American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Laser Irradiation Effect on the Optical Properties of Se88Te10 Al2 Thin Films Islam Uddin* Department of Physics, Deanship of educational services, Qassim University (KSA) Email: islamftp@gmail.com Abstract Laser irradiation effect on optical properties of Se88Te10Al2 thin film has been studied. Thin films have been prepared by physical vapor deposition technique using thermal vapor evaporation unit. Transmission and Absorption spectra has been taken by double beam UV-VIS spectrophotometer. He-Ne laser has been used as irradiation source. Optical Analysis of thin films indicate indirect allowed transition and the optical band gap (Eg) decreases with increasing irradiation time. It is also observed that laser irradiation of 30 minutes time interval changes all optical constant harmonically. Such irradiation effect can be utilize in device applications for laser activated switches and memories. Keywords: Laser Irradiation; Chalcogenide Glasses; Thin films; Irradiation effect; optical properties. 1. Introduction. Recently, there has been an increasing interest in chalcogenide thin films due to their exceptional properties, which are remarkably different from the corresponding bulk materials [1-5]. Chalcogenide materials have wide range of technical importance in various solid-state devices. These materials have wide range of transparency in the far infrared region, making them interesting material for various applications. The interest in these materials is principally due to low phonon energy, extended infrared transparency, high refractive index, high photosensitivity, in reversible phase change optical recording etc [6-9]. Because of these properties, chalcogenide thin films are interesting candidates for applications in the integrated optics, thermoelectric and solar cell applications [10-12].Se-Te based alloys have created extreme interest due to their greater hardness, higher photosensitivity, higher crystallization temperature, and lower aging effects as in comparison to pure amorphous Se [13, 14]. ------------------------------------------------------------------------ * Corresponding author. 34 http://asrjetsjournal.org/ American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 As addition of Te into Se improves the corrosion resistance [15] therefore, Se-Te based alloys are thought to be promising media and used to extend the utility of a-Se. Selenium–Tellurium based semiconductors have been the focus of interest in thin film form because of their properties suitable for device applications [16, 17]. These materials are optically non-linear and sensitive to the laser irradiation [18-20]. In the latest decade, many efforts have been done on the research of laser irradiation effect [21–29] on optical materials. The lifetime of optical elements in high power systems mainly depend on laser-induced damage of optical materials [30]. Absorption and luminescence are two significant methods which have been used in the research of laser-matter interaction. The material absorbs energy from the laser pulse and produces an ionized region that gives rise to broadband emission [31].The absorption of laser irradiation in chalcogenide thin film depends strongly on their electronic structure which in turn changes by the interaction with photons. The most important applications of chalcogenide are now in the field of optics [32 -37] and arising mainly from their exhibited infrared transmitting properties [38, 39]. The energy of the light quanta is expected to induce qualitatively new changes. The aim of the present work is to synthesize Se88Te10 Al2 Thin Filmsand to investigate the laser irradiation effect on optical properties of these films. This includes laser irradiation effect on important optical parameters such as; Absorbance, Transmission, Reflectance, Absorption coefficient, optical band gap, extinction coefficient, refractive index, real and imaginary part of dielectric constant. 