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Optical properties of thermally evaporated SnS thin films
Optical Materials 20 (2002) 159–170 www.elsevier.com/locate/optmat
Optical properties of thermally evaporated SnS thin films M.M. El-Nahass a, H.M. Zeyada a b
b,*
, M.S. Aziz b, N.A. El-Ghamaz
b
Department of Physics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Department of Physics, Faculty of Science at New Damietta, 34517 New Damietta, Egypt
Received 27 April 2001; received in revised form 28 November 2001; accepted 7 January 2002
Abstract Thermally evaporated SnS amorphous chalcogenide films undergo structural transformation upon annealing in the temperature range between 432–573 K. The optical properties of amorphous and annealed films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 250–2500 nm. The films are transparent for a wavelength >1250 nm. The refractive index (n) and the absorption index (k) are independent of film thickness in the measured film thickness range (55–365 nm). The dispersion energy, Ed , of amorphous films increased from 20.2 to 23.85 eV for crystalline films. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions with energy gaps of 1.4 and 2.18 eV for the amorphous films and of 1.38 and 2.33 eV for the crystalline films, respectively. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal evaporation; Optical constants; SnS
1. Introduction SnS is one of the tin chalcogenide layered semiconductors in group IV–VI. It may exhibit the p-type conductivity [4–7,9,10,16], n-type conductivity [1,4] depending on the concentration of tin and it may also changes its type of conduction from p to n-type conduction in accordance with treatment temperature [11,12]. Different values of energy gap have been obtained [1,2,4–7] for SnS ranging from 1 to 2.33 eV depending on the resulting structure obtained by different techniques and the occurring type of
*
Corresponding author. Fax: +20-2-057-403868. E-mail address: [email protected] (H.M. Zeyada).
electron transitions. The requirements imposed on films used as a light absorber are (i) they must have an energy gap of about 1.5 eV with indirect allowed transition and (ii) a high absorption coefficient >104 cm1 . Because SnS crystallizes in orthorhombic structure, it can be used in n–p homojunction [11] and n–p heterojunction [11,12]. The nature is abundant in elements Sn and S which moreover are non-polluting during SnS growing process. SnS thin films can be prepared by a variety of methods [1,2,4,5,7,9] with the purpose of manufacturing thin films suitable for use as a solar absorber in optoelectronic devices and photovoltaic applications [12]. Among these methods, the thermal evaporation technique received little interest in the literature [8,13]. The most recently studies on thermally evaporated SnS films were probably made by Deraman et al. [13] who
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investigated the influence of substrate temperatures on optical band gap of SnS films. They obtained a value of 1.07 eV for the energy gap of treated SnS films. Due to the increasing interest of SnS, the current investigation attempts to find out ways of growing SnS thin films with better properties for photovoltaic and solar cell applications using thermal evaporation under vacuum and controlled deposition conditions with attention given to its structural and optical properties.
2. Experimental procedures Thin films of SnS were prepared by thermal evaporation process using molybdenum boat, on optical flat fused quartz substrates for optical measurements and on glass substrates for structural investigations at room temperature. The chamber was evacuated down to 2 103 Pa. The film thickness and deposition rate were controlled during deposition by means of a quartz balance (model FTM 4) and were also measured interferometrically [14]. The structural analysis of SnS films with thickness of 360 nm was analyzed by Philips X-ray diffraction (XRD) system (model X 00 Pert Pro) equipped with Cu target. A filtered CuKa radia) was used. The X-ray tube tion (k ¼ 1:5408 A voltage and current were 40 KV and 30 mA, respectively. The speed of the detector was 1° per min. The parameters [15] driving the properties of thin films are structure, film thickness, grain size, faults probability, number of layers, annealing temperatures, substrate temperature, deposition rate and the presence of impurities. The parameters under current investigation and influencing the optical constants of thin films are film thickness and annealing temperatures. Different film thicknesses ranging from 55 to 365 nm were grown and investigated. As evaporated films and films annealed at 473 K for 1 h under vacuum were investigated. The measurements of transmittance, T ðkÞ, and reflectance, RðkÞ, of the films deposited on fused quartz substrates were carried out using a double beam spectrophotometer (JASCO, V-570 UV-VIS-NIR), at normal incidence of light and in
the wavelength range between 250 and 2500 nm. All the measurements were carried out at room temperature. The T ðkÞ and RðkÞ were calculated [17,18] according to T ðkÞ ¼ and RðkÞ ¼
Ift ð1 Rq Þ Iq
Ifr 2 RAl ½1 þ ð1 Rq Þ T 2 Rq IAl
ð1Þ
ð2Þ
where Ift , Iq are the intensities of light passing through film-quartz system and reference quartz, respectively, Ifr , IAl are the intensities of light reflected from the sample and that from the reference mirror, respectively, and Rq being the reflectance of quartz. The refractive index, n, and the absorption index, k, were computed by a modified version [19] of Abeles et al. technique [20] based on solving the two non-linear equations: ft ðn; kÞ ¼ Tn;k Texp ¼ 0
ð3Þ
fr ðn; kÞ ¼ Rn;k Rexp ¼ 0
ð4Þ
where Tn;k and Rn;k refer to Murmann’s exact equations [21,22]. The experimental errors [23] were taken into account as follows: 2.2% for film thickness measurements, 0.1% for T and R calculations, 3% for refractive index and 2.5% for absorption index measurements.
