Po-Tsung Lee

Small-Subthreshold-Swing and Low-Voltage Flexible Organic Thin-Film Transistors Which Use HfLaO as the Gate Dielectric

Pentacene organic thin-film transistors (OTFTs) with a high-kappa HfLaO dielectric were integrated onto flexible polyimide substrates. The pentacene OTFTs exhibited good performance, such as a low subthreshold swing of 0.13 V/decade and a threshold voltage of -1.25 V. The field-effect mobility was 0.13 cm2/Vmiddots at an operating voltage as low as only 2.5 V. These characteristics are attractive for high-switching-speed and low-power applications.

Plasmonic Coupling in Gold Nanoring Dimers: Observation of Coupled Bonding Mode

We investigate the optical properties of gold nanoring (NR) dimers in both simulation and experiment. The resonance peak wavelength of gold NR dimers is strongly dependent on the polarization direction and gap distance. As the gold NR particles approach each other, exponential red shift and slight blue shift of coupled bonding (CB) mode in gold NR dimers for longitudinal and transverse polarizations are obtained. In finite element method analysis, a very strong surface plasmon coupling in the gap region of gold NR dimers is observed, whose field intensity at the gap distance of 10 nm is enhanced 23% compared to that for gold nanodisk (ND) dimers with the same diameter. In addition, plasmonic dimer system exhibits a great improvement in the sensing performance. Near-field coupling in gold NR dimers causes exponential increase in sensitivity to refractive index of surrounding medium with decreasing the gap distance. Compared with coupled dipole mode in gold ND dimers, CB mode in gold NR dimers shows higher index sensitivity. This better index sensing performance is resulted form the additional electric field in inside region of NR and the larger field enhancement in the gap region owing to the stronger coupling of collective dipole plasmon resonances for CB mode. These results pave the way to design plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.

High sensitivity plasmonic index sensor using slablike gold nanoring arrays

We investigate the index sensing characteristics of plasmonic arrays based on square lattice slab-like gold nanorings with different ring widths. High sensitivity of 691 nm per RIU is obtained for nanorings with 199 nm width.

Lithographic tuning of a two-dimensional photonic crystal laser array

One attraction of photonic crystals is the ability to control optical device characteristics by lithographically varying the geometry. In this letter, we demonstrate a 10/spl times/10 array of optically pumped two-dimensional (2-D) photonic crystal defect lasers with varying lattice parameters. By adjusting the photonic crystal interhole spacing as well as the hole diameter we are able to tune the laser wavelength from 1500 to 1625 nm on a monolithic InP-InGaAsP wafer. A wavelength resolution of 10 nm from device to device was obtainable, limited by the lithography and etching tolerances of our fabrication method.

Ultra-high sensitivity optical stress sensor based on double-layered photonic crystal microcavity

In this report, we present the design principles to achieve a highly sensitive optical stress sensor. The structure we use is a double-layered (DL) photonic molecule with optical bonding and anti-bonding states based on whispering-gallery mode in photonic crystal microcavity. By applying finite-difference time-domain and finite-element methods, we simulate the change of optical properties (including wavelength and quality (Q) factor) of bonding mode caused by the DL structural variation due to the applied stress in two DL geometries. In the end, we summarize an optical stress sensor design with high Q factor, large structural response due to the applied stress, and large optical spectrum change due to the DL structural variation. The minimum detectable stress variation is estimated to be as small as 0.95 nN.

Low Subthreshold Swing HfLaO/Pentacene Organic Thin-Film Transistors

We have integrated a high-kappa HfLaO dielectric into pentacene-based organic thin-film transistors. We measured good device performance, such as a low subthreshold swing of 0.078 V/dec, a threshold voltage of -1.3 V, and a field-effect mobility of 0.71cm2/ Vldrs . This occurred along with an ON-OFF state drive current ratio of 1.0 times 105, when the devices were operated at only 2 V. The performance is due to the high gate-capacitance density of 950 nF/cm2 that is given by the HfLaO dielectric, which is achieved at an equivalent oxide thickness of only 3.6 nm with a low leakage current of 5.1 times 10-7 at 2 V.

Operation of photonic crystal membrane lasers above room temperature

Operation of photonic crystal lasers for substrate temperatures as high as 50 °C is reported. The temperature dependence of the lasing wavelength and the threshold pump power is also investigated. The characteristic temperature To is 37.7 K.

A flexible organic pentacene nonvolatile memory based on high-κ dielectric layers

We report a pentacene thin film transistor nonvolatile memory fabricated on a flexible polyimide substrate. This device shows a low program/erase voltage of 12 V, a speed of 1/100 ms, an initial memory window of 2.4 V, and a 0.78 V memory window after 48 h. This has been achieved by using a high-κ dielectric as charge trapping, blocking, and tunneling gate insulator layers.

High quality factor microcavity lasers realized by circular photonic crystal with isotropic photonic band gap effect

The photonic band gap (PBG) effect and its isotropy of sunflower-type circular photonic crystal (CPC) are obtained and investigated from the transmission spectra performed by finite-difference time-domain (FDTD) method. The PBG directional width variation is found to be only 6.7%. A well-confined whispering gallery mode (WGM) with azimuthal number of 6 is obtained by FDTD simulation from the CPC microcavity formed by seven missing air holes (C2). Ascribed to the deep and isotropic PBG confinement, the WGM lasing with very-low threshold (∼0.13mW) and very-high-quality (Q) factor (>10000) is obtained from well-fabricated CPC C2 microcavity lasers.

Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.