Chiao-Shun Chuang

Terahertz emission from vertically aligned InN nanorod arrays

Terahertz emission from indium nitride (InN) nanorods and InN film grown by molecular-beam epitaxy on Si(111) substrates has been investigated. Terahertz emission from InN nanorods is at least three times more intense than that from InN film and depends strongly on the size distribution of the nanorods. Surface electron accumulation at the InN nanorods effectively screens out the photo-Dember field in the accumulation layer formed under the surface. The nanorods with considerably large diameter than the thickness of accumulation layer are found to be dominant in the emission of terahertz radiation from InN nanorod arrays.

Intense terahertz emission from a-plane InN surface

We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis (a-plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p-polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry.

Terahertz spectroscopic study of vertically aligned InN nanorods

Terahertz time-domain spectroscopy has been used to investigate terahertz conductivity and dielectric response of indium nitride (InN) nanorod array and epitaxial film. The complex terahertz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using the Drude-Smith model. The electron mobility of the InN film is 1217±58cm2∕Vs, while that of the InN nanorods is 80±5cm2∕Vs. The reduced mobility of carriers for the latter can be attributed to the restricted carrier transport within the nanorods.Terahertz time-domain spectroscopy has been used to investigate terahertz conductivity and dielectric response of indium nitride (InN) nanorod array and epitaxial film. The complex terahertz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using the Drude-Smith model. The electron mobility of the InN film is 1217±58cm2∕Vs, while that of the InN nanorods is 80±5cm2∕Vs. The reduced mobility of carriers for the latter can be attributed to the restricted carrier transport within the nanorods.

Photoluminescence and photoluminescence excitation studies of as-grown and P-implanted GaN: On the nature of yellow luminescence

We have studied optical and electronic properties of isoelectronic P-implanted GaN films grown by metalorganic chemical vapor phase epitaxy. After rapid thermal annealing, a strong emission band around 430 nm was observed, which is attributed to the recombination of exciton bound to isoelectronic P-hole traps. From the Arrhenius plot, the hole binding energy of ∼180 meV and the exciton localization energy of 28 meV were obtained. According to first-principle total-energy calculations, the implantation process likely introduced NI and P-related defects. By using photoluminescence excitation technique, we found that the P-implantation-induced localized states not only increase the yellow luminescence but also suppress the transitions from the free carriers to deep levels.

Photocurrent Suppression of Transparent Organic Thin Film Transistors

Organic thin-film transistors (OTFTs) with high transmittance and low photosensitivity have been demonstrated. By using titanium dioxide nanoparticles as the additives in the polymer gate insulators, the level of device photoresponse has been reduced. The device shows simultaneously a high transparence and a minimal threshold voltage shift under white light illumination. It is inferred that the localized energy levels deep in the energy gap of pentacene behave as the recombination centers, enhancing substantially the recombination process in the conducting channel of the OTFTs. Therefore, the electron trapping is relieved and the shift of threshold voltage is reduced upon illumination.

Free Carrier Dynamics of InN Nanorods Investigated by Time-Resolved Terahertz Spectroscopy

Ultrafast time-resolved terahertz spectroscopy is employed to investigate the carrier dynamics of indium nitride (InN) nanorod arrays and an epitaxial film. Transient differential transmission of terahertz wave shows that hot carrier cooling and defect-related nonradiative recombination are the common carrier relaxation processes for InN film and nanorods. However, the electrons confined in the narrow structure of nanorods are significantly affected by the carrier diffusion process near the surface, which causes the abnormally long relaxation time for nanorods.

Time-resolved photoluminescence study of isoelectronic In-doped GaN films grown by metalorganic vapor-phase epitaxy

Time-resolved photoluminescence spectra were used to characterize isoelectronically doped GaN:In films. Our results indicate that the recombination lifetime of the donor-bound-exciton transition of undoped GaN exhibits a strong dependence on temperature. When In is doped into the film, the recombination lifetime decreases sharply from 68 to 30 ps, regardless of the measured temperature and In source flow rate. These observations might be related to the isoelectronic In impurity itself in GaN, which creates shallow energy levels that predominate the recombination process.

Enhanced field-effect mobility in pentacene based organic thin-film transistors on polyacrylates

We reported on organic thin-film transistors (OTFTs) with high dielectric constant polymer, poly(2,2,2-trifluoroethyl methacrylate) (PTFMA), as the gate dielectric. In top-contact OTFTs, the field-effect mobility was enhanced by applying a dielectric buffer layer poly(α-methylstyrene) to the bare PTFMA. After improving interfacial affinity within the active layer/dielectrics, deposited pentacene grain size and device performance were enhanced dramatically. The corresponding mobility, threshold voltage, and on/off current ratio were 0.70 cm2 V−1 s−1, −10.5 V, and 5.4×105, respectively. The moderately improved interface also suppressed the hole-trapping effect, which led to less hysteresis and minimized threshold voltage shift.

Organic thin-film transistors with color filtering functional gate insulators

We developed color filtering functional organic thin-film transistors exhibiting both high field-effect mobilities and color-filtering ability. The conventional colorant inks were utilized as the materials for the color filter/dielectric multifunction layers. In order to improve the electrical performance, a high dielectric polymeric insulator, poly(2,2,2-trifluoroethyl methacrylate), was introduced to modify the surface of the dielectric layer. Further, the Commission Internationale de L’Eclairage chromaticity coordinates were (0.64, 0.34), (0.36, 0.54), and (0.14, 0.15) for red, green, and blue devices, respectively, covering 49.2% National Television Systems Committee standard. This work represents one potential example for multifunctional organic electronics.

P‐10: CMOS‐Like Ambipolar Organic/Inorganic TFTs for AMLCD and AMOLED Applications

Air-stable ambipolar thin-film transistors (TFTs) based on double active layer of pentacene / a-IGZO (amorphous In2O3-Ga2O3-ZnO) have been fabricated on SiO2 /p-Si substrates. The a-IGZO exhibits n-channel behavior, while pentacene presents p-channel characteristics. Most n-type organic materials are easily affected by moisture and oxygen, thus the measurement of ambipolar devices in ambience air is difficult. However, a-IGZO not only has outstanding mobility but also has good stability while being measured in ambient air. In our work, a CMOS-like inverter was constructed by using two identical ambipolar transistors and the voltage gain up to 70 was obtained. The inverter can be operated in both the first and the third quadrants simplifying circuit design for AMFPD applications.