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Laser-induced changes on optical band gap of amorphous and crystallized thin films of Se75S25−xAGx
ARTICLE IN PRESS Physica B 404 (2009) 4262–4266
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Laser-induced changes on optical band gap of amorphous and crystallized thin films of Se75S25xAGx A.A. Al-Ghamdi, Shamshad A. Khan ,1 Department of Physics, Faculty of Science, King Abdul Aziz University, Jeddah 21589, Saudi Arabia
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 January 2009 Received in revised form 31 July 2009 Accepted 3 August 2009
Optical band gap of amorphous, crystallized, laser induced amorphous and laser induced crystallized films of Se75S25xAgx (x ¼ 4, 6 and 8) glassy alloys was studied from absorption spectra. The amorphous and crystallized films were induced by pulse laser for 10 min. After laser irradiation on amorphous and crystalline films, optical band gap was measured. It has been found that the mechanism of the optical absorption follows the rule of indirect transition. The amorphous thin films show an increase in the optical band gap, while the crystallized (thermally annealed) thin films show a decrease in the optical band gap by inducing laser irradiation. Crystallization and amorphization of chalcogenide films were accompanied with the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. These results are interpreted in terms of concentration of localized states due to shift in Fermi level. & 2009 Elsevier B.V. All rights reserved.
Keywords: Crystallization Optical band gap Thin films Chalcogenides Laser irradiation
1. Introduction Chalcogenide glasses are one of the most widely known families of amorphous materials. Thin films of chalcogenide glasses have been extensively studied because of their interesting fundamental properties and their potential applications in optical imaging, optical recording, integrated optics, microelectronics, optical communications and nanotechnology [1–6]. Several photo-induced and laser-induced phenomena are observed in amorphous chalcogenide thin films. These changes are accompanied with the change in the optical constants, i.e., change in the optical band gap, refractive index and optical absorption coefficient. Annealing and laser irradiation can affect the photo-induced changes, in particular irreversible effects occur in amorphous thin films, while reversible effects occur in crystallized thin films. Laser irradiation to chalcogenide glasses has been regarded as a process for spatially selected structural modification and/or crystallization in glasses. Laser induced changes in amorphous chalcogenide are an object of systematic investigation with a view to better understanding the mechanism of the phenomena taking place in them as well as their practical applications. In the production of flat panel displays, laser crystallization increases the carrier mobility in thin film transistors. Suitable laser intensity profiles
Corresponding author. Tel./fax: +966 26952286.
E-mail address: [email protected] (S.A. Khan). Permanent address: Department of Physics, St. Andrew’s P.G. College, Gorakhpur, UP 273001, India. 1
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.009
in combination with multi-pulse scanning sequence have been used to reduce the number of grain boundaries. Development of information technology demands new optical recording materials and, therefore, good knowledge of their linear optical properties is of great interest. The determination of the optical band gap during the phase transformation is of great importance for understanding the mechanism of the optical processing and for their application in practice. A lot of research work [7–11] is going on for the effect of laser irradiation, annealing, ultraviolet irradiation, g-irradiation, etc. on optical and electrical properties of amorphous thin films. Takahashi et al. [12] have studied increase in the fluorescence intensity of ZnO nanoparticle by laser irradiation. Huajun et al. [13] have studied the structural change of laserirradiated Ge2Sb2Te5 films studied by electrical property measurement. The work on femtosecond laser-induced microfeatures in glasses and their applications by Qiu et al. [14], writing of crystal line patterns in glass by laser irradiation by Honma et al. [15] are also worth mentioning. The aim of the present research work is to study the effect of laser irradiation on optical band gap of amorphous and crystallized thin films of Se75S25xAgx (x ¼ 2, 4 and 6) chalcogenide. Selenium has been selected because of its wide commercial applications. Its device applications like switching, memory and xerography, etc. made it attractive. It also exhibits a unique property of reversible transformation. This property is very useful in optical memory devices. Here we have chosen sulfur as an additive material with selenium. In the present research work, we have incorporated silver in the Se–S system. The addition of third element will create compositional and configurational disorder in
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
2000 Se75S21Ag4
Heat Flow (mW)
the material with respect to the binary alloys. Silver doped chalcogenide glasses have become attractive chalcogenide materials for fundamental research of structure properties and preparation. They have much current and potential application in optics and optoelectronics such as photo-doping, optical imaging, photo lithography and phase change optical recording. Chalcogenide glasses containing silver, generally, exhibit single glass transition and single crystallization temperatures, which is an important condition for rewritable disks. Thin films of chalcogenide glasses containing Ag have found application in erasable PC optical recording [16,17].
