Fabrication, analysis and characterization of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures deposited on GaN

Abstract

The Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with different Cd concentrations were prepared using spin coating technique on GaN substrate. The structural properties of Cu₂Zn_1−x Cd _x SnS₄/GaN were investigated by X-ray diffraction (XRD) and field emission-scanning electron microscope. XRD studies indicated that kesterite phase of Cu₂ZnSnS₄ and stannite phase of Cu₂CdSnS₄ were formed. The optical properties were studied through photoluminescence technique, and indicated that the band gap shifted as Cd concentration increases from 1.75 eV in Cu₂ZnSnS₄ to 1.65 eV in Cu₂CdSnS₄. The electrical characterization of the Ag/n-GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag diode through current to voltage (I–V) characterization showed the highest photoresponse of (value if any) at Cu₂Zn_0.4Cd_0.6SnS₄ composition.

Introduction

Stoichiometric Cu₂–II–IV–VI₄ (II = Zn, Cd, Hg; IV = Si, Ge, Sn; VI = S, Se, Te) chalcogenide semiconductors have drawn the increasing interest during the past few decades due to their potential applications in photovoltaic [1–4] and thermoelectric [5, 6] devices. Especially, ongoing efforts have been devoted to the Cu₂ZnSnS₄ (CZTS) solar cells in view of their power conversion efficiencies, even as high as 12.0 % [3]. The fabrication techniques for CZTS thin films become flourishing correspondingly, e.g. sputtering [7], thermal evaporation [8, 9], chemical vapour deposition [10], atomic layer deposition [11], electrodeposition [4], spray coating [2] and spray pyrolysis [12, 13].

Theoretical calculations indicated that the best fit with solar spectrum and the optimal band gap for absorber materials is around 1.4 eV. To reach the ideal band gap for CZTSSe-based single-junction solar cells, the ratio of S/Se is modified in several studies [14–16]. The synthesis of Cu₂Cd _x Zn_1−x SnS₄ (0 ≤ x ≤ 1) (CZCTS) alloy by sol–gel method has been accomplished [17] to achieve the goal of bandgap engineering in Cu₂ZnSnS₄-based solar cells. Ibraheam et al. [17] have reported that the optical band gap of CZCTS alloy can be also modified continuously from 1.55 to 1.09 eV as Cd varies from 0 to 1.

The low-cost Cu₂CdSnS₄ nanocrytal thin film with a stannite structure has been successfully fabricated by Zhao et al. [18]. The selenized Cu₂CdSn(S,Se)₄ thin film shows large densely packed grains and has a suitable bandgap value of 1.01 eV. The Cu₂CdSn(S,Se)₄ thin-film solar cell with a proof-of-concept power conversion efficiency of 3.1 % was fabricated. Recently, Guo et al. [19] have fabricated Cu₂ZnSnS₄ (CZTS) thin films by sulphurization of direct current and pulse reverse co-electrodeposited CZTS precursors on Mo-coated glass substrates. The CZTS thin film synthesized using pulse reverse co-electrodeposition exhibits homogeneous and large grains, Cu-poor and Zn-rich compositions and free of Cu₂S secondary phase compared with that obtained from direct current co-electrodeposition. By using pulse reverse co-electrodeposition, the performance of pen junction is improved with low diode quality factor (A) and reverse saturation current density (J ₀), and the CZTS device first reaches an efficiency of 6.28 % (V _oc = 609 mV, J _sc = 18.4 mA/cm², FF = 56.1 %) threshold, which is higher than that of 4.69 % (V _oc = 561 mV, J _sc = 16.4 mA/cm², FF = 50.9 %) using direct current co-electrodeposition. This result offers a novel research direction for preparing high-efficiency CZTS thin-film solar cells.

