Sequential cation cross-substitution in zinc-blende chalcogenide semiconductors, from binary to ternary to quaternary compounds, is systematically studied using first-principles electronic structure calculations. Several universal trends are found for the ternary and two classes of quaternary chalcogenides, for example, the lowest-energy structure always has larger lattice constant $a$, smaller tetragonal distortion parameter $\ensuremath{\eta}=c/2a$, negative crystal-field splitting at the top of the valence band, and larger band gap compared to the metastable structures for common-row cation substitution. The band structure changes in the cation substitution are analyzed in terms of the band offsets and band character decomposition, showing that although the band gap decreases from binary II-VI to ternary ${\text{I-III-VI}}_{2}$ are mostly due to the $p\ensuremath{-}d$ repulsion in the valence band, the decreases from ternary ${\text{I-III-VI}}_{2}$ to quaternary ${\text{I}}_{2}{\text{-II-IV-VI}}_{4}$ chalcogenides are due to the downshift in the conduction band caused by the wave-function localization on the group IV cation site. We propose that common-row-cation ${\text{I}}_{2}{\text{-II-IV-VI}}_{4}$ compounds are more stable in the kesterite structure, whereas the widely assumed stannite structure reported in the literature for experimental samples is most likely due to partial disorder in the I-II (001) layer of the kesterite phase.

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Electronic structure and stability of quaternary chalcogenide semiconductors derived from cation cross-substitution of II-VI and I-III-VI2 compounds

Optical constants of polycrystalline thin film CuIn1−xGaxSe2 alloys with Ga/(Ga+In) ratios from 0 to 1 have been determined by spectroscopic ellipsometry over an energy range of 0.75–4.6 eV. CuIn1−xGaxSe2 films were deposited by simultaneous thermal evaporation of elemental copper, indium, gallium and selenium. X-ray diffraction measurements show that the CuIn1−xGaxSe2 films are single phase. Due to their high surface roughness, the films are generally not suitable for ellipsometer measurements. A method is presented in which spectroscopic ellipsometer measurements were carried out on the reverse side of the CuIn1−xGaxSe2 films immediately after peeling them from Mo-coated soda lime glass substrates. A detailed description of multilayer optical modeling of ellipsometric data, generic to ternary chalcopyrite films, is presented. Accurate values of the refractive index and extinction coefficient were obtained and the effects of varying Ga concentrations on the electronic transitions are presented.

Optical characterization of CuIn1−xGaxSe2 alloy thin films by spectroscopic ellipsometry

https://physics.ucf.edu/%7Elc/TSF24417.pdf

Characterization of CdS thin films grown by chemical bath deposition using four different cadmium sources

CuInSe2 (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO3)2 and InCl3 dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absor...

CuInSe2 (CIS) Thin Film Solar Cells by Direct Coating and Selenization of Solution Precursors

Composition, structure and optical properties of sputtered thin films of CuInSe2

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of CuInSe2 and CuGaSe2 were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free CuInSe2 giving an exciton binding energy of 7.5 meV. The band gap energy of both CuInSe2 and CuGaSe2 was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich (Cu/In<0.88) CuInSe2 thin films was explained by plasma screening of Coulomb interactions. A slight decrease in the band gap energy of the In-rich films was attributed to a degradation of film quality such as high-density defects, grains, and structural disordering. The fundamental band gap energy in strained CuInSe2 and CuGaSe2 epilayers was shown to decrease due to in-plane biaxial tensile strain.

Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2

Pinless friction spot joining (FSpJ) has proven to be a competitive friction-based joining technique for fabricating metal/carbon fiber-reinforced thermoplastics hybrid joints. The present study investigated the feasibility of tool modification to improve the property of the silane-coated 6061-T6 Al alloy and carbon fiber-reinforced polyamide-6 (CF/PA6) FSpJ joint. The interfacial structure, bonding strength and fracture surface of the joints obtained by four tools, viz, old flat (OT), new flat (NT), concave-shaped (CT) and ring-shaped tools (RT) with different rotation speeds, were analyzed and compared. An experimentally validated three-dimensional finite element model was also built to help understand FSpJ process. Despite modifying the tool shape, covalent bonding predominated the interfacial bonding force where the formation of nanoscale diffusion layers was witnessed with no new type of covalent bonds generated. The resultant fracture surface was dominated by the cohesive failure mode, suggesting strong chemical bonding between the re-solidified resin and silanized Al surface. The decrease in tensile shear force in the case of NT was due to the extensive formation and coalescence of microscale voids and porosities. This was mitigated by using CT and RT tools, from which the heat input was controlled leading to a more homogenized heat field. From the combined evaluation of thermal process and mechanical characteristics, CT tool was recommended for the FSpJ of Al and CF/PA6 because it offered a more uniform temperature distribution and improved macro-mechanical interlocking at the interface with good wetting conditions, sufficient chemical reaction and effective joining area, and lower tensile residual stresses, in comparison with other tools, and hence showed the highest shear force. 

K. Murakami

Papers

Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2

Improving bonding strength of Al/CFRTP hybrid joint through modifying friction spot joining tools