C. Tessarek

High quality factor whispering gallery modes from self-assembled hexagonal GaN rods grown by metal-organic vapor phase epitaxy

Self-assembled GaN rods were grown on sapphire by metal-organic vapor phase epitaxy using a simple two-step method that relies first on a nitridation step followed by GaN epitaxy. The mask-free rods formed without any additional catalyst. Most of the vertically aligned rods exhibit a regular hexagonal shape with sharp edges and smooth sidewall facets. Cathodo- and microphotoluminescence investigations were carried out on single GaN rods. Whispering gallery modes with quality factors greater than 4000 were measured demonstrating the high morphological and optical quality of the self-assembled GaN rods.

Controlling morphology and optical properties of self-catalyzed, mask-free GaN rods and nanorods by metal-organic vapor phase epitaxy

A simple self-catalyzed and mask-free approach will be presented to grow GaN rods and nanorods based on the metal-organic vapor phase epitaxy technique. The growth parameter dependent adjustment of the morphology of the structures will be discussed. Rods and nanorods with diameters reaching from a few μm down to 100 nm, heights up to 48 μm, and densities up to 8⋅107 cm–2 are all vertically aligned with respect to the sample surface and exhibiting a hexagonal shape with smooth sidewall facets. Optical properties of GaN nanorods were determined using cathodoluminescence. It will be shown that the optical properties can be improved just by reducing the Ga precursor flow. Furthermore, for regular hexagonal shaped rods and nanorods, whispering gallery modes with quality factors up to 500 were observed by cathodoluminescence pointing out high morphological quality of the structures. Structural investigations using transmission electron microscopy show that larger GaN nanorods (diameter > 500 nm) contain threadi...

Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts.

Optical properties of vertical, tilted and in-plane GaN nanowires on different crystallographic orientations of sapphire

Self-catalysed and self-organized GaN nanowires were grown on c-, a-, m- and r-plane sapphire by metal-organic vapour phase epitaxy. In dependence on the crystallographic orientation of the sapphire substrate, vertical, tilted and in-plane GaN nanowires were achieved. The nanowire orientation is visualized by scanning electron microscopy and analysed by x-ray diffraction. The influence of the sapphire nitridation step on the nanowire formation is investigated. Spatially and spectrally resolved cathodoluminescence studies are carried out on the GaN nanowires to analyse the influence of the GaN nanowire orientation as well as the presence of both N- and Ga-polar sections in a single nanowire on the optical properties.

Study of iron-catalysed growth of ?-Ga2O3 nanowires and their detailed characterization using TEM, Raman and cathodoluminescence techniques

In this paper, we demonstrate a new catalyst (Fe) to grow single crystalline beta-gallium oxide (?-Ga2O3) nanowires (NWs) via the vapour?liquid?solid mechanism using the chemical vapour deposition technique. The structural studies of these NWs showed the highly crystalline monoclinic phase of Ga2O3. This was confirmed by detailed scanning transmission electron microscope investigations demonstrating the NW to be single crystalline ?-Ga2O3, growing along the normal of the plane. We also compared Raman and cathodoluminescence (CL) properties of the as-grown ?-Ga2O3 NWs with a bulk Ga2O3 single crystal grown by the Czochralski method. It was observed that Raman peak positions of a single ?-Ga2O3 NW had a red frequency shift of about 0.3?1.4?cm?1 as compared to a bulk Ga2O3 single crystal, which was in fact quite small. In addition, the CL measurements of ?-Ga2O3 NWs and the bulk Ga2O3 single crystal exhibited similar spectra, having a strong broad UV?blue emission band and a weak red emission band. Moreover, the structural, morphological and optical properties of Fe-catalysed ?-Ga2O3 NWs were comparable to those of Au-catalysed ?-Ga2O3 NWs.

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double-pump approach. Wide bandgap gallium nitride (GaN), zinc oxide (ZnO), and cadmium sulfide (CdS) nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier-phonon coupling in ZnO leads to the fastest laser onset time of ∼1 ps in comparison to CdS and GaN exhibiting values of ∼2.5 and ∼3.5 ps, respectively. These values are constant between nanowires of different sizes implying independence from any optical influences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from ∼5 to ∼15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization.

