Hydrogen Behavior at Crystalline/Amorphous Interface of Indium Oxide and Its Role in Carrier Transport, Photoconductivity and Crystallization

Revealing the microscopic behavior of hydrogen in wide-bandgap metal oxides that have been employed as transparent electrodes in many optoelectronic devices, including highly efficient silicon heterojunction solar cells, and that recently became competitive in large-area flexible displays, represents a formidable problem. In oxides, hydrogen may passivate under-coordinated oxygen or metal atoms, forming covalent or ionic H bonds, respectively, and the resulting macroscopic properties depend on the relative formation, distribution, and behavior of the competing H defects. Although the structure and properties of substitutional and interstitial H defects in crystalline oxides, primarily, in In₂O₃, SnO₂ or ZnO, have been studied theoretically, the results cannot be transferred to disordered phases. The ionic nature of metal-oxygen bonding renders a substantial disorder within the short-range structure of the amorphous oxide phases that feature large fractions of under-coordinated oxygen (O) and metal (M) atoms. The wide coordination distributions and strong distortions in the M-O and O-M polyhedra dramatically increase the number of possible locations for hydrogen and the diversity of its immediate neighbor environment. The latter affects the formation, activation and stability of various H defects as well as H mobility through the disordered structure. Therefore, quantum-mechanical and statistically-significant calculations are required to elucidate the complex H behavior in wide-bandgap amorphous oxide semiconductors.<br/>In this work, the role of crystalline-amorphous interfaces in the resulting microscopic behavior of hydrogen in indium oxide is investigated using <i>ab-initio</i> molecular dynamics simulations and hybrid density functional calculations. The results of this computationally-intensive work reveal that disorder plays a decisive role in H-induced extended bond reconfiguration which: (i) significantly outweighs the energy gains from passivation of under-coordinated oxygen atoms and nonbonding O–p-orbitals; (ii) stabilizes H defects with ionic bonding (In-H-In) even in the stoichiometric amorphous oxide; (iii) improves the morphology of disordered oxide that widens the conduction bandwidth---in accord with the observed improved mobility; and, most strikingly, (iv) favors the formation of covalent OH bonds that introduce deep trap defects, limiting the number of free carriers.<br/>The results help explain a puzzling experimental observation of atypical semiconductor behavior in H-doped indium oxide, namely, a decreasing number of free carriers as temperature raises. Furthermore, the crystalline/amorphous interfacial model developed in this work in order to establish the structural characteristics of the interface and their effect on the formation, stability, distribution, and properties of H defects, is indispensable to resolve the controversial experimental observations regarding the H role in crystallization, namely, faster crystallization rates upon H doping, yet, decreasing grain sizes with increasing H concentration. Our statistically-significant calculations combined with molecular-dynamics simulations at various temperatures help identify the H defects and defect complexes that are responsible for H pinning, switching and segregation, governing the complex crystallization processes in indium oxide. Last but not least, the results of this work help predict which defects in H-doped indium oxide will be sensitive to photo-illumination and may contribute to the observed negative bias illumination stress instability behavior in thin film transistor devices, one of the major drawbacks hampering commercialization of the wide-bandgap amorphous oxide semiconductors.