The vibrational properties study of kesterite Cu2ZnSnS4 single crystals by using polarization dependent Raman spectroscopy

The vibrational properties study of kesterite Cu2ZnSnS4 single crystals by using polarization dependent Raman spectroscopy

Optical Materials xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/...

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Optical Materials xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

The vibrational properties study of kesterite Cu2ZnSnS4 single crystals by using polarization dependent Raman spectroscopy Dumitru Dumcenco, Ying-Sheng Huang ⇑ Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

a r t i c l e

i n f o

Article history: Received 9 August 2012 Received in revised form 19 September 2012 Accepted 22 September 2012 Available online xxxx Keywords: CZTS Kesterite Raman spectroscopy Vibrational modes Polarization selection rules

a b s t r a c t This work reports a study of the vibrational properties of kesterite Cu2ZnSnS4 (CZTS) single crystals by using polarization-dependent Raman scattering measurements. The CZTS crystals with several mirrorlike planes were grown by chemical vapor transport technique using iodine trichloride as a transport agent. The detailed analysis of the experimental spectra and comparison with the results of recent theoretical calculations, have allowed us to determine the wavenumber and symmetry assignment of the observed Raman-active modes of CZTS. The results may be used to clarify the existence of structural or phase inhomogeneities in CZTS absorber film of the solar cells. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The quaternary compound Cu2ZnSnS4 (CZTS) belongs to the family of I–II–IV–VI semiconductors. It has attracted a great interest due to its potential applications for sustainable thin-film solar cell devices [1–4]. CZTS has large direct band gap (Eg  1.5 eV), high absorption coefficients (>104 cm1), intrinsic p-type conductivity and low thermal conductivity [5–7]. Comparatively to the other important solar cell materials, the composition of naturally abundant and inexpensive elements such as Zn and Sn makes CZTS particularly attractive candidate for large-scale commercial application [8]. The vibrational properties of CZTS have been addressed in recent detailed theoretical investigations [9,10]. Some infrared (IR) [11] and Raman [10–13] spectroscopy experiments on CZTS thin films were also performed for the analysis of the crystal structure. However, no detailed experimental data of vibrational properties of CZTS single crystals has been reported yet. This paper presents the detailed experimental results of polarization-dependent backscattering Raman spectroscopy on CZTS single crystals. The CZTS crystals with several mirror-like planes were grown by chemical vapor transport (CVT) technique using iodine trichloride (ICl3) as a transport agent. By applying the selection ⇑ Corresponding author. Address: Department of Electronic Engineering, Taiwan Tech, 43 Keelung Road Section 4, Taipei 106, Taiwan. Tel.: +886 2 27376385; fax: +886 2 27376424. E-mail address: [email protected]w (Y.-S. Huang).

rules to the Raman active modes and comparing the experimental results with the recent theoretical calculations [9,10], the observed modes detected from (1 0 0), (0 0 1), (1 1 0) and (1 1 2) planes were classified. 2. Experimental The single crystals of CZTS with well-developed faces were grown by CVT technique using ICl3 as a transport agent [7]. The stoichiometric compositions of selected Cu2ZnSnS4 samples were determined by using energy dispersive X-ray analysis (EDX). EDX measurements showed some variations between the ratio of Cu and Zn among the examined samples. However, the average atomic ratio of Cu:Zn:Sn:S was found to be closed to 2:1:1:4. The crystals were X-ray pre-oriented with respect to the directions and polarizations of the incident and scattered light. A representative crystal showing the as-grown basal (1 1 2) plane, comparativelybig-area (0 0 1), (1 1 0), (1 0 1) and (0 1 1) planes, as well as the small (with a right-angle triangle-shape) (1 0 0) plane together with the prescribed polarization configurations is illustrated in Fig. 1. Raman measurements were performed at room temperature utilizing the back-scattering configuration on a Renishaw in Via micro-Raman system with 1800 grooves/mm grating and an optical microscope with a 50 objective. A linearly polarized Ar+ laser beam of the 514.5 nm excitation line with a power of 1.5 mW was focused into a spot size 5 lm in diameter. For the polarization measurement of the scattering light, a polarizer and a halfwave plate were used. Prior to the measurement, the system was

0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.09.031

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Fig. 1. The images of the CZTS single crystal and the configurations used for Raman measurements: (a) right-angled triangle (1 0 0) plane; (b) top view; (c) schematic representation of polarization-dependent Raman spectroscopy measurements at backscattering configuration; (d) (1 1 0) plane; (e) (1 1 2) basal plane; (f) (0 0 1) plane. Arrows indicate the X = [1 0 0], Y = [0 1 0], Z = [0 0 1] principal crystallographic axes.

calibrated by means of the 520 cm1 Raman peak of a polycrystalline Si.

