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Composition effect on the structure and optical parameters of Ge–Se–Te thin films
Materials Science in Semiconductor Processing 27 (2014) 288–292
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Composition effect on the structure and optical parameters of Ge–Se–Te thin films M. Mohamed n, M.A. Abdel-Rahim Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
a r t i c l e in f o
Keywords: Optical properties Thin films Ge–Se–Te chalcogenide glasses
abstract The present work reported the influence of Ge content variation on the optical properties of GexSe50Te50-x (x¼ 0, 5, 15, 20, 35 at%). Vacuum thermal evaporation technique was employed to prepare amorphous GexSe50Te50 x thin films. The stoichiometry of the chemical composition was checked by energy dispersive X-ray spectroscopy (EDX), whereas the thin films structure was determined by an X-ray diffraction and a scanning electron microscope (SEM). The optical absorption measurements were performed at room temperature in the wavelength range of 200–900 nm. Many optical constants were calculated for the studied thin films utilizing the optical absorption data. It was observed that the optical absorption mechanism follows the rule of the allowed direct transition. The optical band gap was found to increase from 2.31 to 2.60 eV as the Ge content increases from 0 to 35 at%. This result was explained in terms of the chemical bond approach. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Chalcogenide semiconducting glasses are technologically important materials due to their wide range of applications, including memory devices, fiber optics, reversible phase change and optical recording [1,2]. The common feature of these materials is the presence of the localized states in the mobility gas as a result of the absence of long-range order as well as various inherent defects [3,4]. Ge–Se–Te system belongs to chalcogenide glasses and has received great attention because of its technological importance [5–8]. This system can be produced by alloying of Ge, Se, and Te elements. The chalcogen element Se is believed to be an interesting element due to its important commercial applications [9]. However, it has short lifetime as well as low sensitivity and therefore several researchers have used certain additives such as Bi,
n
Corresponding author.
http://dx.doi.org/10.1016/j.mssp.2014.06.058 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
Ga, and Ge for alloying Se to some extent, giving high sensitivity at high crystallization temperature and smaller aging effects [4,10,11]. The addition of the third chalcogen element (Te) could create chemical and topological disorder in Ge–Se alloys. The accurate determination of the optical parameters of Ge–Se–Te thin films is important not only to get better understanding of the physical properties of these materials but also to exploit and develop their interesting applications. Furthermore, the dependence of optical properties of such chalcogende glasses on composition is important, because the continuously variable composition of these alloys may be utilized to prepare materials for particular applications. The investigation of optical parameters of thin films often requires the use of highly refined computer numerical techniques applied to both optical transmittance and reflectance data. Several papers have appeared in the literature reporting the optical and electrical properties of Ge–Se–Te [3,4,10–15]. For example, the effect of annealing on the optical properties of Ge5Se95 xTex has been reported. It
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
has been found that the optical energy gap increases with increasing annealing temperature [15]. Moreover, Min et al. [14] have studied the optical and electrical properties of Ge–Se–Te glasses. They have found a relationship between the glass constituents and the optical band gaps [14]. Further, the effect of composition on the optical absorption behavior of Ge10þ xSe40Te50 x thin film (x ranging from 0.0 to 16.65) has been investigated [15]. The optical band gap was found to increase from 0.19 to 0.32 eV with increasing Te content [15]. On the other hand, the optical properties of amorphous Ge10Se90 xTex (x¼0, 5, 10 and 15 at%) has been investigated using a relatively simple and straightforward method utilizing the transmission spectra [4]. It was found that the non-direct optical band gap decreases by increasing Te content. The aim of the present work is to study the effect of composition on the structure and optical parameters of GexSe50Te50 x (x ¼0, 5, 15, 20, 35 at%) thin films prepared by thermal evaporation technique utilizing the optical transmittance and reflectance spectra.
289
Fig. 1. The energy dispersive spectral X-ray spectroscopy (EDX) of Ge15Se50Te35 thin films.
