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Effect of Sn content on some optical properties of Se90Pb10-x thin films
Optical Materials 100 (2020) 109672
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Effect of Sn content on some optical properties of Se90Pb10-x thin films Ebraheem O. Hawarat a, Mousa M.A. Imran a, Omar A. Lafi a, *, Hassan K. Juwhari b, Bashar I. Lahlouh b, N. Chandel c, N. Mehta c a
Department of Physics, Faculty of Science, Al-Balqa Applied University, Al-Salt, 19117, Jordan Department of Physics, Faculty of Science, University of Jordan, Amman, 11942, Jordan c Department of Physics, Institute of Science, Banaras Hindu University, Varanasi, 221005, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous materials Chalcogenide glasses Optical band gap energy Refractive index Extinction coefficient Dielectric constant
Se90Pb10 and Se90Pb8Sn2 thin films of thickness 400 nm were evaporated on glass substrates. The glassy nature of the prepared films was ascertained by differential scanning calorimeter thermograms and X-ray diffraction patterns. Spectra of reflection and transmission of the prepared films were recorded using FilmTek3000 in the wavelength range of 300–900 nm. The absorption coefficient was calculated and used to evaluate the optical band gap energy which comes out to be 2.1 and 2.2 eV for SePb and SePbSn thin films, respectively. The extinction coefficient (k), refractive index (n), real (ε0 ) and imaginary (ε") dielectric constants were also obtained. Results indicate that the addition of Sn content reduces the value of (n), (ε0 ) and (ε"), while that of k increases. The observed changes in the studied optical parameters were interpreted depending on the chemical bond approach of Tichy and Ticha.
1. Introduction Theoretical and experimental investigation of thin films and bulk forms of chalcogenide glasses have extensively been reported by re searchers over the past few decades due to their interesting properties. However, it is known [1,2] that, the physical properties of amorphous chalcogenide thin films are related to their structural units which can be tailored to control the performance of the produced films that are ulti mately, used as an active or passive-component in devices technology. Indeed, it is found that Se and/or Te-based binary and ternary chlaco genide thin films are promising materials for many applications such as infrared lenses, optical fibers communication, optical recording systems, data storage disks, solar energy conversion, environmental sensors and detectors, and quantum dots hybrid devices [1–8]. It is evident that amorphous semiconducting chalcogenide glasses have such wide range of applications because of their optical properties which are controlled by the localized states concentration in the tails of the bands. These localized states result when the loan pair electrons interact with the different atoms that lie in their local environment [9]. Pure amorphous Se thin films are sensitive to narrow wavelengths of light and limited to that of the visible region [10,11]. However, Se-based thin films found to exhibit photosensitivity and absorption of light over a larger wavelength range of the spectrum which extends the application
of such films to infrared region. In fact, it is reported [12] that doping of Se with heavy metals (such as Bi, Pb and Sn) decreases the phonons energy and hence longer wavelengths. According to Alvi and Khan [13], and the references therein, not only direct allowed transitions are involved in lead chalcogenide thin films, but they also exhibit prodi gious physical properties including high values of dielectric constants and high carrier mobility. This may be the reason which motivated the authors to prepare and study Se–Pb based thin films of chalcogenide glasses. According to Murali and Ramanathan [14], and the references therein, lead chalcogenides and their solid solutions have generated considerable interest because of their use in long wavelength imaging, IR gas spectroscopy, thermo-photovoltaic energy converters; and in photovoltaic and photoconductive detectors. The purpose of this work is to examine the optical parameters of Se–Pb thin films doped with Sn content at low atomic percentage. The spectra of reflection and trans mission were recorded in 300–900 nm wavelength range and used to calculate band gap energy, refractive index, extinction coefficient and dielectric constants. 2. Experimental Pure materials (99.999% impurities free) of Selenium (Se), Lead (Pb) and Tin (Sn) were used for preparation of bulk samples using common
* Corresponding author. E-mail addresses: [email protected], [email protected] (O.A. Lafi). https://doi.org/10.1016/j.optmat.2020.109672 Received 31 August 2019; Received in revised form 30 December 2019; Accepted 8 January 2020 Available online 20 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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Optical Materials 100 (2020) 109672
melt quenching technique, which depends on the idea for supper cooling liquid. For preparing Se90Pb10 and Se90Pb8Sn2 glasses, the constituent elements were weighed according to their atomic weight percentage and the actual amount of each element to get a mass of 5 g for each of the above samples was taken. Depending on their atomic weight percentage, the samples were sealed inside quartz ampoules, of length 10 cm and diameter 8 mm, under a vacuum of 10 5 Torr. The ampoules containing the samples were kept in the furnace and rocked frequently in order to ensure the homogeneity of the samples. The mixture of the elements was � heated up to 750 � C by increasing the temperature at a rate of 3–4 C/ � min and the obtained samples remained at this temperature (750 C) for 8 h. After that, the ampoules were quenched using iced water and then the formed glasses were obtained by breaking the ampoules. SCT BC-1800C electro thermal powder evaporator is used to prepare the films on glass substrate. The samples, in powder form, were inserted into the boat and high electric charge arc is used to evaporate the powder materials under vacuum of ~10 4 Torr to create the required films of thicknesses 400 nm. The thickness of each film was measured using a single crystal thickness monitor and the elemental composition was ascertained through the use of EDX. X-ray scanning was done for the obtained films by using SHIMADZU XRD-7000 diffractometer to obtain a pattern consists of humps with no sharp peaks matching known crystalline material. However, to ensure this result small amount, of about 20 mg, of the samples were subjected to NETZSCH DSC 200 F3 differential scanning calorimeter to get the required thermograms as shown in Fig. 1. FilmTek 3000 DUV-NIR, which was initially calibrated prior to scanning process, is used to record the reflection and transmission spectra which were saved in a coupled computer. The photons produced from the source (tungsten filament lamp (for Ultra Violet) and deute rium lamp (for Visible and Near Infra-Red)) were guided to incident normally on the film and the ratio of reflected and transmitted photons was collected by sensors.
