Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Thermal annealing effect on the optical properties of Ag10As30S60 thin film M.I. Abd-Elrahman a,⇑, A.Y. Abdel-Latief a, Rasha M. Khafagy b, Noha Younis a, M.M. Hafiz a a b
Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt Materials Science Laboratory, Physics Department, Girls College for Arts, Science, and Education, Ain Shams University, Cairo, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Ag10As30S60 films are deposited
600
as 453 K 500 K 573 K
1/2
400
(α h ν )
by thermal evaporation technique. The transparency upon annealing for films is improved. The optical parameters are sensitive to the annealing temperatures. The results suggest to control the optical properties of Ag10As30S60 film.
200
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy, hν (eV)
a r t i c l e
i n f o
Article history: Received 8 April 2014 Received in revised form 13 July 2014 Accepted 7 August 2014 Available online 19 August 2014 Keywords: Chalcogenide Thin films Transmission spectra Optical properties
a b s t r a c t Chalcogenide Ag10As30S60 thin films are prepared using the thermal evaporation technique from the bulk alloy. Deferential Scanning Calorimetry (DSC) curve reveals two crystallization stages for the bulk. The X-ray examination of the as-prepared and annealed films shows that the sample is crystallized in preferential orientations indicated with peaks corresponding the S8 and ternary AsAg3S3 phases. Transmission spectra show that the as-prepared and annealed films have highly transparent over the visible region. The presence of a sharp absorption edge for all films in the transmission spectra recommends Ag10As30S60 thin films as a good optical filter material. The improvement in transparency upon annealing is due to the enhancement in the crystallinity. The decrease in both optical band gap and refractive index of annealed films after crystallization temperatures is discussed in accordance with the structure changes upon annealing. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Chalcogenide based sulfide materials has applications in hologram recording, optical imaging, electro-optic information storage devices and optical mass memories [1-4]. Arsenic sulfide (AsS) chalcogenide glass is a technically important class of materials because of its good transparency in the visible and infrared regions ⇑ Corresponding author. Tel.: +20 1092808520; fax: +20 882342708. E-mail address: [email protected] (M.I. Abd-Elrahman). http://dx.doi.org/10.1016/j.saa.2014.08.015 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
and their applications as inorganic resists [5]. The Ag–As–S ternary composition is very promising to be applied for preparation of films used for the rewritable high resolution optical memories. Introducing Ag forms states within the band gap of the chalcogenide glasses thus changing the optical properties and conductivity of the glassy medium. Conductivity measurements of Ag doped chalcogenide glasses show that introduction of small amount of Ag drastically changes the conductivity of the hosting glass [6]. Hence, Ag additives make chalcogenide glasses very attractive for short wavelength radiation applications, for example imaging in X-ray
30
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
radiology [7]. Most applications are based on the well-known optical properties in this type of materials. The optical properties of the Ag doped AsS thin films regarding different Ag content have been reported [8-10]. Refractive-index [11,12], and undoped and silver photodoped AsS amorphous thin films [13] have been studied. However, studies on the optical properties of the Ag doped AsS thin films changes as the variation of thermal annealing conditions are still required. Therefore, in this work, we report on the annealing dependence of the optical properties of the Ag10As30S60 thin film. Experimental Chalcogenide bulk Ag10As30S60 alloy was prepared using the well known melt-quench technique. In this technique, high purity (99.99%) of Ag, As and S (from Aldirch Co., UK) in atomic proportions were weighed into a quartz glass ampoule. The content of ampoule was sealed in a vacuum of 105 Torr and heated in a rotating furnace at around 1000 °C for 24 h. The ampoule was then quenched in ice-cold water. The powdered samples were prepared by grinding of the resulting bulk alloy sample in a mortar. Ag10As30S60 thin films of 450 nm in thickness were prepared by thermal evaporation of the Ag10As30S60 bulk at 105 Torr using an Edwards E-306 coating system. The film thickness is controlled and measured using a quartz crystal monitor, Edwards model FTMS. The earthed face of the crystal monitor was faced the source of the evaporated materials and was placed at the same height as the substrate. The thermal behavior of the bulk alloy was investigated using a DU Pont 1090 Differential Scanning Calorimeter (DSC). The crystallization thermograms are recorded as the temperature of the sample was increased at uniform rate. To identify the crystallization phases, X-ray investigation of the as-prepared and the annealed samples is performed using Philips Diffractometer, PW1710, Netherlands, with Ni filtered Cu Ka source (k = 0.154 nm). The transmittance (T) and reflectance (R) are recorded at room temperature using a double-beam spectrophotometer (Shimadzu UV-2101, Japan). Results and discussion Thermal analyses of the bulk alloy A typical trace of the DSC of the powdered Ag10As30S60 at heating rate of 15 K/min is shown in Fig. 1. It can be noticed a broad endothermic peak centered at T = 438 K corresponding the glass transition temperature, Tg. The figure shows two crystallization
