Investigations on co-evaporated Cu2SnSe3 and Cu2SnSe3–ZnSe thin films

Applied Surface Science 257 (2011) 8529–8534

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Investigations on co-evaporated Cu2 SnSe3 and Cu2 SnSe3 –ZnSe thin films P. Uday Bhaskar, G. Suresh Babu, Y.B. Kishore Kumar, V. Sundara Raja ∗ Solar Energy Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517502, India

a r t i c l e

i n f o

Article history: Received 18 February 2011 Received in revised form 28 April 2011 Accepted 2 May 2011 Available online 7 May 2011 Keywords: Cu2 SnSe3 Cu2 SnSe3 –ZnSe Co-evaporation X-Ray Diffraction Raman studies Optical studies

a b s t r a c t Cu2 SnSe3 is an important precursor material for the growth of Cu2 ZnSnSe4 , an emerging solar cell absorber layer via solid state reaction of Cu2 SnSe3 and ZnSe. In this study, we have grown Cu2 SnSe3 (CTSe) and Cu2 SnSe3 –ZnSe (20%) films onto soda-lime glass substrates held at 573 K by co-evaporation technique. The effect of annealing of these films at 723 K for an hour in selenium atmosphere is also investigated. XRD studies of as-deposited Cu2 SnSe3 and Cu2 SnSe3 –ZnSe films indicated SnSe as secondary phase which disappeared on annealing. The direct optical band gap of annealed Cu2 SnSe3 and Cu2 SnSe3 –ZnSe films were found to be 0.90 eV and 0.94 eV respectively. Raman spectroscopy studies were used to understand the effect of ZnSe on the properties of Cu2 SnSe3 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction Ternary and multinary compound semiconducting materials with direct optical band gap between 1.1 to 1.5 eV are being explored as candidates for absorber layer in thin film heterojunction solar cells. CuInGaSe2 (CIGS) thin film solar cells achieved a record efficiency of 20.3% [1] at laboratory level. However, the elements In and Ga present in this material are expensive and scarce. Cu2 ZnSnS4 (CZTS) and Cu2 ZnSnSe4 (CZTSe) have received much attention in recent years as alternative solar cell absorber layers, owing to their suitable properties and non-toxic nature. CZTS, CZTSe and Cu2 ZnSn(S,Se)4 based thin film solar cells with laboratory efficiencies of 6.77% [2], 3.22% [3] and 9.45% [4] have been reported. Hergert and Hock [5] predicted the formation reaction of Cu2 ZnSnX4 (X = S, Se) and suggested the following reaction scheme as one of the routes for the synthesis of CZTSe. Cu2 SnSe3 + ZnSe → Cu2 ZnSnSe4 Thus Cu2 SnSe3 (CTSe) is an important precursor layer for the growth of CZTSe via solid state reaction with ZnSe and a thorough understanding of the growth and properties of these precursor layers is very much essential. Techniques like co-evaporation [6], DC sputtering [7,8] have been used earlier to deposit CTSe thin films. The present paper reports the structural and optical properties of CTSe films deposited using co-evaporation technique. The effect of

∗ Corresponding author. Tel.: +91 877 2289472; fax: +91 877 2248485. E-mail address: [email protected] (V. Sundara Raja). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.008

addition of a small amount of ZnSe (20%) on the properties of CTSe is also investigated in order to understand how ZnSe influences the properties of CTSe. The effect of annealing CTSe and CTSe–ZnSe films on properties of these films is also discussed.

