Local compositional environment of Er in ZnS:ErF3 thin film electroluminescent phosphors

ZnS:Er thin film electroluminescent phosphors have been shown to exhibit a marked maximum in the near infrared emission (NIR) after a 425 degrees C post-deposition anneal with a very narrow temperature window of +/- 25 degrees C for optimal NIR emission. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been obtained from both the Zn and Er edges in order to examine the local structure of the host and dopant in this NIR phosphor material. Interestingly, the addition of only similar to 0.5 mol. % of Er as ErF3 into the host is found to reduce the Zn-S bond length of one of the two nearest Zn-S shells by 0.6 angstrom relative to high-quality, atomic layer epitaxy (ALE) grown, pure ZnS. The coordination number of this shorter Zn-S bond increases after the optimal 425 degrees C anneal. Longer range fits indicate a highly disordered structure, overall, consistent with earlier TEM results. Erbium-L-3 EXAFS data from the second and third shells show increasing crystallinity with increasing annealing temperature in the vicinity of the Er dopant. Data from the first shell cannot be fit with S atoms, but are fit equally well with either O or F. Comparison with earlier analyses indicates that the Er is most likely surrounded by F in the first shell. Based on these data and previous studies, we develop a model in which the Er dopant is present as an Er:F-x complex with associated S vacancies, which may include one sulfur atom remaining in the Er nearest shell. Upon annealing, there is a reduction in the F present and a rearrangement of the crystal structure in the vicinity of the Er atom. Optimum annealing conditions occur when optimal crystalline environment is achieved prior to the loss of too much F from the Er:Fx complex. (C) 2011 American Institute of Physics. [doi:10.1063/1.3549726]