Deep-level defect distribution as a function of oxygen partial pressure in sputtered ZnO thin-film transistors

Deep-level defect states in sputtered ZnO thin-film transistors were investigated as a function of oxygen partial pressure during sputtering growth. Photo-induced threshold voltage-shift measurements under monochromatic illumination were used to characterize the deep-level defect distribution. Intrinsically, the defect states of oxygen vacancies were ionized to Vo+ and Vo2+ while the photon energy was absorbed within the bandgap, resulting in the shift of threshold voltage. Extracted deep-level defect distribution from this shift of threshold voltage was clearly confirmed in the range of 1.8–2.1 eV below the conduction band minimum and this region was suppressed with increasing oxygen partial pressure. These deep-level defect states can have a detrimental effect on device performance, such as threshold voltage shift and photo-induced leakage current. The photo instability of the devices occurred under visible light due to the photo-ionization of deep-level trapped charges associated with oxygen vacancies.