2. Experimental Thin films of CTSe and CTSe–ZnSe (20%) were prepared by co-evaporation technique onto soda-lime glass substrates using Hind Hivac Box coater unit (BC-300). Chemically and ultrasonically cleaned glass substrates were used to deposit the films. Prior to the deposition, the glass slides were subjected to ion bombardment. Spectroscopically pure Cu, Sn, Se and ZnSe (Sigma-Aldrich, USA) were used as the source materials to deposit CTSe and CTSe–ZnSe thin films at Ts = 573 K. To achieve the desired evaporation rates, each source is pre-calibrated using a quartz crystal thickness monitor. Selenium evaporation rate was kept slightly higher than the stoichiometric requirement to compensate for the loss of selenium due to re-evaporation. The base pressure in the vacuum coating unit was maintained at 5 × 10−6 m bar and working pressure as 2 × 10−5 m bar. A few films were taken out from the chamber after deposition for the sake of characterizing the as-deposited films to understand their properties and the remaining films were annealed in selenium atmosphere starting from room temperature to 723 K at the rate of 10 K/min where they were kept for an hour and slowly cooled to room temperature at the rate of 5 K/min using the programmable integrative derivative (PID) controller. The films were analyzed by studying their composition, structural and optical properties. The film thickness was determined

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Table 1 Elemental composition of as-deposited and annealed CTSe and CTSe–ZnSe films. Sample code

CTSe CTSe a CTSe–ZnSe CTSe–ZnSe a

Elemental composition (at%) Cu

Zn

Sn

Se

28.6 30.8 20.6 23.7

– – 3.43 4.90

18.5 18.3 21.7 18.5

52.9 50.9 54.4 52.9

from its deposited mass measured using METTLER microbalance (Model-AE240) and bulk density. Spectral transmittance and reflectance of the films was recorded using UV–Vis–NIR double beam spectrophotometer (PerkinElmer, Model-LAMDA 950) in the wavelength region 300–2500 nm. X-Ray diffraction (XRD) patterns of the films were taken using SEIFERT X-ray diffractometer (Model3003TT) with Cu K␣ radiation ( = 0.15406 nm). Microstructure of the films was recorded using Carl Zeiss scanning electron microscope (SEM) (Model-EVO MA15) and elemental composition was determined using energy dispersive spectrometer (Oxford Instruments, UK, Model-INCA 250) attached to the SEM. Raman spectra were recorded using confocal microscope Raman spectrometer (Horiba Jobin Yvon Model T64000). The spectra were recorded in back scattered mode using Argon ion laser source ( = 514.532 nm). 3. Results and discussion 3.1. Composition analysis

3.2. Structural analysis Fig. 1(a) shows the X-ray diffraction pattern of as-deposited CTSe films deposited at Ts = 573 K. Films are polycrystalline in nature and contain SnSe as the secondary phase as indicated by its (4 0 0) peak in the diffraction pattern [9]. On annealing these films at 723 K, the SnSe phase disappeared and there is a slight improvement in crystallinity as seen from Fig. 1(b). The observed d-spacings agree well with d-spacings of intense lines reported by Marcano et al. [10] who reported the structure to be monoclinic. These intense lines of monoclinic structure are also common to sphalerite superstructure [11] and the less intense lines which are characteristic of monoclinic structure alone are not present in our case. Palatnik et al. [12] and Sharma et al. [13] reported the structure of Cu2 SnSe3 to be sphalerite. From a detailed Reitveld refinement analysis of XRD pattern of Cu2 SnSe3 ingots, Delgado et al. [14] confirmed the structure of Cu2 SnSe3 to be monoclinic. From structural investigations on Cu2 Sn1−x Gex Se3 alloys (x = 0–0.5), Skoug et al. [15] reported the structure of slow water cooled Cu2 SnSe3 samples as monoclinic while water quenched samples to be disordered zincblende-like structure indicating that the structure depends on growth conditions of the sample. A more recent single-crystal XRD analysis of Cu2 SnSe3 by Gulay et al. [16] confirmed again the structure to be monoclinic and explained the reasons for these modifications. Since the Cu2 SnSe3 thin films in present study are not grown at very slow growth rates usually employed to grow single crystals, the CTSe films in present study crystallized in dis-

Fig. 1. X-ray diffraction patterns of as-deposited and annealed films. CTSe, (b) annealed CTSe, (c) CTSe–ZnSe, (d) annealed CTSe–ZnSe.

