Cu2ZnSnS4 formation by laser annealing in controlled atmosphere

Laser annealing is an attractive process to form high-quality semiconductor films because of localized annealing area and short annealing time. In a previous study, a Cu2ZnSnS4 (CZTS) polycrystalline semiconductor film was realized using laser annealing in air as a light absorption layer for solar cells, although the crystallization was not sufficient in comparison with CZTS formed by the conventional thermal sulfurization process. In this study, we demonstrate a newly developed gas-atmosphere-controlled laser annealing system. A Cu–Zn–Sn–S-based precursor was formed, followed by laser annealing of the system. Laser annealing in air, Ar, and 5% H2S/Ar gas was performed to investigate the influence of the gas species on the crystallization of the precursor. A 5% H2S/Ar atmosphere promoted the crystallization of CZTS with the suppression of S desorption and Cu sulfide formation, while air and Ar atmospheres allowed the formation of Cu sulfide.


Introduction
Compound thin-film semiconductors have a strong lightabsorption coefficient, which make them attractive as lightabsorption layers for solar cells.High-performance solar cells are indispensable for the sustainable development of societies.The safety of the constituents should be considered because they are potentially harmful to humans during production or wastage.Moreover, scarcity is considered because its distribution has a significant influence on production stability.From the viewpoint of safety and Earth abundance, Cu-Zn-Sn-S based compound semiconductors are attractive as light absorption layers for solar cells.][3][4][5] These characteristics contribute to the expected high energy-conversion efficiency when applied to solar cells.However, the reported energy conversion efficiency remains 11.4%. 6)The reason for this gap between the expected performance and the actual demonstrated cell performance is often considered to be the short minority carrier lifetime and low open circuit voltage. 7,8)In general, the CZTS formation process consists of two steps.][11][12] In the sulfurization process, an amorphoustype precursor changes to a polycrystal known as a kesterite structure.The crystal grain size is predominantly determined by the sulfurization process.][15] However, larger crystal grains are thought to be suitable to suppress carrier recombination because of their smaller grain boundaries.][18] Mo is a high-mp metal, and its thermal stability against the following processes, including CZTS formation, is suitable for solar cell processes.However, CZTS often peels off from the Mo surface during sulfurization or post processing because of the difference in thermal expansion coefficient between Mo and CZTS, or from volume expansion when the precursor changes to a polycrystalline structure with the incorporation of sulfur from the atmosphere (H 2 S or S) in the sulfurization process. 19)This severely degrades the performance of solar cells.
In this study, we suggest laser annealing instead of the conventional thermal sulfurization process to enhance crystallization. 20)This approach can make the thermal process design of solar cell manufacturing more flexible because annealing is limited to a very local area for a very short time.Polycrystalline CZTS formation is demonstrated by laser irradiation of the precursor and CZTS solar cells without the conventional sulfurization process.However, crystal growth is not sufficient compared to the conventional thermal sulfurization process.However, the influence of the atmosphere on laser annealing is not clear.Laser annealing in air should lead to oxidation of the precursor or desorption of constituents such as sulfur.In this study, the influence of the atmosphere during laser annealing is investigated to realize a practical laser annealing process for high-quality CZTS compound thin-film solar cells.

