Catalyst-free Metal-organic Chemical Vapor Deposition Research Articles

Axial InAs(Sb) inserts in selective-area InAsP nanowires on InP for optoelectronics beyond 25 µm

In this work, we report on the growth of high yield small bandgap InAs and InAsSb inserts embedded in InAsP nanowires grown on an InP substrate by catalyst-free selective-area metal-organic chemical vapor deposition. It is observed that the growth of the inserts with high aspect ratios can be achieved by properly tuning the V/III ratio. Nanowire arrays with InAs(Sb) inserts exhibit strong photoluminescence at 77 K from interband transitions, spanning a wavelength range of 2.30–3.70 µm. In addition, the InAsP/InAs heterointerfaces are characterized by a scanning transmission electron microscope and an energy-dispersive X-ray spectroscopy. We believe that these results pave the way to engineering interband transitions and enabling hybrid integration for nanoscale optical devices at the mid-wavelength infrared.

Seeding layer assisted selective-area growth of As-rich InAsP nanowires on InP substrates.

We present the first demonstration of arsenic-rich InAs1-xPx (0 ≤ x ≤ 0.33) nanowire arrays grown on InP (111)B substrates by catalyst-free selective-area metal-organic chemical vapor deposition. It is shown that by introducing a thin InAs seeding layer prior to the growth of the nanowire, an extremely high vertical yield is achieved by eliminating rotational twins between (111)A and (111)B crystal orientations. InAsP nanowire arrays show strong emission of photoluminescence (PL) at room temperature, suggesting a significant reduction of surface state density compared with InAs nanowires. The phosphorus composition deduced from the PL peak energy is verified by energy-dispersive X-ray spectroscopy. The growth temperature shows a strong impact on the aspect ratio of InAs1-xPx nanowires with different phosphorus compositions. In addition, no PL emission is observed from nanowires grown with arsenic overpressure, likely due to an exchange of phosphorus with arsenic atoms at the surface which results in an increase in the surface state density. These results provide a path for the growth of heterojunctions based on As-rich InAs1-xPx for nanoscale short-wavelength infrared and mid-wavelength infrared optical devices.

Self-assembled growth and structural analysis of inclined GaN nanorods on nanoimprinted m-sapphire using catalyst-free metal-organic chemical vapor deposition

In this study, self-assembled inclined (1-10-3)-oriented GaN nanorods (NRs) were grown on nanoimprinted (10-10) m-sapphire substrates using catalyst-free metal-organic chemical vapor deposition. According to X-ray phi-scans, the inclined GaN NRs were tilted at an angle of ∼57.5° to the [10-10]sapp direction. Specifically, the GaN NRs grew in a single inclined direction to the [11-20]sapp. Uni-directionally inclined NRs were formed through the one-sided (10-11)-faceted growth of the interfacial a-GaN plane layer. It was confirmed that a thin layer of a-GaN was formed on r-facet nanogrooves of the m-sapphire substrate by nitridation. The interfacial a-GaN nucleation affected both the inclined angle and the growth direction of the inclined GaN NRs. Using X-ray diffraction and selective area electron diffraction, the epitaxial relationship between the inclined (1-10-3) GaN NRs and interfacial a-GaN layer on m-sapphire substrates was systematically investigated. Moreover, the inclined GaN NRs were observed to be mostly free of stacking fault-related defects using high-resolution transmission electron microscopy.

Monolithically Integrated InGaAs Nanowires on 3D Structured Silicon-on-Insulator as a New Platform for Full Optical Links.

Monolithically integrated III-V semiconductors on a silicon-on-insulator (SOI) platform can be used as a building block for energy-efficient on-chip optical links. Epitaxial growth of III-V semiconductors on silicon, however, has been challenged by the large mismatches in lattice constants and thermal expansion coefficients between epitaxial layers and silicon substrates. Here, we demonstrate for the first time the monolithic integration of InGaAs nanowires on the SOI platform and its feasibility for photonics and optoelectronic applications. InGaAs nanowires are grown not only on a planar SOI layer but also on a 3D structured SOI layer by catalyst-free metal-organic chemical vapor deposition. The precise positioning of nanowires on 3D structures, including waveguides and gratings, reveals the versatility and practicality of the proposed platform. Photoluminescence measurements exhibit that the composition of ternary InGaAs nanowires grown on the SOI layer has wide tunability covering all telecommunication wavelengths from 1.2 to 1.8 μm. We also show that the emission from an optically pumped single nanowire is effectively coupled and transmitted through an SOI waveguide, explicitly showing that this work lays the foundation for a new platform toward energy-efficient optical links.