X-ray diffraction The X-ray diffraction (XRD) patterns of the as-prepared Ag10As30S60 thin film and those annealed at 453, 500 and 573 K for 1 h are shown in Fig. 2. These temperatures represent the region above Tg, the first crystallization peak and the fully completed crystallization process, respectively. The XRD patterns of the as-prepared Ag10As30S60 thin film show three crystallization peaks indicating the process of the deposition film can induce crystallization. The X-ray examination of the as-prepared and annealed films shows that the sample is crystallized in preferential orientations indicated with peaks corresponding the S8 and ternary AsAg3S3 phases. The association of the phases is determined on the basis of best agreement between both observed of 2h and inter-planar spacing with those of American Society for Testing and Materials (ASTM). Transmission spectra of the films Fig. 3 shows the transmittance (T) of the as prepared Ag10As30S60 thin film and thermally annealed thin films at 453, 500 and 573 K. It is seen that the transmittance increases with increasing wavelength. Also, all films are highly transparent over the visible (VIS) and infrared (IR) regions. The optical transmittance values of all films are higher than 80% in these regions. It is also observed that the film transparency increases up to a certain wavelength and then it becomes almost constant for all annealing temperatures. The properties of high transmittance in VIS and IR regions make the films good materials for antireflection coating and also for thermal control window coatings for cold climates. However, the presence of a sharp absorption edge for all films in the transmission spectra T(k) recommends Ag10As30S60 thin films as a good optical filter material. As seen from Fig. 3, there is no transmittance in the UV region; this makes the films suitable for eye glass coating for the protection of the skin around the eye from UV radiation. It is observed that, the transmittance of the as prepared film is higher than that of annealed at 453 K. The decrease in the transmittance after annealing below crystallization transition temperature is considered to be due to the nucleation of scattering centers
Intensity (Arb. units)
Exo
stages characterized by two exothermic peaks with onset of the first and second crystallization temperature, Tc1 at T = 490 K and Tc2 at T = 516 K. The temperature peaks of crystallization transitions occur at Tp1 = 502 K and Tp2 = 522 K. The last endothermic peak at T = 653 K corresponds the melting peak temperature, Tm.
15 K/min
ΔQ
♦ AsAg3S3
°
° S8
ann-573K
ann-500K
°
♦
♦♦
°
ann-453K
♦
as-prepared
Endo
20 400
450
500
550
600
650
40
60
80
2θ (degree)
Temp. [K] Fig. 1. DSC traces for Ag10As30S60 bulk alloy at heating rate of 15 K/min.
Fig. 2. X-ray diffraction pattern of Ag10As30S60 film for as-prepared and after annealing at different temperatures for 1 h.
31
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
Revractive index, n
Transmittance, T
0.8
0.6
as 453 K 500 K 573 K
0.4
0.2 400
800
1200
1600
2000
or crystal growth in the bulk of the material [14]. The spectral features of the films annealed at 500 and 573 K correspond to the structural aspects of the modification through the phase transition. The grain boundaries formed during crystallization, which indicates the enhancement in the crystallinity, affect the transmission in the annealed films [15,16]. This means the improvement in transmittance can be attributed to the enhancement in the crystallinity. This is probably due to refilling and homogenizing of the structure upon annealing [17]. Annealing dependence of the optical energy gap For indirect transition, the absorption coefficient (a) has been reported to relate the optical energy gap (Eg) by the following equation [18]:
¼ Bðhm Eg Þ;
ð1Þ
where B is a constant. The variation of (ahm)1/2 with photon energy (hm) for the as-prepared and annealed films of Ag10As30S60 are shown in Fig. 4. The indirect Eg can be calculated from the intercept of the resulting straight lines with the energy axis at (ahm)1/2 = 0. It
2
1000
1500
2000
2500
Fig. 5. The dependence of the refractive index (n) on the wavelength (k) for asprepared and annealed Ag10As30S60 films.
