Atom Probe Tomography of Zinc Oxide Nanowires

Wide-bandgap zinc oxide (ZnO) semiconductors and nanowires have become important materials for electronic and photonic device applications. In this work, we report the growth of well-aligned single-crystal ZnO nanowire arrays on sapphire substrates by chemical vapor deposition and the development of atom probe tomography, an emerging nanoscale characterization method capable of providing deeper insight into the three-dimensional distribution of atoms and impurities within its structure. Using a metal-catalyst-free approach, the influence of the growth parameters on the orientation and density of the nanowires were studied. The resulting ZnO nanowires were determined to be single crystalline, with diameter on the order of 50 nm to 150 nm and length that could be controlled between 0.5 μm to 20 μm. Their density was on the order of high 10⁸ cm⁻² to low 10⁹ cm⁻². In addition to routine characterizations using scanning and transmission electron microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy, we developed the atom probe tomography technique for ZnO nanowires, comparing the voltage pulse and laser pulse modes. In-depth analysis of the data was carried out to determine the accurate chemical composition of the nanowires and reveal the incorporation of nitrogen impurities. The current–voltage characteristics of individual nanowires were measured to determine their electrical properties.     

Synthesis and Properties of High-Quality InN Nanowires and Nanonetworks

We report on the growth of high-quality InN nanowires by the vapor–liquid–solid mechanism at rates of up to 30 μm/h. Smooth and horizontal nanowire growth has been achieved only with nanoscale catalyst patterns, while large-area catalyst coverage resulted in uncontrolled and three-dimensional growth. The InN nanowires grow along the [110] direction with diameters of 20 to 60 nm and lengths of 5 to 15 μm. The nanowires bend spontaneously or get deflected from other nanowires at angles that are multiples of 30°, forming nanonetworks. The gate-bias-dependent mobility of the charge carriers ranges from 55 cm²/V s to 220 cm²/V s, and their concentration is ∼10¹⁸ cm⁻³.     

Device Characteristics of GaInSb/AlGaSb Quantum Well Lasers Monolithically Grown on GaAs Substrates by Using an Interfacial Misfit Array

We report the device characteristics of GaInSb/AlGaSb quantum well (QW) lasers monolithically grown on GaAs substrates by using an interfacial misfit (IMF) array. The IMF array localized at the GaSb/GaAs interface can accommodate the 7.8% lattice mismatch between GaAs substrates and GaSb buffer layers, resulting in the formation of a GaSb buffer with a very low defect density on GaAs substrates. Top-top and top-bottom metal contact methods are applied to the Ga_0.9In_0.1Sb/GaSb QW edge-emitting lasers monolithically grown on GaAs substrates for characterizing current–voltage (I–V) and output power–current (L–I) curves. The potential drop at the IMF array of ~0.7 V is elucidated by comparing I–V characteristics with these two contact methods. L–I characteristics and electroluminescence spectra shows room-temperature lasing at 1.83 μm from a 1.25-mm-long top-top contact device containing six-layer Ga_0.9In_0.1Sb QWs with a threshold current density (J _th) of 860 kA/cm². This IMF technique will enable a wide range of lasing wavelengths from near- to mid-wavelength infrared regimes on a GaAs platform.     

Nanowires in the CdHgTe Material System

HgTe nanowires have been grown by molecular beam epitaxy (MBE). They are nucleated at Au particles on Si or GaAs substrates and subsequently self-organize and grow laterally on the surface into 20–50 nm wide, 0.5–1 μm long twisted, but single-crystal, wires. Further growth gives longer, wider, and straighter polycrystalline wires. When unimpeded by Au particles on the surface, the wires become straight and consist of segments of cubic 〈111〉 HgTe and hexagonal 〈001〉 Te parallel to the wire. Te nanowires and Au␣nanowires have also occasionally been formed. All attempts to grow CdHgTe on Si substrates with or without Au particles have resulted in polycrystalline layers. The phase diagrams and diffusion coefficients imply that CdHgTe or HgTe nanowires will not grow by the vapor–liquid–solid technique at the low MBE growth temperatures. SiO₂ functions as a mask for selective growth of HgTe, but not for CdHgTe.     

