Effect of heat treatment on the structural and optical properties of amorphous Sb2Se3 and Sb2Se2S thin films

ARTICLE IN PRESS Physica B 404 (2009) 1119–1127

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Effect of heat treatment on the structural and optical properties of amorphous Sb2Se3 and Sb2Se2S thin films E.A. El-Sayad a,, A.M. Moustafa a, S.Y. Marzouk b a b

Solid State Physics Department, National Research Center, Dokki, Cairo, Egypt Electron Microscope and Thin Films Department, National Research Center, Dokki, Cairo, Egypt

a r t i c l e in fo

abstract

Article history: Received 2 August 2008 Received in revised form 5 November 2008 Accepted 13 November 2008

Nearly stoichiometric thin films of Sb2Se3 and Sb2Se2S were deposited, by conventional thermal evaporation of the presynthesized materials, on glass substrates held at room temperature. X-ray diffraction studies on the annealed films, at Ta ¼ 373, 423, and 473 K, revealed an amorphous-tocrystalline phase transition at Ta ¼ 423 and 473 K for Sb2Se3 and Sb2Se2S, respectively. The crystal structure of the polycrystalline films of Sb2Se3 and Sb2Se2S is single-phase of an orthorhombic type structure, with space group Pbnm (62) and Z ¼ 4, as that of their source materials. The optical constants (n,k) and the thickness (t) of the investigated films were determined from optical transmittance data, in the spectral range 600–2500 nm using the Swanepoel method. The dispersion parameters of the indicated compositions were determined from the analysis of the refractive index. The analysis of the optical absorption spectra revealed that the polycrystalline films exhibited both direct and indirect energy gaps, on the contrary to the amorphous films which showed only a non-direct energy gap with a relatively higher Urbach’s tail. & 2008 Elsevier B.V. All rights reserved.

PACS: 61.10.Nz 68.55.Nq 61.66.Fn 78.20.Ci 78.66.Li Keywords: Chalcogenides Semiconductors Amorphous-to-crystalline Sb2Se3 and Sb2Se2S films Optical properties

1. Introduction Sb2Se3 and Sb2S3, which belong to V2–VI3 family are layerstructured semiconductors with orthorhombic crystal structure, in which each Sb-atom and each Se/S-atom is bound to three atoms of the opposite kind that are then held together in the crystal by weak secondary bonds [1]. In the last few years, Sb2Se3 has received a great deal of attention due to its switching effects [2] and its excellent photovoltaic properties and high thermoelectric power [3], which make it possess promising applications in solar selective and decorative coating, optical and thermoelectric cooling devices [4]. On the other hand, Sb2S3 has attracted attention for its applications as a target material for television cameras [5,6], as well as in microwave [7], switching [8], and optoelectronic devices [9–11]. A survey of the literature indicates that most of the Sb2Se3 and Sb2S3 thin films deposited at low or room temperature are amorphous, regardless of the deposition process. It was reported

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E-mail address: [email protected] (E.A. El-Sayad). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.086

that the amorphous-to-crystalline phase transition occurs at 413 [12] and 498 K [13] for Sb2Se3 and Sb2S3, respectively. The early optical studies revealed that Sb2Se3 single crystals have either indirect allowed [14] (Eg ¼ 1.2 eV) or indirect forbidden [15,16] (Eg ¼ 1.11 and 1.18 eV) optical transitions. However, several optical absorption studies have been reported on Sb2Se3 thin films. It was reported [12,17–21] that the as grown amorphous films of Sb2Se3 exhibit non-direct optical transition with energy band gap values 1.2–1.35, 1.35, 1.2, and 1.29 eV. On the other hand, it was reported that the crystalline films of Sb2Se3 exhibit either direct [12,22] or indirect [23,24] optical transitions with energy gap values of 1 and 1.5 eV for direct and 1.1 and 1–1.2 eV for indirect transition, respectively. Consequently, the reported [23,25] optical band gaps for Sb2Se3, due to both direct and indirect transitions in the range of 1–1.13 eV, makes it suitable for use as an absorber material in polycrystalline thin film solar cells [26]. Some of the earlier reports on Sb2S3 single crystals indicate that they have either the direct [27] (Eg ¼ 1.63–1.72 eV) or indirect [28] (Eg ¼ 1.75 eV) optical transitions. However, some optical absorption studies have been reported on Sb2S3 thin films. Reported values and types of band gaps (Eg) of amorphous and

