Coaxial Heterostructures Research Articles

Selective formation of GaN-based nanorod heterostructures on soda-lime glass substratesby a local heating method

We report on the fabrication of high-quality GaN on soda-lime glass substrates, heretoforeprecluded by both the intolerance of soda-lime glass to the high temperatures requiredfor III-nitride growth and the lack of an epitaxial relationship with amorphousglass. The difficulties were circumvented by heteroepitaxial coating of GaN onZnO nanorods via a local microheating method. Metal-organic chemical vapordeposition of ZnO nanorods and GaN layers using the microheater arrays producedhigh-quality GaN/ZnO coaxial nanorod heterostructures at only the desired regions onthe soda-lime glass substrates. High-resolution transmission electron microscopyexamination of the coaxial nanorod heterostructures indicated the formation ofan abrupt, semicoherent interface. Photoluminescence and cathodoluminescencespectroscopy was also applied to confirm the high optical quality of the coaxial nanorodheterostructures. Mg-doped GaN/ZnO coaxial nanorod heterostructure arrays, whoseGaN shell layers were grown with various different magnesocene flow rates, werefurther investigated by using photoluminescence spectroscopy for the p-type dopingcharacteristics. The suggested method for fabrication of III-nitrides on glass substratessignifies potentials for low-cost and large-size optoelectronic device applications.

Swingback in magnetization reversal in MnAs–GaAs coaxial nanowire heterostructures

The reversal processes of magnetization in epitaxial MnAs nanotubes prepared by anovergrowth on the sidewall of GaAs nanowires having a diameter of 26 nm are investigated.While the magnetic hard axis is aligned in the direction of the nanowire axis, we apply anexternal magnetic field perpendicular to the axis to examine the flipping characteristicsof magnetic moments. We determine the contributions from the substrate bya direct measurement in order to extract the magnetization of the core–shellheterostructures. The abrupt change in the thus-obtained magnetization due to a flipwhen the field is varied exhibits an overshoot at about 0.4 kOe for samples with athickness of the ferromagnetic shell (40–50 nm) larger than the diameter of the core.Moreover, the peak value exceeds the value when the field is swept in the oppositedirection. The magnetic hysteresis loop consequently involves line crossings. Wespeculate that the spin textures of domain walls in such thick hollow cylindersand their movement at the magnetization flip are affected by the geometry andmagnetostatic interactions of various origins, giving rise to the anomalous behaviour.

Fabrication of Coaxial Si1−x Ge x Heterostructure Nanowires by O2 Flow-Induced Bifurcate Reactions

We report on bifurcate reactions on the surface of well-aligned Si1−xGex nanowires that enable fabrication of two different coaxial heterostructure nanowires. The Si1−xGex nanowires were grown in a chemical vapor transport process using SiCl4 gas and Ge powder as a source. After the growth of nanowires, SiCl4 flow was terminated while O2 gas flow was introduced under vacuum. On the surface of nanowires was deposited Ge by the vapor from the Ge powder or oxidized into SiO2 by the O2 gas. The transition from deposition to oxidation occurred abruptly at 2 torr of O2 pressure without any intermediate region and enables selectively fabricated Ge/Si1−xGex or SiO2/Si1−xGex coaxial heterostructure nanowires. The rate of deposition and oxidation was dominated by interfacial reaction and diffusion of oxygen through the oxide layer, respectively.

Growth Mechanism and Photoluminescent Properties of AlN/ZnO Heterostructures

We have illustrated a new approach to fabricate two forms of AlN/ZnO heterostructures―AlN/ZnO coaxial nanotubular heterostructures (CNHs) and AlN-nanotube/ZnO-nanoparticles heterostructures (AlN/ZnO NPs). X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that ZnO nanotubes and nanoparticels have grown on the inner surface of amorphous and polycrystalline AlN shell layer, respectively. A possible growth mechanism on the formation of different AlN/ZnO heterostructures is given. Compared with bare ZnO nanofibers, AlN/ZnO CNHs exhibited remarkable enhanced ultraviolet (UV) emission, while AlN/ZnO NPs showed significant visible emission. With the aid of classical optical diffraction effect theory, it can be calculated that nanoscale luminescent materials have a higher external luminescent efficiency with increasing surface/volume ratio. The influence of carrier confinement effect and surface defects for the PL properties is also investigated in the AlN/ZnO heterostructures. In addition, the photoluminescent (PL) properties of AlN/ZnO CNHs with various AlN shell layers thickness are further discussed.