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Temperature (°C) 3000 Se75S17Ag8
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Fig. 1. DSC plots for of Se75S21Ag4, Se75S19Ag6 and Se75S17Ag8 glasses at heating rate of 15 K/min.
Se75S18Ag6
Relative Intensity
Se75S25xAgx bulk glasses with x ¼ 2, 4 and 6 were prepared by melt quenching technique. High purity materials (99.999%) were weighted according to their atomic percentage and sealed in quartz ampoules under a vacuum of 105 Torr. The sealed ampoules were kept inside a microprocessor-controlled programmable muffle furnace, where the temperature was raised to 1000 1C at the rate of 3 K/min and kept at temperature for 12 h with frequent rocking to ensure the homogenization of the melt. After that the quenching was done in ice water to obtain the amorphous state. Quenching samples were removed from the ampoules by breaking the quartz ampoules. Differential scanning calorimeter (Model DSC Plus, Reheometric Scientific Company, UK) was used for measuring the glass transition and crystallization temperatures of bulk samples of Se75S21Ag4, Se75S19Ag6 and Se75S17Ag8. DSC scan using non-isothermal measurements was obtained by heating 5 mg of the powdered sample sealed in an aluminum pans at constant heating rate of 15 K/min, shown in Fig. 1. The annealing temperature was taken in between the glass transition and crystallization temperature of the prepared samples. Thin films of thickness 3000 A˚ were prepared by using an Edward Coating Unit E-306, onto glass substrate at room temperature on a base pressure of 106 Torr using molybdenum boat. The substrates were thoroughly cleaned in a detergent solution and then in a chromic acid and finally, cleaned using trichloroethylene. Double distilled water was used throughout in different stages of cleaning. To avoid the fractionation of the alloy during evaporation and, thereby, to ensure the correct average composition of the films formed, a high deposition rate was used to prepare the studied films. The thickness of the films was measured by using a quartz crystal monitor (Edward model FTM 7). The earthed face of the crystal monitor was facing the source and was placed at the same height as the substrate. The evaporation was controlled by using the same FTM 7 quartz crystal monitor. Thin film of glassy alloys were crystallized by thermal annealing at 373 K temperature, which is in between the glass transition and crystallization temperature of the samples for 2 h in a vacuum furnace under a vacuum of 103 Torr. The X-ray diffraction patterns give valuable information about the nature and structure of the samples. A Philips Model PW 1710 X-ray diffractometer was employed for studying the structure of amorphous and thermally annealed films. The copper target was used as a source of X-rays with l ¼ 1.5404 A˚ (Cu Ka1). The scanning angle was in the range of 20–801. A scan speed of 21/min and a chart speed of 1 cm/min were maintained. The X-ray diffraction traces of all samples were taken at room temperature and found to show almost similar trends and hence only one of them is shown in Fig. 2. The absence of sharp structural peaks in as-prepared films confirm the amorphous nature and the presence of sharp structural peaks in annealed films confirm the crystalline nature of the films. The amorphous and crystallized thin films were induced by pulsed TEA N2 laser (wavelength
80
20
30
40
50
60
70
80
Angle (2θ) (Degree) Fig. 2. X-ray diffractrograms of the amorphous and crystallized films of Se75S19Ag6.
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
337.1 nm, power 100 kW, pulse width 1 ns) for 10 min. A JASCO, V-500, UV/VIS/NIR computerized spectrophotometer is used for measuring optical absorption of thin films. The optical absorption was measured as a function of wavelength of the incidence photon energy.
3. Results and discussion The optical absorption of amorphous, crystallized, laser induced amorphous and laser induced crystallized films of Se75S25xAgx glasses was measured in the wavelength range 300–1000 nm as a function of incidence photon energy. Infact the ‘‘absorbance’’ reading (i.e. photometric value) is a measure of the amount of light absorbed by the sample under specified conditions. The Beer–Lambert law is the basis of the quantitative of UV/visible spectroscopy. In UV/visible spectroscopy, a spectrophotometer passes a double beam of light through a sample with a fixed path length. The spectrophotometer then monitors the absorption of light in terms of optical density at the particular thickness of the sample. The absorption has been measured in terms of optical density. Here we have kept the samples (films) and reference (glass substrate) in the chamber at the appropriate sample holder. The absorption coefficient (a) has been obtained directly from the absorbance against wavelength curve using the relation [18,19]
ðahvÞ1=2 ¼ Bðhv Eg Þ
Se75S19Ag6 Se25S17Ag8
300
ð3Þ
200
100
0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 3. (ahn)1/2 against photon energy in Se75S25xAgx: amorphous films.
