Figure 1 and Table 1 show the results of characterization by EDS of the Cu(In_1−xGa_x)₃Se₅ films.

The good stœchiometry is well observed in our samples and its magnitude deviation Δy is slight; so, our samples present a nearly perfect stœchiometry (Δy = 0) [3] . The composition x in gallium for the studied samples, from EDS measurements, is very close to the starting composition of the elements in the sealed tube. The CuIn₃Se₅ samples show a conductivity of type n and the other samples (x ≠ 0) are all of type p.

The X-Ray diffraction spectra of different samples Cu(In_1−xGa_x)₃Se₅ are illustrated in Figure 2(a). Our samples are of good crystalline quality and all excitation peaks in the resulting spectra are the same as those found in references [4] [5] [6] [7] . Thus, X-Ray diffraction patterns show the presence of many preferential orientations according to the planes (112), (220) and (312) of all the samples. Also, it shows a linear shifting of peaks towards the higher magnitudes of 2θ when the x composition increases.

The (112) peak position as function of the composition x (0 ≤ x ≤ 1) is shown on Figure 2(b) and it may be written under the following linear form:

2 θ = 26.84 + 1.2 x (1)

The compounds Cu(In_1−xGa_x)₃Se₅ do not have a chalcopyrite structure as for Cu(In_1−xGa_x)Se₂ since the characteristic peaks concerning the chalcopyrite

structure could not be observed. These compounds can be of stannite structure [6] [7] [8] or an Ordered Vacancy Chalcopyrite structure (OVC) or Ordered Defect Chalcopyrite Structure (ODC).

The lattice parameters a and c for Cu(In_1−xGa_x)₃Se₅ given in Table 2, have been calculated from our spectra. These values decrease when the Gallium concentration increases [9] [10] [11] [12] . They vary between: a = 5.76, c = 11.2 for the CuIn₃Se₅ and a = 5.49, c = 10.93 for the CuGa₃Se₅; i.e. the Gallium atom is smaller than the Indium one. Our values obtained for a and c are in good agreement with those reported in the literature [9] [10] [11] [12] .

Zott et al. [13] [14] and Orsal et al. [15] have studied the gap variation of CuInSe₂ and CuGaSe₂ as a function of the ratio Cu/In and Cu/Ga respectively; they have observed an increase of the gap value with the concentration of Indium or Gallium. The compounds Cu(In_1−xGa_x)₃Se₅ are equivalent to the compounds Cu(In_1−xGa_x)Se₂ with an excess of Indium or Gallium according to the composition x. Then, the Cu(In_1−xGa_x)₃Se₅ compounds shows a gap value higher than that of Cu(In_1−xGa_x)Se₂ for each value of the composition x.

The different Photoluminescence response spectra of the alloy Cu(In_1−xGa_x)₃Se₅ (0 ≤ x ≤ 1) at liquid helium temperature (4.2 K) are presented in Figure 3(a). For x = 0 we have been able to detect the exciton position which allows us to specify the exact position of the band gap. The arrows pointing upward indicate the gap values of literature [10] [12] , and the arrows pointing downward indicate our values (for x = 1 and x = 0). These spectra shift towards higher energies when gallium concentration increases. Each of the spectra is formed by one large peak, which is probably caused by the impurity band transitions. Figure 3(b) presents the PL dominant peak variation as function of the composition x. We notice that our samples have band gap energy values that are well conform to those of literature [10] [12] .

We have determined the band gap energy value at room-temperature by analyzing

our compounds using spectral Photoconductivity [16] .

Figure 4(a) illustrates the different spectra of Photoconductivity ((αhν)² as a function hν) of the alloy Cu(In_1−xGa_x)₃Se₅. These spectra denote high speeds of surface recombination. We do not observe a saturation level at high energy. In these cases, the gap value is given by an approximate value by taking the abscissa of each curve at PC_max/2. We have observed a variation of band gap energy as a function of the composition x at room temperature, the band gap value varies from 1.23 eV [6] [7] [12] for x = 0 to 1.85 eV for x = 1, these values match those found by Photoluminescence and in literature [4] [6] [7] [10] . Figure 4(b) shows the variations of band gap energy at 300 K as a function of the composition x. These variations can be expressed by the following equation:

