Near-infrared photoluminescence of vertically aligned InN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy

We demonstrate that vertically aligned InN nanorods have been grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy (PA-MBE) at low and high growth temperatures (LT- and HT-InN nanorods). High-resolution scanning electron microscopy images clearly show that InN nanorods grown on Si(111) are hexagonal in shape, vertically aligned, well separated and densely distributed on the substrate. The size distribution of LT-InN nanorods is quite uniform, while the HT-InN nanorods exhibit a broad, bimodal distribution. The structural analysis performed by Raman scattering indicates that PA-MBE grown InN nanorods have the wurtzite-type InN single-crystal structure with the rod axis (growth direction) along the c-axis. In addition, both types of nanorods contain high concentrations of electrons (unintentionally doped). Compared to the HT-InN nanorods and the PA-MBE-grown InN epitaxial film, the LT-grown InN nanorods have a considerable number of structural defects. Near-infrared photoluminescence (PL) from LT- (∼ 0.77 eV) and HT-InN (∼ 0.70 eV) nanorods is clearly observed at room temperature. In comparison with the LT-InN nanorods, the PL efficiency of HT-InN nanorods is better and the PL peak energy is closer to that of InN-on-Si epitaxial films (∼ 0.66 eV). We also find that the PL band at low temperatures from nanorods is significantly weaker (compared to the InN film case) and exhibits anomalous temperature effects. We propose that these PL properties are results of considerable structural disorder (especially for the LT-InN nanorods) and strong surface electron accumulation effect (for both types of nanorods).     

Study of surface morphology control and investigation of hexagonal indium nitride nanorods grown on GaN/sapphire substrate

Heteroepitaxial growth of metal-catalyst-free indium nitride (InN) nanorods on GaN/sapphire substrates by radio-frequency metal-organic molecular beam epitaxy (RF-MOMBE) system was investigated. We found that different N/In flow ratios together with the growth temperatures greatly influenced the surface morphology of InN nanorods and their structural properties. The InN nanorods have been characterized in detail using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM). Optical property was evaluated by photoluminescence (PL) measurements. At lower growth temperatures, InN nanorods were successfully grown. A pronounced two-dimensional growth mode was observed at higher growth temperature of 500 degrees C, and these films showed preferred orientation along the c-axis. XRD patterns and SEM images reveal that InN nanorods has high quality wurtzite structure with FWHM approaching 900 arcsec, and they have uniform diameters of about 150 nm and length of about 800 nm. Meanwhile, no metallic droplet was observed at the end of the nanostructured InN, and this is strong evidence that the nanorods are grown via the self-catalyst process. The PL peak at 0.8 eV is attributed to the quantum confinement and Moss-Burstein effects. These observations provide some valuable insights into the physical-chemical process for manufacturing InN nanorods devices.     

Formation of InN nanoparticle and nanorod structures by nitrogen plasma annealing method

In the present study, a novel method involving nitrogen plasma annealing has been reported for preparing InN nanoparticle/nanorod structures and for improving the properties of InN nanoparticle layers. Plasma annealed structures have been characterized by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy techniques. InN nanoparticle layers have been prepared using activated reactive evaporation set up. It has been observed that there is a remarkable improvement in the conductivity and crystallinity of InN nanoparticle layers on annealing in nitrogen plasma. This has been attributed to the increase in the nitrogen content of the samples. Experiments involving plasma annealing of In nanorods deposited oxide template has also been carried out. It was found that on plasma treatment In nanorods get converted to mixed phase InN nanorods with hexagonal and cubic fractions.     

Catalyst-free metal-organic chemical vapor deposition growth of InN nanorods

We demonstrated the successful growth of catalyst-free InN nanorods on (0001) Al2O3 substrates using metal-organic chemical vapor deposition. Morphological evolution was significantly affected by growth temperature. At 710 degrees C, complete InN nanorods with typical diameters of 150 nm and length of approximately 3.5 microm were grown with hexagonal facets. theta-2theta X-ray diffraction measurement shows that (0002) InN nanorods grown on (0001) Al2O3 substrates were vertically aligned along c-axis. In addition, high resolution transmission electron microscopy indicates the spacing of the (0001) lattice planes is 0.28 nm, which is very close to that of bulk InN. The electron diffraction patterns also revealed that the InN nanorods are single crystalline with a growth direction along (0001) with (10-10) facets.     