500
Se75S21Ag4
400
ð2Þ
where n is the frequency of the incident beam (o ¼ 2pn), B is a constant, Eg is optical band gap and n is an exponent, which can be assumed to have values of 1/2, 3/2, 2 and 3 depending on the nature of electronic transition responsible for the absorption: n ¼ 1/2 for allowed direct transition, n ¼ 3/2 for forbidden direct transition, n ¼ 2 for allowed indirect transition and n ¼ 3 for forbidden indirect transition. The best fit of the experimental result of Se75S25xAgx thin films using Eq. (2), with n ¼ 2, i.e. variation curve of (ahn)1/2 with photon energy (hn), is found to be identical to that of the elemental amorphous semiconductor [22,23]. This indicates that the absorption in Se75S25xAgx thin films is due to non-direct transition and the relation between the optical gap, optical absorption coefficient (a) and the energy (hn) of the incident photon is given by
Se75S21Ag4
400
ð1Þ
where OD is the optical density measured for given layer thickness (t). In the absorption process, a photon of known energy excites an electron from a lower to a higher energy state, corresponding to an absorption edge. In chalcogenide glasses, a typical absorption edge can be broadly ascribed to one of three processes, firstly residual below-gap absorption; secondly Urbach tails and thirdly interband absorption. Chalcogenide glasses have been found to exhibit highly reproducible optical edges which are relatively insensitive to preparation conditions and only the observable absorption [20] with a gap under equilibrium conditions account for the first process. In the second process the absorption edge depends exponentially on the photon energy according to the Urbach relation [21]. In amorphous materials, a increases exponentially with the photon energy near the energy gap. The variation of a with photon energy can be explained in term of: (i) fundamental absorption, (ii) exciton absorption and (iii) valence band acceptor absorption. The fundamental absorption edge in most amorphous semiconductors follows an exponential law. Above the exponential tail, the absorption coefficient has been reported to obey the following equation: ðahvÞ1=n ¼ Bðhv Eg Þ
500
(αh)1/2
a ¼ OD=t
The variation of (ahn)1/2 with photon energy (hn) for amorphous, crystallized films, laser induced amorphous films and laser induced crystallized films of Se75S25xAgx is shown in Figs. 3, 4, 5 and 6, respectively. The value of indirect optical band gap (Eg) has been calculated by taking the intercept on the x-axis. The calculated values of Eg for all four cases (amorphous, crystallized, laser induced amorphous and laser induced crystallized films) are given in Table 1. It is evident from this table that the value of optical band gap increases when the film crystallized. The increase in the optical band gap may be due to the increase in grain size, the reduction in the disorder and decrease in density of defect states grain size, the reduction in the disorder and decrease in density of defect states (which results in the reduction of tailing of bands) [24,25]. The increase in the optical band gap could also be discussed on the basis of density of state model proposed by Mott and Davis [24]. Chalcogenide thin films always contain a high concentration of unsaturated bonds or defects. These defects are responsible for the presence of localized states in the amorphous band gap. During thermal annealing at a temperature below the crystallization temperature, the unsaturated defects are
Se75S19Ag6 Se25S17Ag8
(αh)1/2
4264
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200
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0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 4. (ahn)1/2 against photon energy in Se75S25xAgx: crystallized films by thermal annealing.
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
gradually annealed out producing a large number of saturated bonds. The reduction in the number of unsaturated defects decreases the density of localized states in the band structure consequently increasing the optical band gap. The increase of optical band gap is due to the reduction in disorder in the atomic bonding between neighbors and thus a decrease of the density of tail states adjacent to the band edge was attributed to the amorphous–crystalline phase transformations. During thermal annealing of amorphous solids at temperature in between the
500
400
Se75S21Ag4
(αh)1/2
Se75S19Ag6
300
Se25S17Ag8
200
100
0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 5. (ahn)1/2 against photon energy in Se75S25xAgx: after laser irradiation on amorphous films.
500
Se75S19Ag6
4. Conclusion
Se25S17Ag8
(αh)1/2
glass transition and crystallization temperature, enough vibrational energy is present to break some of the weaker bonds, thus introducing some translational degree of freedom to the system. Consequently, crystallization via nucleation and growth becomes possible. After laser irradiation on amorphous thin films of Se75S25xAgx the optical band gap also increases, which shows that after laser irradiation, the amorphous films were crystallized. After laser irradiation on crystallized films of Se75S25xAgx the optical band gap decreases. The decrease in the optical band gap will lead to a shift in absorption edge towards lower photon energy and consequently decrease in the optical energy gap, which can be explained by the increased tailing of the conduction band edge into the gap due to the laser irradiation. Since the optical absorption also depends on short-range order in the amorphous states and defects associated with it, the decrease in optical band gap may also be explained on the basis of ‘‘density of state model’’ proposed by Mott and Davis [24]. According to this model, the width of the localized states near the mobility edge depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as the result of an insufficient number of atoms deposited in the amorphous film [26]. The unsaturated bonds are responsible for the formation of some defects in the films, producing localized states in the amorphous solids. The presence of high concentration of localized states in the band structure is responsible for the low values of optical band gap in the case of the amorphous films. These low values may also be due to the shift in Fermi level, whose position is determined by the distribution of electrons over the localized states [27], which shows that after laser irradiation, the crystalline films become amorphized. Crystallization and amorphization of chalcogenide films are accompanied by the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase.