E g ( x ) [ e V ] = 1.22 + 0.42 x + 0.21 x 2 (2)

Spectroscopic Ellipsometry (SE) is a characterization technique which allows the

determination of energy levels above the gap in the band scheme. The real part of the dielectric pseudo-function 〈 ε r 〉 is related to the substrate densities of state D n . The calculation of the 3^rd order derivative of the quantity E 2 ε r ( E ) [17] , is given by:

D n ( E ) = 1 E 2 d 3 d E 3 [ E 2 ε r ( E ) ]

where E = ℏ ω is the photon energy, allows the determination of the energy levels of optical transitions that correspond to the peaks of D n . In order to illustrate this fact, a harmonic oscillator model, centred at the frequency ω 0 is used. Its’ Lorentz nature is characterized by an elastic amplitude α e and an absorption one α a :

α e = ω 0 2 − ω 2 ( ω 0 2 − ω 2 ) 2 + r 2 ω 2

α a = r 2 ω 2 ( ω 0 2 − ω 2 ) 2 + r 2 ω 2

where r is the damping constant.

The Cu(In_1−xGa_x)₃Se₅ bulk compounds were characterized by Spectroscopic Ellipsometry, and the observed transitions were determined by the method described above. Figure 5 illustrates the evolution of the imaginary part ( ε i ) and

the real one ( ε r ) for the dielectric function in terms of the composition of Cu(In_1−xGa_x)₃Se₅ alloy for different values of x (x = 0, 0.21, 0.36, 0.5, 0.64, 0.78 and 1). For the sake of clarity of spectra, each spectrum was shifted by the quantity given in parenthesis on the left.

The spectrum of the imaginary part of dielectric function was observed to shift towards higher energies as gallium concentration increases.

The method described above has been applied to the case of Cu(In_1−xGa_x)₃Se₅ bulks. Figure 6 shows the results of the spectra at 3^rd and 4^th order derivatives of ε i for a sample of CuIn₃Se₅.and CuGa₃Se₅.

Zeaiter et al. [18] were studied the stœchiometry effect on the dielectric functions. They found that the imaginary dielectric function ( ε i ) of CuInSe₂ evolves for different values of Cu/In ratio. In fact, the peaks widen when the percentage of indium is increased, because the dielectric function is directly related to the crystal structure. Since the crystal is rich in In, involves a disorder in the crystal lattice and a loss of crystallinity which proved more significantly for high energies. This effect was observed in the case of silicon [19] [20] .

Therefore, the implementation has the effect of widening the peaks and simultaneously decreasing the intensity of the peaks of the imaginary dielectric function (ε_i). The same effects were noticed on the spectra of Cu(In_1−xGa_x)₃Se₅ compounds [21] . Cu(In_1−xGa_x)₃Se₅ compounds are like CuIn_1−xGa_xSe₂ compounds.

Table 3 shows the observed transitions values (E₁, E₂ and E₃) for different samples of the Cu(In_1−xGa_x)₃Se₅ (0 £ x £ 1) alloy. Figure 7 illustrates the variation of these transitions as a function of the initial composition x. Parabolic variations are obtained for E₁, E₂ and E₃ (as a function of the composition x). The equations for the fitted curves are:

E 1 = 2.76 + 0.29 x − 0.06 x 2 E 2 = 3.73 + 0.27 x − 0.146 x 2 E 3 = 4.64 + 0.245 x − 0.033 x 2

The transitions E₁ and E₃ can be attributed to the ionization of levels d of Cu. In contrary, the transition E₂ is related to the hybridization s-s between the s-levels of indium and that of Cu.

The highest levels of the valence band are essentially composed of the 4p-levels of Se and the localized strong density d-levels of Cu. Thus, the valence band is nearly independent of the variation of composition.

The lowest levels of the conduction band are essentially composed of s-levels for copper and indium. Gallium is specified by a 4 s band with higher energy than that of 3 s of indium. When a gallium atom replaces an indium atom, one assumes globally that the levels related to selenium and copper are unchanged. Conversely, the levels corresponding to the conduction band are shifted towards higher energies. Thus, the gap increase as the composition of gallium increases.