Structural and Optical Properties in thin-lms of (GeGa2)100-x(Sb2Ga3)x (x = 15, 30, 45, 60) Alloys

Investigation of structural morphology and optical parameters of non-oxide (GeGa 2 ) 100-x (Sb 2 Ga 3 ) x (x = 15, 30, 45, 60) glass alloys have been studied in the present paper. Thin films of prepared bulk samples by melt quenching technique were obtained using thermal evaporation method. XRD, DSC and SEM were used to investigate the structural morphology and thermal properties of materials. The linear and non-linear optical properties of prepared samples have been studied for optoelectronic applications. FTIR transmittance spectra defines the impurities present in the prepared glassy alloys. It was observed that energy bandgap (E g ) obtain by Tauc’s plot varies from 2.9 eV to 1.25 eV. The calculated value of k, E g decreases whereas the α and Urbach energy increases with increasing doping Sb concentration. The decrease in energy bandgap is discussed on the basis of Mott and Davis model.


Introduction
The non-oxide glasses are appealing materials for various applications like distribution of thermal images,infrared power, all such optical Raman amplification, optical switching limiting, etc [1]. All these glasses are extremely photosensitive and mechanisms of photosensitivity is due to changes impacting local materials and these materials are complex to transforming its thermal, photochemical and photo-structural, electrical, electro-optic phenomena. The function of eachkind of mechanism depends on various functions such as the sample preparation process, excitation wavelengths and so on. The most important photoinduced alterations of glasses are shown in physical thickness by chalcogenide (volume modifications such as photo-expansion or photo-contraction), photo-darkening, (PD) (red shift in the gap in the band) and changes in photoinduced the band state of phase [2]. Such effects are preferred in glasses of chalcogenide due to structural flexibility with elevated lone-pair (p states) in the valence band [3]. In amorphous glasses, the photoinduced effects of glasses are becoming an important issue for investigation because of itspotential use for optoelectronics of such effects (photoresists, memory optics, optoelectronic circuits, etc.) [4][5][6].
Stated by A.S Tverjanovicha et al in Ga-Ge-S, energy exposure is greater than that of band gap resulting in the substitution of S by Se leads to photoinduced bleaching and concerning photo-induced darkening. Exposure to energy, Ge20Se72.5Bi7.5 films contributes to a decrease in the optical values of the bandgap [7]. The Ga5Sb10Ge25Se60 glass is very stable for crystallization and has a high temperature transition glass, making it a suitable candidate for the development and manufacturing of IR optical devices [8]. The transformation phase of the Ge-Sb-S non-oxide glasses has recently been reported [9]. Ge-Sb-S glassy films are thereforeof particular interest because those who have received a remarkable application also as antioxidants over cladding layers of micro-resonator-based sensing [10][11].
Several other glasses compositions such as Ge11.5As24Se64.5, Ge10As35Se55 and Ge-As-Se have also been shown to significantly boost properties such as high nonlinearity and photo stability [4]. However, Arsenic is toxic and environmentally un-recognized when As-based devices are disregarded, Antimony is an acceptable element. It has been found that the Sb transmission line can strengthen the optical nonlinearity of the glasses due to the more ionic existence of Sb [12]. In this work, we focused on Ga and Sb-doped (GeGa2)100-x(Sb2Ga3)x (x = 15, 30, 45, 60) glass alloys system designed to discover for glass both with lower Tg and higher Tc, which is important for both the active and passive optical fibre drawing. The compositional dependence of parameters such as Tg, Tc, and the optical band gap was investigated. The thermal and optical effects of the addition of Sb to the (GeGa2)100-x(Sb2Ga3)x (x = 15, 30, 45, 60) have been comprehensively analyzed by means of DSC, UV-VIS and optical spectra of FTIR.

Experimental
These glasses were processed through the melting of compounds of strong purity (5 N) elements of Ge, Ga, and Sb in the evacuated elements (<10 5 mbar) and silica tubes flame-sealed in a rocking furnace. The mixture was molted in a muffle furnace for 12 h at 850 °C. The tubes were retained for a further 1h at this temperature. After that, the molten tubes were quenched in ice-cooled water after freezing and formed homogenization. To achieve strong homogenization of the glasses were the oven was rocked slowly during the time the ampoule was at high temperatures, producing glass with uniform compositions [13].
Study on photo-induced development in optical properties was carried out on thermal evaporated techniquewhich produces thin films have vacuum at pressure of 10 6 mbar. All the thin films have thickness 500 nm and were deposited at the rate of deposition controlled at 5MHz using a quartz crystal monitor deposited on glass substrate. First X-ray diffractometer measurements were analyzed for all the bulk samples. X'Pert Pro of Cu-ka (1.54056 Å) emits light radiation with such a 0.2 ° step in the scanning range of 10° to 80° here. Using the technique of differential scanning calorimetry, DSC thermograms were collected (SHIMADZU DSC-60 Plus) of the as prepared samples. All observations were measured at room temperature. Using the ultraviolet-visible-near-infrared (UV-VIS) spectrophotometer (SHIMADZU UV-2600) in the 400-1000nm range, the absorbance and transmittance spectra of the samples were reported. In the range (400-5500 cm -1 ) of samples, the IR transmission spectrum was obtained.