Micro-Structural Properties of YBa2Cu3O(7-x)/ZnO Nanorods on SrTiO3.

We examined the local structural and the interfacial properties of YBa2Cu3O(7-x) (YBCO)/ZnO nanorods on SrTiO3 (STO) substrates using various measurements. Vertically aligned ZnO nanorods were synthesized on STO substrates using a catalyst-free metal-organic chemical vapor deposition. YBCO films were deposited ex-situ on the ZnO nanorods/STO templates using a DC magnetron sputtering deposition. X-ray diffraction revealed that the YBCO films were crystallized along their c-axes on the ZnO nanorods/STO templates. Transmission electron microscopy measurements demonstrated that YBCO filled the space between ZnO nanorods and that both interfaces of YBCO/ZnO nanorods and ZnO nanorods/STO were quite clean with no disorder. Polarization-dependent extended X-ray absorption fine structure measurements at the Cu K edge showed extra disorder in the CuO2 planes of YBCO/ZnO nanorods/STO, compared with that of YBCO/STO. The superconductivity transition temperature (T(c)) of YBCO/ZnO nanorods/STO was approximately 50 K whereas that of YBCO/STO was 93 K. The decrease of T(c) of YBCO/ZnO nanorods/STO was ascribed to the structural disorder of CuO2 planes as well as grain boundaries in the YBCO films.

Vertically aligned two-dimensional ZnO nanowall networks: Controllable catalyst-free growth and optical properties

We report the fabrication of vertically aligned two-dimensional (2D) ZnO nanowall networks (NNWs) by a catalyst-free metal–organic chemical vapor deposition system via a two-step growth method. The growth mechanism of the resulting NNWs was investigated detailedly by monitoring the morphology evolution at different growth stages. The formation of rough and striped grains in the initial stage was the determinant factor, which facilitated and guided the subsequent growth of NNWs. In our case, the obtained ZnO NNWs were of single-crystal wurtzite structure with a small full width at half-maximum (481arcsec) of (0002) ω-rocking curve. We also demonstrated the feasibility of controlling the morphology, structural and optical features of ZnO NNWs by adjusting the oxygen flow rate. A controllable evolution of micro-hole size and density of ZnO NNWs was observed. In addition, possible microcavity-related light oscillation and contaminants adsorption behaviors were proposed to explain the shift of ultraviolet peak and deep-level emission variation for the produced ZnO NNWs. Temperature-dependent photoluminescence results behaving with strong exciton effects further confirmed the relatively excellent quality of the obtained ZnO NNWs. It was reasonably believed that the novel 2D nanostructures featuring high crystallinity and remarkable optical properties can be employed in reliable optoelectronic nanodevices.

Local structural properties of Co-ion-implanted ZnO nanorods

We examined the local structural properties around Co and Zn ions in Co-ion-implanted ZnO nanorods by using an X-ray absorption fine structure (XAFS) analysis. Vertically-aligned ZnO nanorods were synthesized on Al2O3 substrates by using a catalyst-free metal-organic chemicalvapor deposition. Co ions (Co+ and Co2+) with energies of 50 and 100 keV and fluxes of 1013 and 1015 particles/cm2 were implanted in the ZnO nanorods, and the ion-implanted ZnO nanorods were annealed at 400–650°C. X-ray absorption near edge structure (XANES) analyses demonstrated that the chemical valence state of the Co ions were mostly 2+. An extended XAFS (EXAFS) analysis revealed that the Co ions were mostly substituted at the Zn sites of ZnO nanorods at a Coion flux of 1015 particles/cm2. However, at a flux of 1013 particles/cm2, Co ions formed Co-O and Co-Co clusters. These results were in contrast to the Co distribution in Co-added ZnO predicted by using a Monte Carlo method.

Heteroepitaxial fabrication and structural characterizations of ultrathin GaN nanotube arrays

We report on the heteroepitaxial fabrication and the structural characterizations of ultrafine GaN nanotube arrays. The ultrathin GaN nanotube arrays were fabricated by using epitaxial growth of a ultrathin GaN layer on ZnO nanoneedles and etching core Zno nanoneedles. Epitaxial growth of GaN and precise etcing control of core ZnO nanoneedles were obtained by using low-pressure metalorganic vapor-phase epitaxy. ZnO nanoneedles were grown on Si and sapphire substrates by using catalyst-free metal-organic chemical vapor deposition. Furthermore, the structural properties of the nanostructures were investigated using both synchrotron-radiation X-ray diffraction and highresolution transmission electron microscopy. The strain induced during the GaN deposition on ZnO was nearly sustained even after the removal of core ZnO.