Photoluminescence properties of non-tapered InN nanorods grown by plasma-assisted metalorganic chemical vapor phase deposition

We have successfully grown non-tapered InN nanorods on Si substrate using an RF plasma assisted metalorganic chemical vapor deposition technique. Employment of 50 W nitrogen plasma reduces the optimal growth temperature to 500 degrees C. In order to study the temperature dependent bandgap and thermal quenching mechanism in relation to the localized states, photoluminescence measurement over a temperature range from 7 to 160 K are conducted. The photoluminescence at 7 K shows a strong near-band-emission energy of 0.682 eV with a narrow band width of 0.027 eV, which reveals excellent optical and structural qualities of the InN nanorods.     

Growth of cubic and hexagonal InN nanorods

InN nanorods with polyhedral ends were grown by evaporating indium in the flow of ammonia gas on Si substrates coated with Au catalyst. The samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). XRD shows that the samples contain both cubic and hexagonal phases. From the TEM results, it can be found that the cubic nanorods grow along [010] direction with a lattice constant of 0.497 nm, while the hexagonal nanorods grow along [112¯0] direction. The growth mechanism of the obtained nanostructures is discussed.     

The effect of the shape of nanorod arrays on the nanocarpet effect

The bundling of densely packed free-standing nanorods/nanotubes in a liquid environment, or the 'nanocarpet effect', has a direct impact on the stability of nanostructures used for chemical and biological sensors. Using glancing angle deposition, we prepared four different structures: vertically aligned, tilted, zigzag, and square spring Si nanorod arrays, and compared their stabilities after water treatment. We found that although the tilted nanorods were bent in the nanorod tilting direction, they did not form nanorod bundles, and this structure was the most stable one. The larger the tilting angle, i.e., the more inclined the nanorod was to the surface, the more stable the structure. We also found that the quasi-vertical nanorod structures, the zigzag and square spring structures, showed improved stabilities compared to vertically aligned nanorods. Furthermore, by properly depositing a capping layer on top of the vertically aligned nanorods, the structure became mechanically very stable while the high porosity nature of the nanorod array was maintained. This work is helpful for designing stable nanostructures used in a liquid environment.     

Growing InN films by plasma-assisted metalorganic vapor-phase epitaxy on Al2O3 and YSZ substrates in plasma generated by gyrotron radiation under electron cyclotron resonance conditions

Hexagonal indium nitride (InN) films on (111)- and (100)-oriented yttria-stabilized zirconia (YSZ) substrates and (0001)-oriented Al₂O₃ substrates have been grown for the first time at a rate of 1 μm/h by the method of metalorganic vapor-phase epitaxy with plasma-assisted nitrogen activation in an electron cyclotron resonance discharge generated by gyrotron radiation at low-temperature (350°C) growth. InN films grown without buffer layers possess a textured polycrystalline structure. Using an InN/GaN double buffer layer, single-crystalline InN films have been obtained on Al₂O₃(0001) substrates. Data on the morphology, structure, and photoluminescent properties of the obtained InN films are presented.     

Raman Spectral Mapping of III–V Nitride and Graphene Nanostructures

Among several spectroscopic imaging techniques to visualise the nanostructures, Raman spectral imaging is one of the most indispensible non-destructive tools. We discuss the limitations and the importance of each step involved in the Raman imaging in the visualization of different nanostructures and illustrated with examples. Raman spectroscopic imaging of nanostructures is demonstrated for differentiation of morphology in InN nanorods, crystallographic orientation for single square faceted GaN nanotube and layer thickness of graphene layers. The limitations of the spatial and spectral resolutions of the Raman maps are evaluated in the illustration.     

