Ab-initio study of the bandgap engineering of Al(1-x)Ga(x)N for optoelectronic applications

A theoretical study of Al(1-x)Ga(x)N, based on full-potential linearized augmented plane wave method, is used to investigate the variations in the bandgap, optical properties and non-linear behavior of the compound with the variation of Ga concentration. It is found that the bandgap decreases with the increase of Ga in Al(1-x)Ga(x)N. A maximum value of 5.5 eV is determined for the bandgap of pure AlN which reaches to minimum value of 3.0 eV when Al is completely replaced by Ga. The static index of refraction and dielectric constant decreases with the increase in bandgap of the material, assigning a high index of refraction to pure GaN when compared to pure AlN. The refractive index drops below 1 for photon energies larger than 14 eV results group velocity of the incident radiation higher than the vacuum velocity of light. This astonishing result shows that at higher energies the optical properties of the material shifts from linear to non-linear. Furthermore, frequency dependent reflectivity and absorption coefficients show that peak value of the absorption coefficient and reflectivity shifts towards lower energy in the UV spectrum with the increase in Ga concentration. This comprehensive theoretical study of the optoelectronic properties of the alloys is presented for the first time which predicts that the material can be effectively used in the optical devices working in the visible and UV spectrum.Comment: 18 pages, 7 figure

Structure-dependent optoelectronic properties of perylene, di-indenoperylene (DIP) isolated molecule and DIP molecular crystal

Theoretical simulations were designed by first principles approach of density functional theory to investigate the structural and optoelectronic properties of different structural classes of perylene; isolated perylene, diindeno[1,2,3-cd:1',2',3'-lm]perylene (DIP) molecule and DIP molecular crystal. The presence of molecular interactions in DIP crystal proved its structure-dependent behaviours. The herringbone molecular arrangement of DIP crystal has influenced the electronic properties by triggering the intermolecular interactions that reduced the energy gaps between HOMO and LUMO of the crystal. Strong hybridization resulting from dense charges population near zero Fermi energy has pushed valence band maxima in the density of states of all perylene structures to higher energies. Under small energy input, charges are transferred continuously as observed in the spectra of conductivity and dielectric. The existence of strong absorption intensities are consistent with the former works and supported by the obtained polarized reflectivity and loss spectra

Ab initio study of structural parameters and gap bowing in zinc-blende AlxGa1−xN and AlxIn1−xN alloys

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Electronic structure and mechanical properties of ternary ZrTaN alloys studied by ab initio calculations and thin-film growth experiments

The structure, phase stability, and mechanical properties of ternary alloys of the Zr-Ta-N system are investigated by combining thin-film growth and ab initio calculations. Zr1-xTaxN films with 0≤x≤1 were deposited by reactive magnetron cosputtering in Ar+N2 plasma discharge and their structural properties characterized by x-ray diffraction. We considered both ordered and disordered alloys, using supercells and special quasirandom structure approaches, to account for different possible metal atom distributions on the cation sublattice. Density functional theory within the generalized gradient approximation was employed to calculate the electronic structure as well as predict the evolution of the lattice parameter and key mechanical properties, including single-crystal elastic constants and polycrystalline elastic moduli, of ternary Zr1-xTaxN compounds with cubic rocksalt structure. These calculated values are compared with experimental data from thin-film measurements using Brillouin light scattering and nanoindentation tests. We also study the validity of Vegard's empirical rule and the effect of growth-dependent stresses on the lattice parameter. The thermal stability of these Zr1-xTaxN films is also studied, based on their structural and mechanical response upon vacuum annealing at 850°C for 3 h. Our findings demonstrate that Zr1-xTaxN alloys with Ta fraction 0.51≤x≤0.78 exhibit enhanced toughness, while retaining high hardness ∼30 GPa, as a result of increased valence electron concentration and phase stability tuning. Calculations performed for disordered or ordered structures both lead to the same conclusion regarding the mechanical behavior of these nitride alloys, in agreement with recent literature findings [H. Kindlund, D. G. Sangiovanni, L. Martinez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J. E. Greene, V. Chirita, and L. Hultman, APL Materials 1, 042104 (2013)10.1063/1.4822440]

Engineering the electronic band structures of novel cubic structured germanium monochalcogenides for thermoelectric applications

Germanium mono-chalcogenides have received considerable attention for being a promising replacement for the relatively toxic and expensive chalcogenides in renewable and sustainable energy applications. In this paper, we explore the potential of the recently discovered novel cubic structured (π-phase) GeS and GeSe for thermoelectric applications in the framework of density functional theory coupled with Boltzmann transport theory. To examine the modifications in their physical properties, the across composition alloying of π-GeS and π-GeSe (such as π-GeS1- xSex for x =0, 0.25, 0.50, 0.75, and 1) has been performed that has shown important effects on the electronic band structures and effective masses of charge carriers. An increase in Se composition in π-GeS1- xSex has induced a downward shift in their conduction bands, resulting in the narrowing of their energy band gaps. The thermoelectric coefficients of π-GeS1- xSex have been accordingly influenced by the evolution of the electronic band structures and effective masses of charge carriers. π-GeS1- xSex features sufficiently larger values of Seebeck coefficients, power factors and figures of merit (ZTs), which experience further improvement with an increase in temperature, revealing their potential for high-temperature applications. The calculated results show that ZT values equivalent to unity can be achieved for π-GeS1- xSex at appropriate n-type doping levels. Our calculations for the formation enthalpies indicate that a π-GeS1- xSex alloying system is energetically stable and could be synthesized experimentally. These intriguing characteristics make π-GeS1- xSex a promising candidate for futuristic thermoelectric applications in energy harvesting devices

Engineering of highly mismatched alloy with semiconductor and semi-metallic substituent's for photovoltaic applications

Highly mismatched alloys (HMAs) are getting a substantial interest of researchers because of holding competence of rapid change in physical properties with minor compositional change and consequently showing their potential for solar energy and photovoltaic applications. In the present density functional theory based work, we design HMAs from the extremely dissimilar GaP (semiconductor) and GaBi (semi-metal). The alloying of the two compounds with unmatched electronic characteristics has triggered a rapid reduction in the energy gap of GaPBi. The energy gap is reduced by 39.3 meV for every 1% increase in Bi composition. The semiconductor behavior of GaPBi based HMAs is found to be transformed to semi-metallic for replacing 64.6% of P atoms by Bi. Unlike the conventional alloys, the variation in the electronic energy gap of GaP1-xBix shows deviation from the Vegard's formalism. Where the optical properties are strongly influenced with the narrowing energy gap of GaPBi. For the Bi-rich GaPBi, the notable red shift is observed in optical dielectric function and absorption spectra. Moreover, the larger atomic size of Bi has enhanced the lattice parameters of Bi-rich GaPBi. The GaPBi based HMAs with tunable energy gap in the span of 2.51 eV–0 eV and the interesting optical properties highlight them prospective materials for optoelectronic applications