3. Theoretical considerations  or stanCZTS can be crystallized in kesterite (KS/space group I4)  nite (ST/space group I42m) structure. The KS and ST crystallorgraphic forms are very close with the only difference in the distribution of the cations in the tetrahedral sites [14]. Specifically, the KS structure consists of two alternating cation layers each containing Cu and Zn or Cu and Sn, whereas in the ST structures, a layer of Cu alternates with a layer of Zn and Sn. In the ST structure, the Zn and Sn atoms from the same layer switch their positions layer by layer. As a result, within experimental broadening of the diffraction peaks, it is a challenge to differentiate KS and ST structures by using X-ray diffraction (XRD). Phonon dispersion in solids is sensitive to the coupling between atoms within the lattice. Thus, variations in phonon dispersion in KS and ST may help to distinguish these structures. For dispersion measurements, either neutron diffraction [15] or Raman spectroscopy [9,10] can be utilized. Specifically, Raman scattering is observed at wavenumbers corresponding to the phonon modes at the C point. Moreover, Raman spectroscopy is a convenient and widely available technique. Since CZTS has eight atoms per primitive cell, there is a total of 24 vibrational modes. Amongst these modes, there are Raman active modes, i.e. 15 for KS structure and 14 for ST [9,10]. A and A1 modes for KS and ST, respectively, result from symmetric vibrations of only anion lattice and they are responsible for the strongest lines observed in the experimental Raman spectra of CZTS [10,12]. In B1 modes of ST, half of the Cu atoms is displaced toward the positive Z axis and the other half is displaced toward the negative Z axis, while the Zn and Sn atoms remain stationary [9]. In the meantime, anions move only in XY plane. In B modes of KS and B2 modes of ST, the cations only move along Z direction, whereas these cations move only within XY plane in E modes for the both

structures. Thus the main difference between KS and ST structures is related to the nonexistence of B1 mode structure caused by absence of the individual layers of Cu in KS. For accurately determination of the positions and origin of Raman active modes, polarization-dependent backscattering measurements have been carried out. The widely used Porto notation [16] has been utilized in this study for the designation of the crystal and polarization directions. The [1 0 0], [0 1 0], and [0 0 1] crystallographic axes are denoted as X, Y, and Z, respectively (see Fig. 1a). The notations of the polarization configurations are adopted from the Bilbao Crystallographic Server [17], and it is directly related to crystallographic axes presented in Fig. 1c. For the measurements from (1 0 0) plane (Fig. 1c), the notation XðY; YÞX indicates that the direction of incident radiation is along X; the first and second symbols in the brackets denote the polarization of the incident and scattered light, respectively; and X represents the direction of the scattered light, which is opposite to X for back-scattering measurements. For XðY; YÞX configuration (case I in Fig. 1c), the analyzer is placed in front of the charge coupled device (CCD) camera such that its polarization axis is parallel to the polarization axis of the incident linearly polarized laser beam. XðY; ZÞX configuration (case II in Fig. 1c) is obtained by placing the half-wavelength plate directly just in front of the analyzer. To determine the origin of the observed Raman modes, the polarization selection rules for backscattering configuration along [1 0 0], [0 0 1], [1 1 0] and [1 1 2] crystallographic directions of the crystals with KS and ST structure have been used [17]. Thus for (1 0 0) and (0 0 1) planes the orthogonal system O with X = [1 0 0], Y = [0 1 0], Z = [0 0 1] axes have been used; for (0 0 1) and (1 1 0)  1 0], Z0 = [0 0 1] axes; for planes – system O0 with X0 = [1 1 0], Y0 = [1  1 0], Z00 = [1 1  1] (1 1 2) plane – system O00 with X00 = [1 1 2], Y00 = [1 axes. The Raman tensors for KS and ST structures in the orthogonal system O are presented in Table 1 [18]. The transformation of the Raman tensors from the principal axes (system O), with the help of a general rotational tensor, gives the possibility to find the polarization selection rules for O0 and O00 systems [17]. The selection rules for the various polarization configurations derived from the