the usefulness of it for certain application. When a light of intensity Io incident on a thin film, transmittance T, the reflectance F, and the absorbance A can be defined as [15]: T ¼ IT =Io
(1)
F ¼ IR =Io
(2)
A ¼ IA =Io
(3)
The transmittance and reflectance for both Se90Pb10 and Se90Pb8Sn2 thin films were recorded using FilmTek™ 3000, as mentioned earlier. The data were obtained and plotted in Fig. 2 and Fig. 3, respectively. Depending on these figures, it is clear that Se90Pb8Sn2 is more trans parent than Se90Pb10 for photons at wavelengths between 600 and 850 nm, while both samples have nearly zero transmittance for any photon at wavelengths less than 550 nm. On the other hand, according to the plot of reflectance (Fig. 3), it appears that Se90Pb8Sn2 has lower reflec tance than Se90Pb10 for all wavelengths. The reflectance of Se90Pb8Sn2 is about half of that of Se90Pb10 for wavelengths between 350 and 550 nm. The absorption coefficient is an important optical property because it describes how radically photon absorbed in matter which is the initial step in studying the electro-photo conducting phenomena. The absorp tion coefficient α can be obtained from transmittance T and reflectance F according to the following relation [16]: 1 d
α ¼ ln
FÞ2
ð1 T
(4)
where d is the thickness of the film, which was measured and found to be nearly � 400 nm for the prepared samples. The absorption coefficient α which was estimated according to the above mentioned relation is plotted as a function of the wavelength as shown in Fig. 4. It is evident that Se90Pb8Sn2 has higher absorption coefficient than that of Se90Pb10 for photons with wavelengths between 350 and 630 nm. However, the observed values of the absorption coefficient were found to be high (α > 104 cm 1) which is an indication that the prepared films have the ability for storing data and so may be used in fabrication of optical disks as reported by Imran and the references therein [10]. Fig. 4 shows that three distinct absorption regions are present, depending on the value of the absorption coefficient. The first one is the weak absorption region, which refers to absorption tail and is generally called Urbach tail. The second region is the exponential part of the plot which shows intermediate absorption such that (1 cm 1< α < 104 cm 1). This is called Urbach edge, and it obeys the Urbach relation α ¼
3. Results and discussion In general, each matter interacts with light in three ways such that; reflecting a part, allowing to another part to be transmitted and absorbing the rest of the incident light. The portion of each interacting way depends on the optical properties of the material under study giving it a characteristic fingerprint. Therefore, studying the optical properties of thin films of semiconducting chalcogenides allows one to determine
Fig. 1. DSC thermograms for Se90Pb10 and Se90Pb8Sn2 at a heating rate of 10 � C/min.
Fig. 2. Transmittance vs. incident photon wavelength for the studied films. 2
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Optical Materials 100 (2020) 109672
and the bottom of conduction band are not at the same momentum (k) value then the energy gap is indirect, the electrons at the top of valence band will need to absorb an energy (equal to the band gap energy) from incident photon to make quantum leap and then get a proper momentum from a phonon to settle down in the bottom of conduction band [19]. For both cases of direct and indirect transition, the energy and momentum are conserved when the electron transmitted from valence band to conduction band, and also the same situation happened in amorphous solids through mobility band gap. The nature of the transition in the band gap of the studied films can be obtained by using Eq. (5) from determine the value of p. The value of p which best fit the above mentioned equation for the present thin films data comes out to be 1/2 indicating that direct allowed transitions occur in the studied films. The absorption coefficient (α) was used to evaluate (αhv)2, which in turn plotted as a function of the incident photon energy (hv) as shown in Fig. 5 and Fig. 6 for Se90Pb10 and Se90Pb8Sn2, respec tively. The characteristics of the obtained figures agree fairly well with the observations reported by Saini et al. [20]. According to them for direct band gap materials the plot of (αhν)1lp against hν shows a linear portion which is extrapolated to get Eg, while indirect band gap films show two linear portions of different slopes. From these figures, the width of the band gap of the studied films can be obtained by extrapo lated the straight line region of the plotted data. The obtained values of (Eg) for Se90Pb10 and Se90Pb8Sn2 glasses are found to be 2.1 and 2.2 eV, respectively. It is clear that the energy band gap slightly increases upon doping Se–Pb by small amount of Sn content. However, the obtained values of Eg, are also close to that of the mobility gap for a-Se [21]. The refractive index of a medium, n ¼ c/v where v and c are the speed of light in the medium and in vacuum, respectively. The refractive index, in loss media, becomes complex and can be written as N ¼ n – i k where i pffiffiffiffiffiffi ¼ 1 and k is the extinction coefficient. In the case of normal incident of a light beam on a surface of such medium, n and k can be determined from the reflection coefficient r, which is expressed as [22,23]:
Fig. 3. Reflectance vs.incident photon wavelength for the studied films.