Se75S21Ag4
400
300
200
100
0
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1
1.4
1.8
2.2 2.6 Energy (hν)
3
3.4
3.8
Fig. 6. (ahn)1/2 against photon energy in Se75S25xAgx: after laser irradiation on crystallized films.
The critical study on the optical band gap of Se75S25xAgx thin films indicated that the absorption mechanism is due to indirect transition. Crystallization and amorphization of chalcogenide films are accompanied by a change in the optical band gap. The effect of laser irradiation is interpreted on the basis of amorphous–crystalline and crystalline–amorphous transformation. The optical band gap increases during crystallization of the thin films. After laser irradiation on amorphous films the optical band gap increases, which shows that the amorphous films became crystallized. After laser irradiation on crystallized films, the optical band gap decreases, which shows that the crystalline films became amorphized. The decrease of optical band gap after laser irradiation can be understood in terms of the generation of excess electronic delocalized states. The increased and decreased value of optical band gap is explained on the basis of the Mott and
Table 1 Optical band gap in Se75S25xAgx films in different cases: amorphous film, crystallized films, laser induced amorphous films and laser induced crystallized films. Sample
Se75S21Ag4 Se75S19Ag6 Se75S17Ag8 a
Ref. [9].
Optical band gap (Eg) (eV) Amorphous filmsa
Crystallized films
After laser irradiation on amorphous films
After laser irradiation on crystalline films
1.60 1.78 1.89
1.99 2.14 2.32
1.81 1.92 2.15
1.69 1.83 2.04
ARTICLE IN PRESS 4266
A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
Davis model for the density of states in amorphous materials. This change in the optical band gap may be due to the increase in the grain size and the reduction in the disorder of the system.
Acknowledgments The authors are thankful to Prof. M. Husain and Prof. M. Zulfequar, Department of Physics, Jamia Millia Islamia, New Delhi, India, for allowing the laser experiment in their laboratory and their useful discussion and valuable suggestions. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
K.A. Aly, H.H. Amer, A. Dahshan, Mater. Chem. Phys. 113 (2009) 690. M. Abdel Rafea, H. Farid, Mater. Chem. Phys. 113 (2009) 868. A.A. Alnajjar, Renewable Energy 34 (2009) 71. A.M. Salem, S.Y. Marzouk, S.H. Moustafa, M.S. Selim, Nuclear Instr. Methods Phys. Res. B 262 (2007) 225. A. Dahshan, K.A. Aly, Acta Materialia 56 (2008) 4869. I. Sharma, S.K. Tripathi, P.B. Barman, Appl. Surf. Sci. 255 (2008) 2791. K.A. Aly, M.A. Osman, A.M. Abousehly, A.A. Othman, J. Phys. Chem. Solids 69 (2008) 2514. M.R. Balboul, Radiation Measurements 43 (2008) 1360. S.A. Khan, F.S. Al-Hazmi, A.M. Al-Sanosi, A.S. Faidah, S.J. Yaghmour, A.A. AlGhamdi, Physica B 404 (2009) 1415.