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crystalline Sb2S3 vary. The amorphous phase has been reported to be indirect with Eg values (1.7, 1.77, 1.83, 1.78, 1.67, 1.88 eV) [21,29–33] and also direct with Eg values (1.7–2.07, 2.24 eV) [13,34]. The crystalline phase has also be reported to be direct with Eg (1.88, 1.97, 2.03, 1.8, 1.42–1.65, 1.73 eV) [13,29–32,34] and also indirect with Eg values (1.5, 1.1 eV) [35]. So, the reported range of energy values and band type gives the variety of potential applications for both amorphous and crystalline Sb2S3 thin films. Up to our knowledge, no previous work has been done on the solid solutions of both Sb2Se3 and Sb2S3 except lastly the same author had reported [21] the compositional dependence of the optical properties of amorphous Sb2Se3xSx films. Through this work we will try to find the effect of annealing up to 473 K and to find whether amorphous-to-crystalline phase transition occur and study the structural and optical properties of those films which show crystalline nature (only Sb2Se3 and Sb2Se2S).

2. Experimental procedures Polycrystalline ingots of Sb2Se2xSx with x ¼ 0, 1, 2, and 3 were prepared by the direct fusion and cooling cycle of a mixture of the

constituent elements, in stoichiometric ratio and purity 99.999%, in vacuum-sealed silica tubes. Thin films of Sb2Se2xSx with x ¼ 0, 1, 2, and 3 were deposited by conventional thermal evaporation of the presynthesized materials onto precleaned glass substrates held at room temperature (293 K), in 5  104 Pa vacuum, using a high vacuum coating unit (Edwards 306 A). The deposition rate was kept constant during evaporation process at nearly 2 nm/s. The film thickness, t, was monitored by a quartz crystal thickness monitor (Edwards, FTM4) and it was also measured interferometrically [36]. The asdeposited films with different compositions were annealed at Ta ¼ 373, 423, and 473 K for 1 h in a vacuum of 1.5  104 Pa. X-ray diffraction (XRD) of the investigated films were recorded at 300 K using a computer controlled X-ray diffractometer (Diano Corporation, USA) with Fe-filtered CoKa (l ¼ 1.79026 A˚) radiation operated at 9 mA and 45 kV. The elemental composition of as-deposited films on a polished surface of a graphite stub were determined using an energydispersive X-ray spectrometer (EDXS) unit, interfaced to a scanning electron microscope (SEM; Philips XL) operating at an accelerating voltage of 30 kV. The relative error of determining the indicated elements does not exceed 5%.

Fig. 1. X-ray diffractograms of (a) Sb2Se3 and (b) Sb2Se2S films: 1: as-deposited; 2: Ta ¼ 373 K; 3: Ta ¼ 423 K; 4: Ta ¼ 473 K; 5: for powder.

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1121

Table 1 (a) X-ray analysis of Sb2Se3 in powder and thin films form compared to the reported values in JCPDS card no. (72–1184) [39] and (b) X-ray analysis of Sb2Se2S in powder and thin films form.. (a) Experimental data (bulk and thin films)

Standard (JCPDS) Card no. (72-1184)

T.F. (Ta ¼ 423 K)

Powder

T.F. (Ta ¼ 473 K)

d (A˚)

I/I0%

d (A˚)

I/I0%

d (A˚)

I/I0%

d (A˚)

I/I0%

hkl

8.28473 5.89384 – 5.26006 4.14212 3.76376 3.72260 3.68730 – – 3.25549 3.16357 – – 2.86798 – – 2.77657 2.70293 2.62817 2.60902 2.51931 – 2.36664 – 2.33939 2.31969 – – – – – 2.18356 – 2.16460 2.06936 – – 2.00894 2.00418 – 1.98935 – 1.96434 1.93782 1.91444 – – 1.86012 – 1.83714 – – – – 1.78838 1.76132

10 83 – 100 10 10 56 14 – – 91 61 – – 83 – – 45 14 95 19 26 – 20 – 11 28 – – – – – 33 – 13 6 – – 21 20 – 26 – 7 7 4 – – 6 – 6 – – – – 7 58