Morphological and electrical characterization of ZnO nanocomposites in dye-sensitized solar cells

ZnO nanocomposites were fabricated as photoanodes for dye-sensitized solar cells. ZnO nanowires grown by catalyst-free metal-organic chemical vapor deposition were uniformly covered with ZnO nanoparticles by chemical bath deposition. ZnO nanocomposites exhibit the highest cell efficiency of 2.44% with a 16.7 mA/cm2 short circuit current density due to a higher specific surface area as compared to bare ZnO nanowires. The relatively low open circuit voltage of 0.48 V is associated with a significant decrease in the electron effective life time as shown by electrochemical impedance spectroscopy measurements, pointing out the importance to control the interface quality in dye-sensitized solar cells.

Bottom-up Photonic Crystal Cavities Formed by Patterned III–V Nanopillars

We report on the formation and optical properties of bottom-up photonic crystal (PC) cavities formed by III-V nanopillars (NPs) via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. This method of NP synthesis allows for precise lithographic control of NP position and diameter enabling simultaneous formation of both the photonic band gap (PBG) region and active gain region. The PBG and cavity resonance are determined by independently tuning the NP radius r, pitch a, and height h in the respective masked areas. Near-infrared emission at 970 nm is achieved from axial GaAs/InGaAs heterostructures with in situ passivation by laterally grown InGaP shells. To achieve out-of-plane optical confinement, the PC cavities are embedded in polydimethylsiloxane (PDMS) and removed from the growth substrate. Spatially and spectrally resolved 77 K photoluminescence demonstrates a strong influence of the PBG resonance on device emission. Resonant peaks are observed in the emission spectra of PC cavities embedded in PDMS.

InGaAs heterostructure formation in catalyst-free GaAs nanopillars by selective-area metal-organic vapor phase epitaxy

We investigate axial GaAs/InGaAs/GaAs heterostructures embedded in GaAs nanopillars via catalyst-free selective-area metal-organic chemical vapor deposition. Structural characterization by transmission electron microscopy with energy dispersive x-ray spectroscopy (EDS) indicates formation of axial InxGa1−xAs (x∼0.20) inserts with thicknesses from 36 to 220 nm with ±10% variation and graded Ga:In transitions controlled by In segregation. Using the heterointerfaces as markers, the vertical growth rate is determined to increase linearly during growth. Photoluminescence from 77 to 290 K and EDS suggest the presence of strain in the shortest inserts. This capability to control the formation of axial nanopillar heterostructures is crucial for optimized device integration.

Poole−Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires

We present electronic transport measurements of GaAs nanowires grown by catalyst-free metal-organic chemical vapor deposition. Despite the nanowires being doped with a relatively high concentration of substitutional impurities, we find them inordinately resistive. By measuring sufficiently high aspect ratio nanowires individually in situ, we decouple the role of the contacts and show that this semi-insulating electrical behavior is the result of trap-mediated carrier transport. We observe Poole-Frenkel transport that crosses over to phonon-assisted tunneling at higher fields, with a tunneling time found to depend predominantly on fundamental physical constants as predicted by theory. By using in situ electron beam irradiation of individual nanowires, we probe the nanowire electronic transport when free carriers are made available, thus revealing the nature of the contacts.

Zero-dimensional field emitter based on ZnO quantum dots

ZnO quantum dots (QDs) with diameter around 6 nm were grown by a catalyst-free metalorganic chemical vapor deposition method and applied as electron field emitters. The Ga doping resulted in a considerable improvement of the field emission properties. Macroscopic field emission measurements revealed that the Ga-doped ZnO QDs have a low turn-on field of 1.3 V/μm and a large field enhancement factor up to 104. Probe force microscopy and scanning field emission probe microscopy were used for mapping at submicrometer scale of the spatial distribution of the work function and the field emission, respectively. The local measurements indicated a spatial correlation between both properties that could be linked to spatial variations of the Ga doping level.

