Synthesis, structure and optical properties of Sb2Se3

Materials Science in Semiconductor Processing 16 (2013) 179–184

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Synthesis, structure and optical properties of Sb2Se3 H. Maghraoui-Meherzi a,n, T. Ben Nasr b, M. Dachraoui a a b

Laboratoire LCAE, De´partement de Chimie, Faculte´ des Sciences, Universite´ Tunis El-Manar 2092, Tunisia Laboratoire LPMC, De´partement de Physique, Faculte´ des Sciences, Universite´ Tunis El-Manar 2092, Tunisia

a r t i c l e i n f o

abstract

Available online 23 May 2012

Antimony selenide (Sb2Se3) films were deposited by chemical bath deposition on glass substrates. Deposited films were characterized by X-ray diffraction, scanning electron microscopy and UV–vis–NIR spectroscopy. X-ray analysis of these films confirms an orthorhombic structure. Evaluation of band gap from optical spectra shows absorption due to indirect transition occurring at 1.16 eV. For the first time, optical properties including refractive index, extinction coefficient, optical conductivity, reflectivity, absorption coefficient and energy-loss spectrum were calculated by a self-consistent approach using the Wien2k code. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Chalcogenides Chemical bath deposition (CBD) Structural and optical properties Wien2k code

1. Introduction Antimony selenide (Sb2Se3) belongs to V–VI family with orthorhombic crystal structure, in which each Sbatom and each Se-atom is bound to three atoms of the opposite kind that are then held together in the crystal by weak secondary bonds [1]. It finds applications as a thermoelectric and as a pristine material for memory switching [2]. Also, it is used for optical coatings in thermophotovoltaic applications due to its high refractive index [3]. Optical band gaps due to both direct and indirect transitions in the range 1–1.2 eV are reported [4–5]. This makes it an alternative material for use as an absorber in solar cells [6]. A photoelectrochemical solar cell, employing antimony selenide thin films as photoanode, has shown short circuit current of  0.45 mA/cm2 and open circuit voltage of  0.37 V [7]. Polycrystalline thin films of the material have been prepared by vacuum evaporation [8], electrodeposition methods [9] and spray pyrolysis [10] methods. In this paper, we present the synthesis, structure and optical properties of Sb2Se3

n

Corresponding author. Tel.: þ216 98538091, fax: þ 216 71 885 008. E-mail address: [email protected] (H. Maghraoui-Meherzi). 1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.04.019

prepared by CBD. In general, deposited films obtained by this technique are reported as amorphous [11,12] and a heat treatment was carried out to enhance crystallinity. In this work, we report a polycrystalline films deposited without annealing. Diverse and sometimes contradictory interpretations of the experimental optical data are commonly found in the literature. This is why a theoretical study based on first principles calculations has been conducted to determine the optical band gap and some of the optical constants. Indeed, the calculated band gap is useful for our investigations to determine the nature of the optical transitions.

2. Experimental methods 2.1. Synthesis of Sb2Se3 Antimony (III) chloride precipitates as oxochloride (SbOCl) in water. Presence of strong ligands such as citrate [4], tartrate [13], triethanolamine [4,14] and thiosulfate that form soluble complexes using SbCl3 as starting material in the deposition bath prevents precipitation of basic salts in aqueous solutions. Dissociation equilibrium involving these complexes produces antimony (III) ions in the bath.

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Sodium selenosulfate used as a selenide precursor was prepared in the following manner: a stoichiometric amount of powdered selenium (Aldrich, Z99.999%) was added to 0.5 mol/L aqueous solution of sodium sulfite (Sigma-Aldrich, Z98%). The mixture was subjected to constant stirring and heated at 90 1C for 4 h [15]. Selenosulfate is obtained according to the following reaction schema: (1)

After cooling to room temperature, unreacted selenium was filtered off and selenosulfate solution was placed and stored in a dark bottle. The formation of the Sb2Se3 film is based on slow release of Sb3 þ and Se2  in an aqueous ammonia medium and then subsequent condensation on substrates. When citrate is complexed with antimony (III), it gives (2)

In alkaline medium, hydrolysis of sodium selenosulfate gives Se2  ions as Na2SeSO3 þOH  $Na2SO4 þHSe 

(3)

HSe  þOH  $H2OþSe2 

(4)

Finally, Sb(III)(citrate) complex reacts with Se2  ions in the reaction bath to give Sb2Se3 film formation as