2. Experimental details Different compositions of GexSe50Te50 x (x¼0, 5, 15, 20, and 35 at%) chalcogenide bulk glasses were prepared by a melt-quenching technique. This process was carried as follows: high purity materials (99.999%) of Ge, Se and Te (from Aldrich UK) were weighed according to their atomic percentages and introduced into cleaned quartz ampoules then sealed off under a vacuum of 10 5 Torr. After that, these ampoules were heated in an oven at 1373 K for 24 h. During the heating process the ampoules were manually shaken many times to maintain the homogeneity of the melt. Then the ampoules were quenched in ice cooled water at zero degree to get the glassy state. Amorphous thin films were prepared from GexSe50Te50 x bulk glasses by thermal evaporation method under vacuum of 10 5 Torr. The films were deposited on ultrasonically cleaned glass substrates (microscope slides) to avoid any impurities which affect the quality of the thin films. The film thickness was determined using a quartz crystal thickness monitor and equal to 200 nm for the studied films. The atomic percentage of elements in the studied compositions was determined by energy dispersive X-ray spectroscopy. The surface microstructure was examined using a Jeol (JSM)–T200 scanning electron microscope (SEM). A Philips X-ray diffractometer of type-1710 was used to investigate the thin films structure. The optical transmittance and reflectance of GexSe50Te50 x were measured using a computerized Shimadzu UV-2100 double beam spectrophototometer in the wavelength range of 190–900 nm. 3. Results and discussion 3.1. Structural studies The actual stoichiometric composition of bulk glasses was checked by EDX. The spectral distribution of the constitute elements of Ge15Se50Te35 is shown in Fig. 1 (similar figures were also obtained for other compositions and are not shown here). The EDX spectrum shows the peaks of Ge, Se, and Te, thereby, confirming the presence of these elements in the bulk glasses. The atomic percentage ratio of the elements of
Fig. 2. X-ray diffraction pattern of GexSe50Te50 x thin films.
GexSe50Te50 x was almost the same as that of the starting chalcogenide glass compositions. The structure of GexSe50Te50 x thin films was investigated by X-ray diffraction. Analysis of the diffraction pattern reveals the amorphous nature of the films as shown in Fig. 2. The existence of some crystalline peaks may be due to Te and Se phases [10]. The number of these peaks decreases as Ge content increases. In addition, this result was confirmed by SEM data obtained for the studied thin films as shown in Fig. 3. The images of SEM display the surface microstructure of Se50Te50, Ge5Se50Te45, and Ge35Se50Te15 samples. It is clear that the surface of thin films contains small crystallites dispersed in an amorphous matrix. The number and size of these crystallites decreases with increasing Ge concentration.
290
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
Fig. 4. Plot of transmittance (T) and reflectance (R) versus wavelength (λ) for GexSe50Te50 x thin films.
spectrophototometer. The transmittance and reflectance spectra for GexSe50Te50 x films are shown in Fig. 4(a and b). One can see that the transmittance and reflectance increases as Ge content increases. This is an indication that the increase of Ge concentration in the compositions leads to a decrease of the defects inside the chalcogenide materials. In addition, Fig. 4(a) shows that the addition of Ge shifts the transmittance spectra to lower wavelengths (higher energies). The optical absorption coefficient (α) was calculated from the experimentally measured values of the transmittance (T) and reflectance (R) according to the following equation [19]. Fig. 3. SEM photographs for (a) Se50Te50, (b) Ge5Se50Te45, and (c) Ge35Se50Te15 thin films.
In other words, the appearance of these crystalline particles in the samples could be due to increasing Te content in the composition, which leads to increasing the crystallization ability. This result is in a good agreement with the conclusions reached by others [10,16–18]. 3.2. Optical studies The optical parameters of chalcogenide thin films such as absorption coefficient, extinction coefficient and optical band gap can be determined from knowledge of the transmittance and reflectance data measured by a
T ¼ ½ð1 RÞ2 eð αdÞ =½1 R2 eð 2αdÞ
ð1Þ
where d is the film thickness. The extinction coefficient of semiconducting thin films is given by the following relation [20,21]: k ¼ αλ=4π
ð2Þ
where λ is the wavelength of the incident radiation. The relationship between the extinction coefficient of GexSe50Te50 x films and the wavelength at different compositions is shown in Fig. 5. The value of the extinction coefficient decreases as Ge content increases. The optical band gap (Eg ) of semiconducting thin films can be determined according to its dependence on absorption coefficient and the incident photon energy hν, as
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
291
Fig. 7. lnα vs. photon energy (hν) of GexSe50Te50 x films. Fig. 5. Relation between the extinction coefficient (k) and the wavelength for GexSe50Te50 x films.
