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Effect of Sn addition on the optical gap and far-infrared reflectivity spectra of amorphous Sb–Se films
LETTER TO THE EDITOR Journal of Non-Crystalline Solids 356 (2010) 1611–1613
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Letter to the Editor
Effect of Sn addition on the optical gap and far-infrared reflectivity spectra of amorphous Sb–Se films Praveen Kumar a, R. Thangaraj a,⁎, T.S. Sathiaraj b a b
Semiconductors Laboratory, Department of Physics, G.N.D. University, Amritsar-143005, India Department of Physics, University of Botswana, Gaborone, Botswana
a r t i c l e
i n f o
Article history: Received 18 July 2009 Received in revised form 26 April 2010 Available online 22 June 2010 Keywords: Amorphous alloys; Glassy semiconductors; Thin films; Band gap; Optical properties; Reflectivity
a b s t r a c t This paper reports the spectroscopic studies on the thermally evaporated SnxSb20Se80 − x (2 ≤ x ≤ 8) films. Optical gap and band tailing parameter decreases except for x = 8, where it increases. The red shift in peak reflectivity spectrum in the interband transition region (200–800 nm) has been observed. The position and shape of the reflectivity bands (80–378 cm− 1) indicate the change in the local structure of the films. The Kramer–Kronig transformations have been used to evaluate the dielectric constants from the far-infrared reflectivity spectrum. Splitting of the longitudinal optic and transverse optic mode characterizes the partial ionic character for Sn–Sb–Se system. The relationship between the optical properties and structure has been revealed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growing interest in understanding chalcogenide glasses arises from the actual and potential applications of these materials in optical and electronic devices. The threshold/memory switching behavior and infrared transmission make these glasses a potential material for memory elements and fiber optics applications respectively [1,2]. All possible applications emerge from their optical and electrical properties which are closely related to their structure and composition. A decrease in the optical gap of amorphous Se upon alloying with Sb has been reported and attributed to the increase in the localized state density in the gap [3]. The addition of Sb increases the connectivity of Se chains with the formation of Sb2Se3 units to form the three dimensional random network. The third impurity also leads to a decrease in the optical gap due to the increase in the compositional disorder in In– Sb–Se system [4]. The addition of Sn inhibits the crystallization of the Sb2Se3 phase in the Sn–Sb–Se system, and results in the formation of glassy alloys [5]. The transformation from the crystalline to amorphous nature of the bulk alloys has not been understood properly, although some electrical, optical properties and the effect of phase separation in this ternary system have already been discussed [6,7]. In the present study, an attempt has been made to elucidate the structure from the far-infrared spectrum for Sn substituted Sb20Se80 films. An important correlation
⁎ Corresponding author. E-mail address: [email protected] (R. Thangaraj). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.046
between the optical gap and molecular structure of the material has been established.
2. Experimental details Thin films of bulk SnxSb20Se80 − x (2 ≤ x ≤ 8) samples [5] were deposited in a high vacuum (∼10− 5 Torr) by using HINDHIVAC coating unit (Model no. 12A4D). The well-cleaned glass substrates placed at room temperature as substrates and the source material taken in molybdenum boats were used for the deposition. After the deposition, the films were kept inside the vacuum chamber for 24 h to attain metastable equilibrium as suggested by Abkowitz [8]. The thickness of the films was measured by using a surface profiler (KLA Tencor P15) and found to be ∼ 2.6 µm. The amorphous nature of the samples was confirmed by the absence of diffraction peaks in the X-ray diffraction studies. A Philips XL30 ESEM system with EDAX attachment was used for studying the chemical composition of the as-prepared films and found ∼ 1% error as compared to the bulk samples. The transmittance (T) and the specular reflectance (R) of the asprepared films were carried out by using UV–VIS–NIR spectrophotometer (VARIAN Cary 500) in the wavelength range 200–2500 nm. The infrared reflection spectra (50–700 cm− 1) of the thick films were taken on the IFS 60v/s vacuum Fourier Transform Interferometer (Bruker, Germany) equipped with an 11° off normal reflectance attachment and appropriate sources and detectors. All data were measured at room temperature with 2 cm− 1 resolution against a high reflectivity aluminum mirror and represent the average of 200 scans for each film sample. All the measurements were performed under
LETTER TO THE EDITOR 1612
P. Kumar et al. / Journal of Non-Crystalline Solids 356 (2010) 1611–1613
vacuum ∼ 10− 2 mbar. The least square curve fitting was used to estimate the errors in the present work.
