Influence of Al₂O₃ layer insertion on the electrical properties of Ga-In-Zn-O thin-film transistors

To investigate the influence of ionic/covalent interface of Al₂O₃/SiO₂ gate insulator on the electrical properties of thin-film transistors (TFTs) with ionic Ga-In-Zn-O (GIZO) semiconducting channel layers, Al₂O₃ layers of different thickness were introduced between SiO₂ and GIZO using plasma-enhanced atomic layer deposition. The GIZO layers were obtained by DC magnetron sputtering using a GIZO target (Ga:In:Zn = 1:1:1 mol. %). The GIZO TFTs with an Al₂O₃/SiO₂ gate insulator exhibited positive threshold voltage (V_th) shift (about 1.1 V), V_th hysteresis suppression (0.23 V), and electron mobility degradation (about 13%) compared with those of a GIZO TFT with SiO₂ gate insulator by the influence of ionic/ionic and ionic/covalent interface at Al₂O₃/GIZO and Al₂O₃/SiO₂, respectively. To clarify the origin of the positive V_th shift, the authors estimated the shifts of flatband voltage (0.4 V) due to the dipole and the fixed charge (−1.1 × 10¹¹/cm²) at Al₂O₃/SiO₂ interface, from capacitance–voltage data for Pt/Al₂O₃/SiO₂/p-Si capacitors. Based on these experimental data, the authors found that the positive V_th shift (1.1 V) could be divided into three components: the dipole (−0.4 V) and fixed charge (0.15 V) at the SiO₂/Al₂O₃ interface, and the fixed charge (1.35 V) at the Al₂O₃/GIZO interface. Finally, it is noted that heterointerface of SiO₂/Al₂O₃/GIZO stacks is important not only to recognize mechanism of V_th shift but also to design future TFTs with high-k dielectrics and low operating voltage.