The influence of film deposition temperature on the subsequent post-annealing and crystallization of sputtered Sb2S3 thin films

Accepted Manuscript The influence of film deposition temperature on the subsequent post-annealing and crystallization of sputtered Sb2S3 thin films M.I. Medina-Montes, Z. Montiel-González, N.R. Mathews, X. Mathew PII:

S0022-3697(16)31215-X

DOI:

10.1016/j.jpcs.2017.07.035

Reference:

PCS 8153

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 2 December 2016 Revised Date:

29 April 2017

Accepted Date: 31 July 2017

Please cite this article as: M.I. Medina-Montes, Z. Montiel-González, N.R. Mathews, X. Mathew, The influence of film deposition temperature on the subsequent post-annealing and crystallization of sputtered Sb2S3 thin films, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/ j.jpcs.2017.07.035. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The influence of film deposition temperature on the subsequent post-annealing and

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crystallization of sputtered Sb2S3 thin films

M.I. Medina-Montesa, §, Z. Montiel-Gonzálezb, N. R. Mathewsa, and X. Mathewa, * a

Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco,

b

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Morelos 62580, México.

CONACYT-Centro de Investigación en Materiales Avanzados S.C., Unidad Monterrey,

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Apodaca, Nuevo León 66628, México.

* Corresponding author (X. Mathew), e-mail: [email protected]

Abstract:

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§ e-mail: [email protected]

Sputter-deposited Sb2S3 thin films were studied to understand the role of the initial film deposition temperature on the subsequent crystallization during the post-annealing in N2-S

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ambient. The films were deposited with substrate temperatures in the range 200 to 350 oC. The as-deposited films were amorphous independent of the substrate temperature, however, after annealing at 300 oC all the films turned in to polycrystalline. It was observed that the

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thermal history (deposition temperature) of the films have a notable influence on the crystallization and grain growth due to post-annealing at 300 oC. The material properties of the annealed film such as: crystallite size, strain, grain size, refractive index, and film stoichiometry showed a dependence on the original film deposition temperature. Furthermore, AFM and SEM micrographs revealed a direct dependence of the morphological features such as grain growth, uniformity and compactness on the thermal history. Studies by variable-angle spectroscopic ellipsometry (VASE) provided some optical parameters including inter-band transitions in the Sb2S3 thin films. We present the

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parameterization of the dielectric function of Sb2S3 using a multi-oscillator model composed by one Tauc-Lorentz and three Lorentz oscillators.

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Keywords: Sb2S3 thin films; sputtering; spectroscopic ellipsometry; optical constants

1. Introduction

Antimony sulfide (Sb2S3) is a low-toxic and earth-abundant semiconductor material,

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used in technological applications such as photo-catalysis [1], photovoltaics [2], and thermoelectric devices [3]. It has a direct band gap with large absorption coefficient (>104 cm-1) in the visible region. Sb2S3 thin films have been developed by chemical methods such

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as CBD [2], spray pyrolysis [4], pulse electrodeposition [5], thermal evaporation [6, 7], and sputtering [8]. Concerning to the physical methods, the as-grown films deposited by thermal evaporation at low substrate temperatures (<225 oC) are amorphous. However, polycrystalline films were obtained with substrate temperature above 225 oC [6, 7] or by a post-annealing of the amorphous films [9, 10]. In contrary to this, the sputtered films were

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amorphous even at substrate temperatures as high as 350 oC [8]. During the post-deposition thermal processing, in addition to the structural transformation of Sb2S3 thin films, morphological, optical and electrical properties are also changed significantly [7, 11]. Uniform and highly adherent thin films were obtained by RF magnetron sputtering, a

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technique with the great potential of scaling to large area for further technological applications. However, the processing of Sb2S3 films by this method has been under-

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explored so far [8, 12].