Fig. 8. Variation of the band gap and the width of tail states vs. Ge content. Fig. 6. (αhν)2 vs. photon energy (hν) for amorphous GexSe50Te50 x films. The straight lines are linear Fit of the absorption edges.
empirical relation [25]: α ¼ α0 eðhν=Et Þ
expressed by the following relation [22]: αhν ¼ Cðhν Eg Þn
ð3Þ
where C is a constant and n is an index associated with both the type of electronic transition responsible for the absorption and the profile of the electron density in the valence and conduction bands. The possible values of n are 1/2 for allowed direct transition, 3/2 for forbidden direct transition, 2 for allowed indirect transition and 3 for forbidden indirect transition [23]. In the present study an allowed direct electronic transition process is involved for which n¼ 1/2 would be the best fit. These results are in good agreement with the previous works [10,24]. The optical band gap of direct transitions can be obtained from the intercept of the ðαhνÞ 2 vs. the photon energy ðhνÞ plots with the energy axis at ðαhνÞ 2 ¼ 0 as shown in Fig. 6. The composition dependence of the direct energy gap of amorphous GexSe50Te50 x films is shown in Fig. 8. The optical band gap increases as Ge content increases. Furthermore, the absorption coefficients of the optical absorption near the absorption edge in many amorphous semiconductors show an exponential dependence on the photon energy of the incident radiation and obey Urbach's
ð4Þ
where α0 is a constant, and Et is the width of the band tails of the localized states in the band gap. The energy Et characterizing the slope of the exponential edge region and is almost temperature independent at low temperature (usually below room temperature). The relationship between ln α and photon energy (hν) for GexSe50Te50 x films is shown in Fig. 7. The calculated values of Et are plotted against Ge content as shown in Fig. 8. It is clear that Et decreases as Ge content increases and accordingly the width of the localized states depends on the composition. The change of the direct Eg and Et associated with the Ge content has a direct effect on the amount and strength of the possible different bonds formed in the network structure of the investigated compositions. According to the chemical bond approach proposed by Bicermo and Ovshinsky [26] which assumed that (i) the atoms of one type combine more favorably with atoms of a different type (ii) bonds are formed in sequence of decreasing bond energy until all the available valences of the atoms are filled and (iii) each constituent atom is coordinated by (8N) atoms. N is the number of outer shell electrons and this is equivalent to neglecting the dangling bonds and other valence defects.
292
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
Keeping the above assumption in mind, the increasing Eg can be attributed to the progressive replacement of the weaker homonuclear bonds by the strong hetronuclear bonds. The hetronuclear bond energies DðA BÞ are calculated by using the following relation [27]:
concentrations. For instance, it was found that the energy gap increases with increasing Ge content. The chemical bond approach was employed to explain the dependence of the optical band gap on composition.
DðA BÞ ¼ ½DðA AÞDðB BÞ1=2 þ 30ðχ A χ B Þ2
References
ð5Þ
where D(A A) and D(B B) are the energies of the homonuclear bonds [28] for atoms A and B (44.04, 37.60 and 33.00 kcal/mol for Se, Ge and Te, respectively [4]). χ A and χ B are the electronegativities of the atoms; 2.55 for Se, 2.01 for Ge and 2.10 for Te [29]. The types of bonds expected to occur in Ge–Se–Te are Ge–Se, Ge–Ge, Ge–Te, Se–Te, Se–Se, and Te–Te. In the present compositions, Ge–Se bonds with the highest energy (49.4 kcal/mol) are expected to form first then Se–Te (44.1 kcal/mol). Based on the chemical bond approach, the bond energies are assumed to be additive. Consequently, the cohesive energies were calculated by summing the bond energies over all the bonds expected in the studied compositions. The increase of the cohesive energies with increasing Ge content is responsible for increasing the Eg and strength of rigidity of the lattice and subsequent glass softening temperature [30]. It should be noted that, the chemical bond approach neglects dangling bond and other valence defects as a first approximation. Furthermore, van der walls interactions are neglected, which can provide a means for further stabilization by the formation of much weaker links than regular covalent bonds. 