Till now, there are no reports on the optical and morphological properties of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures deposited on n-type GaN substrates; therefore, in this study, high-quality Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures were deposited on GaN substrates by spin coating technique to study the structural properties given by X-ray diffraction (XRD) and field emission-scanning electron microscope (FE-SEM), and optical properties of Cu₂Zn_1−x d _x SnS₄ quinternary alloy nanostructures by photoluminescence (PL), in addition to investigating the electrical properties of Ag/n-GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag diode at x = 0, 0.6, 1 for photodetector applications through (I–V) characterization.

Experimental process

Spin coating technique is used to deposit the Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures onto GaN (0001) substrate. First, a solution of Cu₂Zn_1−x Cd _x SnS₄ precursors was prepared from copper(II) chloride dihydrate, zinc(II) chloride dihydrate, tin(II) chloride dihydrate thiourea, 2-methoxyethanol (2-metho) and monoethanolamine (MEA). The 2-metho and MEA were used as solvent and stabilizer, respectively. The molar ratio of MEA to solution of Cu₂Zn_1−x Cd _x SnS₄ was at 0.5. The precursor solution (0.3 M) was stirred at 50 °C for 3 h to completely dissolve the metals during stirring, and after adding thiourea, the milk solution became yellow and transparent. The molar ratio of Cu, (Zn + Cd), Sn and S in the solution was 2:1:1:4. To obtain solutions with different Cd concentrations (x), the molar ratio of Cd to (Zn + Cd) in the solution varied to give x values of 0, 0.6, 0.8 and 1. The precursor solutions were mixed based on the required ratio of the following formula Cu ²⁺₂ (Zn_1−x Cd _x )²⁺ Sn⁴⁺ S₄ ⁸⁻. This means that the solution consisted of two parts of Cu and one part of (Zn,Cd) depending on the value of x, one part Sn and four parts S. The solution was then dropped onto a GaN (0001) substrate (20 mm × 20 mm × 1 mm) rotating at 2500 rpm for 30 s. After deposition by the spin coating, the nanostructures were dried at 250 °C for 80 min on a hot plate. The coating and drying processes were repeated seven times to obtain a film of 1 µm thickness.

The crystal structure of the Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures was examined by X-ray diffraction (XRD; PW 1710 X-ray diffractometer, Phillips, USA) with Cu Kα radiation (λ = 1.54 Å). The optical properties were measured at room temperature using UV–Visible spectrophotometry (Lambda 950, Perkin Elmer, USA) and photoluminescence spectroscopy (Jobin Yvon model HR 800 UV system, Jobin Yvon, USA) at room temperature using a He–Cd laser (λ = 325 nm). Surface morphology and grain size were investigated by field emission-scanning electron microscopy (FE-SEM) (NOVA NANO SEM 450, USA). The topography of the films was characterized by atomic force microscopy (AFM) (SII SPI 3800N Probe, Seiko Instruments Inc., USA). Thickness measurement was carried out using the weight method given by

$$ t = \frac{\Delta m}{A \times \rho }, $$

(1)

where t is thickness, ∆m is a difference of substrate weight (substrate after deposition—substrate before deposition), A is area of sample and ρ is the density of deposited material. Finally, heat treatment was conducted in an elevator furnace under N₂ gas flow (5 % gas atmosphere) for 1 h at 300 °C, and after annealing, the samples were cooled to below 40 °C in the chamber. After that, Ag metal contacts were formed on Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with Cd concentration equals 0, 0.6, 1. PVD-HANDY/2STE (Vaksis Company, USA) vacuum thermal evaporation in the pressure of 4.5 × 10⁵ Torr was used for deposition on GaN, and the contacts were formed in the form of zig-zag with 5 mm length and 100 nm thickness as shown in Fig. 1. The contact area of the diode was found to be 3.14 × 10² cm². For the current to voltage (I–V) characterization, the fabricated device was connected in parallel with the Kiethly (2400 source meter, USA). The reading was recorded from −6 to 6 V. For current to time (I–t) analysis, the device was connected in series with the multimeter and the value was recorded by switching the LED on and off. The wavelength and power of the LED used for conducting the experiment were 490 nm wavelength and 3 mW power, respectively.

