II-VI Semiconductor Nanowires Research Articles

Short-wave infrared cavity resonances in a single GeSn nanowire

Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present an experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content ( ~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of short-wave infrared (SWIR).

Asymmetric waveguide and the dual-wavelength stimulated emission for CdS/CdS0.48Se0.52 axial nanowire heterostructures

Semiconductor axial nanowire heterostructures are important for realizing the high-performance nano-photonics and opto-electronics devices. Although different IV and III-V semiconductor axial nanowire heterostructures have been successfully prepared in recent decade, few of them focused on the optical properties, such as the waveguide, due to their low light emission efficiencies. The II-VI semiconductor nanowires grown by chemical vapor deposition strategy, such as CdS, CdSe and their alloys, can act as nanoscale waveguide, nanolasers, etc., because of their high optical gains and atomically smooth surfaces. However, it is still a challenge to growing the high-quality II-VI semiconductor axial nanowire heterostructures, owning to the poor controllability of the vapor growth techniques. Here, the CdS/CdSSe axial nanowire heterostructures are prepared with well controlled CVD method under the catalysis of annealed Au nanoparticles. The scanning electron microscope characterization shows that the wires have smooth surfaces with Au particles at the tips, indicating the vapor-liquid-solid growth mechanism for the nanowire heterostructures. The microscope images of the dispersed wires illuminated with a 405 nm laser show that the red and the green segment align axially with a sharp interface, demonstrating the axial alignment of CdS and CdSSe segments. The position related micro-photoluminescence spectra exhibit near band edge emissions of CdS and CdSSe without obvious emission from defect states, which suggests that the wires have highly crystalline quality. The waveguide of the nanowire heterostructures is studied through respectively locally exciting the two ends of the wire with a focused 488 nm laser. The local illuminations at both the CdS end and the CdSSe end result in red emission at the corresponding remote ends of the wires, with the emission intensity of the former being one order lower than that of the later, which is caused by the reabsorption of the green light emission (from CdS segment) in the CdSSe segment. This indicates the asymmetric waveguide in these heterosturctures, which implies that the CdS/CdSSe nanowire heterostructures have the potential applications in the photodiode. Under the pumping of 470 nm femtosecond laser, dual-color (red and green) lasing is realized based on these wires, with the lasing threshold of red light lasing being lower than that of the green one, which results from the larger round-trip loss for the green light arising from the self-absorption in CdSSe segment. To prove that the light can be transfer between the two segments with different refractivities, the waveguide of the nanowire heterostructure is simulated by the COMSOL. The result shows that the light can effectively propagate between CdS and CdSSe segments, which ensures the light-matter interaction in the axial CdS/CdSSe nanowire heterostructures as discussed above. These high-quality CdS/CdSSe axial nanowire heterostructures can be found to have the potential applications in photodiodes, dual-color nanolasers and photodetectors.

Ab initio study of structural and electronic properties of single crystal and core/shell II-VI semiconductor nanowires

Structural and electronic properties of pristine and H-passivated wurtzite type ZnSe, ZnTe nanowires and ZnX/ZnY (X=Se(Te) and Y=Te(Se)) core/shell nanowires oriented along the [0001] direction have been investigated using first-principles calculations. The changes in the electronic structure of the nanowires due to the quantum confinement and morphology have been searched. Quantum confinement increases the band gap energy as the diameters of ZnSe and ZnTe nanowires decrease. Both homostructured and heterostructured nanowires are found to show a semiconducting character with direct band gaps at Γ-point. Changing the morphology from homostructured nanowires to heterostructured core/shell nanowires has an important impact on the electronic structure. For instance, the charge separation of electrons and holes along the infinite direction of core/shell nanowires shows a strong preference for electron(hole) states localized inside ZnSe(ZnTe) regions.

Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG)

Zinc Oxide (ZnO) provides unique properties owing to its wide bandgap, large resistivity range and possibility to tune the physical properties. The surfactant assisted carbon doping was made possible due to the lowering of surface energy. The ZnO and carbon doped ZnO (C-ZnO) nanowires fabricated by hydrothermal process, Poly Ethylene Glycol (PEG) is used as surfactant in hydrothermal synthesis followed by post growth annealing treatment at 600 °C–700 °C. At 5%–10% of diluted PEG carbon is doped in ZnO. The crystallinity, structural morphology and elemental composition analysis for ZnO and C-ZnO nanowires were carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques respectively. Carbon doping in ZnO nanowires in the presence of different percentage of surfactant is explained by calculating the change in surface energy with respect to change in PEG molecule concentration. It was found that the surface energy per molecule modulates from 3.92 × 10−8 J/m2 to 8.16 × 10−7 J/m2 in the PEG concentration range between 5% and 10%. Our results provides a new theoretical calculations, implemented on real system, to observe the details of PEG-assisted Carbon doping in II-VI semiconductor nanowires.

Structural and magnetic properties of hybrid ferromagnetic metal/semiconductor (ZnTe)/Co core-shell nanowires

We report on the study of two-step grown crystalline II-VI semiconductor nanowires (NWs) covered with ferromagnetic metal. In the first step ZnTe nanowires were grown by a vapor-liquid-solid mechanism (VLS) on GaAs (111)B substrates covered with Au thin film. Solid source molecular beam epitaxy and Au-based nanocatalysts were used for these purposes. The nanowires were smooth, slightly tapered and mostly oriented along the axis perpendicular to the substrate. The diameters of the obtained nanowires were from 30 to 70nm and their length was around 1µm. In the second step the nanowires were covered with Co shell of different thicknesses. As a result magnetic nanotubes were formed. Structural characterization of the nanowires was performed using scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy techniques. Initial roughness of the nanowires influences the growth of Co shell with the increment of deposition thickness. Deposited Co formed nanograins having hexagonal closed packed structure, however face cubic centered grains were also observed. Magnetic force microscopy experiments on an individual Co nanotube revealed magnetic anisotropy of the investigated structures.

Creating Single Boundary between Two CdTe (111) Wafers with Controlled Orientation by Wafer Bonding

Zinc-blende (ZB) CdTe has drawn great attention as optoelectronic and solar energy conversion materials since it has a near optimum band gap of 1.6 eV and a absorption coefficient greater than 5x10 5 /cm. CdTe can be either ZB or wurtzite (WZ) structures, resulting in different electronic properties. It has been reported that the twin superlattice with numerous twin boundaries in III-V and II-VI semiconductor nanowires along growth orientation of ZB structures considerably enhance band gap engineering and mechanical behavior in quasi-one-dimensional materials [1]. This opens new possibilities for properties and functionalities at the atomic and quantum scales by controlling the twin boundary and modulating twin densities. Therefore, it is an in-demand and challenging feat to “create” a single twin boundary in a nanowire or bulk material in order to understand the electronic and mechanical characteristics of the III-V and II-VI quantum well or barrier. Wafer bonding, which enables the direct integration of two or more single crystal wafers with controlled surfaces and orientation, is a key technique in creating a single boundary [2]. In this study, we show the creation of a single boundary between two identical CdTe single crystals.

Growth and Characterisation of ZnSe Semiconductor Nanowires

The growth and characteristics of wide gap II-VI semiconductor nanowires prepared by the so-called Vapour-Liquid-Solid (VLS) technique was presented ZnSe nanowires were prepared on Si (111) by using Au as catalyst. Vapor-Liquid-Solid (VLS) process under certain conditions to form the desired nanowires. The as-synthesized products were characterized by SEM and EDX. The SEM analysis of ZnSe nanowires indicated that nanowires grow randomly at angles widely different from the vertical.

ChemInform Abstract: Various Synthetic Methods for One‐Dimensional Semiconductor Nanowires/Nanorods and Their Applications in Photovoltaic Devices

AbstractReview: 98 refs.