Electronic transport mechanism and photocurrent generations of single-crystalline InN nanowires

Nanodevices using individual indium nitride nanowires are fabricated by e-beam lithography. The nanowires have diameters of 40-80?nm, lengths up to several tens of micrometres and single-crystalline nature. We observed ohmic I-V behaviour of InN nanowires above nearly 100?K, which is consistent with the pinning Fermi level of the metal electrode near the conduction band edge of InN nanowire. At low temperatures, the device shows typical semiconductor behaviour along with a quantum tunnelling effect through the Schottky barrier rather than thermally activated transport. The activation energy calculated above and below 80?K is 28.2 and 5.08?meV, respectively. We have also fabricated a photocurrent generation device using InN nanowires. The photocurrent of an acceptor-sensitizer dyad with di-(3-aminopropyl)-viologen (DAPV) and a Ru complex on an InN nanowires/ITO plate was 8.3?nA?cm(-2), which increased by 62.7% compared to that without InN nanowire layers.     

Low-temperature growth of highly c-oriented InN films on glass substrates with ECR-PEMOCVD

High quality InN films are deposited with an interlayer of high c-orientation (002) AZO (Aluminium-doped Zinc Oxide; ZnO:Al) films on glass substrates by electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) at low temperature. AZO films used as a buffer layer are effective for the epitaxial growth of InN films. The influence of Trimethyl Indium (TMIn) flux on the properties of InN films is systematically investigated by reflection high energy electron diffraction (RHEED), X-ray diffraction analysis (XRD), atomic force microscopy (AFM) and optical transmittance spectra. The results indicate that high quality InN films with high c-orientation and small surface roughness are successfully achieved at an optimized Trimethyl Indium (TMIn) flux of 5.5 sccm. The InN/AZO structures have great potential for the development of full spectra solar cells.     

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.     

Chiral Seeded Growth of Gold Nanorods Into Fourfold Twisted Nanoparticles with Plasmonic Optical Activity

A robust and reproducible methodology to prepare stable inorganic nanoparticles with chiral morphology may hold the key to the practical utilization of these materials. An optimized chiral growth method to prepare fourfold twisted gold nanorods is described herein, where the amino acid cysteine is used as a dissymmetry inducer. Four tilted ridges are found to develop on the surface of single-crystal nanorods upon repeated reduction of HAuCl₄, in the presence of cysteine as the chiral inducer and ascorbic acid as a reducing agent. From detailed electron microscopy analysis of the crystallographic structures, it is proposed that the dissymmetry results from the development of chiral facets in the form of protrusions (tilted ridges) on the initial nanorods, eventually leading to a twisted shape. The role of cysteine is attributed to assisting enantioselective facet evolution, which is supported by density functional theory simulations of the surface energies, modified upon adsorption of the chiral molecule. The development of R-type and S-type chiral structures (small facets, terraces, or kinks) would thus be non-equal, removing the mirror symmetry of the Au NR and in turn resulting in a markedly chiral morphology with high plasmonic optical activity.     

Growth of InN Nanowires with Uniform Diameter on Si(111) Substrates: Competition Between Migration and Desorption of In Atoms

The effects of the growth parameters on the uniformity and the aspect ratio of InN nanowires grown on Si(111) substrates have been studied systematically, and a modified quasi‐equilibrium model is proposed. The growth temperature is of great importance for both the nucleation of the nanowires and the migration of In and N atoms, thus affecting the uniformity of the InN nanowires. In order to improve the uniformity of the InN nanowires, both traditional substrate nitridation and pre‐In‐droplet deposition have been implemented. It is found that the substrate nitridation is favorable for the nucleation of InN nanowires. However, the initial In atoms adhered to the substrate are insufficient to sustain the uniform growth of the InN nanowires. We have found that the initial In droplet on the substrate is not only advantageous for the nucleation of the InN nanowire, but also favorable for the In atom equilibrium between the initial In droplets and the direct In flux. Therefore, InN nanowires with a uniform aspect ratio and optimal diameter can be achieved. The results reported herein provide meaningful insights to understanding the growth kinetics during the InN nanowires growth, and open up great possibilities of developing high‐performance group III‐nitride‐based devices.     