Experimental methods
SLG was used as a substrate with a size of 2.5 × 2.5 cm 2 .The SLG was treated by wet cleaning using detergent, followed by deionized water cleaning with a spin dryer, and was finally treated by UV light treatment.Then, a 200 nm thick CZTS precursor was formed by RF sputtering (50 W, Ar 20 standard cubic centimeters per minute (sccm), 0.4 Pa 30 min) using a Cu-Zn-Sn-S based sintered target.][23] One of the precursors was heat-treated at 560 °C for 60 min at a 5% H 2 S/N 2 flow rate of 20 sccm at 1 atm to form the precursor CZTS polycrystal.This process is the "sulfurization process."CZTS polycrystal was formed from the precursor, and the formed sample was a conventional CZTS, suitable as a reference.Other precursors were loaded into the chamber and laser irradiation was performed using an XY scanning laser annealing system, as shown in Fig. 1.To investigate the influence of the atmosphere around the sample during laser irradiation, three atmospheres-air, Ar, and 5% H 2 S diluted with Ar, were introduced into the chamber, respectively.The laser wavelength was 445 nm.Laser irradiation was performed through a quartz window on the top side of the chamber, and the laser module was scanned in the X-Y direction above the chamber.The scanning area was 20 mm long and 5 mm wide, as shown in Fig. 2(a).Within this area, scanning was performed with line and space patterns at a scanning speed of 100 mm min −1 and scanning pitch of 0.1 mm.The laser output power was 128-164 mW, as evaluated using a power meter.The spot size was 0.20-0.24mm corresponding to the laser power.After laser irradiation, the samples were divided into small pieces of laser irradiation condition areas of 4-5 mm width and 7-8 mm length, cutting off the area not irradiated by the laser.X-ray diffraction (XRD; Rigaku, Miniflex) and field emission scanning electron microscopy (Zeiss, Ultra-55) were employed to investigate the crystallinity and morphology of the cross section, respectively.The atomic composition was analyzed using energy-dispersive spectroscopy (EDS, JEOL, IT-200).Laser Raman analysis (JASCO, NRS7200) was performed to investigate the species formed in the film with an excitation laser of 532 nm wavelength.For the light absorption coefficient, samples with a scanning area of 20 mm × 20 mm were prepared, as shown in Fig. 2(b), and analyzed by UV-vis spectroscopy (SHIMADZU, SolidSPEC3700).The band gap energy was estimated from the Tauc plot using the light absorption coefficient.