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D. Dumcenco, Y.-S. Huang / Optical Materials xxx (2012) xxx–xxx Table 1 Raman tensors for the compounds of the tetragonal space group in the orthogonal system O (X = [1 0 0], Y = [0 1 0], Z = [0 0 1] principal axes). Structure

Raman tensors

 Kesterite (I4)

A 2

 Stannite (I42m)

a 40 0

0 a 0

3 0 05 b

B(z) 2 c d 4 d c 0 0

3 0 05 0

A1 2 a 40 0

0 a 0

3 0 05 b

B1 2 c 40 0

3 0 05 0

0 c 0

B2(z) 2 0 d 4d 0 0 0

3 0 05 0

E(x) 2 0 0 40 0 e f

3 e f5 0

E(y) 2 0 0 40 0 f e

E(x) 2 0 0 40 0 0 e

3 0 e5 0

E(y) 2 0 0 40 0 e 0

3 f e 5 0 3 e 05 0

Table 2 Polarization selection rules for backscattering along the [1 0 0], [0 0 1], [1 1 0] and [1 1 2] crystallographic directions of single crystals with tetragonal in different polarization configurations: for (1 0 0) and (0 0 1) planes the orthogonal system O with X = [1 0 0], Y = [0 1 0], Z = [0 0 1] axes have been used; for (0 0 1) and (1 1 0) planes – system O0 with  1 0], Z0 = [0 0 1] axes; for (1 1 2) plane – system O00 with X00 = [1 1 2], Y00 = [1  1 0], Z00 = [1 1  1]. X0 = [1 1 0], Y0 = [1 Plane

(1 0 0)

Geometry

XðY; YÞX XðY; ZÞX; XðZ; YÞX XðZ; ZÞX

(0 0 1)

ZðX; XÞZ; ZðY; YÞZ ZðX; YÞZ; ZðY; XÞZ Z 0 ðX 0 ; X 0 ÞZ 0 ;Z 0 ðY 0 ; Y 0 ÞZ 0 Z 0 ðX 0 ; Y 0 ÞZ 0 ;Z 0 ðY 0 ; X 0 ÞZ 0

(1 1 0)

X 0 ðY 0 ; Y 0 ÞX 0 X 0 ðY 0 ; Z 0 ÞX 0 ; X 0 ðZ 0 ; Y 0 ÞX 0 X 0 ðZ 0 ; Z 0 ÞX 0

(1 1 2)

X 00 ðY 00 ; Y 00 ÞX 00 X 00 ðY 00 ; Z 00 ÞX 00 ; X 00 ðZ 00 ; Y 00 ÞX 00 X 00 ðZ 00 ; Z 00 ÞX 00

Allowed modes  Kesterite (I4)

 Stannite (I42m)

A + B(TO) E(TO) + E(LO) A

A1 + B1 E(LO) A1

A + B(LO) B(LO) A + B(LO) B(LO)

A1 + B1 B2(LO) A1 + B2(LO) B1

A + B(TO) E(TO) + E(LO) A

A1 + B2(TO) E(TO) + E(LO) A1

A + B(TO) + B(LO) B(TO) + B(LO) + E(TO) + E(LO) A + B(TO) + B(LO) + E(TO) + E(LO)

A1 + B2(TO) + B2(LO) B1 + E(TO) + E(LO) A1 + B2(TO) + B2(LO) + E(TO) + E(LO)

Fig. 2. Polarization-dependent Raman spectra at backscattering configuration from (1 0 0) plane using orthogonal system O. Arrows above the curves show the positions of the dominated A and E(TO)/E(LO) modes. The inset represents the spectrum obtained by subtraction of the spectra at XðY; YÞX and XðZ; ZÞX configurations normalized to A mode. The subtracted spectrum clearly shows two features: unknown X and B(TO) mode.

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Raman tensors at backscattering along the [1 0 0], [0 0 1], [1 1 0] and [1 1 2] crystallographic directions for KS and ST structures, are summarized in Table 2.