氮化铟薄膜的p型掺杂和铁磁性研究进展

Ⅲ族氮化物半导体材料(InN、GaN、AlN)由于能带结构的特殊性,使其在光电器件与微波等领域得到广泛应用。其中,研究和发展InN材料及器件已被公认是占领光电信息技术领域战略至高点的重要途径,InN材料的p型导电以及室温铁磁性研究更是成为Ⅲ族氮化物中新颖的研究课题。首先简单介绍InN的晶体结构和制备方法,并分析其目前所遇到的挑战,然后重点阐述国际上关于InN在p型掺杂以及铁磁性领域的研究进展,同时介绍本课题组在该方面的研究,最后进行了简要总结和展望。     

Localized oblique-angle deposition: Ag nanorods on microstructured surfaces and their SERS characteristics

In this paper, we demonstrate a simple and convenient method of depositing Ag nanorods on a substrate inside a standard evaporation chamber with the substrate resting on a leveled stage. Microstructuring the substrate prior to the deposition imparts a large incidence angle (>70°) between the collimated vapor atoms and the local surface normal, which is essential to induce the shadowing effect. Thereby, a localized oblique-angle deposition (LOAD) is realized, forming nanorods selectively on the steep sidewalls of surface microcavities patterned via standard photolithography and silicon dry etching. We also demonstrate that these nanorods can boost SERS activity of the underlying substrate and thus perform comparable to those fabricated via advanced patterning techniques or conventional OAD whereby the entire substrate has to be tilted with respect to the incident vapor atoms. Our results suggest the viability of decorating microchannel sidewalls with SERS-active nanorods for integrated sample processing and SERS detection.     

Controllable Synthesis of [11−2−2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high‐quality InN samples. Here, high‐quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single‐crystals. The as‐prepared InN nanopyramids have well‐defined exposed facets of [0001], [11?2?2], [1?212], and [?2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First‐principles density functional calculations reveal that the nonpolar [11?2?2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n–n junction is naturally formed at the InN/ZnO interface. The formation of the n–n junction and the built‐in electric field is ascribed to the efficient separation of the photogenerated electron–hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm^?2 h^?1, which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.     

Tuning the surface charge properties of epitaxial InN nanowires

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ～0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ～0.95-1.3 eV above the valence band maximum. Through these trends, well captured by the effective mass and ab initio materials modeling, we have unambiguously identified the definitive role of surface doping in tuning the surface charge properties of InN.     

Multilayered Si/Ni nanosprings and their magnetic properties

A two-turn, eight-armed, rectangular Si/Ni heterogeneous nanospring structure on Si(100) has been fabricated using a multilayer glancing-angle deposition technique. The multilayered nanosprings with a height of approximately 1.98 mum were composed of alternating layers of amorphous Si nanorods approximately 580 nm in length and face-centered cubic Ni nanorods approximately 420 nm in length, both with a diameter of approximately 35 nm. The magnetic anisotropy of the nanosprings showed that the in-plane easy and hard axes were parallel and perpendicular to the Ni nanorod plane, respectively. The out-of-plane magnetic hysteresis loop was very sensitive to the applied magnetic field direction when rotating the nanosprings about their in-plane hard axis, and the magnetization measurement revealed that the nanosprings tilted at approximately 7.5 degrees toward the plane of the Si nanorods. The magnetic anisotropy of the nanosprings is determined by their structure, and the experimental results can be interpreted by the shape anisotropy energy.     

Origin of n‐type conductivity in nominally undoped InN

The influence of different contributions to the high electron concentration in state‐of‐the‐art InN layers grown by molecular‐beam epitaxy is investigated. Surface accumulation has a crucial influence for thin InN layers < 300 nm and superimposes the background concentration. For air‐exposed InN it can be assigned to a surface near doping by oxygen. For InN layers in the micron range the density of dislocations is the major doping mechanism. Finally, point defects like vacancies and impurities have minor influence on the carrier concentration and would dominate the free electron concentration only for InN > 10 μm.