[10] J. Xu, Y. Rui, D. Chen, J. Mei, L. Zhou, Z. Cen, W. Li, K. Chen, Mater. Lett. 61 (2007) 5010. [11] S.A. Khan, M. Zulfequar, M. Husain, Physica B 324 (2002) 336. [12] S. Takahashi, H. Wada, A. Noguchi, O. Odawara, Mater. Lett. 62 (2008) 3407. [13] S. Huajun, H. Lisong, W. Yiqun, W. Jingsong, J. Non-Cryst. Solids 354 (2008) 5563. [14] J. Qiu, K. Miura, K. Hirao, J. Non-Cryst. Solids 354 (2008) 1100. [15] T. Honma, R. Ihara, Y. Benino, R. Sato, T. Fujiwara, T. Komatsu, J. Non-Cryst. Solids 354 (2008) 468. [16] R.M. Almeida, L.F. Santos, A. Ganjoo, H. Jain, J. Non-Cryst. Solids 353 (2007) 2066. [17] A.E. Bekheet, N.A. Hegab, M.A. Afifi, H.E. Atyia, E.R. Sharaf, Appl. Surf. Sci. 255 (2009) 4590. [18] D.P. Gosain, T. Shimizu, M. Ohmura, M. Suzuki, T. Bando, S. Okano, J. Mater. Sci. 26 (1991) 3271. [19] E.K. Shokr, M.M. Wakkad, J. Non-Crys, J. Non-Cryst. Solids 23 (1992) 1197. [20] J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum Press, New York, 1979, p. 159. [21] F. Urbach, Phys. Rev. 92 (1953) 1324. [22] M. Ilyas, M. Zulfequar, M. Husain, J. Modern Opt. 47 (2000) 663. [23] A.S. Maan, D.R. Goyal, T.P. Sharma, J. Physique III 4 (1994) 493. [24] N.F. Mott, E.A. Davis, Electronics Processes in Non-Crystalline Materials, vol. 428, Clarendon, Oxford, 1979. [25] K. Oe, Y. Toyoshiman, J. Non-Cryst. Solids 58 (1973) 304. [26] T.T. Nang, M. Okuda, T. Matsushita, S. Yokota, A. Suzuki, Japan. J. Appl. Phys. 14 (1976) 849. [27] M.L. Theye, in: Proceedings of Fifth International Conference on Amorphous and Liquid Semiconductors, vol. 1, 1973, p. 479.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Laser-induced changes on optical band gap of amorphous and crystallized thin films of Se75S25xAGx A.A. Al-Ghamdi, Shamshad A. Khan ,1 Department of Physics, Faculty of Science, King Abdul Aziz University, Jeddah 21589, Saudi Arabia
a r t i c l e in f o
a b s t r a c t
Article history: Received 30 January 2009 Received in revised form 31 July 2009 Accepted 3 August 2009
Optical band gap of amorphous, crystallized, laser induced amorphous and laser induced crystallized films of Se75S25xAgx (x ¼ 4, 6 and 8) glassy alloys was studied from absorption spectra. The amorphous and crystallized films were induced by pulse laser for 10 min. After laser irradiation on amorphous and crystalline films, optical band gap was measured. It has been found that the mechanism of the optical absorption follows the rule of indirect transition. The amorphous thin films show an increase in the optical band gap, while the crystallized (thermally annealed) thin films show a decrease in the optical band gap by inducing laser irradiation. Crystallization and amorphization of chalcogenide films were accompanied with the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. These results are interpreted in terms of concentration of localized states due to shift in Fermi level. & 2009 Elsevier B.V. All rights reserved.
Keywords: Crystallization Optical band gap Thin films Chalcogenides Laser irradiation
1. Introduction Chalcogenide glasses are one of the most widely known families of amorphous materials. Thin films of chalcogenide glasses have been extensively studied because of their interesting fundamental properties and their potential applications in optical imaging, optical recording, integrated optics, microelectronics, optical communications and nanotechnology [1–6]. Several photo-induced and laser-induced phenomena are observed in amorphous chalcogenide thin films. These changes are accompanied with the change in the optical constants, i.e., change in the optical band gap, refractive index and optical absorption coefficient. Annealing and laser irradiation can affect the photo-induced changes, in particular irreversible effects occur in amorphous thin films, while reversible effects occur in crystallized thin films. Laser irradiation to chalcogenide glasses has been regarded as a process for spatially selected structural modification and/or crystallization in glasses. Laser induced changes in amorphous chalcogenide are an object of systematic investigation with a view to better understanding the mechanism of the phenomena taking place in them as well as their practical applications. In the production of flat panel displays, laser crystallization increases the carrier mobility in thin film transistors. Suitable laser intensity profiles
Corresponding author. Tel./fax: +966 26952286.
