Thin Solid Films 518 (2010) 6567–6572

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Thin Solid Films

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Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films

Hyesun Yoo, JunHo Kim ⁎

Department of Physics, University of Incheon,12-1 Songdo-Dong Yeonsu-Gu, Incheon 406-840, Republic of Korea

⁎ Corresponding author.E-mail address: jhk@incheon.ac.kr (J. Kim).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.03.058

a b s t r a c t

a r t i c l e i n f o

Available online 25 March 2010

Keywords:Cu2ZnSnS4SulfurizationSputteringStacked metallic filmsRaman scatteringXRD

We fabricated Cu2ZnSnS4 (CZTS) thin films through sulfurization of stacked metallic films. Three types of Cu–Zn–Sn metallic films, i.e., Cu-rich, Cu-correct and Cu-poor precursor films were sputtered onto Mo-coatedglass. The sulfurization of stacked Cu–Zn–Sn alloy films was performed at a relatively high temperature,570 °C, with S-powder evaporation. CZTS films from Cu-rich and Cu-correct precursors showed a Cu2− xSphase on the film surface, while CZTS films from Cu-poor precursors didn't show the Cu2− xS phase.However, all films didn't exhibit any extra secondary phase and exhibited good crystalline textures evenwith Cu-ratio differences in metallic precursor films. Fabricated CZTS films were characterized by X-raydiffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), andRaman scattering measurements. SEM cross-section images of CZTS films showed that Cu-poor CZTS filmswere grown with more smooth film surface compared with other types of CZTS films.

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1. Introduction

Currently, researches on low-cost thin-film solar cells areincreasing significantly. Among the thin-film solar cells, Cu2ZnSnS4(CZTS) is very promising as a photo-absorbing layer for low-cost solarcell [1–3], since this quaternary compound has a band gap of 1.4–1.5 eV and an absorption coefficient of 104 cm−1 in visible spectrumrange. In addition, CZTS contains less toxicmaterial S instead of Se andmore abundant elements, Zn, Sn, than In, which are included inCuIn1− xGaxSe2 (CIGS) based solar cell. The best conversion efficiencyof CZTS solar cell is reported to be ∼6.7%, where CZTS layer wasproduced by sputtering deposition and subsequent sulfurization [4].

Compared with CIGS solar cell, study on film growth of CZTS is stillinsufficient, and it needs more systematic investigations to enhanceconversion efficiency. For the deposition of CZTS layer, severaltechniques such as sputtering deposition [3,4], spray pyrolysis [5],electro-deposition [6,7], co-evaporation [8] and pulsed laser deposi-tion [9] were utilized. Considering that sputtering deposition is easilyscaled up to large area deposition, and best conversion efficiency isachieved by conversion of sputtered stacked layers into sulfide filmsvia sulfurization, it is worth focusing on the method of sputtering andsubsequent sulfurization. On the other hand, in the fabrication processof CIGS solar cell, the quality control of absorber layer has been doneby X-ray diffraction and Raman spectroscopy. Both measurementslead us to estimate crystalline quality and to judge existence ofsecondary phase with high reliability.

We deposited various stacked Cu–Zn–Sn layers with differentdeposition sequences, and transformed the stacked films into CZTSfilms by sulfurization. The fabricated CZTS filmswere characterized byX-ray diffraction (XRD), scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDS), and Raman scattering measure-ments. Based on these measurements, we report growth properties ofCZTS films.

2. Experimental

The stacked metallic films were fabricated by rf sputteringdeposition, where Cu, Zn and Sn elemental targets were used. Forsputtering deposition, the vacuum chamber was evacuated up to10−6 Torr, and sputtering was carried out under Ar atmosphere withpressure of 6–7 mTorr. The distance between target and substrate was∼8 cm, and sputtering power was maintained at 40 W for alldepositions. The fabricated films were summarized in Table 1.Basically, three different types of CZTS films were prepared, whichwere Cu-rich Cu2ZnSnS4 (CR-CZTS), Cu-correct Cu2ZnSnS4 (CC-CZTS),and Cu-poor Cu2ZnSnS4 (CP-CZTS). For the three types of CZTS, wefabricated several multilayer films with different stacking sequences.However, some stacked films were not transformed into good qualityCZTS films after sulfurizations, and those samples are excluded in thispaper. The deposited multilayer films were sulfurized in vacuumfurnace with two heating zone. At one zone sulfur powder was placed,and at the other zone stacked metallic film was located. The sulfurpowder zone was heated up to 240 °C within 20 min, and the samplezone was heated up to 570 °C within 20 min simultaneously. Thenthey were cooled naturally to room temperature. The end of samplezone was opened to rotary pump, whereas that of sulfur zone wasclosed. Before every sulfurization the furnace was evacuated to

Table 1Atomic compositions of deposited CZTS films.