Figure 1

p–n injection

Results and discussion

The typical XRD pattern of the Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures is shown in Fig. 2. It can be seen that the major diffraction peaks at 2θ = 17.46°, 22.93°, 28.26°, 34.39°, 39.85° and 44.13° can be attributed to the (101), (110), (112), (004), (102) and (213) plans of kesterite phase of Cu₂ZnSnS₄ and stannite phase of Cu₂CdSnS₄. Owing to the presence of four or five elements in the materials, there was a possibility for the secondary phases like SnS, attributed to the (110) was observed at 2θ = 41.99° corresponding to JSPDS cards (00-001-1244). According to the Debye–Scherrer formula, (112) peak was calculated to increase the accuracy, and the average crystallite size of the CZCTS nanostructures can be presented in Table 1. The relatively intense and sharper (112) and (200) diffraction peaks were observed for nanostructures deposited at x = 0.6 and 1. However, when the concentration of Cd was further increased to 0.6 and 1, the intensity of diffraction peaks increases, which indicates that the crystallinity increase with increasing in concentration of Cd. The peak (112) was shifted to the lower angle side with increasing Cd concentration in the CZCTS solid solutions, which was attributed to the increasing lattice constant. This was due to the radius of Cd ion (1.53 Å) which is larger than that of Zn (1.33 Å) as supported by previous work [17]. The simplest possibility is that Cd substitutes other metals at their sites in crystal lattice of Cu₂Cd _x Zn_1−x SnS₄ quinternary alloy nanostructures. As the theoretical calculated substitution energies of Cd atoms at Cu, Sn and Zn atom sites in CZCTS lattice are E _sub (Cd_Cu) = 0.69 eV, E _sub (Cd_Sn) = 1.07 eV and E _sub (Cd_Zn) = 0.53 eV [20], the most likely is the isoelectronic substitution of Cd at the Zn site. Lattice constants a and c were calculated from XRD data for the (112) plane, which are given in Table 1.

Figure 2

XRD patterns of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with different Cd concentrations (x = 0, 0.6, 1)

Table 1 The structural properties of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures using XRD for different values of x

Figure 3 shows FE-SEM micrographs of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with Cd concentration equals (a) 0, (b) 0.6 and (c) 1. From FE-SEM images, the obtained Cu₂Zn_1−x Cd _x SnS₄ layer on GaN substrate was found to be homogeneous with good adherence to the substrate. Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures are clearly discernible, indicating an isolated grain growth at the surface. With the addition of Cd, the shape of the grains changed significantly (Fig. 3). The grain shape becomes rounder as well as bigger for 0.6. The presence of nanowire rod shaped when x = 1 might lead to the formation of more densely packed nanostructures with less porosity.

Figure 3

FE-SEM images of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures for different Cd concentrations a 0, b 0.6, c 1

The elemental analysis of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with different Cd concentrations was investigated using EDAX and their compositions are illustrated in Fig. 4. The presence of Cu, Zn, Cd, Sn and S elements is confirmed by EDAX analysis. Furthermore, it is also interesting to note that the composition of Zn in Cu₂Zn_1−x Cd _x SnS₄ varies upon increasing the Cd concentration. The nanostructures were copper poor and zinc rich at x (0, 0.6) concentrations [23, 24].

Figure 4

EDX of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures at different Cd concentrations x = 0, 0.6, 1

Figure 5 represents the three-dimensional topographic AFM images Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with Cd concentration equals 0, 0.6 and 1. For a detailed study on average roughness properties of Cu₂Zn_1−x Cd _x SnS₄ with Cd concentration, line profiles were recorded. From the line profile analysis, the calculated average roughness values are 3.56, 4.356 and 5.167 nm for x = 0, 0.6, 1, respectively. Minimum average surface roughness value is 3.56 nm. It was found that the incorporation of Cd results in the increasing of surface roughness.

