Ahmed Y. Alyamani

Heterogeneous Lollipop-like V2O5/ZnO Array: A Promising Composite Nanostructure for Visible Light Photocatalysis

ZnO/V(2)O(5) core-shell nanostructures have been prepared by a two-step synthesis route through combined hydrothermal growth and magnetron sputtering. After annealing under oxygen ambience, a ZnO/V(2)O(5) heterogeneous lollipop-like nanoarray formed. The microstructure and crystal orientation of those nanolollipops were investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), which show single crystal structure. The optical properties were characterized by UV-vis spectroscopy and showed quite different absorption curves for the as-deposited and annealed samples. The ZnO/V(2)O(5) nanolollipops demonstrated excellent photocatalytic activity in terms of decomposing 2,6-dichlorophenol (2,6-DCP) under visible light, indicating their promising potential as catalysts for industrial wastewater and soil pollution treatments.

Improved Open- Circuit Voltage in ZnO-PbSe Quantum Dot Solar Cells by Understanding and Reducing Losses Arising from the ZnO Conduction Band Tail.

Colloidal quantum dot solar cells (CQDSCs) are attracting growing attention owing to significant improvements in efficiency. However, even the best depleted-heterojunction CQDSCs currently display open-circuit voltages (VOCs) at least 0.5 V below the voltage corresponding to the bandgap. We find that the tail of states in the conduction band of the metal oxide layer can limit the achievable device efficiency. By continuously tuning the zinc oxide conduction band position via magnesium doping, we probe this critical loss pathway in ZnO–PbSe CQDSCs and optimize the energetic position of the tail of states, thereby increasing both the VOC (from 408 mV to 608 mV) and the device efficiency.

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light-Emitting Diodes

Solution-processed polymer organic light-emitting diodes (OLEDs) doped with triplet-triplet annihilation (TTA)-upconversion molecules, including 9,10-diphenylanthracene, perylene, rubrene and TIPS-pentacene, are reported. The fraction of triplet-generated electroluminescence approaches the theoretical limit. Record-high efficiencies in solution-processed OLEDs based on these materials are achieved. Unprecedented solid-state TTA-upconversion quantum yield of 23% (TTA-upconversion reaction efficiency of 70%) at electrical excitation well below one-sun equivalent is observed.

Achieving Uniform Carrier Distribution in MBE-Grown Compositionally Graded InGaN Multiple-Quantum-Well LEDs

We investigated the design and growth of compositionally graded InGaN multiplequantum-well (MQW)-based light-emitting diodes (LEDs) without an electron-blocking layer. Numerical investigation showed uniform carrier distribution in the active region and higher radiative recombination rate for the optimized graded-MQW design, i.e., In0→xGa1→(1-x)N/InxGa(1-x)N/Inx→0Ga(1-x)→1N, as compared with the conventional stepped-MQW-LED. The composition-grading schemes, such as linear, parabolic, and Fermi-function profiles, were numerically investigated for comparison. The stepped- and graded-MQW-LEDs were then grown using plasma-assisted molecular beam epitaxy through surface-stoichiometry optimization based on reflection high-energy electron diffraction in situ observations. Stepped- and graded-MQW-LED showed efficiency roll over at 160 and 275 A/cm2, respectively. The extended threshold current density rollover (droop) in graded-MQW-LED is due to the improvement in carrier uniformity and radiative recombination rate, which is consistent with the numerical simulation.

Droop-free Al x Ga 1-x N/Al y Ga 1-y N quantum-disks-in-nanowires ultraviolet LED emitting at 337 nm on metal/silicon substrates

Currently the AlGaN-based ultraviolet (UV) solid-state lighting research suffers from numerous challenges. In particular, low internal quantum efficiency, low extraction efficiency, inefficient doping, large polarization fields, and high dislocation density epitaxy constitute bottlenecks in realizing high power devices. Despite the clear advantage of quantum-confinement nanostructure, it has not been widely utilized in AlGaN-based nanowires. Here we utilize the self-assembled nanowires (NWs) with embedding quantum-disks (Qdisks) to mitigate these issues, and achieve UV emission of 337 nm at 32 A/cm2 (80 mA in 0.5 × 0.5 mm2 device), a turn-on voltage of ~5.5 V and droop-free behavior up to 120 A/cm2 of injection current. The device was grown on a titanium-coated n-type silicon substrate, to improve current injection and heat dissipation. A narrow linewidth of 11.7 nm in the electroluminescence spectrum and a strong wavefunctions overlap factor of 42% confirm strong quantum confinement within uniformly formed AlGaN/AlGaN Qdisks, verified using transmission electron microscopy (TEM). The nitride-based UV nanowires light-emitting diodes (NWs-LEDs) grown on low cost and scalable metal/silicon template substrate, offers a scalable, environment friendly and low cost solution for numerous applications, such as solid-state lighting, spectroscopy, medical science and security.

