Jung Min Bae

Strongly Enhanced THz Emission caused by Localized Surface Charges in Semiconducting Germanium Nanowires

A principal cause of THz emission in semiconductor nanostructures is deeply involved with geometry, which stimulates the utilization of indirect bandgap semiconductors for THz applications. To date, applications for optoelectronic devices, such as emitters and detectors, using THz radiation have focused only on direct bandgap materials. This paper reports the first observation of strongly enhanced THz emission from Germanium nanowires (Ge NWs). The origin of THz generation from Ge NWs can be interpreted using two terms: high photoexcited electron-hole carriers (Δn) and strong built-in electric field (Eb) at the wire surface based on the relation . The first is related to the extensive surface area needed to trigger an irradiated photon due to high aspect ratio. The second corresponds to the variation of Fermi-level determined by confined surface charges. Moreover, the carrier dynamics of optically excited electrons and holes give rise to phonon emission according to the THz region.

The modulation of Si1−xGexnanowires by correlation of inlet gas ratio with H2 gas content

Si1−xGexnanowires (NWs) were prepared by a Vapor–Liquid–Solid (VLS) procedure using Au as the catalyst at a fixed growth temperature of 400 °C. The alloy composition was adjusted and the growth rate of the Si1−xGex NWs was achieved by varying the inlet gas ratio and the H2 flow rate. The growth of Si1−xGex NWs can be explained by two mechanisms that are related to growth kinetics; first, collisional activation is a dominant factor at flow rates of H2 100 sccm and second, in the case of a H2 flow rate of 200 sccm, the reaction is unimolecular. In addition, a Ge concentration (0.56 < x < 0.91) in Si1−xGex NWs is observed at a relatively high growth temperature of 400 °C as compared with data reported in the literature. The findings herein indicate that the high Ge concentration (x) can be attributed to the presence of interstitial Ge atoms in the Si1−xGex NWs, when they are grown under non-equilibrium conditions. This was confirmed by comparing the measured Ge concentration between EDX and XRD, Raman and strongly demonstrated by XPS results indicating the development of Ge interstitial states at lower binding energy, rather than bulk-like bonding.

The oxidation characteristics of silicon nanowires grown with an au catalyst

AbstractIn this study, we investigated the thermal oxidation of silicon nanowires (SiNWs) grown via the vapor-liquid-solid (VLS) method with an Au catalyst. We systematically analyzed the oxidation mechanism of the SiNWs in both the radial and axial directions and mapped the behavior of the Au atoms on the sidewall and at top of the wire as a function of oxidation time. After thermal oxidation at a temperature of 900 °C, two kinds of oxidation behavior in SiNWs were observed: one was conventional radial oxidation and the other was axial oxidation. In particular, the axial oxidation rate at the Si/Au interface increased dramatically compared with the radial oxidation rate, which can be explained by the reaction between the Si atoms precipitated from the Au tip and the O2 gas injected in the area surrounding the Au tip. Additionally, we observed that the oxidation rate in the axial direction was inversely proportional to the wire diameter, which is related to the SiO2 surrounding the Si wire. Moreover, the Au shape changed with respect to the wire diameter, suggesting that both the stress in the Au-Si alloy and the SiO2 shell thickness of the wire critically affect the growth of SiO2 on Au.

Induction of the surface plasmon resonance from C-incorporated Au catalyst in Si1−xCx nanowires

