Wei Hsiu Hung

Potential-controlled electrodeposition of gold dendrites in the presence of cysteine

Crystalline Au dendrites were formed by electrodeposition on a glassy carbon electrode from a solution of HAuCl(4) containing cysteine. The Au dendrites possessed a hierarchical architecture with three-fold symmetry; they comprised trunks, branches, and nanorod leaves, which all grew along the <111> direction.

Electrodeless wet etching of GaN assisted with chopped ultraviolet light

Electrodeless photoelectrochemical (PEC) etching of GaN was studied in a K2S2O8/KOH solution irradiated with ultraviolet (UV) light either continuously or periodically. The rate of recombination of electrons and holes at dislocation defects is greater than for crystalline GaN, resulting in a rough etched surface with hexagonal pyramids. The shorter the interval of UV irradiation, the smoother is the etched GaN surface; a chopped UV source thus serves to improve the morphology of the etched surface. A uniform and smooth GaN surface was obtained with a root-mean-square roughness 0.37 nm through electrodeless PEC etching in a solution (KOH 0.01 M, K2S2O8 0.05 M) with a chopper frequency 2500 Hz (i.e., duration of irradiation 0.2 ms).

Adsorption and thermal decomposition of H2S on Si(100)

Adsorption and thermal decomposition of H2S on Si(1 0 0)-2 � 1 are studied by means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation. The H2S molecule dis- sociates to form H and HS on the Si surface at adsorption temperature of 115 K. The Si(1 0 0)-2 � 1 surface structure is conserved upon the adsorption of H2S due to bonding of dissociative H and HS on two Si atoms in a dimer without breaking the Si-Si dimer bond. H2 and SiS are the only desorption products of thermal decomposition of H2S with peaks at 780 and 820 K, respectively. On the basis of TPD and XPS results, intermediates involved in decomposition of H2S and their adsorption configurations are proposed and discussed. 2002 Elsevier Science B.V. All rights reserved.

Surface etching of InP(100) by chlorine

The cleaning and etching of the InP(100) surface by chlorine gas is investigated using synchrotron-radiation photoemission spectroscopy. A clean InP surface with a 4◊2 configuration is obtained by ion sputtering or chlorine etching, followed by annealing to 650 K. The clean surface obtained by chlorine etching and annealing is indium-rich with the surface indium atoms showing metallic characterics. The chemisorption of chlorine leads to the formation of various InCl x (x=1‐3) and PCl species on the InP surface at 110 K and their corresponding chemical shifts are assigned. The chlorination of the InP surface causes surface band bending by about 0.36 eV at the saturation coverage. Argon-ion sputtering enhances the surface reactivity so that the sputtered surface can be chlorinated to a higher extent than the clean surface.

Probing the electronic structures of III–V-nitride semiconductors by x-ray photoelectron spectroscopy

The electronic structures of III–V-nitride semiconductors, including InGaAsN, GaAsN, and InAsN grown by molecular beam epitaxy, were investigated by high-resolution x-ray photoelectron spectroscopy using synchrotron radiation beam and low energy Ar+ sputtering. The N(1s) core-level photoelectron spectra show a single peak with a binding energy (Eb)∼398.0 eV of N–Ga bonding for GaAsN sample. For the InAsN samples, a single N(1s) peak at Eb∼397.0 eV of N–In bonding is observed. For the InGaAsN samples, the N(1s) spectra exhibit two peaks with a major component corresponding to N–In bonding, and a minor one to N–Ga bonding. The integrated N(1s) intensity of N–In bonding has a higher value than that of N–Ga bonding, in spite of the lower mole fraction of In for the InGaAsN sample. The data indicate that N has a bonding configuration with In-rich nearest neighbors in the InGaAsN samples.

Photoelectrochemical etching of InxGa1−xN

Photoelectrochemical (PEC) etching of InxGa1−xN in the KOH solution under illumination of a Hg-arc lamp is studied. An indium oxide surface layer is formed during PEC etching, which slows down the etching rate. The PEC etch rate of InxGa1−xN is determined by dissolution of indium oxides into the solution. Increase of the solution temperature results in an increase of solubility of indium oxides and thus enhances the PEC etch rate. It is found that stirring the solution can accelerate indium oxides to dissolve into the solution and increase the etch rate. The thick indium oxide layer on the PEC-etched InxGa1−xN surface can be effectively removed by the treatment of using a hot 6N HCl solution. A post-treatment by using a 3.2 M KOH solution can provide a smooth sidewall on the PEC-etched surface for the potential application to laser cavity.