ordered cubic zincblende-like (sphalerite) structure. The lattice parameter is found to be 0.569 nm. Fig. 1(c) shows the XRD pattern of as-deposited CTSe–ZnSe (20%) films. As in the case of as-deposited CTSe films, SnSe is observed as the secondary phase. The characteristic peaks due to ZnSe are

179 187 235

c Intensity (a.u)

The elemental composition of CTSe and CTSe–ZnSe (20%) determined through EDS analysis is shown in Table 1. The top two rows show the composition of as-deposited (Ts = 523 K) and annealed (TA = 723 K) CTSe films. The films are Cu-poor and slightly Sn-rich. On annealing, film composition improved slightly but still the films are Cu-poor and Sn-rich. The bottom two rows, shown in bold, indicate the composition of as-deposited and annealed CTSe–ZnSe films.

179 187

235

b

179 235

a 100

200

300

400

500

Wavenumber (cm-1) Fig. 2. Raman spectra of CTSe and CTSe–ZnSe films as-deposited CTSe, (b) asdeposited CTSe–ZnSe, (c) annealed CTSe–ZnSe.

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Fig. 3. SEM images of as-deposited and annealed films. CTSe, (b) annealed CTSe, (c) CTSe–ZnSe, (d) annealed CTSe–ZnSe.

not explicitly seen. Fig. 1(d) shows the XRD pattern of CTSe–ZnSe films annealed at TA = 723 K. On annealing, crystallinity of these films improved and the peak corresponding to tin selenide phase disappeared. Raman spectroscopy is used as a complimentary tool to know the presence of different phases present in the film. Fig. 2(a) shows the unpolarized, room temperature Raman spectrum of asdeposited CTSe films. It shows an intense peak at 179 cm−1 , a small

CTSe 30

CTSe a

b

25

a

Transmittance (T%)

I 20

15

II 10

5

0 300

600

900

1200

1500

1800

2100

2400

Wavelength (nm) Fig. 4. Spectral transmittance curves of (a) as-deposited CTSe and (b) annealed CTSe films.

hump at 235 cm−1 . The intense peak at 179 cm−1 and the hump at 235 cm−1 are attributed to CTSe phase which is in agreement with the work reported earlier [17,18]. XRD pattern of as-deposited CTSe film showed the peak corresponding to SnSe phase also. Raman lines for SnSe crystals were reported to occur at 37, 74, 142 and 156 cm−1 [19] at 20 K and 33, 71, 133 and 151 cm−1 [20] at room temperature, the intense observed among them being at 37 and 74 cm−1 [19]. Raman signatures corresponding to this phase could not be observed in Fig. 2(a) probably due to its minute presence or difference in sensitivity. Raman spectrum of annealed CTSe films is not quite different from the one shown in Fig. 2(a) and hence it is not shown. Fig. 2(b) shows the Raman spectrum of as-deposited CTSe–ZnSe films. There are two broad peaks at 179 and 187 cm−1 along with a hump at 235 cm−1 . The peak at 179 cm−1 and the hump at 235 cm−1 are due to CTSe phase. The CTSe peak is not as sharp as in Fig. 2(a) where it is only pure CTSe phase. Raman peaks of ZnSe were reported to occur at 252 cm−1 [21]. We could not see explicitly any Raman signature corresponding to ZnSe. However, a broad peak centered around 187 cm−1 is seen in addition to CTSe peak at 179 cm−1 . The absence of Raman signatures corresponding to ZnSe, broadening of Raman line at 180 cm−1 corresponding to CTSe and the additional broad peak indicate the incorporation of ZnSe in the CTSe lattice. Raman peaks of CZTSe were reported [17] to occur at 172 cm−1 , 196 cm−1 and 231 cm−1 , the intense being 196 cm−1 which obviously indicates that CZTSe phase is not formed due to lack of required quantity of ZnSe as per stoichiometry for the total conversion of CTSe to CZTSe. We thus feel that the additional peak at 187 cm−1 is due to the inclusion of ZnSe into CTSe lattice. Fig. 2(c) shows the Raman spectrum of annealed CTSe–ZnSe. On annealing, the intensity of CTSe peak at 179 cm−1 increased slightly while there is no noticeable improvement in intensity of the peak at 187 cm−1 corresponding to CTSe–ZnSe. The increase in intensity of Raman lines at 179 cm−1 with marginal change