3. Results and discussion Fig. 1 shows the EDAX spectrum of evaporated SnS, only Sn and S peaks of nearly equal intensities were observed. The evaluated elemental composition was Sn––50.05 at.% and S––49.95 at.%. The XRD studies of the bulk SnS after fine powdering is shown in Fig. 2. Table 1 gives the calculated d-spacing for the above pattern, as well as those obtained from ICDD pattern No. 39-354 for comparison. The XRD studies indicated that the fine powder of the bulk material corresponds to a herbenzite SnS with an orthorhombic crystal , structure of lattice parameters, a ¼ 4:329 A b ¼ 11:193 A and c ¼ 3:984 A [24]. This XRD
M.M. El-Nahass et al. / Optical Materials 20 (2002) 159–170
Fig. 1. EDAX spectrum of evaporated SnS.
Fig. 2. XRD pattern of SnS as powder.
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Table 1 The calculated d-spacing for SnS in comparison to ICDD card no. 39-354 No.
Powder form ) d (A I/I0
ICDD card no. 39-345 ) d (A I/I0 hkl
1 2 3 4 5 6 7 8 9
3.423 3.243 2.928 2.833 2.797 2.789 2.304 2.125 2.024
3.423 3.244 2.931 2.835 2.794 – 2.305 2.125 2.024
8 4 43 76 100 52 4 1 4
50 65 70 100 50 – 50 25 25
120 021 101 111 040 – 131 210 141
profile of evaporated SnS is shown in Fig. 3. The profile indicated that the film is amorphous in nature and it exhibited a broad peak around 2h ¼ 23°. Annealing led to partial crystallization beginning at 369 K, as it is shown in differential scanning calorimetric (Fig. 4). Fig. 3 also shows the XRD pattern of annealed specimens at 423, 473, 523 and 573 K for 1 h in vacuum of 103 Pa. The films are brownish dark in colour, films annealed at 423 K exhibited mainly two peaks corresponding to the planes (1 1 1) and (1 0 1) of SnS. On raising the annealing temperature in steps of 50 K up to a temperature of 573 K, the intensity of
Fig. 3. XRD pattern of treated films: I––as deposited, II––annealed at 423 K, III––annealed at 473 K, IV––annealed at 523 K and V––annealed at 573 K.
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163
Fig. 4. Differential scanning calorimetric of amorphous SnS.
the halo corresponding to the amorphous phase decreased. The halo at 523 K coincides with that one at 573 K. The highest peak intensity on diffracting planes (1 1 1) and (1 0 1) is obtained at 473 K. At 523 and 573 K, planes (0 0 2) and (0 6 1) started to grow at the expense of (1 1 1) and (1 0 1)
planes, resulting in a decrease in their peak intensity as compared to intensity on corresponding planes for films annealed at 473 K, as shown in Fig. 3 and Table 2. This ensures the increase in the volume fraction of its crystallized phase at 473 K. Fig. 3 shows that SnS is the only structural phase
Table 2 Influence of annealing temperature on crystallization process Annealing temperature 423 K ) d-spacing (A hkl I/I0 Peak height (counts/s)
2.92 101 76 26
473 K 2.82 111 100 34
2.9 101 60 45
523 K 2.81 111 100 74
2.9 101 71 37
573 K 2.81 111 100 52
1.987 002 12 9
2.9 101 58 27
2.81 111 100 47.5
1.99 002 7 3
1.66 061 7.5 3.5
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formed during annealing in the temperature range (373–573 K). Reddy [3] studied the dependence of substrate temperature in the range between 373 and 673 K on structural behaviour of SnS layers grown by spray pyrolysis. They found that for temperatures lower than 573 K the films exhibited Sn2 S3 and SnS2 phases. The SnO2 phase was predominant in the films grown at temperatures above 633 K. The SnS phase was formed in the temperature range 500–633 K by spray pyrolysis. Deraman et al. [9] studied the dependence of substrate temperatures in the range 323–573 K on structural behaviour of SnS layers grown by the thermal evaporation technique. They showed that films prepared at temperatures lower than 573 K have significant proportion of Sn2 S3 , SnS2 and Sn3 S4 . Evaporated films prepared from SnS at a substrate temperature of 573 K are primarily SnS. Thermal evaporation of SnS films followed by annealing in the temperature range (373–573 K)
differs from both spray pyrolysis [1,2] and thermal evaporation at substrate temperature [9] by forming SnS phase only, which is considered as a good light absorber phase. The spectral behaviour of the normal incidence transmittance, T ðkÞ, and reflectance, RðkÞ, for amorphous SnS films and for the same films annealed at 473 K in 103 Pa of vacuum for 1 h and in the wavelength range of (250–2500 nm), are shown in Figs. 5 and 6, respectively. It is clear from Figs. 5 and 6 that both of T ðkÞ and RðkÞ for all thicknesses under investigation increase with increasing wavelength up to 575 nm. Increasing the thickness of the films decreases both of T ðkÞ and RðkÞ for a given wavelength in the amorphous and annealed conditions, and the sum of T ðkÞ and RðkÞ for each value of thickness is less than unity. This means that there is absorption in that wavelength range. In the wavelength range from 575 to 1250 nm, T ðkÞ increases and RðkÞ roughly levels off
Fig. 5. Spectral behaviour of transmittance, T, and reflectance, R, of amorphous SnS thin films.