3. Results and discussion (5)

10

15

20

25

30

40

(251)

(510)

(240)

35

(241)

where m is the mass, s is the area and r is the density of films deposited. Structural characterization was carried out by analyzing the X-ray diffraction (XRD) pattern, obtained using a Philips X-ray diffractometer model XPert Pro (l ¼0.15405 nm for Cu-ka). Morphological studies were done using a Quanta 200 scanning electron

(400)

The film thickness is determined by double weight method, using the relation m e¼ ð6Þ rs

(221)

2.2. Films characterization

Fig. 1 shows XRD spectrum of the film deposited for 60 min under previously defined conditions. The film thickness was 0.3 mm. It appears that the film is polycrystalline with an orthorhombic crystal structure and well crystallized with narrow peaks indicating a large grain size. The peak positions obtained experimentally from Fig. 1 are in close agreement with theoretical values for Sb2Se3 thin films (ASTM standard 72-1184). Indexation of the peaks according to this structure is reported in

(230)

The bath for antimony selenide was prepared as follows: 1 g of SbCl3 (Sigma-Aldrich, Z99%) was added with stirring to 37 ml of 1 M sodium citrate solution (Fisher BioReagents, Z99%). This was followed by a sequential addition of 20 ml ammonia (Applichem, 25% pure), 25 ml of 0.4 M sodium selenosulfate and the rest with deionized water to reach 100 ml as final volume. Bath solution was clear and devoid of any precipitate at the beginning. Clean glass substrates were introduced vertically in the bath supported against the wall of the beaker. The bath was maintained at 50 1C for 60 min. At the end, the coated substrates were taken out, washed well with distilled water and dried to give brown adherent uniform films.

(310)

2[Sb3 þ (citrate)]3 þ þ3Se2  -Sb2Se3 þ2citrate

The well-established scheme to calculate electronic properties of solids is based on the DFT [16], for which Walter Kohn has received the Nobel Prize in chemistry in 1998. One among most precise schemes for solving the Kohn–Sham equations is the full potential linearized augmented plane wave (FP-LAPW) method [17]. Calculations presented in this work were performed using the FPLAPW method. We use the Wien2k [18] implementation of the method, which allows inclusion of local orbital in the basis, improving upon linearization and making possible a consistent treatment of semicore and valance states in an energy window, hence ensuring proper orthogonality. We have performed calculations using the generalized gradient approximation (GGA) proposed by Perdew et al. [19]. The electronic configurations for Sb and Se are as follows: Sb: [Kr] 5s24d105p3; Se: [Ar] 4s23d104p4. Values of the atomic radii were 2.4 for Sb and 2 a.u for Se. Convergence of self-consistent iterations was performed for 120 k points inside the irreducible brillouin zone within 10  4 Ry with a cut-off at  7 Ry between valence and core states. In calculations of the optical properties, a dense mesh of uniformly distributed k-points is required. We present calculations with 300 k-points.

(111)

Sb3 þ þcitrate-[Sb(III)(citrate)]3 þ

2.3. Theoretical method

(220)

Na2SO3 þSe$Na2SeSO3

microscopy (SEM). Optical transmittance (T) and reflectance (R) were measured by a Varian UV–vis–NIR spectrophotometer in the 200–2500 nm range.

Intensity (arb. unit)

180

45

2θ (deg.) Fig. 1. X-ray spectra of Sb2Se3 prepared at 50 1C for 60 min.

50

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Fig. 1. One can observe a slight preferential orientation along the (310) plane while in the powder diagram of Sb2Se3 the most intense diffraction peak is (221). SEM images of Sb2Se3 films were shown in Fig. 2 at two magnifications. The films were found to be smooth, dense and without cracks to cover the glass substrate well. Some interesting observation on the nucleation stage may be noted. It is seen that film growth proceeds by nucleation of clusters, which subsequently coagulate to cover the entire substrate surface showing a dense structure of islands uniformly distributed (Fig. 2a). Magnified image of these islands given in Fig. 2b depicts the spindle-shape crystallites of Sb2Se3 phase. There are contradictory reports regarding the nature of the band gap (direct/indirect), as well as a significant range of values for its energy. For instance, in case of thin films, Rajpure et al. [20] reported a direct band gap of

