Study of optical constants in Se- In –Pb thin films of chalcogenide glasses

Bulk glasses of Se75 In25-x Pbx (when n = 0, 4, 6, 10) systems and thin films have been prepared by the conventional melt quenching technique and by thermal evaporation technique, respectively. The optical constants (absorption coefficient (α), refractive index (n), extinction coefficient (k)) have been studied for Se-In-Pb thin films as a function of photon energy in the wavelength range (300-900 nm). Spectral dependence of transmittance (T) and reflectance (R) in the spectral range (300-900 nm) of glass Se-InPb have also been observed at various concentrations of Te and Pb. The effect of compositional variation on some optical constant has been observed and discussed in the present investigation.


INTRODUCTION
Chalcogenide glasses have been studied intensively in the last few years owing to their interesting properties and technological applications (Al-Ghamdi, 2006).They are materials used in optical and photonic devices.They have good electro-optic, thermo-optic, magneto-optic, acousto-optic properties, high refractive index (Barman and Sharma, 2013) and I R transparency (Zakery and Elliott, 2003).A good deal of information regarding the synthesis and opto-electro-structural properties of thin films of lead chalcogenides have been revealed by Kumar et al. (2005).The development of laser technology had opened up new application for narrow gap lead salts and their alloys.The polycrystalline thin films were deposited onto optically plane and chemically clean glass substrates by vacuum evaporation technique.The films were thin, uniform, smooth and tightly adherent to the substrates.Optical absorption spectroscopy, X-ray diffraction technique and current-voltage characteristics method were used to characterize the films.
Chalcogenide glasses material heating to melting is the form of a liquid and at some instance; it freezes the position of every atom by quenching.Even in this freezed position, they retain short range order and the position of the nearest neighbour remains nearly the same.These glasses are useful in the preparation of passive devices like lenses, windows, fibers (Books et al., 1999;Hoede et al., 2001), active devices like laser fiber amplifiers (Schweitzer et al., 2001) and non-linear components (Dixit and Vijaya, 2001).The present work has been carried out in order to investigate some optical properties of Se 75 In 25-x Pb x system.The effect of incorporation of lead in Se-In alloy can change the structural and optical properties of the system.4,6,10) have been prepared by melt-quenching techniques.Extra pure materials (99.99%) were weighed according to their atomic percentage and sealed in 6 cm long quartz ampoules with diameter 6 mm in the vacuum of 10 -5 torr.The sealed ampoules were kept in a furnace at 700°C and were held at the same temperature for about 10 h.The temperature of the furnace was raised slowly at a rate of 3-4°C/min.During heating, all the ampoules were constantly rocked for better homogenization of the alloys.This was achieved with the help of a *Corresponding author.E-mail: rita_awasthi2000@yahoo.co.in ceramic rod to which all the ampoules were tucked away in the furnace.Rotating the same rod after every five minutes interval of time did this.Thereafter, the obtained melts were cooled rapidly by removing the ampoules from the furnace and quenching into the ice cold water. Te quenched samples were taken out by breaking the ampoules.The amorphous nature of the glassy alloys was identified by X -Ray diffraction.Thin films of glassy alloys of thickness (~300 nm) were prepared by thermal evaporation technique in which substrate was kept at room temperature at a base pressure of 10 -4 torr using the molybdenum boat; the film was kept inside the deposition chamber for 24 h to achieve the stable equilibrium (Ogusk et al., 2004).The thickness of the film was measured by a single crystal thickness monitor.In order to measure the optical absorption reflection, transmission as a function of wavelength (300-900 nm), a double UV/VIS/NIR computerized spectrophotometer (JASCO V-500) has been used.

