Kai Cui

Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy.

We report on the achievement of wafer-level photocatalytic overall water splitting on GaN nanowires grown by molecular beam epitaxy with the incorporation of Rh/Cr(2)O(3) core-shell nanostructures acting as cocatalysts, through which H(2) evolution is promoted by the noble metal core (Rh) while the water forming back reaction over Rh is effectively prevented by the Cr(2)O(3) shell O(2) diffusion barrier. The decomposition of pure water into H(2) and O(2) by GaN nanowires is confirmed to be a highly stable photocatalytic process, with the turnover number per unit time well exceeding the value of any previously reported GaN powder samples.

p-Type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111).

Full-color, catalyst-free InGaN/GaN dot-in-a-wire light-emitting diodes (LEDs) were monolithically grown on Si(111) by molecular beam epitaxy, with the emission characteristics controlled by the dot properties in a single epitaxial growth step. With the use of p-type modulation doping in the dot-in-a-wire heterostructures, we have demonstrated the most efficient phosphor-free white LEDs ever reported, which exhibit an internal quantum efficiency of ∼56.8%, nearly unaltered CIE chromaticity coordinates with increasing injection current, and virtually zero efficiency droop at current densities up to ∼640 A/cm(2). The remarkable performance is attributed to the superior three-dimensional carrier confinement provided by the electronically coupled dot-in-a-wire heterostructures, the nearly defect- and strain-free GaN nanowires, and the significantly enhanced hole transport due to the p-type modulation doping.

Controlling Electron Overflow in Phosphor-Free InGaN/GaN Nanowire White Light-Emitting Diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.

One-step overall water splitting under visible light using multiband InGaN/GaN nanowire heterostructures.

The conversion of solar energy into hydrogen via water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. Therefore, development of visible light-responsive and efficient photocatalyst material has been of immense interest, but with limited success. Here, we show that overall water splitting under visible-light irradiation can be achieved using a single photocatalyst material. Multiband InGaN/GaN nanowire heterostructures, decorated with rhodium (Rh)/chromium-oxide (Cr2O3) core-shell nanoparticles can lead to stable hydrogen production from pure (pH ∼ 7.0) water splitting under ultraviolet, blue and green-light irradiation (up to ∼560 nm), the longest wavelength ever reported. At ∼440-450 nm wavelengths, the internal quantum efficiency is estimated to be ∼13%, the highest value reported in the visible spectrum. The turnover number under visible light well exceeds 73 in 12 h. Detailed analysis further confirms the stable photocatalytic activity of the nanowire heterostructures. This work establishes the use of metal-nitrides as viable photocatalyst for solar-powered artificial photosynthesis for the production of hydrogen and other solar fuels.

Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon

We report on the achievement of a new class of nanowire light emitting diodes (LEDs), incorporating InGaN/GaN dot-in-a-wire nanoscale heterostructures grown directly on Si(111) substrates. Strong emission across nearly the entire visible wavelength range can be realized by varying the dot composition. Moreover, we have demonstrated phosphor-free white LEDs by controlling the indium content in the dots in a single epitaxial growth step. Such devices can exhibit relatively high internal quantum efficiency (>20%) and no apparent efficiency droop for current densities up to ~ 200 A cm(-2).

Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon.

In this paper, we have performed a detailed investigation of the temperature- and current-dependent emission characteristics of nanowire light-emitting diodes, wherein InGaN/GaN dot-in-a-wire nanoscale heterostructures and a p-doped AlGaN electron blocking layer are incorporated in the devices active region to achieve white-light emission and to prevent electron overflow, respectively. Through these studies, the Auger coefficient is estimated to be in the range of ∼10(-34) cm(6) s(-1) or less, which is nearly four orders of magnitude smaller than the commonly reported values of planar InGaN/GaN heterostructures, suggesting Auger recombination plays an essentially negligible role in the performance of GaN-based nanowire light-emitting diodes. It is observed, however, that the performance of such nanowire LEDs suffers severely from Shockley-Read-Hall recombination, which can account for nearly 40% of the total carrier recombination under moderate injection conditions (∼100 A cm(-2)) at room temperature. The Shockley-Read-Hall nonradiative lifetime is estimated to be in the range of a few nanoseconds at room temperature, which correlates well with the surface recombination velocity of GaN and the wire diameters used in this experiment.

High efficiency ultraviolet emission from AlxGa1−xN core-shell nanowire heterostructures grown on Si (111) by molecular beam epitaxy

High crystalline quality, vertically aligned AlxGa1−xN nanowire heterostructures are grown on GaN nanowire templates on Si (111) substrates by plasma-assisted molecular beam epitaxy. The nanowires exhibit unique core-shell structures, with enhanced Al compositions in the near-surface region. The emission wavelength can be varied across nearly the entire ultraviolet A (∼3.10–3.94 eV) and B (∼3.94–4.43 eV) spectral range by controlling the Al compositions. Such nanowire structures can exhibit extremely high internal quantum efficiency (up to ∼58%) at room-temperature, which is attributed to the superior carrier confinement offered by the core-shell structures and to the use of defect-free GaN nanowire templates.

High-Efficiency InGaN/GaN Dot-in-a-Wire Red Light-Emitting Diodes

We report on the achievement of high-performance InGaN/GaN dot-in-a-wire red light-emitting diodes on Si(111) substrates. Owing to the superior 3-D carrier confinement offered by the self-organized dot-in-a-wire heterostructures, the devices exhibit relatively high (~18%-32%) internal quantum efficiency at room temperature. Moreover, no efficiency droop was observed for injection current up to ~480A/cm2 under pulsed biasing conditions. We have also demonstrated that, by controlling the inhomogeneous broadening of the dot-in-a-wire heterostructures, the devices can exhibit relatively stable emission characteristics with increasing current.

Tunnel injection InGaN/GaN dot-in-a-wire white-light-emitting diodes

We report on the demonstration of InGaN/GaN dot-in-a-wire tunnel injection white-light-emitting diodes on Si, wherein electrons and holes are injected into the quantum dot active region through two separate InGaN injector wells. Significantly reduced electron overflow is realized without the use of any Al-containing electron blocking layer. Moreover, the InGaN hole injector well, which is positioned between the quantum dot active region and p-GaN, can harvest electrons that have escaped through the near-surface region of nanowires.

Molecular beam epitaxial growth and characterization of InGaN/GaN dot-in-a-wire nanoscale heterostructures: toward ultrahigh efficiency phosphor-free white light emitting diodes

One of the grand challenges for future solid state lighting is the development of high efficiency, phosphor-free white light emitting diodes (LEDs). In this context, we have investigated the molecular beam epitaxial growth and characterization of nanowire LEDs on Si, wherein intrinsic white-light emission is achieved by incorporating selforganized InGaN quantum dots in defect-free GaN nanowires on a single chip. We have further demonstrated that, with the incorporation of p-type modulation doping and AlGaN electron blocking layer, InGaN/GaN dot-in-a-wire white LEDs can exhibit nearly zero efficiency droop and significantly enhanced internal quantum efficiency (up to ~57%) at room-temperature.