Facile and Stable n+ Doping Process Via Simultaneous Ultraviolet and Thermal Energy for Coplanar ALD-IGZO Thin-Film Transistors

We fabricated atomic layer deposition-derived coplanar In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) using UV irradiation for the n + doping region. To investigate the effects of different temperatures of the n + doping region process on the electrical characteristics of TFTs, we intentionally controlled the UV irradiation temperature at 25 °C (device A) and 200 °C (device B). The subthreshold swing (S.S) of both devices showed slight differences (device A: 0.47 V/dec and device B: 0.36 V/dec), which results from differences in the normalized contact resistances ( R SD W ). The reliability of the devices was evaluated under negative bias temperature stress (NBTS) and positive bias temperature stress (PBTS). The NBTS stability tests showed a negligible change of less than −0.1 V of negative threshold voltage ( V TH ) shift for both devices. However, a large negative V TH shift of −6.1 V accompanied by abnormal hump characteristics were observed under PBTS in device A. This phenomenon could be originated from hydrogen (H) diffusion from the n + region to the active layer. The movable H i + (H interstitial, H bonded with an oxygen) that induced abnormal device reliability mainly exists at lower UV irradiation temperature, whereas robust H o + (H substitutional, H located at an oxygen vacancy) is formed at higher UV irradiation temperature. Therefore, the binding states of H can be changed by UV irradiation temperature, and consequently, the excellent reliability of device B that is treated with UV at high temperature is attributed to robust H o + sites in the n + doping region.