2. Material and Methods Alloy of Se88Te10Al2 has been prepared by melt quenching technique. Highly pure (99.999%) source materials with desired compositional ratio of elements has been sealed in a quartz ampoules under a vacuum of 10-5 torr. The sealed ampoules are kept inside a programmable furnace where the temperature is raised up to 1200 K at the rate of 5 K / minute for 10 hours with frequent rocking to ensure the homogenization of the melt. The quenching has been done in ice-cool water. As-quenched Alloy has been grounded and the resulting fine powder has been used to prepare the thin films by Physical vapour deposition method. Thin films of Se88Te10Al2 alloy of thickness 500 nm has been deposited on a well cleaned glass substrate in the shape of squares at room temperature and in a vacuum of ~10-5 torr. Films have been kept inside the deposition chamber under vacuum for 24 h to achieve metastable equilibrium. The thickness of the film has been measured under a single-crystal thickness monitor. He-Ne laser with wavelength 632.8 nm has been used as irradiation source. The thin films have been irradiated for four different time duration of 30, 60, 90 and 120 minutes. Thin film has been placed in a specially designed sample holder, which kept at a distance of 10 cm from output laser head. For irradiation a spot of 5 mm diameter has been adjusted by focusing lens. Double beam UV/VIS/NIR Scanning Spectrophotometer (UV1900) has been used for optical measurements of the thin films. The optical spectrum has been measured as a function of wavelength (200-1100 nm) of incident light. 3. Results and discussion Absorption spectrum of Se88Te10Al2 with and without irradiation has been showed in figure 1. It can be seen that laser irradiation causes significantly changes in absorption edge. Absorbance has been decreases after laser irradiation however there are no well-defined pattern with laser irradiation time. A close look of absorption with 35 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 irradiation time reveals an alternate relation of being increasing and decreasing absorption at every 30 minutes of irradiation time interval. This harmonic pattern of absorption with irradiation time can be seen in inset pic of figure 1 which is measured near absorption edge at 700 nm and given in table 1. Figure 1: Variation of Absorbance with wavelength for Se88Te10Al2 with and without laser irradiation. Table 1: Absorbance of Se88Te10Al2 with and without laser irradiation at 700 nm Sample (s) Pristine sample Irradiated sample 30 minutes 60 minutes 90 minutes 120 minutes Absorbance 0.5952 0.2722 0.5677 0.0468 0.3535 Transmittance spectrum of these films before and after laser irradiation has been shown in figure 2.Transmittance is almost zero up to 500 nm and then started increasing with increasing wavelength. It suggests 36 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 that this material can be used as blocking film for wavelength up to 500 nm. Analysis of spectrum reveals that laser irradiation with a time duration of 30 minutes produces harmonic pattern in transmittance as shown in inset figure 2. Figure 2: Variation of Transmittance with wavelength for Se88Te10Al2 with and without laser irradiation. Table 2: %Transmittance of Se88Te10Al2 with and without laser irradiation at 550 nm Sample (s) Pristine sample Irradiated sample 30 minutes 60 minutes 90 minutes 120 minutes % T 1.2063 5.7195 2.4139 14.0159 9.7256 Reflectance of thin films can be calculated as below [40]- 𝑅 = 1 − 𝐴 − 𝑇 Reflection spectra for Se88Te10Al2 thin films before and after laser irradiation has been shown in figure 3. Laser irradiation effect with time on reflectance can be seen from inset figure 3. 37 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 Figure 3: Reflectance of Se88Te10Al2 with and without laser irradiation Here also irradiation of 30 minutes time interval produces harmonic effect in reflectance. Values of reflectance with different irradiation time is given in table 3 below. Table 3: Reflectance of Se88Te10Al2 with and without laser irradiation at 550 nm Sample (s) Pristine sample Irradiated sample 30 minutes 60 minutes 90 minutes 120 minutes R -0.93061 -0.29984 -0.64143 0.00646 -0.10934 Absorption coefficient α can be calculated as – [41, 42] 𝛼 = 𝑂𝐷 𝑡 Where OD is optical density and t is thickness of film in cm. The