Yung-Ting Chen

Double side electroluminescence from p-NiO/n-ZnO nanowire heterojunctions

Double side light emission devices based on p-NiO/n-ZnO nanowire heterojunctions have been fabricated on indium tin oxide substrate by radio frequency magnetron sputtering combined with hydrothermal process. According to the energy band alignment, the detected broad visible and narrow ultraviolet electroluminescence arise from defect and band edge transitions in ZnO nanowires, respectively. The unique property of the double side emission is due to the nature of the large band gap of NiO film. It provides a good opportunity for the emission of a light emitting device with different colors on the top and back sides, simultaneously.

Mechanically tunable surface plasmon resonance based on gold nanoparticles and elastic membrane polydimethylsiloxane composite

Surface plasmon in nanoscaled materials has recently attracted a great deal of attention due to its possibility in a wide range of application. From a practical standpoint, it is desirable for the devices having a tunability of surface plasmon frequency. To achieve this goal, in this study, a composite consisting of two-dimensional gold nanoparticles array embedded in elastic polydimethylsiloxane (PDMS) membrane has been synthesized. Because the elastic PDMS membrane has a high malleability, with an external stress, it is very easy to regulate the interparticle distance in the gold nanoparticle array. The change in the distance between each nanoparticle will alter the surface plasmon interaction, and hence surface plasmon frequency can be manipulated. It is found that when the interparticle distance increases, the enhanced surface plasma mutual coupling will cause the blueshift of surface plasmon resonance frequency. The observed result satisfies the forecast based on electromagnetic theory.

Graphene-lead zirconate titanate optothermal field effect transistors

We have developed a pyroelectric field effect transistor (FET) based on a graphene-lead zirconate titanate (PZT) system. Under the incidence of a laser beam, the drain current can be increased or decreased depending on the direction of the polarization of the PZT substrate. The drain current sensitivity of the optothermal FET can reach up to 360 nA/mW at a drain field of 6.7 kV/m more than 5 orders of magnitude higher than that of the photogating transistors based on carbon nanotube on SiO2/Si substrate. Graphene is an excellent component for pyroelectric FET due to its high optical transparency and conductance.

Enhanced random lasing in ZnO nanocombs assisted by Fabry–Perot resonance

The ultraviolet random lasing behavior of an ensemble of ZnO nanocombs has been demonstrated. It is found that the Fabry-Perot resonance induced by nanocomb geometry can greatly enhance random lasing action with a low threshold condition. Besides, the emission spectra exhibit few sharp lasing peaks with a full width at half maximum (FWHM) of less than 0.3 nm and a narrow background emission with a FWHM of about 5 nm. Cathodoluminescence mapping images are utilized to analyze the Fabry-Perot resonance phenomenon. The resonant effect on the lasing system is further confirmed by nanocombs with different resonant cavity lengths. The unique lasing behavior induced by the simultaneous occurrence of Fabry-Perot resonance and random laser action shown here may open up a new possibility for the creation of highly efficient light emitting devices.

Giant white and blue light emission from Al2O3 and ZnO nanocomposites

A new and general approach enabling us to amplify not only the bandgap emission of ZnO nanorods but also the defect emission of Al(2)O(3) is proposed. The light intensity of the band edge emission of ZnO nanorods can be improved by as much as 19 times after the decoration of Al(2)O(3) layers. Moreover, white light emission arising from Al(2)O(3) defects in ZnO/Al(2)O(3) nanostructures also shows a large enhancement factor of 12 times. Our new strategy offers an alternative possibility to create strong white and blue light-emitting devices.