RESULTS AND DISCUSSION
The value of optical absorption coefficient () as a function of incident photon (hv) for deposited thin films of Se 75 In 25-x Pb x where x= 0, 4, 6, 10 at room temperature are shown in Table 1.The absorption coefficient () has been obtained directly from the absorbance against wavelength curves using the relation Where OD is the optical density measured at a given layer thickness (t).It has been observed in Table 1 that the value of absorption coefficient () increases linearly with the increase in photo energy of Se 75 In 25-x Pb x .According to Mott and Davis (1979), the absorption of light by amorphous solids depends on the energies of the incident photon and on the optical band gap of the material.Thus the behavior may be represented by the following equation.
Where B is a constant which depends on the transition probability, hv is energy of incident photon, Eg is optical band gap of the material, r is an index which depends on the nature of transition (Hasegawa et al., 1978) where r = 1/2 and r = 2 for allowed direct transition and indirect transition, respectively.The present system obeys the rule of indirect transition and the relation between optical gap, optical coefficient (), and the energy h of the incident photon is given by Where  obeys -Urbach relation (Urbach, 1953) Where A is a constant of the order of unity,  0 is the constant corresponding to the lowest excitonic frequency.The variation of (h) 1/2 with photon energy h for Se 75 -In 25 -x Pb x films are shown in Figure 1.The value of indirect optical band gap (Eg) has been calculated by taking the intercept on the x-axis.The calculated values of Eg are given in Table 2.It is evident from the table that the value of optical band (Eg) increases from 1.24 to 1.32 with increasing Pb content in Se 75 In 25 -x Pb x .The increase in the Eg with increasing Pb may be due to increase in grain size, the reduction in the disorder, and decrease in density of defect states.
The value of Eg decreases from 1.32 to 1.25 with increasing Pb concentration in Se 25 In 25 -x Pb X sample, since the optical absorption depends on the short range order in the amorphous state and defects associated with it.The decrease in the optical band gap in the present system may be due to reduction of the amount of disorder in the system and increase in the density of defect state.It may be due to (Borgogro et al., 1982) the shift in Fermi level whose position is determined by the distribution of electrons over the localized states (Miller et al., 1988).The values of the variation of (h) 1/2 with photon energy (hv) for Se 75 In 25 -x Pb x films are given in Table 3.The variation of reflectance (R) in percentage for the present system of Se 75 In 25-x Pb x is shown in Table 4.The table shows that the reflectance (R) increases with the increase in wavelength.Similarly, the transmittance (T) has also been shown for Se 75 In 25-x Pb x in Table 3.It is evident from the table that the percentage in the transmission increases with respect to wavelength.It is clear from the table that the value of the transmission Table 3. Variation of ( h) 1/2 with photon energy in a -se75 In25-x Pbx thin films at room temperature.

S/N
Photon  starts decreasing after about 600 nm (Table 5).The refractive index (n) and extinction coefficient (k) have been calculated by using the theory of reflectivity of light for the above sample discussed.According to this theory, the reflectance of light from this film can be expressed in turn of fresnel's coefficient.The reflectivity (Scott et al., 1982) on a interface can be given by: R= Where  is the wavelength The spectral dependence of refractive index (n) and extinction coefficient (k) for Se 75 In 25 -x Pb x thin films are shown in Figures 2 and 3, respectively.The value of refractive index (n) and the value of extinction coefficient (k) increases with increasing photon energy conclusion.

Conclusion
In this paper we systematically studied the chemical modification of Se 75 In 25 -x Pb x glass system incorporated with Pb.It was found in the optical investigations that the optical band gap increases up to 4% of Pb concentration in Se75 In25-x Pbx; with further increase of Pb content, the optical band gap decreases in the present system.The increase in the optical band gap with increasing Pb content may be due to the increase in the grain size, the reduction in the disorder and decrease in the density of defect states.The value of optical band gap (Eg) decreases at higher Pb concentration.The decrease in the optical band gap in the present system may be due to reduction in the amount of disorder in the system and increase in the density of defect states.From the reflectance and transmittance studies of the film of Se 75 In 25 -x Pb x , it may be concluded that the refractive index (n) decreases, while the value of the extinction coefficient k increases with photon energy.

Figure 1 .
Figure 1.Variation of ( h) 1/2 with photon energy in a Se75 In25-x Pbx thin films at room temperature.

Figure 2 .
Figure 2. Variation of refractive index (n) with photon energy in a Se75In25-x Pbx thin films at room temperature.

Figure 3 .
Figure 3. Variation of extinction coefficient (K) with photon energy in a Se75In25-x Pbx thin films at room temperature.

Table 1 .
Variation of Absorption coefficient () with photon energy in a se75 In25-x Pbx thin films at room temperature.

Table 2 .
Optical parameter in Se75In25-x PbX thin film at 700 nm at RT.

Table 4 .
Variation of reflection (R) in % with wavelength in a-se75 In25-x Pbx thin films at room temperature.

Table 5 .
Variation of transmission (T) in % with wave length in a se75In25-x Pbx thin films at room temperature.