4. Conclusions Influence of the composition on the structure and optical parameters of GexSe50Te50 x thin films was presented and discussed. The results reported in this paper showed that the amorphous state of the films was confirmed using X-ray diffraction. However, some crystallites were observed in the films which could be due to Te and Se phases. The number and size of these particles decrease as Ge content increases. The optical absorption results showed that the allowed-direct electronic transition is mainly responsible for the photon absorption inside the studied thin films. In addition, the optical parameters of GexSe50Te50 x were affected by changing Ge
[1] P. Nermec, M. Frumar, J. Jedelsky, M. Jelinek, J. Lancok, I. Gregora, J. Non-Cryst. Solids 299–302 (2002) 1013. [2] M.S. Kamboj, G. Kaur, R. Thangaraj, Thin Solid Films 350 (2002) 420. [3] M.A. Abdel-Rahim, J. Phys. Chem. Solids 60 (1999) 29. [4] K.A. Aly, J. Non-Cryst. Solids 355 (2009) 1489. [5] D. Ležal, J. Kašik, J. Gotz, J. Non-Cryst. Solids 90 (1987) 557. [6] V. Trnovcova, T. Pažurova, T. Šramkova, D. Ležal, J. Non-Cryst. Solids 90 (1987) 561. [7] J.L. Adam, Chem. Rev. 102 (2002) 2461. [8] A. Zakery, S.R. Elliott, J. Non-Cryst. Solids 330 (2003) 1. [9] J. Savage, J. Non-Cryst. Solids 47 (1982) 101. [10] A. El-korashy, A. Bakry, M.A. Abdel-Rahim, M. Abd El-Sattar, Physica B 391 (2007) 266. [11] M. Kubliha, P. Kostka, V. Trnovcová, J. Zavadil, J. Bednarcik, V. Labaš, J. Pedlíková, A.-Ch Dippel, H.-P. Liermann, J. Psota, J. Alloys Compd. 586 (2014) 308. [12] F.A. Al-Agel, Mater. Sci. Semicond. Process. 18 (2014) 36. [13] S.M. El-Sayed, Vacuum 72 (2004) 169. [14] S. Min, H. Yang, Z. Cheng, J. Non-Cryst. Solids 52 (1982) 181. [15] Shamshad A. Khan, M. Zulfquar, M. Husain, Physica B 324 (2002) 336 . [16] M.A. Abdel-Rahim, Physica B 239 (1997) 238. [17] R.A. Ligero, J. Vasquez, P. Villares, R. Jimenez-Garay, Mater. Lett. (1989) 6. [18] J. Vasquez, R.A. Ligero, P. Villares, R. Jimenez-Garay, Thermochim. Acta 157 (1990) 181. [19] J.I. Pankove, Optical Processes in SemiconductorsDover Puble., New York, 1973. [20] Shamshad A. Khan, F.A. Al-Agel, A.A. Al-Ghamdi, Superlattices Microstruct. 47 (2010) 695. [21] Shamshad A. Khan, F.A. Al-Agel, A.S. Faidah, S.J. Yaghmour, A.A. AlGhamdi, Mater. Lett. 64 (2010) 1391. [22] J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974. [23] R.A. Smith, Philos. Mag. Suppl. 2 (1953) 81. [24] M.A. Abdel-Rahim, M.M. Hafiz, A. Elwhab., B. Alwany, Opt. Laser Technol. 44 (2012) 1116. [25] F. Urbach, Phys. Rev. 92 (1953) 1324. [26] J. Bicermo, S.R. Ovshinsky, J. Non-Cryst. Solids 74 (1985) 75. [27] L.Pauling, The Nature of the Chemical Bond3rd Edition, Cornell University Press, Ithaca, NY, 1960. [28] L. Tichy, A. Triska, H. Ticha, M. Frumar, J. Klikorka, Solid State Commun. 41 (1982) 751. [29] A.L. Allred, J. Inorg. Nucl. Chem. 1 (1961) 215. [30] M.A. Abdel-Rahim, M.M. Hafiz, A.M. Shamekh, Physica B 369 (2005) 143.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Composition effect on the structure and optical parameters of Ge–Se–Te thin films M. Mohamed n, M.A. Abdel-Rahim Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
a r t i c l e in f o
Keywords: Optical properties Thin films Ge–Se–Te chalcogenide glasses
