Yong-Lim Foo

An effective surface-enhanced Raman scattering template based on a Ag nanocluster–ZnO nanowire array

An effective surface-enhanced Raman scattering (SERS) template based on a 3D hybrid Ag nanocluster (NC)-decorated ZnO nanowire array was fabricated through a simple process of depositing Ag NCs on ZnO nanowire arrays. The effects of particle size and excitation energy on the Raman scattering in these hybrid systems have been investigated using rhodamine 6G as a standard analyte. The results indicate that the hybrid nanosystem with 150 nm Ag NCs produces a larger SERS enhancement factor of 3.2 x 10(8), which is much higher than that of 10 nm Ag NCs (6.0 x 10(6)) under 532 nm excitation energy. The hybrid nanowire arrays were further applied to obtain SERS spectra of the two-photon absorption (TPA) chromophore T7. Finite-difference time-domain simulations reveal the presence of an enhanced field associated with inter-wire plasmon coupling of the 150 nm Ag NCs on adjacent ZnO nanowires; such a field was absent in the case of the 10 nm Ag NC-coated ZnO nanowire. Such hybrid nanosystems could be used as SERS substrates more effectively than assembled Ag NC film due to the enhanced light-scattering local field and the inter-wire plasmon-enhanced electromagnetic field.

Investigation of HfO2 high-k dielectrics electronic structure on SiO2/Si substrate by x-ray photoelectron spectroscopy

In this Letter, x-ray photoelectron spectroscopy (XPS) was used to investigate electronic structures of HfO2 with various thicknesses on SiO2/Si substrate prepared by atomic layer deposition with post deposition annealing of 600 °C, which is compatible with a gate last process in the fabrication of complementary metal oxide semiconductor. The hafnium silicate formed between HfO2 and SiO2 interface plays a key role in generating an internal electric field. This can be attributed to the presence of oxygen vacancies (Vo) at the interface. The XPS binding energy shifts of O 1s, Si 2p, and Hf 4f spectra with increasing HfO2 thicknesses can be explained by the presence of the internal field and differential charging effects. Electrical measurements of capacitor structures support the XPS result.

Formation and evolution of epitaxial Co5Ge7 on Ge(001) surface by reactive deposition inside an ultrahigh-vacuum transmission electron microscope

Cobalt was deposited on single-crystal Ge(001) surface at ∼350°C by electron-beam evaporation in an ultrahigh-vacuum transmission electron microscope. The deposited Co reacts with Ge to form nanosized islands with the cobalt germanide Co5Ge7 phase. The Co5Ge7 islands show square and rectangular shapes. Two epitaxial orientation relationships between Co5Ge7 and Ge were observed: Co5Ge7 ⟨110⟩(001)‖Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)‖Ge⟨100⟩(001).

High quality silicon-germanium-on-insulator wafers fabricated using cyclical thermal oxidation and annealing

An improved fabrication scheme for forming strained SiGe on insulator (SGOI) is demonstrated. Cyclical thermal oxidation and annealing (CTOA) process is introduced to mitigate issues associated with surface roughening and nonuniformity due to increased germanium (Ge) content during SiGe oxidation. Annealing in an inert ambient can be introduced between each oxidation phase to homogenize the Ge content. The root-mean-square surface roughness of the SGOI layer is evaluated to be 0.41nm. With CTOA, a high quality SGOI substrate is obtained. This technique is promising for the fabrication of dislocation-free SGOI layers for applications in high mobility metal-oxide-semiconductor field-effect transistors.

Formation and evolution of epitaxial Co5Ge7 film on Ge (001) surface by solid-state reaction in an in situ ultrahigh-vacuum transmission electron microscope

A thin metallic cobalt (Co) layer was deposited on a single-crystal Ge (001) surface at room temperature by the electron-beam evaporation of a pure Co metal source in an ultrahigh-vacuum transmission electron microscope. The formation and epitaxial growth of a cobalt germanide Co5Ge7 phase on the Ge (001) surface was studied in situ by gradually heating the sample from room temperature to ∼350°C. The occurrence of an epitaxial hexagonal-close-packed Co and the reaction between Co and Ge were observed at ∼225°C. After annealing at ∼300°C for 26.5 h, a continuous epitaxial Co5Ge7 film formed on the Ge (001) substrate. With further annealing at a higher temperature, the continuous Co5Ge7 layer broke up and formed three-dimensional islands in order to relieve the strain energy in the epitaxial Co5Ge7 layer. Two epitaxial relationships between Co5Ge7 and Ge, i.e., Co5Ge7⟨110⟩(001)∕∕Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)∕∕Ge⟨100⟩(001) were found by electron diffraction.