Table 1 Value of average coordination number (Z), R-value, thickness (t), optical gap (Eg), tailing parameter (B1/2), and reflectivity maximum (Rmax) for amorphous SnxSb20Se80 − x films.
3. Results
x (at.%)
Z
R-value
t (µm)
Eg (±0.02 eV)
B1/2 (cm− 1/2 eV− 1/2)
Rmax (nm)
From the transmittance and reflectance of the films, absorption coefficient (α) of the films has been calculated by using the relation [9]:
2 4 6 8
2.24 2.28 2.32 2.36
2.29 2.00 1.76 1.56
2.34 2.60 3.05 2.80
1.57 1.48 1.38 1.36
5561.2 5120.9 4791.9 5175.1
416 419 485 522
α=
( )1 = 2 ) ( 2 2 1 ð1−R Þ ð1−R Þ 2 ln + + R t ð2TÞ ð2TÞ2
where ‘t’ is the thickness of the films, and ‘T’ and ‘R’ represents the transmittance and reflectance respectively. The absorption coefficient so obtained has been found to lie between 104 and 105 cm− 1 near the fundamental absorption region. In this region, the absorption coefficient is given by αhv = B(hv − Eg)2; where Eg is the optical gap and B1/2 is the slope in the extended region or measure of the extent of the band tailing for a given material. Fig. 1 shows the Tauc plots of (αhv)1/2 versus hv for all the films are drawn and the values of the optical gap (Eg) and slope (B1/2) are determined. Both the parameters along with the average coordination number (Z) and R-value [5] for different compositions are summarized in Table 1. The value of the optical gap decreases with an inverse relation for the tailing parameter as the Sn content increases. The value of optical gap for amorphous Sb20Se80 film is found to be consistent with that reported by Fouad et al. (1.57 eV) [3]. Similar effect in the optical gap has also been observed with the Sn substitution for Ge in amorphous Ge–Se system [10]. The inset of Fig. 1 shows the reflectivity spectrum and exhibits a red shift in the reflectivity maximum with the increase in Sn content. The experimental far-IR reflection spectra of the as-prepared films are presented in the Fig. 2. The broad reflectivity bands were analyzed by de-convoluting those located between 230 and 120 cm− 1 giving the vibrational bands at 160 and 178 cm− 1 which can be assigned for stretching modes for Sn-tetrahedral units (149–172 cm− 1) [11], Sbpyramidal unit (172–192 cm− 1) [12] and homopolar Sb–Sb bonds (170 cm− 1) [13] respectively. On the other hand, the de-convolution of band (560–230 cm− 1) gives 290, 388 cm− 1 and others, which can be assigned for SbSe3 units [12] and SeO2 (v2) structural units [14]
Fig. 1. Tauc plots between (αhv)1/2 and hv and the inset picture shows reflectivity spectra for thermally evaporated SnxSb20Se80 − x films.
generally due to the surface adsorbed oxygen present in 1–2 atomic layers for x = 2 films. The spectral domain 50–110 cm− 1 is characterized by weak bands for Sen chains (90 cm− 1), Sb(Se1/2)3 pyramids (78 cm− 1) bending modes of Sn(Se1/2)4 tetrahedra (102–123 cm− 1), respectively. No major change in the shape and position of the characteristic vibration bands with the increase in Sn content has been observed. It is now interesting to study the effect of composition on the position of different optical mode frequencies i.e. transverse optic (TO) and longitudinal optic (LO) from the reflectivity spectrum. In the TO modes, the atomic displacements are perpendicular to the direction of the periodicity of the elastic wave, whereas in the LO the displacements are parallel to the wave vector (defined for the crystalline materials, not defined for glasses) [15]. From the far-IR reflectivity spectra, the values of n (refractive index) and k (extinction coefficient) have been derived by Kramers–Kronig transformations [11,15,16]. The real part of the complex dielectric constant, ε1 and imaginary part, ε2 are computed and the peaks in the ε2 and ε2 / (ε21 + ε22) correspond to the TO and LO split frequencies respectively. Fig. 3 shows such a plot for all the samples. The splitting at the lower frequencies is for the SbSe3 vibrational modes. Position of both TO/LO mode frequencies does not change appreciably, while shape of bands for ε2 with frequency broadens with the increase in composition. 4. Discussion The experimental results can be discussed by using the chemically ordered network model (CONM) by considering the chemical bond energies involved. The bonds i.e. Sn–Se, Sb–Se and Se–Se corresponding to different bond energies 401.2, 346.2, 332.6 kJ/mol respectively are supposed to be present in this system. Even though, there is an increase in the average bond energy, the decrease in the optical gap can be due to the increase in the localized state density (as evident from the increase in tailing parameter) within the band gap [3]. The broad spectral dependence and red shift in the reflectivity spectrum with the Sn
Fig. 2. Far-infrared reflection spectra for amorphous SnxSb20Se80 − x films.