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165
Fig. 6. Spectral behaviour of transmittance, T, and reflectance, R, of SnS thin films annealed at 473 K for 1 h.
and their sum is less than unity. At longer wavelength, (k > 1250 nm), T ðkÞ increases and RðkÞ decreases and their sum is unity, indicating that the investigated films are expected to be transparent. Fig. 7 shows the spectral behaviour of both n and k in the wavelength range of (250–2500 nm), for amorphous and annealed films and summarize the average curves of both n and k for the investigated thickness range (55–365 nm) as a function of wavelength, k. These investigations proved that the optical constants (n and k) are independent of film thickness in the film thickness range (55–365 nm). This last finding is in contradiction to results obtained by Goswami et al. [8], where they reported that there is a dependence of ðn; kÞ and energy gap, Eg , on film thickness. The annealing process shifts the curves of n and k to shorter wavelength, decreasing the value of k and increasing the value of n, in comparison to amorphous films for all wavelength range.
El-Nahass [25] showed that there is a good agreement between experimentally and theoretically calculated values of refractive index in the transmission and low absorption region. Hence, it is possible to apply Wemple and Di Domenico model [26,27] in which the refractive index, nðkÞ, is related to the dispersion parameters (E0 and Ed ) by the relation: n2 1 ¼
E0 E d E02 E2
ð5Þ 1
A plot of ðn2 1Þ versus E2 of amorphous and crystalline films is illustrated in Fig. 8. E0 and Ed are directly determined from the slope, ðE0 Ed Þ1 , and the intersection, (E0 /Ed ), with the vertical axis. The calculated values of the dispersion parameters (E0 and Ed ), as well as the corresponding optical dielectric constant (e1 ¼ n21 Þ for the amorphous and annealed films are listed in Table 3.
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Fig. 7. Spectral behaviour of refractive index, n, and absorption index, k, of SnS thin films.
Fig. 8. Plot of 1/(n2 1) versus E2 of SnS thin films.
The oscillator energy, E0 , to a very good approximation scales with the energy gap, Eg , E0 2Eg , as it was found by Tanaka [29]. The dispersion energy, Ed , also obeys an empirical relationship [27,28], which states that Ed ¼ bNc Za Ne
ðeVÞ
ð6Þ
Wemple and Di Domenico [27] have found empirically that the coefficient b in relation (6) takes the values bi ¼ 0:26 0:04 eV for ionic binding and bc ¼ 0:37 0:05 eV for covalent binding and
varies between these two limits for a large number of different materials, Nc is the coordination
Table 3 Values of the oscillator energy, E0 , dispersion energy, Ed , and the dielectric constant at infinite frequency, e1 ¼ n21 , for amorphous and annealed SnS thin films Film condition
e1
E0 (eV)
Ed (eV)
Amorphous Annealed
8:11 0:01 8:87 0:01
2:84 0:01 3:03 0:01
20:20 0:01 23:85 0:01
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167
Fig. 9. Spectral behaviour of the absorption coefficient, a, of SnS thin films.
number of the cations (nearest neighbor to the anion), Za is the formal chemical valence of the anion, and Ne is the effective number of valence electrons per anion. Table 3 shows that the value of dispersion energy, Ed , is 20.2 eV for amorphous films and it increases upon annealing to 23.85 eV. Assuming Ne ¼ ð6 50:05 þ 4 49:95Þ=49:95 ¼ 10 and Za ¼ 2; these values must not be changed by annealing, it is well known that SnS consists of two dimensional structural layers [31], in which the Sn coordination number is 4, substituting these values in (6) we obtained a value for b ¼ 0:25 indicating an ionic binding in SnS and a value for the coordination number of annealed films equal to 4.77. The increase in the dispersion energy, Ed , can be attributed to the increase in coordination number upon annealing. The increase in Nc could certainly be explained in terms of increase of the interaction between the structural layers of SnS as a consequence of annealing. The absorption coefficient, a, was calculated from the average absorption index as a ¼ 4pk=k. The spectral behaviour of the absorption coefficient as a function of energy, hm, is shown in Fig. 9, for amorphous and annealed films. The films have a highest absorption coefficient >105 cm1 in the wavelength up to 800 nm. At a given frequency in the absorption range, the absorption coefficient of amorphous films is higher than that of the annealed films.
To determine the energy gap, Eg , and the type of optical transition responsible for optical absorption, the equation by Bardeen et al. [30] was used. It states that ahm ¼ Aðhm Eg Þ
x
ð7Þ
where x ¼ 1=2 and 3/2 for direct allowed and forbidden transitions, respectively, x ¼ 2 and 3 for
Fig. 10. Plot of ðahmÞ1=2 and ðahmÞ2=3 versus hm of SnS thin films.