Study of the Piezoelectric Power Generation of ZnO Nanowire Arrays Grown by Different Methods

The piezoelectric power generation from ZnO nanowire arrays grown on different substrates using different methods is investigated. ZnO nanowires were grown on n‐SiC and n‐Si substrates using both the high‐temperature vapor liquid solid (VLS) and the low‐temperature aqueous chemical growth (ACG) methods. A conductive atomic force microscope (AFM) is used in contact mode to deflect the ZnO nanowire arrays. No substrate effect was observed but the growth method, crystal quality, density, length, and diameter (aspect ratio) of the nanowires are found to affect the piezoelectric behavior. During the AFM scanning in contact mode without biasing voltage, the ZnO nanowire arrays grown by the VLS method produced higher and larger output voltage signal of 35 mV compared to those grown by the ACG method, which produce smaller output voltage signal of only 5 mV. The finite element (FE) method was used to investigate the output voltage for different aspect ratio of the ZnO nanowires. From the FE results it was found that the output voltage increases as the aspect ratio increases and starts to decreases above an aspect ratio of 80 for ZnO nanowires.     

Oxidation characteristics of Si0.85Ge0.15 nanowires

Oxidation characteristics of Si_0.85Ge_0.15 nanowires were investigated using transmission electron microscopy (TEM) analyses. Si_0.85Ge_0.15 nanowires were grown in a tube furnace by vapor–liquid–solid (VLS) method and thermally oxidized at 925 °C for 1–8 h. After oxidation, oxide thicknesses were measured using TEM images. Si_0.85Ge_0.15 nanowires showed a thicker oxide than Si nanowires, for the whole range of oxidation time. The oxidation rate of Si_0.85Ge_0.15 nanowires significantly decreased in nanowires with diameters less than 150 nm. Long-term oxidation in Si_0.85Ge_0.15 nanowire resulted in the oxidation of germanium atoms.     

Controlled Growth of ZnO Nanowires and Their Optical Properties

This article surveys recent developments in the rational synthesis of single‐crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power‐dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm^–2). These short‐wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 10¹⁰ cm^–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single‐crystalline, well‐facetted nanowires can function as self‐contained optical resonance cavities. This argument is further supported by our recent near‐field scanning optical microscopy (NSOM) studies on single nanowires.     

Molecular Dynamics Simulation of Mechanical Properties of Single-Crystal Bismuth Telluride Nanowire

The mechanical properties of single-crystal bismuth telluride nanowires have been studied by molecular dynamics methods. The mechanical behavior of the Bi₂Te₃ nanowire for two principal axes was simulated under different strain rates at low temperature and the results compared with those of bulk Bi₂Te₃. The simulation results show that, due to its marked anisotropy, the nanowire shows quite different failure behaviors in the two directions, with the failure stress for the a-axis (4.7 GPa) being three times that for the c-axis (1.4 GPa). The stress–strain curve of the nanowire is different from that for Bi₂Te₃ bulk, as surface stress induced by atomic rearrangement significantly reduces the strength of the nanowire. The effect of strain rate on the mechanical properties of the nanowire has also been analyzed, showing that the failure stress and failure strain decrease with decreasing strain rate, a behavior not apparent in the bulk Bi₂Te₃ simulation.     