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3.50E+009

a(I)

6.00E+010

CTSe

3.00E+009

-2

4.00E+010

(αhν) ev cm

-2

2

2.00E+009

3.00E+010

2

2

2

CTSe

5.00E+010

2.50E+009

(αhν) ev cm

a(II)

1.50E+009

2.00E+010

1.00E+009 1.00E+010 5.00E+008 0.6

0.8

1.0

1.2

0.00E+000 1.0

1.2

hν (eV) 1.00E+010

b

1.4

1.6

1.8

hν (eV) CTSe a

6.00E+009

2

2

(αhν) ev cm

-2

8.00E+009

4.00E+009

2.00E+009

0.00E+000 0.4

0.6

0.8

1.0

1.2

1.4

h ν (eV) Fig. 5. (˛h)2 versus h plots of (a) as-deposited CTSe and (b) annealed CTSe films.

in the intensity of the peak at 187 cm−1 indicates that improvement in CTSe phase is more on annealing compared to CTSe–ZnSe phase.

4. Microstructure Fig. 3(a) and (b) shows the microstructure of as-deposited and annealed CTSe films respectively. Almost uniform and distinct grains are seen in the case of as-deposited CTSe films with very finely dispersed grains in the interspaces. On annealing, clustering of grains took place with small void formation. These voids might be due to loss of SnSe from the film. Figs. 3(c) and (d) show the microstructure of as-deposited and annealed CTSe–ZnSe films respectively. The extent of uniform, distinct grains decreased on inclusion of ZnSe into the matrix and a few large grains have emerged. On annealing CTSe–ZnSe film, there is a marginal improvement in crystallinity but there is no void formation as observed in the case of annealed CTSe films.

4.1. Optical absorption studies Fig. 4 shows the spectral transmittance (T ) curves of asdeposited and annealed CTSe films. The optical absorption coefficient (˛) was determined from the measured spectral transmittance (T ) and reflectance (R ) data using the formula [22].



˛=

(1 − R )2 1 ln t T



(1)

where ‘t’ is the thickness of the film. For direct-allowed transitions between parabolic bands, the dependence of (˛h) is given by the relation ˛h = A(h − Eg )

1/2

(2)

where A is constant. The spectral transmittance curve (a) in Fig. 4 corresponding to as-deposited CTSe films shows two transitions labeled I and II. Fig. 5(a) shows the plot of (˛h)2 versus h corresponding to these two transitions. The optical band gaps obtained by extrapolating the linear region onto h-axis are found to be 0.92 eV and 1.32 eV which are attributed to direct optical band gaps of CTSe and SnSe phases respectively. The reported values of direct

P. Uday Bhaskar et al. / Applied Surface Science 257 (2011) 8529–8534

1.40E+009

70

b 50

I

40

a 30

II

20

10

0 300

600

900

1200

1500

a(II)

CTSe - ZnSe

1.30E+009 4.00E+010

-2

(αhν) ev cm

3.00E+010

2

2

(αhν) ev cm

-2

1.20E+009

2

2

1.00E+009 9.00E+008

2.00E+010

8.00E+008 1.00E+010

7.00E+008 6.00E+008

0.00E+000 0.6

0.8

1.0

1.2

1.0

1.2

hν ( eV) 1.00E+009

1.4

1.6

1.8

hν ( eV)

CTSe - ZnSe a

b(I)

b(II)

6.00E+010

CTSe - ZnSe a

8.00E+008

-2

4.00E+010

2

(αhν) ev cm

4.00E+008

3.00E+010

2

(αhν)2 ev2 cm-2

5.00E+010

6.00E+008

2.00E+010

2.00E+008 1.00E+010 0.00E+000

0.00E+000 0.6

0.8

1.0

hν ( eV)

1.2

2100

2400

Fig. 6. Spectral transmittance curves of (a) as-deposited CTSe–ZnSe and (b) annealed. CTSe–ZnSe films.