Group IV semiconductor nanowire arrays: epitaxy in different contexts

Epitaxy can be used to direct nanowire deposition and to influence the crystallographic orientation of nanowires during their nucleation and growth via the vapor–liquid–solid mechanism. We have investigated rapid thermal chemical vapor deposition of epitaxial Ge nanowires and have used it to separately study nanowire nucleation and growth. This work has provided important insights into deep-subeutectic Ge nanowire growth using Au catalyst particles. Germanium nanowires have also been studied as the cores in epitaxial Ge core/Si shell nanowires. We have studied the conditions under which strain-driven surface roughening and dislocations formation occur in these coaxial nanowire heterostructures. Our results indicate that suppression of Si shell surface roughening can lead to fully strained, coherent core/shell nanowires. Recently, we have used vertical arrays of Ge ⟨1 1 1⟩ nanowires grown at low temperatures on Si substrates to seed liquid-phase epitaxy of large-area amorphous Ge islands above the substrate surface. This work demonstrates a potential approach for dense vertical integration of Ge-based devices on Si substrates, for on-chip optoelectronics or 3D integrated circuit applications.

Position-controlled AlN/ZnO coaxial nanotube heterostructure arrays for electron emitterapplications

We studied the fabrication and field-emission characteristics of position-controlledAlN/ZnO nanotube heterostructure arrays. AlN layers with various thicknesses from 20 to52 nm were deposited coaxially on the position-controlled ZnO nanotube arrays. Thefield-emission properties of the coaxial AlN/ZnO nanotube arrays were controlled using theAlN thickness and the nanotube interdistance. As compared to the bare ZnO nanotubearrays, the AlN-coated coaxial nanotube arrays exhibited enhanced electron emission, andthe optimum AlN coating layer thickness on the nanotube tips was 26 nm. The improvedfield emission from the coaxial nanotube heterostructures is attributed to the lowelectron affinity and the thickness modulation effect of the AlN coating layer.

Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications

We report on the structural and optical characteristics of position-controlled GaN/ZnO coaxial nanotube heterostructure and GaN/InxGa1−xN coaxial nanotube quantum structure arrays for light-emitting diode (LED) applications. The GaN/ZnO nanotube heterostructures were fabricated by growing a GaN layer on the entire surface of position-controlled ZnO nanotube arrays using low-pressure metal-organic vapour-phase epitaxy. As determined by transmission-electron microscopy (TEM), an abrupt and coherent interface between the core ZnO and the GaN overlayer was observed. The optical characteristics of heteroepitaxial GaN/ZnO nanotube heterostructures were also investigated using cathodoluminescence (CL) spectroscopy. This position-controlled growth of high-quality single crystalline GaN/ZnO coaxial nanotube heterostructures allowed the fabrication of artificial arrays of high-quality GaN-based coaxial quantum structures by the heteroepitaxial growth of GaN/InxGa1−xN multiple quantum wells along the circumference of the GaN/ZnO nanotubes. The optical and structural characteristics of the position-controlled GaN/InxGa1−xN coaxial nanotube quantum structures were investigated by using CL spectroscopy and TEM analysis, respectively. The green LED microarrays were successfully fabricated by the controlled heteroepitaxial coaxial coatings of GaN/InxGa1−xN coaxial nanotube quantum structures and the outermost Mg-doped p-type GaN layer onto the GaN/ZnO coaxial nanotube heterostructures, presumably implying that the position-controlled growth of high-quality GaN/ZnO coaxial nanotube heterostructure arrays provides a general and rational route of integrating vertical nanodevices for nanoscale electronics and optoelectronics.

Epitaxial growth of coaxial GaInN-GaN hetero-nanotubes

We report about the successful realization of a coaxial hetero structure grown by MOVPE around ZnO nanocolumns. At higher overgrowth temperatures, the ZnO cores completely dissolved leaving GaN nanotube structures with excellent properties. Such tubes could be sheathed by a GaInN-GaN single quantum well structure, as confirmed by photoluminescence and transmission electron microscopy.

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was ∼200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu 3 ions was not present. Transmission electron microscopy shows crystalline Eu 2O 3 formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp 5D O– 7F 2 transition of the Eu 3+ ion at around 615 nm was observed.