Crystallinity and morphology
The overview of the XRD patterns and enlarged XRD patterns at approximately 28.6°are shown in Figs.3(a) and 3(b), respectively.Strong peaks are observed at 37.1°, 43.3°, 63.84°, and 77.03°for the samples subjected to laser irradiation.These stronger diffractions originate from Aluminum, which is described in the International Centre for Diffraction Data (ICDD) # 00-004-0787 and used in the sample holder of the XRD equipment.The X-ray focus size was 10 mm wide and 1 mm long, and the samples for the XRD measurements were 7-8 mm wide and 4-5 mm long.Therefore, a part of the sample holder was exposed to X-rays, which resulted in the diffraction of Al, which was the material of the sample holder.However, the sample size of the precursor and conventional sulfurization was 25 × 25 mm 2 .Therefore, Al diffraction was detected only in the samples subjected to laser irradiation owing to their small sample size.The diffraction patterns of the samples treated by conventional sulfurization show obvious peaks at 28.59°with very weak peaks at 47.47°and 56.30°.The diffraction patterns at approximately 28.6°, 47.5°, and 56.4°are assigned to the kesterite phases of CZTS( 112), (220)/(204), and (312)/(116), respectively, as described in ICDD #01-075-4122 of CZTS.The diffraction pattern assigned to CZTS(112) can be observed more clearly in the enlarged patterns shown in Fig. 3(b).Although the film thickness in this study is as small as 200 nm, the diffraction intensity is relatively weak.All the samples subjected to laser irradiation show diffraction near 28.6°.ZnS exhibits the same diffraction pattern as CZTS, as described in ICDD # 01-071-5976.Raman spectroscopy was employed to determine the species formed in the CZTS and ZnS films.The Raman spectra are shown in Fig. 4. Except the precursor, sharp peaks are found clearly at 337 cm −1 .][26][27][28] Therefore, diffraction peaks at 28.64-28.70°canbe assigned to CZTS(112) because the Raman spectra of all samples with laser irradiation show A/A1 mode peaks at 337 cm −1 .Samples with laser irradiation in Ar and 5% H 2 S/Ar show peaks at 285 cm −1 and 369 cm −1  02SP16-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd also.From these results, those indicate that CZTS is formed by laser irradiation, irrespective of the gas atmosphere.However, the Raman spectra show peaks at 472 cm −1 in the samples subjected to laser irradiation in air and some of the Ar samples.This Raman shift corresponds to the Cu 2−x S species A1(LO) mode. 24,25)This means that the Cu 2−x S secondary phase is formed in the case of laser irradiation in air and Ar, whereas laser irradiation in 5% H 2 S/Ar can suppress Cu 2−x S phase formation.The diffraction angle of CZTS(112) at approximately 28.6°is shown in Fig. 5(a).In the range of laser power 128-164 mW, the diffraction angle slightly decreases by 0.02°-0.04°withincreasing laser power.A similar tendency is observed, irrespective of the annealing atmosphere.The diffraction angles of the samples subjected to laser irradiation are slightly higher than those of the samples subjected to conventional sulfurization.This suggests that the distance (d-spacing) between the atomic layers of the laser-irradiated samples is smaller than that in conventional sulfurization.The full width at half maximum (FWHM) is estimated from the XRD pattern, as shown in Fig. 5(b).The FWHM values of the samples with laser irradiation in air are 0.18°-0.29°largerthan that of conventional sulfurization.This indicates that the crystallite size in the sample subjected to laser irradiation in air is smaller than that in the sample subjected to conventional sulfurization.With laser irradiation in Ar or 5% H 2 S/Ar, the FWHM at 128 mW is 0.04°-0.05°largerthan that of conventional sulfurization.The FWHM with a power of 146-164 mW is 0.03°-0.05°smallerthan that for conventional sulfurization.The crystallite size is estimated using Scherrer's equation 29) where  02SP16-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd D is the crystallite size; K is the Scherrer factor and a value of 0.9 is employed in this study; l is the wavelength of Cu k α (0.1542 nm) used as an X-ray source in the XRD measurement; b is the FWHM; q is the Bragg angle.Figure 5(c) shows the effect of the laser irradiation conditions on the crystallite size.With increasing laser power, the crystallite size of laser irradiation in air shows no clear dependency on the laser power, and is 15-19 nm smaller than that of conventional sulfurization.For the samples subjected to laser irradiation in Ar or 5% H 2 S/Ar, the crystallite size increases with laser power and becomes 4-10 nm larger than that of conventional sulfurization.Figure 6 shows the cross-sectional SEM images of the samples.In comparison with the conventional sulfurization sample, the sample with laser irradiation in air has smaller grains and does not show a clear dependence on the laser power.In the samples subjected to laser irradiation in Ar and 5% H 2 S/Ar, the grains with a laser power of 128 mW are small or similar to those obtained by conventional sulfurization.At 146 and 164 mW, the interfaces between the grains are not clear.This implies that the CZTS crystal grains combine with each other, and crystallite growth is enhanced by laser irradiation in Ar or 5% H 2 S/Ar.
Figures 7(a)-7(d) show the atomic composition ratios determined by EDS measurements.With increasing laser power, the Cu, Zn, and Sn to metal (Cu+Zn+Sn) ratio changes insignificantly, and shows a similar value to that of conventional sulfurization.However, the dependency of the sulfur-to-metal ratio on the laser power varies with the gas atmosphere.In the case of laser irradiation in air, the sulfur content clearly decreases with increasing laser power.This indicates that sulfur desorption is enhanced by high-power laser irradiation in air.However, in the case of laser irradiation in Ar, the sulfur-to-metal ratio decreases slightly.This suggests that inactive gases, such as Ar, do not enhance  02SP16-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd sulfur desorption.In air, active gas components such as oxygen or water may react with the precursor, which helps to desorb sulfur.Yu et al. reported that in an experiment of annealing CZTS in an oxygen-containing atmosphere, part of the sulfur in CZTS was replaced by oxygen, and the grain size reduced with higher oxygen content. 30)In the present study involving laser annealing in air, sulfur reduction and smaller grain size are both observed, and we suspect that oxygen is incorporated into CZTS by replacing sulfur.In the EDS measurements, the oxygen incorporation from air into CZTS by laser annealing could not be distinguished because the oxygen included in the SLG was strongly detected.In this study, the precursor thickness is 200 nm and this is sufficiently thin to detect the characteristic X-rays from SLG.To observe the influence of oxygen incorporation by air more clearly, a sufficiently thick precursor over the EDS detection depth (>μm) would be required.In the case of 5% H 2 S/Ar, the sulfur content increases with laser power.This suggests that H 2 S facilitates the incorporation of sulfur into the precursor during laser irradiation.
The absorption coefficients are shown in Fig. 8.The absorption coefficient of laser irradiation in air is smaller than or similar to that of conventional sulfurization.In contrast, laser irradiation in Ar and 5% H 2 S/Ar shows higher absorption than conventional sulfurization.In particular, in 5% H 2 S/Ar over a range of 300-1100 nm wavelength, the absorption coefficient is higher than that of conventional sulfurization.4][35][36] Band gap of laser irradiation of 128 mW in the case of laser annealing in air is not shown because clear absorption edge could not be defined in Tauc plot.With increasing laser power, the band gap energy of samples with laser irradiation in air is almost constant in the range of 1.49-1.51eV.For Ar and 5% H 2 S/Ar, the bandgap energy decreases with increasing laser power. These wo cases show quite similar values in the range of 1.40-1.49eV.However, the Urbach energies show clear differences among air, Ar, and 5% H 2 S/Ar.The Urbach energy of 5% H 2 S/Ar is the smallest, although it shows an increasing dependency on the laser power.The Urbach energy indicates the localized states in the forbidden band.Thus, it is influenced by crystal imperfections and disorder.In this study, CZTS formation is confirmed using Cu 2−x S species in air and Ar from the Raman spectra at 472 cm −1 .This may have contributed to the observed high crystallinity disorder.For the case of 5% H 2 S/Ar, the Urbach energy decreases with larger crystallites.However, it is still higher than that of conventional sulfurization.This means that there might be another limiting process, except for the crystallite size, which influences the Urbach energy.An in-depth investigation of this limitation process is important to determine the potential of lightabsorbing materials for solar cell applications.