relation between absorption coefficient and 38 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 photon energy is shown in figure 4 below. Figure 4: Variation of absorption coefficient with photon energy for Se88Te10Al2with and without laser irradiation. Optical energy gap Eg can be calculated by following expression [43-45] 𝛼ℎ𝜈 = 𝐴�ℎ𝜈 − 𝐸𝑔� 𝑛 Where A is the constant, Eg is the optical energy gap of the material and n is a number which characterizes the transition process involved, n has the value 1 2� for the direct allowed transition and 2 for an indirect allowed transition. According to Tauc [46] the absorption tail related to localized states into the pseudo-gap, which localized states can arise from the existence of vacancy defects and/or impurities. The optical band gap (𝐸𝑔) 39 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 have been measured from the plot (𝛼ℎ𝜈)1 2⁄ versus ℎ𝜈 by extrapolating the curves to ℎ𝜈 axis at (𝛼ℎ𝜈)1 2⁄ = 0 for all samples before and after laser irradiation with different time as shown in figure 5 below . Figure 5: Variation of (𝛼ℎ𝜈)1 2⁄ versus ℎ𝜈 for Se88Te10Al2 with and without laser irradiation. The calculated data shows that the optical energy gap decreases and increases alternately with laser irradiation time interval of 30 minutes and given in table 4 below. Table 4: Optical band gap of Se88Te10Al2 with and without laser irradiation. Sample (s) Pristine sample Irradiated sample 30 minutes 60 minutes 90 minutes 120 minutes Eg (eV) 1.65 1.61 1.63 1.55 1.59 Valance band of chalcogenides forms by lone pair orbital whereas conduction band is formed by anti-bonding orbital. Laser irradiation excite the electron from the lone pair of bonding state to higher energy states and hence 40 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 vacancies created in these states are immediately filled by the outer electrons by Auger process that in turns induce more holes in the lone pair bonding orbitals leading to a vacancy cascade process which makes easier bond breaking and ionization of atoms and changes the local structure order of the amorphous network causing a decrease in the optical energy gap. Influence of laser irradiation on the optical properties is connected with higher degree of disorder in the alloy. Hence the increase in transition probability due to disorderness produced by laser irradiation leads to narrowing the optical band gap of Se88Te10Al2 alloy. Figure 6: Variation of optical band gap with laser irradiation time in Se88Te10Al2 The overall band gap has been decreases from 1.65 eV to 1.59 eV by laser irradiation but this change is not so smooth. A harmonic pattern has been observed as a laser irradiation effect on optical band gap as shown in figure 6. Every 30 minutes time interval of laser irradiation shows remarkable harmonic pattern which might be useful for switching devices. The optical properties of the solid are governed by the interaction between the solid and the electric field of the electromagnetic wave. The extinction coefficient k is related to the damping of the oscillation amplitude of the incident electric field. The extinction coefficient (k) is a measure of the damping factor, which indicates the amount of absorption loss when the electromagnetic wave propagates through the material, has been calculated using well known relation [47, 48] 41 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 𝑘 = 𝛼𝜆 4𝜋 Variation of extinction coefficient with incident energy as well as with laser irradiation time is shown in figure 7 below. Figure 7: Variation of extinction coefficient with incident energy for different time of laser irradiation. Extinction coefficient increases with increasing incident photon energy however laser irradiation causes harmonic pattern with remarkable overall decrease in extinction coefficient. The refractive index (n) has been calculated using- 𝑛 = 1 + √𝑅 1 − √𝑅 Figure 8 shows variation of refractive index with energy for different time of laser irradiation. After irradiation dose for 30 minutes refractive index increased rapidly and for next irradiation of 60 minutes it decreases. This pattern considerable repeated at every 30 minutes of irradiation time. This shows that Se88Te10Al2 acts as laser irradiation time dependent switching material. 