Electronic structure and properties of Cu_2-xS thin films: Dependence of phase structures and free-hole concentrations

Cu_2-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 Cu_2-xS with varying Cu stoichiometries are surprisingly complex, and consensus on these aspects are still lacking. In this work, p-type polycrystalline Cu_2-xS thin films with different phase structures and a wide range of free-hole concentrations N_Hall (10¹⁹ ∼ 10²² cm⁻³) were grown by controlling the Cu stoichiometries. We comprehensively studied the correlation of electronic structures and optoelectrical properties of Cu_2-xS thin films with their phase structures and N_Hall. With increasing N_Hall of Cu_2-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 E_p 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 m_h^∗ increases from ∼ 0.25 to 3.7 m₀; v) the direct optical bandgap E_G^Opt slightly increases from ∼ 1.9 to ∼ 2.45 eV. Our results also reveal that the surface Fermi level of these Cu_2-xS thin films is located above the corresponding valence band maximum, likely due to the presence of surface oxidation and/or the surface defects.