Microchip for the Measurement of Seebeck Coefficients of Single Nanowires

Bismuth nanowires were electrochemically grown in ion track-etched polycarbonate membranes. Micromachining and microlithography were employed to realize a newly developed microchip for Seebeck coefficient measurements on individual nanowires. By anisotropic etching of a (100) Si wafer, an 800-nm-thick SiO₂/Si₃N₄ membrane was prepared in the chip center. The low thermal conductivity of the membrane is crucial to obtain the required temperature difference ΔT along the nanowire. The wire is electrically contacted to thin metal pads which are patterned by a new method of microscopic exposure of photoresist and a lift-off process. A ΔT between the two pairs of contact pads, located on the membrane, is established by a thin-film heater. Applying the known Seebeck coefficient of a reference film, the temperature difference at this gap is determined. Using ΔT and the measured Seebeck voltage U of the nanowire, its Seebeck coefficient can be calculated.     

Uncertainty quantification in nanowire growth modeling – A precursor to quality semiconductor nanomanufacturing

Mitigating the uncertainties associated with nanowire growth models have significant ramifications for the quality and reliability of nanomanufacturing of semiconductor nanowires. This research is focused on the development of a sectional-based mechanistic model of nanowire growth and the determination of the level of impacts the model parameters have on the growth of nanowires, characterized in terms of their weight, diameter and length. After testing the model with experimental growth data of silica (Si) nanowire weight, ZnO average diameter and length, it was observed that the direct top impingement growth coefficient (α_im) had the largest influence on the nanowire growth, in comparison to other model parameters → sidewall diffusion growth coefficient (α_sw), maximum allowable growth weight or length (W^(max)) and initial weight or length (W₀). The knowledge of the impact of uncertainty in these parameters on the overall growth of the nanowire can be leveraged on for robust design of the nanofabrication process that will impact on the quality, reliability, yield and cost of nanomanufacturing.     

Self-catalyzed molecular beam epitaxy growth and their optoelectronic properties of vertical GaAs nanowires on Si(111)

Self-assembled GaAs nanowires were grown by molecular beam epitaxy (MBE) on un-pretreated Si(111) substrates under different As₄/Ga flux ratios (V/III ratios). It has been found that the fraction of vertical wires would be nearly 100% when the As₄/Ga ratio arrives 90. The transmission electron microscopy (TEM) and micro-photoluminescence (PL) spectra results have indicated that the GaAs nanowires grown under a larger V/III ratio (90) have a pure ZB structure. Field-effect transistors (FET) based on single nanowire were fabricated with GaAs nanowires grown under the larger V/III ratio (90). The characteristics of the FET reveal a hole concentration of 3.919×10¹⁷ cm⁻³ and a hole mobility of 0.417 cm² V⁻¹s⁻¹. Photodetectors based on single nanowire and multiple nanowires structure with a metal-semiconductor-metal (MSM) electrode configuration have been proposed and demonstrated. All the photodetectors operating at room temperature exhibit good photoconductive performance, excellent stability, reproducibility and superior peak responsivity (87.67 A/W under 5 V for single nanowire photodetector).     

Electrical transport properties of single GaN and InN nanowires

The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×10¹⁷ cm⁻³, mobility of 30 cm²/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10⁻⁴ Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10⁻⁷ Ω cm². For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.     

Growth and Characterization of GaN Nanowires for Hydrogen Sensors

We report on the growth and characterization of high-quality GaN nanowires for hydrogen sensors. We grew the GaN nanowires by catalytic chemical vapor deposition (CVD) using gold thin films as a catalyst on a Si wafer with an insulating SiO₂ layer. Structural characterization of the as-grown nanowires by several methods shows that the nanowires are single-crystal wurtzite GaN.␣Photoluminescence measurements under 325 nm excitation show a near-band-edge emission peak around ∼3.4 eV. The hydrogen sensors are fabricated by contacting the as-grown GaN nanowires by source and drain electrodes and coating them with a thin layer of Pd. Hydrogen sensing experiments using the fabricated devices show high sensitivity response (ppm detection limit at room temperature) and excellent recovery. This work opens up the possibility of using high-quality GaN nanowire networks for hydrogen sensing applications.     