In this study, we report the development of Sb2S3 thin films by RF magnetron

sputtering at different substrate temperatures, followed by a thermal treatment in sulfur atmosphere at identical conditions. This particular work is motivated by our previous study where we found that the sputter deposited Sb2S3 films are always amorphous even at deposition temperatures as high as 350 oC, and change to polycrystalline after annealing at relatively low temperatures [8]. Hence we undertook this work to explore if the film deposition temperature has any influence on the material properties when the film is converted from amorphous-to-polycrystalline. We found that the morphological evolution

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of the post-annealed film has a strong dependence on the thermal history (deposition temperature) of the film. The extent of grain growth and reduction in inter-grain voids during post-annealing is strongly related to the original deposition temperature. In addition to exploring the structural properties, emphasis was given to study the dependence of

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optical properties of the films on annealing temperature. We used Spectroscopic Ellipsometry (SE), a non-destructive optical technique to derive the optical constants and relate them to the corresponding structural and morphological changes [13]. The analysis by SE requires appropriate dispersion models for parameterization of the dielectric function

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of materials. Specifically, some generalized oscillators such as Tauc-Lorentz and Lorentz dispersion functions have been employed to describe the real and imaginary parts of the

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dielectric function, and to determine the optical band gap and electronic band to band transitions (if applicable) of amorphous and polycrystalline semiconductors [14-16]. A brief review of the theory of these oscillators, which were used for the optical parameterization of our Sb2S3 films is also provided. 2. Experimental details

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Sb2S3 thin films were deposited on glass substrates by RF magnetron sputtering. Films were deposited at substrate temperatures (Ts) in the range 200 to 350 oC in steps of 50 oC while the RF power and working pressure (argon ambient) were maintained at 20 W and 10 mTorr respectively. In order to have films of comparable thickness, the deposition

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rate for each substrate temperature was determined in a prior step and adjusted the deposition time accordingly. The as-deposited Sb2S3 films were amorphous even at Ts =

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350 oC, and changed into polycrystalline after thermal treatment at 300 oC for 30 min in low vacuum (10 Torr) in a nitrogen-sulfur (N2-S) environment. The structure of the films was determined by X-ray diffraction (XRD) using a Rigaku D-MAX 2200 with Cu anode (Cu Kα = 1.54184 Å). Raman spectra were recorded using an integrated micro-Raman system with a 632.8 nm He-Ne laser as the excitation source. Surface morphology images were acquired by atomic force microscopy (AFM) with a DI-Veeco Nanoscope IV system and scanning electron microscopy (Hitachi FESEM S-5500). Elemental composition of the films was studied by energy dispersive X-ray spectroscopy (EDXS). Variable-angle spectroscopic ellipsometry (VASE) measurements were performed using an alpha-SE

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ellipsometer (J. A. Woollam Co.). For electrical measurements, Au/Cu electrodes of 2 mm length and 2 mm separation were thermally evaporated on the films in a coplanar configuration. In order to evaluate the photoresponse of the films, current in dark and under illumination were measured using a computer controlled photoresponse system. The type of

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majority carriers was determined by measurements of thermoelectric voltage using a homebuilt Seebeck system. Photo Luminescence (PL) spectra were taken using a Perkin Elmer LS-55 spectrometer equipped with 325 nm light source. 3. Results and discussion

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3.1.Structural and morphological studies

The as-deposited Sb2S3 thin films at substrate temperatures Ts = 200, 250, 300 and o

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350 C are amorphous as seen in Fig. S1 in supplementary file. However, the films changed to polycrystalline after annealing at 300 oC for 30 min in N2-S environment (Fig. 1). As seen in the XRD patterns, the films crystallized in orthorhombic system (PDF#42-1393)

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with all the diffraction peaks corresponding to stibnite phase.

o

Ts=200 C

o

Ts=250 C

0 100

o

o

(530)

(520)

Ts=350 C (141)

(420)

Ts=300 C

(310) (320) (211) (221)

500

(220)

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(200)

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1000

(120)

Intensity (arb. units)

1500

PDF#42-1393-stibnite Sb2S3

50 0 10

20

30

2θ (degrees)

40

50

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Fig. 1. XRD patterns of the Sb2S3 films deposited at different substrate temperatures (Ts) in the range 200 to 350 oC, and subsequently subjected to annealing in N2-S environment at 300 oC for 30 min.

