Mohamed Ebaid

Towards highly efficient photoanodes: the role of carrier dynamics on the photoelectrochemical performance of InGaN/GaN multiple quantum well coaxial nanowires

The carrier dynamics in highly active InGaN/GaN coaxial nanowire photoanodes were studied for photoelectrochemical water splitting applications that can provide deeper insight to enhance the photon-to-electron conversion efficiency. The carrier dynamics in InGaN/GaN multiple quantum well coaxial nanowires (MQW-CNWs) with three different quantum well (QW) thicknesses and the same barrier thickness were studied optically using temperature-dependent and time-resolved photoluminescence spectroscopies. The role of the carrier dynamics on the photoelectrochemical water splitting (PEC-WS) performance of the MQW-CNWs was also investigated. The dependence of the PEC-WS performance and carrier dynamics on the QW thickness provided results indicative of the impact of the exciton localization and the defect states in the photoanodic performance of the MQW-CNWs. Strong localization effects and defect-induced recombination have been shown using samples with a thin QW with thicknesses up to 3 nm. During the PEC-WS, the samples showed a large onset potential and a low photocurrent density that led to low incident-photon-to-current conversion efficiency (IPCE). As the QW thickness approached 6 nm, negligible localization as well as improved photoemission quality were achieved, which lead to a small overpotential and a high IPCE of approximately 15%. The result demonstrated that an efficient photoanode requires a high crystal quality and weak localization, which can be achieved through careful structural optimization.

Optically pumped GaN vertical cavity surface emitting laser with high index-contrast nanoporous distributed Bragg reflector

Laser operation of a GaN vertical cavity surface emitting laser (VCSEL) is demonstrated under optical pumping with a nanoporous distributed Bragg reflector (DBR). High reflectivity, approaching 100%, is obtained due to the high index-contrast of the nanoporous DBR. The VCSEL system exhibits low threshold power density due to the formation of high Q-factor cavity, which shows the potential of nanoporous medium for optical devices.

Efficient energy harvesting of a GaN p–n junction piezoelectric generator through suppressed internal field screening

A high-efficiency GaN-based thin film piezoelectric energy harvester was demonstrated by suppressed screening of a piezoelectric field with the aid of a p–n diode junction. Piezoelectric field screening was effectively controlled by the deposition of highly resistive p-type GaN. The semi-intrinsic property of Mg-doped GaN and improved junction quality successfully suppressed internal screening, which resulted in a significantly enhanced output voltage up to 8.1 V and a maximum output current density of 3.0 μA cm−2. The energy-harvesting capabilities of the device were evaluated by charging a commercial capacitor, and self-powered light-emitting diode operation was demonstrated using the fabricated generator.

Vertically aligned InGaN nanowires with engineered axial In composition for highly efficient visible light emission.

We report on the fabrication of novel InGaN nanowires (NWs) with improved crystalline quality and high radiative efficiency for applications as nanoscale visible light emitters. Pristine InGaN NWs grown under a uniform In/Ga molar flow ratio (UIF) exhibited multi-peak white-like emission and a high density of dislocation-like defects. A phase separation and broad emission with non-uniform luminescent clusters were also observed for a single UIF NW investigated by spatially resolved cathodoluminescence. Hence, we proposed a simple approach based on engineering the axial In content by increasing the In/Ga molar flow ratio at the end of NW growth. This new approach yielded samples with a high luminescence intensity, a narrow emission spectrum, and enhanced crystalline quality. Using time-resolved photoluminescence spectroscopy, the UIF NWs exhibited a long radiative recombination time (τr) and low internal quantum efficiency (IQE) due to strong exciton localization and carrier trapping in defect states. In contrast, NWs with engineered In content demonstrated three times higher IQE and a much shorter τr due to mitigated In fluctuation and improved crystal quality.

Fabrication of vertical light emitting diode based on thermal deformation of nanoporous GaN and removable mechanical supporter.

A GaN vertical light emitting diode (LED) based on the novel lift-off method was demonstrated by high temperature regrowth over nanoporous (NP) GaN template formed by electrochemical (EC) etching. A two-step EC etching process was employed on a SiO2 patterned GaN surface to fabricate a nanoporous template with a controlled porosity profile, which enabled better structural stability than a single NP GaN. During the regrowth of LED structures, the high porosity GaN layer produced large coalesced voids due to the thermal deformation of nanopores. LED layers were then separated from the sapphire substrate and transferred to a Mo substrate by the removal of the SiO2 mechanical supporters that held the LED structure to suppress cracks and damage during the process. The vertical LEDs fabricated using this technique showed improved optical power emission as well as low series resistance.