r¼
1 N 1 n þ ik ¼ 1 þ N 1 þ n ik
which is used to calculate the reflectance F as follows: � � 2 2 �1 n þ ik�2 � ¼ ð1 nÞ þ k F ¼ jrj2 ¼ �� � 2 1 þ n ik ð1 þ nÞ þ k2 Fig. 4. Absorption coefficient vs. photon wave length of the studied thin films.
(6)
(7)
Using the calculated values of reflectance R and with some algebra one can get n from the relation given below [22,23]:
αoexp(hv/Ee) [17], where α, αo, h, v and Ee, respectively, are absorption coefficient, physical constant, Planck constant, photon frequency and Urbach energy. From the slope, one can determine the localized states tail-width in the gap region. The third region is characterized by strong absorption in which the absorption coefficient is (>104 cm 1) and is used to evaluate the optical band gap energy through the well known relation of Tauc [18]:
αhv ¼ Xðhv
EgÞp
(5)
where X is the slope and it depends on the width of localized states in the band gap and is indicated as Eg in the above Tauc equation. The power p is important since it specifies the nature of optical transition between the conduction band and that of the valence; p ¼ 1/2 for direct allowed transition, p ¼ 2 for indirect allowed transition, p ¼ 3 for indirect forbidden transition, and p ¼ 3/2 for direct forbidden transition. In crystalline solids, the periodicity in atoms potentials produces two distinct energy bands that can be occupied by electrons and separated by quantized energy called band gap energy. When the top of valence band and the bottom of conduction band occur at the same momentum (k) value then the energy gap is direct. In such case, the electrons at the top of valence band need to absorb the proper energy from incident photon (equal to the energy band gap) to make quantum leap and settle down in the bottom of conduction band. Otherwise, if the top of valence band
Fig. 5. (αhv)2 as a function of the incident photon energy (hv) for Se90Pb10. 3
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Optical Materials 100 (2020) 109672
Fig. 6. (αhv)2 as a function of the incident photon energy (hv) for Se90Pb8Sn2.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � sffi� � 2 1þF 1þF þ n¼ ð1 þ kÞ2 1 F 1 F
Fig. 7. The real part of refractive index (n) as a function of photon wavelength for both thin film samples.
(8)
exist in the films can be calculated using Pauling principle [27]:
The extinction coefficient k is obtained from the relation:
αλ k¼ 4π
(9)
In addition, the real and imaginary parts of dielectric constant ε were also deduced using the following relations [10,24]:
ε’ ¼ n2 ε} ¼ 2nk
k2
BSe
Sn
¼ ½ðBSe
1=2 Se ÞðBSn Sn Þ�
þ 30ðχSe
χSn Þ2
(12)
BSe
Pb
¼ ½ðBSe
1=2 Se ÞðBPb Pb Þ�
þ 30ðχSe
χPb Þ2
(13)
Where χSe, χPb and χSn are the electronegativities which, according to ref. [28], are found to be 2.55, 1.87, and 1.96, respectively. Meanwhile, BSe–Se ¼ 1.911, BPb–Pb ¼ 3.644, and BSn–Sn ¼ 2.15 eV are the homopolar bond energies of the constituent elements according to the same refer ence. The calculated bond energies are 4.29 and 3.27 eV for BSe–Sn and BSe–Pb, respectively. The structure of Se–Pb glass consists of Se chains and rings with some PbSe4/2 tetrahedrally bonded atoms. However, the addition of Sn at lower percentage causes the formation a small amount of another tetrahedral structural units composed of SnSe4/2 with bond energy higher than that of Se–Pb units. The optical band gap is found to be sensitive to the bond energy [11] and this explains why it increases with the addition of Sn. To assure this result, one has to calculate the cohesive P energy ðCE ¼ Ci Bi Þ) of the studied samples, where Ci and Bi are the distribution of the possible bonds and the bond energy, respectively. The
(10) (11)
The real part of the refractive index n was calculated using equation (8) for both of Se90Pb10 and Se90Pb8Sn2 thin films. The obtained values of (n) were plotted in Fig. 7 versus photon wavelength. The (average) refractive index (n) (between wavelengths 350 and 575 nm) for Se90Pb10 (~5.3) is greater than that of Se90Pb8Sn2 (~2.7). This was expected because one can see that the Sn addition to the Se–Pb system reduces the reflectance (R) and increased the absorption (α) as mentioned earlier. The extinction coefficient k (the imaginary part of refractive index) was calculated using Eq.(11) for both as-prepared samples of Se90Pb10 and Se90Pb8Sn2 thin films and is plotted in Fig. 8 versus photon wavelength. It is obvious that Se90Pb8Sn2 has greater extinction coefficient than that of Se90Pb10 for photons with wavelengths between 350 and 630 nm which is in consistent with the earlier result of absorption shown in Fig. 4. The dielectric constants, real (ε0 ) and imaginary (ε"), are calcu lated using Eq.(10) and Eq.(11), respectively for both samples of Se90Pb10 and Se90Pb8Sn2 and the obtained values were plotted in Fig. 9 and Fig. 10 versus photon wavelength, respectively. The values of all optical parameters for the studied thin films, that obtained above, are listed in Table 1. The correlation between physical properties of amorphous semi conducting glasses and their composition was studied by researchers using two different models. The first is the mechanical threshold which was suggested by Phillips and Thorpe [25]. The glassy compounds, ac cording to this model, transform from polymeric (floppy material) to rigid cross-linked layers at an average coordination number z ¼ 2.40, which was extended by Tanaka to 2.67 [26]. The other model is the chemical threshold which suggests that atoms in chalcogenide glasses prefer to construct heteroploar (strong) bonds between chalcogen atoms with non-chalcogen atoms rather than homopolar bonds. This makes the glass rigid and also lowers the free energy. However, it is known that the change in the optical band gap is related to the types of bonds that are formed in a glassy material. In the present work the heteroploar bonds
Fig. 8. The imaginary photon wavelength. 4
part
of
refractive
index
k
(extinction)
vs.