E-mail address: [email protected] (S.A. Khan). Permanent address: Department of Physics, St. Andrew’s P.G. College, Gorakhpur, UP 273001, India. 1
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.009
in combination with multi-pulse scanning sequence have been used to reduce the number of grain boundaries. Development of information technology demands new optical recording materials and, therefore, good knowledge of their linear optical properties is of great interest. The determination of the optical band gap during the phase transformation is of great importance for understanding the mechanism of the optical processing and for their application in practice. A lot of research work [7–11] is going on for the effect of laser irradiation, annealing, ultraviolet irradiation, g-irradiation, etc. on optical and electrical properties of amorphous thin films. Takahashi et al. [12] have studied increase in the fluorescence intensity of ZnO nanoparticle by laser irradiation. Huajun et al. [13] have studied the structural change of laserirradiated Ge2Sb2Te5 films studied by electrical property measurement. The work on femtosecond laser-induced microfeatures in glasses and their applications by Qiu et al. [14], writing of crystal line patterns in glass by laser irradiation by Honma et al. [15] are also worth mentioning. The aim of the present research work is to study the effect of laser irradiation on optical band gap of amorphous and crystallized thin films of Se75S25xAgx (x ¼ 2, 4 and 6) chalcogenide. Selenium has been selected because of its wide commercial applications. Its device applications like switching, memory and xerography, etc. made it attractive. It also exhibits a unique property of reversible transformation. This property is very useful in optical memory devices. Here we have chosen sulfur as an additive material with selenium. In the present research work, we have incorporated silver in the Se–S system. The addition of third element will create compositional and configurational disorder in
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
2000 Se75S21Ag4
Heat Flow (mW)
the material with respect to the binary alloys. Silver doped chalcogenide glasses have become attractive chalcogenide materials for fundamental research of structure properties and preparation. They have much current and potential application in optics and optoelectronics such as photo-doping, optical imaging, photo lithography and phase change optical recording. Chalcogenide glasses containing silver, generally, exhibit single glass transition and single crystallization temperatures, which is an important condition for rewritable disks. Thin films of chalcogenide glasses containing Ag have found application in erasable PC optical recording [16,17].
4263
0
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-4000
-6000 40
2. Experimental
120
160
200
240
200
240
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Heat Flow (mW)
Se75S18Ag6
0 -3000 -6000 -9000 80
40
120
160
Temperature (°C) 3000 Se75S17Ag8
Heat Flow (mW)
0
-3000
-6000
-9000 40
90
140 Temperature (°C)
190
240
Fig. 1. DSC plots for of Se75S21Ag4, Se75S19Ag6 and Se75S17Ag8 glasses at heating rate of 15 K/min.
Se75S18Ag6
Relative Intensity
Se75S25xAgx bulk glasses with x ¼ 2, 4 and 6 were prepared by melt quenching technique. High purity materials (99.999%) were weighted according to their atomic percentage and sealed in quartz ampoules under a vacuum of 105 Torr. The sealed ampoules were kept inside a microprocessor-controlled programmable muffle furnace, where the temperature was raised to 1000 1C at the rate of 3 K/min and kept at temperature for 12 h with frequent rocking to ensure the homogenization of the melt. After that the quenching was done in ice water to obtain the amorphous state. Quenching samples were removed from the ampoules by breaking the quartz ampoules. Differential scanning calorimeter (Model DSC Plus, Reheometric Scientific Company, UK) was used for measuring the glass transition and crystallization temperatures of bulk samples of Se75S21Ag4, Se75S19Ag6 and Se75S17Ag8. DSC scan using non-isothermal measurements was obtained by heating 5 mg of the powdered sample sealed in an aluminum pans at constant heating rate of 15 K/min, shown in Fig. 1. The annealing temperature was taken in between the glass transition and crystallization temperature of the prepared samples. Thin films of thickness 3000 A˚ were prepared by using an Edward Coating Unit E-306, onto glass substrate at room temperature on a base pressure of 106 Torr using molybdenum boat. The substrates were thoroughly cleaned in a detergent solution and then in a chromic acid and finally, cleaned using trichloroethylene. Double distilled water was used throughout in different stages of cleaning. To avoid the fractionation of the alloy during evaporation and, thereby, to ensure the correct average composition of the films formed, a high deposition rate was used to prepare the studied films. The thickness of the films was measured by using a quartz crystal monitor (Edward model FTM 7). The earthed face of the crystal monitor was facing the source and was placed at the same height as the substrate. The evaporation was controlled by using the same FTM 7 quartz crystal monitor. Thin film of glassy alloys were crystallized by thermal annealing at 373 K temperature, which is in between the glass transition and crystallization temperature of the samples for 2 h in a vacuum furnace under a vacuum of 103 Torr. The X-ray diffraction patterns give valuable information about the nature and structure of the samples. A Philips Model PW 1710 X-ray diffractometer was employed for studying the structure of amorphous and thermally annealed films. The copper target was used as a source of X-rays with l ¼ 1.5404 A˚ (Cu Ka1). The scanning angle was in the range of 20–801. A scan speed of 21/min and a chart speed of 1 cm/min were maintained. The X-ray diffraction traces of all samples were taken at room temperature and found to show almost similar trends and hence only one of them is shown in Fig. 2. The absence of sharp structural peaks in as-prepared films confirm the amorphous nature and the presence of sharp structural peaks in annealed films confirm the crystalline nature of the films. The amorphous and crystallized thin films were induced by pulsed TEA N2 laser (wavelength
80
20
30
40
50
60
70
80
Angle (2θ) (Degree) Fig. 2. X-ray diffractrograms of the amorphous and crystallized films of Se75S19Ag6.