Controlled growth mode of high-aspect-ratio GaN nanorods by Ni/In/Ga catalyst

Gallium nitride nanorods (GaN NRs) were grown by employing a low melting point Ni/In/Ga alloy via metalorganic chemical vapour deposition. A small growth temperature window was observed in the range of 720?765??C, which is lower than typical temperatures used for the growth of GaN NRs assisted by metal catalyst. Tapered GaN NRs with triangular cross-section were produced at 750??C by vapour?solid (VS) growth mechanism. A slight increase of temperature to 765??C was able to change the growth mode to vapour?liquid?solid (VLS) and quasi-aligned GaN NRs with high aspect ratio were produced. Photoluminescence of both GaN NR morphologies measured at 10?K revealed only near band edge emission centred at 3.48?eV, which was blue-shifted from that of the bulk GaN estimated at 3.46?eV. Micro-Raman spectroscopy performed at 300?K exhibited that GaN NRs grown either by VS or VLS growth mechanisms are relatively free of strain.

Controlled synthesis of GaN-based nanowires for photoelectrochemical water splitting applications

Photoelectrochemical (PEC) water splitting using semiconductor materials as light absorbers have been extensively studied. Several semiconducting materials have been proposed, such as TiO2, ZnO, and GaN. Because the efficiency of PEC water splitting is dependent on visible light absorption, the ability to tune the bandgap of GaN by alloying with In makes it advantageous over other wide bandgap semiconductors. The fabrication of GaN-based materials with nanoscale geometry offers more merit for their use in PEC water splitting. In this review, we provide an overview of the recent progress made in the synthesis and application of GaN-based nanomaterials in PEC water splitting. The outstanding challenges and the future prospects of this field will also be addressed.

Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen

Water splitting using InGaN-based photocatalysts may make a great contribution to future renewable energy production systems. Among the most important parameters that need to be optimized are those related to substrate lattice-matching compatibility. Here, we directly grow InGaN nanowires (NWs) on a metallic Ti/Si template, for improving the water splitting performance compared to a bare Si substrate. The open circuit potential of the epitaxially grown InGaN NWs on metallic Ti was almost two times higher than when directly grown on the Si substrate. The interfacial transfer resistance was also reduced significantly after introducing the metallic Ti interlayer. An applied-bias-photon-to-current conversion efficiency of 2.2% and almost unity faradaic efficiency for hydrogen generation were achieved using this approach. The InGaN NWs grown on Ti showed improved stability for hydrogen generation under continuous operation conditions, when compared to those grown on Si, emphasizing the role of the semiconductor-on-metal approach in enhancing the overall efficiency of water splitting devices.

Quantified Hole Concentration in AlGaN Nanowires for High-Performance Ultraviolet Emitters

p-Type doping in wide bandgap and new classes of ultra-wide bandgap materials has long been a scientific and engineering problem. The challenges arise from the large activation energy of dopants and high densities of dislocations in materials. We report here, a significantly enhanced p-type conduction using high-quality AlGaN nanowires. For the first time, the hole concentration in Mg-doped AlGaN nanowires is quantified. The incorporation of Mg into AlGaN was verified by correlation with photoluminescence and Raman measurements. The open-circuit potential measurements further confirmed the p-type conductivity, while Mott-Schottky experiments measured a hole concentration of 1.3 × 1019 cm-3. These results from photoelectrochemical measurements allow us to design prototype ultraviolet (UV) light-emitting diodes (LEDs) incorporating the AlGaN quantum-disks-in-nanowire and an optimized p-type AlGaN contact layer for UV-transparency. The ∼335 nm LEDs exhibited a low turn-on voltage of 5 V with a series resistance of 32 Ω, due to the efficient p-type doping of the AlGaN nanowires. The bias-dependent Raman measurements further revealed the negligible self-heating of devices. This study provides an attractive solution to evaluate the electrical properties of AlGaN, which is applicable to other wide bandgap nanostructures. Our results are expected to open doors to new applications for wide and ultra-wide bandgap materials.

Enhanced photoelectrochemical performance of InGaN-based nanowire photoanodes by optimizing the ionized dopant concentration

InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at TSi = 1120 °C exhibited almost 9 times higher current density than that of the undoped sample and a stoichiometric evolution of hydrogen and oxygen gases. Our systematic findings suggest that the PEC performance can be significantly improved by optimizing the Si doping level of InGaN-based NW photoanodes.InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at T...