Woo-Jung Lee

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.

Effects of hydrogen on Au migration and the growth kinetics of Si nanowires

Silicon n nanowires (SiNWs) were synthesized by means of a Vapor–Liquid–Solid (VLS) procedure using Au as a catalyst in a UHV-CVD system. The growth of the SiNWs was critically dependent on the flow rate of H2. In our system, the nanowires showed novel features such as faceting phenomenon and the migration of Au atoms on the lateral surface, which were dependent on the flow rate of H2. In particular, Au that diffused from the catalyst tip to the facet edge was transformed into Au clusters, resulting in the formation of numerous Au stripes that entirely surrounded the pillar during the growth process. A one step synthesis of branched SiNWs can be achieved via the presence of Au clusters at the lateral surface under conditions of a high flow rate of H2. The growth kinetics of SiNWs as a function of H2 flow rate lead to the conclusion that the influence of the effect of hydrogen is a major factor in controlling the growth of SiNWs under conditions of a low partial pressure of silane.

Carrier Mobility Enhancement of Tensile Strained Si and SiGe Nanowires via Surface Defect Engineering

Changes in the carrier mobility of tensile strained Si and SiGe nanowires (NWs) were examined using an electrical push-to-pull device (E-PTP, Hysitron). The changes were found to be closely related to the chemical structure at the surface, likely defect states. As tensile strain is increased, the resistivity of SiGe NWs deceases in a linear manner. However, the corresponding values for Si NWs increased with increasing tensile strain, which is closely related to broken bonds induced by defects at the NW surface. Broken bonds at the surface, which communicate with the defect state of Si are critically altered when Ge is incorporated in Si NW. In addition, the number of defects could be significantly decreased in Si NWs by incorporating a surface passivated Al2O3 layer, which removes broken bonds, resulting in a proportional decrease in the resistivity of Si NWs with increasing strain. Moreover, the presence of a passivation layer dramatically increases the extent of fracture strain in NWs, and a significant enhancement in mobility of about 2.6 times was observed for a tensile strain of 5.7%.

Effects of Surface Chemical Structure on the Mechanical Properties of Si1–xGex Nanowires

The Youngs modulus and fracture strength of Si(1-x)Ge(x) nanowires (NWs) as a function of Ge concentration were measured from tensile stress measurements. The Youngs modulus of the NWs decreased linearly with increasing Ge content. No evidence was found for a linear relationship between the fracture strength of the NWs and Ge content, which is closely related to the quantity of interstitial Ge atoms contained in the wire. However, by removing some of the interstitial Ge atoms through rapid thermal annealing, a linear relationship could be produced. The discrepancy in the reported strength of Si and Ge NWs between calculated and experimented results could be related to SiO(2-x)/Si interfacial defects that are found in Si(1-x)Ge(x) NWs. It was also possible to significantly decrease the number of interfacial defects in the NWs by incorporating a surface passivated Al2O3 layer, which resulted in a substantial increase in fracture strength.

Interfacial reaction induced phase separation in LaxHfyO films

Amorphous LaxHfyO films containing La at concentrations (x) of 50 and 20% were prepared by atomic layer deposition on ultrathin SiO2 films (1 nm). We examined the electronic structures and microstructures of the LaxHfyO films by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Phase separation into La2O3 and HfO2 was observed in the LaxHfyO films subjected to annealing temperatures over 900 °C, although the mixture of La2O3 and HfO2 is thermodynamically stable. The structural changes that occurred as the result of phase separation were dependent on the concentrations of La and Hf in the films. During the annealing treatment, silicate was produced due to interfacial reactions and the interfacial reactions were found to be dependent on the La2O3 content in the LaxHfyO films, which has a significant influence on the phase separation process and resulting film structure.

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.

Changes in Electronic Structure of LaxAlyO Films as a Function of Postdeposition Annealing Temperature

Amorphous La x Al y O films, containing 100, 50, and 30% La, were deposited by atomic layer deposition (ALD) on ultrathin SiO 2 films (1 nm). Changes in the depth profile, as a function of composition, and structure were examined by medium energy ion scattering and transmission electron microscopy. The electronic structure of the La x Al y O films was investigated by X-ray photoelectron spectroscopy as a function of La concentration and postdeposition annealing temperature. In the case of a pure La 2 0 3 film without Al 2 O 3 , the Si0 2 at the interfacial layer had been converted to SiO 2―x and La silicate during the ALD process. However, in case of a La 2 O 3 film with Al 2 O 3 , interfacial reactions were significantly suppressed. In particular, silicate formation in the La 2 O 3 films gradually increased with the increasing annealing temperature, while that in La 2 O 3 films incorporated Al 2 O 3 was suppressed up to an annealing temperature of 800°C.

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.

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.

Electronic and structural characteristics of Zr-incorporated Gd2O3 films on strained SiGe substrates

Zr-incorporated Gd(2)O(3) films were grown on various substrates as a function of Zr content. The extent of interfacial reactions was found to be critically dependent on both the incorporated Zr content and the substrate type. Specifically, the silicide layer was suppressed and the Gd(2)O(3) phase was changed to ZrO(2) on a Si substrate with increasing Zr content. Crystalline Gd(2)Ge(2)O(7) was grown on a Ge substrate, as the result of interfacial reactions between Gd-oxide and the Ge substrate. However, interfacial reactions were not affected by the amount of Zr incorporated. On the SiGe/Si substrate, reactions between Gd-oxide and Si could be controlled effectively by the incorporation of Zr, while the extent of reactions with Ge was significantly enhanced as the Zr content increased. The formation of an interfacial layer between the film and the SiGe substrate resulted in a textured crystalline growth.