– 5.89415 – 5.25520 – – 3.72170 – – – 3.25355 – – – 2.85204 – – – – 2.62656 – – – – – – – – – – – – 2.18324 – – – – – – – – – – – 1.93309 – – – – – – – – – – – –

– 100 – 26 – – 32 – – – 9 – – – 14 – – – – 15 – – – – – – – – – – – – 10 – – – – – – – – – – – 6 – – – – – – – – – – – –

– 5.89697 – 5.25683 — — 3.71977 – – – 3.25318 – – – 2.85241 – – – – 2.62618 – – – – – – 2.31572 – – – – – 2.18348 – – – – – – – – – – – – – – – – – – – – – – – –

– 44 – 100 – – 63 – – – 35 – – – 22 – – – – 54 – – – – – – 15 – – – – – 23 – – – – – – – – – – – – – – – – – – – – – – – –

8.2691 5.8850 5.8100 5.2500 4.1345 3.7500 3.7171 3.6792 3.5730 3.2865 3.2514 3.1536 2.9425 2.9050 2.8606 2.8524 2.8203 2.7696 2.6960 2.6250 2.6049 2.5134 2.5060 2.3622 2.3430 2.3346 2.3149 2.3071 2.2976 2.2799 2.2626 2.1883 2.1817 2.1766 2.1615 2.0672 2.0167 2.0114 2.0045 1.9995 1.9937 1.9810 1.9761 1.9616 1.9366 1.9109 1.8774 1.8585 1.8534 1.8396 1.8327 1.8289 1.8237 1.8278 1.7989 1.7905 1.7629

9 18 6 55 7 12 21 15 1 8 48 78 1 1 100 65 2 63 20 46 16 29 17 18 5 9 29 17 9 3 4 5 15 23 15 5 8 28 28 24 12 36 20 2 3 6 1 2 5 2 3 2 2 1 3 12 31

110 020 200 120 220 101 130 310 111 021 230 211 040 400 221 140 410 301 311 240 420 231 321 041 340 430 141 150 411 510 331 241 250 421 520 440 341 350, 431 501 530 151 002 511 060 160, 600 251, 610 022, 202 260 122, 212 620 441 450 540 450 531 351 601

(b) Powder

Thin film

dobs. (A˚)

I/I0%

dcal. (A˚)

hkl

dobs. (A˚)

I/I0%

dcal. (A˚)

8.21562 5.84095 5.21232 4.1026 3.68978 – 3.65374 3.55805

9 68 100 18 72 – 34 4

8.20836 5.84046 5.21082 4.10418 3.68921 – 3.65284 3.54940

110 020 120 220 130 101 310 111

– 5.83170 5.20492 – 3.68089 3.68089 – –

– 100 45 – 46

– 5.83205 5.20475 – 3.68441 3.68422 – –

– –

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Table 1 (continued ) (b) Powder

Thin film

dobs. (A˚)

I/I0%

dcal. (A˚)

hkl

dobs. (A˚)

I/I0%

dcal. (A˚)

3.22623 – 3.13095 2.83840 – 2.74844 2.67445 2.60467 2.58538 2.49465 2.34420 2.31665 2.29585 2.24700 2.16319 – – 2.14326 2.05072 1.99263 1.96698 1.94721 1.92019 1.92019 1.89611 1.89611 1.84463 1.77245 1.74438

75 – 55 79 – 41 13 86 17 22 15 11 25 4 26 – – 11 6 25 20 9 7

3.22728 – 3.13244 2.84094 – 2.75085 2.67761 2.60541 2.58612 2.49574 2.34533 2.31766 2.29833 2.24671 2.16535 – 2.16141 2.14602 2.05209 1.99065 1.96823 1.94682 1.92286 1.91968 1.89733 1.89725 1.84460 1.77244 1.74507

230 201 211 221 140 301 311 240 420 231 041 430 141 331 250 241 421 520 440 501 002 060 600 160 610 251 260 531 061

– 3.22387 – – 2.82813 – – 2.60170 – – – – – – 2.16193 2.16193 – – – – – 1.94113 – 1.91489 – – – – –

– 9 – – 5 – – 15 – – – – – – 5

– 3.22387 – – 2.82711 – – 2.60237 – – – – – – 2.16264 2.16261 – – – – – 1.94402 – 1.91699 – – – – –

