An enhanced surface passivation effect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley-Read-Hall recombination.

We present a detailed study of the effects of dangling bond passivation and the comparison of different sulfide passivation processes on the properties of InGaN/GaN quantum-disk (Qdisk)-in-nanowire based light emitting diodes (NW-LEDs). Our results demonstrated the first organic sulfide passivation process for nitride nanowires (NWs). The results from Raman spectroscopy, photoluminescence (PL) measurements, and X-ray photoelectron spectroscopy (XPS) showed that octadecylthiol (ODT) effectively passivated the surface states, and altered the surface dynamic charge, and thereby recovered the band-edge emission. The effectiveness of the process with passivation duration was also studied. Moreover, we also compared the electro-optical performance of NW-LEDs emitting at green wavelength before and after ODT passivation. We have shown that the Shockley-Read-Hall (SRH) non-radiative recombination of NW-LEDs can be greatly reduced after passivation by ODT, which led to a much faster increasing trend of quantum efficiency and higher peak efficiency. Our results highlighted the possibility of employing this technique to further design and produce high performance NW-LEDs and NW-lasers.

InGaN/GaN nanowires grown on SiO(2) and light emitting diodes with low turn on voltages.

GaN nanowires and InGaN disk heterostructures are grown on an amorphous SiO2 layer by a plasma-assisted molecular beam epitaxy. Structural studies using scanning electron microscopy and high-resolution transmission electron microscopy reveal that the nanowires grow vertically without any extended defect similarly to nanowires grown on Si. The as-grown nanowires have an intermediate region consisting of Ga, O, and Si rather than SiNx at the interface between the nanowires and SiO2. The measured photoluminescence shows a variation of peak wavelengths ranging from 580 nm to 635 nm because of non-uniform indium incorporation. The nanowires grown on SiO2 are successfully transferred to a flexible polyimide sheet by Au-welding and epitaxial lift-off processes. The light-emitting diodes fabricated with the transferred nanowires are characterized by a turn-on voltage of approximately 4 V. The smaller turn-on voltage in contrast to those of conventional nanowire light-emitting diodes is due to the absence of an intermediate layer, which is removed during an epitaxial lift-off process. The measured electroluminescence shows peak wavelengths of 610-616 nm with linewidths of 116-123 nm.

Formation and nature of InGaN quantum dots in GaN nanowires.

InGaN/GaN disk-in-nanowire heterostructures on silicon substrates have emerged as important gain media for the realization of visible light sources. The nature of quantum confinement in the disks is largely unknown. From the unique nature of the measured temperature dependence of the radiative lifetime and direct transmission electron microscopy, it is evident that such self-organized islands (disks) behave as quantum dots. This is confirmed by the observation of single photon emission from a single disk-in-nanowire and the presence of a sharp minimum in the line width enhancement factor of edge emitting lasers having the InGaN disks as the gain media.

Structural and optical properties of disc-in-wire InGaN/GaN LEDs.

This study examines the role of the microstructure and optical properties of InGaN/GaN nanowire LED structures on Si(111) having different nanowire coverages. Cathodoluminescence (CL) measurements show that all samples exhibit broad emission around the intended energy, 1.95 eV (635 nm). While the absolute emission intensity is hard to compare for CL measurement, the bandgap emission (∼3.4 eV) coming from the GaN root is more pronounced as coverage of nanowires decreases, which has less coalescence formation. The width of the emission peak is likely due to variations in the morphology of the InGaN discs within the wires, as faceted layers with different thicknesses and quantum dots are observed by transmission electron microscopy. Nonepitaxial six-fold symmetric lateral branching, called "nanocrowns," emanate from stacking faults within the active regions. These features likely reduce optical emission as a result of grain boundaries between the nanocrown and nanowire.

Monolithic electrically injected nanowire array edge-emitting laser on (001) silicon.

A silicon-based laser, preferably electrically pumped, has long been a scientific and engineering goal. We demonstrate here, for the first time, an edge-emitting InGaN/GaN disk-in-nanowire array electrically pumped laser emitting in the green (λ = 533 nm) on (001) silicon substrate. The devices display excellent dc and dynamic characteristics with values of threshold current density, differential gain, T0 and small signal modulation bandwidth equal to 1.76 kA/cm(2), 3 × 10(-17) cm(2), 232 K, and 5.8 GHz respectively under continuous wave operation. Preliminary reliability measurements indicate a lifetime of 7000 h. The emission wavelength can be tuned by varying the alloy composition in the quantum disks. The monolithic nanowire laser on (001)Si can therefore address wide-ranging applications such as solid state lighting, displays, plastic fiber communication, medical diagnostics, and silicon photonics.

Room-temperature polariton lasing from GaN nanowire array clad by dielectric microcavity.

Room-temperature polariton lasing from a GaN-dielectric microcavity is demonstrated with optical excitation. The device is fabricated with a GaN nanowire array clad by Si3N4/SiO2-distributed Bragg reflectors. The nanowire array is initially grown on silicon substrate by molecular beam epitaxy. A distinct nonlinearity in the lower polariton emission is observed at a threshold optical energy density of 625 nJ/cm(2), accompanied by significant line width narrowing to 5 meV and a small blue shift of ~1 meV. The measured polariton dispersion is characterized by a Rabi splitting of 40 meV and a cavity exciton detuning of -17 meV. The device described here is a demonstration of exciton-photon strong coupling phenomenon in an array of light emitters and paves the way for the realization of a room temperature electrically injected polariton laser.