LETTER TO THE EDITOR P. Kumar et al. / Journal of Non-Crystalline Solids 356 (2010) 1611–1613
1613
the enhancement in the partial ionic character of the network with Sn content (Fig. 3). The broadening in the TO mode frequency to the higher wavenumber side can be due to the superposition and relative increase in concentration of Sn-tetrahedra as compared to Sbpyramidal units in the network. Therefore, the mixing of different molecular clusters or structural changes plays an important role for the glass formation in Sn–Sb–Se system. Thus, the broadening of band (TO mode) can be taken as a change in the structural ordering at the local level or formation of glassy samples for x = 8 sample of this ternary system [3]. These changes can be interpreted as a change in the intermediate range ordering of the network structure leading to the formation of glassy samples as well as reins the majority of the electronic and optical phenomena in this semiconducting system. 5. Conclusions
Fig. 3. Dependence of ε2 and ε2 / (ε21 + ε22) with wave number for SnxSb20Se80 − x films.
concentration has been observed (Fig. 1). The peaks in the reflection spectra for the crystalline semiconductors in the interband transition region correspond to the highest symmetry points for the Brillion zone of the crystal [17]. The absence of long range order in amorphous materials leads to exhibit the characteristic broad spectrum and the peak reflectivity shift can be attributed to the weakening of the intermolecular interactions or the oscillator strengths in the interband transition region. The spectra of these glasses could be described as a result of superposition of the spectra of their structural components. The vibrational spectra for Sn–Sb–Se system has been discussed by taking the formation of two basic structural units, SbSe3 pyramids with threefold coordinated Sb atom at the apex and SnSe4 tetrahedrons with Sn in the center. The vibrational bands reported in the literature for their crystalline accompaniment were taken as reference for the discussion of the spectrum [11–13,18]. Corredor et al. have theoretically calculated the vibrational bands for SbSe3 pyramidal units as 112, 315, 215 and 78 cm− 1 respectively for the Sb–As–Se glasses [12]. Similarly, Smith et al. have reported the Raman mode frequencies 107.9, 187.0 cm− 1 and IR active mode frequencies as 145.6, 239.8 cm− 1 for the SnSe2 crystals respectively [11,18]. There is a relative increase in the area of first band (160 cm− 1) as compared to the second one (178 cm− 1) with an access of Se-content (R-value greater than one) for all the compositions, therefore diminishing the presence of the vibrations of homopolar Sb–Sb bonds at 170 cm− 1 [13]. Thus, the vibrational band (160 cm− 1) could be the combined effect of the stretching modes of Sb-pyramidal units and/or Sn-tetrahedra while the band (178 cm− 1) has been assigned to the stretching modes of the Sb-pyramidal units [12]. Strong nearest-neighbor forces serve as mechanical constraints in stabilizing a glass network [19], while weak Coulomb forces promote only space filling of the network glasses. The shift in the transverse optic mode frequency (TO) to lower wavenumber side leads to increase the difference between LO/TO mode frequencies revealing
The composition dependence of the optical gap and tailing parameter has been studied for thermally evaporated SnxSb20Se80 − x (2 ≤ x ≤ 8) films. The optical gap decreases while the slope or tailing parameter decreases except for x = 8 compositions where it increases. The far-infrared reflectivity spectrum indicates the presence of vibrational modes due to SbSe3, SnSe2 structural units and the surface adsorbed oxide bands for all the compositions. The splitting of the different optical modes (LO/TO) indicates partial ionic character for this system. Acknowledgments The authors are thankful to Prof. V. Venkataraman (Co-ordinator) and Dr. R. Ganesan (Scientific Officer) at the DST-National Facility for Low Temperatures and High Magnetic Fields, Department of Physics, Indian Institute of Science, Bangalore for the accessing the FTIR facility. One of the authors (PK) is thankful for the financial assistance from the CSIR, New Delhi, India. References [1] A. Zakery, S.R. Elliott, J. Non-Cryst. Sol. 330 (2003) 1. [2] P. Boolchand, Insulating and Semiconducting Glasses, World Scientific, Singapore, 