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Optical Materials 100 (2020) 109672
Table 2 Distribution of expected chemical bonds ratio and the cohesive energy (CE), for Se90Pb10-xSnx (x ¼ 0, 2). x 0 2
Distribution of chemical bonds
CE (eV/molecule)
Se–Pb
Se–Sn
Se–Se
0.222 0.178
0.000 0.044
0.778 0.778
2.213 2.258
(14)
Q ¼ fðCNSe Þðat:%SeÞg = f½ðCNPb Þðat:%PbÞ�½ðCNSn Þðat:% SnÞ�g
The coordination number CNSe, CNPb, and CNSn are 2, 4, and 4, respectively. Hence the calculated value of Q is 4.5 for both of the studied samples. Since the value of Q > 1 means that the studied glasses contain only heteroploar bonds and chalcogen-chalcogen bonds as suggested by Tichy and Ticha [30], so the chemical threshold model can be applied to the samples under investigation. According to this model, the overall mean bond energy ¼ þ , where and are the bond energy of the average crosslinking per atom and the mean bond energy per atom of the remaining matrix, respectively. The value ¼ Prich Ehp where Prich and Ehp are given as below [31]:
Fig. 9. Real part of dielectric constant (ε0 ) as a function of wavelength for both thin film samples.
(15)
Prich ¼ fðCNPb Þðat:%PbÞ þ ðCNSn Þðat:%SnÞg Ehp ¼ fðCNPb Þðat:%PbÞðBSe Pb Þ þ ðCNSn Þðat:%SnÞðBSe � fðCNPb Þðat%PbÞ þ ðCNSn Þðat:%SnÞg
Sn Þg
(16)
The mean bond energy per atom of the remaining matrix can be estimated from the following relation: < Erm > ¼ f2ð0:5 < CN >
Prich Þ * ðBSe
Se Þg =
< CN > ¼ 1:216eV
(17)
where is the average coordination number of the studied sam ples, which was deduced as described in Ref. [32] and comes out to be 2.2 for both samples since Sn and Pb have the same coordination number. Table 3 based on the above calculations, the overall mean bond energy ¼ 2.524 and 2.606 eV for Se90Pb10 and Se90Pb8Sn2 films, respectively. From the above calculations it is obvious that as Sn is added to Se–Pb glass, the overall mean bond energy is slightly increased. This further supports our earlier argument regarding the observed in crease in the band gap energy. 4. Conclusions The prepared thin films (with measured thickness ~ 400 nm) were subjected to x-ray and DSC examination and they showed their amor phous nature. From the obtained reflectance and transmittance data, it has been found that the Sn addition to the binary Se–Pb at 2 at. % re duces the reflectance, increases the transmittance (maximum trans mittance and minimum reflectance, good transparency occurs between 700 and 800 nm) and the absorption coefficient is significantly increased. The refractive index (real part) and both real and imaginary dielectric constants also reduced, but the extinction coefficient increased after the addition of Sn to the binary Se–Pb at 2 at. % supporting the result of increases absorption coefficient. No significant change in the optical energy gap is observed and this may be due to similarity of Pb and Sn atoms in coordination numbers and valence electrons. In spite of that, the slight change may be refers to the difference in atoms size which slightly affects the defects in the amorphous extended and local
Fig. 10. Imaginary part of dielectric constant (ε") as a function of wavelength for both thin film samples. Table 1 Values of absorption coefficient (α), refractive index (n), extinction coefficient (k), dielectric constant (ε0 ), dielectric loss (ε"), optical band gap (Eg) for Se90Pb10xSnx (x ¼ 0,2) thin film. . X
λ (nm)
α(104cm 1)
n
k
ε0
ε"
Eg(eV)
0 2
583.7 581.5
4.21 5.55
5.15 2.61
0.20 0.26
26.47 6.72
2.01 1.34
2.1 2.2
distribution of the possible chemical bonds ratio was calculated and listed along with the cohesive energy in Table 2. From this table, it is obvious that the cohesive energy slightly increases with the addition of Sn to Se–Pb glass which is why the band gap slightly increases. According to Tichy et al. [29], the mean bond energy and stoichiometry deviation number Q, which is the ratio of bonding elec trons of chalcogen atom to non-chalcogen atom, are related to the cohesive energy and glass transition temperature. The value of Q is calculated using the relation:
Table 3 The average coordination number, Prich, Erm, Ehp, and Ecl for Se90Pb10xSnx (x ¼ 0, 2).
