Man Wong

Nickel induced crystallization of amorphous silicon thin films

Nickel (Ni) induced crystallization of amorphous silicon (a-Si) has been studied by selective deposition of Ni on a-Si thin films. The a-Si under and near the Ni-covered regions was found to be crystallized after heat treatment at 500 °C from 1 to 90 h. Micro-Auger electron spectroscopy revealed that a large amount of Ni stayed in the region under the original Ni coverage, but no Ni was detected either in the crystallized region next to the Ni coverage or in the amorphous region beyond the front of the laterally crystallized Si. X-ray photoelectron spectroscopy revealed a nonuniform Ni distribution through the depth of the crystallized film under the original Ni coverage. In particular, a Ni concentration peak was found to exist at the interface of the crystallized Si and the buried oxide. It was found that a layer of 5-nm-thick Ni could effectively induce lateral crystallization of over 100 μm of a-Si, but the lateral crystallization rate was found to decrease upon extended heat treatment. Transmission e...

Forced convection boiling in a microchannel heat sink

Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 /spl mu/m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.

Highly efficient organic light-emitting diodes with a silole-based compound

Efficient light emission was obtained in a silole-based organic light-emitting diode. A high luminous current efficiency of 20 cd/A, corresponding to an external quantum efficiency of 8%, was achieved. The apparent violation of the upper theoretical limit of 5.5% for the external quantum efficiency of a singlet emitter is discussed. With a suitably designed cathode, a high power efficiency of ∼14 lm/W was obtained. A strong dependence of the power efficiency on the thickness of Alq3 layer is also observed and explained.

Surface-chemistry technology for microfluidics

A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.

Characterization of the MIC/MILC interface and its effects on the performance of MILC thin-film transistors

Process and material characterization of the crystallization of amorphous silicon by metal-induced crystallization (MIC) and metal-induced lateral crystallization (MILC) using evaporated Ni has been performed. An activation energy of about 2 eV has been obtained for the MILC rate. The Ni content in the MILC area is about 0.02 atomic %, significantly higher than the solid solubility limit of Ni in crystalline Si at the crystallization temperature of 500/spl deg/C. A prominent Ni peak has been detected at the MILC front using scanning secondary ion mass spectrometry. The MIC/MILC interface has been determined to be highly defective, comprising a continuous grain boundary with high Ni concentration. The effects of the relative locations of this interface and the metallurgical junctions on TFT performance have been studied.

Phase change in microchannel heat sinks with integrated temperature sensors

A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels.

Coupling efficiency enhancement in organic light-emitting devices using microlens array-theory and experiment

Microlens arrays are introduced on glass substrates to improve the out-coupling efficiency of organic light-emitting devices (OLEDs). The microlenses suppress waveguiding loss in the substrate. A theoretical model, based on electromagnetic wave propagation and geometric ray tracing, is developed to simulate the enhancement effects and optimize the structure parameters of the lens pattern. A simple soft-lithography approach is employed to fabricate the microlens array on glass substrates. With the use of an optimized lens pattern, an increase of over 85% in the coupling efficiency of the OLED is expected theoretically. An increase of 70% in the coupling efficiency is achieved experimentally, without detrimental effect to the electrical performance of the OLED.

High-efficiency microcavity top-emitting organic light-emitting diodes using silver anode

Top-emitting organic light-emitting diodes (TOLEDs) employing highly reflective Ag as anode and semitransparent LiF∕Al∕Ag as cathode were fabricated. The hole injection efficiency of Ag anode can be significantly improved with surface modification using a CF4 plasma. With C545T-doped Alq3 emitter, the top-emitting device shows a low turn-on voltage of 2.65V. The optimized microcavity TOLED shows a current efficiency enhancement of 65% and a total outcoupling efficiency enhancement of 35%, compared with a conventional OLED. No color variation was observed in the forward 140° forward viewing cone. Strong dependence of efficiency on Ag cathode thickness was observed, in good agreement with numerical simulations.

Effects of longitudinal grain boundaries on the performance of MILC-TFTs

Compared to conventional solid phase crystallized (SPC) thin-film transistors (TFTs), metal induced laterally crystallized (MILC) TFTs exhibit significantly enhanced performance at reduced processing temperature. It is concluded that the major improvements in MILC-TFTs result from the growth of the crystal grains in a direction longitudinal to that of the current flow, whereas in SPC-TFTs, the grain boundaries are randomly oriented. It is also observed in this work that while the MILC-TFTs are less sensitive to short-channel effects (SCEs), their leakage current exhibits higher sensitivity to channel length reduction. These differences again can be traced to the different arrangements of the grain boundaries in the two types of devices.

Vanadium pentoxide modified polycrystalline silicon anode for active-matrix organic light-emitting diodes

Recently, polycrystalline silicon (p-Si) has been demonstrated to be an efficient anode for organic light-emitting diode (OLED) [X. L. Zhu, J. X. Sun, H. J. Peng, Z. G. Meng, M. Wong, and H. S. Kwok, Appl. Phys. Lett. 87, 083504 (2005)]. In this letter, we show that, by depositing an ultrathin vanadium pentoxide (V2O5) layer on the p-Si anode, the performance of the OLED can be greatly improved. Detailed x-ray photoelectron spectroscopy study shows that strong band bending occurs at the p-Si∕V2O5 interface, leading to much stronger hole injection. This modified p-Si anode can be integrated with the active p-Si layer of thin-film transistors in active-matrix OLED displays.