ZnO nanostructures with controlled morphologies on a glass substrate

We report morphology-controlled selective growth of ZnO nanostructures on glasssubstrates by using catalyst-free metal-organic chemical vapor deposition. For themorphology-controlled selective growth, a microheating method using a series ofmicroheaters was developed, which provided well-controlled local heating based on themicroheater geometry and spatial arrangement. ZnO nanostructure morphology dependedon the local growth temperature, so various nanostructure morphologies were obtainedselectively at specific positions on glass substrates by using local microheating. Themonolithic integration of nanostructures with different morphologies will have greatpotential for applications in multifunctional devices.

Heteroepitaxial ZnO nano hexagons on p-type SiC

ZnO single crystal nanohexagons have been grown heteroepitaxially on p-type Si-face 4H–SiC substrates with 8 ° miscut from [0 0 0 1] by catalyst-free atmospheric pressure metalorganic chemical vapor deposition and characterized by x-ray diffraction, scanning and transmission electron microscopy as well as energy disperse x-ray and cathodoluminescence analyses. The as-grown ZnO nanohexagons have a pillar shape terminated by a and c plane facets, and are aligned along the growth direction with the epitaxial relation [0 0 0 1] ZnO‖[0 0 0 1] 4H–SiC and [ 1 0 1 ¯ 0 ] ZnO ∥ [ 1 0 1 ¯ 0 ] 4 H - SiC . The ZnO nanohexagons demonstrate intense UV emission ( λ NBE=376 nm) and negligible defect-related luminescence.

In-situ and ex-situ ZnO nanorod growth on ZnO homo-buffer layers

Vertically well-aligned ZnO nanorods were fabricated in-situ and ex-situ on ZnO homo-buffer layers using catalyst-free metal-organic chemical vapor deposition. Field-emission electron microscopy measurements demonstrated that the nanorods were well aligned and had a uniform diameter of 70–100 nm depending on the growth temperature, irrespective of growth conditions, in-situ and ex-situ. X-ray diffraction measurements demonstrated that the ZnO nanorods and the ZnO buffer layers had a wurtzite structure, and that the crystal quality of the nanorods grown on a smooth surface was better than that of the nanorods grown on a rough surface. Field-emission transmission electron microscopy measurements revealed the presence of a disordered layer at the interface of the nanorod and the buffer layer.

Photoluminescent characteristics of MgxZn1−xO (0 ⩽ x ⩽ 0.18) nanorods

We report on continuous wave and time-resolved photoluminescent characteristics of MgxZn1−xO (0 ⩽ x ⩽ 0.18) alloy nanorods grown on Si substrates using catalyst-free metal–organic chemical vapor deposition. Low temperature time-integrated photoluminescence (PL) spectra of the alloy nanorods clearly exhibited a dominant PL peak of a single MgxZn1−xO phase without showing any PL peak from ZnO. In addition, dominant PL peak positions of the alloy nanorods were blueshifted with increasing Mg content in the nanomaterials. Using temperature-dependent PL spectra and time-resolved PL spectra of MgxZn1−xO nanorods, we discuss the composition fluctuations in MgxZn1−xO (0 ⩽ x ⩽ 0.18) nanorods.

Structural and Optical Properties of Nitrogen-Ion-Implanted ZnO Nanorods

We present a study of the structural and the optical properties of nitrogen-ion-implanted ZnO nanorods. Vertically-aligned ZnO nanorods were synthesized on Al2O3 substrates by using a catalyst-free metal-organic chemical vapor deposition (MOCVD) procedure. Nitrogen ions with an energy of 120 keV and an ion ux of 1 1016 particles/cm2 were implanted on the verticallyaligned ZnO nanorods. The structural properties were studied by using various techniques, including eld emission transmission electron microscopy (FE-TEM), X-ray di raction (XRD), and X-ray absorption ne structure (XAFS) analyses. The XRD measurements revealed that the crystalline properties of the nitrogen-ion-implanted ZnO nanorods were comparable with those of the as-grown ZnO nanorods. However, the XAFS analysis demonstrated that the bonding lengths of atomic pairs on the c-axis were slightly elongated and that the ion implantation caused an extra structural disorder of Zn-O pairs. The FE-TEM revealed that the nanorods had structurally-damaged spots along the entire length of the nanorods. From the photoluminescence (PL) spectra, we observed a considerably weak main donor-acceptor transition peak at 3.02 eV in the temperature range of 5 300 K. The blue shift and the intensity decrease of the main transition peak strongly suggest that the implanted nitrogen ions play a role in the optical property changes of the nanorods.

Charge Influence and Growth Mechanism of ZnO Nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystalization and formation of ZnO nanorods.

Charge Influence and Growth Mechanism of ZnO Nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystallization and formation of ZnO nanorods.