42 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 Figure 8: Variation of refractive index with energy for different time of laser irradiation Also, the complex dielectric constant 𝜀described the optical properties of material and calculated in the following equation 𝜀 = 𝜀𝑟 + 𝑖𝜀𝑖 Where 𝜀𝑟 and 𝜀𝑖 represent the real and imaginary parts of dielectric constant respectively, and determined by the following equations 𝜀𝑟 = 𝑛2 − 𝑘2 and, 𝜀𝑖 = 2𝑛𝑘 Variation of these constants with photon energy for different irradiation time has been plotted in Figures 9 and 10 respectively. 43 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 Figure 9: Variation of 𝜀𝑟 with photon energy for different time of laser irradiation. Figure 10: Variation of 𝜀𝑖 with photon energy for different time of laser irradiation. 44 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 It has been observed that the behavior of these constants is the same of refractive index and extinction coefficient; however the value of real dielectric constant is higher than all other constants. 4. Conclusion In this study the effect of laser irradiation on optical properties of Se88Te10Al2films in different irradiation times have been studied. The absorbance, transmittance, reflectance, absorption coefficient, optical band gap, extinction coefficient, refractive index, real part of dielectric constant and imaginary part of dielectric constant this alloy has been affected with exposed to He - Ne laser. All optical constant studied here shows interesting pattern after Laser irradiation of 30 minutes time interval. These constants are changing in a harmonic pattern which can be of great interest for laser activated switches and memories. These changes can be utilized as 0 and 1 stages in laser tuned digital memory devices fabrication. Acknowledgement This work is supported by SABIC Company & Deanship of Scientific research at Qassim University, Saudi Arabia under contract No. SR-S-14-03. Author is thankful to Dr. H. Howari and Dr. Gufran A. Ansari for their help in carrying out this work. References [1] A. H. Ammar, M. S. Abo Ghazala, A. A. M. Farag , A. Eleskandrany,” Influence of composition on structural, electrical and optical characterizations of Bi48−x Sb x Se52 ternary chalcogenide system” Indian Journal of Physics, Vol. 87, Issue 12, December 2013, pp 1169-1175. [2] A.A. Al-Ghamdi, Shamshad A. Khan, S. Al-Heniti, F.A. Al-Agel, M. Zulfequar,” Annealing and laser irradiation effects on optical constants of Ga15Se85 and Ga15Se83In2 chalcogenide thin films” Current Applied Physics, Vol 11, Issue 3, May 2011, Pages 315-320. [3] Atsufumi Hirohata, Jagadeesh S. Moodera, Geetha P. Berera,” Structural and electrical properties of InSe polycrystalline films and diode fabrication” Thin Solid Films, Volume 510, Issues 1–2, 3 July 2006, Pages 247-250. [4] Maninder Singh Kamboj, G Kaur, R Thangaraj, “Dark and photoconductivity of amorphous Se–Te–Pb thin films” Thin Solid Films, Volumes 420–421, 2 December 2002, Pages 350-353. [5] A. Zakery, S.R. Elliott, “Optical properties and applications of chalcogenide glasses: a review” Journal of Non-Crystalline Solids, Volume 330, Issues 1–3, 15 November 2003, Pages 1-12. [6] Neetu, Adam A. Bahishti, M. Zulfequar, “Photoconductivity of Se85−xTe15Hgx thin films” Physica B: Condensed Matter, Volume 407, Issue 12, 15 June 2012, Pages 2267-2271. [7] Zishan H. Khan, Numan Salah, Sami Habib, A.A. Al-Ghamdi, Shamshad A. Khan, “Electrical and 45 http://link.springer.com/journal/12648 http://link.springer.com/journal/12648 http://link.springer.com/journal/12648/87/12/page/1 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 optical properties of a-SexTe100–x thin films” Optics & Laser Technology, Volume 44, Issue 1, February 2012, Pages 6–11. [8] KarunapatiTripathi, Adam A. Bahishti, M.A. Majeed Khan, M. Husain, M. Zulfequar, “Optical properties of selenium–tellurium nanostructured thin film grown by thermal evaporation” Physica B: Condensed Matter, Volume 404, Issue 16, 1 August 2009, Pages 2134-2137. [9] C. Dohare, N. Mehta,” A study