Oxygen Defects and Instability in Very Thin a-IGZO TFTs

Abstract

Amorphous oxide semiconductor (AOS) thin-film transistors (TFT) have gained significant attention for their potential in capacitor-free next-generation memory applications. However, improving threshold voltage (V_TH) stability and precisely controlling carrier concentration in ultra-thin channels remain critical challenges. In this study, an extraordinarily large positive-bias-stress (PBS) instability in hydrogen-free amorphous IGZO (a-IGZO)-TFTs that emerges as the channel thickness decreases is reported. This instability can be attributed to acceptors interacting with donors at shallow levels below the conduction band minimum (CBM). This model, based on temperature-dependent Hall effect measurements, reveals an unusual correlation between donor concentration and donor energy levels. Unlike in previously reported semiconductors, the energy difference between the CBM and donor energy level increases linearly in proportion to (donor concentration)^1/3. The O 1s core-level hard X-ray photoemission measurements suggest that the entity of the acceptors is oxygen vacancies without two electrons (V_O²⁺) formed during deposition. These vacancies result from strong donor−acceptor interactions arising from the formation of oxygen Frenkel defects in the thinner films. It is demonstrated that low-temperature extra-annealing effectively suppresses PBS instability by inducing structural relaxation of the Frenkel defects, thereby stabilizing the TFTs.