Feng-Wen Huang

High-performance GaN metal–insulator–semiconductor ultraviolet photodetectors using gallium oxide as gate layer

In this study, gallium nitride (GaN)-based metal-insulator-semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with a gallium oxide (GaO(x)) gate layer formed by alternating current bias-assisted photoelectrochemical oxidation of n-GaN are presented. By introducing the GaO(x) gate layer to the GaN MIS UV PDs, the leakage current is reduced and a much larger UV-to-visible rejection ratio (R(UV/vis)) of spectral responsivity is achieved. In addition, a bias-dependent spectral response results in marked increase of the R(UV/vis) with bias voltage up to ~10(5). The bias-dependent responsivity suggests the possible existence of internal gain in of the GaN MIS PDs.

Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin Oxide ohmic contacts

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.

Characteristics of InGaN-based concentrator solar cells operating under 150X solar concentration

InGaN/sapphire-based photovoltaic (PV) cells with blue-band GaN/InGaN multiple-quantum-well absorption layers grown on patterned sapphire substrates were characterized under high concentrations up to 150-sun AM1.5G testing conditions. When the concentration ratio increased from 1 to 150 suns, the open-circuit voltage of the PV cells increased from 2.28 to 2.50 V. The peak power conversion efficiency (PCE) occurred at the 100-sun conditions, where the PV cells maintained the fill factor as high as 0.70 and exhibited a PCE of 2.23%. The results showed great potential of InGaN alloys for future high concentration photovoltaic applications.

Effect of Growth Pressure of Undoped GaN Layer on the ESD Characteristics of GaN-Based LEDs Grown on Patterned Sapphire

The effect of growth pressure of underlying undoped GaN(u-GaN) layer on the electrical properties of GaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSS) is evaluated. The electrostatic discharge (ESD) endurance voltages could increase from 4000 to 7000 V when the growth pressure of u-GaN layers is increased from 100 to 500 torr, while the forward voltages and light output powers remain almost the same. Poor ESD endurance ability could be attributed to the underlying GaN layer grown under relative low pressure, which leads to significant surface pits. This could be further attributed to the imperfect coalescence of crystal planes above the convex sapphire patterns. The pits are associated with TDs behaving as a leakage path to degrade electrical performance.

Linear photon up-conversion of 450 meV in InGaN/GaN multiple quantum wells via Mn-doped GaN intermediate band photodetection.

Up-converted heterostructures with a Mn-doped GaN intermediate band photodetection layer and an InGaN/GaN multiple quantum well (MQW) luminescence layer grown by metal-organic vapor-phase epitaxy are demonstrated. The up-converters exhibit a significant up-converted photoluminescence (UPL) signal. Power-dependent UPL and spectral responses indicate that the UPL emission is due to photo-carrier injection from the Mn-doped GaN layer into InGaN/GaN MQWs. Photons convert from 2.54 to 2.99 eV via a single-photon absorption process to exhibit a linear up-conversion photon energy of ~450 meV without applying bias voltage. Therefore, the up-conversion process could be interpreted within the uncomplicated energy level model.

Enhanced Light Output of GaN-Based Light-Emitting Diodes With Embedded Voids Formed on Si-Implanted GaN Layers

GaN-based LEDs grown on Si-implanted GaN templates form air gaps beneath the active layer to enhance light-extraction efficiency. GaN-based epitaxial layers grown on selective Si-implanted regions had lower growth rates compared with those grown on implantation-free regions, resulting in selective growth and the subsequent over the Si-implanted regions. Accordingly, air gaps were formed over the Si-implanted regions after the meeting of laterally growing GaN facet fronts. The experimental results indicate that the light-output power of the LEDs grown on the Si-implanted GaN templates was enhanced by 36% compared with conventional LEDs. This enhancement in output power was attributed mainly to the air gaps, which led to a higher escape probability for the photons.