5 5 9 33

– – – – – 3 – 4 – – – – –

Table 2 The observed lattice parameters of Sb2Se3 and Sb2Se2S in powder and thin films form compared to the reported values in JCPDS card no. (72–1184) [39]. Compound

Sb2Se3

Sb2Se2S

Sample

a (A˚)

b (A˚)

c (A˚)

Source

Powder Powder Thin films (Ta ¼ 423 K) Thin films (Ta ¼ 473 K)

11.62(7) 11.643(2) 11.603(5) 11.59(2)

11.77(1) 11.788(1) 11.788(1) 11.792(4)

3.962(1) 3.977(0) 3.939(2) 3.95(1)

Ref. [39] This work This work This work

Powder Thin films (Ta ¼ 473 K)

11.537(2) 11.54(1)

11.681(1) 11.664(2)

3.936(2) 3.888(2)

This work This work

The optical transmittance, T, of the films, was measured at room temperature with unpolarized light at normal incidence in the wavelength range 500–2500 nm using a double beam spectrophotometer (Type JASCO Corporation, Model V-570) with estimated error 70.5%. All measurements (whether XRD or T) were performed on the as-deposited films as well as those annealed at different temperatures.

3. Results and discussions 3.1. Structural properties XRD studies were carried out on the annealed Sb2Se3xSx films (x ¼ 0, 1, 2, and 3) at Ta ¼ 373, 423 and 473 K for 1 h in a vacuum of 1.5  104 Pa. Annealing at higher temperatures (Ta4473 K) show a film decomposition which may be due to the sublimation of S as indicated from EDXS. X-ray diffractograms of the annealed films of Sb2Se3xSx with different compositions revealed an amorphous-to-crystalline

Fig. 2. Transmission spectra for the annealed films of (a) Sb2Se3 and (b) Sb2Se2S compared to their previously reported [21] as-deposited ones.

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phase transition for x ¼ 0 at Ta ¼ 423 K, while for x ¼ 1 the crystalline phase appears only at Ta ¼ 473 K. On the other hand for x ¼ 2 and 3 annealing up to Ta ¼ 473 K has no effect on the amorphous nature of the films. The identical results of the transmittance, T, on both as-deposited films and those annealed up to Ta ¼ 473 K for x ¼ 2 and 3, confirm that no phase transition occur and that annealing has no effect on the amorphous nature of the films (This agrees well with X-ray diffractograms). This behaviour may be explained by the following: In case of sample with S ¼ 1, the sulphur substitution of Se in the structure lead to contraction of the unit cell (as indicated by X-ray data, Table 2) as the ionic radius of S (1.84 A˚) is smaller than that of Se (1.98 A˚) [37]. This situation may lead to higher distortion of the lattice and in turn needs more thermal energy to begin the crystallization process. This may explain the higher annealing temperature of the sample S ¼ 1 (473 K) than those without sulphur. Any how the highly distortion due to the replacement of Se by S (S ¼ 2 and 3) needs to higher thermal energy to be crystallized (T a b473 K which could not be attained). This situation explains the impossibility to attain crystallization for thin films with S more than 1. Figs. 1a and b show X-ray diffractograms of as-deposited and annealed films of Sb2Se3 and Sb2Se2S compared to their corresponding source materials in fine powder forms, respectively.

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The powder diffraction traces (see Figs. 1a and b) showed no extra peaks corresponding to any precipitation of elements or binary alloys. This may indicate the complete miscibility of the constituent elements for the synthesized compounds. The unitcell parameters were determined from X-ray powder data and all the planes were indexed, (see Tables 1a and b), using the DICVOL91 computer programme [38] and they are listed in Table 2. The observed interplanar spacing, d, obtained from X-ray data for Sb2Se3, were found to be in quite well agreement with the standard XRD data for Sb2Se3 (JCPDS card no. 72-1184) [39]. The analysis has indicated that the prepared ingot materials of Sb2Se3 and Sb2Se2S are single-phase polycrystalline materials corresponding to the orthorhombic type structure with space group Pbnm (62) and Z ¼ 4. For Sb2Se3 and Sb2Se2S, the observed interplanar spacing, d, obtained from X-ray data for films with polycrystalline structure, were found to be in quite well agreement with the observed X-ray data of their corresponding bulk ingot materials. The good matching between the observed reflecting planes of the indicated films and that of their corresponding source materials data revealed that the diffraction traces showed no extra peaks appeared corresponding to any change in composition and that the single-phase orthorhombic structure is retained. The unit-cell parameters of crystalline films of Sb2Se3 and Sb2Se2S were determined, as described before, from their X-ray data (see Tables 1a and b) and they are given in Table 2 compared to those