5
x
Prich
Erm (eV)
Ehp (eV)
Ecl (eV)
0 2
2.2 2.2
0.4 0.4
1.216 1.216
3.270 3.474
1.308 1.390
Optical Materials 100 (2020) 109672
E.O. Hawarat et al.
states at the mobility gap, and is reflected as higher absorption values for Se90Pb8Sn2 than that of Se90Pb10 and allow the use of ternary Se–Pb–Sn films as optical record material which required high absorption.
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Effect of Sn content on some optical properties of Se90Pb10-x thin films Ebraheem O. Hawarat a, Mousa M.A. Imran a, Omar A. Lafi a, *, Hassan K. Juwhari b, Bashar I. Lahlouh b, N. Chandel c, N. Mehta c a
Department of Physics, Faculty of Science, Al-Balqa Applied University, Al-Salt, 19117, Jordan Department of Physics, Faculty of Science, University of Jordan, Amman, 11942, Jordan c Department of Physics, Institute of Science, Banaras Hindu University, Varanasi, 221005, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Amorphous materials Chalcogenide glasses Optical band gap energy Refractive index Extinction coefficient Dielectric constant
Se90Pb10 and Se90Pb8Sn2 thin films of thickness 400 nm were evaporated on glass substrates. The glassy nature of the prepared films was ascertained by differential scanning calorimeter thermograms and X-ray diffraction patterns. Spectra of reflection and transmission of the prepared films were recorded using FilmTek3000 in the wavelength range of 300–900 nm. The absorption coefficient was calculated and used to evaluate the optical band gap energy which comes out to be 2.1 and 2.2 eV for SePb and SePbSn thin films, respectively. The extinction coefficient (k), refractive index (n), real (ε0 ) and imaginary (ε") dielectric constants were also obtained. Results indicate that the addition of Sn content reduces the value of (n), (ε0 ) and (ε"), while that of k increases. The observed changes in the studied optical parameters were interpreted depending on the chemical bond approach of Tichy and Ticha.
1. Introduction Theoretical and experimental investigation of thin films and bulk forms of chalcogenide glasses have extensively been reported by re searchers over the past few decades due to their interesting properties. However, it is known [1,2] that, the physical properties of amorphous chalcogenide thin films are related to their structural units which can be tailored to control the performance of the produced films that are ulti mately, used as an active or passive-component in devices technology. Indeed, it is found that Se and/or Te-based binary and ternary chlaco genide thin films are promising materials for many applications such as infrared lenses, optical fibers communication, optical recording systems, data storage disks, solar energy conversion, environmental sensors and detectors, and quantum dots hybrid devices [1–8]. It is evident that amorphous semiconducting chalcogenide glasses have such wide range of applications because of their optical properties which are controlled by the localized states concentration in the tails of the bands. These localized states result when the loan pair electrons interact with the different atoms that lie in their local environment [9]. Pure amorphous Se thin films are sensitive to narrow wavelengths of light and limited to that of the visible region [10,11]. However, Se-based thin films found to exhibit photosensitivity and absorption of light over a larger wavelength range of the spectrum which extends the application
of such films to infrared region. In fact, it is reported [12] that doping of Se with heavy metals (such as Bi, Pb and Sn) decreases the phonons energy and hence longer wavelengths. According to Alvi and Khan [13], and the references therein, not only direct allowed transitions are involved in lead chalcogenide thin films, but they also exhibit prodi gious physical properties including high values of dielectric constants and high carrier mobility. This may be the reason which motivated the authors to prepare and study Se–Pb based thin films of chalcogenide glasses. According to Murali and Ramanathan [14], and the references therein, lead chalcogenides and their solid solutions have generated considerable interest because of their use in long wavelength imaging, IR gas spectroscopy, thermo-photovoltaic energy converters; and in photovoltaic and photoconductive detectors. The purpose of this work is to examine the optical parameters of Se–Pb thin films doped with Sn content at low atomic percentage. The spectra of reflection and trans mission were recorded in 300–900 nm wavelength range and used to calculate band gap energy, refractive index, extinction coefficient and dielectric constants. 2. Experimental Pure materials (99.999% impurities free) of Selenium (Se), Lead (Pb) and Tin (Sn) were used for preparation of bulk samples using common
* Corresponding author. E-mail addresses: [email protected], [email protected] (O.A. Lafi). https://doi.org/10.1016/j.optmat.2020.109672 Received 31 August 2019; Received in revised form 30 December 2019; Accepted 8 January 2020 Available online 20 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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Optical Materials 100 (2020) 109672