Stack sequence(/Mo/glass)

Cu-rich Cu2ZnSnS4 (CR-CZTS) Cu-correct Cu2ZnSnS4 (CC-CZTS) Cu-poor Cu2ZnSnS4 (CP-CZTS)

Cu:Zn:Sn:S Cu:Zn:Sn:S Cu:Zn:Sn:S

Cu/(Zn+Sn) Zn/Sn S/metal Cu/(Zn+Sn) Zn/Sn S/metal Cu/(Zn+Sn) Zn/Sn S/metal

I Cu/Sn/Cu/Zn 2.6:1.3:1:5 2.1:1:1.1:5 1.5:1:1:4.21.13 1.3 1.02 1 0.9 1.19 0.75 1 1.2

II Cu/Sn/Zn 2.1:1:1.1:4.6 2.1:1:1.1:4.7 1.6:1:1:4.81 0.9 1.1 1 0.9 1.12 0.8 1 1.33

III Cu/Zn/Sn 2.1:1.1:1:4.6 1.9:1:1:4.8 1.8:1:1:5.21 1.1 1.1 0.95 1 1.23 0.9 1 1.37

IV Sn/Cu/Zn/Cu 2.4:1:1.1:4.8 1.9:1:1.1:3.8 1.5:1:1:3.91.08 0.9 1.06 0.9 0.9 0.95 0.75 1 1.11

V Sn/Cu/Zn 2.5:1:1.1:5.2 1.8:1:1:4.8 1.7:1.1:1:4.51.19 0.9 1.13 0.9 1 1.26 0.81 1.1 1.15

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∼5 mTorr, and it was kept being evacuated until the end ofsulfurization. The flux of S-vapor reached sample zone without acarrier gas.

The fabricated CZTS films were characterized by XRD, SEM, EDSand Raman scattering measurements. For XRD measurements, θ–2θscans were carried out from 15° to 70° with a scan rate of 3°/min, anda Cu–Kα (λ=0.15406 nm) line was used as an X-ray source. ForRaman spectroscopy, a micro-Raman spectrometer was used in

Fig. 1. Results of XRD (a) and Raman scattering (b) measurements for the CR-CZTS films. Besito check small peaks in the bottom line.

backscattering configuration, where the wavelength of excitationlight was 514 nm. The penetration depth of CZTS film for 514 nm wasestimated to be 500 nm [10].

3. Results and discussion

Table 1 shows EDS results of all fabricated samples, where Cu-ratiodifference between each type is apparent. The uniformity of film

de full XRD 2θ scans, enlarged graphs in the 2θ-angle range from 28° to 34° are displayed

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composition is assumed for the EDS study. For sulfurizations, weselected higher temperature, 570 °C, which could result in homoge-neous film composition [11–13]. The compositional ratio of constit-uent element is normalized by atomic percent of Sn. In the stacksequence, the last element is firstly deposited onto the Mo-coatedglass (Mo/glass). For the CR-CZTS-II film, the deposited layers are laidas Cu/Sn/Zn/Mo/glass. For the CR-CZTS films the maximum of Cu ratiois observed to be ∼2.6, while for the CP-CZTS films, the minimum ofCu ratio is detected to be ∼1.5. The control of sputtering time for eachlayer seems effective to engineer a CZTS film with pre-designedatomic ratio, especially for the CC- and CP-CZTS films. For the CR-CZTSfilms, Cu-ratio deviation is prominent compared with other types ofCZTS films. In previous articles, increase of Cu ratio is reported to bedue to the loss of Sn or Zn during sulfurization [14,15]. In this sense,under our sulfurization process the re-evaporation of Sn or Zn mightbe occurred for CR-CZST-I, -IV, -V films, where Sn is located at theupper layer of multilayer precursor.

