Synthesis and characterization of new indium gallium selenides of the InSe-GaSe system

Abstract

The new solid solution phases crystallized in the hexagonal and tetragonal system were revealed in the GaSe-InSe system, and a new orthorhombic phase with a new type of structural blocks has been proposed. The structural features of the In1-xGaxSe solid solutions (where 0 ​≤ ​x ​≤ ​1) were studied using X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA), and Raman spectroscopy. The chemical interaction between Ga and In atoms in the hexagonal In1-xGaxSe phases was identified. The reversible phase transition from hexagonal to the tetragonal structure was found to occur in the In0.7Ga0.3Te phase. Using the Density Functional Theory (DFT) method, the electronic, optical, and vibrational properties for the Pnnm orthorhombic InGaSe2 phase were studied. Modified Becke-Johnson exchange potential and generalized gradient approximations (mBJ-GGA) were used to calculate the electronic band structure, dielectric function, and density of states (DOS) of InGaSe2, which solve the band-gap problem found within GGA approach. The InGaSe2 stoichiometric phase was found to be a direct band-gap semiconductor and the band gap of this compound can be tuned via the different stacking sequences of InSe/GaSe layers. The optical absorption edges which are observed in the visible light region make this compound promising material for photovoltaic applications. The calculated phonon spectra of the InGaSe2 revealed that the frequencies of the optic modes are close to frequencies of acoustic modes, which carry the heat flow, and in this case, the acoustic modes will be strongly scattered by low-frequency optic modes. The latter event may lead to a low lattice thermal conductivity that is important for thermoelectric applications. The phonon dispersion of this compound shows no imaginary modes, suggesting that it is dynamically stable.