CoIn₂S₄. In addition, there has been no study yet on the influence of cobalt, a constituent element of CoIn₂S₄, on the optical properties of this material. In the previous work [19], we have studied the effects of cobalt constituent element in Co _x Cd_1-x Ga₂Se₄ mixed crystals on their optical properties. In order to further enhance the applicability of CoIn₂S₄, therefore, more information about the influence of cobalt constituent element including band gap of this material is needed. And the spray pyrolysis technique for the thin film growth of the material should be established. Among a variety of methods for thin film growth, the spray pyrolysis technique is used for several compound semiconductors because of a simple and low-cost method enabling large-area production with highly uniform [20].

In the present paper, we report on the thin film growth of CoIn₂S₄ by the spray pyrolysis technique. The grown thin films were characterized by X-ray diffraction and field-emission scanning electron micro-scope measurements. The band gap energy at 292 K was first obtained by optical absorption spectroscopy measurements. Also, we discuss the absorption bands observed in the near-infrared region, which are originated in cobalt ions of the constituent elements of CoIn₂S₄. These absorption bands are analyzed in connection with d-d optical transitions of tetrahedral Co²⁺ ions in the CoIn₂S₄ lattice.

CoIn₂S₄ thin films were grown by the spray pyrolysis method [21]. A spraying solution was prepared by dissolving CoCl₂ (GA, Alfa Products), InCl₃ (5N, Alfa Products) and thiourea (GA, Alfa Products) in 1:1 solvent solution composed of methanol (Merck-Analysis) and distilled water to give 0.2 molar solutions of each solute. Then these solutions were mixed in a volume ratio of 1:2:4, and at the same time an excess thiourea solution of 20 % was added in order to compensate the loss of sulphur caused by vaporization during the deposition. The final mixture of the solutions was magnetically stirred at 70 °C for 30 min. Then this solution has sprayed on a slide glass (Corning-2948) with a fixed spray rate of 6 ml⋅min⁻¹ under ambient air. The substrate temperature was varied from 250 to 320 °C for film deposition.

The structural properties such as surface morphology and layer thickness of the grown thin films were characterized by using field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800), and their crystalline phases and qualities were studied by X-ray diffraction (XRD, X’pert pro MPD) measurements. Also, the band gaps of CoIn₂S₄ and the d-d optical transitions of Co²⁺ ions were investigated by the measurements of optical absorption spectra in the wavelength region of 250 to 3200 nm at 292 K using a UV-VIS-NIR spectropho-tometer (Hitachi, U-3501).

Figure 1 shows the XRD patterns of the CoIn₂S₄ thin films depending on the substrate temperature. The XRD patterns for the thin films grown at 250 °C of Fig. 1(a) shows a broad and weak peak near 2θ angle of 20~30∘, which is considered to be typical of amorphous thin films. As the substrate temperature increases, the XRD peaks become intense. For the thin films grown at the substrate temperature of 320 °C, as shown in Fig. 1(c), the XRD pattern shows a lot of planes (111), (220), (311), (400), (511), (440), (531), (533), and (444) of cubic spinel, except for the diffraction peak with a relatively weak intensity at 2θ = 22.336 originated from In₂O₃. It is in good agreement with that in JCPDS 03-065-5369 for CoIn₂S₄ crystals. The strong and sharp diffraction peaks suggest that the grown thin films were well crystallized. Also, the FE-SEM micrographs of surface and cross-section of the CoIn₂S₄ thin films grown at 320 °C are given in Fig. 2. The result depicts that the morphological features of the CoIn₂S₄ thin films have almost a uniform and homogeneous surface, and a thickness of 1.90 µm.

Figure 1. XRD patterns of CoIn₂S₄ thin films grown at (a) 250, (b) 280, and (c) 320 °C.

Figure 2. SEM micrographs of (a) surface and (b) cross-sectional of CoIn₂S₄ thin films grown at 320 °C.

Figure 3 depicts optical absorption spectrum of the CoIn₂S₄thin films in the wavelength range of 250 to 3200 nm at 292 K. In Fig. 3 we can see some characteritics, i.e., a rapid rise in absorption near the regions of 780 and 2700 nm, and a relatively sharp absorption band in the wavelength range of 600 to 850 nm and a broad absorption band in the wavelength range of 1200 to 2400 nm. Among them, the rapid absorption near the 2700 nm-wavelength region originates in a slide glass used as a substrate, and also the arising of abrupt absorption near the 780 nm-wavelength region refers to the fundamental absorption. And the two absorption bands observed in the wavelength range of 650 to 850 nm and 1200 to 2400 nm are concerned with the d-d transitions of Co²⁺ ions. In order to obtain the band gap of the CoIn₂S₄ thin films, firstly, we examined the absorption spectrum in the wave-length range of 500 to 680 nm near the fundamental absorption edge. However, unfortunately the fundamental absorption is influenced by the d-d transitions of Co²⁺ions. Thus, we extracted the absorption effects due to Co²⁺ions from the optical absorption spectrum near the fundamental absorption edge. The corrected absorption spectrum is shown as a dotted line in Fig. 3, and is used for plotting of the relation between the optical absorption coefficient β and the incident photon energy ℎv. Figure 4 shows the relation (ahv)² for a direct band gap, assuming that the CoIn₂S₄has a direct band gap in consideration of a characteristic of a rapid absorption near the fundamental absorption edge for the case of direct band-to-band transitions. The direct band gap is then obtained by extrapolation of the plot to (ahv)² = 0 in Fig. 4 from the relation (ahv)² = A(hv - Eg) for a direct band gap [22], and is found to be 1.59 eV. It is considered that the obtained band gap energy is a reasonable value considering the absorption edges for the crystals of x = 0.46 in Co _x Zn_1-x In₂S₄ (0.07 ≤ x ≤ 0.46) mixed crystals [14].

