Effect of Killer Impurities on Optical Properties of ZnO at Low Temperature

Optical characterizations for ZnO doped with killer impurities (Co or Fe or Ni) having doping concentrations 0.05–1.00% by weight have been carried out at 77 K. Optical properties such as excited state lifetime, trap-depth and decay constant values have been measured using the pulse excitation method. The effect of killer dopants, concentration of killer ions as well as the effect of temperature on various optical parameters values has been observed. Multi-exponential decay curves have been observed. Lifetime values are found to be in the micro- and nano-second time domain and a reverse trend is obtained with increase in concentration of killer impurities. With decreases in temperature from 300 to 77 K, lifetime shortening takes place. The effect of killer impurities is more prominent at higher concentrations. A mixed trend in decay constant values is observed with the increase proportional to the concentration of killer impurities. The values of the decay constant show a mixed effect with change in impurity concentrations. The value of the decay constant 0.76, corresponding to ZnO: Ni (0.7%) is the highest among all the doped ZnO phosphors.