181

2.14 eV for polycrystalline films, Rodriguez-Lazcano et al. [4,21] showed Sb2Se3 films with an indirect band gap of 1–1.2 eV, whereas El-Sayed et al. [22] revealed that both direct and indirect energy gaps exist with values of 1.50 and 1.15 eV, respectively. In order to evaluate the energy band gap and optical transitions, theoretical studies of Sb2Se3 crystal have been conducted using FP-LAPW method. As shown in Fig. 3, the lowest of the conduction band (CBM) is in the G–X region, the highest energy of the valence band (VBM) is located in the regions G–X, U–S, which leads to many possible transitions within a very close energy range. An indirect gap of 0.96 eV (Eg,th: theoretical band gap) was shown with the VBM in U–Y region and the CBM in G–X region. Moreover, a direct gap of 0.95 eV is found in G–X region. The indirect (0.96 eV) and direct (0.95 eV) transitions differ only by 0.01 eV. This diverse and complex nature of optical transitions of Sb2Se3 could possibly be the reason behind the wide variety of Tauc fittings [23] and the incoherency of reported energy band gaps. To get more information about the optical properties of Sb2Se3, transmittance and reflectance have been measured in the 250–2500 nm range (Fig. 4). At low wavelengths (l o500 nm), Sb2Se3 transmittance (T) is close to 0%, whereas the reflectance (R) is ranging between 8 and 10%. Assuming that light scattering at the films surface can be neglected, the absorbance (A) is estimated from A ¼ 12T2R

ð7Þ

Thus, Sb2Se3 films exhibit high absorbance (nearly 90%) at low wavelengths. After the absorption edge, the absorbance strongly decreases. Energy band gap and optical transition type of the film can be determined experimentally by the following

Fig. 2. SEM micrographs of Sb2Se3 thin film prepared at 60 min at: (a) 300  and (b) 5000  magnifications.

Fig. 3. GGA calculated band structure of Sb2Se3.

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80

1000

T% R%

800

60

T, R (%)

600

40 400

20

200

0

0 500

1000

1500 λ (nm)

2000

1 .0

2500

1.5

2 .0

2.5

hν (eV) Fig. 6. Plots of (ahv)(1/2) against hv for Sb2Se3 films prepared at 60 min.

Fig. 4. Optical transmission and reflection spectra of Sb2Se3.

40 13

30 slope = 2.2

12

20

ε (ω)

11

10

10

0 9

-10 8

0 -0.6

-0.4

-0.2

0.0

0.2

0.4

5

0.6

10

15

20

25

20

25

Energy (eV) 40

Fig. 5. Plot of ln(ahv) versus ln(hv  Eg,th).

30

relation [24]:

ahn ¼ Aðhn2Eg Þ

ð8Þ

where A is an energy-independent constant and Eg is the optical band gap. m is a constant that determines type of optical transitions. For indirect allowed transition, m ¼2, and for indirect forbidden transition, m¼3; for direct allowed transition, m ¼1/2 and for direct forbidden transition, m¼3/2. To determine type of optical transition, ln(ahv) versus ln(hv  Eg,th) was plotted in Fig. 5. The m value can be directly determined from slope of the straight line, it was found to be about 2. This suggests that indirect-allowed transitions are dominant in the fundamental absorption edge of the Sb2Se3. An optical band gap of 1.16 eV was found by extrapolating the linear portion of the plot (ahv)1/2 versus (hn) to zero (Fig. 6). However, the band gap of this material is underestimated by DFT, when compared with experimental data. In fact, self-consistent calculations usually underestimate the energy gap [25]. Optical properties of matter can be described by the complex dielectric function e(o), which represents the system linear response to an external electromagnetic

ε2 ω)

m

20

10

0 0

5

10

15

Energy (eV) Fig. 7. Real part (a) and imaginary part (b) of the dielectric function of Sb2Se3.

field with a small wave vector. It can be expressed as

eðoÞ ¼ e1 ðoÞ þ ie2 ðoÞ

ð9Þ

The imaginary part e2(o) was calculated from momentum matrix elements between the occupied and unoccupied wave functions within selection rules using the expression given in Ref. [26]. The real part e1(o) of the