Fig. 3. Spectral distribution of refractive index, n, for the annealed films of (a) Sb2Se3 and (b) Sb2Se2S. The points represent the experimental values of n and the solid line represents the fitting of these points using the two-term Cauchy function.

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of their corresponding source materials in powder form. The determined unit-cell lattice parameters a, b, and c of Sb2Se3 films were found to be in suitable agreement with the bulk material and the reported values [39] as displayed in Table 2. Moreover, for Sb2Se3, the analysis of X-ray revealed that the polycrystalline films annealed at 423 K have a preferred orientation plane (2 0 0), while for those annealed at higher temperature (473 K) have a preferred orientation plane (2 1 0) as those for their bulk ingot material. Such behaviour may be due to the fact that crystallization starts at 423 K with a preferred orientation plane (2 0 0), on increasing the annealing temperature, i.e., increasing the thermal energy, the crystallites in the thin film acquire higher energy which leads to their randomness and consequently losing their preferred orientation and become similar to the bulk material. On the other hand, for Sb2Se2S, the analysis of X-ray data revealed that polycrystallization starts in films annealed at 473 K with a preferred orientation plane (2 0 0), while for bulk ingot material the preferred orientation plane is (2 1 0). In order to confirm the situation happened in Sb2Se3 thin films, i.e., increasing the annealing temperature leads to complete randomness of the crystallites. The thin film has to be annealed at Ta4473 K but unfortunately this cannot be done in our case (as explained before). The elemental analysis of the as-deposited films of Sb2Se3 and Sb2Se2S has led to the chemical formula Sb1.97Se3.03 and Sb1.865Se2.043S1.092, respectively, revealing a very nearly stoichiometric composition. 3.2. Optical properties The measured transmittance data of the investigated films of Sb2Se3 and Sb2Se2S were found to exhibit interference effects for

photon energies below the fundamental absorption edge (Figs. 2a and b). From Figs. 2a and b, it is revealed that the optical transmittance, T, decreases with increasing the annealing temperature for Sb2Se3 films which may be due to the increase of the partial crystallization of films with increasing the annealing temperature. While the change in transmittance for the as-deposited and annealed films at Tap423 K of Sb2Se2S was found to be within the experimental error and T decreases for the annealed films at Ta ¼ 473 K indicating that annealing up to 423 K has no effect on the amorphous nature of the films, i.e., no partial crystallizations occur. So, in this work we investigate the optical properties of Sb2Se3 films annealed at Ta ¼ 373, 423, and 473 K, while for Sb2Se2S we study only the films annealed at 473 K. 3.2.1. Determination of the refractive index, n, and film thickness, t The refractive index, the film thickness, and the order of interference of the investigated films were computed using Swanepoel method [40] with ns ¼ 1.51. The details of calculations have been reported in our previous paper [21]. The obtained values of thickness are 620, 610.5, and 590 nm and 649.3 nm for the investigated films of Sb2Se3 and Sb2Se2S, respectively; and they are found to be in reasonable agreement with the interferometrically measured ones. The calculated values of n, with an experimental error 70.2%, were found to fit a two-term Cauchy formula: n ¼ a/l2+c, with a correlation coefficient 0.98. The dispersion of the refractive index, n, of the investigated films of Sb2Se3 and Sb2Se2S is shown in Figs 3a and b, respectively. Fig. 3a shows that n increases on increasing the annealing temperature throughout the investigated range. While Fig. 3b does not show that behaviour as only one annealing (Ta ¼ 473 K)

Table 3 The dispersion parameters of the annealed films of Sb2Se3 and Sb2Se2S compared to the previously reported [21] their as-deposited ones. Compound

Ta (K)

E0 (eV)

Ed (eV)

Nc

eN

M1

M3 (eV2)