Vertical InGaN light-emitting diodes with a sapphire-face-up structure

Vertical GaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using the wafer-bonding technique. Lapping and dry-etching processes were performed for thinning the sapphire substrate instead of removing this substrate using the laser lift-off technique and the thinning process associated with the wafer-bonding technique to feature LEDs with a sapphire-face-up structure and vertical conduction property. Compared with conventional lateral GaN/sapphire-based LEDs, GaN/Si-based vertical LEDs exhibit higher light output power and less power degradation at a high driving current, which could be attributed to the fact that vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction. In addition, with an injection current of 350 mA, the output power (or forward voltage) of fabricated vertical LEDs can be enhanced (or reduced) by a magnitude of 60% (or 5%) compared with conventional GaN/sapphire-based LEDs.

Immersed finger-type indium tin oxide ohmic contacts on p-GaN photoelectrodes for photoelectrochemical hydrogen generation

In this study, we demonstrated photoelectrochemical (PEC) hydrogen generation using p-GaN photoelectrodes associated with immersed finger-type indium tin oxide (IF-ITO) ohmic contacts. The IF-ITO/p-GaN photoelectrode scheme exhibits higher photocurrent and gas generation rate compared with p-GaN photoelectrodes without IF-ITO ohmic contacts. In addition, the critical external bias for detectable hydrogen generation can be effectively reduced by the use of IF-ITO ohmic contacts. This finding can be attributed to the greatly uniform distribution of the IF-ITO/p-GaN photoelectrode applied fields over the whole working area. As a result, the collection efficiency of photo-generated holes by electrode contacts is higher than that of p-GaN photoelectrodes without IF-ITO contacts. Microscopy revealed a tiny change on the p-GaN surfaces before and after hydrogen generation. In contrast, photoelectrodes composed of n-GaN have a short lifetime due to n-GaN corrosion during hydrogen generation. Findings of this study indicate that the ITO finger contacts on p-GaN layer is a potential candidate as photoelectrodes for PEC hydrogen generation.

Enhanced output power of GaN-based LEDs with embedded AlGaN pyramidal shells

In this article, the characteristics of GaN-based LEDs grown on Ar-implanted GaN templates to form inverted Al0.27Ga0.83N pyramidal shells beneath an active layer were investigated. GaN-based epitaxial layers grown on the selective Ar-implanted regions had lower growth rates compared with those grown on the implantation-free regions. This resulted in selective growth, and formation of V-shaped concaves in the epitaxial layers. Accordingly, the inverted Al0.27Ga0.83N pyramidal shells were formed after the Al0.27Ga0.83N and GaN layers were subsequently grown on the V-shaped concaves. The experimental results indicate that the light-output power of LEDs with inverted AlGaN pyramidal shells was higher than those of conventional LEDs. With a 20 mA current injection, the output power was enhanced by 10% when the LEDs were embedded with inverted Al0.27Ga0.83N pyramidal shells. The enhancement in output power was primarily due to the light scattering at the Al0.27Ga0.83N/GaN interface, which leads to a higher escape probability for the photons, that is, light-extraction efficiency. Based on the ray tracing simulation, the output power of LEDs grown on Ar-implanted GaN templates can be enhanced by over 20% compared with the LEDs without the embedded AlGaN pyramidal shells, if the AlGaN layers were replaced by Al0.5Ga0.5N layers.

Mn-doped GaN as photoelectrodes for the photoelectrolysis of water under visible light

Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical (PEC) cells made of Mn-doped GaN photoelectrodes. In addition to its absorption of the ultraviolet spectrum, Mn-doped GaN photoelectrodes could absorb photons in the visible spectrum. The photocurrents measured from PEC cells made of Mn-doped GaN were at least one order higher than those measured from PEC cells made of undoped GaN-working electrodes. Under the visible light illumination and a bias voltage below 1.2 V, the Mn-doped GaN photoelectrodes could drive the water splitting reaction for hydrogen generation. However, hydrogen generation could not be achieved under the same condition wherein undoped GaN photoelectrodes were used. According to the results of the spectral responses and transmission spectra obtained from the experimental photoelectrodes, the enhanced photocurrent in the Mn-doped GaN photoelectrodes, compared with the undoped GaN photoelectrodes, was attributable to the Mn-related intermediate band within the band gap of GaN that resulted in further photon absorption.