5.00E+010

1.10E+009

1800

Wavelength (nm)

CTSe - ZnSe

a(I)

CTSe - ZnSe CTSe - ZnSe a

60

Transmittance (T%)

optical band gap of CTSe are in the range 0.74 eV–0.843 eV [6–8,11] and for SnSe the reported values are 1.21 eV [23], 1.3 eV [24] and 1.18 eV–1.50 eV [25]. In our earlier investigations on Cu2 SnSe3 [6], we observed two direct optical transitions, 0.74 eV and 1.12 eV, which were attributed to free to bound transition and from spinorbit splitting level of the valence band to the conduction band respectively. The Cu/Sn ratio in these samples is 1.94. Marcano et al. [11] reported a band gap of 0.84 eV for CTSe single crystals with Cu/Sn ratio 1.7. Kuo et al. [7] reported the band gap of CTSe films to be 0.76 eV with Cu/Sn = 2.2. A close look of these reported values clearly shows that the band gap increases with decrease in Cu/Sn ratio. In the present investigations, Cu/Sn = 1.68 and observed band gap of CTSe is 0.92 eV which is attributed to band to band transition. From PL measurements on CZTSe monograins, Grossberg et al. [26] reported the band to band transition energy of Cu2 SnSe3 phase as 0.86 eV. The slight difference in the band to band energy might be due to difference in composition and/or crystallinity. On annealing, the transition II due to SnSe phase disappeared (Fig. 4 Curve b) and the band gap is found to be 0.90 eV in this case (Fig. 5b). Fig. 6 shows the spectral transmittance curves of as-deposited and annealed CTSe–ZnSe films. The optical band gaps corresponding to transitions I and II observed in the as-deposited (Curve a) are found to be 0.90 eV and 1.30 eV [Fig. 7(a)] which are attributed to CTSe and SnSe phases respectively as in the earlier case. The direct

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1.0

1.2

1.4 1.6 hν ( eV)

1.8

Fig. 7. (˛h)2 versus h plots of (a) as-deposited CTSe–ZnSe and (b) annealed CTSe–ZnSe.

2.0

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band gaps of annealed CTSe–ZnSe films obtained from Fig. 7(b) are found to be 0.94 eV and 1.42 eV which are due to CTSe–ZnSe and SnSe phases respectively. The inclusion of ZnSe might be the reason for the slight increase in the band gap of CTSe and SnSe phases on annealing. 5. Conclusions Cu2 SnSe3 and Cu2 SnSe3 –ZnSe (20%) films were successfully obtained through co-evaporation technique and subsequent annealing of these films at 723 K for an hour in selenium atmosphere. Powder XRD studies indicated that as-deposited CTSe and CTSe–ZnSe contained SnSe as the secondary phase which on annealing disappeared. Raman spectroscopy studies of CTSe–ZnSe film showed a broad peak centered around 187 cm−1 in addition to the one corresponding to CTSe indicating the inclusion of ZnSe into CTSe matrix. The optical band gap of annealed Cu2 SnSe3 and Cu2 SnSe3 –ZnSe were found to be 0.90 eV and 0.94 eV respectively. However, further studies with higher ZnSe content in Cu2 SnSe3 films are needed to draw much more precise conclusions on the influence of ZnSe in CTSe films. We feel such studies with higher percentage of ZnSe in CTSe will pave the way for better understanding of the growth of CZTSe through this approach. Acknowledgments One of the authors Mr. P. Uday Bhaskar gratefully acknowledges the National Renewable Energy fellowship grant provided by the Ministry of New and Renewable Energy Sources, New Delhi, India. We gratefully acknowledge the help of Dr. P.S.R. Prasad, ScientistEII, NGRI, Hyderabad in arranging Raman spectra on Jobin Yvon (Model T64000). References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. Res. Appl. (2011), doi:10.1002/pip.1078.

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