Electroluminescence from zinc sulfide thin films doped with erbium trifluoride

Near infrared emission at a wavelength of 1.55 mum from zinc sulfide (ZnS) doped with erbium trifluoride (ErF3) has been studied in alternating current thin film electroluminescent devices (ACTFELDs). The thin film phosphors were deposited by radio frequency (RF) planar magnetron sputtering of a 1.5 mol% ErF3-doped ZnS target and an undoped ZnS target. The intensity of near infrared emission is related to the processing parameters used in film deposition and post-deposition annealing. Thin films, approximately 650 nm thick, were deposited at 100 W from both targets. Deposition temperature, duty cycle, sputtering gas pressure and post-deposition annealing temperature were varied in a design-of-experiment. As deposition temperature was increased from 120°C to 150°C, the near infrared electroluminescence increased by ∼180% at an operating voltage 20 volts above the threshold voltage (B20). This is attributed to better activation of erbium luminescent centers on zinc lattice sites results in a greater number of radiative events, increasing luminescence. Increasing the duty cycle applied to the ZnS:ErF3 doped target from 25% to 75%, increased the electroluminescence at B20 by ∼650%. This likely results from a greater number of ErF3 dopants incorporated into the phosphor. As the sputtering gas pressure increases from 1 mTorr to 24 mTorr, the electroluminescence at B20 increases by ∼80%. At lower pressure, the phosphor undergoes negative ion resputtering removing zinc and sulfur from the film. This creates a film with a higher proportion of ErF3 dopant incorporated. After film deposition, a portion of the phosphor films underwent annealing for 1 hour in ultra high purity nitrogen at 425°C. Annealing films increased electroluminescence at B20 by ∼60%. The increase in brightness correlated with a reduction in fluorine incorporation in the film. Additionally the number of sulfur vacancies is likely reduced. This probably alters the local crystal field around the erbium dopant allowing for more adequate pathways for the near-infrared transitions, thus increasing the overall near infrared luminance. The highest brightness was observed for a deposition temperature of 150°C, a duty cycle of 75%, argon sputtering gas pressure of 24 mTorr and post-deposition annealing at 425°C. These conditions resulted in a brightness of the 1550 nm emission peak of 146.46 muW/cm2 at 20 V above threshold.