Lung-Hsing Hsu

Enhanced photocurrent of a nitride-based photodetector with InN dot-like structures

The InN dot-like layer was applied in the gallium nitride based material for the purpose of infrared photodetectors (PDs). This InN layer was grown by a low-pressure metal organic chemical vapor deposition technology under different growth temperatures. The X-ray diffraction patterns provide the information of crystal structure and the hexagonal orientation was detected. The Raman shifts and photoluminescence were also used to characterize the quality of InN film. Finally, the fabricated Schottky-type photodetector was tested under a solar simulator and a long-wavelength laser (λ = 1550nm). The measurements show a highly linear relation between photo-generated currents and laser powers for the wavelength of 1550 nm. In the photonic detection range suitable for optical fiber communiation, a quantum efficiency of 9.2% can be observed.

Frequency tracking and stabilization of a tunable dual-wavelength external-cavity diode laser

Abstract We show a unique dual-wavelength external-cavity laser diode with frequency tracking capability and obtain a stable beat frequency between the dual-wavelength output. By using a Fabry–Perot interferometer as the frequency discriminator and the time-gating technique in a servo loop, the peak-to-peak frequency fluctuations were stabilized, with respect to the Fabry–Perot cavity, to 86 kHz in the dual-wavelength output at 802.5 and 804.5 nm, and to 17 kHz in their 0.9 THz beat signal. Similar performance was achieved for tuning of the dual wavelength separation ranging from 0.2 to 4 nm.

Influence of the microstructure geometry of patterned sapphire substrates on the light extraction efficiency of GaN LEDs

The influence of the microstructure geometry of patterned sapphire substrates (PSS) on the light extraction efficiency (LEE) of GaN light-emitting diodes (LEDs) is numerically analyzed. Cone structures of various dimensions are studied, along with dome and mixed microstructures. LEE is found to mainly depend on the microstructure surface slope. LEE rises quickly with slope and flattens out when the slope exceeds 0.6. Scaling down the microstructure has little effect on LEE. Light rays are found to travel longer distances in PSS LEDs, as compared with LEDs grown on a flat substrate. Keeping GaN absorption loss low is important for LEE optimization.

Enhanced power conversion efficiency in InGaN-based solar cells via graded composition multiple quantum wells.

This work demonstrates the enhanced power conversion efficiency (PCE) in InGaN/GaN multiple quantum well (MQWs) solar cells with gradually decreasing indium composition in quantum wells (GQWs) toward p-GaN as absorber. The GQW can improve the fill factor from 42% to 62% and enhance the short current density from 0.8 mA/cm2 to 0.92 mA/cm2, as compares to the typical MQW solar cells. As a result, the PCE is boosted from 0.63% to 1.11% under AM1.5G illumination. Based on simulation and experimental results, the enhanced PCE can be attributed to the improved carrier collection in GQW caused by the reduction of potential barriers and piezoelectric polarization induced fields near the p-GaN layer. The presented concept paves a way toward highly efficient InGaN-based solar cells and other GaN-related MQW devices.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Crown-Shaped Patterned Sapphire Substrates

In this letter, we report the high performance GaN-based light-emitting diodes (LEDs) with embedded air void array grown by metal-organic chemical vapor deposition. The donut-shaped air void was formed at the interface between crown-shaped patterned sapphire substrates (CPSS) and the GaN epilayer by conventional photolithography. The transmission electron microscopy images demonstrate that the threading dislocations were significantly suppressed by epitaxial lateral overgrowth (ELOG). The Monte Carlo ray-tracing simulation reveals that the light extraction of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.

InN nanopillar photodetector with enhanced infrared response using indium-tin oxide nanorods

Enhanced infrared photoresponse is observed in InN pillars/ITO rods photodetectors fabricated by LP-MOCVD and oblique-angle electron beam evaporation. The enhanced IR portion photocurrent as high as 19% can be measured via AM1.5G solar simulated spectra.

InN nanopillar devices with strong photoresponse

The InN pillars/p-GaN is promising for extended visible and infrared absorption. The higher growth temperature and more V/III could trend toward hexagonal InN pillars epitaxy, fabricated by a low-pressure metal organic chemical vapor deposition. The tensile strain effect and the peak energy blue-shift phenomenon due to higher Fermi-level to acceptor emission were investigated via Raman and photoluminescence (PL) measurements. The high quality InN pillars reveal no indium droplets by X-ray diffraction pattern. A InN pillars photodetection device is demonstrated with extended IR response, and the portion photocurrent of InN detection as high as 13% measured via AM1.5G solar simulated spectra.

Numerical study on doping and positioning effect of type-II GaSb/GaAs quantum ring layer on solar cell performances

In this work, we demonstrate the doping concentration and positioning effect of the type-II quantum well (QR) on the solar cell performances in terms of numerical simulation. The variation in doping concentration and location can affect the band diagram seriously and possibly form the back surface field which can either facilitate or deteriorate the carrier collection. A wide range of parameters are calculated to reveal this trend and the previous experimental results are also discussed.

A single InN nanopillar photodetector with extended infrared response grown by MOCVD

An extended infrared photoresponse is observed in a high quality InN pillar/p-GaN photodetector with self-assembly epitaxy grown by LP-MOCVD. The IR portion photocurrent as high as 14.2% can be measured via AM1.5G solar simulated spectra.

Optical properties of InN-based photodetection devices

The InN capped with GaN structures is promising for extended visible and infrared absorption. The low growth temperature of GaN/InN epitaxy was fabricated by a low-pressure metal organic chemical vapor deposition system. The strain effect and mixed InxGa1-xN complex of the GaN/InN layer were investigated via Raman and photoluminescence (PL) measurements. The capping GaN with similar growth temperature of previous InN growth induces the gallium diffusion mechanism to form InxGa1-xN complex. The blue-shift phenomenon of multiple GaN/InN peaks with increasing growth temperature was attributed to residual strain and higher Ga content of InGaN. A ZnO/GaN/InN photodetection device is demonstrated with extended IR response, and the quantum efficiency is 2.28%.