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Figure 2a shows the evolution in shape and position of the (310) diffraction peak with respect to deposition temperature Ts; as mentioned in section 2 all the films were postannealed at same temperature under same conditions. A powder sample (raw material from Sigma-Aldrich) was used as the reference for the (310) peak position (2θpowder = 25.042 deg). The average crystallite size (D) was calculated from Debye-Scherrer equation


=  , where K=0.9 is the shape factor, λ = 1.54184 Å is the Cu Kα radiation, θ is the

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Bragg angle and β is the Full Width at Half Maximum (FWHM) of the diffraction peak.

The relative change of inter-planar spacing or strain (ε) was estimated according to the equation =

   



, where dfilm and dpowder are the inter-planar spacing for the film

(strained lattice) and powder sample (unstrained lattice), respectively. The tendency of D, FWHM and ε with respect to Ts are shown in Fig. 2b as well. The overall trend establishes that D and ε increase and FWHM decrease as Ts increases, indicating recrystallization

o

(310)

25

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20

1200

15 0.6

250

0.5

800

0.4

300

0.3 0.000

400

0 24

FWHM (deg)

200

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Intensity (arb. units)

(b)

Ts ( C ):

D (nm)

(a)

2θpowder 25

2θ (degrees)

-0.002

350

-0.004 26 200

250

300 o

Ts ( C)

350

ε

1600

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leading to grain growth with Ts [17].

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Fig. 2: (a) - Evolution in shape and position of the (310) diffraction peak with deposition temperature Ts; (b) - Dependence of D, FWHM and ε on Ts. All the films were postannealed at 300 oC for 30 min. in N2-S environment. Orthorhombic lattice constants a, b and c of the Sb2S3 polycrystalline films were   /   /   / 

, and are shown in Table 1. These

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calculated from the equation  =

values are in agreement with the JCPD card # 42-1393 (a =11.239 Å, b =11.313 Å, c =3.8411 Å).

Lattice parameters (Å) a 11.195 11.237 11.209 11.237

b 11.347 10.897 11.313 11.230

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Film deposition temperature (oC) Ts 200 250 300 350

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Table 1. Lattice parameters of the Sb2S3 polycrystalline films. All the films were postannealed as mentioned in section 2.

c 3.8112 3.8268 3.8302 3.8291

In addition, the transition from amorphous to polycrystalline structure after the post-

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annealing treatment was assessed by Raman spectroscopy. Fig. 3a shows the Raman spectra of the films deposited at different temperatures (as-deposited films). The spectra are dominated by broad bands, characteristic of amorphous Sb2S3 [10]. Fig. 3b is the Raman

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spectra of the annealed films showing characteristic bands of Sb2S3. The well-defined sharp bands support the transformation from amorphous to polycrystalline structure. The main observed bands centered at 156, 189, 237, 281 and 310 cm-1 are associated to stibnite

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structure [18]. By de-convoluting the spectra corresponding to the annealed films with Ts 200 oC and 350 oC, some additional bands belonging to stibnite appear at 207 and 300 cm-1 as observed in the spectra shown in figures (c) and (d). The only notable difference between these two spectra is the increase in intensity for the weak bands 156, 207 and 300 cm-1. The very weak band centered at 115 cm-1 (figure b) is attributed to the senarmontite phase (αSb2O3) [19].

o

1000

(a)

Ts ( C): 350 300

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200 o

Ts ( C):

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(b) (c) (d)

(b) (d)

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350 C 156

189

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156

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3000 1500 0 3000

(a) 200 (b) 250 (c) 300 (d) 350

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Intensity (arb. units)

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150

-1

Raman shift (cm )

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Fig. 3: (a) Raman spectra of the as-deposited Sb2S3 films corresponding to different substrate temperatures (Ts); (b) Raman shift of the same Sb2S3 films after annealing at 300 o C for 30 min. in N2-S environment, (c) and (d) deconvolution of Raman spectra of the films deposited at Ts=200 oC and 350 oC and post-annealed at 300 oC for 30 min. in N2-S environment. The excitation was with a 632.8 nm laser.