2000. [3] S.S. Fouad, A.H. Ammar, M. Abo-Ghazala, Phys. B: Condensed Matter 229 (1997) 249–255. [4] M.S. Kamboj, G. Kaur, R. Thangaraj, D.K. Awasthi, J. Phys. D: App. Phys. 35 (2002) 477–479. [5] P. Kumar, R. Thangaraj, J. Non-Cryst. Solids 252 (2006) 2288–2291. [6] M.M. Wakkad, E.Kh. Shokr, H.A. Abd El Ghani, M.A. Awad, Eur. Phys. J. Appl. Phys. 43 (2008) 23–30. [7] P. Kumar, J. Kumar, R. Thangaraj, Eur. Phys. J. Appl. Phys. 32 (2007) 1–5. [8] M. Abkowitz, Polym. Eng. Sci. 24 (1984) 1149. [9] G. Lucovsky, Phys. Rev. B 15 (1977) 5762. [10] M.H. Ali, S.A. Fayek, Phys. Stat. Solidi (a) 147 (2) (1995) 577–584. [11] J.Y. Harbec, S. Jandl, Phys. Rev. B 25 (10) (1982) 6126. [12] C. Corredor, I. Quiroga, J. Vazquez, J. Galdon, P. Villares, R. Jimenez-Garay, Matt. Lett. 42 (2000) 229. [13] S.M. El-Sayed, Semicond. Sci. Tech. 18 (2003) 337. [14] R.J.M. Konigs, A.S. Booij, A. Kovacs, Chem. Phys. Lett. 292 (4–6) (1998) 447–453. [15] R.M. Almeida, Phys. Rev. B 45 (1) (1992) 161. [16] T.S. Moss, Optical Properties of Semiconductors, Academic Press, New York, 1959. [17] D.L. Greenaway, G. Harbeke, Optical Properties and Band Structure of Semiconductors, Pergamon, Oxford, 1970. [18] A.J. Smith, P.E. Meek, W.Y. Liang, J. Phys. C: Sol. Stat. Phys. 10 (1977) 1321. [19] M. Kastner, D. Adler, H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Letter to the Editor
Effect of Sn addition on the optical gap and far-infrared reflectivity spectra of amorphous Sb–Se films Praveen Kumar a, R. Thangaraj a,⁎, T.S. Sathiaraj b a b
Semiconductors Laboratory, Department of Physics, G.N.D. University, Amritsar-143005, India Department of Physics, University of Botswana, Gaborone, Botswana
a r t i c l e
i n f o
Article history: Received 18 July 2009 Received in revised form 26 April 2010 Available online 22 June 2010 Keywords: Amorphous alloys; Glassy semiconductors; Thin films; Band gap; Optical properties; Reflectivity
a b s t r a c t This paper reports the spectroscopic studies on the thermally evaporated SnxSb20Se80 − x (2 ≤ x ≤ 8) films. Optical gap and band tailing parameter decreases except for x = 8, where it increases. The red shift in peak reflectivity spectrum in the interband transition region (200–800 nm) has been observed. The position and shape of the reflectivity bands (80–378 cm− 1) indicate the change in the local structure of the films. The Kramer–Kronig transformations have been used to evaluate the dielectric constants from the far-infrared reflectivity spectrum. Splitting of the longitudinal optic and transverse optic mode characterizes the partial ionic character for Sn–Sb–Se system. The relationship between the optical properties and structure has been revealed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growing interest in understanding chalcogenide glasses arises from the actual and potential applications of these materials in optical and electronic devices. The threshold/memory switching behavior and infrared transmission make these glasses a potential material for memory elements and fiber optics applications respectively [1,2]. All possible applications emerge from their optical and electrical properties which are closely related to their structure and composition. A decrease in the optical gap of amorphous Se upon alloying with Sb has been reported and attributed to the increase in the localized state density in the gap [3]. The addition of Sb increases the connectivity of Se chains with the formation of Sb2Se3 units to form the three dimensional random network. The third impurity also leads to a decrease in the optical gap due to the increase in the compositional disorder in In– Sb–Se system [4]. The addition of Sn inhibits the crystallization of the Sb2Se3 phase in the Sn–Sb–Se system, and results in the formation of glassy alloys [5]. The transformation from the crystalline to amorphous nature of the bulk alloys has not been understood properly, although some electrical, optical properties and the effect of phase separation in this ternary system have already been discussed [6,7]. In the present study, an attempt has been made to elucidate the structure from the far-infrared spectrum for Sn substituted Sb20Se80 films. An important correlation
⁎ Corresponding author. E-mail address: [email protected] (R. Thangaraj). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.046
between the optical gap and molecular structure of the material has been established.