is found to be 2.415 eV for the as prepared film; and 2.370, 1.651 and 0.984 eV for annealed films at 453, 500 and 573 K, respectively. The decrease of Eg with increasing of annealing temperature can be explained as follows; the annealing above the glass transition temperature results in a crystallization of thin films with the formation of dangling bonds around the surfaces of the crystallites [19,20]. Further annealing causes the crystallites to break into smaller ones increasing the number of surface dangling bonds which responsible for the formation of defects causing a decrease in the optical band gap. The decrease of the band gap with increase in annealing temperature shows the capability of this film to be used as an absorber layer in photovoltaic application. Annealing dependence of the refractive index The refractive index (n) is related to the measured reflectance (R) by the relation [21],
n¼
12 1þR 4R 2 þ k ; 1R 1 R2
ð2Þ
where k is extinction coefficient and is given in terms of the wavelength (k) as k = ak/4p. The dependence of the refractive index on the wavelength for the as-prepared and annealed films of Ag10As30S60 is shown in Fig. 5. The increase of n with k at short wavelengths is showing anomalous behavior for n. On other hand, the decrease in n as increasing of k shows the normal dispersion behavior of the material. In the normal dispersion range, the annealing at crystallization temperatures leads to a remarkable decrease in the value of n. This result may be attributed to the changes of structure and bonding arrangement [22].
600
as 453 K 500 K 573 K
(α h ν )
1/2
400
3
Wavelength, λ (nm)
2400
Fig. 3. Transmission spectra for as-prepared and annealed Ag10As30S60 films.
1=2
4
1 500
Wavelength, λ (nm)
ðahmÞ
as 453 K 500 K 573 K
5
1.0
Conclusion 200
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy, hν (eV) Fig. 4. Absorption coefficient plotted as (ahm)1/2 vs. (hm) for as-prepared and annealed Ag10As30S60 films.
The bulk Ag10As30S60 chalcogenide glass has two crystallization stages shown on the DSC curve. Transmission spectra of the Ag10As30S60 films show improvement in transparency upon annealing, which is due to the enhancement in the crystallinity. The decrease in the optical band gap of the films after annealing is due to the formation of defects cased by the surface dangling bonds. The decrease of refractive index with increasing the annealing temperature is attributed to the changes of structure and bonding arrangement. The dependence of optical parameters on the annealing temperature of the film suggests using the thermal annealing to control the optical properties of Ag10As30S60 film.
32
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
S. Maruno, T. Yamada, M. Noda, Y. Kondo, J. Appl. Phys. 10 (1971) 653. K. Tanaka, Appl. Phys. Lett. 26 (1975) 243. K. Tanaka, Y. Ohtsuka, Thin Solid Films 57 (1979) 59. V.A. Danko, I.Z. Indutnyi, A.A. Kudryavstev, V.I. Minko, Phys. Status Solidi 124 (1991) 235. A.V. Kolobov, Photo-Induced Metastability in Amorphous Semiconductors, Wiley-WCH, Weinheim, 2003. M. Ribes, E. Bychkov, A. Pradel, J. Optoelectron. Adv. Mater. 3 (2001) 665. S.O. Kasap, J.A. Rowlands, IEEE Proc. – Circ., Devices Syst. 149 (2002) 85–96. T. Wágner, A. Macková, V. Perˇina, E. Rauhala, A. Seppälä, S.O. Kasap, M. Frumar, Mir Vlcˇek, Mil Vlcˇek, J. Non-Cryst. Solids 299 (2002) 1028. N.N. Syrbu, I.G. Stamov, V.V. Ursaki, V. Parvan, Yu. Ivanenco, Physica B 407 (2012) 3371. J. Orava, T. Wagner, M. Krbal, T. Kohoutek, Mil. Vlcek, L. Benes, E. Kotulanova, P. Bezdicka, P. Klapetek, M. Frumar, J. Phys. Chem. Solids 68 (2007) 1008.