The surface morphology of the post-annealed Sb2S3 polycrystalline films was

studied by AFM as shown in Fig. 4. Images were acquired in two scan sizes, 10 µm x 10 µm and 50 µm x 50 µm (insets). The evolution of surface morphology after annealing in relation to the thermal history is clear in the low magnification insets: enhancement in size and uniformity of grains as well as better compactness are achieved when Ts was high. The

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grain boundaries and surface features are clearly seen in the magnified images. The 200 oC deposited film (Fig. 4a) lacks uniformity as a whole, the surface morphology consists of a combination of features dissimilar in shape and size, and possibly some voids. As the Ts increased from 250 to 350 oC, smaller grains coalesce to form larger faceted grains due to

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post-annealing [20, 21]. The morphology of the film corresponding to 350 oC reveals a compact and uniform surface, free of voids with large grains of the order of micrometers. It is clear from the AFM (Fig. 4) and SEM (Fig. 5) images that the thermal history has a strong influence on the recrystallization process during post-annealing. It can be seen from

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AFM images that the grain growth is higher for films deposited at higher temperature, which is supported by the reduction in FWHM of the XRD peak corresponding to the (310)

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plane (Fig. 2).

o

(a) Ts= 200 C

o

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(b) Ts=250 C

o

Ts = 200 C

o

o

(d) Ts=350 C

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(c) Ts= 300 C

Fig. 4. AFM images of the Sb2S3 films at scan sizes: 10 µm x10 µm and 50 µm x 50 µm (insets). The deposition temperature (Ts) is shown in each image. All the films were postannealed at 300 oC for 30 min. in N2-S environment.

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SEM images of two representative films (Ts=200 oC and 350 oC) after postannealing are shown in Fig. 5. The images reveal very clearly the role of deposition temperature on the evolution of morphological features after annealing. Although the films are post-annealed at the same conditions, the morphology is very distinct and depends on

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the deposition temperature, this is consistent with the AFM images shown in Fig 4.

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Fig. 5. SEM images of two representative Sb2S3 films deposited at 200 oC and 350 oC, and post-annealed at 300 oC for 30 min. in N2-S environment. Elemental composition of the films was determined by EDXS analysis. The asdeposited films at all substrate temperatures were sulfur-deficient [7] and amorphous (Fig. S1, supplementary file) with relative amount of S slightly higher for the film with Ts =350 C than that of 200 oC. The compositional ratio S/Sb was in the range of 1.2-1.3 as shown in

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Table 2 (and Fig. S2 in supplementary file). However, the deficiency of S was compensated after annealing in sulfur environment. Annealing in N2-S changed not only crystallinity and

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morphology but also the composition.

Table 2. Elemental composition and S/Sb ratios of the Sb2S3 films deposited at different temperatures and post-annealed at 300 oC for 30 min in N2-S environment. Ts (oC) 200 250

as deposited S (at %) Sb (at %) 55.5 44.5 55.3 44.7

S/Sb 1.2 1.2

S (at %) 65.4 63.1

post-annealed Sb (at %) 34.6 36.9

S/Sb 1.9 1.7

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300 350

55.2 57.3

44.8 42.7

1.2 1.3

63.6 60.9

36.4 39.1

1.7 1.5

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3.2.Optical properties Lorentz oscillator:

Lorentz model describes the radiation absorption due to inter-band transitions for

changing its k-vector in the first Brillouin´s zone [22].

