Electronic structure and properties of Cu2-xS thin films: Dependence of phase structures and free-hole concentrations

Abstract

Cu2-xS has received increasing attentions in recent years, owing to its promising applications in a broad range of optoelectronic devices. However, the phase structures and the electronic structures of Cu2-xS with varying Cu stoichiometries are surprisingly complex, and consensus on these aspects are still lacking. In this work, p-type polycrystalline Cu2-xS thin films with different phase structures and a wide range of free-hole concentrations NHall (1019 ∼ 1022 cm−3) were grown by controlling the Cu stoichiometries. We comprehensively studied the correlation of electronic structures and optoelectrical properties of Cu2-xS thin films with their phase structures and NHall. With increasing NHall of Cu2-xS with different phase structures, it is observed that i) the hole mobility μ reduces from ∼ 6 to ∼ 1 cm2 V−1 s−1; ii) the plasma energy Ep increases from ∼ 0.3 to 1.2 eV; iii) the high-frequency dielectric constant ε∞ decreases linearly from ∼ 10 to ∼ 2.5; iv) the hole effective mass mh∗ increases from ∼ 0.25 to 3.7 m0; v) the direct optical bandgap EGOptslightly increases from ∼ 1.9 to ∼ 2.45 eV. Our results also reveal that the surface Fermi level of these Cu2-xS thin films is located above the corresponding valence band maximum, likely due to the presence of surface oxidation and/or the surface defects.