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Dependence of electroluminescence efficiency and memory effect on Mn concentration in ZnS:Mn ACTEL devices

We have studied experimentally the dependence of the electroluminescence efficiency and the memory effect on the Mn concentration in ZnS:Mn ac thin-film electroluminescence (ACTEL) devices. With other device parameters kept approximately constant, we find that both luminous efficiency and memory loop width (expressed as a percentage of threshold voltage) exhibit a maximum as a function of Mn concentration. In addition, the memory loop width was found to depend on the ZnS film thickness. Under 1-kHz sine-wave excitation, the peak luminous efficiency in these devices was ∼0.5 l/W and occurred at ∼0.2-wt % Mn, while the peak memory loop width was ∼11 percent of the threshold voltage and occurred at ∼1.1-wt % Mn. The luminous efficiency and memory loop width were found to depend on the frequency and waveform of the excitation. The optimum Mn concentration chosen far a ZnS :Mn memory device depends on a compromise between a high luminous efficiency and a wide memory loop width. A key new result is that the average threshold electric field for electroluminescence increases logarithmically with Mn concentration beyond ∼0.2-wt % Mn. The above results suggest that the Mn doping is modifying the carrier-conduction process in the ZnS:Mn thin film.