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which the electron moves to a final state corresponding to a different band without

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Considering a single transition, the imaginary part of the dielectric constant ε 2 is given by [23]:

!" #$ % =

&'( )'   *' '( + +)  '



(1)

Where E is the photon energy, A, C, and E0 are the amplitude, the broadening parameter,

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and the center energy of the oscillator, respectively. Tauc Lorentz oscillator dispersion model:

The Tauc-Lorentz dispersion approach models the main absorption of amorphous materials using a broad Lorentzian line shape with zero absorption below a defined

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bandgap energy. The model is based on the Tauc joint density of states and the Lorentz oscillator [23]. The imaginary part of the dielectric constant ε 2 is given by:

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!" #$% = -



&'( )*''. +

∙ , $ > $2 , 

5 0, $ ≤ $2 

*'  '( + +)  ' 

'

(2)

Where E and Eg are the photon energy and optical band gap; A, C and E0 are the amplitude, the broadening parameter, and the peak transition energy (the energy of the Lorentz peak) of the oscillator, respectively.

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The real part of the dielectric function ε 1 for the Lorentz and Tauc-Lorentz oscillator models is obtained by Kramers-Kroning integration of the ε 2 functions ! #$% = ! #∞% + 9 :' 8 "

< ξ; #ξ% .

ξ '

ξ ,

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represented by the equations (1) and (2), and is given by [23]: (3)

Where ! #∞% is the high frequency dielectric constant; P stands for the Cauchy principal

part of the integral. The derivation of the integral of equation (3) can be reviewed in [23].

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The VASE measurements were carried out at room temperature in the range of 380900 nm. The ellipsometric spectra Psi (Ψ) and Delta (∆) were measured at 65o, 70o and 75o incidence angles. Fig. 6 shows the plot of Ψ and ∆ data for the films deposited at Ts= 200 C and 350 oC and post-annealed at 300 oC for 30 min. in N2-S environment. The best-fit

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between the experimental values (symbols) of Ψ and ∆ and those generated by the model (solid lines) is also shown. The physical model consisted of a roughness layer, rf (Sb2S3 50%vol – air 50%vol)/ homogeneous Sb2S3 film, tf / glass substrate system. The roughness layer was modeled with the Bruggeman effective medium approximation [24]. The

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dielectric function of the Sb2S3 film was parameterized using a multi-oscillator model composed by one Tauc-Lorentz (TLosc.) and three Lorentz oscillators (Losc. # 1, Losc. # 2 and Losc. # 3), both expressions being Kramers-Kroning consistent. Sb2S3 is an anisotropic biaxial material, in other words, the dielectric function has three components along the

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principal axes a, b and c of its crystalline structure. Therefore, the dielectric function determined in the present study, represents an average of the contribution of the dielectric

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functions along the three directions.

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o

300

o

200 C

200 C o

65

o

65 20 o

75

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350 C

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Psi, Ψ ( degrees )

70

200

Delta, ∆ ( degrees )

30

-100

400 500 600 700 800 900 400 500 600 700 800 900

65 Exp.

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wavelength (nm) o

o

70 Exp.

o

75 Exp.

Best fit

Fig. 6. Ellipsometric spectra (Ψ and ∆) at three angles of incidence 65, 70 and 75o for the films deposited at substrate temperatures 200 oC and 350 oC. The films were post-

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annealed at 300 oC for 30 min. in N2-S environment.

Figure 7 shows the imaginary part of the dielectric function ε 2, and its parameterization using the multi-oscillator model for the films deposited at 200 oC and 350 o

C substrate temperatures and post-annealed at 300 oC for 30 min. in N2-S environment.