abstract The present work reported the influence of Ge content variation on the optical properties of GexSe50Te50-x (x¼ 0, 5, 15, 20, 35 at%). Vacuum thermal evaporation technique was employed to prepare amorphous GexSe50Te50 x thin films. The stoichiometry of the chemical composition was checked by energy dispersive X-ray spectroscopy (EDX), whereas the thin films structure was determined by an X-ray diffraction and a scanning electron microscope (SEM). The optical absorption measurements were performed at room temperature in the wavelength range of 200–900 nm. Many optical constants were calculated for the studied thin films utilizing the optical absorption data. It was observed that the optical absorption mechanism follows the rule of the allowed direct transition. The optical band gap was found to increase from 2.31 to 2.60 eV as the Ge content increases from 0 to 35 at%. This result was explained in terms of the chemical bond approach. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Chalcogenide semiconducting glasses are technologically important materials due to their wide range of applications, including memory devices, fiber optics, reversible phase change and optical recording [1,2]. The common feature of these materials is the presence of the localized states in the mobility gas as a result of the absence of long-range order as well as various inherent defects [3,4]. Ge–Se–Te system belongs to chalcogenide glasses and has received great attention because of its technological importance [5–8]. This system can be produced by alloying of Ge, Se, and Te elements. The chalcogen element Se is believed to be an interesting element due to its important commercial applications [9]. However, it has short lifetime as well as low sensitivity and therefore several researchers have used certain additives such as Bi,
n
Corresponding author.
http://dx.doi.org/10.1016/j.mssp.2014.06.058 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
Ga, and Ge for alloying Se to some extent, giving high sensitivity at high crystallization temperature and smaller aging effects [4,10,11]. The addition of the third chalcogen element (Te) could create chemical and topological disorder in Ge–Se alloys. The accurate determination of the optical parameters of Ge–Se–Te thin films is important not only to get better understanding of the physical properties of these materials but also to exploit and develop their interesting applications. Furthermore, the dependence of optical properties of such chalcogende glasses on composition is important, because the continuously variable composition of these alloys may be utilized to prepare materials for particular applications. The investigation of optical parameters of thin films often requires the use of highly refined computer numerical techniques applied to both optical transmittance and reflectance data. Several papers have appeared in the literature reporting the optical and electrical properties of Ge–Se–Te [3,4,10–15]. For example, the effect of annealing on the optical properties of Ge5Se95 xTex has been reported. It
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
has been found that the optical energy gap increases with increasing annealing temperature [15]. Moreover, Min et al. [14] have studied the optical and electrical properties of Ge–Se–Te glasses. They have found a relationship between the glass constituents and the optical band gaps [14]. Further, the effect of composition on the optical absorption behavior of Ge10þ xSe40Te50 x thin film (x ranging from 0.0 to 16.65) has been investigated [15]. The optical band gap was found to increase from 0.19 to 0.32 eV with increasing Te content [15]. On the other hand, the optical properties of amorphous Ge10Se90 xTex (x¼0, 5, 10 and 15 at%) has been investigated using a relatively simple and straightforward method utilizing the transmission spectra [4]. It was found that the non-direct optical band gap decreases by increasing Te content. The aim of the present work is to study the effect of composition on the structure and optical parameters of GexSe50Te50 x (x ¼0, 5, 15, 20, 35 at%) thin films prepared by thermal evaporation technique utilizing the optical transmittance and reflectance spectra.
289
Fig. 1. The energy dispersive spectral X-ray spectroscopy (EDX) of Ge15Se50Te35 thin films.
