G. S. Chang

Co and Al co-doping for ferromagnetism in ZnO:Co diluted magnetic semiconductors

Co and Al co-doped ZnO diluted magnetic semiconductors are fabricated by a pulsed laser deposition and their electronic structure is investigated using x-ray absorption and emission spectroscopy. The Zn(0.895)Co(0.100)Al(0.005)O thin films grown under oxygen-rich conditions exhibit ferromagnetic behavior without any indication of Co clustering. The Co L-edge and O K-edge x-ray absorption and emission spectra suggest that most of the Co dopants occupy the substitutional sites and the oxygen vacancies are not responsible for free charge carriers. The spectroscopic results and first principles calculations reveal that the ferromagnetism in Co and Al co-doped ZnO semiconductors mainly arises from Al interstitial defects and their hybridization with Co substitutional dopants.

Adhesion enhancement of ion beam mixed Cu/Al/polyimide

Cu (400 A)/polyimide was mixed with 80 keV Ar+ and N2+ from 1.0×1015 to 2.0×1016 ions/cm2. The same processes were repeated for the Cu (400 A)/Al (50 A)/polyimide system which has Al as a buffer layer. The quantitative adhesion strength was measured by a standard scratch test. X-ray photoelectron spectroscopy was employed to investigate the change in the chemical bonds of the ion beam mixed polyimide substrate and the intermediate effects for the adhesion enhancement in Cu/Al/polyimide. Two distinct tendencies are observed in the adhesion strength: Cu/Al/polyimide is more adhesive than Cu/polyimide after ion beam mixing, and N2+ ions are more effective in the adhesion enhancement than Ar+. The formation of an interlayer compound of CuAl2O4 accounts for the former, while the latter is understood by the fact that N2+ ions produce more pyridinelike moiety, amide group and tertiary amine moiety which are known as adhesion promoters.

Buffer layer effect on the structural and electrical properties of rubrene-based organic thin-film transistors

The structural and electrical properties of organic thin-film transistors with rubrene/pentacene and pentacene/rubrene bilayered structures were investigated using x-ray diffraction, atomic force microscopy, and x-ray emission spectroscopy. High-quality rubrene thin films with orthorhombic structure were obtained in the rubrene/pentacene bilayer while the pentacene/rubrene bilayer only had an amorphous rubrene phase present. The rubrene/pentacene thin-film transistor shows more desirable current-voltage characteristics compared to the pentacene/rubrene transistor. The overall results suggest that the presence of a chemically active organic buffer layer and its associated crystal structure are crucial in enhancing the structural and electrical properties of rubrene-based transistors.

Universal Temperature Crossover Behavior of Electrical Conductance in a Single Oligothiophene Molecular Wire

We have observed and analyzed a universal temperature crossover behavior of electrical conductance in a single oligothiophene molecular wire. The crossover between the Arrhenius-type temperature dependence at high temperature and the temperature-invariant behavior at low temperature is found at a critical molecular wire length of 5.6 nm, where we found a change from the exponential length dependence to the length-invariant behavior. We have derived a scaling function analysis for the origin of the crossover behavior. After assuring that the analysis fits the explanation of the Keldysh Greens function calculation for the temperature dependence, we have applied it to our experimental results and found successfully that our scaling function gives a universal description of the temperature dependence for all over the temperature range.

Species- and tissue-specific bioaccumulation of arsenicals in various aquatic organisms from a highly industrialized area in the Pohang City, Korea.

Contamination of water and sediment with arsenic (As) in a highly industrialized area of Pohang City, Korea was investigated, with emphasis on in situ bioaccumulation of arsenicals by various aquatic organisms. Species- and tissue-specific concentrations of arsenicals were determined by use of HPLC-ICP/MS and μ-X-ray absorption near-edge structure (μ-XANES). Concentrations of arsenic in aquatic organisms were strongly associated with corresponding water concentrations, which indicates point sources associated with land use and activities. Arsenobetaine was the most dominant form of arsenic found in fishes, bivalves, crabs, and shrimps, while As(III) was predominant in freshwater snails. The μ-XANES analysis provided additional information about the unidentified arsenicals such as As-thiol. Arsenicals were mainly localized in intestine of mullet and marsh clam. Distribution and bioaccumulation of arsenic were strongly correlated with salinity, which indicates that natural processes controlling biogeochemistry of arsenic would be important in estuarine lotic system.

