Thermal Stability of Black Aluminum Films and Growth of Aluminum Nanowires from Mechanical Defects on the Film Surface during Annealing

The thermal stability of black aluminum (Black Al) films deposited on unheated fused silica substrates by pulsed DC magnetron sputtering is investigated. The deposited films have porous moth-eye morphology and a relatively soft surface that can be easily damaged mechanically. Photothermally stimulated exoelectron emission (PTSE) spectra of the films have emission peaks at 180 and 300 degrees C, which can be associated with processes of aluminum recrystallization occurring at these temperatures. The total emitted charge of PTSE decreases according to a power law with each subsequent heating, indicating that the most pronounced structural changes on the film surface occur during its first annealing. The shape of the mechanically intact pores of the films does not change after heating. However, spiral-shaped aluminum nanowires grow from scratches and abrasions on the film surface already after heating up to 200 degrees C due to the stress-induced migration of aluminum atoms. When scratches are deliberately applied to the surface of an already annealed film, the nanowires do not grow when the film is reheated. The results of this study show that mechanically intact as-deposited Black Al films are thermally stable up to temperatures of at least 400 degrees C.