4. Results and discussion Polarization-dependent Raman spectra from (1 0 0) plane (see Fig. 1a) of the CZTS are shown in Fig. 2. For XðY; YÞX and XðZ; ZÞX configurations, the spectra with the dominated features at 334 and 285 cm1 have been observed. In the case of KS/ST, the difference between configurations is related to B(TO)/B1 modes (Table 2). Subtracting the spectrum of XðZ; ZÞX configuration from XðY; YÞX; where both are normalized to A mode, the resultant

spectrum clearly shows two features: unidentified feature X at 280 cm1 and B(TO) mode (inset of Fig. 2). The difference between the intensities of A modes at XðY; YÞX and XðZ; ZÞX is related to a different values of a and b used in Raman tensors (Table 1). The disparity of feature X to B1 mode of ST will be shown later. The theoretical calculations [9,10] show that the highest frequency B1 mode is expected at 291.1 (324.1) cm1 whereas B(TO) mode at 355.8 (354.8) cm1. Comparison of the mode observed at 352 cm1 with the literature data on theoretical calculations allow us to assign it as B(TO) mode. Thus the presence of B(TO) mode in the subtracted spectrum indicates that the investigated sample has a KS structure. So the designation of the modes appropriate to KS has been used. According to the polarization selection rules, two prominent features at 334 and 285 cm1 are related to A mode. There is an

Fig. 3. Polarization-dependent Raman spectra at backscattering configuration from (0 0 1) plane using orthogonal system O. Arrows above the curves show the positions of the dominated A and B(LO) modes.

Fig. 4. Polarization-dependent Raman spectra at backscattering configuration from (0 0 1) plane using orthogonal system O0 . Arrows above the curves show the positions of the dominated A and B(LO) modes as well as unknown feature X.

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Fig. 5. Polarization-dependent Raman spectra at backscattering configuration from (1 1 0) plane using orthogonal system O0 . Arrows above the curves show the positions of the dominated A, B(TO) and E(TO)/E(LO) modes. The top inset represents the spectrum obtained by subtraction of the spectra at X 0 ðY 0 ; Y 0 ÞX 0 and X 0 ðZ 0 ; Z 0 ÞX 0 configurations normalized to A mode. The subtracted spectrum shows clear two modes: unknown X and B(TO) modes. The bottom inset represents the spectrum obtained by subtraction of the spectra at XðY; ZÞX and X 0 ðY 0 ; Z 0 ÞX 0 configurations. The subtracted spectrum clearly shows E(TO) mode.

additional small feature at 306 cm1 that is also most probably associated with A mode. For XðY; ZÞX and XðZ; YÞX configurations, the Raman spectra are dominated by E(TO) and E(LO) modes. To distinguish E(TO) and E(LO) modes, the measurements from other planes should be done. In despite of the selection rules (Table 2), the trace of the most intensive A mode is still observed (Fig. 2). The reason of the forbidden A mode observation will be discussed later. Figs. 3 and 4 show the polarization-dependent results from (0 0 1) plane (see Fig. 1f). In accordance with the selection rules for KS (Table 2), the identical spectra for ZðX; XÞZ and ZðY; YÞZ configurations are dominated by A modes while B(LO) mode is rel-

atively small (Fig. 3). For ZðX; YÞZ and ZðY; XÞZ configurations, the spectra are also identical. According to the selection rules, the features at 353, 250 and 162 cm1 correspond to B(LO) modes. The trace of the forbidden A mode is also observed as a shoulder. At Z 0 ðX 0 ; X 0 ÞZ 0 and Z 0 ðY 0 ; Y 0 ÞZ 0 configurations (Fig. 4), A and B(LO) modes have been clearly observed and their positions correspond to the values for ZðX; YÞZ and ZðY; XÞZ. While for Z 0 ðX 0 ; Y 0 ÞZ 0 and Z 0 ðY 0 ; X 0 ÞZ 0 configurations, the intensities of the allowed B(LO) modes are very small so the forbidden A modes as well as unknown feature X have been clearly observed. Polarization-dependent data for (1 1 0) plane (see Fig. 1d) are shown in Fig. 5. For both X 0 ðY 0 ; Y 0 ÞX 0 and X 0 ðZ 0 ; Z 0 ÞX 0 configurations,

Fig. 6. Polarization-dependent Raman spectra at backscattering configuration from (1 1 2) plane using orthogonal system O00 . Arrows above the curves show the positions of the dominated A, B(TO)/B(LO), and E(TO)/E(LO) modes.

