Schorl tourmaline thin films deposited by electron beam evaporation and RF magnetron sputtering: Structural, chemical and photoconductive properties

Thin films of schorl tourmaline, a complex iron-rich borosilicate mineral, were deposited using electron beam evaporation (E-Beam) and RF magnetron sputtering in order to evaluate their structural, morphological, chemical, and electrical characteristics. SEM and XRD analyses revealed that E-Beam deposition produced highly discontinuous amorphous films composed of isolated nanoparticles, whereas sputtering resulted in dense and continuous amorphous coatings. EDS measurements showed partial congruent evaporation of Fe-, Mg-, Mn-, Al-, Na-, and B-containing species in E-Beam films, while sputtered samples exhibited strong Cu incorporation originating from erosion of the backing plate due to non-conformal target coverage. Electrical characterization demonstrated that E-Beam films exhibited low dark currents and modest photoconductive response, consistent with their island-like microstructure and limited charge percolation. In contrast, sputtered films showed dark currents up to three orders of magnitude higher and a pronounced photocurrent enhancement under halogen illumination, indicating bulk-limited transport and efficient photon-driven carrier generation. The results demonstrate that microstructural continuity more than chemical congruence is the dominant factor governing the functional behavior of schorl thin films. Overall, RF magnetron sputtering proved to be the most effective method for producing electrically active schorl-based coatings, revealing the technological potential of natural tourmaline as a low-cost precursor for optoelectronic and sensing applications.