Figure 1 represents the DTA thermogram of the investigated system. It was obtained for a bulk glass sample by heating 8mg using a heating rate of 20 k/min. By increasing Se content T_g shows tendency to increase Figure 1. The temperature was found to be 433 K, 445 K, and 447 K respectively.

The X-ray diffraction pattern of theGa_80−xSe_xTe₂₀ (where x = 10, 15 and 20 at %) glass composition was presented in Figure 2. It is clear that the absence of sharp diffraction lines and the presence of hump only confirm the amorphous nature of the prepared samples.

Morphological inspections performed by SEM on as-grown films have shown a flat surface without the presence of cracks agglomerates and precipitated Figure 3. SEM image has confirmed the absence of spurious phase. The morphology of thin films of different compositions has been found to be of coniform contrast. The diffraction pattern for the as-prepared thin films of different compositions is similar, indicating the amorphous nature of as-prepared thin films of different compositions. The elemental compositions of Ga_80−xSe_xTe₂₀ (where x = 10, 15 and 20 at %) chalcogenide glasses were checked by energy dispersive X-ray analysis (EDX). It is noticed that Figure 4 which clearly show that amount of selenium increase with the increase of X.

The possible bonds formed in chalcogenids system Ga_80−xSe_xTe₂₀ are Se-Ga, Se-Se, Se-Te. According to chemical bond approach (CBA) [12] combination in different type in the atoms takes place more easily rather in the atoms of same type. These bonds are formed in the sequence of decreasing bond energy until the available valence of atoms is saturated. The Ga-Se glassy system is covalent chalcogenide system. The bond energy of homopolar bonds can be estimated by The Pauling method in terms of the bond energy of homopolar bonds and the electronegativity of the atoms involved.

The bond energy E(A-B) of hetero nuclear bond, can be calculated by Equation [13]

E A − B = ( E A − A − E B − B ) 1 / 2 + 30 ( x A − x B ) 2 (1)

E_A−A and E_B−B are the bonds energies of homo nuclear bonds, x_A and x_B are electronegativity values of A and B Elements.

The bond energy of the homo polar bonds E_Ga−Ga is 31.82, E_Se−Se is 44, and E_Te−Te is 33 Kal/mol [14] [15] .

The electronegativity value are 1.81, 2.55, and 2.10 for Ga, Se and Te respectively [13] .

By using Equation (1) values of E_Ga−Se, E_Se−Te and E_Te−Se are gives as 53.85, 44.93 and 44.18 respectively.

The bonds are formed in order of decreasing bond energy.

Ga-Se bonds having maximum energy are first followed by E_Te−Se then E_Ga−Te bonds.

As these bond energies are assumed to be addition, so cohesive energies calculated by summing the bond energies over all possible in a compound. Cohesive energies calculated as:

C E = ∑ C i E i (2)

where C_i is the probability of formation of expected bonds, and E_i is the energy of the corresponding bond present in the system.

Table 1 listed the chemical distribution of bonds, electronegativity, and cohesive energy of the sample.

It is observed that by increasing Se content leads to increasing the cohesive energy in chalcogenide system Ga_80−xSe_xTe₂₀.

Optical properties of Ga_80−xSe_xTe₂₀ (where x = 10, 15 and 20 at %) have been investigated in the wavelength (500 - 2500) nm. Figures 5(a)-(c) show reflectivity R and transmition T of investigated then films with wavelength.

The data of (Figure 5(a)) Ga₇₀Se₁₀Te₂₀ reveled reflectivity R decrees by increasing wavelength, while transimtion T is increase by increasing wavelength. It is worthy to mention that the observed edg in transmtion spectra extended over wide wavelength ranged, confirming a non-crystalline nature of the films.

Figure 5(b) and Figure 5(c) of composition Ga₆₅Se₁₅Te₂₀ and Ga₆₀Se₂₀Te₂₀ show that the dependence of R and T is non-linear, by increasing the selenium content, the transimtion increase up to 1750 nm with minimum reflection at the same wavelength. Such observed nonlinearity allows to conclude that we are dealing with more than one absorption mechanism.

The obtained spectra were used to estimate the optical band gap energies as 0.721 ev, 0.97 ev, and 1.007 ev for Ga₇₀Se₁₀Te₂₀, Ga₆₅Se₁₅Te₂₀ and Ga₆₀Se₂₀Te₂₀ respectively.

It is worthy to mention that the band gap increases by increase Se content, i.e. by increasing C.E the letter represent the average binding energy of the system.