Spectroscopic Ellipsometry Study of the Dielectric Function of Cu(In_1–xGa_x)₃Se₅ Bulk Compounds: Identification of Optical Transitions

Abstract

Using Spectroscopic Ellipsometry (SE), the optical properties of Cu(In 1 − x Ga x ) 3 Se 5 bulk compounds, grown by the Bridgman method, were analyzed by varying x composition (0 ≤ x ≤ 1). Energy levels above the gap in the band scheme were determined by measuring the complex dielectric function at room-temperature for energies between 1.5 and 5.5 eV using a variable angle of incidence ellipsometer. The transitions values E 1 , E 2 and E 3 were observed above the gap for different samples of Cu(In 1 − x Ga x ) 3 Se 5 alloy. 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 increases as the composition of gallium increases. Spectroscopic Ellipsometry (SE) gave evidence for the interpretation of the choice of gap values which were compatible with that obtained from solar spectrum. Several other characterization methods like Energy Dispersive Spectrometry (EDS), hot point probe method, X-ray diffraction, Photoluminescence (PL), Optical response (Photoconductivity) were presented in this paper. The Cu(In 1 − x Ga x ) 3 Se 5 have an Ordered Vacancy Chalcopyrite-type structure with lattice constants varying as a function of the x composition. The band gap energy of Cu(In 1 − x Ga x ) 3 Se 5 compounds is found to vary from 1.23 eV to 1.85 eV as a function of x.