Wei-Chih Lai

InGaN-GaN multiquantum-well blue and green light-emitting diodes

InGaN-GaN multiquantum-well (MQW) blue and green light-emitting diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and the properties of these LEDs were evaluated by photoluminescence (PL), double crystal X-ray diffraction, and electroluminescence (EL) measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of quantum well (QW) increased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well was high for the green MQW LEDs. The forward voltage V/sub f/ of green MQW LEDs was also found to be larger than that of blue MQW LEDs due to the same reason.

Influence of Si-doping on the characteristics of InGaN-GaN multiple quantum-well blue light emitting diodes

A detailed study on the effects of Si-doping in the GaN barrier layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs) has been performed. Compared with unintentionally doped samples, X-ray diffraction results indicate that Si-doping in barrier layers can improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs. It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier and an unintentionally doped barrier, respectively. These results suggests that one can significantly improve the performance of InGaN-GaN MQW LEDs by introducing Si doping in the GaN barrier layers.

InGaN-AlInGaN multiquantum-well LEDs

InGaN-GaN and InGaN-AlInGaN multiquantum-well (MQW) light-emitting diodes (LEDs) were both fabricated and their optical properties were evaluated by photoluminescence (PL) as well as electroluminescence (EL). We found that the PL peak position of the InGaN-AlInGaN MQW occurs at a much lower wavelength than that of the InGaN-GaN MQW. The PL intensity of the InGaN-AlInGaN MQW was also found to be larger. The EL intensity of the InGaN-AlInGaN MQW LED was also found to be larger than that of the InGaN-GaN MQW LED under the same amount of injection current. Furthermore, it was found that EL spectrum of the InGaN-AlInGaN MQW LED is less sensitive to the injection current. These observations all suggest that we can improve the properties of nitride-based LEDs by using AlInGaN as the barrier layer.

Demonstration of GaN-Based Solar Cells With GaN/InGaN Superlattice Absorption Layers

In this letter, we display InGaN/GaN-based photovoltaic (PV) devices with active layers in absorbing the solar spectrum around blue regions. The GaN/In0.25Ga0.75 N superlattice layers grown by metalorganic vapor-phase epitaxy are designed as the absorption layers with the same total thickness. The PV effect is almost absent when the In0.25Ga0.75N single layer is used as the absorption layer. This could be due to the large leakage current caused by the poor material quality and the relatively small shunt resistance. Devices with superlattice structure illuminated under a one-sun air-mass 1.5-G condition exhibit an open-circuit voltage of around 1.4 V and a short-circuit current density of around 0.8 mA/cm2 corresponding to a conversion efficiency of around 0.58%.

InGaN/GaN tunnel-injection blue light-emitting diodes

A charge asymmetric resonance tunneling (CART) structure was applied to nitride-based blue light emitting diodes (LEDs) to enhance their output efficiency. It was found that with a 20-nm-thick In/sub 0.18/Ga/sub 0.82/N electron emitter layer, we could increase the LED output intensity from 28.3 minicandela (mcd) to 43.2 mcd (i.e., a 53% increase). However, a further increase in electron emitter layer thickness will reduce the intensity due to relaxation. It was also found that we could decrease the 20 mA forward voltage from 4.16 V to 3.58 V with a proper electron emitter layer.

Oxidized Ni/Au Transparent Electrode in Efficient CH3NH3PbI3 Perovskite/Fullerene Planar Heterojunction Hybrid Solar Cells

UNLABELLED The successful application of a Ni/Au transparent electrode for fabricating efficient perovskite-based solar cells is demonstrated. Through interdiffusion of the Ni/Au bilayer, Au forms an interconnected metallic network structure as the transparent electrode. Ni diffuses to the bilayer surface and oxidizes into NiOx becoming an appropriate electrode interlayer. These ITO- and PEDOT PSS-free devices have potential applications in the design of future cost-effective, low-weight, and stable solar cells.

GaN-Based Light-Emitting Diode With Sputtered AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN-based light-emitting diodes (LEDs) were improved using a sputtered AlN nucleation layer. Replacing the in situ AlN nucleation layer with the sputtered AlN nucleation layer reduced the (002) and (102) X-ray rocking curve widths of the GaN layer from 318.0 to 201.1 and 412.5 to 225.0 arcsec, respectively. The -20-V reverse leakage current of the LEDs with the sputtered AlN nucleation layer is about three orders less than that of the LEDs with the in situ AlN nucleation layer. In addition, the LEDs with sputtered AlN nucleation layer could sustain more than 60% passing yield on the ESD test of under a -600-V machine mode, whereas the LEDs with the in situ AlN nucleation layer sustained less than 40% passing yield. Moreover, the 20-mA output power of the LEDs with the sputtered AlN nucleation layer also improved by approximately 5.73% compared with that of the LEDs with the in situ AlN nucleation layer.

Improved Performance of GaN-Based Blue LEDs With the InGaN Insertion Layer Between the MQW Active Layer and the n-GaN Cladding Layer

In this study, we demonstrate the effect of GaN-based blue light-emitting diodes (LEDs), using an InGaN layer inserted between the n-type GaN cladding layer and the active layer (InGaN/GaN multiple quantum well), on improving device performances. With a 20-mA current injection, the results indicate that the typical output power (or forward voltage) of light-emitting diodes grown with, and without, the InGaN insertion layer are approximately 18.1 (3.1) and 15.3(3.5) mW (V), respectively. This corresponds to an enhancement in output power (wall-plug efficiency) of around 18% (33%), with the use of the InGaN insertion layer. In addition, the electrostatic discharge (ESD) endurance voltages increased from 1000 V to 6000 V when the InGaN insertion layer was applied to the GaN/sapphire-based LEDs. The improvement of output power and ESD endurance voltage could be mainly due to the fact that the Si-doped InGaN insertion layer played the role of a current-spreading layer, which led to a lower possibility of junctions suffering a large current density in specific local sites.

Laser-induced periodic structures for light extraction efficiency enhancement of GaN-based light emitting diodes

The laser-induced periodic surface structure technique was used to form simultaneously dual-scale rough structures (DSRS) with spiral-shaped nanoscale structure inside semi-spherical microscale holes on p-GaN surface to improve the light-extraction efficiency of light-emitting diodes (LEDs). The light output power of DSRS-LEDs was 30% higher than that of conventional LEDs at an injection current of 20 mA. The enhancement in the light output power could be attributed to the increase in the probability of photons to escape from the increased surface area of textured p-GaN surface.

GaN-based light emitting diodes with embedded SiO2 pillars and air gap array structures

We demonstrated GaN-based light emitting diodes (LEDs) with different embedded heights of SiO2 pillars and air gap array structures. The air gap on top of the SiO2 pillars were also realized using the enhanced epitaxial lateral overgrowth mode. With the embedded SiO2 pillars and air gap array structures, we achieved a smaller reverse leakage current due to the lateral growth-induced crystal quality improvement. Moreover, under 20 mA current injections, the output powers were 3.04, 4.23, 4.66, and 4.44 mW for conventional LED, LEDs with embedded 200 and 500 nm height of SiO2 pillars and air gaps, 500 nm height of SiO2 pillars and air gaps, and 700 and 400 nm height of SiO2 pillars and air gaps, respectively. We found that the embedded 500 nm height SiO2 pillars and 500 nm height air gap array structures could enhance LED output power by more than 50% due to the enhanced guided-light scattering efficiency in our study.