Nonpolar InGaN/GaN Core-Shell Single Nanowire Lasers.

We report lasing from nonpolar p-i-n InGaN/GaN multi-quantum well core-shell single-nanowire lasers by optical pumping at room temperature. The nanowire lasers were fabricated using a hybrid approach consisting of a top-down two-step etch process followed by a bottom-up regrowth process, enabling precise geometrical control and high material gain and optical confinement. The modal gain spectra and the gain curves of the core-shell nanowire lasers were measured using micro-photoluminescence and analyzed using the Hakki-Paoli method. Significantly lower lasing thresholds due to high optical gain were measured compared to previously reported semipolar InGaN/GaN core-shell nanowires, despite significantly shorter cavity lengths and reduced active region volume. Mode simulations show that due to the core-shell architecture, annular-shaped modes have higher optical confinement than solid transverse modes. The results show the viability of this p-i-n nonpolar core-shell nanowire architecture, previously investigated for next-generation light-emitting diodes, as low-threshold, coherent UV-visible nanoscale light emitters, and open a route toward monolithic, integrable, electrically injected single-nanowire lasers operating at room temperature.

Quantum-size-controlled photoelectrochemical fabrication of epitaxial InGaN quantum dots.

We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm).

Multi-colour nanowire photonic crystal laser pixels.

Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form "super-pixels". Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.

Spatial mapping of efficiency of GaN/InGaN nanowire array solar cells using scanning photocurrent microscopy.

GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.

Three-dimensional mapping of quantum wells in a GaN/InGaN core-shell nanowire light-emitting diode array.

Correlated atom probe tomography, cross-sectional scanning transmission electron microscopy, and cathodoluminescence spectroscopy are used to analyze InGaN/GaN multiquantum wells (QWs) in nanowire array light-emitting diodes (LEDs). Tomographic analysis of the In distribution, interface morphology, and dopant clustering reveals material quality comparable to that of planar LED QWs. The position-dependent CL emission wavelength of the nonpolar side-facet QWs and semipolar top QWs is correlated with In composition.

III-nitride core­shell nanowire arrayed solar cells.

A solar cell based on a hybrid nanowire­film architecture consisting of a vertically aligned array of InGaN/GaN multi-quantum well core­shell nanowires which are electrically connected by a coalesced p-InGaN canopy layer is demonstrated. This unique hybrid structure allows for standard planar device processing, solving a key challenge with nanowire device integration, while enabling various advantages by the nanowire absorbing region such as higher indium composition InGaN layers by elastic strain relief, more efficient carrier collection in thinner layers, and enhanced light trapping from nano-scale optical index changes. This hybrid structure is fabricated into working solar cells exhibiting photoresponse out to 2.1 eV and short-circuit current densities of ~1 mA cm(-2) under 1 sun AM1.5G. This proof-of-concept nanowire-based device demonstrates a route forward for high-efficiency III-nitride solar cells.

Nonradiative energy transfer between colloidal quantum dot-phosphors and nanopillar nitride LEDs.

We present in this communication our study of the nonradiative energy transfer between colloidal quantum dot (QD) phosphors and nitride nanopillar light emitting diodes (LEDs). An epitaxial p-i-n InGaN/GaN multiple quantum-well (QW) heterostructure was patterned and dry-etched to form dense arrays of nanopillars using a novel etch mask consisting of self-assembled In3Sn clusters. Colloidal QD phosphors have been deposited into the gaps between the nanopillars, leading to sidewall coupling between the QDs and InGaN QW emitters. In this approach, close QW-QD contact and a low-resistance design of the LED contact layer were achieved simultaneously. Strong non-radiative energy transfer was observed from the InGaN QW to the colloidal QD phosphors, which led to a 263% enhancement in effective internal quantum efficiency for the QDs incorporated in the nanopillar LEDs, as compared to those deposited over planar LED structures. Time-resolved photoluminescence was used to characterize the energy transfer process between the QW and QDs. The measured rate of non-radiative QD-QW energy-transfer agrees well with the value calculated from the quantum efficiency data for the QDs in the nanopillar LED.

Identifying threading dislocations in GaN films and substrates by electron channelling.

Electron channelling contrast imaging of threading dislocations in GaN (0002) substrates and epitaxial films has been demonstrated using a conventional polepiece-mounted backscatter detector in a commercial scanning electron microscope. The influence of accelerating voltage and diffraction vector on contrast features denoting specific threading dislocation types has been studied. As confirmed by coordinated transmission electron microscopy analysis, electron channelling contrast imaging contrast features for edge-type threading dislocations are spatially smaller than mixed-type threading dislocations in GaN. This ability to delineate GaN edge threading dislocations from mixed type was also confirmed by defect-selective etch processing using molten MgO/KOH. This study validates electron channelling contrast imaging as a nondestructive and widely accessible method for spatially mapping and identifying dislocations in GaN with wider applicability for other single-crystal materials.

Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays.

Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94% of the nanorod LEDs are dislocation-free and a reduced quantum confined Stark effect is observed due to reduced piezoelectric fields. Despite these advantages, the IQE of the nanorod LEDs measured by photoluminescence is comparable to the planar LED, perhaps due to inefficient thermal transport and enhanced nonradiative surface recombination.

Nanocrystal-based light-emitting diodes utilizing high-efficiency nonradiative energy transfer for color conversion.

We report a practical implementation of high-efficiency color conversion in an electrically pumped light-emitting diode (LED) using nonradiative energy transfer. On the basis of a new LED design that offers both strong energy-transfer coupling and efficient carrier injection, we show that a hybrid structure comprising a single monolayer of CdSe nanocrystals assembled on top of an InGaN/GaN quantum well provides nearly 10% color conversion efficiency. This value is significantly higher than that for a traditional absorption-re-emission color-conversion scheme in a similar device structure. Furthermore, these hybrid devices can also provide improved efficiencies, compared not only to phosphor-based structures but also to stand-alone LEDs.

Multicolor light-emitting diodes based on semiconductor nanocrystals encapsulated in GaN charge injection layers.

Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.

Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well.

As a result of quantum-confinement effects, the emission colour of semiconductor nanocrystals can be modified dramatically by simply changing their size. Such spectral tunability, together with large photoluminescence quantum yields and high photostability, make nanocrystals attractive for use in a variety of light-emitting technologies--for example, displays, fluorescence tagging, solid-state lighting and lasers. An important limitation for such applications, however, is the difficulty of achieving electrical pumping, largely due to the presence of an insulating organic capping layer on the nanocrystals. Here, we describe an approach for indirect injection of electron-hole pairs (the electron-hole radiative recombination gives rise to light emission) into nanocrystals by non-contact, non-radiative energy transfer from a proximal quantum well that can in principle be pumped either electrically or optically. Our theoretical and experimental results indicate that this transfer is fast enough to compete with electron-hole recombination in the quantum well, and results in greater than 50 per cent energy-transfer efficiencies in the tested structures. Furthermore, the measured energy-transfer rates are sufficiently large to provide pumping in the stimulated emission regime, indicating the feasibility of nanocrystal-based optical amplifiers and lasers based on this approach.