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

Abstract

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 degrees C (device A) and 200 degrees 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 (RSDW). 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 Hi(+) (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.