Mechanism for interfacial adhesion strength of an ion beam mixed Cu/polyimide with a thin buffer layer

A Cu (400 A)/Al (50 A)/polyimide system showed larger adhesion strength than that of Cu (400 A)/polyimide after N2+ ion beam mixing. X-ray emission spectroscopy was performed to elucidate the mechanism of adhesion enhancement of the ion beam mixed Cu (400 A)/polyimide with a thin Al buffer layer. Cu L2,3 x-ray emission spectra showed the formation of a CuAl2O4 layer which is strongly correlated with the large adhesion strength of a Cu/Al/polyimide. A decrease in adhesion strength at an ion dose higher than 5×1015 cm−2 was also explained by the formation of an amorphous carbon. This was understood by investigating C Kα x-ray emission spectra. The overall spectroscopic results were in accordance with the behavior of quantitative adhesion strength.

Clustering of impurity atoms in Co-doped anatase TiO2 thin films probed with soft x-ray fluorescence

The electronic structure of Co-doped anatase TiO(2) epitaxial thin films grown at different partial oxygen pressures is investigated using soft x-ray emission spectroscopy. The resonantly excited Co L(2,3) x-ray emission spectra of ferromagnetic Ti(0.96)Co(0.04)O(2) samples for the oxygen-deficient regime show that the ratio of integral intensities for Co L(2) and L(3) emission lines significantly decreases with respect to nonmagnetic samples in the oxygen-rich regime. This is due to L(2)L(3)M(4,5) Coster-Kronig transitions and suggests that ferromagnetic Ti(0.96)Co(0.04)O(2) samples have n-type charge carriers and Co-Co bonds between substitutional and interstitial Co atoms are present while Co-O bonds are dominant in nonmagnetic Ti(0.96)Co(0.04)O(2) samples in the oxygen-rich regime. Electronic structure calculations show that the presence of free charge carriers and Co segregation play a crucial role in strong ferromagnetism at room temperature in Co-doped TiO(2).

Analysis of octadecyltrichlorosilane treatment of organic thin-film transistors using soft x-ray fluorescence spectroscopy

The effect of octadecyltrichlorosilane (OTS) treatment on the electronic properties of organic thin-film transistors is investigated using soft x-ray absorption and emission spectroscopy. Analysis of Carbon Kα x-ray emission spectra reveals that treating the SiO2 layer with OTS prior to pentacene deposition increases the number of π-bonding states in the active pentacene layer, which is strongly correlated with the conduction of charge carriers. The role of the increased π-bonding states is verified by measuring the current-voltage characteristics of pentacene thin-film transistors with and without OTS treatment. Drain current and field-effect mobility of OTS-treated samples are significantly enhanced as anticipated from the spectroscopic analysis.

Probing the highly transparent and conducting SnOx/Au/SnOx structure for futuristic TCO applications

A SnOx/Au/SnOx transparent conductive oxide (TCO) multilayered film was fabricated with a total thickness of 75 nm using both e-beam and thermal evaporation techniques. X-ray diffraction confirms the amorphous nature of SnOx with a crystalline peak attributed to the presence of diffraction from the Au (111) plane. The morphological studies using atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed a smooth top layer of the sandwich structure. A Rutherford backscattering spectrum has been used to probe the thickness of individual TCO layers, and reveals an oxygen deficient structure in the SnOx layer. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) measurements confirm the formation an SnO-rich phase and the presence of oxygen vacancies. The specimen exhibited resistivity and sheet resistance of 3.9 × 10−4 Ω cm and 52 Ω sq.−1, respectively, with an optical transparency of 83% beyond 475 nm. The superior parameters exhibited by this stacked multilayer are due to relatively lower oxygen concentration in the tin oxide layer, and it is therefore proposed as a necessary ingredient to increase the overall conductivity in metal oxide multilayer thin films.

Efficient energy transfer from ZnO to Nd3+ ions in Nd-doped ZnO films deposited by magnetron reactive sputtering

In this paper, a detailed study of the luminescent properties of Nd3+ ions in sputtered ZnO thin films is reported for the first time. Experimental evidence is provided showing that Nd is inserted and optically active in the ZnO matrix. Despite the small amount (<2%) of rare earth in these thin ZnO films, intense luminescence signals have been collected, indicating efficient infrared emission of Nd3+ in ZnO. Direct excitation of Nd3+ ions in the ZnO matrix was possible, suggesting that most of the Nd atoms are in the 3+ form at all deposition temperatures. Moreover, intense Nd3+ emission has been recorded also when the host was excited, indicating that an efficient energy transfer occurs from ZnO to Nd ions. Both the transfer efficiency and the Nd3+ concentration seem to depend on the deposition temperature. In particular, indirect excitation of the sample deposited at 400 °C generates a richer emission pattern compared to lower temperatures. The careful analysis of the luminescence data indicated that the new pattern comes from Nd sites that cannot be efficiently directly excited, but that are characterized by intense emission under indirect excitation of the host. The possible transfer mechanisms leading to this behavior will be outlined.