The multi-functional stack design of a molybdenum back contact prepared by pulsed DC magnetron sputtering

Abstract

The purpose of this work is to study the effect of deposition parameters on the properties of molybdenum (Mo) films based on direct-current (DC) pulsed magnetron sputtering and to develop a suitable Mo back contact structure with the requisite properties for Cu(In, Ga)Se2 (CIGS) thin films fabricated by post-selenization of electrodeposited Cu/In/Ga stack precursors. The electrical, microstructural and stress properties of the Mo films were investigated as a function of two deposition parameters: the working pressure and film thickness. First, the resistivity of the Mo films decreased with decreasing working pressure or increasing film thickness. Second, the films deposited at higher pressures with porous microstructures are under compressive stress, whereas those films deposited at lower pressures with densely packed networks are under tensile stress. Third, the films deposited at low pressure beyond a certain thickness exhibit tensile stress cracks. Given the above results, a multi-functional stack of Mo films was introduced as the back contact. In this process, it was determined that a bilayer Mo back contact with opposing stress contributions could simultaneously eliminate surface flaws and achieve low resistivity; however, electrodeposition of Cu/In/Ga stack precursors from a chemical solution onto the Mo layer failed and the Mo film fell off the substrate during the high temperature selenization process. To improve adhesion at the interface, the surface and bottom of the bilayer Mo stack were designed to contain different Mo functional structures and a four-layer Mo back contact was created for post-selenized CIGS thin film.