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indirect allowed and forbidden transitions, respectively. The types of transition and the value of the optical energy gap are illustrated in Fig. 10. This figure shows an indirect allowed transition with energy gaps of 1.4 and 1.38 eV, accompanied with phonon absorption of energies 0.05 and 0.07 eV for the amorphous and crystalline films, respectively. This indirect allowed transition occurs in the energy range from 1.38 to 2 eV. A direct forbidden transition with energy gaps of 2.18 and 2.33 eV for the amorphous and crystalline films,
respectively and occurring in the energy range from 2.18 to 5 eV. Figs. 11 and 12 clearly show that the slopes of the straight lines are 2 and 1.5, ensuring the occurrence of both indirect allowed transition and direct forbidden transition in the energy range concerned. Comparing the obtained values of energy gap for amorphous and annealed films with the results of oscillator energy, E0 , given in Table 3. It is clear that it agrees with Tanaka [29] approximations, which states that E0 2Eg . Table 4, gives the values of energy gap obtained in
Fig. 11. The relation between logðahmÞ and logðhm Egind Þ.
Fig. 12. The relation between logðahmÞ and logðhm Egdir Þ.
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169
Table 4 Values of energy gaps for SnS films prepared by different techniques Film condition
Egind (eV)
Egdir (eV)
Preparation technique
Reference
Amorphous Crystalline Crystalline Crystalline Amorphous Amorphous Amorphous Amorphous Crystalline
1.4 1.38 1.0 – 1.51 1.1 1.1 1.2 1.07
2.18 2.33 – 1.32 – – – – –
Thermal evaporation Thermal evaporation Spray pyrolysis Spray pyrolysis Chemical deposition Chemical deposition Chemical deposition Chemical deposition Thermal evaporation
Present work Present work [1] [2] [4] [5] [6] [7] [13]
the present work in comparison to those given by other workers [1,2,4–7,13]. Careful study of Table 4 shows that there are different values of energy gap depending on the technique of preparation and the mechanism by which the light is absorbed in the film. The values of energy gap obtained by thermal evaporation technique adapted in this work (Table 4) are near from the optimum value of 1.5 eV for efficient light absorption and they are also greater than those values obtained by the same technique [13] by about 30%. 4. Conclusion The main conclusion of the current investigation can be summarized as follows: Thermal evaporation of bulk crystalline SnS materials resulted in amorphous films with energy gap of 1.4 eV and absorption coefficient >105 cm1 . SnS is the only structural phase formed during annealing of SnS films in the temperature range 373–573 K. A significant growth of planes (1 0 1) and (1 1 1) is noticed upon annealing of amorphous SnS films at 473 K. Annealing of thermally evaporated SnS films at 473 K resulted in films with energy gap of 1.38 eV and absorption coefficient >105 cm1 . Both amorphous and annealed SnS films are transparent for wavelength >1250 nm. The value of dispersion energy, Ed , for amorphous films is 20.25 eV and that for annealed films is 23.85 eV, respectively. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions.
References [1] B. Thangaraju, P. Kaliannan, J. Phys. D: Appl. Phys. 33 (2000) 1054. [2] N. Koteswara Reddy, K.T. Ramakrishna Reddy, Thin Solid Films 325 (1998) 4. [3] N. Koteswara Reddy, K.T. Ramakrishna Reddy, G. Fisher, J. Phys. D: Appl. Phys. 32 (1999) 988. [4] P. Pramanik, P.K. Basu, S. Biswas, Thin Solid Films 150 (1987) 269. [5] Z. Zainal, M.Z. Hussein, A. Ghazali, Solar Energy Mater. Solar Cells 40 (1996) 347. [6] Z. Zainal, M.Z. Hussein, A. Kasseim, A. Ghazali, J. Mater. Sci. Lett. 16 (1997) 1446. [7] A. Ghazali, Z. Zainal, M.Z. Hussein, A. Kasseim, Solar Energy Mater. Solar Cells 55 (1998) 237. [8] A. Goswami, A. Mitra, Indian J. Pure Appl. Phys. 13 (1975) 508. [9] K. Deraman, S. Sakrani, B.B. Ismail, Y. Wahab, R.D. Gould, Int. J. Electron. 76 (5) (1994) 917. [10] A. Ortiz, J.C. Alonso, M. Garcia, J. Toriz, Semicond. Sci. Technol. (UK) 11 (2) (1996) 243. [11] M. Ristov, Gj. Sinadinovski, I. Grozdanov, M.M. Mitreski, Thin Solid Films 173 (1989) 53. [12] I. Grozdanov, M. Ristov, Gj. Sinadinovski, M. Mitreski, Fizika 21 (1989) 320. [13] K. Deraman, S. Sakrani, B. Ismail, SPIE 2364 (1994) 357. [14] S. Tolansky, in: Multiple-Beam Interference Microscopy of Metals, Academic, London, 1970, p. 55. [15] I. Pop, C. Nascu, V. Ionescu, E. Indrea, I. Pratu, Thin Solid Films 307 (1997) 240. [16] M. Parenteau, C. Carlone, Phys. Rev. B 41 (1990) 5227. [17] L.A. Agiev, I.N. Shklyarevskii, J. Prekel Spekt. 76 (1978) 380. [18] I.N. Shklyarevski, T.I. Kornveeva, K.N. Zozula, Opt. Spect. 27 (1969) 174. [19] M.M. El-Nahass, H.S. Soliman, N. El-Kadry, A.Y. Morsy, S. Yaghmour, J. Mater. Sci. Lett. 7 (1988) 1050. [20] F. Abeles, M.L. Theye, Surf. Sci. 5 (1966) 325. [21] O.S. Heavens, in: G. Hass, R. Thus (Eds.), Physics of Thin Films, Academic, New York, 1964, p. 193.
