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-P63/mmc 194) phase with lattice parameters a0 = b0 = 0.3939 nm, c0 = 1.7250 nm; however, electron diffraction in the TEM surprisingly indicated the β-Cu2-xSe phase (Cu1.95Se-R3m 166) with lattice parameters a0 = b0 = 0.412 nm, c0 = 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-Cu2-xSe 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-Cu2-xSe (Cu1.95Se-Fm3m) phase co-existing together with the β-Cu2-xSe phase.