melt quenching technique, which depends on the idea for supper cooling liquid. For preparing Se90Pb10 and Se90Pb8Sn2 glasses, the constituent elements were weighed according to their atomic weight percentage and the actual amount of each element to get a mass of 5 g for each of the above samples was taken. Depending on their atomic weight percentage, the samples were sealed inside quartz ampoules, of length 10 cm and diameter 8 mm, under a vacuum of 10 5 Torr. The ampoules containing the samples were kept in the furnace and rocked frequently in order to ensure the homogeneity of the samples. The mixture of the elements was � heated up to 750 � C by increasing the temperature at a rate of 3–4 C/ � min and the obtained samples remained at this temperature (750 C) for 8 h. After that, the ampoules were quenched using iced water and then the formed glasses were obtained by breaking the ampoules. SCT BC-1800C electro thermal powder evaporator is used to prepare the films on glass substrate. The samples, in powder form, were inserted into the boat and high electric charge arc is used to evaporate the powder materials under vacuum of ~10 4 Torr to create the required films of thicknesses 400 nm. The thickness of each film was measured using a single crystal thickness monitor and the elemental composition was ascertained through the use of EDX. X-ray scanning was done for the obtained films by using SHIMADZU XRD-7000 diffractometer to obtain a pattern consists of humps with no sharp peaks matching known crystalline material. However, to ensure this result small amount, of about 20 mg, of the samples were subjected to NETZSCH DSC 200 F3 differential scanning calorimeter to get the required thermograms as shown in Fig. 1. FilmTek 3000 DUV-NIR, which was initially calibrated prior to scanning process, is used to record the reflection and transmission spectra which were saved in a coupled computer. The photons produced from the source (tungsten filament lamp (for Ultra Violet) and deute rium lamp (for Visible and Near Infra-Red)) were guided to incident normally on the film and the ratio of reflected and transmitted photons was collected by sensors.
the usefulness of it for certain application. When a light of intensity Io incident on a thin film, transmittance T, the reflectance F, and the absorbance A can be defined as [15]: T ¼ IT =Io
(1)
F ¼ IR =Io
(2)
A ¼ IA =Io
(3)
The transmittance and reflectance for both Se90Pb10 and Se90Pb8Sn2 thin films were recorded using FilmTek™ 3000, as mentioned earlier. The data were obtained and plotted in Fig. 2 and Fig. 3, respectively. Depending on these figures, it is clear that Se90Pb8Sn2 is more trans parent than Se90Pb10 for photons at wavelengths between 600 and 850 nm, while both samples have nearly zero transmittance for any photon at wavelengths less than 550 nm. On the other hand, according to the plot of reflectance (Fig. 3), it appears that Se90Pb8Sn2 has lower reflec tance than Se90Pb10 for all wavelengths. The reflectance of Se90Pb8Sn2 is about half of that of Se90Pb10 for wavelengths between 350 and 550 nm. The absorption coefficient is an important optical property because it describes how radically photon absorbed in matter which is the initial step in studying the electro-photo conducting phenomena. The absorp tion coefficient α can be obtained from transmittance T and reflectance F according to the following relation [16]: 1 d
α ¼ ln
FÞ2
ð1 T
(4)
where d is the thickness of the film, which was measured and found to be nearly � 400 nm for the prepared samples. The absorption coefficient α which was estimated according to the above mentioned relation is plotted as a function of the wavelength as shown in Fig. 4. It is evident that Se90Pb8Sn2 has higher absorption coefficient than that of Se90Pb10 for photons with wavelengths between 350 and 630 nm. However, the observed values of the absorption coefficient were found to be high (α > 104 cm 1) which is an indication that the prepared films have the ability for storing data and so may be used in fabrication of optical disks as reported by Imran and the references therein [10]. Fig. 4 shows that three distinct absorption regions are present, depending on the value of the absorption coefficient. The first one is the weak absorption region, which refers to absorption tail and is generally called Urbach tail. The second region is the exponential part of the plot which shows intermediate absorption such that (1 cm 1< α < 104 cm 1). This is called Urbach edge, and it obeys the Urbach relation α ¼
3. Results and discussion In general, each matter interacts with light in three ways such that; reflecting a part, allowing to another part to be transmitted and absorbing the rest of the incident light. The portion of each interacting way depends on the optical properties of the material under study giving it a characteristic fingerprint. Therefore, studying the optical properties of thin films of semiconducting chalcogenides allows one to determine
Fig. 1. DSC thermograms for Se90Pb10 and Se90Pb8Sn2 at a heating rate of 10 � C/min.