激光烧蚀法制备半导体纳米丝的研究进展

简要介绍了当前国内外激光烧蚀法制备半导体纳米丝的研究现状。介绍了目前激光烧蚀法制备的实验装置及所采用的激光束参数。比较了不同实验结果中在纳米丝结构和生长方向等方面的差异，并分析了半导体纳米丝生长的VLS金属催化机理和氧化物辅助生长模型，我们认为Si-金属混合物作靶时金属催化作用对纳米丝的生长起主要作用，而在Si-氧化物混合物作靶时，氧化物辅助作用将占主导地位。     

Dual-Color InAs/GaSb Superlattice Focal-Plane Array Technology

Within a very few years, InAs/GaSb superlattice technology has proven its suitability for high-performance infrared imaging detector arrays. At the Fraunhofer Institute for Applied Solid State Physics (IAF) and AIM Infrarot-Module GmbH, efforts have been focused on developing mature fabrication technology for dual-color InAs/GaSb superlattice focal-plane arrays for simultaneous, colocated detection at 3 μm to 4 μm and 4 μm to 5 μm in the mid-wavelength infrared atmospheric transmission window. Integrated into a wide-field-of-view missile approach warning system for an airborne platform, a very low number of pixel outages and cluster defects is mandatory for bispectral detector arrays. Process refinements, intense root-cause analysis, and specific test methodologies employed at various stages during the process have proven to be the key for yield enhancements.     

Low-Noise Mid-Wavelength Infrared Avalanche Photodiodes

Mid-wavelength infrared (MWIR) p ⁺–n ⁻–n ⁺ avalanche photodiodes (APDs) were fabricated using two materials systems, one with mercury cadmium telluride (HgCdTe) on a silicon (Si) substrate and the other with an indium arsenide/gallium antimonide (InAs/GaSb) strained layer superlattice (SLS). Diode characteristics, avalanche characteristics, and excess noise factors were measured for both sets of devices. Maximum zero-bias resistance times active area (R ₀ A) of 3 × 10⁶ Ω cm² and 1.1 × 10⁶ Ω cm² and maximum multiplication gains of 1250 at −10 V and 1800 at −20 V were measured for the HgCdTe and the SLS, respectively, at 77 K. Gains reduce to 200 in either case at 120 K. Excess noise factors were almost constant with increasing gain and were measured in the range of 1 to 1.2.     

Performance of MnO<Subscript>2</Subscript> Crystallographic Phases in Rechargeable Lithium-Air Oxygen Cathode

Manganese dioxide (MnO₂) has been shown to be effective for improving the efficiency of cathodes in lithium-air cells. Different crystallographic phases including α-, β-, and γ-MnO₂ nanowires, α-MnO₂ nanospheres, and α-MnO₂ nanowires on carbon (α-MnO₂/C) were synthesized using the hydrothermal method. Their physical properties were examined using x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area measurements, and scanning electron microscopy (SEM) and found to be in agreement with the literature. Electrochemical properties of the synthesized catalyst particles were investigated by fabricating cathodes and testing them in a lithium-air cell with lithium hexafluorophosphate in propylene carbonate (LiPF₆/PC) and tetra(ethylene glycol)dimethyl ether (LiTFSi/TEGDME) electrolytes. α-MnO₂ had the highest discharge capacity in the LiTFSi/TEGDME electrolyte (2500 mAh/g), whilst α-MnO₂/C in LiPF₆/PC showed a significantly higher discharge capacity of 11,000 mAh/g based on total mass of the catalytic cathode. However, the latter showed poor capacity retention compared with γ-MnO₂ nanowires, which was stable for up to 30 cycles. The reported discharge capacity is higher than recorded in previous studies on lithium-air cells.     