NiGe on Ge(001) by reactive deposition epitaxy: An in situ ultrahigh-vacuum transmission-electron microscopy study

We use an ultrahigh-vacuum transmission-electron microscopy, equipped with an electron-beam evaporator directed at a heating stage in the pole piece, to follow the reaction pathway of Ni on Ge(001) substrate at 300 °C. Using reactive deposition, we illustrate that epitaxial orthorhombic NiGe (a=5.381A, b=3.428A, and c=5.811A) phase can be grown directly without the initial formation of metal-rich Ni2Ge phase. The epitaxial orientation of the NiGe islands and the underlying Ge(001) substrate were found to be NiGe(1¯01)∕∕Ge(001) and NiGe[010]∕∕Ge[110].

Room temperature ferromagnetism at self-assembled monolayer modified Ag nanocluster-ZnO nanowire interface

Magnetic characterization of ZnO nanowires (NW) decorated with thiol-capped Ag nanoclusters (NCs) reveals spontaneous field-dependent magnetization and hysteresis at room temperature. The saturation magnetization is temperature independent for the 13nm thiol-capped Ag NCs but unexpectedly increases with temperature for the 4nm thiol-capped Ag NCs. The high magnetic moment results from the efficient dispersal of Ag NCs on the ZnO NW scaffold and charge transfer interaction between Ag and ZnO. The anomalous magnetic behavior in 4nm NCs may be due to the spin reorientation of Ag–S dipoles mediated by Zn–S dipoles.

Kinetically constraint zero-and one-dimensional heteroepitaxial island growth

Direct observation of the dynamics, formation, and selective growth of low dimensional epitaxial Fe13Ge8 structures [zero-dimensional (0D) compact islands or one-dimensional (1D) wires of different aspect ratios] was conducted in real time using in situ ultra high vacuum transmission electron microscopy at 350, 430, 480, and 510°C. Both types of island (0D/1D) share the same epitaxial relation to the underlying Ge substrate. The compact islands are formed preferentially at lower deposition temperature while wires, which are kinetically constrained, at higher temperature. The effective Ea for growth along two orthogonal azimuths of an Fe13Ge8 island are 0.17 and 0.95eV. The temperature dependence in morphological evolution is due to anisotropy in corner barriers and ledge diffusion on orthogonal azimuths during growth.

Distortion analysis of pulsed terahertz signal measured with spectral-encoding technique

The principle of the spectral-encoding technique, one of the single-shot techniques for measurement of arbitrary pulsed terahertz (THz) signals, or T-rays, is demonstrated theoretically and through simulation. For single-cycle input THz pulse (bipolar wave form), THz signal could be retrieved faithfully under certain conditions, while for the multicycle input case, the THz signal cannot be retrieved without any distortion. However, there is an optimal length of the chirped-probe-pulse, corresponding to the characteristic time of the multicycle THz field, to minimize the distortion in the retrieved THz signal. Three main possible sources of distortions from both bipolar and multicycle THz signals are analyzed systematically. The first distortion originates from the mismatch of the optimal duration Tco of the chirped-probe-pulse and the T-ray length T. The second one is relevant to the spectrum bandwidth of the probe pulse. The third one occurs when the modulation depth k⪡1 is not satisfied. It comes from t...

Reduced Contact Resistance and Improved Surface Morphology of Ohmic Contacts on GaN Employing KrF Laser Irradiation

We employ excimer laser annealing for ohmic contact formation to n- and p-type GaN layers grown on sapphire substrates. The laser irradiation of the n-GaN layers led to increased nitrogen vacancies at the nitride surface, which promoted tunneling currents with a less resistive n-contact. For p-GaN layer, the laser irradiation increased the effective hole concentration that resulted in a reduced contact resistivity. The lowest specific contact resistance measured using the transmission line method was about 2.4 ×10-7 and 3.2 ×10-4 Ω cm2 for n- and p-contacts, respectively. Laser irradiation also resulted in a comparatively good surface morphology as compared to rapid thermal annealing, which in turn improved the transmittance of contacts for light extraction from active layers. It was found out that both the electrical and optical characteristics of the p-GaN contacts exhibited a good thermal stability and an improved transmittance in the blue–green spectral range. An increased forward current with a reduced ohmic contact resistance in such high thermal stable contacts enable the fabrication of GaN light emitting diodes.