Fig. 2. Transmittance vs. incident photon wavelength for the studied films. 2
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Optical Materials 100 (2020) 109672
and the bottom of conduction band are not at the same momentum (k) value then the energy gap is indirect, the electrons at the top of valence band will need to absorb an energy (equal to the band gap energy) from incident photon to make quantum leap and then get a proper momentum from a phonon to settle down in the bottom of conduction band [19]. For both cases of direct and indirect transition, the energy and momentum are conserved when the electron transmitted from valence band to conduction band, and also the same situation happened in amorphous solids through mobility band gap. The nature of the transition in the band gap of the studied films can be obtained by using Eq. (5) from determine the value of p. The value of p which best fit the above mentioned equation for the present thin films data comes out to be 1/2 indicating that direct allowed transitions occur in the studied films. The absorption coefficient (α) was used to evaluate (αhv)2, which in turn plotted as a function of the incident photon energy (hv) as shown in Fig. 5 and Fig. 6 for Se90Pb10 and Se90Pb8Sn2, respec tively. The characteristics of the obtained figures agree fairly well with the observations reported by Saini et al. [20]. According to them for direct band gap materials the plot of (αhν)1lp against hν shows a linear portion which is extrapolated to get Eg, while indirect band gap films show two linear portions of different slopes. From these figures, the width of the band gap of the studied films can be obtained by extrapo lated the straight line region of the plotted data. The obtained values of (Eg) for Se90Pb10 and Se90Pb8Sn2 glasses are found to be 2.1 and 2.2 eV, respectively. It is clear that the energy band gap slightly increases upon doping Se–Pb by small amount of Sn content. However, the obtained values of Eg, are also close to that of the mobility gap for a-Se [21]. The refractive index of a medium, n ¼ c/v where v and c are the speed of light in the medium and in vacuum, respectively. The refractive index, in loss media, becomes complex and can be written as N ¼ n – i k where i pffiffiffiffiffiffi ¼ 1 and k is the extinction coefficient. In the case of normal incident of a light beam on a surface of such medium, n and k can be determined from the reflection coefficient r, which is expressed as [22,23]:
Fig. 3. Reflectance vs.incident photon wavelength for the studied films.
r¼
1 N 1 n þ ik ¼ 1 þ N 1 þ n ik
which is used to calculate the reflectance F as follows: � � 2 2 �1 n þ ik�2 � ¼ ð1 nÞ þ k F ¼ jrj2 ¼ �� � 2 1 þ n ik ð1 þ nÞ þ k2 Fig. 4. Absorption coefficient vs. photon wave length of the studied thin films.
(6)
(7)
Using the calculated values of reflectance R and with some algebra one can get n from the relation given below [22,23]:
αoexp(hv/Ee) [17], where α, αo, h, v and Ee, respectively, are absorption coefficient, physical constant, Planck constant, photon frequency and Urbach energy. From the slope, one can determine the localized states tail-width in the gap region. The third region is characterized by strong absorption in which the absorption coefficient is (>104 cm 1) and is used to evaluate the optical band gap energy through the well known relation of Tauc [18]:
αhv ¼ Xðhv
EgÞp
(5)
where X is the slope and it depends on the width of localized states in the band gap and is indicated as Eg in the above Tauc equation. The power p is important since it specifies the nature of optical transition between the conduction band and that of the valence; p ¼ 1/2 for direct allowed transition, p ¼ 2 for indirect allowed transition, p ¼ 3 for indirect forbidden transition, and p ¼ 3/2 for direct forbidden transition. In crystalline solids, the periodicity in atoms potentials produces two distinct energy bands that can be occupied by electrons and separated by quantized energy called band gap energy. When the top of valence band and the bottom of conduction band occur at the same momentum (k) value then the energy gap is direct. In such case, the electrons at the top of valence band need to absorb the proper energy from incident photon (equal to the energy band gap) to make quantum leap and settle down in the bottom of conduction band. Otherwise, if the top of valence band
Fig. 5. (αhv)2 as a function of the incident photon energy (hv) for Se90Pb10. 3
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Optical Materials 100 (2020) 109672
Fig. 6. (αhv)2 as a function of the incident photon energy (hv) for Se90Pb8Sn2.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � sffi� � 2 1þF 1þF þ n¼ ð1 þ kÞ2 1 F 1 F
Fig. 7. The real part of refractive index (n) as a function of photon wavelength for both thin film samples.
(8)
exist in the films can be calculated using Pauling principle [27]:
The extinction coefficient k is obtained from the relation:
αλ k¼ 4π
(9)
In addition, the real and imaginary parts of dielectric constant ε were also deduced using the following relations [10,24]:
ε’ ¼ n2 ε} ¼ 2nk
k2
BSe
Sn
¼ ½ðBSe
1=2 Se ÞðBSn Sn Þ�
þ 30ðχSe
χSn Þ2
(12)
BSe
Pb
¼ ½ðBSe
1=2 Se ÞðBPb Pb Þ�
þ 30ðχSe
χPb Þ2
(13)
