Theoretical study on oxygen-related defects in amorphous oxide semiconductors and novel phases of boron and phosphorus

Abstract

In this dissertation, I suggest a possible origin of instability in amorphous oxide semiconductors and predict novel boron and phosphorus allotropes by performing density-functional theory calculations.

In the first chapter, the atomic and electronic structures of oxygen-related defects in amorphous oxide semiconductors are examined. Amorphous oxide semiconductors have attracted much attention as channel materials for thin-film transistors because their n-type conductivities are robust against disorder, which is in contrast to other crystalline semiconductors. However, they suffer from device instability in that the threshold voltage is shifted positively or negatively, and this still remains as a major issue to overcome. In order to identify a possible origin of the instability, amorphous oxide semiconductor models are generated through ab intio molecular dynamics simulations. Under oxygen-deficient conditions, it is found that undercoordinated cation defects are formed in amorphous oxide semiconductors, and these can capture free electrons, resulting in device instability. Although the undercoordinated cation defects are easily removed by raising oxygen partial pressures, this causes oxygen interstitial defects under oxygen abundant conditions. The oxygen interstitial defects form dimers with host oxygen atoms, and then a dimer gives rise to positive shifts of the threshold voltage by capturing electrons. On the other hand, under light-illumination or negative-bias stress, the original dimer configuration is recovered by capturing hole carriers without any energy barrier, and the stability of current-voltage characteristics is restored. This study's findings provide not only explanations for the origin of device instability under different oxygen conditions but also implications for controlling defect states, which can help improve the stability of amorphous oxide semiconductor based devices.

In the second chapter, the atomic and electronic properties of boron and phosphorous allotropes newly predicted by an ab inito evolutionary crystal structure search method are analyzed. First, I report on new boron allotropes in bulk phases, and then investigate their structural properties. Elemental boron exhibits a variety of allotropes owing to its electron deficiency compared with carbon that results in the ability to form multicenter bonds. Although it has been reported that several allotropes transform into gamma-orthogonal boron under high pressure and high temperature, the detailed kinetics and mechanisms remain poorly understood. The newly discovered metastable allotropes are understood to be a three-dimensional buckled defective honeycomb lattice in which boron vacancies lead to a dynamically and mechanically stable structure with triangular motifs. It is suggested here that the metastable allotropes act as intermediate states on the transition pathway from alpha-rhombohedral boron to gamma-orthogonal boron owing to their structural flexibility and low enthalpies, as in the framework of Ostwald’s step rule. These results can assist in revealing the mechanisms of other unexplained solid-solid transitions. Next, I report on a new phosphorus allotrope, termed green phosphorus (lambda-P), and then investigate its optical and transport properties. Since black phosphorus was successfully exfoliated to a few layers in 2014, it has attracted much attention as an alternative of the graphene and transition metal dichalcogenides for nano-scale device applications. It is found that green phosphorus has a layered structure like black phosphorus and is more stable than other phosphorus allotropes reported previously. A monolayer of green phosphorus exhibits a tunable direct band gap from 0.7 to 2.4 eV and high electron mobility, which is suitable for novel applications in electronic and optical devices. Free-energy calculations show that a phase transition from black to green phosphorene can occur at temperatures above 87 K. I suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than on clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth. On the basis of the results, it is inferred that green phosphorene can serve as a potential material for n-type devices together with black phosphorene.

Lastly, electron-phonon interactions in a hole-doped boron Kagome lattice are presented. The boron Kagome lattice has an intrinsic flat band, which is attributed to the destructive interference below the Fermi energy and shows ferromagnetism. It is demonstrated that spin-dependent electron-phonon coupling strengths are significantly enhanced via hole doping to the flat band. The enhanced electron-phonon interactions come from a large Fermi surface nesting at K point, although the averaged electron-phonon coupling matrices between localized states and bond-stretching modes are weak. On the other hand, the increased Fermi surface nesting at K point results in dynamical instability above the critical doping limit. These results could provide a new platform to help understand the interplay between ferromagnetism and electron-phonon interactions.