Anderson Janotti

We used depth-resolved cathodoluminescence spectroscopy and van-der-Pauw-Hall measurements to determine the effects of molecular beam epitaxy growth and processing conditions on native point defects in indium tin oxide (ITO). The near-nanometer depth resolution of this optical technique enabled us to identify spectral changes associated with removing or creating these defects, leading to the identification of oxygen-related defects in the indium tin oxide bandgap. The combined near-surface detection and processing of ITO together with their correlation with Hall effect mobility indicates an avenue for identifying the physical nature and reducing the density of specific native point defects to increase the free carrier concentration and conductivity of ultrathin ITO.

Hydrogen impurities are known to significantly influence the electronic properties of delafossite oxides CuMO2 (M = Al, Ga, In) by passivating acceptor defects, thereby impacting the p-type conductivity of these materials. Identifying H and its configurations is, therefore, important to understand the properties of these materials. In this study, we employ hybrid density-functional theory calculations to investigate the vibrational properties of various hydrogen-related defects, including interstitial hydrogen (Hi), the hydrogen-copper antisite complex (Hi–CuM), and hydrogen-copper vacancy complexes (Hi–VCu, 2Hi–VCu). Our calculations provide the first theoretical predictions of local vibrational modes associated with these defects, revealing frequencies ranging from approximately 2800 to 3800 cm−1. These results offer valuable insights into the behavior of hydrogen in CuMO2 and serve as a reference for future experimental investigations using techniques such as infrared spectroscopy. By identifying distinct vibrational signatures for different hydrogen configurations, this study lays the groundwork for defect characterization and passivation mechanisms in delafossite oxides, which are critical for optimizing their electronic and optical properties.

The ultrathin structure and efficient spin dynamics of two-dimensional (2D) antiferromagnetic (AFM) materials hold promising opportunities for ultrafast memory devices, artificial intelligence circuits, and advanced computing technology. For example, chromium thiophosphate (CrPS₄) is one of the most promising 2D A-type AFM materials due to its robust stability in diverse environmental conditions and net out-of-plane magnetic moment in each layer, attributed to anisotropy in crystal axes (a and b). However, their net-zero magnetic moment poses a challenge for detecting the Néel state that is used to encode information. In this study, we demonstrate the detection of the Néel vector by detecting the magnetic order of the surface layer by employing localized defects in tungsten diselenide (WSe₂). These defects are ideal candidates for optically active transducers to probe the magnetic order due to their narrow line width and high susceptibility to magnetic field (B-fields). We observed spin-polarized charge transfer in the heterostructure of bulk CrPS₄ and single-layer WSe₂, indicating type-II band alignment as supported by density functional theory (DFT) calculations. In the A-type AFM regime, the intensity of both right-handed and left-handed circularly polarized light emanating from the sample remains constant as a function of the applied B-field, indicating a constant polarized transition behavior. Our results demonstrate an approach to optically characterizing the magnetic states of 2D bulk AFM material by using both localized and delocalized defect excitons as a probe, highlighting avenues for future research and technological applications.

Using density functional theory calculations we investigated possible Weyl semimetal (WSM) phases in antimony arsenide ordered alloys Sb1-xAsx (x=0, 1/6, 1/3, 1/2, 2/3, 5/6, 1). We find WSM phases for all As compositions of Sb1-xAsx with broken inversion symmetry, in contrast to Bi1-xSbx where only compositions x=1/2 and 5/6 were predicted to exhibit WSM phases. The WSM phases in Sb1-xAsx are characterized by the presence of 12 Weyl points, located within 55 meV from the Fermi level in the case of x=1/2. The robust spin-orbit coupling strength and Berry curvature in these alloys produce large spin-Hall conductivity in the range of 176-602 (hbar/e)(S/cm), comparable to that in the BiSb alloys. Finally, Sb0.5As0.5 is predicted to be almost lattice-matched to GaAs(111), with the Fermi level within the gap of the semiconductor, facilitating growth and characterization, and thus, offering promising integration with conventional semiconductors.

Oxygen is a common impurity in AlN samples. Using hybrid density functional calculations, we investigate the role of substitutional oxygen (ON) in the optical absorption. We construct configuration coordination diagrams for ON and related complexes. Our results indicate that an optical transition involving ON− (a DX center) gives rise to an absorption band peaked at 2.22 eV, suggesting it is a source of the absorption band with an onset at ∼ 2 eV observed in oxygen-containing samples. We also propose that neutral ON–DX complexes can form, which would give rise to absorption peaking at 3.06 eV. In addition, we find that oxygen, in spite of its DX character, may behave as an “optically shallow donor” and be involved in optical transitions from deep defect states to the conduction band. This observation provides an alternative physical mechanism for the optical absorption bands observed in AlN samples in the visible and ultraviolet (UV) region.

The Bi2Se3–In2Se3 layered system has garnered significant attention and extensive research due to its versatile properties, yet its structural properties and phase stability remain elusive. Here, using first-principles calculations with van der Waals interactions, we systematically study the phase stability and transition behavior of (BixIn1−x)2Se3 alloys. Our results reveal a contrasting stability profile between Bi2Se3 and In2Se3, with the former exhibiting a distinct preference for the β phase over the α phase, while the latter shows similar stabilities in both phases, thus partially addressing previously reported ground-state inconsistencies. Exploring composition–structure relationships, we demonstrate that Bi incorporation in low concentrations stabilizes the β phase, consistent with early experimental observations. Further analysis based on the cation orbital properties indicates that the preference of Bi for octahedral sites over tetrahedral ones drives the small critical composition for the α→β phase transition. This work enhances our understanding of phase stability in (BixIn1−x)2Se3 alloys, providing insights for future design of monophasic materials and advanced applications.