Where χSe, χPb and χSn are the electronegativities which, according to ref. [28], are found to be 2.55, 1.87, and 1.96, respectively. Meanwhile, BSe–Se ¼ 1.911, BPb–Pb ¼ 3.644, and BSn–Sn ¼ 2.15 eV are the homopolar bond energies of the constituent elements according to the same refer ence. The calculated bond energies are 4.29 and 3.27 eV for BSe–Sn and BSe–Pb, respectively. The structure of Se–Pb glass consists of Se chains and rings with some PbSe4/2 tetrahedrally bonded atoms. However, the addition of Sn at lower percentage causes the formation a small amount of another tetrahedral structural units composed of SnSe4/2 with bond energy higher than that of Se–Pb units. The optical band gap is found to be sensitive to the bond energy [11] and this explains why it increases with the addition of Sn. To assure this result, one has to calculate the cohesive P energy ðCE ¼ Ci Bi Þ) of the studied samples, where Ci and Bi are the distribution of the possible bonds and the bond energy, respectively. The
(10) (11)
The real part of the refractive index n was calculated using equation (8) for both of Se90Pb10 and Se90Pb8Sn2 thin films. The obtained values of (n) were plotted in Fig. 7 versus photon wavelength. The (average) refractive index (n) (between wavelengths 350 and 575 nm) for Se90Pb10 (~5.3) is greater than that of Se90Pb8Sn2 (~2.7). This was expected because one can see that the Sn addition to the Se–Pb system reduces the reflectance (R) and increased the absorption (α) as mentioned earlier. The extinction coefficient k (the imaginary part of refractive index) was calculated using Eq.(11) for both as-prepared samples of Se90Pb10 and Se90Pb8Sn2 thin films and is plotted in Fig. 8 versus photon wavelength. It is obvious that Se90Pb8Sn2 has greater extinction coefficient than that of Se90Pb10 for photons with wavelengths between 350 and 630 nm which is in consistent with the earlier result of absorption shown in Fig. 4. The dielectric constants, real (ε0 ) and imaginary (ε"), are calcu lated using Eq.(10) and Eq.(11), respectively for both samples of Se90Pb10 and Se90Pb8Sn2 and the obtained values were plotted in Fig. 9 and Fig. 10 versus photon wavelength, respectively. The values of all optical parameters for the studied thin films, that obtained above, are listed in Table 1. The correlation between physical properties of amorphous semi conducting glasses and their composition was studied by researchers using two different models. The first is the mechanical threshold which was suggested by Phillips and Thorpe [25]. The glassy compounds, ac cording to this model, transform from polymeric (floppy material) to rigid cross-linked layers at an average coordination number z ¼ 2.40, which was extended by Tanaka to 2.67 [26]. The other model is the chemical threshold which suggests that atoms in chalcogenide glasses prefer to construct heteroploar (strong) bonds between chalcogen atoms with non-chalcogen atoms rather than homopolar bonds. This makes the glass rigid and also lowers the free energy. However, it is known that the change in the optical band gap is related to the types of bonds that are formed in a glassy material. In the present work the heteroploar bonds
Fig. 8. The imaginary photon wavelength. 4
part
of
refractive
index
k
(extinction)
vs.
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Optical Materials 100 (2020) 109672
Table 2 Distribution of expected chemical bonds ratio and the cohesive energy (CE), for Se90Pb10-xSnx (x ¼ 0, 2). x 0 2
Distribution of chemical bonds
CE (eV/molecule)
Se–Pb
Se–Sn
Se–Se
0.222 0.178
0.000 0.044
0.778 0.778
2.213 2.258
(14)
Q ¼ fðCNSe Þðat:%SeÞg = f½ðCNPb Þðat:%PbÞ�½ðCNSn Þðat:% SnÞ�g
The coordination number CNSe, CNPb, and CNSn are 2, 4, and 4, respectively. Hence the calculated value of Q is 4.5 for both of the studied samples. Since the value of Q > 1 means that the studied glasses contain only heteroploar bonds and chalcogen-chalcogen bonds as suggested by Tichy and Ticha [30], so the chemical threshold model can be applied to the samples under investigation. According to this model, the overall mean bond energy
Fig. 9. Real part of dielectric constant (ε0 ) as a function of wavelength for both thin film samples.
(15)
Prich ¼ fðCNPb Þðat:%PbÞ þ ðCNSn Þðat:%SnÞg Ehp ¼ fðCNPb Þðat:%PbÞðBSe Pb Þ þ ðCNSn Þðat:%SnÞðBSe � fðCNPb Þðat%PbÞ þ ðCNSn Þðat:%SnÞg
Sn Þg
(16)
The mean bond energy per atom of the remaining matrix can be estimated from the following relation: < Erm > ¼ f2ð0:5 < CN >
Prich Þ * ðBSe
Se Þg =
< CN > ¼ 1:216eV
(17)
where
Fig. 10. Imaginary part of dielectric constant (ε") as a function of wavelength for both thin film samples. Table 1 Values of absorption coefficient (α), refractive index (n), extinction coefficient (k), dielectric constant (ε0 ), dielectric loss (ε"), optical band gap (Eg) for Se90Pb10xSnx (x ¼ 0,2) thin film. . X
λ (nm)
α(104cm 1)
n
k
ε0
ε"
Eg(eV)
0 2
583.7 581.5
4.21 5.55
5.15 2.61
0.20 0.26
26.47 6.72
2.01 1.34
2.1 2.2
distribution of the possible chemical bonds ratio was calculated and listed along with the cohesive energy in Table 2. From this table, it is obvious that the cohesive energy slightly increases with the addition of Sn to Se–Pb glass which is why the band gap slightly increases. According to Tichy et al. [29], the mean bond energy
Table 3 The average coordination number
5
x
Prich
Erm (eV)
Ehp (eV)
Ecl (eV)
0 2
2.2 2.2
0.4 0.4
1.216 1.216
3.270 3.474
1.308 1.390
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states at the mobility gap, and is reflected as higher absorption values for Se90Pb8Sn2 than that of Se90Pb10 and allow the use of ternary Se–Pb–Sn films as optical record material which required high absorption.
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