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
337.1 nm, power 100 kW, pulse width 1 ns) for 10 min. A JASCO, V-500, UV/VIS/NIR computerized spectrophotometer is used for measuring optical absorption of thin films. The optical absorption was measured as a function of wavelength of the incidence photon energy.
3. Results and discussion The optical absorption of amorphous, crystallized, laser induced amorphous and laser induced crystallized films of Se75S25xAgx glasses was measured in the wavelength range 300–1000 nm as a function of incidence photon energy. Infact the ‘‘absorbance’’ reading (i.e. photometric value) is a measure of the amount of light absorbed by the sample under specified conditions. The Beer–Lambert law is the basis of the quantitative of UV/visible spectroscopy. In UV/visible spectroscopy, a spectrophotometer passes a double beam of light through a sample with a fixed path length. The spectrophotometer then monitors the absorption of light in terms of optical density at the particular thickness of the sample. The absorption has been measured in terms of optical density. Here we have kept the samples (films) and reference (glass substrate) in the chamber at the appropriate sample holder. The absorption coefficient (a) has been obtained directly from the absorbance against wavelength curve using the relation [18,19]
ðahvÞ1=2 ¼ Bðhv Eg Þ
Se75S19Ag6 Se25S17Ag8
300
ð3Þ
200
100
0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 3. (ahn)1/2 against photon energy in Se75S25xAgx: amorphous films.
500
Se75S21Ag4
400
ð2Þ
where n is the frequency of the incident beam (o ¼ 2pn), B is a constant, Eg is optical band gap and n is an exponent, which can be assumed to have values of 1/2, 3/2, 2 and 3 depending on the nature of electronic transition responsible for the absorption: n ¼ 1/2 for allowed direct transition, n ¼ 3/2 for forbidden direct transition, n ¼ 2 for allowed indirect transition and n ¼ 3 for forbidden indirect transition. The best fit of the experimental result of Se75S25xAgx thin films using Eq. (2), with n ¼ 2, i.e. variation curve of (ahn)1/2 with photon energy (hn), is found to be identical to that of the elemental amorphous semiconductor [22,23]. This indicates that the absorption in Se75S25xAgx thin films is due to non-direct transition and the relation between the optical gap, optical absorption coefficient (a) and the energy (hn) of the incident photon is given by
Se75S21Ag4
400
ð1Þ
where OD is the optical density measured for given layer thickness (t). In the absorption process, a photon of known energy excites an electron from a lower to a higher energy state, corresponding to an absorption edge. In chalcogenide glasses, a typical absorption edge can be broadly ascribed to one of three processes, firstly residual below-gap absorption; secondly Urbach tails and thirdly interband absorption. Chalcogenide glasses have been found to exhibit highly reproducible optical edges which are relatively insensitive to preparation conditions and only the observable absorption [20] with a gap under equilibrium conditions account for the first process. In the second process the absorption edge depends exponentially on the photon energy according to the Urbach relation [21]. In amorphous materials, a increases exponentially with the photon energy near the energy gap. The variation of a with photon energy can be explained in term of: (i) fundamental absorption, (ii) exciton absorption and (iii) valence band acceptor absorption. The fundamental absorption edge in most amorphous semiconductors follows an exponential law. Above the exponential tail, the absorption coefficient has been reported to obey the following equation: ðahvÞ1=n ¼ Bðhv Eg Þ
500
(αh)1/2
a ¼ OD=t
The variation of (ahn)1/2 with photon energy (hn) for amorphous, crystallized films, laser induced amorphous films and laser induced crystallized films of Se75S25xAgx is shown in Figs. 3, 4, 5 and 6, respectively. The value of indirect optical band gap (Eg) has been calculated by taking the intercept on the x-axis. The calculated values of Eg for all four cases (amorphous, crystallized, laser induced amorphous and laser induced crystallized films) are given in Table 1. It is evident from this table that the value of optical band gap increases when the film crystallized. The increase in the optical band gap may be due to the increase in grain size, the reduction in the disorder and decrease in density of defect states grain size, the reduction in the disorder and decrease in density of defect states (which results in the reduction of tailing of bands) [24,25]. The increase in the optical band gap could also be discussed on the basis of density of state model proposed by Mott and Davis [24]. Chalcogenide thin films always contain a high concentration of unsaturated bonds or defects. These defects are responsible for the presence of localized states in the amorphous band gap. During thermal annealing at a temperature below the crystallization temperature, the unsaturated defects are
Se75S19Ag6 Se25S17Ag8
(αh)1/2
4264
300
200
100
0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 4. (ahn)1/2 against photon energy in Se75S25xAgx: crystallized films by thermal annealing.