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Table 3 The experimental values of the wavenumbers obtained from polarization-dependent Raman measurements using selection rules at back-scattering configuration from the (1 0 0), (0 0 1), (1 1 0) and (1 1 2) planes. The calculated wavenumber values for kesterite and stannite structures [9,10] are also shown for comparison. Experimental (±1 cm1)

Calculated kesterite [9]/[10]

Calculated stannite [9]/[10]

Modes

Wavenumber (cm1)

Modes

Wavenumber (cm1)

Modes

Wavenumber (cm1)

A

334 306 285 352 – 245 160 – – 341 – 246 143 – –

A

340.04/335.2 284.30/309.0 272.82/302.1 355.80/354.8 309.56/332.7 238.48/269.1 166.65/179.6 98.82/104.2 86.70/92.3 351.55/341.4 281.07/309.7 250.26/278.2 150.53/166.1 105.93/101.4 83.64/79.2

A1

334.08/332.7 277.12/309.1 263.11/304.3 291.12/324.1 74.17/88.1 360.12/358.5 277.08/306.2 149.69/171.0 95.85/96.4 346.01/341.3 264.34/305.3 235.41/268.7 161.68/170.9 97.34/106.9 78.39/74.9

B(TO LO)

E(TO LO)

353 – 250 162 – – 346 – 255 145 – –

B(TO LO)

E(TO LO)

A modes have been clearly distinguished. In accordance with selection rules (Table 2), the difference between presented configurations is related to the presence of B(TO) modes at X 0 ðY 0 ; Y 0 ÞX 0 in comparison with X 0 ðZ 0 ; Z 0 ÞX 0 . The subtracted spectrum (top inset of Fig. 5) obtained by normalizing with A mode intensities clearly shows the unidentified feature X and B(TO) modes of KS. The selection rules (Table 2) confirm disparity of feature X to B1 mode of ST. The values of the B(TO) modes are found to be 352, 245 and 161 cm1. Moreover, the value of the dominated B(TO) mode corresponds to the position determined from the subtracted spectrum at XðY; YÞX and XðZ; ZÞX configurations (inset of Fig. 2). The difference of X/B(TO) ratio between subtracted spectra determined from (1 0 0) and (1 1 0) planes is related to a different values of c and d used in Raman tensor for B mode (Table 1). The identical spectra for X 0 ðY 0 ; Z 0 ÞX 0 and X 0 ðZ 0 ; Y 0 ÞX 0 configurations are observed to be dominated by E(TO) and E(LO) modes. To determine the position of E(TO) and E(LO) modes, the subtracted spectrum (bottom inset of Fig. 5) from (1 0 0) plane at XðY; ZÞX and (1 1 0) plane at X 0 ðY 0 ; Z 0 ÞX 0 has been obtained. As a result, the features at 341, 246 and 143 cm1 correspond to E(TO) modes. Taking into account the values for E(TO), the position of E(LO) modes are found to be 346, 255 and 145 cm1. Fig. 6 shows the polarization-dependent data obtained from (1 1 2) basal plane (see Fig. 1e). In accordance with selection rules (Table 2), the spectra at X 00 ðY 00 ; Y 00 ÞX 00 and X 00 ðZ 00 ; Z 00 ÞX 00 configurations are different mostly because of E(TO) and E(LO) modes. The values of A, B(TO), B(LO), E(TO) and E(LO) modes determined above for (1 0 0), (0 0 1) and (1 1 0) planes agree quite well to the position of modes at X 00 ðY 00 ; Y 00 ÞX 00 and X 00 ðZ 00 ; Z 00 ÞX 00 . For spectra at X 00 ðY 00 ; Z 00 ÞX 00 and X 00 ðZ 00 ; Y 00 ÞX 00 configurations, B(TO), B(LO), E(TO) and E(LO) modes are detected, and their locations also correspond to the positions of the respective modes obtained from (1 0 0) and (1 1 0) planes. The values of the Raman active modes determined by polarization-dependent measurements from different crystallographic planes are presented in Table 3 together with the data obtained from theoretical calculations [9,10]. As one can see, the experimental results correspond for KS structure much better than for ST. That is consistent with polarization-dependent selection rules of Raman-active modes. Should be noted that some of the theoretically predicted B(TO) and B(LO) modes as well as E(TO) and E(LO) modes are undetected experimentally. In presented work, the modes below 130 cm1 cannot be determined because of the edge filter used in Raman system. The small deviation from the polarization selection rules related to the presence of a trace of the most intensive A mode forbidden theoretically at some config-