Structural properties
The measurement of the X-Ray diffraction as prepared (GeGa2)  All values of Tg and Tc compute from the DSC thermograms are shown in the table1 at a heating rate of 20 K/ min for the studied sample. Table 1 shows that the glass alloys (GeGa2)100x(Sb2S3)x, the glass transition temperature has a calculated maximum value for x= 60. Tg is observed to increases as the concentration of doping of x increases (for x = 15 to x = 60). It can therefore be observed that in the prepared sample, Sb performs a dual behaviour. The variability in temperature of the glass transition temperature could be attributed to the bond energy difference shown in Table 2.An significant parameter for analyzing glasses is thermal stability.

Thermal property
To evaluate the thermal stability of these glasses, multiple parameters are used. The two most popular parameters differences between the value Tg and Tc and the second criterion are the Hruby parameter, which gives the ability to forming glass [13]. The greater difference between the computed value of Tc and Tg suggests that the crystallization kinetic resistance is higher or vice versa.These glasses exhibit the crystallization peak near the temperature of the glass transition, they are considered unstable glasses, whereas glasses with a peak near the temperature of melting are considered stable glasses.Since the greater difference between Tc and Tg is x = 60, suggesting the limit of the K parameter. Therefore, (GeGa2)40(Sb2S3)60 has optimum thermal stability and, therefore, the ability to form large glass.

Absorption coefficient and optical band gap analysis
Theabsorption coefficient (α) and transmission (%T) which are recorded from UV-Vis spectrophotometer for all thin film samples (GeGa2)100-x(Sb2Ga3)x (x = 15, 30, 45, 60). The recorded transmittance% spectra of the as prepared thin film of (GeGa2)100-x(Sb2Ga3)x (x = 15, 30, 45, 60) is shown in Fig 4. Due to the doping of Sb constituent the transmission % of thin film is reduced. It is also possible to investigate the electronic characteristics of the material by optimizingessential parameters for electronic transitions, widely used optical absorption measurement procedure. In order to obtain the absorption coefficient (α), UV-Vis spectrum of absorption of the thin films as prepared has been recorded, recognizing that the band gap couldbe calculated by well-known Tauc plot model [14] The coefficient of absorption (α) is followed by [15][16][17] Absorption coefficient (α) = 2.303 x where, Eg is the distance of the optical band gap, ν defines the frequency of emitted photon, h is the constant of Plank, and A is a constant normally associated with the parameter of edge width reflecting thin film quality. The transition from n with a value of 1/2, 2, 3 and 3/2 defines the direct allowed transition, the indirect allowed transition, indirect forbidden transition and the direct forbidden transition respectively. In the current case the best match for n= 1/2 is at λ = 500 nm. So, there is a clear direct allowed transition in the studied sample.
In order to find out the value for Eg along with the Tauc map, the linear region of such curves is interpreted as the photon energy axis (hv), where it cuts the axis of energy which provides optical band gap for the materials. The band gap for (GeGa2)100-x(Sb2Ga3)x (x=15, 30, 45, 60) is given in Table 3. It's clearly visible where the optical band gap decreases as Sb concentration increases. A well-known Mott and Davis [18] method where the density of defect states could determine the significant decrease of the band gap with increased the concentration of Sb. Such deficiencies are responsible for the localized states in the bandgap of the amorphous semiconductors. As concentration of Sb is increased, there are a more number of defect states that cause the material band gap to decrease. The present finding is in good accordance with the findings previously stated by other researchers [19][20][21][22][23][24][25][26][27].
There are three distinct regions of amorphous materials for optical spectra of absorption.

II.
Defects and impurities are present in this area. There is low energy of absorption is not determinedin this research work.
III. α ≥10 4 cm −1 is a region of higher absorption where Eg takes place in between the conduction band and the valance band.
IV. α = 10 2 -10 4 cm −1 , describing the Urbach exponential tail. According to the expression area of absorption depends exponentially on photon energy.

α∼exp[A(hν−hνo)]/KT
Where A = constant having the unity order, and vo is the lowest frequency of excitonic.
The empirical correlation Urbach's, its band tail width from each localized states (Eu) close to the band edges could be determined as where, α0 is a constant while Eu stand for energy of Urbach. Fig.7. shows the synthesized thin film which describes the variation between ln(α) and photon energy (hν).
The reciprocal of the slopes of linear region of the graph ln (α) versus the photon-energy (hv) graph was evaluated as Urbach energy (Eu). Urbach energy (Eu) represents the enhancement of the absorption edge.
The optical band gap (Eg) is accepted to decrease when the energy of Urbach (Eu) increases significantly with the high doping concentration of Sb which is shown by Fig 8. The decrease in band gap with the higher doping concentration of Sb may be due to an increase in grain sizes and a decrease in thin film structural impairment.

Extinction coefficient analysis
The extinction coefficient is a fundamental optical parameter. Itmeasures the behavior of decrease in the emitting light due to absorption coefficient and attenuation and can be calculated using the accompanying formula [28] k = 4 where α states absorption coefficient of thin film and λ provides the data incident wavelength in the thin films. The incident wavelength (λ) vs extinction coefficient (k) shows in Fig 9.