High crystalline quality ZnSe films grown by pulsed laser deposition

We have grown epitaxial ZnSe films on (001)GaAs substrates at 300 °C by pulsed laser deposition (PLD). Before the growth, thin buffer layers of GaAs are also grown by PLD at 300 °C. While the pattern of reflection high energy electron diffraction (RHEED) of the buffer layers is spotty, the pattern of the ZnSe films subsequently grown is streaky, and shows distinct Kikuchi lines and bands. The x‐ray rocking curve width of the films is as narrow as 150 arcsec. Photoluminescence (PL) at 10 K of the films shows free and bound excitons, donor‐acceptor pairs (DAP), and is free of any deep level emissions, indicating good crystalline quality of the films. Scanning electron microscopy (SEM) shows that the particulate number density of the films is only about 1 particulate per 400 μm2.

Ultraviolet electroluminescence from hybrid inorganic/organic ZnO/GaN/poly(3-hexylthiophene) dual heterojunctions.

Based on hybrid inorganic/organic n-ZnO nanorods/p-GaN thin film/poly(3-hexylthiophene)(P3HT) dual heterojunctions, the light emitting diode (LED) emits ultraviolet (UV) radiation (370 nm - 400 nm) and the whole visible light (400 nm -700 nm) at the low injection current density. Meanwhile, under the high injection current density, the UV radiation overwhelmingly dominates the room-temperature electroluminescence spectra, exponentially increases with the injection current density and possesses a narrow full width at half maximum less than 16 nm. Comparing electroluminescence with photoluminescence spectra, an enormously enhanced transition probability of the UV luminescence in the electroluminescence spectra was found. The P3HT layer plays an essential role in helping the UV emission from p-GaN material because of its hole-conductive characteristic as well as the band alignment with respect to p-GaN. With our new finding, the result shown here may pave a new route for the development of high brightness LEDs derived from hybrid inorganic/organic heterojuctions.

Highly sensitive MOS photodetector with wide band responsivity assisted by nanoporous anodic aluminum oxide membrane

A new approach for developing highly sensitive MOS photodetector based on the assistance of anodic aluminum oxide (AAO) membrane is proposed, fabricated, and characterized. It enables the photodetector with the tunability of not only the intensity but also the range of the response. Under a forward bias, the response of the MOS photodetector with AAO membrane covers the visible as well as infrared spectrum; however, under a reverse bias, the near-infrared light around Si band edge dominates the photoresponse. Unlike general MOS photodetectors which only work under a reverse bias, our MOS photodetectors can work even under a forward bias, and the responsivity at the optical communication wavelength of 850nm can reach up to 0.24 A/W with an external quantum efficiency (EQE) of 35%. Moreover, the response shows a large enhancement factor of 10 times at 1050 nm under a reverse bias of 0.5V comparing with the device without AAO membrane. The underlying mechanism for the novel properties of the newly designed device has been proposed.

Single ZnO nanowire–PZT optothermal field effect transistors

A new type of pyroelectric field effect transistor based on a composite consisting of single zinc oxide nanowire and lead zirconate titanate (ZnO NW-PZT) has been developed. Under infrared (IR) laser illumination, the transconductance of the ZnO NW can be modulated by optothermal gating. The drain current can be increased or decreased by IR illumination depending on the polarization orientation of the Pb(Zr(0.3)Ti(0.7))O(3) (PZT) substrate. Furthermore, by combining the photocurrent behavior in the UV range and the optothermal gating effect in the IR range, the wide spectrum of response of current by light offers a variety of opportunities for nanoscale optoelectronic devices.

The optical and electrical studies of hydrogen passivation in GaInP/GaAs heterostructures

It is shown that hydrogen passivation by the photochemical vapor deposition method can have a significant influence on GaInP/GaAs heterostructures. The effect has been investigated by low-temperature photoluminescence and current–voltage and capacitance–voltage experiments. The photoluminescence measurement shows a strong increase in the luminescence intensity after hydrogenation. It is interpreted in terms of the passivation of nonradiative recombination defect centers by atomic hydrogen. The effect is also accompanied by a simultaneous decrease in the carrier concentration as shown from the capacitance–voltage measurements. In addition, the effect of hydrogenation is confirmed by the improvement of the Schottky-diode properties. These results provide concrete evidence to support the passivation of impurities and defects by atomic hydrogen in GaInP/GaAs heterostructures.