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The line shape of each oscillator can be observed in the figure. According to Eq. (1) and (2), the values of Eg are defined by TLosc., the center energies at E1 and E2 of Losc. # 1 and

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Losc. #2, establish electronic transitions above Eg. On the other hand, the values of the center energies at E3 of Losc. # 3, which are lower than those of Eg (E3< Eg), might indicate certain absorption features below Eg as found in some materials [16]. It is important to note the line shape of this oscillator; it has the appearance of a baseline, which extends in all the measurement range. It should be stated that the inclusion of this oscillator in the optical model resulted in best quality for the fit. We propose the significance of this oscillator in terms of light scattering caused by the anisotropic structure of polycrystalline Sb2S3 thin films [25]. The results of Eg, E1, E2, and E3 obtained from the best-fit for the annealed films, are shown in Table 3. The values of E1 and E2 are in agreement with the band to band

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transitions for the three crystallographic directions, reported by Schubert et al. [26]. Furthermore, the values of roughness (rf), film thickness (tf) and the mean-square error (MSE), representing the best matching between experimental and modeled values of Ψ and

20

20

(a)

(b) o

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15

ε2

15

10

5

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10

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∆ spectra, are summarized in Table 3.

5

0

0

1.5

2.0

2.5

3.0

1.5

2.0

2.5

3.0

Photon energy (eV)

Sb2S3 film

TL osc.

Losc. # 2

Losc. # 3

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Losc. # 1

Fig. 7. Parameterization of the imaginary part of the dielectric constant for the films deposited at substrate temperatures: (a) 200 oC and (b) 350 oC, and post-annealed at 300 o

C for 30 min. in N2-S environment. A multi-oscillator model with one Tauc-Lorentz

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(TLosc.) and three Lorentz (Losc. #1, Losc. #2 and Losc. #3) oscillators were used.

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Table 3. Optical constants for the Sb2S3 thin films deposited at different substrate temperatures (Ts) and post-annealed at 300 oC for 30 min. in N2-S environment. Values were obtained from the best fit of the ellipsometry spectra. Ts (oC)

Eg (eV)

200 250 300 350

1.68 1.67 1.66 1.65

E1 (eV) 1.925 1.968 1.956 1.966

E2 (eV) 2.315 2.226 2.280 2.174

E3 (eV) 1.492 1.346 1.374 1.190

rf (nm) 1.26 0.08 9.10 5.00

tf (nm) 138.0 150.0 141.1 146.3

MSE 4.369 3.086 5.066 3.686

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Since the light scattering is not taken into account in the analysis of SE measurements, we treated the contribution of the Losc. #3 as an absorption process, nevertheless it is not related to electronic transitions. In order to validate our proposal that the scattering effect is caused by the crystalline structure of Sb2S3 films, the dielectric

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function of the amorphous films (as-deposited) was parameterized as well. The optical band gaps obtained for the amorphous films are between 1.96 and 2.06 eV, as previously reported [11, 27]. Additionally, the thicknesses of the as-deposited and annealed films determined by spectroscopic ellipsometry are shown in Table 4. The percentage (%)

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lowering in film thickness after annealing is also provided. The thicknesses of the asdeposited films are comparable. Nevertheless, due to annealing a reduction in film thickness was observed which has a dependence on film deposition temperature. The

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thickness loss was 10% in the case of film deposited at 200 oC, where as it was only 2.2% when the deposition temperature was 350 oC. It should be noted that at all substrate temperatures the as-deposited film is amorphous, and changed to polycrystalline after annealing. It is clear from Fig. 4 and 5 that the grain growth has a strong dependence on thermal history of the film, and is independent of the initial film thickness. Further studies

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are needed to give a satisfactory explanation for this observation.