of some physico-chemical properties in amorphous Se-Ag alloy prior to and subsequent to Laserstimulatednano-structuring” Journal of intense pulsed lasers and applications in advanced physics Volume 4, No.1, 2014, pages 1 - 9. [10] S K Chakarvarti, “Track-etch membranes as templates enabled nano/micro technology: a review” Indian J. Phys. Volume 83 (6), 2009, pages 737-749. [11] El-Sayed M. Farag, “Dispersive optical constants of a-Se100−xSbx films” Optics & Laser Technology, Volume 36, Issue 1, February 2004, Pages 35-38. [12] F S Terra, G M Mahmoud, L Mourad and A Tawfik,”Structural and electrical properties of In–Se films deposited by thermal evaporation” Indian Journal of Physics, Volume 86, Issue 12, December 2012, pp 1093- 1100. [13] R K Shukla, S Swarup, A Kumar and A N Nigam,”Structural studies in glassy semiconducting Se-Te alloys” Physica status solidi (a), Volume 115, Issue 1, September 1989 pages K105–K108. [14] Yang Hanmei, Wang Weizhong, Min Szukwei “The effects of As and Te on the crystallization and optical gaps of selenium” Journal of Non-Crystalline Solids, Volume 80, Issues 1–3, March 1986, Pages 503- 508. [15] Reiichi Chiba, Nobuhiro Funakoshi “Crystallization of vacuum deposited Te SeCu alloy fi Journal of Non-Crystalline Solids, Volume 105, Issues 1–2, October 1988, Pages 149-154. [16] K. Yilmaz, M. Parlak, C. Ercelebi, “Space-charge-limited current analysis in amorphous InSe thin films” Journal of Materials Science: Materials in Electronics, Volume 15, Issue 4, April 2004, pp 225-229. [17] C. Amory, J.C. Bernede, E. Halgand, S. Marsillac, “Cu(In,Ga)Se2 films obtained from γ-In2Se3 thin film” Thin Solid Films, Volumes 431–432, 1 May 2003, Pages 22-25. [18] Hui Lu, YaoquanTu, Xian Lin, Bin Fang, Duanbin Luo, AattoLaaksonen, “Effects of laser irradiation on the structure and optical properties of ZnO thin films” Materials Letters, Volume 64, Issue 19, 15 October 2010, Pages 2072-2075. [19] Pengfei Wang, Ming Ding, Lin Bo, YuliyaSemenova, Qiang Wu, and Gerald Farrell, “A silica single- mode fibre-chalcogenide multimode fibre-silica singlemodefibre structure” Photonics letters of poland, Volume 46 http://link.springer.com/journal/12648/86/12/page/1 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 4 (4), 2012 pages 143 - 145. [20] V. Lyubin, M. Klebanov, M. Mitkova, T. Petkova,” Laser-induced polarization-dependent photocrystallization of amorphous chalcogenide films” Journal of Non-Crystalline Solids, Volumes 227–230, Part 2, May 1998, Pages 739-742. [21] Adam A Bahishti, M Husain and M. Zulfequar, “Effects of laser irradiation on optical properties of a- Se100- xTex thin films” Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology, Volume 166, Issue 7, May 2011 pages 529 – 536. [22] Z. Zhang, J. Huang, F. Geng, X.Y. Zhou, S.Q. Feng, X.L. Cheng, X.D. Jiang, W.D. Wu, W.G. Zheng, Y.J. Tang, “Transient absorption and luminescence spectra of K9 glass at sub-damage site by ultraviolet laser irradiation” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volume 318, Part B, January 2014, Pages 219–222. [23] S. Papernov, A.W. Schmid, Proc. SPIE 7132 (2008) pp. 71321J-1. [24] Adam A. Bahishti, M. A. Majeed Khan, B. S. Patel, F.S.Al-Hazmi and M. Zulfequar, “Effect of Laser Irradiation on Thermal and Optical Properties of Selenium-Tellurium Alloy” Journal of Non-Crystalline Solids; Volume 355, Issues 45–47, November 2009, Pages 2314–2317. [25] A. Ritucci, G. Tomassetti, A. Reale, L. Arrizza, P. Zuppella, L. Reale, L. Palladino, F. Flora, F. Bonfigli, A. Faenov, T. Pikuz, J. Kaiser, J. Nilsen, A.F. Jankowski, “Damage and ablation of large bandgap dielectrics induced by a 46.9 nm laser beam”Optics Letters, Vol. 31, Issue 1, 2006 pages 68-70. [26] B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchik, B.W. Shore, M.D. Perry, “Nanosecond-to- femtosecond laser-induced breakdown in dielectrics” Phys. Rev. B, Volume 53 ,1996 pages 1749–1761. [27] S. G. Demos, M. Staggs, and M. R. Kozlowski, “Investigation of processes leading to damage growth in optical materials for large-aperture lasers,” Appl. Opt., Volume 41(18), 2002, pages 3628–3633. [28] Adam A. Bahishti, M. A. Majeed Khan, S. Kumar, M. Husain and