Conclusions
In this study, CZTS formation by 445 nm wavelength laser annealing at a power of 128-164 mW in air, Ar, or 5% H 2 S diluted in Ar was investigated.The Cu-Zn-Sn-S-based precursor was formed 200 nm thick on the SLG substrate by RF sputtering followed by laser annealing, and CZTS was with a bandgap energy of 1.38-1.51eV.Laser annealing in air and Ar results in sulfur desorption and the formation of Cu 2−x S. In the case of laser annealing in 5% H 2 S/Ar, larger crystallites and grains are realized in comparison with the conventional sulfurization process.The Urbach energy of the samples subjected to laser irradiation in 5% H 2 S/Ar is smaller than that in air or Ar, while it is still higher than that in conventional sulfurization.A deeper investigation is required to achieve higher quality than conventional sulfurization.02SP16-6 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .
Fig. 1.Schematic image of the XY scanning laser irradiation equipment.

Fig. 2 .
Fig. 2. Schematic images of the laser irradiation area on the samples.Sample for XRD, EDS, SEM and Raman, (b) sample for UV-vis.

Fig. 3 .
Fig. 3. Dependence of the XRD patterns of the samples on the laser power and irradiation atmosphere.(a) XRD patterns in the range of 10°-90°, (b) XRD patterns in the range of 26°-32°.

Fig. 4 .
Fig. 4. Raman spectra of the samples with laser irradiation at laser power of 128-164 mW in air, Ar and 5% H 2 S/Ar.

Fig. 6 .
Fig.6.Cross sectional images of the scanning electron microscope for samples with laser irradiation at laser power of 128-164 mW in air, Ar and 5% H 2 S/Ar.The acceleration voltage is 1.5 kV and magnification is 100 000.

Fig. 10 .
Fig. 10.Optical band gap energy and Urbach energy of the samples with laser irradiation at a power of 128-164 mW in air, Ar and 5% H 2 S/Ar.