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-Cu_2−xSe phase (Cu_1.95Se – R$\bar{3}$m 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-Cu_2−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-Cu_2−xSe (Cu_1.95Se – Fm$\bar{3}$m) phase co-existing together with the β-Cu_2−xSe phase.

Ion beam irradiation causes Klockmannite CuSe to lose Se and transform into β-Cu₂Se. Caution must be taken when using the dual beam FIB for preparing TEM specimen.