2. Experimental details Different compositions of GexSe50Te50 x (x¼0, 5, 15, 20, and 35 at%) chalcogenide bulk glasses were prepared by a melt-quenching technique. This process was carried as follows: high purity materials (99.999%) of Ge, Se and Te (from Aldrich UK) were weighed according to their atomic percentages and introduced into cleaned quartz ampoules then sealed off under a vacuum of 10 5 Torr. After that, these ampoules were heated in an oven at 1373 K for 24 h. During the heating process the ampoules were manually shaken many times to maintain the homogeneity of the melt. Then the ampoules were quenched in ice cooled water at zero degree to get the glassy state. Amorphous thin films were prepared from GexSe50Te50 x bulk glasses by thermal evaporation method under vacuum of 10 5 Torr. The films were deposited on ultrasonically cleaned glass substrates (microscope slides) to avoid any impurities which affect the quality of the thin films. The film thickness was determined using a quartz crystal thickness monitor and equal to 200 nm for the studied films. The atomic percentage of elements in the studied compositions was determined by energy dispersive X-ray spectroscopy. The surface microstructure was examined using a Jeol (JSM)–T200 scanning electron microscope (SEM). A Philips X-ray diffractometer of type-1710 was used to investigate the thin films structure. The optical transmittance and reflectance of GexSe50Te50 x were measured using a computerized Shimadzu UV-2100 double beam spectrophototometer in the wavelength range of 190–900 nm. 3. Results and discussion 3.1. Structural studies The actual stoichiometric composition of bulk glasses was checked by EDX. The spectral distribution of the constitute elements of Ge15Se50Te35 is shown in Fig. 1 (similar figures were also obtained for other compositions and are not shown here). The EDX spectrum shows the peaks of Ge, Se, and Te, thereby, confirming the presence of these elements in the bulk glasses. The atomic percentage ratio of the elements of
Fig. 2. X-ray diffraction pattern of GexSe50Te50 x thin films.
GexSe50Te50 x was almost the same as that of the starting chalcogenide glass compositions. The structure of GexSe50Te50 x thin films was investigated by X-ray diffraction. Analysis of the diffraction pattern reveals the amorphous nature of the films as shown in Fig. 2. The existence of some crystalline peaks may be due to Te and Se phases [10]. The number of these peaks decreases as Ge content increases. In addition, this result was confirmed by SEM data obtained for the studied thin films as shown in Fig. 3. The images of SEM display the surface microstructure of Se50Te50, Ge5Se50Te45, and Ge35Se50Te15 samples. It is clear that the surface of thin films contains small crystallites dispersed in an amorphous matrix. The number and size of these crystallites decreases with increasing Ge concentration.
290
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
Fig. 4. Plot of transmittance (T) and reflectance (R) versus wavelength (λ) for GexSe50Te50 x thin films.
spectrophototometer. The transmittance and reflectance spectra for GexSe50Te50 x films are shown in Fig. 4(a and b). One can see that the transmittance and reflectance increases as Ge content increases. This is an indication that the increase of Ge concentration in the compositions leads to a decrease of the defects inside the chalcogenide materials. In addition, Fig. 4(a) shows that the addition of Ge shifts the transmittance spectra to lower wavelengths (higher energies). The optical absorption coefficient (α) was calculated from the experimentally measured values of the transmittance (T) and reflectance (R) according to the following equation [19]. Fig. 3. SEM photographs for (a) Se50Te50, (b) Ge5Se50Te45, and (c) Ge35Se50Te15 thin films.
In other words, the appearance of these crystalline particles in the samples could be due to increasing Te content in the composition, which leads to increasing the crystallization ability. This result is in a good agreement with the conclusions reached by others [10,16–18]. 3.2. Optical studies The optical parameters of chalcogenide thin films such as absorption coefficient, extinction coefficient and optical band gap can be determined from knowledge of the transmittance and reflectance data measured by a
T ¼ ½ð1 RÞ2 eð αdÞ =½1 R2 eð 2αdÞ
ð1Þ
where d is the film thickness. The extinction coefficient of semiconducting thin films is given by the following relation [20,21]: k ¼ αλ=4π
ð2Þ
where λ is the wavelength of the incident radiation. The relationship between the extinction coefficient of GexSe50Te50 x films and the wavelength at different compositions is shown in Fig. 5. The value of the extinction coefficient decreases as Ge content increases. The optical band gap (Eg ) of semiconducting thin films can be determined according to its dependence on absorption coefficient and the incident photon energy hν, as
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
291
Fig. 7. lnα vs. photon energy (hν) of GexSe50Te50 x films. Fig. 5. Relation between the extinction coefficient (k) and the wavelength for GexSe50Te50 x films.
