Po g Chen

Vertical InGaN-based green-band solar cells operating under high solar concentration up to 300 suns

InGaN/GaN-based solar cells with vertical-conduction feature on silicon substrates were fabricated by wafer bonding technique. The vertical solar cells with a metal reflector sandwiched between the GaN-based epitaxial layers and the Si substrate could increase the effective thickness of the absorption layer. Given that the thermally resistive sapphire substrates were replaced by the Si substrate with high thermal conductivity, the solar cells did not show degradation in power conversion efficiency (PCE) even when the solar concentrations were increased to 300 suns. The open circuit voltage increased from 1.90 V to 2.15 V and the fill factor increased from 0.55 to 0.58 when the concentrations were increased from 1 sun to 300 suns. With the 300-sun illumination, the PCE was enhanced by approximately 33% compared with the 1-sun illumination.

Improved conversion efficiency of GaN-based solar cells with Mn-doped absorption layer

GaN-based solar cells with Mn-doped absorption layer grown by metal-organic vapor-phase epitaxy were investigated. The transmittance spectrum and the spectral response showed the presence of an Mn-related band absorption property. Power-dependent, dual-light excitation, and lock-in amplifier techniques were performed to confirm if the two-photon absorption process occurred in the solar cells with Mn-doped GaN absorption layer. Although a slight decrease in an open circuit voltage was observed, a prominent increase in the short circuit current density resulted in a significant enhancement of the overall conversion efficiency. Under one-sun air mass 1.5 G standard testing condition, the conversion efficiency of Mn-doped solar cells can be enhanced by a magnitude of 5 times compared with the cells without Mn-doped absorption layer.

Selective Growth of AlGaN-Based p-i-n UV Photodiodes Structures

Al0.25Ga0.75N ultraviolet (UV) p-i-n photodiodes (PDs) with top and inverted p-layer structure are created by selective-area growth on a GaN template layer. All PDs exhibit typical zero bias peak responsivity at 310 nm and the UV-to-visible (310/400 nm) spectral rejection ratio is greater than three orders of magnitude. In contrast to conventional AlGaN-based p-i-n photodiodes, the inverted devices with low-resistivity n-type AlGaN top-contact layers exhibit a nearly flat spectral response at the short wavelength regions (210-310 nm). The proposed device structure can achieve solar-blind, AlGaN/GaN-based p-i-n PDs with high-aluminum content and reduced tensile strain due to the lattice mismatch between AlGaN and GaN layers.

InGaN Flip-Chip Light-Emitting Diodes With Embedded Air Voids as Light-Scattering Layer

The performance of GaN-based flip-chip light-emitting diodes (LEDs) with embedded air voids grown on a selective-area Ar-implanted sapphire (SAS) substrate was demonstrated in this letter. The GaN-based epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on SAS was greater than that of LEDs grown on implantation-free sapphire substrates. The output power of LEDs grown on SAS was enhanced by 20% at an injection current of 700 mA. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs.

Acoustic spectroscopy for studies of vitreous silica up to 740 GHz

Due to the high attenuation in vitreous silica, acoustic attenuations in the THz regime are typically measured by incoherent techniques such as Raman, neutron, and X-ray scattering. Here, we utilized multiple-quantum-well structures to demonstrate acoustic spectroscopy of vitreous silica up to ∼740 THz. The acoustic properties of silica thin films prepared by physical and chemical deposition methods were characterized in the sub-THz regime. This technique can be useful in resolving debated issues relating to Boson peak around 1 THz.

Vertical GaN-Based LEDs With Naturally Textured Surface Formed by Patterned Sapphire Substrate With Self-Assembled Ag Nanodots as Etching Mask

We propose a new pattern-transfer method comprising laser liftoff and wafer bonding process; the former was used to separate GaN-based epitaxial layers from patterned sapphire substrates (PSSs) with self-assembled surface nanodots, and the latter was used to combine the epitaxial layers with the Si substrate. The nanodots on the PSS surface were formed by dry etching with self-assembled Ag nanoislands as an etching mask. The fabricated vertical GaN-based light-emitting diodes (VLEDs) presented an in situ nanotextured surface. At a driving current of 350 mA, the VLEDs increased the output power by ~17% compared with the conventional VLEDs without a nanotextured surface. The main enhancement can be attributed to the increase in light-extraction efficiency of photons emitted inside the LEDs.

Carrier dynamics of Mn-induced states in GaN thin films

GaN-based materials are widely used for light emission devices, but the intrinsic property of wide bandgap makes it improper for photovoltaic applications. Recently, manganese was doped into GaN for absorption of visible light, and the conversion efficiency of GaN-based solar cells has been greatly improved. We conducted transient optical measurements to study the carrier dynamics of Mn-doped GaN. The lifetime of carriers in the Mn-related intermediate bands (at 1.5 eV above the valence band edge) is around 1.7 ns. The carrier relaxation within the Mn-induced bandtail states was on the order of a few hundred picoseconds. The relaxation times of different states are important parameters for optimization of conversion efficiency for intermediate-band solar cells.

Improved light extraction of nitride-based flip-chip light-emitting diodes by forming air voids on Ar-implanted sapphire substrate

GaN-based flip-chip light emitting diodes (FC-LEDs) with embedded air voids grown on a selective-area Arimplanted AlN/sapphire (AIAS) substrate was demonstrated in this study. The proposed FC LED with an embedded light scattering layer can destroy the light interference and thereby increase the LEE of GaN-based flip-chip LEDs. The epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on AIAS was greater than that of LEDs grown on implantation free sapphire substrates. At an injection current of 700 mA, the output power of LEDs grown on AIAS was enhanced by 20% compared with those of LEDs without embedded air voids. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs. This study on FC LEDs with embedded light-scattering layer highlights the potential application of these LEDs as an alternative to conventional patterned sapphire substrates for improving the LEE of GaN/sapphire-based LEDs. Based on ray tracing simulation, if the height and the width of bottom of gaps were increased to 3 μm, the Lop could be enhanced over 60%.

THz acoustic spectroscopy based on GaN nanostructures

Due to the high attenuation in vitreous silica, acoustic attenuations in the THz regime are typically measured by incoherent techniques such as Raman, neutron, and X-ray scattering. Here, we utilized multiple-quantum-well structures to demonstrate acoustic spectroscopy of vitreous silica up to THz regime. The acoustic properties of silica thin films prepared by chemical deposition methods were characterized in the sub-THz regime. This technique may be useful in resolving debated issues relating to Boson peak around 1 THz.