Figure 3. Absorption spectrum in the wavelength range of 250 to 3200 nm at 292 K.

Figure 4. Plots of (ahv)² and band gap energy at 292 K.

Now, we discuss the d-d transitions of Co²⁺ ions in CoIn₂S₄. Two absorption bands in Fig. 3 are assigned as the d-d transitions between the ground state ⁴A₂(⁴F) and the excited states ⁴T₁(⁴F), ⁴T₁(⁴P) of Co²⁺ ions, as shown in the schematic energy-level diagram inserted in Fig. 3. The relatively sharp absorption band, which corresponds to the transition ⁴A₂(⁴F) → ⁴T₁(⁴P) in the wavelength range of 600 to 850 nm, shows three split absorption bands as shown in Fig. 5. The triplet structure is assigned as the transitions of Co²⁺ ions from the ground state ⁴A₂(⁴F) to the split states Γ₇+Γ₈, Γ₈, and Γ₆ of the excited ⁴T₁(⁴P) under the spin-orbit interaction. On the other hand, the broad absorption band, which corresponds to the transition ⁴A₂(⁴F) → ⁴T₁(⁴F) in the wavelength range of 1200 to 2400 nm, is observed to be not resolved, and it gives the crystal-field splitting 18 Dq. The Dq-value is then obtained by 348 cm⁻¹, which is close to Dq ≈ 400 cm⁻¹ for tetrahedral Co²⁺ ions [23]. This implies that the observed absorption bands are originated from Co²⁺ ions with the tetrahedral sites of the CoIn₂S₄ lattice. However, our results are compared to those of CoxCd_1−x In₂S₄ by Porta et al. [10]. The authors reported three group bands due to both octahedral and tetrahedral Co²⁺ in reflectance spectra of CoxCd_1−x In₂S₄ (0.0 ≤ x ≤ 1.0) crystals, which is a different appearance from our results due to only tetrahedral Co²⁺. Also, Fiorani et al. [8] reported that the Co²⁺ ions of the spinel CoIn₂S₄ are distributed between octahedral (63 %) and tetrahedral (37 %) sites of the lattice. In our works, nevertheless, a characteristic of absorption bands due to octahedral Co²⁺ was not observed, but only absorption bands due to tetrahedral Co²⁺ were observed. It is possible that these results may be come from an occurrence of an accidental breaking of the true site symmetry of Co²⁺ ions in the CoIn₂S₄. However, its origin is not clear at present. In Table I, our observed absorption bands due to tetrahedral Co²⁺ are summarized.

Table 1 . Observed absorption bands due to the d-d transitions of tetrahedral Co²⁺ in CoIn₂S₄..

Observed absorption bands					Assignments
Notation		nm	eV	cm−1
P	A	702	1.77	14245	4A2(4F)→ 4T1(4P)
	B	754	1.65	13263
	C	796	1.56	12563
F		1596	0.78	6266	4A2(4F)→ 4T1(4F)

Figure 5. Triplet absorption bands of the transition ⁴A₂(⁴F) → ⁴T₁(⁴P).

CoIn₂S₄ thin films have been successfully grown on a slide glass at an optimal substrate temperature of 320 °C and a spray rate of 6 ml⋅min⁻¹ by the spray pyrolysis method. The XRD analysis revealed that the films grown at 320 °C are well crystallized in single-phase cubic spinel structure. The FE-SEM micrographs depicted that the films have a uniform and homogeneous surface.

The nature and the magnitude of band-to-band transitions of the CoIn₂S₄ thin films were determined from the optical absorption spectrum analysis near the fundamental absorption edge in which the absorption effect due to the cobalt ions of the constituent elements was extracted. The result gives the direct band gap of 1.59 eV. Also, the two absorption bands observed in the near-infrared regions, which are originated in cobalt ions of constituent elements, are well assigned as due to the crystal-field transitions of Co²⁺ ions with the tetrahedral sites of the CoIn₂S₄ lattice. However, our results due to only tetrahedral Co²⁺ are somewhat different from those due to both octahedral and tetrahedral Co²⁺ reported in reflectance spectra of CoIn₂S₄ for x = 1.0 of CoxCd_1−x In₂S₄ crystals [10]. The origin of the differences in the results of each other is not clear at present. In order to clarify these results, further crystallographic studies on this material should be needed.

This research was supported by Research Funds of Mokpo National University in 2019.