YewChung Sermon Wu

Improved crystal quality and performance of GaN-based light-emitting diodes by decreasing the slanted angle of patterned sapphire

Periodic triangle pyramidal array patterned sapphire substrates (PSSs) with various slanted angles were fabricated by wet etching. It was found beside normal wurtzite GaN, zinc blende GaN was found on the sidewall surfaces of PSS. The crystal quality and performance of PSS-LEDs improved with decrease in slanted angle from 57.4° to 31.6°. This is because most of the growth of GaN was initiated from c-planes. As the growth time increased, GaN epilayers on the bottom c-plane covered these pyramids by lateral growth causing the threading dislocation to bend toward the pyramids.

Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes

The effect of thermal annealing on current–voltage properties of GaN light emitting diodes (LEDs) has been studied. At annealing temperatures above 700 °C, the p–n junction of the diodes became very leaky and Ga-contained metallic bubbles were observed on the surface of Ni/Au p-ohmic contact. Transmission electron microscopy and energy dispersive x-ray spectrometer studies revealed that these metallic bubbles resided directly on top of the threading dislocations in GaN and both Ni and Au were indiffused into the LED structure along the cores of the TDs. The conducting paths formed by the metal containing dislocation cores are believed to be the cause for the observed short circuit behavior of p–n junctions at high annealing temperatures.

Improved luminance intensity of InGaN–GaN light-emitting diode by roughening both the p-GaN surface and the undoped-GaN surface

The InGaN–GaN epitaxial films were grown by low-pressure metal-organic chemical vapor deposition on a sapphire substrate, and then the light-emitting diode (LED) with double roughened (p-GaN and undoped-GaN) surfaces was fabricated by surface-roughening, wafer-bonding, and laser lift-off technologies. It was found that the front side luminance intensity of double roughened LED was 2.77 times higher than that of the conventional LED at an injection current of 20mA. The backside luminance intensity was 2.37 times higher than that of the conventional LED. This is because the double roughened surfaces can provide photons multiple chances to escape from the LED surface, and redirect photons, which were originally emitted out of the escape cone, back into the escape cone.

16-microm infrared generation by difference-frequency mixing in diffusion-bonded-stacked GaAs.

Tunable 90-ps 15.6-17.6-microm coherent radiation was generated by means of difference-frequency mixing in diffusion-bonded-stacked GaAs. The sample consisted of 24 alternately rotated layers with a total length of 6 mm and with low optical loss to achieve third-order quasi-phase matching. The wavelength-tuning curve was close to the theoretical prediction, demonstrating that the bonding process maintained nonlinear optical phase matching over the entire interaction length. Maximum conversion efficiency of 0.7%, or 5% internal quantum efficiency, was measured at 16.6 microm, consistent with the theoretical predictions.

Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes

The KrF pulsed excimer laser (248nm) and the frequency-tripled neodymium doped yttrium aluminum garnet laser (355nm) have been used to separate GaN thin films from sapphire substrates and transfer to bond other substrate. However, these processes would increase the dislocation density, resulting in an increase of the leakage current. In this study, the effects of these two laser sources on the reverse-bias leakages of InGaN–GaN light-emitting diodes were studied.

Enhanced performance of an InGaN-GaN light-emitting diode by roughening the undoped-GaN surface and applying a mirror coating to the sapphire substrate

An InGaN–GaN light-emitting diode (LED) with a roughened undoped-GaN surface and a silver mirror on the sapphire substrate was fabricated through a double transfer method. It was found that, at an injection current of 20mA, its luminance intensity was 100% larger than conventional LEDs. Its output power was 49% larger than conventional LEDs.

Enhanced Extraction and Efficiency of Blue Light-Emitting Diodes Prepared Using Two-Step-Etched Patterned Sapphire Substrates

Using a hot acid wet etching method, we have fabricated two types of patterned sapphire substrates: A pyramidal patterned sapphire substrate (PPSS) and a flat-top patterned sapphire substrate (FTPSS). After placing these samples into an atmospheric pressure metallorganic chemical vapor deposition system, we deposited standard InGaN light-emitting diode (LED) structures onto their surfaces. The crystal quality of these two surfaces was enhanced, as evidenced using X-ray diffraction (the full width at half-maximum decreased from 406.8 arcsec for the conventional sapphire to 356.4 and 349.2 arcsec for the PPSS and FTPSS samples, respectively). The output power of InGaN-based blue LEDs incorporating the PPSS and FTPSS improved to 17.9 and 18.7%, respectively, at 20 mA.

Improving the Electrical Properties of NILC Poly-Si Films Using a Gettering Substrate

Ni-metal-induced lateral crystallization (NILC) of amorphous silicon (alpha-Si) has been employed to fabricate poly-crystalline silicon (poly-Si) thin-film transistors. However, current crystallization technology often leads to Ni and NiSi2 precipitates being trapped, thus degrading the performance of the device. We proposed using alpha-Si-coated wafers as Ni-gettering substrates. After bonding the gettering substrate with the NILC poly-Si film, both the Ni-metal impurity within the NILC poly-Si film and the leakage current were greatly reduced, thus increasing the ON/OFF current ratio.

Improved Electrical Characteristics and Reliability of MILC Poly-Si TFTs Using Fluorine-Ion Implantation

In this letter, fluorine-ion (F+) implantation was employed to improve the electrical performance of metal-induced lateral-crystallization (MILC) polycrystalline-silicon thin-film transistors (poly-Si TFTs). It was found that fluorine ions minimize effectively the trap-state density, leading to superior electrical characteristics such as high field-effect mobility, low threshold voltage, low subthreshold slope, and high on/off-current ratio. F+-implanted MILC TFTs also possess high immunity against the hot-carrier stress and, thereby, exhibit better reliability than that of typical MILC TFTs. Moreover, the manufacturing processes are simple (without any additional thermal-annealing step), and compatible with typical MILC poly-Si TFT fabrication processes.

Improved GaAs Bonding Process for Quasi‐Phase‐Matched Second Harmonic Generation

A multilayer stack of bonded GaAs wafers, each layer rotated 180° from the adjacent one, has been proposed for quasi-phase-matched second harmonic generation. Current bonding technology, however, often leads to unacceptable optical losses and, therefore, poor device performance. In this study, three sources of optical losses were investigated: (i) interfacial defects between the wafers, (ii) bulk defects within the wafers, and (iii) decomposition at the exposed outer surfaces. Surface losses due to incongruent evaporation were easily eliminated by repolishing the outer surfaces. However, to minimize the losses from interfacial and bulk defects, it was necessary to investigate the relationship between these defects and the processing parameters. It was found that an increase in temperature and/or time led to a decrease in interfacial defects, but an increase in bulk and surface defects. Optimized processing conditions were developed which permit the preparation of stacks containing over 50 layers of (100) GaAs wafers, and about 40 layers of (110) GaAs wafers. Optical losses as low as 0.1 to 0.3 % per interface (at 5.3 and 10.6 μm) were observed for the (110) oriented multilayer structures.