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[22] H.M. Liddeli, in: Computer-Aided Techniques for the Design of Multilayer Filters, Hilger, Bristol, 1981, p. 118. [23] I. Konstantinov, T. Babeva, S. Kitova, Appl. Opt. 37 (19) (1998) 4260. [24] ICDD Card No. 39-354. [25] M.M. El-Nahass, J. Mater. Sci. 27 (1992) 6597. [26] M. DiDomenico, S.H. Wemple, J. Appl. Phys. 40 (1969) 720.
[27] [28] [29] [30]
S.H. Wemple, M. DiDomenico, Phys. Rev. B 3 (1971) 1338. S.H. Wemple, Phys. Rev. B 7 (1973) 3767. Ke. Tanaka, Thin Solid Films 66 (1980) 271. J. Bardeen, F.J. Blatt, L.H. Hall, in: R. Breckenridge, B. Russel, T. Hahn (Eds.), Proceedings of Photo Conductivity Conference, Wiley, New York, 1956. [31] H. Ahari, G. Ozin, R.L. Bedard, S. Petrov, D. Young, Adv. Mater. 7 (1995) 370.
Optical properties of thermally evaporated SnS thin films M.M. El-Nahass a, H.M. Zeyada a b
b,*
, M.S. Aziz b, N.A. El-Ghamaz
b
Department of Physics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Department of Physics, Faculty of Science at New Damietta, 34517 New Damietta, Egypt
Received 27 April 2001; received in revised form 28 November 2001; accepted 7 January 2002
Abstract Thermally evaporated SnS amorphous chalcogenide films undergo structural transformation upon annealing in the temperature range between 432–573 K. The optical properties of amorphous and annealed films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence in the wavelength range 250–2500 nm. The films are transparent for a wavelength >1250 nm. The refractive index (n) and the absorption index (k) are independent of film thickness in the measured film thickness range (55–365 nm). The dispersion energy, Ed , of amorphous films increased from 20.2 to 23.85 eV for crystalline films. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions with energy gaps of 1.4 and 2.18 eV for the amorphous films and of 1.38 and 2.33 eV for the crystalline films, respectively. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal evaporation; Optical constants; SnS
1. Introduction SnS is one of the tin chalcogenide layered semiconductors in group IV–VI. It may exhibit the p-type conductivity [4–7,9,10,16], n-type conductivity [1,4] depending on the concentration of tin and it may also changes its type of conduction from p to n-type conduction in accordance with treatment temperature [11,12]. Different values of energy gap have been obtained [1,2,4–7] for SnS ranging from 1 to 2.33 eV depending on the resulting structure obtained by different techniques and the occurring type of
*
Corresponding author. Fax: +20-2-057-403868. E-mail address: [email protected] (H.M. Zeyada).
electron transitions. The requirements imposed on films used as a light absorber are (i) they must have an energy gap of about 1.5 eV with indirect allowed transition and (ii) a high absorption coefficient >104 cm1 . Because SnS crystallizes in orthorhombic structure, it can be used in n–p homojunction [11] and n–p heterojunction [11,12]. The nature is abundant in elements Sn and S which moreover are non-polluting during SnS growing process. SnS thin films can be prepared by a variety of methods [1,2,4,5,7,9] with the purpose of manufacturing thin films suitable for use as a solar absorber in optoelectronic devices and photovoltaic applications [12]. Among these methods, the thermal evaporation technique received little interest in the literature [8,13]. The most recently studies on thermally evaporated SnS films were probably made by Deraman et al. [13] who
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 0 3 0 - 7
160
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investigated the influence of substrate temperatures on optical band gap of SnS films. They obtained a value of 1.07 eV for the energy gap of treated SnS films. Due to the increasing interest of SnS, the current investigation attempts to find out ways of growing SnS thin films with better properties for photovoltaic and solar cell applications using thermal evaporation under vacuum and controlled deposition conditions with attention given to its structural and optical properties.