[11] N. Nordman, O. Nordman, JOSA B 18 (2001) 1206. [12] T.I. Kosa, T. Wagner, P.J.S. Ewen, A.E. Owen, Phil. Mag. B 71 (1995) 311. [13] J. Orava, T. Wagner, M. Krbal, T. Kohoutek, Mil. Vlcek, M. Frumar, J. Non-Cryst. Solids 352 (2006) 1637. [14] D. Singh, S. Kumar, R. Thangaraj, J. Non-Cryst. Solids 358 (2012) 2826. [15] A.F. Dericioglu, Y. Kagawa, J. Eur. Ceram. Soc. 23 (2003) 951. [16] C.V. Ramana, R.J. Smith, O.M. Hussain, Phys. Status Solidi A 199 (2003) R4. [17] Y. Ding, J. Erlebacher, J. Am. Chem. Soc. 125 (2003) 7773. [18] G. Sinha, K. Adhikary, S. Chaudhuri, J. Phys.: Condens. Matter 18 (2006) 2409. [19] M.M. Hafiz, A.A. Othman, M.M. El-Nahass, A.T. Al-Motasem, Physica B 390 (2007) 348. [20] M.M. Ibrahim, M.A. Abdel-Rahim, Phys. Scr. 38 (1988) 762. [21] Y. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [22] M. Kincl, L. Tichy, Mater. Chem. Phys. 110 (2008) 322.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Thermal annealing effect on the optical properties of Ag10As30S60 thin film M.I. Abd-Elrahman a,⇑, A.Y. Abdel-Latief a, Rasha M. Khafagy b, Noha Younis a, M.M. Hafiz a a b
Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt Materials Science Laboratory, Physics Department, Girls College for Arts, Science, and Education, Ain Shams University, Cairo, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Ag10As30S60 films are deposited
600
as 453 K 500 K 573 K
1/2
400
(α h ν )
by thermal evaporation technique. The transparency upon annealing for films is improved. The optical parameters are sensitive to the annealing temperatures. The results suggest to control the optical properties of Ag10As30S60 film.
200
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy, hν (eV)
a r t i c l e
i n f o
Article history: Received 8 April 2014 Received in revised form 13 July 2014 Accepted 7 August 2014 Available online 19 August 2014 Keywords: Chalcogenide Thin films Transmission spectra Optical properties
a b s t r a c t Chalcogenide Ag10As30S60 thin films are prepared using the thermal evaporation technique from the bulk alloy. Deferential Scanning Calorimetry (DSC) curve reveals two crystallization stages for the bulk. The X-ray examination of the as-prepared and annealed films shows that the sample is crystallized in preferential orientations indicated with peaks corresponding the S8 and ternary AsAg3S3 phases. Transmission spectra show that the as-prepared and annealed films have highly transparent over the visible region. The presence of a sharp absorption edge for all films in the transmission spectra recommends Ag10As30S60 thin films as a good optical filter material. The improvement in transparency upon annealing is due to the enhancement in the crystallinity. The decrease in both optical band gap and refractive index of annealed films after crystallization temperatures is discussed in accordance with the structure changes upon annealing. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Chalcogenide based sulfide materials has applications in hologram recording, optical imaging, electro-optic information storage devices and optical mass memories [1-4]. Arsenic sulfide (AsS) chalcogenide glass is a technically important class of materials because of its good transparency in the visible and infrared regions ⇑ Corresponding author. Tel.: +20 1092808520; fax: +20 882342708. E-mail address: [email protected] (M.I. Abd-Elrahman). http://dx.doi.org/10.1016/j.saa.2014.08.015 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
and their applications as inorganic resists [5]. The Ag–As–S ternary composition is very promising to be applied for preparation of films used for the rewritable high resolution optical memories. Introducing Ag forms states within the band gap of the chalcogenide glasses thus changing the optical properties and conductivity of the glassy medium. Conductivity measurements of Ag doped chalcogenide glasses show that introduction of small amount of Ag drastically changes the conductivity of the hosting glass [6]. Hence, Ag additives make chalcogenide glasses very attractive for short wavelength radiation applications, for example imaging in X-ray
30
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
radiology [7]. Most applications are based on the well-known optical properties in this type of materials. The optical properties of the Ag doped AsS thin films regarding different Ag content have been reported [8-10]. Refractive-index [11,12], and undoped and silver photodoped AsS amorphous thin films [13] have been studied. However, studies on the optical properties of the Ag doped AsS thin films changes as the variation of thermal annealing conditions are still required. Therefore, in this work, we report on the annealing dependence of the optical properties of the Ag10As30S60 thin film. Experimental Chalcogenide bulk Ag10As30S60 alloy was prepared using the well known melt-quench technique. In this technique, high purity (99.99%) of Ag, As and S (from Aldirch Co., UK) in atomic proportions were weighed into a quartz glass ampoule. The content of ampoule was sealed in a vacuum of 105 Torr and heated in a rotating furnace at around 1000 °C for 24 h. The ampoule was then quenched in ice-cold water. The powdered samples were prepared by grinding of the resulting bulk alloy sample in a mortar. Ag10As30S60 thin films of 450 nm in thickness were prepared by thermal evaporation of the Ag10As30S60 bulk