Sample preparation induced phase transitions in solution deposited copper selenide thin films

Thin films of CuSe were deposited onto GaAs substrate. XRD showed that the as-deposited films were of the Klockmannite (CuSe – P6(3)/mmc 194) phase with lattice parameters a(0) = b(0) = 0.3939 nm, c(0) = 1.7250 nm; however, electron diffraction in the TEM surprisingly indicated the β-Cu(2−x)Se phase (Cu(1.95)Se – R3̄m 166) with lattice parameters a(0) = b(0) = 0.412 nm, c(0) = 2.045 nm. The discrepancy originated from the specimen preparation method, where the energy of the focused ion beam resulted in loss of selenium which drives a phase transition to β-Cu(2−x)Se in this system. The same phase transition was observed also upon thermal treatment in vacuum, as well as when the 200 keV electron beam was focused on a powder sample in the TEM. The initial phase can be controlled to some extent by changing the composition of the reactants in solution, resulting in thin films of the cubic α-Cu(2−x)Se (Cu(1.95)Se – Fm3̄m) phase co-existing together with the β-Cu(2−x)Se phase.