Study of local structure in amorphous indium oxide using first-principles calculations

Zheng-Hong Li^1*, Minamitani Emi², Chun-Liang Lin¹, Chi-Cheng Lee³, Chien-Te Wu¹

¹1Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²SANKEN, Osaka University, Osaka, Japan
³Department of Physics, Tamkang University, Tamkang, Taiwan

* Presenter:Zheng-Hong Li, email:zhenghongli.sc11@nycu.edu.tw

Amorphous indium oxide (a-In₂O₃) has gained popularity in recent years as a transparent oxide semiconductor (a-TOS). Unlike crystalline transparent oxide semiconductors (c-TOS), the amorphous structure offers several advantages, such as high carrier mobility and smooth surfaces [1-2]. The presence of oxygen vacancies in a-In₂O₃ plays an important role in controlling the carrier concentration. Therefore, we investigated the characteristics of defects in a-In₂O₃ based on the density functional theory.
Amorphous structures were created from melt-quench simulations using ab initio molecular dynamics simulations implemented in VASP [3]. We employed the GGA-PBE functional and set the energy cutoff to 400 eV. We set the bixbyite In₂O₃ unit cell as the initial structure, which has 80 atoms, and we created oxygen vacancies in the systems. We investigated the influence of the quench rate in the simulation. In particular, we employed three different quench rates previously reported to create a-In₂O₃ [1, 4-6]. In these three types of models, following the liquid-quench process to 3000 K, the cooling behavior varies significantly. One model undergoes a rapid cool down to 100 K. Another model cools slowly to the melting point at 2000 K and then continues to cool down to 100 K. The final model follows an exponential decay pattern during cooling.
It is widely known that amorphous systems easily give rise to tail states, and we are particularly interested in the formation of these states in a-In₂O₃. To understand the origin of the tail states, we analyzed the partial charge density within a specific energy range. Our results indicate that the tail states in a-In₂O₃ arise from two different scenarios. The first scenario involves the O-O bond and is closely related to the unoccupied state of the oxygen atoms [7]. The second scenario is primarily caused by the In-In bond. We then organized the contributions of the local structures to the tail states, which helps us better understand the composition of a-In₂O₃.

Keywords: first-principles calculations, transparent oxide semiconductor, Amorphous structures