Papers by Author: Lee Siang Chuah

Abstract: Porous tungsten oxide (WO₃) nanofibers were synthesized by electrospinning aqueous solutions of polyvinylpyrrolidone (PVP) and ammonium metatungstate (AMT). The as-spun fibers and their annealing were studied by scanning electron microscope (SEM). SEM results showed that the WO₃ fibers have a large amount of pores with diameters ranging from 50 nm to 100 nm, and pure monoclinic WO₃ nanoscaled fibers formed between 500 and 600 °C. When a higher temperature (700 °C) was used, the tungsten oxide nanofibers totally disintegrated. When heated at 600 °C, the fibers broken into nanorods by the mechanical pressure. However, when the annealing treatment was low (500 °C), the fiber-like crystalline structure that can be seen. When the optimal temperature was used, the structure of the obtained WO₃ nanofibers with novel morphology and crystallinity were captured.

Abstract: ZnO nanosheetlike structures were synthesized on zinc (Zn) foil substrates by electrochemical deposition method in ZnCl₂ aqueous solutions at a temperature of 90 °C. In addition, the synthetic parameters in this work allow additional structural direction for ZnO nanoscaled structures. The morphology growth from smooth plane structures to nanosheet like structures could be accomplished by modifying the current densities of electrodeposition. In the photoluminescence (PL) spectra of the as-synthesized ZnO samples, typically there are few oxygen vacancies or interstitial Zn centers would be produced when the electrochemical deposition was performed out with a low current density. The UV peak is usually considered as the characteristic emission of ZnO nanosheetlike structures and attributed to the band edge emission or the exciton transition. All XRD diffraction peaks of ZnO nanosheetlike structures are shown in a good agreement with hexagonal structure. The average particle size was calculated using the Debye-Scherrer formula. ZnO nanosheetlike structures processed for various current densities have different size.

Abstract: Excellent tin oxide (SnO₂) ohmic and Schottky contacts are need for device utilizations and essential electrical characterization. Up to now, metal contact property studies on SnO₂ are inadequate and provide miscellaneous results. Ohmic contacts have been studied on high quality epitaxial n-type tin oxide thin films that were grown by solid state chemical vapor deposition (SSCVD). To the best of our knowledge, this is the first time that Al-Zn co-doped SnO₂ films grown by SSCVD have been reported. Non-alloyed Al/Ag, Al, and Ag contacts were characterized by current-voltage measurements. Ohmic contacts were realized using Al, Ag , and Al/Ag after an air treatment of the SnO₂ surface.

Abstract: In this work, we present results about the preparation and characterization of stannous oxide (SnO) thin films. SnO thin films were obtained via thermal evaporation method from SnO₂ powder as source material. These thin films were successfully deposited onto well cleaned glass substrates by thermal evaporation technique. The as deposited thin films were of thickness of 2500 Å and film were post-deposition annealed in air ambient at 400°C for 20 min and 40 min, respectively in a furnace. As-deposited films are highly conductive and p type. The best p-type SnO film annealed at 400 °C for 40 min shows a resistivity of 1.05 Ω·cm and a relatively high hole concentration of 2 × 10¹⁷ cm³ at room temperature. The X-ray diffraction (XRD) patterns of annealed films exhibit a polycrystalline hexagonal wurtzite structure without preferred orientation. The scanning electron microscopy (SEM) image shows the presence of uniformly dispersed spherical in shaped SnO particles. The mean grain sizes (diameter) are calculated to be about 80 and 100 nm for the p-type SnO films prepared at 400 °C for 20 min, and 40 min, respectively. Room temperature photoluminescence (PL) spectra of the SnO film exhibit two emission bands, around the wavelength of 300 nm and 450 nm. All spectra were measured at room temperature.

Abstract: A high-quality crack-free AlN cap layer on GaN layer has been achieved using an AlN buffer layer directly grown on a silicon substrate at high temperature by radio frequency (RF) plasma-assisted molecular beam epitaxy. A two dimensional (2D) growth process guide to AlN cap layer of high grade crystal quality. The nucleation and the growth dynamics have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ by high resolution transmission electron microscopy (HR-TEM). The microstructure was investigated by energy-dispersive X-ray spectroscopy (EDX). It was disclosed that AlN is single crystalline with low defect. High densities of V-shaped pits were not detected at the interface between AlN and GaN layers. Contradictory the earlier reported V-shaped defects in nitride-based alloys; these V-shaped pits were condensed on top of the AlN layer because of H2 etching of the surface when a high temperature growth discontinuity between AlN and GaN layers.

Abstract: This paper presents the structural and optical studies of porous GaN sample compared to the corresponding as grown GaN. The samples were investigated by scanning electron microscopy (SEM), high resolution x-ray diffraction (HRXRD), and photoluminescence (PL). The porous area is very uniform, with pore diameter in the range of 80-110 nm. XRD measurements showed that the (0002) diffraction plane peak width of porous samples was slightly broader than the as-grown sample. PL measurements revealed that the near band edge peak of the porous samples were redshifted. Metal-semiconductor-metal (MSM) photodiode was fabricated on the samples. For as grown GaN sample, this detector shows a sharp cut-off wavelength at 362 nm. A maximum responsivity of 0.258 A/W was achieved at 360 nm. For the porous GaN sample, this detector shows a sharp cut-off wavelength at 364 nm. A maximum responsivity of 0.771 A/W was achieved at 363 nm.