Dynamics of evaporation from CuGaSe 2 targets in pulsed electron deposition technique

Author： Pattini F. Bronzoni M. Mezzadri F. Bissoli F. Gilioli E. Rampino S.

ISSN： 0022-3727

Source： Journal of Physics D: Applied Physics, Vol.46, Iss.24, 2013-06, pp. : 245101-245107

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Abstract

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 ^pθ (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 × 10⁸ 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.