H. Maghraoui-Meherzi et al. / Materials Science in Semiconductor Processing 16 (2013) 179–184

dielectric function e(o) can be derived from the Kramer– Kronig relation. From real and imaginary parts of the frequency dependent dielectric function one can calculate the refractive index n(o), the reflectivity R(o), the absorption coefficient a(o) and the energy loss function L(o) using the expressions given in Ref. [27]. Compounds with orthorhombic symmetry have three non-zero components of the dielectric tensor. These compounds correspond to an electric field perpendicular and parallel to the z-axis, which are indexed as exx, eyy and ezz. The calculated imaginary part for Sb2Se3 is shown in

Fig. 7b. From this figure that there is considerable anisotropy between the three spectra corresponding to different polarizations for this compound and its maximum values are around 2.86, 2.39 and 2.59 for E//x, E//y and E// z, respectively. Results for the dispersive part of the dielectric function, e1(o), are given in Fig. 7a. At high frequencies the zero crossing of e1(o) which corresponds to location of the screened plasma frequency is located at 17.66 eV. The static dielectric constant e1(0) is given by low energy limit of e1(o). It is necessary to emphasize that we do not include phonon contributions to the dielectric screening, and e1(0) corresponds to the static optical dielectric constant. The obtained optical dielectric constants along crystal axes resolved into three components are listed in Table 1. We have estimated average value of zero frequency dielectric constant using the relation

Table 1 The calculated static dielectric e1(0), n(0) and the positions of plasmon peaks appearing in L(o) for the three polarization directions.

e1(0) n(0) Plasmon peaks in L(o) (eV)

E//x

E//y

E//z

15.18 3.90 18.53

21.72 4.65 18.33

22.31 4.72 18.46

183

eð0Þ ¼ 1=3ðexx ð0Þ þ eyy ð0Þ þ ezz ð0ÞÞ

ð10Þ

From this relation, we have obtained e(0) equal to 14.59. In order to estimate degree of anisotropy, we

7 4

6

3

4

k (ω)

n(ω)

5

3

2

2 1 1

12000

0.6

10000

0.5

8000

0.4

R(ω)

0 0.7

σ (w)

0 14000

6000

0.3

4000

0.2

2000

0.1

0

0.0 5

160 4 3

L(ω)

α (w)

120 80 40

2 1 0

0 0

5

10

15

Energy (eV)

20

25

0

5

10

15

20

25

Energy (eV)

Fig. 8. Calculated optical constants of Sb2Se3: (a) refractive index, (b) extinction coefficient, (c) optical conductivity, (d) reflectivity, (e) absorption coefficient and (f) energy-loss spectrum.

184

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determine the ratio ðeyy ð0Þ=ezz ð0ÞÞ as a measure for average optical anisotropy. This ratio is equal to 0.98 in our case, if one considers the value reported in Ref. [28] of the ratio equal to 1.05 in case of Sb2S3, we can note that Sb2Se3 is less anisotropic than Sb2S3. Calculated results of refractive index, extinction coefficient, optical conductivity, reflectivity, absorption coefficient and electron energy-loss are displayed in Fig. 8. Refractive index n(o) is shown in Fig. 8a. It increases with energy in transparency region reaching a peak at about 1.56 eV. Beyond this energy, the refractive index drops sharply. At low frequency (o ¼0), we p get the ffiffiffiffiffiffiffiffiffiffi ffi following relation between n(0)and e1 ð0Þ : nð0Þ ¼ e1 ð0Þ. Our calculated n(0) resolved into three polarization directions is given in Table 1. The static refractive index n(0) can be estimated to be 3.82, which is close to 3.90 value reported by El-Shair et al. [29]. The extinction coefficient k(o) is presented in Fig. 8b. The local maxima of k(o) correspond to the zero of e1(o). For photovoltaic purposes, it is very important to determine the region of higher conductivity. Optical conductivity curve is related with imaginary part of dielectric function e2(o) by the relation

sðoÞ ¼

o 4p

e2 ðoÞ

ð11Þ

In Fig. 8c the higher peak of conductivity is located around 2.70 eV. From the reflectivity spectra (Fig. 8d), we note presence of two prominent structures corresponding to inter-band transitions. The maximum reflectivity occurs in energy range between 2.43 and 5.03 eV. In Fig. 8e we display calculated interband absorption coefficient for Sb2Se3. No fine structure appears in this spectrum. Very intense absorption due to two phonon processes occurs between 0.96 and 25 eV. Plot of the energy loss function L(o) shows a sharper peak situated at 18.53 eV (Fig. 8f). This peak defines the screened plasma frequency op [30], it corresponds to the abrupt reduction of the reflectivity spectrum R(o) and to the zero crossing of e1(o). 4. Conclusion Results of our experimental and theoretical investigation of the structural and optical properties of orthorhombic Sb2Se3 have been reported. SEM images show good film coverage that consists of spindle-shape grains. Optical transmittance and reflectance spectra at near normal incidence were performed over a large spectral range. In support of our preliminary measurements, we have performed calculations of the optical properties using the FP-LAPW method. Optical absorption results

