Shea-Lin Ng

New Insights on the Burstein-Moss Shift and Band Gap Narrowing in Indium-Doped Zinc Oxide Thin Films

The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.

Insights on semiconductor-metal transition in indium-doped zinc oxide from x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and x-ray diffraction

ZnO thin films doped with various amounts of In impurities were prepared by magnetron sputtering at a substrate temperature of 150°C. The shift in optical bandgap of the In-doped ZnO films is studied as a function of carrier concentration. Nominally doped ZnO films exhibit an increase in the measured optical band gap known as the Burstein-Moss effect. Dominant band gap narrowing is observed with increased doping. XPS and TOFSIMS analyses confirm that In is incorporated in the ZnO material. The In 3d peaks show that no metallic In is present as a result of heavy doping. The XRD phase analysis shows a preferential c-axis growth but a shift of the ZnO (002) peak to lower 2-theta values with increasing FWHM as the carrier concentration increases indicates the decline in the quality of crystallinity. An elongation of the c lattice constant is also observed and is likely to be caused by intersitital In as the amount of In dopants increases. The incorporation of In induces a semiconductor-metal transition betwee...

Fabrication of titanium dioxide nanotubes in fluoride-free electrolyte via rapid breakdown anodization

This paper presents the study on the growth of bundled titanium dioxide (TiO2) nanotubes via rapid breakdown anodization method by using chloride-based electrolyte. The effects of different ratios of deionized water (DI) to ethylene glycol (EG) on the morphological, structural and photoelectrochemical properties of the TiO2 nanotubes generated were investigated. Different ratios of DI:EG showed significant changes on the diameter of nanotube bundles. Besides that, X-ray diffraction measurements revealed that anatase phase of titanium dioxide appeared within the thermally treated samples. Scherrer method was applied to calculate the mean crystallite size of the crystal growth in this study. The photoelectrochemical properties of TiO2 nanotube bundles were characterized by using three-electrode photoelectrochemical cell and showed good photocurrent response and stability.

Effect of zinc on the growth mechanism of zinc oxide nanostructures in the nitrogen environment

The effect of zinc on the growth mechanism of zinc oxide (ZnO) nanostructures is investigated by annealing zinc-rich sputtered ZnO thin films and ZnO single crystals that are partially coated with metallic zinc (Zn). Evidence from scanning electron microscopy, x-ray photoelectron spectroscopy as well as Raman and photoluminescence measurements suggests that excess Zn may induce the growth of ZnO nanostructures. This mechanism provides the possibility of large area selective growth.

(R)- and (S)-1-(2-(benzyloxy)-3-methoxyphenyl)-2,2,2-trichloroethyl benzenesulfonate

A novel chiral compound was synthesized from the reaction between the new benzimidazole, 2-(2-benzyloxy-3-methoxyphenyl)-1H-benzimidazole 8 and benzenesulfonyl chloride 9 in dry dichloromethane DCM at 45oC for 10 hr in the presence of 4-N,N-dimethyl aminopyridine DMAP 12 as a catalyst was expected to obtain 2-(2-benzyloxy-3-methoxyphenyl)-1-(phenylsulfonyl)-1Hbenzimidazole, 10. Unfortunately, a novel chiral compound (R) and (S) 1-(2-benzyloxy-3methoxyphenyl)-2,2,2-trichloroethyl benzenesulfonate 11 was obtained as a single crystal (59% yield) with melting point of 58.4°C. We suggest preliminary mechanisms of the formation of 11 by two ways: a) It is formed benzimidazolide ion 13 that is attacked from benzene sulfonic acid 15, which is hydrolyzed from 9 to form N-(2-aminophenyl)-2-(benzyloxy)-3-methoxybenzimidine benzenesulfonate 17, or b) that benzimidazole 8 is hydrolyzed to its basic compound benzyl o-vanillin 18, which it attacks the 4-(dimethylamino)-1-(phenylsulfonyloxy) pyridinium chloride 21 to form (2(benzyloxy)-3-methoxyphenyl) (phenylsulfonyloxy) methylium ion 22. However, the mechanism of this reaction still is under investigation.