2. Experimental details Thin films of bulk SnxSb20Se80 − x (2 ≤ x ≤ 8) samples [5] were deposited in a high vacuum (∼10− 5 Torr) by using HINDHIVAC coating unit (Model no. 12A4D). The well-cleaned glass substrates placed at room temperature as substrates and the source material taken in molybdenum boats were used for the deposition. After the deposition, the films were kept inside the vacuum chamber for 24 h to attain metastable equilibrium as suggested by Abkowitz [8]. The thickness of the films was measured by using a surface profiler (KLA Tencor P15) and found to be ∼ 2.6 µm. The amorphous nature of the samples was confirmed by the absence of diffraction peaks in the X-ray diffraction studies. A Philips XL30 ESEM system with EDAX attachment was used for studying the chemical composition of the as-prepared films and found ∼ 1% error as compared to the bulk samples. The transmittance (T) and the specular reflectance (R) of the asprepared films were carried out by using UV–VIS–NIR spectrophotometer (VARIAN Cary 500) in the wavelength range 200–2500 nm. The infrared reflection spectra (50–700 cm− 1) of the thick films were taken on the IFS 60v/s vacuum Fourier Transform Interferometer (Bruker, Germany) equipped with an 11° off normal reflectance attachment and appropriate sources and detectors. All data were measured at room temperature with 2 cm− 1 resolution against a high reflectivity aluminum mirror and represent the average of 200 scans for each film sample. All the measurements were performed under
LETTER TO THE EDITOR 1612
P. Kumar et al. / Journal of Non-Crystalline Solids 356 (2010) 1611–1613
vacuum ∼ 10− 2 mbar. The least square curve fitting was used to estimate the errors in the present work.
Table 1 Value of average coordination number (Z), R-value, thickness (t), optical gap (Eg), tailing parameter (B1/2), and reflectivity maximum (Rmax) for amorphous SnxSb20Se80 − x films.
3. Results
x (at.%)
Z
R-value
t (µm)
Eg (±0.02 eV)
B1/2 (cm− 1/2 eV− 1/2)
Rmax (nm)
From the transmittance and reflectance of the films, absorption coefficient (α) of the films has been calculated by using the relation [9]:
2 4 6 8
2.24 2.28 2.32 2.36
2.29 2.00 1.76 1.56
2.34 2.60 3.05 2.80
1.57 1.48 1.38 1.36
5561.2 5120.9 4791.9 5175.1
416 419 485 522
α=
( )1 = 2 ) ( 2 2 1 ð1−R Þ ð1−R Þ 2 ln + + R t ð2TÞ ð2TÞ2
where ‘t’ is the thickness of the films, and ‘T’ and ‘R’ represents the transmittance and reflectance respectively. The absorption coefficient so obtained has been found to lie between 104 and 105 cm− 1 near the fundamental absorption region. In this region, the absorption coefficient is given by αhv = B(hv − Eg)2; where Eg is the optical gap and B1/2 is the slope in the extended region or measure of the extent of the band tailing for a given material. Fig. 1 shows the Tauc plots of (αhv)1/2 versus hv for all the films are drawn and the values of the optical gap (Eg) and slope (B1/2) are determined. Both the parameters along with the average coordination number (Z) and R-value [5] for different compositions are summarized in Table 1. The value of the optical gap decreases with an inverse relation for the tailing parameter as the Sn content increases. The value of optical gap for amorphous Sb20Se80 film is found to be consistent with that reported by Fouad et al. (1.57 eV) [3]. Similar effect in the optical gap has also been observed with the Sn substitution for Ge in amorphous Ge–Se system [10]. The inset of Fig. 1 shows the reflectivity spectrum and exhibits a red shift in the reflectivity maximum with the increase in Sn content. The experimental far-IR reflection spectra of the as-prepared films are presented in the Fig. 2. The broad reflectivity bands were analyzed by de-convoluting those located between 230 and 120 cm− 1 giving the vibrational bands at 160 and 178 cm− 1 which can be assigned for stretching modes for Sn-tetrahedral units (149–172 cm− 1) [11], Sbpyramidal unit (172–192 cm− 1) [12] and homopolar Sb–Sb bonds (170 cm− 1) [13] respectively. On the other hand, the de-convolution of band (560–230 cm− 1) gives 290, 388 cm− 1 and others, which can be assigned for SbSe3 units [12] and SeO2 (v2) structural units [14]
Fig. 1. Tauc plots between (αhv)1/2 and hv and the inset picture shows reflectivity spectra for thermally evaporated SnxSb20Se80 − x films.
