Nano-structured morphological features of pulsed direct current magnetron sputtered Mo films for photovoltaic applications

Historically, molybdenum thin films have been used as the back contact for Cu(In,Ga)Se₂ based solar cells and as such the properties of these layers play an important role in the overall cell structure. This paper describes the production of molybdenum films using pulsed d.c. magnetron sputtering from compressed molybdenum powder targets. The films were deposited at different substrate temperatures under constant power and constant current modes, and analysed using X-ray diffraction, scanning electron microscopy, atomic force microscopy and four point resistance probe. Mechanical strain and resistivity were found to decrease with substrate temperature together with a shift in the (110) crystallographic plane towards higher diffraction angles. All films were well adhered to the glass substrates irrespective of their high tensile strain. Surface morphology analysis revealed the presence of nano-structured stress relief patterns which can enhance the nucleation sites for subsequent CuInSe₂ deposition. A high-resolution cross sectional image showed the columnar growth of the films. Surface roughness analysis revealed that roughness increased with increase in substrate temperature.