Controlling the directional emission of light by periodic arrays of heterostructured semiconductor nanowires

Epitaxial growth of crystal phase quantum dots in III-V semiconductor nanowires

Crystal phase quantum dots (QDs) are formed during the axial growth of III-V semiconductor nanowires (NWs) by stacking different crystal phases of the same material. In III-V semiconductor NWs, both zinc blende (ZB) and wurtzite (WZ) crystal phases can coexist. The band structure difference between both crystal phases can lead to quantum confinement. Thanks to the precise control in III-V semiconductor NW growth conditions and the deep knowledge on the epitaxial growth mechanisms, it is nowadays possible to control, down to the atomic level, the switching between crystal phases in NWs forming the so-called crystal phase NW-based QDs (NWQDs). The shape and size of the NW bridge the gap between QDs and the macroscopic world. This review is focused on crystal phase NWQDs based on III-V NWs obtained by the bottom-up vapor-liquid-solid (VLS) method and their optical and electronic properties. Crystal phase switching can be achieved in the axial direction. In contrast, in the core/shell growth, the difference in surface energies between different polytypes can enable selective shell growth. One reason for the very intense research in this field is motivated by their excellent optical and electronic properties both appealing for applications in nanophotonics and quantum technologies.

GaInP nanowire arrays for color conversion applications

Color conversion by (tapered) nanowire arrays fabricated in GaInP with bandgap emission in the red spectral region are investigated with blue and green source light LEDs in perspective. GaInP nano- and microstructures, fabricated using top-down pattern transfer methods, are derived from epitaxial Ga0.51In0.49P/GaAs stacks with pre-determined layer thicknesses. Substrate-free GaInP micro- and nanostructures obtained by selectively etching the GaAs sacrificial layers are then embedded in a transparent film to generate stand-alone color converting films for spectrophotometry and photoluminescence experiments. Finite-difference time-domain simulations and spectrophotometry measurements are used to design and validate the GaInP structures embedded in (stand-alone) transparent films for maximum light absorption and color conversion from blue (450 nm) and green (532 nm) to red (~ 660 nm) light, respectively. It is shown that (embedded) 1 μm-high GaInP nanowire arrays can be designed to absorb ~ 100% of 450 nm and 532 nm wavelength incident light. Room-temperature photoluminescence measurements with 405 nm and 532 nm laser excitation are used for proof-of-principle demonstration of color conversion from the embedded GaInP structures. The (tapered) GaInP nanowire arrays, despite very low fill factors (~ 24%), can out-perform the micro-arrays and bulk-like slabs due to a better in- and out-coupling of source and emitted light, respectively.

Directional and Polarized Emission from Nanowire Arrays

Lighting applications require directional and polarization control of the emitted light, which is currently achieved by bulky optical components such as lenses, parabolic mirrors, and polarizers. Ideally, this control would be achieved without any external optics, but at the nanoscale, during the generation of light. Semiconductor nanowires are promising candidates for lighting devices due to their efficient light outcoupling and synthesis flexibility. In this work, we demonstrate a precise control of both the directionality and the polarization of the nanowire array emission by changing the nanowire diameter. We change the angular emission pattern from a large-angle doughnut shape to a narrow-angle beaming along the nanowire axis. In addition, we tune the polarization from unpolarized to either p- or s-polarized. Both the far-field emission pattern and its polarization are controlled by the number and type of guided or leaky modes supported by the nanowire, which are determined by the nanowire diameter.

Optimized emission in nanorod arrays through quasi-aperiodic inverse design

We investigate a new class of quasi-aperiodic nanorod structures for the enhancement of incoherent light emission. We identify one optimized structure using an inverse design algorithm and the finite-difference time-domain method. We carry out emission calculations on both the optimized structure as well as a simple periodic array. The optimized structure achieves nearly perfect light extraction while maintaining a high spontaneous emission rate. Overall, the optimized structure can achieve a 20%-42% increase in external quantum efficiency relative to a simple periodic design, depending on material quality.

Self-catalyzed ternary core-shell GaAsP nanowire arrays grown on patterned Si substrates by molecular beam epitaxy

The growth of self-catalyzed ternary core-shell GaAsP nanowire (NW) arrays on SiO2 patterned Si(111) substrates has been demonstrated by using solid-source molecular beam epitaxy. A high-temperature deoxidization step up to ∼ 900 °C prior to NW growth was used to remove the native oxide and/or SiO2 residue from the patterned holes. To initiate the growth of GaAsP NW arrays, the Ga predeposition used for assisting the formation of Ga droplets in the patterned holes, was shown to be another essential step. The effects of the patterned-hole size on the NW morphology were also studied and explained using a simple growth model. A lattice-matched radial GaAsP core-shell NW structure has subsequently been developed with room-temperature photoluminescence emission around 740 nm. These results open up new perspectives for integrating position-controlled III-V NW photonic and electronic structures on a Si platform.

Local nanotip arrays sculptured by atomic force microscopy to enhance the light-output efficiency of GaN-based light-emitting diode structures

In this work, local nanotip arrays on GaN-based light-emitting (LED) structures were fabricated through nano-oxidation using an atomic force microscope (AFM). The photoluminescence (PL) intensity of the InGaN/GaN multiple quantum wells (MQWs) active layer and the light extraction efficiency of the LED structure were enhanced by forming this nanotips structure to serve as a graded-refractive index layer, which is further validated by the finite-difference time-domain analysis. The PL emission peak of the MQWs active layer has a blue-shift phenomenon that is caused by a partial reduction of the strain on the InGaN well. It is expected that our approach opens a promising route for simultaneously enhancing both the internal quantum efficiency and the light extraction efficiency of GaN-based LEDs. The proposed AFM-based method will be of importance for local patterning the light emitting components for optoelectronic applications.

Enhanced and directional emission of semiconductor nanowires tailored through leaky/guided modes

Photoluminescence from finite semiconductor nanowires is theoretically investigated, exploring and predicting their antenna-like properties for light emission in a variety of configurations of interest in Nanophotonics. The theoretical analysis is based on the leaky/guided mode dispersion relation for infinite nanowires, which govern the local density of available electromagnetic states. Light emission from finite nanowires is then numerically investigated in various scenarios with regard to its enhancement and directionality. A simple analytical model is derived that, upon tuning leaky/guided mode coupling through dipole position/orientation and nanowire length, allows us to predict their antenna-like behavior and thus to tailor photoluminescence (including magnetic dipole transitions) at will, with regard to both enhancement/inhibition and associated radiation patterns.

Hydrazine-Assisted Formation of Indium Phosphide (InP)-Based Nanowires and Core-Shell Composites

Indium phosphide nanowires (InP NWs) are accessible at 440 °C from a novel vapor phase deposition approach from crystalline InP sources in hydrazine atmospheres containing 3 mol % H₂O. Uniform zinc blende (ZB) InP NWs with diameters around 20 nm and lengths up to several tens of micrometers are preferably deposited on Si substrates. InP particle sizes further increase with the deposition temperature. The straightforward protocol was extended on the one-step formation of new core-shell InP–Ga NWs from mixed InP/Ga source materials. Composite nanocables with diameters below 20 nm and shells of amorphous gallium oxide are obtained at low deposition temperatures around 350 °C. Furthermore, InP/Zn sources afford InP NWs with amorphous Zn/P/O-coatings at slightly higher temperatures (400 °C) from analogous setups. At 450 °C, the smooth outer layer of InP-Zn NWs is transformed into bead-shaped coatings. The novel combinations of the key semiconductor InP with isotropic insulator shell materials open up interesting application perspectives in nanoelectronics.