M. Zulfequar, “Effect of Laser Irradiation on The Optical Properties of Amorphous Se96-xTe4Gax Thin Films” Chalcogenide Letters, Vol. 4, No. 12, December 2007, pages 155 – 160. [29] Xiang Gao, Guoying Feng, Jinghua Han, and LinglingZhai,” Investigation of laser-induced damage by various initiators on the subsurface of fused silica”Optics Express, Vol. 20, Issue 20, 2012, pp. 22095-22101. [30] M.A. Stevens-Kalceff, A. Stesmans, J. Wong,” “Defects Induced in Fused Silica By High Fluence Ultraviolet Laser Pulses at 355nm,” Appl. Phys. Lett. Volume 80, 2002, pages758-760. [31] C.W. Carr, H.B. Radousky, A.M. Rubenchik, M.D. Feit, S.G. Demos, “Localized Dynamics during Laser-Induced Damage in Optical Materials” Phys. Rev. Lett., Volume 92, February 2004, page 087401. 47 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 [32] Muneer Ahmad, J Kumar and R Thangaraj,”Electrical and optical properties of Sn10Sb20−xBixSe70 (0 ⩽ x ⩽ 8) glassy films” Journal of Non-Crystalline Solids, Volume 355, Issues 48–49, November 2009, Pages 2345-2348. [33] J Orava, T Kohoutek, T Wagner, Z Cerna, Mil Vlcek, L Benes, B Frumarova and M Frumar, “Optical and structural properties of Ge–Se bulk glasses and Ag–Ge–Se thin films” Journal of Non-Crystalline Solids, Volume 355, Issues 37–42, October 2009, Pages 1951-1954 . [34] Ambika Sharma and P B Barman, “Calorimetric and optical study of amorphous Se85 − xTe15Bix glassy alloy” Thin Solid Films, Volume 517, Issue 9, March 2009, Pages 3020-3023. [35] A Rabhi, M Kanzari and B Rezig, “Optical and structural properties of CuSbS2 thin films grown by thermal evaporation method” Thin Solid Films, Volume 517, Issue 7, February 2009, Pages 2477-2480. [36] M Fadel, S A Fayek, M O Abou-Helal, M M Ibrahim and A M Shakra, “Structural and optical properties of SeGe and SeGeX (X = In, Sb and Bi) amorphous films” Journal of Alloys and Compounds, Volume 485, Issues 1–2, 19 October 2009, Pages 604-609 . [37] Ishu Sharma, S K Tripathi and P B Barman, “Effect of Bi addition on the optical behavior of a-Ge–Se– In–Bi thin films” Applied Surface Science, Volume 255, Issue 5, Part 2, December 2008, Pages 2791-2795. [38] P Nemec and M Frumar, “Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition” Thin Solid Films, Volume 517, Issue 13, 1 May 2009, Pages 3635-3638. [39] P R de Moura, D P Almeida and J C de Lima, “Photo-induced effects in chalcogenide thin films under irradiation by synchrotron light”Journal of Electron Spectroscopy and Related Phenomena, Volume 155, Issues 1–3, March 2007, Pages 129-135 . [40] Mahasin F. Hadi Al-kadhemy, AsrarAbdulMunem Saeed, Farah Jawad Kadhum, Samara Abbas Mazloum, HawraaKhalafAied “The effect of (He–Ne) laser irradiation on the optical properties of methyl orange doped PVA films”Journal of Radiation Research and Applied Sciences, Volume 7, Issue 3, July 2014, Pages 371–375. [41] J Schottmiller, M Tabak, G Lucovsky and A Ward, “The effects of valency on transport properties in vitreous binary alloys of selenium” J Non-Cryst Solids, Volume 4, April 1970, Pages 80-96. [42] E KhShokr and M MWakkad, “Optical properties of Bi2Te2Se thin films” Journal of Materials Science, Volume 27, Issue 5, March 1992, pp 1197-1201. [43] J. Tauc, A. Menth, “States in the gap” Journal of Non-Crystalline Solids, Volumes 8–10, June 1972, Pages 569-585. [44] Davis E A and Mott N F. “Conduction in non-crystalline systems V. Conductivity, optical absorption 48 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 15, No 1, pp 34-49 and photoconductivity in amorphous semiconductors” Philosophical Magazine, Volume 22, Issue 179, 1970, pages 0903-0922. [45] Mott N F and Davis E A. “Electronic Processes in Non-Crystalline Materials 2ndedn” Oxford Univ Press,1979 p.273. [46] Tauc J.” Optical properties of Solids” edited by F Abeles, North-Holland, 1972. [47] S Chaudhuri, S K Biswas and A Choudhury, “On the optical energy gap of thick amorphous selenium film” Journal of Non-Crystalline Solids, Volume 69, Issue 1, December 1984, Pages 169-170. [48] S R Elliott, “A unified model for reversible photostructural effects in chalcogenide glasses”Journal of Non-Crystalline Solids, Volume 81, Issues 1–2, April 1986, Pages 71-98. 49