at 105 Torr using an Edwards E-306 coating system. The film thickness is controlled and measured using a quartz crystal monitor, Edwards model FTMS. The earthed face of the crystal monitor was faced the source of the evaporated materials and was placed at the same height as the substrate. The thermal behavior of the bulk alloy was investigated using a DU Pont 1090 Differential Scanning Calorimeter (DSC). The crystallization thermograms are recorded as the temperature of the sample was increased at uniform rate. To identify the crystallization phases, X-ray investigation of the as-prepared and the annealed samples is performed using Philips Diffractometer, PW1710, Netherlands, with Ni filtered Cu Ka source (k = 0.154 nm). The transmittance (T) and reflectance (R) are recorded at room temperature using a double-beam spectrophotometer (Shimadzu UV-2101, Japan). Results and discussion Thermal analyses of the bulk alloy A typical trace of the DSC of the powdered Ag10As30S60 at heating rate of 15 K/min is shown in Fig. 1. It can be noticed a broad endothermic peak centered at T = 438 K corresponding the glass transition temperature, Tg. The figure shows two crystallization
X-ray diffraction The X-ray diffraction (XRD) patterns of the as-prepared Ag10As30S60 thin film and those annealed at 453, 500 and 573 K for 1 h are shown in Fig. 2. These temperatures represent the region above Tg, the first crystallization peak and the fully completed crystallization process, respectively. The XRD patterns of the as-prepared Ag10As30S60 thin film show three crystallization peaks indicating the process of the deposition film can induce crystallization. The X-ray examination of the as-prepared and annealed films shows that the sample is crystallized in preferential orientations indicated with peaks corresponding the S8 and ternary AsAg3S3 phases. The association of the phases is determined on the basis of best agreement between both observed of 2h and inter-planar spacing with those of American Society for Testing and Materials (ASTM). Transmission spectra of the films Fig. 3 shows the transmittance (T) of the as prepared Ag10As30S60 thin film and thermally annealed thin films at 453, 500 and 573 K. It is seen that the transmittance increases with increasing wavelength. Also, all films are highly transparent over the visible (VIS) and infrared (IR) regions. The optical transmittance values of all films are higher than 80% in these regions. It is also observed that the film transparency increases up to a certain wavelength and then it becomes almost constant for all annealing temperatures. The properties of high transmittance in VIS and IR regions make the films good materials for antireflection coating and also for thermal control window coatings for cold climates. However, the presence of a sharp absorption edge for all films in the transmission spectra T(k) recommends Ag10As30S60 thin films as a good optical filter material. As seen from Fig. 3, there is no transmittance in the UV region; this makes the films suitable for eye glass coating for the protection of the skin around the eye from UV radiation. It is observed that, the transmittance of the as prepared film is higher than that of annealed at 453 K. The decrease in the transmittance after annealing below crystallization transition temperature is considered to be due to the nucleation of scattering centers
Intensity (Arb. units)
Exo
stages characterized by two exothermic peaks with onset of the first and second crystallization temperature, Tc1 at T = 490 K and Tc2 at T = 516 K. The temperature peaks of crystallization transitions occur at Tp1 = 502 K and Tp2 = 522 K. The last endothermic peak at T = 653 K corresponds the melting peak temperature, Tm.
15 K/min
ΔQ
♦ AsAg3S3
°
° S8
ann-573K
ann-500K
°
♦
♦♦
°
ann-453K
♦
as-prepared
Endo
20 400
450
500
550
600
650
40
60
80
2θ (degree)
Temp. [K] Fig. 1. DSC traces for Ag10As30S60 bulk alloy at heating rate of 15 K/min.
Fig. 2. X-ray diffraction pattern of Ag10As30S60 film for as-prepared and after annealing at different temperatures for 1 h.
31
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
Revractive index, n
Transmittance, T
0.8
0.6
as 453 K 500 K 573 K
0.4
0.2 400
800
1200
1600
2000
or crystal growth in the bulk of the material [14]. The spectral features of the films annealed at 500 and 573 K correspond to the structural aspects of the modification through the phase transition. The grain boundaries formed during crystallization, which indicates the enhancement in the crystallinity, affect the transmission in the annealed films [15,16]. This means the improvement in transmittance can be attributed to the enhancement in the crystallinity. This is probably due to refilling and homogenizing of the structure upon annealing [17]. Annealing dependence of the optical energy gap For indirect transition, the absorption coefficient (a) has been reported to relate the optical energy gap (Eg) by the following equation [18]:
¼ Bðhm Eg Þ;
ð1Þ
where B is a constant. The variation of (ahm)1/2 with photon energy (hm) for the as-prepared and annealed films of Ag10As30S60 are shown in Fig. 4. The indirect Eg can be calculated from the intercept of the resulting straight lines with the energy axis at (ahm)1/2 = 0. It
2
1000
1500
2000
2500
Fig. 5. The dependence of the refractive index (n) on the wavelength (k) for asprepared and annealed Ag10As30S60 films.