Figure 5

3D AFM images of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with different Cd contents a 0, b 0.6, c 1

PL spectra of CZCTS quinternary alloy nanostructures with different Cd concentrations measured at room temperature are shown in Fig. 6. PL spectra consist of one broad asymmetric PL band at 1.75 eV in Cu₂ZnSnS₄ and at 1.65 eV in Cu₂CdSnS₄ as given in Table 1. A shift of PL band of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures towards highest wavelengths with increasing Cd concentration is observed in the region 0 ≤ x ≤ 1. This shifting is due to the substitution of Zn atoms with Cd atoms to produce a lower energy gap [25, 26].

Figure 6

The photoluminescence (PL) of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with different Cd concentrations

Figure 7 shows that the I–V characteristics from −6 to 6 V of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with Cd concentration values x = 0, 0.6, 1 measured in the dark and under illuminating were 490 nm and 3 mW. The difference between I _dark and I _ph is increased in Cu₂Zn_0.4Cd_0.6SnS₄ heterojunction compared with Cu₂ZnSnS₄ (x = 0) heterojunction. The photosensitivities of the heterojunction were calculated by

Figure 7

I–V characteristics in dark and under illuminating were 490 nm and 3 mW for Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures with Cd concentration values x = 0, 0.6, 1

$$ S = \frac{{I_{\text{ph}} - I_{\text{dark}} }}{{I_{\text{dark}} }} \times 100\;\%. $$

(2)

Based on the I–V curve (Fig. 7) and Eq. (2), the I _dark and I _ph and S _ph are listed in Table 2. Figure 7 shows that the photosensitivity increases with increasing Cd concentration to 99.9982 for x = 0.6 and decreased to 99.4826 for x = 1. There is no significant difference in photosensitivity when the concentration of Cd increases.

Table 2 Photoresponse properties of the Ag/GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag heterojunction at wavelength 490 nm

To confirm the better performance of the heterojunction and reproducibility of Ag/GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag device, it was examined by cyclically switching the white light on and off. Figure 7 shows the I _ph as a function of time intervals (I–t) of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures at x = 0, 0.6, 1. I _ph sharply increases/decreases to reach the maximum/minimum under 490 nm light on/off. The Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures at x = 0.6 (Fig. 7b) show better performance than at x = 0, 1 (Fig. 8). The result indicated an improvement with increasing Cd concentration, x = 0.6. The I _ph/I _dark ratio was 2.6, 3.6 and 3 for x = 0, 0.6 and 1, respectively, as given in Table 3. The calculated response time (t _Res) and decay time (t _Rec) using 490 nm and 3 V show that all values decreased with increasing of Cd concentration indicating an improved photoresponse. The performance of Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures shows t _Res (0.02654) and t _Rec (0.01064) for x = 0.6 (Table 3).

Figure 8

Photocurrent response spectra of Ag/GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag heterojunction at different Cd concentrations, x = 0, 0.6, 1, under illuminating light were 490 nm and 3 mW turned on and off repeatedly

Table 3 Response and decay time corresponding to I _ph/I _dark ratio of Ag/GaN/Cu₂Zn_1−x Cd _x SnS₄/Ag diode

Conclusion

The Cu₂Zn_1−x Cd _x SnS₄ quinternary alloy nanostructures were prepared on n-GaN substrate using spin coating technique. XRD results revealed that the as-prepared CZCTS exhibits impurity-free kesterite phase of Cu₂ZnSnS₄ and stannite phase of Cu₂CdSnS₄ in polycrystalline nature with the crystallite sizes between 44.69 and 58.22 nm. The band gap was found to decrease with increasing Cd concentrations. It was found that the incorporation of Cd results in an increasing of surface roughness of Cu₂Zn_1−x Cd _x SnS₄ nanostructures from 3.56 to 4.356 nm. Under illumination densities, the n-GaN/CZCTS junction exhibited good photoconductivity. The I _on/I _off ratios confirmed the photosensitivity of the produced structure. Photocurrent measurements showed the highest photoresponse for Cu₂Zn_0.4Cd_0.6SnS₄ quinternary alloy nanostructures.