Source

Sb2Se3

293 373 423 473

2.73470.004 2.71970.004 2.36970.003 2.31970.004

24.270.1 25.270.1 25.670.1 32.770.1

3.5170.01 3.6570.01 3.7070.01 4.7370.02

9.8670.04 10.2770.03 11.8070.05 15.0870.06

8.8670.04 9.2770.03 10.8070.05 14.0870.06

1.18570.006 1.25470.005 1.92470.007 2.61970.008

Ref. [21] This work This work This work

Sb2Se2S

293 473

2.81770.004 2.39470.004

21.570.1 27.070.1

3.1270.01 3.9070.01

8.6470.03 12.2670.05

7.6470.03 11.2670.05

0.96370.004 1.96670.007

Ref. [21] This work

Fig. 4. Optical absorption spectra for the annealed films of (a) Sb2Se3 and (b) Sb2Se2S.

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has been done, any how it shows a higher value than those reported [21] of the amorphous films. This may be attributed to the increase of crystallization by annealing and consequently a decrease of transmittance. Also, the refractive index for each sample decreases with increasing l and reaches a nearly constant long-wavelength value. Moreover, for each sample, below the

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interband absorption edge (hno1.1 eV), it was found that (n21)1 varies linearly with (hn)2 in accordance with the single oscillator model [41], where n2 ðhnÞ ¼ 1 þ

E0 Ed

(1)

E20  ðhnÞ2

Table 4 The optical parameters of the optical gap for the annealed films of Sb2Se3 and Sb2Se2S compared to the previously reported [21] as-deposited ones. Compound

Ta (K)

Steepness parameter (s)

Urbach’s energy (eV)

Egi (eV)

Egd (eV)

Source

Sb2Se3

293 373 423 473

0.05370.003 0.05470.003 0.06070.003 0.08070.003

0.4970.03 0.4870.02 0.4370.03 0.3270.01

1.29270.002 1.28070.002 1.15270.001 1.12470.001

– – 1.50170.002 1.43070.002

Ref. [21] This work This work This work

Sb2Se2S

293 473

0.04470.003 0.05570.003

0.5970.05 0.4770.03

1.44970.002 1.22970.001

– 1.55070.002

Ref. [21] This work

Fig. 5. Plots of (ahn)1/2 and (ahn)2 against hn for the investigated films of (a and c) Sb2Se3 and (b) Sb2Se2S.

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where E0 and Ed are the oscillator energy and dispersion energy, respectively. They are determined from the slope and the zero energy intercept of the fitted straight line. The oscillator energy, E0, is an average energy gap and on close approximation, E0  2Eopt g , as was found by Tanaka [42]. The dispersion energy or oscillator strength, Ed, also follows a simple empirical relationship, Ed ¼ bNcZaNe, where b is a constant, and, according to Wemple [43], for covalent crystalline and amorphous materials has a value of 0.3770.04 eV. Nc is the coordination number of the cation nearest neighbour to the anion, Za is the formal chemical valency of the anion, and Ne is the total number of valence electrons (cores excluded) per anion. The value of the cation coordination number, Nc, could be estimated, where Za ¼ 2, Ne ¼ 9.333 for our samples. On the basis of such model, E0 and Ed are related to the imaginary part of the complex dielectric constant, ei. The momenta M1 and M3 of the ei(hn) optical spectrum [43,44] can be derived from the relations E20 ¼

M1 ; M3

E2d ¼

M 31 M 3

(2)

