Oxidation Mechanism of Si_1–<i>x</i>Ge_<i>x</i> Nanowires with Au Catalyst Tip as a Function of Ge Content

Si_1–<i>x</i>Ge_<i>x</i> nanowires (NWs) (0.22 ≤ <i>x</i> ≤ 0.78) were synthesized using a vapor–liquid–solid procedure with a Au catalyst. We measured the intrinsic physical, chemical, and electrical properties of the oxidized Si_1–<i>x</i>Ge_<i>x</i> NWs using several techniques, including transmission electron microscopy, X-ray photoemission spectroscopy, and optical pump-THz probe spectroscopy. We suggest two distinct oxidation mechanisms depending on the Ge content in the Si_1–<i>x</i>Ge_<i>x</i> NWs: (i) when the Ge content is around 0.22, a Au catalytic effect brings about oxidation in both the axial and lateral directions; and (ii) when the Ge content is greater than 0.22, the Au tip is detached from the NW body and does not act as a catalyst, which is a result of the high degree of Ge-atom participation in the oxidation process. Additionally, we measured the photoconductivity decay time distribution for the Si_1–<i>x</i>Ge_<i>x</i> NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si_1–<i>x</i>Ge_<i>x</i> NWs (0.22 < <i>x</i>), whereas it is maintained for Si-rich Si_1–<i>x</i>Ge_<i>x</i> NWs (<i>x</i> ≈ 0.22) as compared to the as-grown Si_1–<i>x</i>Ge_<i>x</i> NWs. It indicates that the number of defect states is generated with the formation of defective Ge oxide at the oxide-shell-layer/Si_1–<i>x</i>Ge_<i>x</i>-core-NW interface

Structural and Electrical Properties of EOT HfO₂ (<1 nm) Grown on InAs by Atomic Layer Deposition and Its Thermal Stability

We report on changes in the structural, interfacial, and electrical characteristics of sub-1 nm equivalent oxide thickness (EOT) HfO₂ grown on InAs by atomic layer deposition. When the HfO₂ film was deposited on an InAs substrate at a temperature of 300 °C, the HfO₂ was in an amorphous phase with an sharp interface, an EOT of 0.9 nm, and low preexisting interfacial defect states. During post deposition annealing (PDA) at 600 °C, the HfO₂ was transformed from an amorphous to a single crystalline orthorhombic phase, which minimizes the interfacial lattice mismatch below 0.8%. Accordingly, the HfO₂ dielectric after the PDA had a dielectric constant of ∼24 because of the permittivity of the well-ordered orthorhombic HfO₂ structure. Moreover, border traps were reduced by half than the as-grown sample due to a reduction in bulk defects in HfO₂ dielectric during the PDA. However, in terms of other electrical properties, the characteristics of the PDA-treated sample were degraded compared to the as-grown sample, with EOT values of 1.0 nm and larger interfacial defect states (D_it) above 1 × 10¹⁴ cm^–2 eV^–1. X-ray photoelectron spectroscopy data indicated that the diffusion of In atoms from the InAs substrate into the HfO₂ dielectric during the PDA at 600 °C resulted in the development of substantial midgap states

Characterization of Rotational Stacking Layers in Large-Area MoSe₂ Film Grown by Molecular Beam Epitaxy and Interaction with Photon

Transition metal dichalcogenides (TMDCs) are promising next-generation materials for optoelectronic devices because, at subnanometer thicknesses, they have a transparency, flexibility, and band gap in the near-infrared to visible light range. In this study, we examined continuous, large-area MoSe₂ film, grown by molecular beam epitaxy on an amorphous SiO₂/Si substrate, which facilitated direct device fabrication without exfoliation. Spectroscopic measurements were implemented to verify the formation of a homogeneous MoSe₂ film by performing mapping on the micrometer scale and measurements at multiple positions. The crystalline structure of the film showed hexagonal (2H) rotationally stacked layers. The local strain at the grain boundaries was mapped using a geometric phase analysis, which showed a higher strain for a 30° twist angle compared to a 13° angle. Furthermore, the photon–matter interaction for the rotational stacking structures was investigated as a function of the number of layers using spectroscopic ellipsometry. The optical band gap for the grown MoSe₂ was in the near-infrared range, 1.24–1.39 eV. As the film thickness increased, the band gap energy decreased. The atomically controlled thin MoSe₂ showed promise for application to nanoelectronics, photodetectors, light emitting diodes, and valleytronics