GaN / In 1 − x Ga x N / GaN / ZnO nanoarchitecture light emitting diode microarrays

We studied the fabrication and electroluminescent (EL) characteristics of GaN/In1−xGaxN/GaN/ZnO nanoarchitecture light emitting diode (LED) microarrays consisting of position-controlled GaN/ZnO coaxial nanotube heterostructures. For the fabrication of nanoarchitecture LED arrays, n-GaN, GaN/In0.24Ga0.76N multiquantum well (MQW) structures and p-GaN layers were deposited coaxially over the entire surface of position-controlled ZnO nanotube arrays grown vertically on c-plane sapphire substrates. The nanoarchitecture LEDs exhibited strong green and blue emission from the GaN/GaN/In0.24Ga0.76N MQWs at room temperature. Furthermore, the origins of dominant EL peaks are also discussed.

Fabrication and Optical Characteristics of Position‐Controlled ZnO Nanotubes and ZnO/Zn0.8Mg0.2O Coaxial Nanotube Quantum Structure Arrays

AbstractThe position‐controlled growth and structural and optical characteristics of ZnO nanotubes and their coaxial heterostructures are reported. To control both the shape and position of ZnO nanotubes, hole‐patterned SiO2 growth‐mask layers on Si(111) substrates with GaN/AlN intermediate layers using conventional lithography are prepared. ZnO nanotubes are grown only on the hole patterns at 600 °C by catalyst‐free metal–organic vapor‐phase epitaxy. Furthermore, the position‐controlled nanotube growth method allows the fabrication of artificial arrays of ZnO‐based coaxial nanotube single‐quantum‐well structures (SQWs) on Si substrates. In situ heteroepitaxial growth of ZnO and Zn0.8Mg0.2O layers along the circumference of the ZnO nanotube enable an artificial formation of quantum‐well arrays in a designed fashion. The structural and optical characteristics of the ZnO nanotubes and SQW arrays are also investigated using synchrotron radiation X‐ray diffractometry and photoluminescence and cathodoluminescence spectroscopy.

ZnO / Mg 0.2 Zn 0.8 O coaxial nanorod heterostructures for high-performance electronic nanodevice applications

We report on fabrication and electrical characteristics of field effect transistors (FETs) based on ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures. As compared to bare ZnO nanorod FETs, coaxial nanorod heterostructure FETs exhibited the enhanced mobility (∼110 cm2/V s), superior subthreshold swing (∼200 mV/decade), and negligibly small hysteresis to demonstrate very stable operation of high-performance nanorod FETs. In situ surface passivation and carrier confinement effects provided by heteroepitaxially grown Mg0.2Zn0.8O shell layer are presumably responsible for the highly enhanced device performance.

Electrical Injection in Longitudinal and Coaxial Heterostructure Nanowires: A Comparative Study through a Three-Dimensional Simulation

We carried out a comparative study of electrical injection in longitudinal p-i-n and coaxial p-n core-shell nanowires by performing a three-dimensional numerical simulation. In the case of the core-shell structure, we show that both electrons and holes of high density can be efficiently injected into and confined in the structure even without an i-region. The required bias voltage and doping concentrations in the core-shell structure are smaller than those in the longitudinal p-i-n structure to achieve the same carrier injection level. Furthermore, we show that a type-I band alignment, as required in traditional p-i-n structure is not necessary in core-shell structure, allowing more flexibility in nanowire devices design. Our results thus provide a theoretical foundation and understanding that a core-shell structure is far superior to the longitudinal p-i-n structure for electrical injection nanowire lasers.

Synthesis and Strain Relaxation of Ge-Core/Si-Shell Nanowire Arrays

Analogous to planar heteroepitaxy, misfit dislocation formation and stress-driven surface roughening can relax coherency strains in misfitting core-shell nanowires. The effects of coaxial dimensions on strain relaxation in aligned arrays of Ge-core/Si-shell nanowires are analyzed quantitatively by transmission electron microscopy and synchrotron X-ray diffraction. Relating these results to reported continuum elasticity models for coaxial nanowire heterostructures provides valuable insights into the observed interplay of roughening and dislocation-mediated strain relaxation.