Fig. 1(a) shows XRD θ–2θ scans of the CR-CZTS films with differentstacking sequences. All films are observed to be grown well withpolycrystalline textures. For all films, major crystalline texture is(112)-oriented one (JCPDS card No.: 26-0575). Some films showpeaks from the Cu2− xS phases and they are marked as “*” in Fig. 2(a)(JCPDS card No.: 20-0365). Beside full 2θ scans, enlarged graphs in the2θ range of 28–34° are displayed in order to check small peaks inbottom lines. Fig. 2(b) shows results of Raman scattering of the samefilms as in Fig. 2(a). The peak at ∼474 cm−1 is a feature of the Cu2− xS

Fig. 2. Results of XRD (a) and Raman scattering (b)measurements for the CC-CZTS films. Besito check small peaks in the bottom line.

phase, and it is due to Cu-richness in the CR-CZTS film. In Fig 2(b), weplotted dashed lines at 251, 288, 337, 352 and 372 cm−1 for possiblepeaks. Up to now, for kesterite CZTS films, designations of Ramanactive modes, which are proved theoretically and experimentally, arenot reported in public articles yet. However, peaks at ∼252, ∼287,∼338, ∼351, and ∼368 cm−1 are reported to be from kesterite CZTS,and the biggest peak is known to be positioned at ∼338 cm−1 [11,16].In XRD result of Fig. 1(a), at 2θ=32° SnS peak may be concealed, butin Raman scattering result no feature of SnS, which appears at 160,190 and 220 cm−1, is observed [11]. Thus, the peak at 2θ=32° ispurely due to the Cu2− xS phase, and the possibility of the existence ofSnS is excluded. The XRD and Raman scattering results mean thatalthough some Cu2− xS peaks are observed, the CR-CZTS films aregrown well with high crystal quality of kesterite structure.

Fig. 2(a) shows XRD θ–2θ scans of the CC-CZTS films withsame stack sequences as those for the CR-CZTS films. As it is shownin Fig. 2(a), all films were grown well with polycrystalline textures,where the Cu2−xS phasesmarked as “*” are also observed. The CC-CZTSfilms were also observed to be grown with (112)-oriented texturedominant. Fig. 2(b) shows Raman scattering measurements of filmsof Fig. 2(a). Possible peak positions are plotted as dashed lines,which are located at 252, 289, 338, 354 and 374 cm−1. These peakpositions are almost the same as those for the CR-CZTS films. For all theCC-CZTS films including Sn/Cu/Zn/Cu and Sn/Cu/Zn films, peak of theCu2−xS phase at ∼475 cm−1 is also observed, which is attributed tohigh mobility of Cu and its migration to the film surface during

de full XRD 2θ scans, enlarged graphs in the 2θ-angle range from 28° to 34° are displayed

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sulfurization. For Raman spectroscopy, we used the micro-Ramanspectrometer, which had a focused beam diameter of 0.7–1 µm. On theother hand, it is known that the Cu2− xS phase forms as an island-likecapping layer not a continuous film on the top surface [17]. Thus, itmay occur that the intensity of the Cu2−xS phase in Raman spectra can'tbe proportional to that in XRD result in consistent way. The peak at410 cm−1 is close to the peak from MoS2, which implies that MoS2 isformed at the interface of CZTS and Mo [18].

Fig. 3(a) shows XRD θ–2θ scans of the CP-CZTS films. Growthfeatures of the CP-CZTS films look almost the same as other types offilms. However, for the CP-CZTS films, any peak from secondary phaseor impurity is not observed. Fig. 3(b) shows Raman scatteringmeasurements of films of Fig. 3(a). We also plotted dashed lines forpossible peak positions, which are almost the same as those for othertypes of films. Peak at 476 cm−1 is not observed in the CP-CZTS films,which indicates that the CP-CZTS films bear no Cu2− xS phase throughthe entire films.

XRD and Raman scattering measurements showed that all threetypes of CZTS films were grown well with (112)-oriented crystallinetexture. Although three types of CZTS films were fabricated withmetallic precursors with different compositional variations, results ofXRD and Raman scattering measurements showed similar features.These results lead us to conclusion that evenwith some compositionaldeviations in precursor films good crystalline CZTS films could beobtained. However, for the CR- and CC-CZTS films, the Cu2− xS phases

Fig. 3. Results of XRD (a) and Raman scattering (b) measurements for the CP-CZTS films. Besito check small peaks in the bottom line.

occurred after sulfurization. When Cu–Zn–Sn stacked film is con-verted into CZTS by sulfurization, secondary phases such as Cu2SnS3,ZnS and SnS2 may occur at low temperature [11]. However, in ourexperiments for all three types of CZTS films, no secondary phaseexcept Cu2− xS is observed. This can be ascribed to higher sulfuriza-tion temperature [7,11–14].