is found to be 2.415 eV for the as prepared film; and 2.370, 1.651 and 0.984 eV for annealed films at 453, 500 and 573 K, respectively. The decrease of Eg with increasing of annealing temperature can be explained as follows; the annealing above the glass transition temperature results in a crystallization of thin films with the formation of dangling bonds around the surfaces of the crystallites [19,20]. Further annealing causes the crystallites to break into smaller ones increasing the number of surface dangling bonds which responsible for the formation of defects causing a decrease in the optical band gap. The decrease of the band gap with increase in annealing temperature shows the capability of this film to be used as an absorber layer in photovoltaic application. Annealing dependence of the refractive index The refractive index (n) is related to the measured reflectance (R) by the relation [21],
n¼
12 1þR 4R 2 þ k ; 1R 1 R2
ð2Þ
where k is extinction coefficient and is given in terms of the wavelength (k) as k = ak/4p. The dependence of the refractive index on the wavelength for the as-prepared and annealed films of Ag10As30S60 is shown in Fig. 5. The increase of n with k at short wavelengths is showing anomalous behavior for n. On other hand, the decrease in n as increasing of k shows the normal dispersion behavior of the material. In the normal dispersion range, the annealing at crystallization temperatures leads to a remarkable decrease in the value of n. This result may be attributed to the changes of structure and bonding arrangement [22].
600
as 453 K 500 K 573 K
(α h ν )
1/2
400
3
Wavelength, λ (nm)
2400
Fig. 3. Transmission spectra for as-prepared and annealed Ag10As30S60 films.
1=2
4
1 500
Wavelength, λ (nm)
ðahmÞ
as 453 K 500 K 573 K
5
1.0
Conclusion 200
0 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy, hν (eV) Fig. 4. Absorption coefficient plotted as (ahm)1/2 vs. (hm) for as-prepared and annealed Ag10As30S60 films.
The bulk Ag10As30S60 chalcogenide glass has two crystallization stages shown on the DSC curve. Transmission spectra of the Ag10As30S60 films show improvement in transparency upon annealing, which is due to the enhancement in the crystallinity. The decrease in the optical band gap of the films after annealing is due to the formation of defects cased by the surface dangling bonds. The decrease of refractive index with increasing the annealing temperature is attributed to the changes of structure and bonding arrangement. The dependence of optical parameters on the annealing temperature of the film suggests using the thermal annealing to control the optical properties of Ag10As30S60 film.
32
M.I. Abd-Elrahman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 29–32
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
S. Maruno, T. Yamada, M. Noda, Y. Kondo, J. Appl. Phys. 10 (1971) 653. K. Tanaka, Appl. Phys. Lett. 26 (1975) 243. K. Tanaka, Y. Ohtsuka, Thin Solid Films 57 (1979) 59. V.A. Danko, I.Z. Indutnyi, A.A. Kudryavstev, V.I. Minko, Phys. Status Solidi 124 (1991) 235. A.V. Kolobov, Photo-Induced Metastability in Amorphous Semiconductors, Wiley-WCH, Weinheim, 2003. M. Ribes, E. Bychkov, A. Pradel, J. Optoelectron. Adv. Mater. 3 (2001) 665. S.O. Kasap, J.A. Rowlands, IEEE Proc. – Circ., Devices Syst. 149 (2002) 85–96. T. Wágner, A. Macková, V. Perˇina, E. Rauhala, A. Seppälä, S.O. Kasap, M. Frumar, Mir Vlcˇek, Mil Vlcˇek, J. Non-Cryst. Solids 299 (2002) 1028. N.N. Syrbu, I.G. Stamov, V.V. Ursaki, V. Parvan, Yu. Ivanenco, Physica B 407 (2012) 3371. J. Orava, T. Wagner, M. Krbal, T. Kohoutek, Mil. Vlcek, L. Benes, E. Kotulanova, P. Bezdicka, P. Klapetek, M. Frumar, J. Phys. Chem. Solids 68 (2007) 1008.
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