Deep level transient spectroscopy study of the effect of Mn and Bi doping on trap formation in ZnO

Deep Level Transient Spectroscopy (DLTS) characterisation of sintered polycrystalline ZnO, and ZnO doped with Mn, and with Mn plus Bi, has been carried out to investigate the effect of these additions on the formation and activation of electron trap states in ZnO used for varistor applications. Samples were produced using a conventional solid state sintering technique, and sintered at 1100°C and 1200°C, quenching the Bi-free samples from the sintering temperature to preserve high temperature defect distribution and slow cooling the Bi-containing samples to develop varistor behaviour. The two well-known bulk ZnO traps, L1 (0.18 eV below the conduction band edge) and L2 (0.29 eV below the conduction band edge), were observed in both the undoped and doped samples. Detailed characterisation of the L1 and L2 traps indicated that they are due to point defects, since their energy was independent of the length of the fill pulse and the fill bias. The introduction of both 1% Mn and (1% Mn + 2% Bi) caused several additional electron traps, some of which have not been reported previously, to appear deeper in the band gap with energies depending on composition and firing cycle,. The DLTS peaks associated with these additional traps were very broad and had activation energies that varied with fill pulse length: characteristics that indicate they are associated with extended defects.