High-brightness semipolar (2021¯) blue InGaN/GaN superluminescent diodes for droop-free solid-state lighting and visible-light communications

A high-brightness, droop-free, and speckle-free InGaN/GaN quantum well blue superluminescent diode (SLD) was demonstrated on a semipolar (2021¯) GaN substrate. The 447-nm emitting SLD has a broad spectral linewidth of 6.3 nm at an optical power of 123 mW. A peak optical power of 256 mW was achieved at 700 mA CW injection current. By combining YAG:Ce phosphor, SLD-generated white light shows a color-rendering index (CRI) of 68.9 and a correlated color temperature (CCT) of 4340 K. The measured frequency response of the SLD revealed a -3  dB bandwidth of 560 MHz, thus demonstrating the feasibility of the device for both solid-state lighting (SSL) and visible-light communication (VLC) applications.

High-speed 405-nm superluminescent diode (SLD) with 807-MHz modulation bandwidth

III-nitride LEDs are fundamental components for visible-light communication (VLC). However, the modulation bandwidth is inherently limited by the relatively long carrier lifetime. In this letter, we present the 405 nm emitting superluminescent diode (SLD) with tilted facet design on semipolar GaN substrate, showing a broad emission of ~9 nm at 20 mW optical power. Owing to the fast recombination (τe<0.35 ns) through the amplified spontaneous emission, the SLD exhibits a significantly large 3-dB bandwidth of 807 MHz. A data rate of 1.3 Gbps with a bit-error rate of 2.9 × 10-3 was obtained using on-off keying modulation scheme, suggesting the SLD being a high-speed transmitter for VLC applications.

Bandgap measurements and the peculiar splitting of E2H phonon modes of InxAl1-xN nanowires grown by plasma assisted molecular beam epitaxy

The dislocation free InxAl1-xN nanowires (NWs) are grown on Si(111) by nitrogen plasma assisted molecular beam epitaxy in the temperature regime of 490 °C–610 °C yielding In composition ranges over 0.50 ≤ x ≤ 0.17. We study the optical properties of these NWs by spectroscopic ellipsometry (SE), photoluminescence, and Raman spectroscopies since they possesses minimal strain with reduced defects comparative to the planar films. The optical bandgap measurements of InxAl1-xN NWs are demonstrated by SE where the absorption edges of the NW samples are evaluated irrespective of substrate transparency. A systematic Stoke shift of 0.04–0.27 eV with increasing x was observed when comparing the micro-photoluminescence spectra with the Tauc plot derived from SE. The micro-Raman spectra in the NWs with x = 0.5 showed two-mode behavior for A1(LO) phonons and single mode behavior for E2H phonons. As for x = 0.17, i.e., high Al content, we observed a peculiar E2H phonon mode splitting. Further, we observe composition dep...

Semipolar InGaN-based superluminescent diodes for solid-state lighting and visible light communications

III-nitride light emitters, such as light-emitting diodes (LEDs) and laser diodes (LDs), have been demonstrated and studied for solid-state lighting (SSL) and visible-light communication (VLC) applications. However, for III-nitride LEDbased SSL-VLC system, its efficiency is limited by the “efficiency droop” effect and the high-speed performance is limited by a relatively small -3 dB modulation bandwidth (<100 MHz). InGaN-based LDs were recently studied as a droop-free, high-speed emitter; yet it is associated with speckle-noise and safety concerns. In this paper, we presented the semipolar InGaN-based violet-blue emitting superluminescent diodes (SLDs) as a high-brightness and high-speed light source, combining the advantages of LEDs and LDs. Utilizing the integrated passive absorber configuration, an InGaN/GaN quantum well (QW) based SLD was fabricated on semipolar GaN substrate. Using SLD to excite a YAG:Ce phosphor, white light can be generated, exhibiting a color rendering index of 68.9 and a color temperature of 4340 K. Besides, the opto-electrical properties of the SLD, the emission pattern of the phosphor-converted white light, and the high-speed (Gb/s) visible light communication link using SLD as the transmitter have been presented and discussed in this paper.

Semipolar III–nitride quantum well waveguide photodetector integrated with laser diode for on-chip photonic system

A high-performance waveguide photodetector (WPD) integrated with a laser diode (LD) sharing the single InGaN/GaN quantum well active region is demonstrated on a semipolar GaN substrate. The photocurrent of the integrated WPD is effectively tuned by the emitted optical power from the LD. The responsivity ranges from 0.018 to 0.051 A/W with increasing reverse bias from 0 to 10 V. The WPD shows a large 3 dB modulation bandwidth of 230 MHz. The integrated device, being used for power monitoring and on-chip communication, paves the way towards the eventual realization of a III–nitride on-chip photonic system.