Wu-Ching Chou

Photoluminescence of colloidal CdSe/ZnS quantum dots under oxygen atmosphere

The effects of oxygen versus vacuum ambients on colloidal CdSe/ZnS quantum dots (QDs) were studied using both continuous and time-resolved photoluminescence (PL) measurements. The PL intensities were found to be an order of magnitude higher in an oxygen atmosphere, which is explained by the passivation of surface defects by oxygen absorption. The decay of PL intensities can be best fitted by a biexponential function with lifetimes of approximately 1 ns for the fast decay and approximately 10 ns for the slow decay. Based on the emission-energy dependence of carrier lifetimes and of the amplitude ratio of the fast-decay component to the slow-decay component, we suggest that the fast and slow PL decay of colloidal CdSe/ZnS QDs is caused by the recombination of delocalized carriers in the internal core states and the localized carriers in the surface states, respectively.

Investigation of on-current degradation behavior induced by surface hydrolysis effect under negative gate bias stress in amorphous InGaZnO thin-film transistors

This study investigates the electrical instability under negative gate bias stress (NGBS) induced by surface hydrolysis effect. Electrical characteristics exhibit instability for amorphous InGaZnO (a-IGZO) Thin Film Transistors (TFTs) under NGBS, in which on-current degradation and current crowding phenomenon can be observed. When the negative gate bias is applied on the TFT, hydrogen ions will dissociate from ZnO-H bonds and the dissociated hydrogen ions will cause electrical instability under NGBS. The ISE-Technology Computer Aided Design simulation tool and moisture partial pressure modulation measurement are utilized to clarify the anomalous degradation behavior.

Carrier dynamics in isoelectronic ZnSe1−xOx semiconductors

This study explores the effects of both Oxygen and temperature on the carrier dynamics of isoelectronic ZnSe1−xOx (x=0.027 and 0.053) semiconductors using photoluminescence (PL) and time-resolved PL spectroscopy. We find that the Kohlrausch law is highly consistent with the complex decay traces induced by isoelectronic O traps, and the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. Complex recombination processes are clarified using the relaxation model based on various decay channels. These findings are consistent with the initial fall in the stretching exponent β followed by its monotonic increase with increasing temperature.

Robust magnetic polarons in type-II (Zn,Mn)Te/ZnSe magnetic quantum dots

We present a magneto-optical study of magnetic polarons in type-II Zn,Mn Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to 150 K, with a formation energy of 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field 2 meV at 8 T and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperaturedependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

Investigation of channel width-dependent threshold voltage variation in a-InGaZnO thin-film transistors

This Letter investigates abnormal channel width-dependent threshold voltage variation in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. Unlike drain-induced source barrier lowering effect, threshold voltage increases with increasing drain voltage. Furthermore, the wider the channel, the larger the threshold voltage observed. Because of the surrounding oxide and other thermal insulating material and the low thermal conductivity of the IGZO layer, the self-heating effect will be pronounced in wider channel devices and those with a larger operating drain bias. To further clarify the physical mechanism, fast IV measurement is utilized to demonstrate the self-heating induced anomalous channel width-dependent threshold voltage variation.

Effects of growth temperature on InN∕GaN nanodots grown by metal organic chemical vapor deposition

InN nanodots grown on GaN by metal organic chemical vapor deposition using conventional growth mode as well as flow-rate modulation epitaxy at various growth temperatures (550–730°C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN nanodots and their photoluminescence (PL) properties. The sample grown at around 650°C showed the highest growth rate. For samples grown at higher temperatures, the residual carrier concentration was reduced and the PL efficiency was improved. Furthermore, we found that the growth of InN nanodots is still sustainable even at a temperature higher than 700°C while retaining their optical quality.

Local geometric and electronic structures of gasochromic VOx films

VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were amorphous with mixed V(4+) and V(5+) valences. The amorphous VOx had a disordered and expanded lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into orthorhombic V2O5 at 300 °C, and the lamellar structure was eliminated at 400 °C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V(5+) and V(3+). Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5 crystalline phase irreversibly into an H1.43V2O5 phase. Based on the results of this study, amorphous VOx with a lamellar structure is recommended for use in H2 gas sensors.

Rashba spin splitting in parabolic quantum dots

We have investigated the effect of the Rashba spin splitting and a magnetic field on the energy levels of electrons in parabolic quantum dots. We find that with an increase in the Rashba parameter, the spin-orbit interaction mixes states of higher angular momentum together. In the absence of a magnetic field, the energy levels of the electrons are doubly degenerate and decrease as the Rashba parameter increases. In the presence of a magnetic field, this degeneracy is removed and the energy splitting of the spin states increases with the increase in both the Rashba parameter and the magnetic field. The Fermi energy level as a function of the magnetic field shows oscillatory behavior due to the crossings between the energy levels of the system. The magnetization of the electron gas is investigated and shows strong oscillations with the magnetic field for large values of the Rashba parameter.

Mechanism of Electrochemical Deposition and Coloration of Electrochromic V2O5 Nano Thin Films: an In Situ X-Ray Spectroscopy Study.

Electrochromic switching devices have elicited considerable attention because these thin films are among the most promising materials for energy-saving applications. The vanadium oxide system is simple and inexpensive because only a single-layer film of this material is sufficient for coloration. Vanadium dioxide thin films are fabricated by electrochemical deposition and cyclic voltammetry. Chronoamperometric analyses have indicated that the thin V2O5 film demonstrates faster intercalation and deintercalation of lithium ions than those of the thick V2O5 film, benefiting the coloration rate. Despite substantial research on the synthesis of vanadium oxides, the monitoring of electronic and atomic structures during growth and coloration of such material has not been thoroughly examined. In the present study, in situ X-ray absorption spectroscopy (XAS) is employed to determine the electronic and atomic structures of V2O5 thin films during electrochemical growth and then electrochromic coloration. In situ XAS results demonstrate the growth mechanism of the electrodeposited V2O5 thin film and suggest that its electrochromic performance strongly depends on the local atomic structure. This study improves our understanding of the electronic and atomic properties of the vanadium oxide system grown by electrochemical deposition and enhances the design of electrochromic materials for potential energy-saving applications.

Effects of isomeric transformation on characteristics of Alq3 amorphous layers prepared by vacuum deposition at various substrate temperatures

Organic tris(8-hydroxyquinoline)aluminum (Alq3) amorphous layers are prepared by vacuum deposition at various substrate temperatures Tsub from 30 to 180u2009°C. The surface morphology and electrical characteristics of these as-deposited layers are studied by atomic force microscopy and current-density versus electric-field (J-E) curves. The temperature dependence of the dark electrical conductivity σ(T) determined from J-E curves is also examined. These experimental results reveal that the surface and electrical properties of Alq3 amorphous layers deposited at Tsub between 90 and 120u2009°C exhibit an anomalous Tsub dependence. However, this anomalous Tsub dependence is not observed from infrared absorption measurements, and therefore is not the result of chemical degradation. The observed behavior is explained in terms of the property that the vacuum deposition of Alq3 with Tsub between 90 and 120u2009°C involves a thermal interconversion between meridional and facial Alq3 isomers.