The influence of the nitrogen-ion flux on structure and ionic conductivity of vapor deposited lithium phosphorus oxynitride films

Abstract

Thin films of lithium phosphorus oxynitride (Lipon) have been grown using a plasma-assisted, directed vapor deposition (PA-DVD) technique. In this approach, a high voltage electron beam is used to vaporize a Li 3PO 4 source and a supersonic, nitrogen-doped, helium gas jet then transport the vapor towards a substrate. A hollow cathode technique was then used to create an argon plasma just above the substrate. This sufficiently ionized the nitrogen in the gas jet to allow its incorporation into the Li 3PO 4 film reactively forming lithium phosphorus oxynitride. Increasing the nitrogen flux in the gas jet also increased the deposition rate from 113 to 178 nm min −1 for the deposition conditions used here, significantly reduced the pore volume fraction in the films and increased the N/P ratio from 0 to 0.75 as the gas jet nitrogen flux was increased from zero to 4.3 × 10 18 molecules cm −2 s −1. Using substrate rotation, pore and columnar-free dense Lipon films could be grown by this method. The Li-ion conductivity increased from 3.7 × 10 −9 to 5.2 × 10 −7 S cm −1 as the nitrogen concentration was increased from zero to 2.1 × 10 18 molecules cm −2 s −1 and was correlated with an increase in the film's Li/P ratio. An optimum nitrogen flux has been identified. As the nitrogen flux was increased above this value, the Lipon films suffered lithium loss and partial crystallization, resulting in a decrease in their Li-ion conductivity.