Hundred-fold reduction in Iron diffusivity in titanium nitride diffusion barrier on steel by microstructure engineering

Abstract

Titanium nitride (TiN) barrier films can enhance the efficiency of thin-film solar cells on steel by preventing Fe contamination in semiconductor layers. This work uses microstructure engineering to obtain low diffusivity of iron in TiN barrier layers. Three different microstructures were analyzed, each obtained by tuning deposition parameters using pulsed direct current sputtering. We show that metal diffusion in TiN proceeds via two mechanisms – bulk and grain-boundary diffusion. The performance of polycrystalline TiN barrier layers is limited by grain-boundary diffusion. By tuning the microstructure, a remarkably low diffusivity was achieved, only 1.5×10−18cm2s−1 at 700°C, a 100-fold improvement over the state-of-the-art. Activation energy suggests that the diffusion in TiN barrier proceeds via N-vacancies, not grain-boundary defects. For application where electrical insulation is also required, the TiN can be combined with insulating SiNx to form a bilayer barrier, without any loss in performance. The optimized TiN barrier can also be used for stainless steel with a Fe diffusivity of just 9×10−18cm2s−1 at 700°C. However, the TiN barrier on stainless steel suffers from very high chromium diffusion, a previously unknown effect. This suggests that counter to accepted wisdom; stainless steel is not always superior to mild-steel for electronic applications. The work provides quantitative data to aid design of diffusion barriers for electronic devices on steel.