The determined values of the dispersion parameters E0, Ed, Nc, eN, M1, and M3 for Sb2Se3 and Sb2Se2S samples are given in Table 3. 3.2.2. Determination of the absorption coefficient, a Knowing the values of the refractive index, n(l), and the film thickness, t, the absorption coefficient, a(l), can be calculated using Swanepoel method [40]. The details of calculations have been reported in our previous paper [21]. Figs. 4a and b show the dependence of the absorption coefficient, a, on photon energy, hn, for the investigated Sb2Se3 and Sb2Se2S films, respectively. The observed optical absorption spectra of the investigated films show two distinct absorption regions below and above photon energy of 1.3 eV. The first one show slight increase of a upto a o104 cm1, while the other show a sharp increase of a till reaches a constant value of 105 cm1 above 1.85 eV. In the low-absorption region (102oao104 cm1), the analysis of the observed absorption tail of the investigated films revealed that the absorption tail corresponds to an exponential edge according to Urbach’s empirical relation [45,46]. The details of analysis have been reported in our previous paper [21]. From the analysis of the obtained results for Sb2Se3 samples, it is revealed that, on increasing the annealing temperature from 373 to 473 K, the steepness parameter, s, was increased from 0.054 to 0.08, while the Urbach energy, EU ( ¼ kT/s), was decreased from 0.48 to 0.32 eV, which is due to the decrease of randomness. Also, for Sb2Se2S samples annealed at 473 K, the steepness parameter, s, and the Urbach energy, EU, were found to be 0.06 and 0.47 eV, respectively. This may be attributed to the effect of annealing on the amorphous samples which lead to an increase of crytallization process and consequently a decrease of randomness. The determined values of s and EU for Sb2Se3 and Sb2Se2S samples are given in Table 4. In the high-absorption region beyond 1.2 eV (aX104 cm1), the analysis of the absorption coefficients, a (Figs. 5a and b) of the investigated films was found to follow the relation:

aðhnÞ ¼ ðB=hnÞðhn  Eg Þp

(3)

with p ¼ 2, which characterizes an indirect optical transition for the investigated films of Sb2Se3 and Sb2Se2S according to Tauc [47]. A plot of (ahn)1/2 versus hn yields a straight line characterizing the investigated samples (Figs. 5a and b) from which the values of Eg1 could be estimated to be 1.28, 1.152, 1.124 eV, and 1.229 eV for the investigated films of Sb2Se3 and Sb2Se2S, respectively (see Table 4). The obtained indirect energy gaps for crystalline films of Sb2Se3 (1.152 and 1.124 eV) were found to be in

Fig. 6. Dependence of the indirect optical energy gap of Sb2Se3 films on the annealing temperature.

good agreement with the previously reported values of 1.1 and 1.2 eV [23,24]. However, on calculating a1 using B1 and Eg1 determined from Figs. 5a and b for energies above 1.2 eV, we find that a1 is considerably smaller than the absorption coefficients, a, measured experimentally for the investigated crystalline films of Sb2Se3 and Sb2Se2S, indicating the existence of additional absorption, a2 ( ¼ aa1). The analysis of the additional absorption coefficients showed that, the dependence of a2 on hn could be described by the relation (3) with p ¼ 1/2 [48,49]; with a gap energy Eg2 ¼ 1.501, 1.43, and 1.55 eV for polycrystalline films, annealed at Ta ¼ 423 and 473 K, of Sb2Se3 and Sb2Se2S, respectively (Figs. 5b and c). The observed values of the direct energy gap for crystalline films of Sb2Se3 (1.501 and 1.43 eV) were found to be in suitable agreement with the previously reported value of 1.5 eV [22]. Also, it is found that direct band-gap value for crystalline Sb2Se2S films (1.55 eV) lies between individual bandgap values of Sb2Se3 (1.5 eV) [22] and Sb2S3 (1.97 and 2.03 eV) [30,31]. This transition corresponds to an allowed direct transition from the top of the valence band to the conduction band minimum at the center of the Brillouin zone. Calculating ac ( ¼ a1+a2) and comparing it with the experimental values of ae, it was found that no further absorption occurs for the investigated polycrystalline films in the investigated range. Fig. 6 shows the dependence of the indirect optical transition energy on annealing temperature, Ta, for Sb2Se3 samples.

4. Conclusions Nearly stoichiometric thin films of Sb2Se2xSx (x ¼ 0, 1, 2, and 3) were prepared at room temperature by thermal evaporation of the presynthesized materials onto glass substrates. Crystal structural analysis of the annealed films revealed an amorphousto-crystalline phase transition with a single phase of an orthorhombic type structure as that of their source materials for x ¼ 0 and 1 at Ta ¼ 423 and 473 K, respectively. On the other hand for x ¼ 2 and 3 annealing up to Ta ¼ 473 K has no effect on the amorphous nature of the films. Annealing at higher temperatures (Ta4473 K) show a film decomposition. The optical constants and consequently the dispersion parameters of the investigated samples of Sb2Se3 and Sb2Se2S were determined from the analysis

ARTICLE IN PRESS E.A. El-Sayad et al. / Physica B 404 (2009) 1119–1127

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