Hydrogen behavior under X-ray irradiation for a-IGZO thin film transistors

Abstract

We studied hydrogen (H) behavior in amorphous In-Ga-Zn-O (a-IGZO) films under X-ray irradiation by evaluating the threshold voltage (VTH) shift in a-IGZO thin film transistors (TFTs) with different H concentrations in the active layers. We fabricated three types of a-IGZO TFTs: (i) one without a buffer layer and postannealed in N2, (ii) one with a H-resolved buffer layer and postannealed in N2, and (iii) one with a H-resolved buffer layer and postannealed in a mixture of N2 and H2. All three TFTs showed a negative VTH shift after 100 Gy of X-ray exposure. The degree of VTH shift correlated with an increase in conductivity, which, in turn, corresponds to the H concentration in the active layer of the as-fabricated TFTs. Based on spectroscopic ellipsometry analysis, we confirmed a large increase in the donorlike H related D1 state after X-ray irradiation in high-H concentration a-IGZO films. In addition, an increase in the number of H2 molecules in a-IGZO films after X-ray irradiation was observed via thermal desorption spectroscopy analysis. Therefore, we conclude that the increase in conductivity and/or the resulting negative VTH shift in a-IGZO TFTs during X-ray irradiation can be attributed not only to the state transition from acceptorlike to donorlike H in the as-prepared a-IGZO but also to the incorporation of additional H radicals generated by X-ray irradiation.