ZnS:TbOF alternating current thin film electroluminescent (ACTFEL) devices were sputter deposited from a single pressed powder ZnS:TbOF (1.5 mol % of Tb) target using a Ar+He mixed gas. The He gas concentration was changed from 0% to 70% and deposited films were annealed at 500 degreesC for 60 min under a N-2 gas atmosphere. The x-ray diffraction peak full width half maximum (FWHM) of the cubic (111)/hexagonal (0002) plane in as-deposited films decreases from 0.55degrees for 100% Ar, to 0.39degrees for 30% Ar+70% He. The film deposition rate decreased about 25% as the He percentage increases from 0% to 70%. In spite of the improved crystallinity as indicated by the decreased FWHM data, the brightness of alternating current thin film electroluminescent devices (ACTFELDs) decreases from 64 cd/m(2) for a pure Ar atmosphere to 45-54 cd/m(2) at 60%-70% He. This brightness decrease is attributed to an increased dielectric constant (from 12 for pure Ar to 17 at 70% He) and decreased root mean square surface roughness (from 6.7 nm for pure Ar to 4.3 run at 70% He) of films sputter deposited with high He. (C) 2003 American Vacuum Society.