Growth of high-quality GaSb by metalorganic vapor phase epitaxy

The growth of nominally undoped GaSb layers by atmospheric pressure metalorganic vapor phase epitaxy on GaSb and GaAs substrates is studied. Trimethylgallium and trimethylantimony are used as precursors for the growth at 600°C in a horizontal reactor. The effect of carrier gas flow, V/III-ratio, and trimethylgallium partial pressure on surface morphology, electrical properties and photoluminescence is investigated. The optimum values for the growth parameters are established. The carrier gas flow is shown to have a significant effect on the surface morphology. The optimum growth rate is found to be 3–8 μm/ h, which is higher than previously reported. The 2.5 μm thick GaSb layers on GaAs are p-type, having at optimized growth conditions room-temperature hole mobility and hole concentration of 800 cm² V⁻¹ s⁻¹ and 3·10¹⁶ cm^-3, respectively. The homoepitaxial GaSb layer grown with the same parameters has mirror-like surface and the photoluminescence spectrum is dominated by strong excitonic lines.     

MBE-Grown II–VI and Related Nanostructures

Nanostructures of II–VI semiconductor materials could potentially offer novel and superior physical (in particular, optoelectronic) properties with respect to their bulk counterparts. Herein, we present our most recent research on several II–VI and related nanostructures grown by molecular beam epitaxy (MBE) technique. These include a ZnSe nanograting. This nanograting structure was realized at the surface of Fe/ZnSe bilayers grown on GaAs(001) substrates by thermal annealing. A model based on an Ewald construction is presented to explain its unusual reflection high-energy electron diffraction (RHEED) patterns. The formation mechanism of this one-dimensional (1D) nanostructure is possibly related to surface energy minimization, together with an Fe–Se exchange interaction and Fe-induced decomposition of several top ZnSe atomic layers during thermal annealing. Another nanostructure investigated was the ZnS Schottky barrier embedded with Fe quantum dots (QDs). Here, a Au/ZnS/Fe-QDs/ZnS/n ⁺-GaAs(100) Schottky barrier structure containing five layers of spherical Fe quantum dots with a diameter of ~3 nm was fabricated. Its I–V characteristic measured from 5 K to 295 K displays negative differential resistance (NDR) for temperature ≤50 K. Staircase-like I–V characteristics were also observed at low temperature in some devices fabricated from this structure. Possible mechanisms that can account for the observed unusual I-V characteristic in this structure are presented. Finally, laterally grown Fe nanowires (NWs) on a ZnS surface were prepared. Under high growth/annealing temperature, two types of Fe NWs with specific orientations can be grown on the ZnS(100) surface. We propose a mean-field model that the torque exerted by type A Fe NWs could effectively turn the two components of type B Fe NWs slightly toward the ZnS [110] direction, leading to the observed misalignment of type B Fe NWs.     

In Situ Scanning Electron Microscopy Observation of Growth Kinetics and Catalyst Splitting in Vapor–Liquid–Solid Growth of Nanowires

In situ observations during vapor–liquid–solid (VLS) growth of semiconductor nanowires in the chamber of an environmental scanning electron microscope (ESEM) are reported. For nanowire growth, a powder mixture of CdS and ZnS is used as a source material and silver nanoparticles as a metal catalyst. Through tracing growth kinetics of nanowires, it is found that nanowires with a relatively bigger catalyst droplet on the tip grow faster. Intriguingly, it is also found that the growth of nanowires can involve catalyst splitting: while the majority of catalyst remains at the nanowire tip and continues facilitating the growth, a portion of it is removed from the tip due to the splitting. It remains attached to the nanowire at the position where the splitting occurred and subsequently induces the growth of a nanowire branch. As far as it is known, this is the first time that catalyst splitting is revealed experimentally in situ. It is proposed that the instability of catalyst droplet caused by the volume increase is the main reason for the splitting. It is believed that in situ growth inside the ESEM can largely enrich our understanding on the metal‐catalyzed VLS growth kinetics, which may open up more opportunities for morphology‐controlled synthesis of 1D semiconductor nanowires in future study.