For the Cu(In,Ga)(S,Se)2 films, Raman spectroscopy has been usedfor quality control of film growth [19]. These films are known toexhibit chalcopyrite and Cu–Au orderings, where chalcopyriteordering is preferable for high efficiency solar cell. Raman spectros-copy distinguishes these two orderings, since characteristic peaks forthe two orderings are very different in Raman spectra. However, forCZTS filmsmade by our sulfurization process Raman scatterings of theCR-, CC- and CP-CZTS films are observed to be so similar that qualitycontrol of CZTS film is not so effective as that of the Cu(In,Ga)(S,Se)2film.

Fig. 4 shows SEM images of cross sections of all CZTS films. Thescale bars in all images are 500 nm in length. Features of voids areobserved for all CZTS films, especially for CZTS films from Cu/Zn/Sn/Mo/glass and Sn/Cu/Zn/Cu/Mo/glass. For the former, Cu is notadjacent to Sn, whereas for the latter Cu is the first layer to Mo/glass.When Cu is not adjacent to Sn, it's difficult to form Cu2SnS3 which isreactant to produce CZTS with ZnS, and resultantly creation of largegrain of CZTS is inhibited [20]. And when Cu is firstly deposited ontoMo, “Cu-migrations” to upper surface during sulfurization leave voids

de full XRD 2θ scans, enlarged graphs in the 2θ-angle range from 28° to 34° are displayed

Fig. 4. SEM images of cross-sections for all fabricated CZTS films.

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at the interface [21]. On the other hand, the CZTS films from the groupof Cu/Sn/Zn/Mo/glass show relatively large thickness. This may bedue to the topmost thick Cu layer, which prevents loss of Sn and Znduring sulfurizations. For the CP-CZTS films void features are relativelyweak compared with the CR- and CC-CZTS films. Further, the CP-CZTSfilms showmost smooth film surfaces, which are very desirable for thefabrication of a high efficiency solar cell. Recently, Schurr et al. reportedthat Cu-poor and Cu-rich CZTS films follow different reaction process,where binary sulfides, which will be involved in the reactions for CZTSformation, are produced easily than Cu-rich precursors [7]. Thesmoothness of the film surface free of the Cu2−xS phases in the CP-CZTS films correspond to the fact that best conversion efficiency wasachieved by Cu-poor CZTS film with Cu/(Zn+Sn) of ∼0.85 [4].

4. Conclusion

We fabricated several Cu–Zn–Sn stacked films and convertedprevious metallic films to CZTS films by sulfurization process, whereprecursor films were heated up to 570°C during sulfurization. Thefabricated CZTS films were sorted as Cu-rich (CR), Cu-correct (CC) andCu-poor (CP) CZTS films according to their relative Cu ratios.

The Cu2− xS phase was observed in the CR- and CC-CZTS films,whereas it was not seen in the CP-CZTS films. Except the Cu2− xSphase, all CZTS films showed no secondary phase such as Cu2SnS3,ZnS, and SnS, and exhibited (112)-orientation dominant growth,which were confirmed by XRD and Raman scattering measurements.These results indicate that higher temperature is preferable for highquality of kesterite CZTS film. Considering that removing the Cu2− xSphase by cyanide etchant may leave possible voids or defects, the CP-CZTS films are desirable for realization of high efficiency solar cell.

We studied stack sequence effect on film growth of CZTS film withSEMmeasurements. For this purpose, cross-section images of all CZTSfilms were taken by SEM. Many voids (or porous characteristics) were

observed in all samples, especially in CZTS films from Cu/Zn/Sn/Mo/glass and Sn/Cu/Zn/Cu/Mo/glass. The reason of this result can befound in separation of Cu and Sn by Zn for the former precursor film,and in firstly deposited Cu for the later precursor film. In the cross-section SEM images, the CP-CZTS films showed relatively uniform andsmooth film-surface lines compared with other types of CZTS films.

Results of above experiments lead us to conclude that the CP-CZTSfilms have more smooth film surface than other types of CZTS filmsand no Cu2− xS phase on or near the film surface, which are verydesirable for high efficiency CZTS based solar cells in some ways. ForCZTS films with larger grains and fewer voids inside, more systematicstudies on film growth of the CP-CZTS films are required.

Acknowledgement

This work was supported by the University of Incheon ResearchGrant in 2008.

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