374.05/366.4 313.19/336.1 254.73/285.1 168.21/179.9 98.83/104.3 87.51/93.1 366.35/353.2 293.44/314.1 257.85/289.8 151.05/166.2 106.00/101.4 83.65/79.2

A2 B1 B2(TO LO)

E(TO LO)

370.63/364.2 291.82/320.6 150.91/171.1 95.86/96.4 364.87/353.7 275.52/311.9 246.58/283.3 162.63/171.0 97.38/106.9 78.73/75.5

urations can be explained by the extinction ratio of the polarizer. Moreover the origin of feature X as well as of the features higher 360 cm1 is unclear at present. In comparison with the previous Raman measurements from CZTS compounds obtained by different synthesized methods [8,10–13,19–21], the most intense A mode has been observed between 331 and 338 cm1. The value of 338 cm1 determined for CZTS films is the most common [8,10,12,13,20]. However, the most recent publication on KS CZTS single crystal shows the presence of the intensive A mode centered at 336 cm1 [21]. From polarization-dependent measurements of CZTS powder samples [22], changing the location of the measuring spot the Raman spectrum is dominated by 331 cm1 or 337 cm1 related to the existence of local structural inhomogeneities. While for the presented CZTS single crystal, A mode showing the same position even measured from different planes that confirms the homogeneity of the sample. In addition, the features identified for CZTS powder samples [22] with E or B symmetry are in a good agreement with the modes assigned in present work. Some of the single-phase CZTS thin films fabricated through sulfurization of stacked metallic films also demonstrate the dominated peak at the values lower 338 cm1 [23]. The difference observed between the position of A mode for bulk and thin film materials is still unclear. But comparing different CZTS samples one can observe a trend. Once A mode shift to lower value the unidentified mode at 371 cm1 shifts to the higher value and its intensity increases [23]. In the presented work A mode is centered at 334 cm1 and the mode at 371 cm1 is weaker with unclear polarization dependence. It is noted that recently Grossberg et al. reported the observation an additional Raman feature from CZTS polycrystals and attributed to the coexistence of kesterite and disordered kesterite phases [24]. Hence more work needs to be done to determine the shift phenomena of A mode and the origins of the additional features. 5. Summary The polarization-dependent Raman spectra from (1 0 0), (0 0 1), (1 1 0) and (1 1 2) planes of CZTS single crystals are measured. From a comprehensive analysis of the experimental spectra and comparison with the results of theoretical calculations, the positions and symmetry assignment of the observed Raman features are determined and identified. According to the selection rules, the values of A mode are found to be 334, 306 and 285 cm1; B(TO) mode are 352, 245 and 160 cm1; B(LO) mode are 353, 250 and 162 cm1; E(TO) mode are 341, 246 and 143 cm1, and E(LO) modes are 346, 255 and 145 cm1. The results reveal that the

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investigated CZTS single crystals have been crystallized in the kesterite structure. The obtained data may be used to clarify the existence of structural or phase inhomogeneities in CZTS absorber films of the solar cells. Acknowledgements The authors acknowledge the support of National Science Council of Taiwan under projects NSC 100-2112-M-011-001-MY3 and NSC 101-2811-M-011-002. The authors acknowledge also Sergiu Levcenco for the contribution to the growth of single crystals and useful discussion. References [1] A. Weber, H. Krauth, S. Perlt, B. Schubert, I. Kötschau, S. Schorr, H.W. Schock, Thin Solid Films 517 (2009) 2524–2526. [2] H. Katagiri, K. Jimbo, W.S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Thin Solid Films 517 (2009) 2455–2560. [3] G. Suresh Babu, Y.B. Kishore Kumar, P. Uday Bhaskar, V. Sundara Raja, Sol. Energy Mater. Sol. Cells 94 (2010) 221–226. [4] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, Sol. Energy Mater. Sol. Cells 95 (2011) 1421–1436. [5] K. Ito, T. Nakazawa, Jpn. J. Appl. Phys. 27 (1988) 2094–2097.

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