Dynamics of evaporation from CuGaSe₂targets in pulsed electron deposition technique

The evaporation mechanisms from a solid target in pulsed electron deposition (PED) technique have been investigated by analysing the chemical composition and the thickness distribution of CuGaSe 2 (CGS) films grown at different discharge voltages on glass substrates. The behaviour of the plasma plume generated from the target can be described by a linear combination of two coexisting processes: incongruent thermal evaporation and congruent ablation, which exhibit different weights depending on the PED voltage. The first component arises from the thermodynamic liquid-to-vapour transition involving the very first layers of the target surface, while the second one is due to the subsurface target penetration of the pulsed e-beam. The chemical composition of the thermally evaporated cloud, according to the CuGaSe 2 phase diagram, exhibits an incongruent Cu depletion during the solid-to-gas phase transition with respect to the target, thus forming the ordered vacancy compound CuGa 3 Se 5 , while the sublimation of ablated species is perfectly stoichiometric. The thermally evaporated plasma follows a typical surface source spatial distribution, while the expansion of the ablation products exhibit a forward-peaked angular behaviour proportional to cos p θ (4 < p < 7). The incongruent component becomes negligible by enhancing the discharge voltage, where the e-beam is able to more deeply penetrate the target, and the electron power density exceeds the threshold ablation value of 1 × 10 8 W cm −2 . The proposed mechanism for PED process is compared to other models describing the plume generation in pulsed high-energy-induced growth technique. This study represents a remarkable result to better understand and control the PED process.