2. Experimental procedures Thin films of SnS were prepared by thermal evaporation process using molybdenum boat, on optical flat fused quartz substrates for optical measurements and on glass substrates for structural investigations at room temperature. The chamber was evacuated down to 2 103 Pa. The film thickness and deposition rate were controlled during deposition by means of a quartz balance (model FTM 4) and were also measured interferometrically [14]. The structural analysis of SnS films with thickness of 360 nm was analyzed by Philips X-ray diffraction (XRD) system (model X 00 Pert Pro) equipped with Cu target. A filtered CuKa radia) was used. The X-ray tube tion (k ¼ 1:5408 A voltage and current were 40 KV and 30 mA, respectively. The speed of the detector was 1° per min. The parameters [15] driving the properties of thin films are structure, film thickness, grain size, faults probability, number of layers, annealing temperatures, substrate temperature, deposition rate and the presence of impurities. The parameters under current investigation and influencing the optical constants of thin films are film thickness and annealing temperatures. Different film thicknesses ranging from 55 to 365 nm were grown and investigated. As evaporated films and films annealed at 473 K for 1 h under vacuum were investigated. The measurements of transmittance, T ðkÞ, and reflectance, RðkÞ, of the films deposited on fused quartz substrates were carried out using a double beam spectrophotometer (JASCO, V-570 UV-VIS-NIR), at normal incidence of light and in
the wavelength range between 250 and 2500 nm. All the measurements were carried out at room temperature. The T ðkÞ and RðkÞ were calculated [17,18] according to T ðkÞ ¼ and RðkÞ ¼
Ift ð1 Rq Þ Iq
Ifr 2 RAl ½1 þ ð1 Rq Þ T 2 Rq IAl
ð1Þ
ð2Þ
where Ift , Iq are the intensities of light passing through film-quartz system and reference quartz, respectively, Ifr , IAl are the intensities of light reflected from the sample and that from the reference mirror, respectively, and Rq being the reflectance of quartz. The refractive index, n, and the absorption index, k, were computed by a modified version [19] of Abeles et al. technique [20] based on solving the two non-linear equations: ft ðn; kÞ ¼ Tn;k Texp ¼ 0
ð3Þ
fr ðn; kÞ ¼ Rn;k Rexp ¼ 0
ð4Þ
where Tn;k and Rn;k refer to Murmann’s exact equations [21,22]. The experimental errors [23] were taken into account as follows: 2.2% for film thickness measurements, 0.1% for T and R calculations, 3% for refractive index and 2.5% for absorption index measurements.
3. Results and discussion Fig. 1 shows the EDAX spectrum of evaporated SnS, only Sn and S peaks of nearly equal intensities were observed. The evaluated elemental composition was Sn––50.05 at.% and S––49.95 at.%. The XRD studies of the bulk SnS after fine powdering is shown in Fig. 2. Table 1 gives the calculated d-spacing for the above pattern, as well as those obtained from ICDD pattern No. 39-354 for comparison. The XRD studies indicated that the fine powder of the bulk material corresponds to a herbenzite SnS with an orthorhombic crystal , structure of lattice parameters, a ¼ 4:329 A b ¼ 11:193 A and c ¼ 3:984 A [24]. This XRD
M.M. El-Nahass et al. / Optical Materials 20 (2002) 159–170
Fig. 1. EDAX spectrum of evaporated SnS.
Fig. 2. XRD pattern of SnS as powder.
161
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Table 1 The calculated d-spacing for SnS in comparison to ICDD card no. 39-354 No.
Powder form ) d (A I/I0
ICDD card no. 39-345 ) d (A I/I0 hkl
1 2 3 4 5 6 7 8 9
3.423 3.243 2.928 2.833 2.797 2.789 2.304 2.125 2.024
3.423 3.244 2.931 2.835 2.794 – 2.305 2.125 2.024
8 4 43 76 100 52 4 1 4
50 65 70 100 50 – 50 25 25
120 021 101 111 040 – 131 210 141
profile of evaporated SnS is shown in Fig. 3. The profile indicated that the film is amorphous in nature and it exhibited a broad peak around 2h ¼ 23°. Annealing led to partial crystallization beginning at 369 K, as it is shown in differential scanning calorimetric (Fig. 4). Fig. 3 also shows the XRD pattern of annealed specimens at 423, 473, 523 and 573 K for 1 h in vacuum of 103 Pa. The films are brownish dark in colour, films annealed at 423 K exhibited mainly two peaks corresponding to the planes (1 1 1) and (1 0 1) of SnS. On raising the annealing temperature in steps of 50 K up to a temperature of 573 K, the intensity of
Fig. 3. XRD pattern of treated films: I––as deposited, II––annealed at 423 K, III––annealed at 473 K, IV––annealed at 523 K and V––annealed at 573 K.
