Trap states and space charge limited current in dispersion processed zinc oxide thin films

Abstract

The electric transport properties of nanoparticulate zinc oxide (ZnO) thin films are investigated in nitrogen and ambient atmosphere with respect to the effects of polymer adsorbates, in order to study the origin of hysteresis behavior of ZnO thin film transistors. A strong dependence on the polymer adsorbate of the conductivity in nitrogen atmosphere is observed. Utilizing the space charge limited current theory, the trap depth and concentration in the films have been estimated. According to this analysis, the low conductivity of polymer free thin films in ambient atmosphere is caused by an increase in deep traps, compensating free charge carriers and not by a reduction in donorlike defect states. Furthermore, polymeric additives seem to induce similar trap states, which make the transport properties less sensitive against atmospheric influences. However, the strongly compensated semiconductor created in this way, causes a slow trap and release behavior resulting in a strong hysteresis in the transistor characteristics and long-term instabilities. It is shown, that ignoring these time-dependent characteristics, straight forward derived transistor parameters like the field effect mobility can be easily overestimated.