Table 4. Thicknesses of the Sb2S3 films determined by SE

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As-deposited (nm)

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Substrate temperature, Ts (oC) 200 250 300 350

153.4 155.8 148.7 149.6

Annealed (nm)

138.0 150.0 141.1 146.3

Percent decrease in thickness (%) 10.0 3.7 5.1 2.2

Fig. 8 shows the refractive index (n) and extinction coefficient (k) of the asdeposited (broken lines) and annealed (solid lines) films. The differences between their n and k spectra are clearly observed. When the films are annealed, the re-crystallization

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promoted small grain coalescence, reduction in grain boundaries and voids, leading to an increase in film density. As described in previous sections, the recrystallization was strongly influenced by the original deposition temperature. Changes in film density caused by post-deposition annealing resulted in an increase of refractive index and extinction

), where = is density

? 

? "

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coefficient, as established in the Lorentz-Lorenz relationship (= ∝ of the medium [28].

The effect of post-annealing on the microstructure of the films is clearly observed,

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the detailed critical points structure (produced by the band-to-band transitions) observed in the n and k spectra of the annealed films is not seen for the as-deposited samples (Fig. 8). Additionally, with respect to the increase in deposition temperature, an improvement in

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crystallinity of the annealed films (example: a more detailed critical points structure at 350 o

C) and film compactness (increase in n) is observed. These results are in agreement with

those from XRD and AFM of Figs. 1, 2, and 4. On the other hand, there is no absorption (k = 0) for energy values below Eg (wavelength > 611 nm of Fig. 8) in the amorphous films, irrespective of the substrate temperature. However, the spectra of the annealed films

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indicate a non-zero extinction coefficient at energies below Eg (wavelength >750 nm in Fig. 8). Taking the contribution of Losc. #3 as a scattering instead of an absorption process in polycrystalline Sb2S3 films, this observed feature below Eg slightly decreases as the film deposition temperature increases, that is, k approaches zero for energies < Eg (see Fig. 7

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and the bottom plot of Fig. 8). We would expect this result due to the fact that both the crystal and grain sizes increase with Ts. The role of grain size on light scattering has been previously reported in ZnO polycrystalline films [29, 30]. The vertical arrows in the k

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spectra of Fig. 8 indicate the position of Eg and the center energies at E1 and E2 (band to band transitions).

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4.5 o

o

250 C

o

300 C

o

350 C

4.0

3.5

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wavelength (nm) o

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Eg 0

600

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Extinction coefficient, k

200 C

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Refractive index, n

200 C

800 400

600

800 400

600

800 400

600

800

wavelength (nm)

as deposited

annealed

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Fig. 8. Optical constants n and k of the as-deposited (dotted lines) and post-annealed (solid lines) Sb2S3 films. The vertical arrows indicate the positions of Eg, and the center energies at E1 and E2. The film deposition temperature is shown in each figure.

Figure 9 depicts the influence of deposition temperature on the film compactness after the films are annealed. The refractive index at λ=632.8 nm (absorption region) increases from 3.9 to 4.3 with Ts. indicating a higher film densification in the case of film

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deposited at 350 oC. When the films are post-processed, the rearrangement promoted by the annealing temperature results in the crystallization of the films and a subsequent increase of

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density.

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4.2

4.0

3.8 200

250

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λ = 632.8 nm

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Refractive index, n

4.4

300

350

o

Ts ( C)

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Fig. 9. Refractive index at λ= 632.8 nm of the Sb2S3 films deposited at different temperatures and post-annealed at 300 oC for 30 min. in N2-S environment.

Figure 10 shows the absorption coefficient of the films, derived from the k spectra (Fig. 8). The band gaps determined from the TL oscillator are also given in the figure.

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Above Eg (hv >1.69 eV) the values of α is in the range of 104-105 cm-1.

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10

5

10

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Eg=1.68 eV 2

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250 C

350 C

Eg=1.67 eV Eg=1.66 eV

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Photon energy (eV)

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Absorption coefficient, α (cm )

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Fig. 10. Absorption coefficient and optical band gap of the Sb2S3 films.