Si1−xCx nanowires (NWs) were synthesized by varying the ratio of SiH4 and CH3SiH3 gases using a vapor–liquid–solid (VLS) procedure using Au as a catalyst. The growth rate of the Si1−xCx NWs and the change in the wire shape from straight to helical near the Au tip were found to be closely related to the ratio of the CH3SiH3 content. The large concentration of C in the Si1−xCx NWs was proportional to the CH3SiH3 content, overcoming the extremely low solubility of C in Si, resulting in an interstitial incorporation of C atoms in the wire. This incorporation can be attributed to the cleavage of Si–C bonds in the CH3SiH3 compound through the Au catalyst (an Au–Si liquid-state cluster of about 70–100 nm) during wire growth by the VLS method. Simultaneously supplying CH3SiH3 and SiH4 gases enhanced the diffusion of Au atoms from the tip to the sidewall of the wire, while also deforming the shape of the Au tip. When the CH3SiH3 gas was increased to 1.5 sccm, the number of Au nanoparticles (2–3 nm in size) at the lateral surface induced a surface plasmon resonance (SPR) and improved the optical conductivity (σ) of the Si1−xCx NWs. For 2 sccm of CH3SiH3, a remarkable increase in the number of C atoms incorporated in the Au nanoparticles along the sidewall red shifted the SPR peak, suggesting that the SPR can be modulated by the Au–C interactions in the nanoparticles.

Structural evolution and carrier scattering of Si nanowires as a function of oxidation time

We investigated the morphological characteristics of the cross-sectional shape of Si-core nanowires (NWs) as a function of oxidation time. In the case of as-grown Si NWs, the Si cores were hexagons with a 3-fold symmetry, not a 6-fold symmetry, and this shape was transformed into a triangular shape with rounded edges after thermal oxidation. Structural changes in the cross-section of the Si-core NWs were related to the surface free energy. Moreover, the morphological change in the oxide shell during the oxidation treatment suggested that the stress relaxation process is closely related to the structural evolution of the Si core. The change in defect states generated in Si/SiO2 core/shell NWs during structural evolution was investigated by low-temperature photoluminescence and optical pump–THz probe spectroscopy measurements. In particular, surface defect states formed in the interfacial region between the Si core and the oxide shell were reduced during the oxidation process. Appropriate control of the surface state affected carrier scattering: the relaxation time of photogenerated carriers was significantly increased due to the reduction in surface defects.

The effect of structural and chemical bonding changes on the optical properties of Si/Si1−xCx core/shell nanowires

Si/Si1−xCx core/shell nanowires (CS NWs) were synthesized. First, a Si NW was grown via a Vapor–Liquid–Solid (VLS) procedure using Au as a catalyst. Next, a Si1−xCx shell was deposited by a chemical vapor deposition (CVD) method after the removal of the Au tip at the top of the Si NW. We investigated the physical, chemical, and optical properties of the Si/Si1−xCx CS NWs as a function of annealing temperature. The Si1−xCx shell was initially deposited on the Si core with small clusters of an amorphous state, which were remarkably transformed into larger clusters by recrystallization after annealing under vacuum. To relieve the strain induced by the huge difference between the atomic sizes of Si and C, substitutionally incorporated C atoms can combine with another C atom at the third-nearest-neighbor distance in the Si1−xCx shell with increasing annealing temperature. Furthermore, the THz pulse emitted from the Si/Si1−xCx CS NWs was observed and analyzed. In the case of annealing treatment at 600 °C, the THz pulse intensity was substantially increased, which is not ascribed to Drude absorption but to mid-IR absorption. Moreover, based on the simulation results, we suggest that the existence of substitutional C atoms and control of the shell thickness is a viable method to enhance the THz pulse amplitude.

Oxidation Mechanism of Si1–xGex Nanowires with Au Catalyst Tip as a Function of Ge Content

Si1-xGex nanowires (NWs) (0.22 ≤ x ≤ 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 Si1-xGex 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 Si1-xGex 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 Si1-xGex NWs before and after oxidation process; the decay time is significantly shortened in oxidized Si1-xGex NWs (0.22 < x), whereas it is maintained for Si-rich Si1-xGex NWs (x ≈ 0.22) as compared to the as-grown Si1-xGex NWs. It indicates that the number of defect states is generated with the formation of defective Ge oxide at the oxide-shell-layer/Si1-xGex-core-NW interface.