Dynamics of evaporation from CuGaSe2 targets 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 CuGaSe2 (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 CuGaSe2 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 CuGa3Se5, 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) theta (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 x 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.