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163
Fig. 4. Differential scanning calorimetric of amorphous SnS.
the halo corresponding to the amorphous phase decreased. The halo at 523 K coincides with that one at 573 K. The highest peak intensity on diffracting planes (1 1 1) and (1 0 1) is obtained at 473 K. At 523 and 573 K, planes (0 0 2) and (0 6 1) started to grow at the expense of (1 1 1) and (1 0 1)
planes, resulting in a decrease in their peak intensity as compared to intensity on corresponding planes for films annealed at 473 K, as shown in Fig. 3 and Table 2. This ensures the increase in the volume fraction of its crystallized phase at 473 K. Fig. 3 shows that SnS is the only structural phase
Table 2 Influence of annealing temperature on crystallization process Annealing temperature 423 K ) d-spacing (A hkl I/I0 Peak height (counts/s)
2.92 101 76 26
473 K 2.82 111 100 34
2.9 101 60 45
523 K 2.81 111 100 74
2.9 101 71 37
573 K 2.81 111 100 52
1.987 002 12 9
2.9 101 58 27
2.81 111 100 47.5
1.99 002 7 3
1.66 061 7.5 3.5
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formed during annealing in the temperature range (373–573 K). Reddy [3] studied the dependence of substrate temperature in the range between 373 and 673 K on structural behaviour of SnS layers grown by spray pyrolysis. They found that for temperatures lower than 573 K the films exhibited Sn2 S3 and SnS2 phases. The SnO2 phase was predominant in the films grown at temperatures above 633 K. The SnS phase was formed in the temperature range 500–633 K by spray pyrolysis. Deraman et al. [9] studied the dependence of substrate temperatures in the range 323–573 K on structural behaviour of SnS layers grown by the thermal evaporation technique. They showed that films prepared at temperatures lower than 573 K have significant proportion of Sn2 S3 , SnS2 and Sn3 S4 . Evaporated films prepared from SnS at a substrate temperature of 573 K are primarily SnS. Thermal evaporation of SnS films followed by annealing in the temperature range (373–573 K)
differs from both spray pyrolysis [1,2] and thermal evaporation at substrate temperature [9] by forming SnS phase only, which is considered as a good light absorber phase. The spectral behaviour of the normal incidence transmittance, T ðkÞ, and reflectance, RðkÞ, for amorphous SnS films and for the same films annealed at 473 K in 103 Pa of vacuum for 1 h and in the wavelength range of (250–2500 nm), are shown in Figs. 5 and 6, respectively. It is clear from Figs. 5 and 6 that both of T ðkÞ and RðkÞ for all thicknesses under investigation increase with increasing wavelength up to 575 nm. Increasing the thickness of the films decreases both of T ðkÞ and RðkÞ for a given wavelength in the amorphous and annealed conditions, and the sum of T ðkÞ and RðkÞ for each value of thickness is less than unity. This means that there is absorption in that wavelength range. In the wavelength range from 575 to 1250 nm, T ðkÞ increases and RðkÞ roughly levels off
Fig. 5. Spectral behaviour of transmittance, T, and reflectance, R, of amorphous SnS thin films.
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165
Fig. 6. Spectral behaviour of transmittance, T, and reflectance, R, of SnS thin films annealed at 473 K for 1 h.
and their sum is less than unity. At longer wavelength, (k > 1250 nm), T ðkÞ increases and RðkÞ decreases and their sum is unity, indicating that the investigated films are expected to be transparent. Fig. 7 shows the spectral behaviour of both n and k in the wavelength range of (250–2500 nm), for amorphous and annealed films and summarize the average curves of both n and k for the investigated thickness range (55–365 nm) as a function of wavelength, k. These investigations proved that the optical constants (n and k) are independent of film thickness in the film thickness range (55–365 nm). This last finding is in contradiction to results obtained by Goswami et al. [8], where they reported that there is a dependence of ðn; kÞ and energy gap, Eg , on film thickness. The annealing process shifts the curves of n and k to shorter wavelength, decreasing the value of k and increasing the value of n, in comparison to amorphous films for all wavelength range.
El-Nahass [25] showed that there is a good agreement between experimentally and theoretically calculated values of refractive index in the transmission and low absorption region. Hence, it is possible to apply Wemple and Di Domenico model [26,27] in which the refractive index, nðkÞ, is related to the dispersion parameters (E0 and Ed ) by the relation: n2 1 ¼
E0 E d E02 E2
ð5Þ 1
A plot of ðn2 1Þ versus E2 of amorphous and crystalline films is illustrated in Fig. 8. E0 and Ed are directly determined from the slope, ðE0 Ed Þ1 , and the intersection, (E0 /Ed ), with the vertical axis. The calculated values of the dispersion parameters (E0 and Ed ), as well as the corresponding optical dielectric constant (e1 ¼ n21 Þ for the amorphous and annealed films are listed in Table 3.
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Fig. 7. Spectral behaviour of refractive index, n, and absorption index, k, of SnS thin films.
Fig. 8. Plot of 1/(n2 1) versus E2 of SnS thin films.