3.3. Photoresponse

Figure 11 shows the photoresponse of the annealed films, plotted as conductivity (σ)

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vs. time. The current was collected by applying a constant bias of 10 V in dark and under illumination as follows: 10 s in dark, 10 s under illumination and 60 s in dark. All the films were photosensitive, however, no specific relation could be established between the original deposition temperature and photosensitivity.

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The type of electrical conductivity was assessed from thermally generated electromotive force measurements (thermo-emf). In all the annealed Sb2S3 films, a positive

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thermoelectric voltage (∆V > 0) was obtained under a temperature gradient (∆T). The thermoelectric power (S), which is defined as S = ∆V/∆T, is a positive value, which means that the films have p-type electrical conductivity.

o

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light off light off light on

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Electrical conductivity, σ (ohm cm )

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160

200

240

280

320

time (s)

Fig. 11. Electrical conductivity of the Sb2S3 films deposited at 200, 250, 300 and 350 oC, and post-annealed at 300 0C for 30 min in N2-S ambient.

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3.4. Photoluminescence

The PL spectra at room temperature of the annealed Sb2S3 films originally deposited at 200 oC and 350 oC are shown in Fig. 12. The excitation wavelength was 325 nm and the emission spectra appeared in the range 320-600 nm, as observed in the figure. Four bands

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at 362, 431, 490, 525nm were observed for the film deposited at 200 oC, and in the case of film deposited at 350 oC the four bands were at 365, 436, 484, 522nm. The detected signals

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are in agreement with PL emission features already reported for Sb2S3 material [31-33].

362 nm

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350

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350 C 522 nm

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484 nm

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525 nm

490 nm

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365 nm

PL Intensity (arb. units)

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200 C 431 nm

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Fig. 12. Photoluminescence of the Sb2S3 films deposited at 200 oC and 350 oC, and postannealed at 300 0C for 30 min.

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4. Conclusions

In this study, we report the development of Sb2S3 thin films by RF magnetron sputtering followed by a post-deposition thermal annealing in presence of S. The asdeposited films were amorphous even at substrate temperature of 350 oC. Polycrystalline

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and phase pure films were obtained after annealing at 300 0C in N2-S ambient. It was demonstrated that the thermal history of the film has a role on the post-deposition annealing

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and crystallization. The material properties such as crystallinity, compactness, spatial uniformity, grain growth and stoichiometry showed a dependence on the original film deposition temperature. The S/Sb ratio decreased from 1.9 of the 200 oC deposited film to 1.5 for the film deposited at 350 oC. Furthermore, AFM and SEM micrographs of the annealed films revealed an enhancement in film morphology such as grain size, spatial uniformity and compactness with increase in deposition temperature. All the films showed p-type conductivity and photoresponse. Studies by variable-angle spectroscopic ellipsometry (VASE) provided some optical constants including inter-band transitions in the Sb2S3 thin films. We present the parameterization of the dielectric complex function of

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Sb2S3 using a multi-oscillator model composed by one Tauc-Lorentz and three Lorentz oscillators. The absorption coefficient and refractive index of the annealed films changed from 8.5 to 9.6 x104 cm-1 and from 3.9 to 4.3 respectively with a difference of 150 oC in

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deposition temperature.

Acknowledgments

This work at IER-UNAM was partially supported by the projects CeMIE-Sol

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207450/P28, and PAPIIT IN 107815. Medina-Montes thanks the postdoctoral fellowship of UNAM. The authors thank Gildardo Casarrubias Segura for technical assistance in

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sputtering and AFM equipments; Maria Luisa Ramón for XRD measurements; José Campos Alvarez for SEM images and EDXS measurements, and Mou Pal (IFUAP-BUAP)

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Highlights: Thermal history of the film influence the material properties after recrystallization.

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Sb2S3 films deposited by sputtering are amorphous and change to polycrystalline after annealing even at temperature lower than deposition

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Crystallite size, strain, grain size, refractive index, and film stoichiometry showed a dependence on the thermal history.

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