ARTICLE IN PRESS A.A. Al-Ghamdi, S.A. Khan / Physica B 404 (2009) 4262–4266
gradually annealed out producing a large number of saturated bonds. The reduction in the number of unsaturated defects decreases the density of localized states in the band structure consequently increasing the optical band gap. The increase of optical band gap is due to the reduction in disorder in the atomic bonding between neighbors and thus a decrease of the density of tail states adjacent to the band edge was attributed to the amorphous–crystalline phase transformations. During thermal annealing of amorphous solids at temperature in between the
500
400
Se75S21Ag4
(αh)1/2
Se75S19Ag6
300
Se25S17Ag8
200
100
0 1
1.4
1.8
2.2
2.6
3
3.4
3.8
Energy (hν) Fig. 5. (ahn)1/2 against photon energy in Se75S25xAgx: after laser irradiation on amorphous films.
500
Se75S19Ag6
4. Conclusion
Se25S17Ag8
(αh)1/2
glass transition and crystallization temperature, enough vibrational energy is present to break some of the weaker bonds, thus introducing some translational degree of freedom to the system. Consequently, crystallization via nucleation and growth becomes possible. After laser irradiation on amorphous thin films of Se75S25xAgx the optical band gap also increases, which shows that after laser irradiation, the amorphous films were crystallized. After laser irradiation on crystallized films of Se75S25xAgx the optical band gap decreases. The decrease in the optical band gap will lead to a shift in absorption edge towards lower photon energy and consequently decrease in the optical energy gap, which can be explained by the increased tailing of the conduction band edge into the gap due to the laser irradiation. Since the optical absorption also depends on short-range order in the amorphous states and defects associated with it, the decrease in optical band gap may also be explained on the basis of ‘‘density of state model’’ proposed by Mott and Davis [24]. According to this model, the width of the localized states near the mobility edge depends on the degree of disorder and defects present in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as the result of an insufficient number of atoms deposited in the amorphous film [26]. The unsaturated bonds are responsible for the formation of some defects in the films, producing localized states in the amorphous solids. The presence of high concentration of localized states in the band structure is responsible for the low values of optical band gap in the case of the amorphous films. These low values may also be due to the shift in Fermi level, whose position is determined by the distribution of electrons over the localized states [27], which shows that after laser irradiation, the crystalline films become amorphized. Crystallization and amorphization of chalcogenide films are accompanied by the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase.
Se75S21Ag4
400
300
200
100
0
4265
1
1.4
1.8
2.2 2.6 Energy (hν)
3
3.4
3.8
Fig. 6. (ahn)1/2 against photon energy in Se75S25xAgx: after laser irradiation on crystallized films.
The critical study on the optical band gap of Se75S25xAgx thin films indicated that the absorption mechanism is due to indirect transition. Crystallization and amorphization of chalcogenide films are accompanied by a change in the optical band gap. The effect of laser irradiation is interpreted on the basis of amorphous–crystalline and crystalline–amorphous transformation. The optical band gap increases during crystallization of the thin films. After laser irradiation on amorphous films the optical band gap increases, which shows that the amorphous films became crystallized. After laser irradiation on crystallized films, the optical band gap decreases, which shows that the crystalline films became amorphized. The decrease of optical band gap after laser irradiation can be understood in terms of the generation of excess electronic delocalized states. The increased and decreased value of optical band gap is explained on the basis of the Mott and
Table 1 Optical band gap in Se75S25xAgx films in different cases: amorphous film, crystallized films, laser induced amorphous films and laser induced crystallized films. Sample
Se75S21Ag4 Se75S19Ag6 Se75S17Ag8 a
Ref. [9].
Optical band gap (Eg) (eV) Amorphous filmsa
Crystallized films
After laser irradiation on amorphous films
After laser irradiation on crystalline films
1.60 1.78 1.89
1.99 2.14 2.32
1.81 1.92 2.15
1.69 1.83 2.04
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Davis model for the density of states in amorphous materials. This change in the optical band gap may be due to the increase in the grain size and the reduction in the disorder of the system.
Acknowledgments The authors are thankful to Prof. M. Husain and Prof. M. Zulfequar, Department of Physics, Jamia Millia Islamia, New Delhi, India, for allowing the laser experiment in their laboratory and their useful discussion and valuable suggestions. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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