Aging of electroluminescent ZnS:Mn thin films deposited by atomic layer deposition processes.

Electroluminescent ZnS:Mn thin films were deposited by the atomic layer deposition (ALD) technique. The deposition processes were based on ZnI₂ or ZnCl₂ as the Zn source and Mn(thd)₃ (thd=2,2,6,6-tetramethyl-3,5-heptanedionato) as the Mn source. The ZnI₂ process was found to have a wide temperature range between 300 and 490 °C where the growth rate was independent of the deposition temperature, which offers the possibility to select the deposition temperature according to the thermal stability of the dopant precursor without reducing growth of ZnS. The electro-optical measurements suggested that the amount of space charge was lower within the phosphors made with the iodide process, which resulted in higher efficiency of the “iodide” devices as compared to the “chloride” devices. Brightness and efficiency of the best iodide device after 64 h aging were 378 cd/m² and 2.7 lm/W, respectively, measured at 60 Hz and at 40 V above threshold voltage. Conversely, brightness and efficiency of the best chloride device after 64 h aging were 355 cd/m² and 1.6 lm/W, respectively. On the other hand, changes in the emission threshold voltages indicated that the chloride devices aged slower than the iodide devices. Though the samples were annealed later at high temperature, the deposition temperature was found to be a significant parameter affecting the grain size, luminance, and efficiency of the devices. Overall, the results of this study show that a relatively small change in the Zn precursor can have a clear impact on the electro-optical properties of the devices, and that a mixed halide/metalorganic ALD process can produce an electroluminescent device that ages relatively slowly. [ABSTRACT FROM AUTHOR]