[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this letter, the mechanism for improvement of the dynamic performance of GaN-based light-emitting diodes with an InGaN insertion layer is investigated using the very fast electrical-optical pump-probe technique. Our measurements indicate that, when the bias current is relatively low (100 A/cm<sup>2</sup>), the device with the InGaN insertion layer (device A) exhibits a shorter response time than does the control (device B) without such a layer. However, when the bias current density reaches 0.5 kA/cm<sup>2</sup>, devices A and B exhibit exactly the same response time during operation from room temperature to 200 <sup>°</sup>C. These results indicate that, under low current density (100 A/cm<sup>2</sup>), the piezoelectric (PZ) field inside device A will be stronger, which should result in a lower effective barrier height with a shorter carrier escape time than is the case for device B. On the other hand, under high bias current density, both devices have the same internal response time, which indicates the screening of the PZ field inside due to injected carriers. These dynamic measurement results suggest that the origin of the efficiency droop in our device under low and high bias current densities is carrier leakage and the Auger effect, respectively.
No preview · Article · Jun 2011 · IEEE Electron Device Letters
[Show abstract][Hide abstract] ABSTRACT: In this study, hetero-structure p-i-n type epitaxy wafers were deposited on the GaN/sapphire templates with different buffer layers by the MOVPE system. The absorption layers sandwiched in top p-GaN and bottom n+-GaN layers were designed into different short-period InGaN/GaN superlattice structures with specific pair numbers to maintain a total absorption thickness of 200 nm. As the buffer layer was properly adjusted, the VOC and JSC were enhanced by 35% and 95%, respectively. In addition to material qualities, the thickness of GaN buffer layers and piezoelectric-induced stain in the InGaN film itself also influenced the PV device performance.
Full-text · Article · Feb 2011 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: In this study, GaN-based light-emitting diodes (LEDs) with naturally formed oblique sidewall facets (OSFs) were fabricated through a selective regrowth process. The SiO₂ mask layer was patterned on a heavily doped n-GaN template layer rather than on a sapphire substrate. As a result, the periphery of the LED included several OSFs around the regrown GaN mesa. While processing the device, dry etching was unnecessary for exposing the n-GaN underlying layer in order to form the n-type Ohmic contacts. This could be attributed to the fact that the n-GaN template layer with an electron concentration of around 8 × 10¹⁸/cm³ was exposed after the removal of the SiO₂ mask layer. With an injection current of 20 mA, GaN-based LEDs with OSFs exhibited a 21% enhancement in light output compared with those that have vertical sidewall facets. The enhancement is attributed to the fact that photons extracted from OSFs can reduce internal absorption loss.
[Show abstract][Hide abstract] ABSTRACT: Inverted Al0.25Ga0.75N/GaN ultraviolet (UV) p-i-n photodiodes (PDs) were grown by selective-area regrowth on p-GaN template. The inverted devices with low-resistivity n-type AlGaN top-contact layers exhibited a typical zero-bias peak responsivity of 66.7 mA/W at 310 nm corresponding to the external quantum efficiency of 26.6%. The typical UV-to-visible (310/400 nm) spectral rejection ratio at zero-bias was over three orders of magnitude. The differential resistance and detectivity were obtained at approximately 6.2×1012 Ω and 3.4×1013 cm Hz1/2 W−1, respectively. Compared with conventional AlGaN/GaN-based UV p-i-n PDs, the proposed device structure can potentially achieve solar-blind AlGaN/GaN-based p-i-n PDs with low-aluminum content or aluminum-free p-contact layer and reduce excessive tensile strain due to the lattice mismatch between AlGaN and GaN layers.
[Show abstract][Hide abstract] ABSTRACT: In this letter, we display InGaN/GaN-based photovoltaic (PV) devices with active layers in absorbing the solar spectrum around blue regions. The GaN/In<sub>0.25</sub>Ga<sub>0.75</sub> 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 In<sub>0.25</sub>Ga<sub>0.75</sub>N 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/cm<sup>2</sup> corresponding to a conversion efficiency of around 0.58%.