Efficiency Enhancement of Light Extraction in LED With a Nano-Porous GaP Surface

Electrodeless photoelectrochemical etching is developed to produce a nano-porous n-GaP surface. Pores of diameter 300-700 nm are distributed on the surface with a density 1.2 times 108 cm-2. Such a porous surface structure exhibits a short mean free path for the transport of visible light and enhances photon scattering in a red AlInGaP-based light-emitting device. The efficiency of extraction of light emitted from the active layer becomes about 30%-50% greater than that without an etching treatment at a current of 20-40 mA.

Electrochemically fabricated gold dendrites with underpotential deposited silver monolayers for a bimetallic SERS-active substrate

With electrochemical deposition, cysteine-directed crystalline gold dendrites (Au-Ds) on glassy carbon electrodes were fabricated. The Au-Ds surfaces were further modified with Ag adatoms by underpotential deposition (UPD) for Ag-covered Au-Ds (Ag–Au-Ds). The Ag–Au-Ds possessed a hierarchical architecture comprising trunks, branches, and nanoleaves for a threefold-symmetry, resulting in a high density of sharp tips and edges for hot spots of surface-enhanced Raman scattering (SERS). Prominent SERS was observed with p-nitrothiophenol (p-NTP) adsorption onto either Au-Ds or Ag–Au-Ds, for a best p-NTP detection limit down to 5–10 nM at 785 nm laser excitation. However, at specific 633 nm laser excitation, SERS with p-NTP adsorption on Ag–Au-Ds exhibited a three-fold higher enhancement over that measured for p-NTP adsorbed on unmodified Au-Ds, suggesting an increased chemical SERS enhancement with the Ag–p-NTP bonding. Furthermore, adsorption isotherms of p-NTP with Au-Ds and Ag–Au-Ds adsorption in solution are established from solution p-NTP-concentration dependent SERS; from which, comparable binding constants of p-NTP to Au-Ds and Ag–Au-Ds are extracted.

Fabrication of Octadecyl and Octadecanethiolate Self-Assembled Monolayers on Oxide-Free Si(111) with a One-Cell Process

Self-assembled monolayers (SAM) of 1-octadecene (ODE) and 1-octadecanethiol (ODT) were deposited on an oxide-free Si(111) surface with a one-cell method. The etching and SAM deposition of Si(111) were performed in one cell containing immiscible solutions in two layers: an aqueous solution of NH4F and a toluene solution of organic SAM precursors (ODE and ODT). To remove surface Si oxides, the Si(111) surface was initially etched in the lower layer of NH4F solution. The Si as etched was subsequently moved directly to the upper solution of the precursors for deposition of the SAM under illumination of white light. This one-cell approach avoids the Si surface, as etched, coming in contact with the atmosphere, so eliminating oxidation and contamination. The ODE and ODT SAM were characterized with measurements with an atomic force microscope (AFM) and X-ray photoelectron spectra (XPS). The resulting ODE SAM was more stable than the ODT SAM and exhibited satisfactory resistance to oxidation under the ambient atmosphere. The ODT SAM prepared with this one-cell method exhibited a resistance to atmospheric oxidation better than with a two-cell method.

An 18dBm transmitter frontend with 29% PAE for 24GHz FMCW radar applications

A CMOS transmitter frontend integrated circuit suitable for 24-GHz frequency-modulated continuous-wave (FMCW) radar applications is presented in this paper. With a transformer-based power-combining technique and pseudo-differential class-AB power amplifiers (PAs), the proposed circuit exhibits enhanced output power and efficiency at a nominal supply voltage. In addition, a voltage-controlled oscillator is incorporated for frequency modulation purposes while differential amplifiers are utilized as the driver stages for the PAs to facilitate near-saturation circuit operation. The transmitter frontend is fabricated in a 65-nm CMOS process, demonstrating an output power of 18 dBm and a power-added efficiency (PAE) of 29% at 1.2-V supply voltage.