Zn2SiO4/ZnO Core/Shell Coaxial Heterostructure Nanobelts Formed by an Epitaxial Growth

Si-doped ZnO can be synthesized on the surface of the early grown Zn2SiO4 nanostructures and form core/ shell coaxial heterostructure nanobelts with an epitaxial orientation relationship. A parallel interface with a periodicity array of edge dislocations and an inclined interface without dislocations can be formed. The visible green emission is predominant in PL spectra due to carrier localization by high density of deep traps from complexes of impurities and defects. Due to band tail localization induced by composition and defect fluctuation, and high density of free-carriers donated by doping, especially the further dissociation of excitons into free-carriers at high excitation intensity, the near-band-edge emission is dominated by the transition of free-electrons to free-holes, and furthermore, exhibits a significant excitation power-dependent red-shift characteristic. Due to the structure relaxation and the thermalization effects, carrier delocalization takes place in deep traps with increasing excitation density. As a result, the green emission passes through a maximum at 0.25I(0) excitation intensity, and the ratio of the violet to green emission increases monotonously as the excitation laser power density increases. The violet and green emission of ZnO nanostructures can be well tuned by a moderate doping and a variation in the excitation density.

Synthesis and optical properties of ZnO and carbon nanotube based coaxial heterostructures

Carbon nanotubes and ZnO based functional coaxial heterostructured nanotubes have been fabricated by using atomic layer deposition. An irregular structured shell composed of ZnO nanocrystals was deposited on pristine nanotubes, while a highly defined ZnO shell was deposited on the tubes after its functionalization with Al2O3. Photoluminescence measurements of the ZnO shell on Al2O3/nanotube show a broad green band emission, whereas the shell grown on the bare nanotube shows a band shifted to the orange spectral range.

Observation of Single-Electron Transport in a Zn0.8Mg0.2O/ZnO Coaxial Heterostructure Nanorod

Observation of Single-Electron Transport in a Zn0.8Mg0.2O/ZnO Coaxial Heterostructure Nanorod

MOVPE growth of GaN around ZnO nanopillars

A multi-layer growth process is established for the growth of GaN on ZnO films and implemented to the growth of GaN around ZnO nanopillars using metalorganic vapor phase epitaxy (MOVPE). We have successfully grown GaN/ZnO coaxial nanopillar heterostructures and, furthermore, high quality single-crystal GaN nanotubes by increasing the growth temperature. Scanning electron microscopy, transmission electron microscopy, low temperature (10 K) photoluminescence spectroscopy, and high-resolution X-ray diffraction exhibit good crystalline and morphological properties of both of these structures.

Synthesis and Characterization of Al2O3/SiO2 Coaxial Nanowire Heterostructures with Periodical Twinning Structures

Al2O3/SiO2 coaxial nanowire heterostructures were synthesized on a silicon substrate by a simple thermal evaporation method. The structure and morphology of the as-synthesized nanostructure were characterized using scanning electron microscopy and transmission electron microscopy. The growth of Al2O3/SiO2 coaxial nanowire heterostructures follows a vapor-solid (VS) process. Studies indicate that typical nanostructure consists of single twinning-crystalline Al2O3 nanowires (core) with diameter of about 50 nm and amorphous SiO2 shell. Photoluminescence properties were also investigated at room temperature. The photoluminescence spectrum reveals the product has a blue emission band and two UV emission bands.

Controlled Growth and Characterization Methods of Semiconductor Nanomaterials

One-dimensional (1D) semiconductor nanomaterials attract much attention because they are ideal systems for investigation and studying the relationship between properties and structures and having extensive application future in the high technical field. They are expected to play an important role in fabrication of the next generation nanocircuits, nanotools, nanowires lasers, photon tunneling devices, near-field photo-waveguide devices, etc. This article described controlled growth, characterization of structures and morphologies and properties of 1D semiconductor nanomaterials based on our previous works. This article is organized into two parts: The first part is complicated nanostructures of semiconductors, which includes coaxial nanocables, heterostructure nanowires and nanowires with metal-semiconductor junction behavior, hierarchical structures, doping of the nanowires and nanobelts, porous materials and periodically twined nanowires and asymmetrical polytypic nanobelts. The second part contains semiconductor nanoarrays based on anodic alumina membrane (AAM) templates. Finally, we propose that further investigation of the influence of nanomaterial morphologies on properties and how to design the morphology of nanostructures to meet the property requirements of nanodevices are our future research directions in this field.