The oscillator energy, E0 , to a very good approximation scales with the energy gap, Eg , E0 2Eg , as it was found by Tanaka [29]. The dispersion energy, Ed , also obeys an empirical relationship [27,28], which states that Ed ¼ bNc Za Ne
ðeVÞ
ð6Þ
Wemple and Di Domenico [27] have found empirically that the coefficient b in relation (6) takes the values bi ¼ 0:26 0:04 eV for ionic binding and bc ¼ 0:37 0:05 eV for covalent binding and
varies between these two limits for a large number of different materials, Nc is the coordination
Table 3 Values of the oscillator energy, E0 , dispersion energy, Ed , and the dielectric constant at infinite frequency, e1 ¼ n21 , for amorphous and annealed SnS thin films Film condition
e1
E0 (eV)
Ed (eV)
Amorphous Annealed
8:11 0:01 8:87 0:01
2:84 0:01 3:03 0:01
20:20 0:01 23:85 0:01
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167
Fig. 9. Spectral behaviour of the absorption coefficient, a, of SnS thin films.
number of the cations (nearest neighbor to the anion), Za is the formal chemical valence of the anion, and Ne is the effective number of valence electrons per anion. Table 3 shows that the value of dispersion energy, Ed , is 20.2 eV for amorphous films and it increases upon annealing to 23.85 eV. Assuming Ne ¼ ð6 50:05 þ 4 49:95Þ=49:95 ¼ 10 and Za ¼ 2; these values must not be changed by annealing, it is well known that SnS consists of two dimensional structural layers [31], in which the Sn coordination number is 4, substituting these values in (6) we obtained a value for b ¼ 0:25 indicating an ionic binding in SnS and a value for the coordination number of annealed films equal to 4.77. The increase in the dispersion energy, Ed , can be attributed to the increase in coordination number upon annealing. The increase in Nc could certainly be explained in terms of increase of the interaction between the structural layers of SnS as a consequence of annealing. The absorption coefficient, a, was calculated from the average absorption index as a ¼ 4pk=k. The spectral behaviour of the absorption coefficient as a function of energy, hm, is shown in Fig. 9, for amorphous and annealed films. The films have a highest absorption coefficient >105 cm1 in the wavelength up to 800 nm. At a given frequency in the absorption range, the absorption coefficient of amorphous films is higher than that of the annealed films.
To determine the energy gap, Eg , and the type of optical transition responsible for optical absorption, the equation by Bardeen et al. [30] was used. It states that ahm ¼ Aðhm Eg Þ
x
ð7Þ
where x ¼ 1=2 and 3/2 for direct allowed and forbidden transitions, respectively, x ¼ 2 and 3 for
Fig. 10. Plot of ðahmÞ1=2 and ðahmÞ2=3 versus hm of SnS thin films.
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indirect allowed and forbidden transitions, respectively. The types of transition and the value of the optical energy gap are illustrated in Fig. 10. This figure shows an indirect allowed transition with energy gaps of 1.4 and 1.38 eV, accompanied with phonon absorption of energies 0.05 and 0.07 eV for the amorphous and crystalline films, respectively. This indirect allowed transition occurs in the energy range from 1.38 to 2 eV. A direct forbidden transition with energy gaps of 2.18 and 2.33 eV for the amorphous and crystalline films,
respectively and occurring in the energy range from 2.18 to 5 eV. Figs. 11 and 12 clearly show that the slopes of the straight lines are 2 and 1.5, ensuring the occurrence of both indirect allowed transition and direct forbidden transition in the energy range concerned. Comparing the obtained values of energy gap for amorphous and annealed films with the results of oscillator energy, E0 , given in Table 3. It is clear that it agrees with Tanaka [29] approximations, which states that E0 2Eg . Table 4, gives the values of energy gap obtained in
Fig. 11. The relation between logðahmÞ and logðhm Egind Þ.
Fig. 12. The relation between logðahmÞ and logðhm Egdir Þ.
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Table 4 Values of energy gaps for SnS films prepared by different techniques Film condition
Egind (eV)
Egdir (eV)
Preparation technique
Reference
Amorphous Crystalline Crystalline Crystalline Amorphous Amorphous Amorphous Amorphous Crystalline
1.4 1.38 1.0 – 1.51 1.1 1.1 1.2 1.07
2.18 2.33 – 1.32 – – – – –
Thermal evaporation Thermal evaporation Spray pyrolysis Spray pyrolysis Chemical deposition Chemical deposition Chemical deposition Chemical deposition Thermal evaporation
Present work Present work [1] [2] [4] [5] [6] [7] [13]
the present work in comparison to those given by other workers [1,2,4–7,13]. Careful study of Table 4 shows that there are different values of energy gap depending on the technique of preparation and the mechanism by which the light is absorbed in the film. The values of energy gap obtained by thermal evaporation technique adapted in this work (Table 4) are near from the optimum value of 1.5 eV for efficient light absorption and they are also greater than those values obtained by the same technique [13] by about 30%. 4. Conclusion The main conclusion of the current investigation can be summarized as follows: Thermal evaporation of bulk crystalline SnS materials resulted in amorphous films with energy gap of 1.4 eV and absorption coefficient >105 cm1 . SnS is the only structural phase formed during annealing of SnS films in the temperature range 373–573 K. A significant growth of planes (1 0 1) and (1 1 1) is noticed upon annealing of amorphous SnS films at 473 K. Annealing of thermally evaporated SnS films at 473 K resulted in films with energy gap of 1.38 eV and absorption coefficient >105 cm1 . Both amorphous and annealed SnS films are transparent for wavelength >1250 nm. The value of dispersion energy, Ed , for amorphous films is 20.25 eV and that for annealed films is 23.85 eV, respectively. The types of optical transition responsible for optical absorption are indirect allowed and direct forbidden transitions.
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