Fig. 8. Variation of the band gap and the width of tail states vs. Ge content. Fig. 6. (αhν)2 vs. photon energy (hν) for amorphous GexSe50Te50 x films. The straight lines are linear Fit of the absorption edges.
empirical relation [25]: α ¼ α0 eðhν=Et Þ
expressed by the following relation [22]: αhν ¼ Cðhν Eg Þn
ð3Þ
where C is a constant and n is an index associated with both the type of electronic transition responsible for the absorption and the profile of the electron density in the valence and conduction bands. The possible values of n are 1/2 for allowed direct transition, 3/2 for forbidden direct transition, 2 for allowed indirect transition and 3 for forbidden indirect transition [23]. In the present study an allowed direct electronic transition process is involved for which n¼ 1/2 would be the best fit. These results are in good agreement with the previous works [10,24]. The optical band gap of direct transitions can be obtained from the intercept of the ðαhνÞ 2 vs. the photon energy ðhνÞ plots with the energy axis at ðαhνÞ 2 ¼ 0 as shown in Fig. 6. The composition dependence of the direct energy gap of amorphous GexSe50Te50 x films is shown in Fig. 8. The optical band gap increases as Ge content increases. Furthermore, the absorption coefficients of the optical absorption near the absorption edge in many amorphous semiconductors show an exponential dependence on the photon energy of the incident radiation and obey Urbach's
ð4Þ
where α0 is a constant, and Et is the width of the band tails of the localized states in the band gap. The energy Et characterizing the slope of the exponential edge region and is almost temperature independent at low temperature (usually below room temperature). The relationship between ln α and photon energy (hν) for GexSe50Te50 x films is shown in Fig. 7. The calculated values of Et are plotted against Ge content as shown in Fig. 8. It is clear that Et decreases as Ge content increases and accordingly the width of the localized states depends on the composition. The change of the direct Eg and Et associated with the Ge content has a direct effect on the amount and strength of the possible different bonds formed in the network structure of the investigated compositions. According to the chemical bond approach proposed by Bicermo and Ovshinsky [26] which assumed that (i) the atoms of one type combine more favorably with atoms of a different type (ii) bonds are formed in sequence of decreasing bond energy until all the available valences of the atoms are filled and (iii) each constituent atom is coordinated by (8N) atoms. N is the number of outer shell electrons and this is equivalent to neglecting the dangling bonds and other valence defects.
292
M. Mohamed, M.A. Abdel-Rahim / Materials Science in Semiconductor Processing 27 (2014) 288–292
Keeping the above assumption in mind, the increasing Eg can be attributed to the progressive replacement of the weaker homonuclear bonds by the strong hetronuclear bonds. The hetronuclear bond energies DðA BÞ are calculated by using the following relation [27]:
concentrations. For instance, it was found that the energy gap increases with increasing Ge content. The chemical bond approach was employed to explain the dependence of the optical band gap on composition.
DðA BÞ ¼ ½DðA AÞDðB BÞ1=2 þ 30ðχ A χ B Þ2
References
ð5Þ
where D(A A) and D(B B) are the energies of the homonuclear bonds [28] for atoms A and B (44.04, 37.60 and 33.00 kcal/mol for Se, Ge and Te, respectively [4]). χ A and χ B are the electronegativities of the atoms; 2.55 for Se, 2.01 for Ge and 2.10 for Te [29]. The types of bonds expected to occur in Ge–Se–Te are Ge–Se, Ge–Ge, Ge–Te, Se–Te, Se–Se, and Te–Te. In the present compositions, Ge–Se bonds with the highest energy (49.4 kcal/mol) are expected to form first then Se–Te (44.1 kcal/mol). Based on the chemical bond approach, the bond energies are assumed to be additive. Consequently, the cohesive energies were calculated by summing the bond energies over all the bonds expected in the studied compositions. The increase of the cohesive energies with increasing Ge content is responsible for increasing the Eg and strength of rigidity of the lattice and subsequent glass softening temperature [30]. It should be noted that, the chemical bond approach neglects dangling bond and other valence defects as a first approximation. Furthermore, van der walls interactions are neglected, which can provide a means for further stabilization by the formation of much weaker links than regular covalent bonds. 4. Conclusions Influence of the composition on the structure and optical parameters of GexSe50Te50 x thin films was presented and discussed. The results reported in this paper showed that the amorphous state of the films was confirmed using X-ray diffraction. However, some crystallites were observed in the films which could be due to Te and Se phases. The number and size of these particles decrease as Ge content increases. The optical absorption results showed that the allowed-direct electronic transition is mainly responsible for the photon absorption inside the studied thin films. In addition, the optical parameters of GexSe50Te50 x were affected by changing Ge
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