generally due to the surface adsorbed oxygen present in 1–2 atomic layers for x = 2 films. The spectral domain 50–110 cm− 1 is characterized by weak bands for Sen chains (90 cm− 1), Sb(Se1/2)3 pyramids (78 cm− 1) bending modes of Sn(Se1/2)4 tetrahedra (102–123 cm− 1), respectively. No major change in the shape and position of the characteristic vibration bands with the increase in Sn content has been observed. It is now interesting to study the effect of composition on the position of different optical mode frequencies i.e. transverse optic (TO) and longitudinal optic (LO) from the reflectivity spectrum. In the TO modes, the atomic displacements are perpendicular to the direction of the periodicity of the elastic wave, whereas in the LO the displacements are parallel to the wave vector (defined for the crystalline materials, not defined for glasses) [15]. From the far-IR reflectivity spectra, the values of n (refractive index) and k (extinction coefficient) have been derived by Kramers–Kronig transformations [11,15,16]. The real part of the complex dielectric constant, ε1 and imaginary part, ε2 are computed and the peaks in the ε2 and ε2 / (ε21 + ε22) correspond to the TO and LO split frequencies respectively. Fig. 3 shows such a plot for all the samples. The splitting at the lower frequencies is for the SbSe3 vibrational modes. Position of both TO/LO mode frequencies does not change appreciably, while shape of bands for ε2 with frequency broadens with the increase in composition. 4. Discussion The experimental results can be discussed by using the chemically ordered network model (CONM) by considering the chemical bond energies involved. The bonds i.e. Sn–Se, Sb–Se and Se–Se corresponding to different bond energies 401.2, 346.2, 332.6 kJ/mol respectively are supposed to be present in this system. Even though, there is an increase in the average bond energy, the decrease in the optical gap can be due to the increase in the localized state density (as evident from the increase in tailing parameter) within the band gap [3]. The broad spectral dependence and red shift in the reflectivity spectrum with the Sn
Fig. 2. Far-infrared reflection spectra for amorphous SnxSb20Se80 − x films.
LETTER TO THE EDITOR P. Kumar et al. / Journal of Non-Crystalline Solids 356 (2010) 1611–1613
1613
the enhancement in the partial ionic character of the network with Sn content (Fig. 3). The broadening in the TO mode frequency to the higher wavenumber side can be due to the superposition and relative increase in concentration of Sn-tetrahedra as compared to Sbpyramidal units in the network. Therefore, the mixing of different molecular clusters or structural changes plays an important role for the glass formation in Sn–Sb–Se system. Thus, the broadening of band (TO mode) can be taken as a change in the structural ordering at the local level or formation of glassy samples for x = 8 sample of this ternary system [3]. These changes can be interpreted as a change in the intermediate range ordering of the network structure leading to the formation of glassy samples as well as reins the majority of the electronic and optical phenomena in this semiconducting system. 5. Conclusions
Fig. 3. Dependence of ε2 and ε2 / (ε21 + ε22) with wave number for SnxSb20Se80 − x films.