Growth of GaN Nanorods and Wires and Spectral Tuning of Whispering Gallery Modes in Tapered GaN Wires

This paper reports on the growth of GaN nanorods and wires by metal–organic vapor phase epitaxy. Density, height and diameter are strongly influenced by the growth time. A deposition time of a few minutes leads to the formation of GaN nanorods. Increasing the deposition time up to 1 h yields wires with heights exceeding 47 µm. Transmission electron microscopy and convergent beam electron diffraction measurements are showing the presence of N- and Ga-polar GaN in a single nanorod. Cathodoluminescence measurements are performed showing the appearance of whispering gallery modes. Due to slight tapering of the wires the whispering gallery modes can be spectrally tuned by changing the position of the exposing electron beam at the sidewall facet of the rod.

Efficient Nitrogen Doping of Single-Layer Graphene Accompanied by Negligible Defect Generation for Integration into Hybrid Semiconductor Heterostructures

While doping enables application-specific tailoring of graphene properties, it can also produce high defect densities that degrade the beneficial features. In this work, we report efficient nitrogen doping of ∼11 atom % without virtually inducing new structural defects in the initial, large-area, low defect, and transferred single-layer graphene. To shed light on this remarkable high-doping-low-disorder relationship, a unique experimental strategy consisting of analyzing the changes in doping, strain, and defect density after each important step during the doping procedure was employed. Complementary micro-Raman mapping, X-ray photoelectron spectroscopy, and optical microscopy revealed that effective cleaning of the graphene surface assists efficient nitrogen incorporation accompanied by mild compressive strain resulting in negligible defect formation in the doped graphene lattice. These original results are achieved by separating the growth of graphene from its doping. Moreover, the high doping level occurred simultaneously with the epitaxial growth of n-GaN micro- and nanorods on top of graphene, leading to the flow of higher currents through the graphene/n-GaN rod interface. Our approach can be extended toward integrating graphene into other technologically relevant hybrid semiconductor heterostructures and obtaining an ohmic contact at their interfaces by adjusting the doping level in graphene.

Self-Catalyzed Growth of Vertically Aligned InN Nanorods by Metal–Organic Vapor Phase Epitaxy

Vertically aligned hexagonal InN nanorods were grown mask-free by conventional metal-organic vapor phase epitaxy without any foreign catalyst. The In droplets on top of the nanorods indicate a self-catalytic vapor-liquid-solid growth mode. A systematic study on important growth parameters has been carried out for the optimization of nanorod morphology. The nanorod N-polarity, induced by high temperature nitridation of the sapphire substrate, is necessary to achieve vertical growth. Hydrogen, usually inapplicable during InN growth due to formation of metallic indium, and silane are needed to enhance the aspect ratio and to reduce parasitic deposition beside the nanorods on the sapphire surface. The results reveal many similarities between InN and GaN nanorod growth showing that the process despite the large difference in growth temperature is similar. Transmission electron microscopy, spatially resolved energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy have been performed to analyze the structural properties. Spatially resolved cathodoluminescence investigations are carried out to verify the optical activity of the InN nanorods. The InN nanorods are expected to be the material of choice for high-efficiency hot carrier solar cells.

Carrier-induced refractive index change observed by a whispering gallery mode shift in GaN microrods

Vertical oriented GaN microrods were grown by metal-organic vapor phase epitaxy with four different n-type carrier concentration sections above 1019 cm−3 along the c-axis. In cathodoluminescence investigations carried out on each section of the microrod, whispering gallery modes can be observed due to the hexagonal symmetry. Comparisons of the spectral positions of the modes from each section show the presence of an energy dependent mode shift, which suggest a carrier-induced refractive index change. The shift of the high energy edge of the near band edge emission points out that the band gap parameter in the analytical expression of the refractive index has to be modified. A proper adjustment of the band gap parameter explains the observed whispering gallery mode shift.