J. M. Merino

Structural characterization of CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 compounds

Abstract Polycrystalline CuIn2Se3.5, CuIn3Se5 and CuIn5Se8 have been synthesized in vacuum under different cooling conditions resulting in different phases. X-ray diffraction and Rietveld refinement of the diagrams have been used to characterize the different structures obtained. The two former compounds were refined using two different types of structure: stannite (S.G.: I42m) and the proposed P–chalcopyrite (S.G.: P42c). The determined bond distances after refinements indicate a higher tetragonal distortion for stannite type structure than P–chalcopyrite structure. These results seem to indicate that the Cu–In–Se structure with these two stoichiometries belongs to the P42c S.G. On the other hand, for the CuIn5Se8 compound a far different XRD diagram was obtained where all the reflections have been assigned to two structures, named γ-phases, one trigonal (γ) and one hexagonal (γH).

COMPOSITION EFFECTS ON THE CRYSTAL STRUCTURE OF CUINSE2

By x‐ray powder diffraction and the Rietveld refinement method, the atomic positions in CuInSe2 were determined for compositions close to stoichiometry. The Se position, x(Se), was found to be correlated to the Cu content. According to a model proposed by Jaffe and Zunger [Phys. Rev. B 29, 1882 (1984)], changes in x(Se) induce a variation in the optical band gap, Eg. This could explain the spread in energy gap, Eg, found by many authors for this compound. The increase in the lattice parameter, a, correlated with the difference between the energy dispersive analysis of x rays and x‐ray diffraction determined Cu contents, suggests the presence of a fraction of Cu atoms as interstitials.

Optical constants of Cu2ZnGeS4 bulk crystals

The dielectric functions of Cu2ZnGeS4 bulk crystals grown by the Bridgman method were measured over the energy range 1.4 to 4.7 eV at room temperature using variable angle spectroscopic ellipsometry. The observed structures in the dielectric functions were adjusted using the Adachi’s model and attributed to interband transitions E0, E1A, and E1B at Γ:(000), N(A):2π/a(0.5 0.5 0.5), and T(Z):2π/a(0 0 0.5) points of the first Brillouin zone, respectively. The model parameters (threshold energy, strength, and broadening) have been determined using the simulated annealing algorithm. The decrease in the first gap, E0, has been attributed to a higher Ge–S hybridization. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived.

Dielectric functions and fundamental band gaps of Cu2In4Se7, CuGa3Se5 and CuGa5Se8 crystals

Room temperature pseudodielectric function spectra e(ω) = e1(ω) +i e2(ω) of the ordered defects compounds Cu2In4Se7, CuGa3Se5 and CuGa5Se8 have been measured by spectroscopic ellipsometry. The values of refractive index n and extinction coefficient k are given. The structures observed in e(ω) spectra have been analysed using different methods, including fitting the numerically differentiated experimental spectrum (second derivative) to analytical line shapes. As a result, the energies corresponding to the fundamental gap (E0) and higher critical points have been determined. A linear correlation of the fundamental gap values with Ga/Cu atomic ratio contents in CuGaxSey samples is deduced.

Optical properties of monocrystalline CuIn5Se8

Single crystals of CuIn5Se8 have been grown by chemical vapor transport. The crystals show a deviation from stoichiometry. The temperature dependence of their optical absorption spectra was investigated in the temperature range of 10–300K. The variation of the energy gap with temperature was studied by means of a three-parameter thermodynamic model, the Einstein model, and the Passler model. The values of the band gap at T=0K, a dimensionless constant related to the electron-phonon coupling, an effective and a cutoff phonon energy have been estimated. It was also found that the major contribution of phonons to the shift of Eg vs T in CuIn5Se8 is mainly from optical phonons. The presence of Urbach’s tail just below the band edge in the absorption spectra of CuIn3Se5 has been observed. It was shown that the static structural disorders contribute mainly to the absorption below the direct band gap.

Dielectric functions and optical constants modeling for CuIn3Se5 and CuIn5Se8

The complex dielectric functions, e(ω)=e1(ω)+ie2(ω), of CuIn3Se5 and CuIn5Se8 crystals with different Cu contents have been determined in the 0.8–4.7 eV photon energy range by using spectroscopic ellipsometry. The spectral dependence of the real, e1(ω), and imaginary, e2(ω), parts of e(ω), as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity, has been modeled by using a modification of Adachi’s model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap, E0, and to higher critical points, have been determined by using the simulated annealing algorithm.

Crystal structure of synthesized CuGaTe2 determined by X-ray powder diffraction using the Rietveld method

A full profile X-ray powder diffraction structure refinement has been carried out on a sample of synthesized CuGaTe2 using graphite monocromatized CuKα step-scan data and a profile shape of the Pearson VII type. The most satisfactory convergence was achieved at Rp = 0.0666, Rwp = 0.0884, RB = 0.0106 and RF = 0.0102. The derived structural parameters at 26.5°C are: a = 0.602348(7), c = 1.193979(2) nm and x(Te) = 0.256(6). The ratio between lattice parameters, η = c/2a = 0.9911 (0), differs from 1.0, indicating a tetragonal distortion, and non-ideal anion displacements, x(Te)≠1/4, is manifested by the existence of bond alternation of Cu-Te and Ga-Te with interatomic distances of 0.262(5) and 0.2578(5) nm, respectively. These results show a light tetrahedral deformation produced by four-fold tetrahedra of the copper cation in the CuGaTe2 chalcopyrite-type structure.

Electronic properties of correlated metals in the vicinity of a charge-order transition : Optical spectroscopy of α -(BEDT, -TTF ))2MHg( SCNn )4 (M=NH4, Rb, TI)

The infrared spectra of the quasi-two-dimensional organic conductors

Preparation and optical studies on flash evaporated Sb2S3 thin films

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Modeling the optical constants of Cu2In4Se7 and CuGa3Se5 crystals

-(BEDT-TTF)