suggest that the indirect allowed transitions are dominant in the fundamental absorption edge with optical band gap value of 1.16 eV. References [1] O. Madelung, Semiconductors data handbook, 3rd ed., Springer, 2004. [2] N.S. Platakis, H.C. Gatos, Physica Status Solidi A 13 (1972) k1. [3] P.M. Fourspring, D.M. DePoy, T.D. Rahmlow, J.E. Lazowasem, E.J. Gratrix, Applied Optics 45 (2006) 1356. [4] Y. Rodriguez-Lazcano, Y. Pena, M.T.S. Nair, P.K. Nair, Thin solid Films 493 (2005) 77. [5] S. Messina, M.T.S. Nair, P.K. Nair, Journal of the Electrochemical Society 156 (2009) H327. [6] P. Arun, A.G. Vedeshwar, Thin Solid Films 335 (1998) 270. [7] R.N. Bhattacharya, P. Pramanik, Solar Energy Materials 6 (1982) 317. [8] J.G.N. Braithwite, Proceedings of the Physical Society Section B 64 (1951) 274. [9] A.P. Torane, K.Y. Rajapure, C.H. Bhosale, Materials Chemistry and Physics 61 (1999) 219. [10] K.Y. Rajapure, C.H. Bhosale, Materials Chemistry and Physics 62 (2000) 169. [11] R.S. Mane, C.D. Lokhande, Materials Chemistry and Physics 65 (2000) 1. [12] P. Pramanik, R.N. Bhattacharya, Journal of Solid State Chemistry 44 (1982) 425. [13] J. Lu, Q. Han, X. Yang, L. Lu, X. Wang, Materials Letters 62 (2008) 2415. [14] C.D. Lokhande, B.R. Sankapal, R.S. Mane, H.M. Pathan, M. Muller, M. Giersig, V. Ganesan, Applied Surface Science 193 (2002) 1. [15] B. Pejova, M. Najdoski, I. Grozdanov, S.K. Dey, Materials Letters 43 (2000) 269. [16] K. Schwarz, Journal of Solid State Chemistry 176 (2003) 319. [17] D.J. Singh, Plane Waves, Pseudopotential and the LAPW Method, Kluwer Academic Publisher, Boston, Dortrecht, London, 1994. [18] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kavanicka, J. Luitz, WIEN2K, an augmented plane wave plus local orbitals program for calculating crystal properties, Vienna University of Technology, Austria, 2001. [19] J.P. Perdew, S. Burke, M. Ernzerhaf, Physical Review Letters 77 (1996) 3865. [20] K.Y. Rajapure, C.D. Lokhande, C.H. Bhosale, Thin solid Films 311 (1997) 114. [21] Y. Rodriguez-Lazcano, L. Guerrero, O. Gomez Daza, M.T.S. Nair, P.K. Nair, Superficies y Vacio 9 (1999) 100. [22] E.A. El-Sayed, A.M. Moustafa, S.Y. Marzouk, Physica B 404 (2009) 1119. [23] J. Tauc, Amorphous and liquid semiconductors, Plenum, New York, 1974 [chapter 4]. [24] J.I. Pankove, Optical processes in semiconductors, Prentice-Hall, New Jersey, 1971. [25] P. Dufek, P. Blaha, K. Schwarz, Physical Review B 50 (1994) 7279. [26] C. Ambrosch-Draxl, J.O. Sofo, Computer Physics Communications 175 (2006) 1. [27] S. Saha, T.P. Sinha, Physical Review B 62 (2000) 8828. [28] T. Ben Nasr, H. Maghraoui-Meherzi, H. Ben Abdallah, R. Bennaceur, Physica B 406 (2011) 287. [29] H.T. El-Shair, A.M. Ibrahim, E.A. El-Wahabb, M.A. Afify, F.A. ElSalam, Vacuum 42 (1991) 911. [30] M. Fox, Optical Properties of Solids, Oxford University Press, New York, 2001.