concentration has been observed (Fig. 1). The peaks in the reflection spectra for the crystalline semiconductors in the interband transition region correspond to the highest symmetry points for the Brillion zone of the crystal [17]. The absence of long range order in amorphous materials leads to exhibit the characteristic broad spectrum and the peak reflectivity shift can be attributed to the weakening of the intermolecular interactions or the oscillator strengths in the interband transition region. The spectra of these glasses could be described as a result of superposition of the spectra of their structural components. The vibrational spectra for Sn–Sb–Se system has been discussed by taking the formation of two basic structural units, SbSe3 pyramids with threefold coordinated Sb atom at the apex and SnSe4 tetrahedrons with Sn in the center. The vibrational bands reported in the literature for their crystalline accompaniment were taken as reference for the discussion of the spectrum [11–13,18]. Corredor et al. have theoretically calculated the vibrational bands for SbSe3 pyramidal units as 112, 315, 215 and 78 cm− 1 respectively for the Sb–As–Se glasses [12]. Similarly, Smith et al. have reported the Raman mode frequencies 107.9, 187.0 cm− 1 and IR active mode frequencies as 145.6, 239.8 cm− 1 for the SnSe2 crystals respectively [11,18]. There is a relative increase in the area of first band (160 cm− 1) as compared to the second one (178 cm− 1) with an access of Se-content (R-value greater than one) for all the compositions, therefore diminishing the presence of the vibrations of homopolar Sb–Sb bonds at 170 cm− 1 [13]. Thus, the vibrational band (160 cm− 1) could be the combined effect of the stretching modes of Sb-pyramidal units and/or Sn-tetrahedra while the band (178 cm− 1) has been assigned to the stretching modes of the Sb-pyramidal units [12]. Strong nearest-neighbor forces serve as mechanical constraints in stabilizing a glass network [19], while weak Coulomb forces promote only space filling of the network glasses. The shift in the transverse optic mode frequency (TO) to lower wavenumber side leads to increase the difference between LO/TO mode frequencies revealing
The composition dependence of the optical gap and tailing parameter has been studied for thermally evaporated SnxSb20Se80 − x (2 ≤ x ≤ 8) films. The optical gap decreases while the slope or tailing parameter decreases except for x = 8 compositions where it increases. The far-infrared reflectivity spectrum indicates the presence of vibrational modes due to SbSe3, SnSe2 structural units and the surface adsorbed oxide bands for all the compositions. The splitting of the different optical modes (LO/TO) indicates partial ionic character for this system. Acknowledgments The authors are thankful to Prof. V. Venkataraman (Co-ordinator) and Dr. R. Ganesan (Scientific Officer) at the DST-National Facility for Low Temperatures and High Magnetic Fields, Department of Physics, Indian Institute of Science, Bangalore for the accessing the FTIR facility. One of the authors (PK) is thankful for the financial assistance from the CSIR, New Delhi, India. References [1] A. Zakery, S.R. Elliott, J. Non-Cryst. Sol. 330 (2003) 1. [2] P. Boolchand, Insulating and Semiconducting Glasses, World Scientific, Singapore, 2000. [3] S.S. Fouad, A.H. Ammar, M. Abo-Ghazala, Phys. B: Condensed Matter 229 (1997) 249–255. [4] M.S. Kamboj, G. Kaur, R. Thangaraj, D.K. Awasthi, J. Phys. D: App. Phys. 35 (2002) 477–479. [5] P. Kumar, R. Thangaraj, J. Non-Cryst. Solids 252 (2006) 2288–2291. [6] M.M. Wakkad, E.Kh. Shokr, H.A. Abd El Ghani, M.A. Awad, Eur. Phys. J. Appl. Phys. 43 (2008) 23–30. [7] P. Kumar, J. Kumar, R. Thangaraj, Eur. Phys. J. Appl. Phys. 32 (2007) 1–5. [8] M. Abkowitz, Polym. Eng. Sci. 24 (1984) 1149. [9] G. Lucovsky, Phys. Rev. B 15 (1977) 5762. [10] M.H. Ali, S.A. Fayek, Phys. Stat. Solidi (a) 147 (2) (1995) 577–584. [11] J.Y. Harbec, S. Jandl, Phys. Rev. B 25 (10) (1982) 6126. [12] C. Corredor, I. Quiroga, J. Vazquez, J. Galdon, P. Villares, R. Jimenez-Garay, Matt. Lett. 42 (2000) 229. [13] S.M. El-Sayed, Semicond. Sci. Tech. 18 (2003) 337. [14] R.J.M. Konigs, A.S. Booij, A. Kovacs, Chem. Phys. Lett. 292 (4–6) (1998) 447–453. [15] R.M. Almeida, Phys. Rev. B 45 (1) (1992) 161. [16] T.S. Moss, Optical Properties of Semiconductors, Academic Press, New York, 1959. [17] D.L. Greenaway, G. Harbeke, Optical Properties and Band Structure of Semiconductors, Pergamon, Oxford, 1970. [18] A.J. Smith, P.E. Meek, W.Y. Liang, J. Phys. C: Sol. Stat. Phys. 10 (1977) 1321. [19] M. Kastner, D. Adler, H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504.