Shang-Ju Tu

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%.

Inverted Al0.25Ga0.75N/GaN ultraviolet p-i-n photodiodes formed on p-GaN template layer grown by metalorganic vapor phase epitaxy

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.

Enhancement of the conversion efficiency of GaN-based photovoltaic devices with AlGaN/InGaN absorption layers

InGaN/sapphire-based p-i-n type photovoltaic (PV) devices were shown to have Al0.14Ga0.86N/In0.21Ga0.79N heterostructures that enhance the extraction of photogenerated carriers from active layers. With an appropriately increased barrier height in AlGaN/InGaN absorption layers, PV devices exhibit lower RS despite the increase in conduction-band discontinuity compared with GaN/InGaN superlattice absorption layers. This improvement can be attributed to polarization-induced electric fields enhanced by the incorporated aluminum in barrier layers. The enhancement is beneficial to increase built-in electric fields. Subsequently, the photogenerated carriers can escape more easily from recombination or scattering centers. Under 1 sun air-mass 1.5 standard testing conditions, the Al0.14Ga0.86N/In0.21Ga0.79N PV device exhibits high VOC (2.10 V) as well as an enhanced fill factor (0.66) and JSC (0.84 mA/cm2) corresponding to a power conversion efficiency of 1.16%.

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.

Femtosecond excitation of radial breathing mode in 2-D arrayed GaN nanorods

Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement observed a much reduced elastic stiffness constant (C11) of 193 ± 24 GPa in 35nm diameter nanorods, compared with the 365 ± 2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.

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.