Nanometer-scale Islands Research Articles

Growth of 2D GaN Single Crystals on Liquid Metals.

Two-dimensional (2D) gallium nitride (GaN) has been highly anticipated because its quantum confinement effect enables desirable deep-ultraviolet emission, excitonic effect and electronic transport properties. However, the currently obtained 2D GaN can only exist as intercalated layers of atomically thin quantum wells or nanometer-scale islands, limiting further exploration of its intrinsic characteristics. Here, we report, for the first time, the growth of micrometer-sized 2D GaN single crystals on liquid metals via a surface-confined nitridation reaction and demonstrate that the 2D GaN shows uniformly incremental lattice, unique phonon modes, blue-shifted photoluminescence emission and improved internal quantum efficiency, providing direct evidence to the previous theoretical predictions. The as-grown 2D GaN exhibits an electronic mobility of 160 cm2·V-1·s-1. These findings pave the way to potential optoelectronic applications of 2D GaN single crystals.

Morphology Analysis of Si Island Arrays on Si(001).

The formation of nanometer-scale islands is an important issue for bottom-up-based schemes in novel electronic, optoelectronic and magnetoelectronic devices technology. In this work, we present a detailed atomic force microscopy analysis of Si island arrays grown by molecular beam epitaxy. Recent reports have shown that self-assembled distributions of fourfold pyramid-like islands develop in 5-nm thick Si layers grown at substrate temperatures of 650 and 750°C on HF-prepared Si(001) substrates. Looking for wielding control and understanding the phenomena involved in this surface nanostructuring, we develop and apply a formalism that allows for processing large area AFM topographic images in a shot, obtaining surface orientation maps with specific information on facets population. The procedure reveals some noticeable features of these Si island arrays, e.g. a clear anisotropy of the in-plane local slope distributions. Total island volume analysis also indicates mass transport from the substrate surface to the 3D islands, a process presumably related to the presence of trenches around some of the pyramids. Results are discussed within the framework of similar island arrays in homoepitaxial and heteroepitaxial semiconductor systems.

A large-scale quantum dot IR focal plane array

Potential uses for light detectors operating in the 8–15μmwavelength range include groundand space-based applications such as night vision, temperature detection, early warning systems, navigation, flight control systems, weather monitoring, as well as security and surveillance. In addition, they can be used to monitor and measure pollution, relative humidity profiles, and the distribution of different gases (such as ozone, carbonmonoxide, and nitrous oxide) in the atmosphere. This is due to the fact that most of the absorption lines of gas molecules lie in this IR spectral region. The earth’s atmosphere is opaque to most of the IR. Of its few transparent windows, the 8–12μm is one of the clearest. Cameras operating in this wavelength range and used in ground-based telescopeswill be able to see through the earth’s atmosphere, image distant stars and galaxies, and help in the search for cold objects such as planets orbiting nearby stars. Traditional quantum well IR photodetectors are made from a combination of group III and group V elements and already offer extremely high operability, mature fabrication technology, very large formats, and material production that is increasingly high volume and low cost. However, operating temperatures are moderate (40–80K), and efficiency is limited. Past attempts at solving these issues focused on improving materials quality and designing new devices. In this work we exploited the artificial atomlike properties of self-assembling quantum dots (QDs) to developmore efficient large-scale (640×512 pixel) highoperating-temperature long-wavelength infrared (LWIR) focal plane arrays (FPAs) without sacrificing the economic advantages of the mature III-V IR imaging system pipeline. QDs are nanometer-scale islands that form spontaneously on a semiconductor substrate due to lattice mismatch. QDs can confine the carriers (i.e., electrons or holes) in 3D, modifying the optical transition selection rule that forbids a quantum well from absorbing normal incident radiation and enhancing quantum efficiency. The 3D confinement of the photoexcited carrier by a quantum dot IR photodetector (QDIP) also increases its lifetime Figure 1. An image taken with the first 640×512 pixel QDIP LWIR FPA operating at 60K.

Mild oxygen plasma treated PEDOT:PSS as anode buffer layer for vacuum deposited organic light-emitting diodes

The surface morphology of PEDOT:PSS after mild oxygen plasma treatment were investigated by scanning electron microscopy and atomic force microscopy. The nanometer-scale islands on the surface of treated PEDOT:PSS were observed. Vacuum deposited organic light-emitting diodes (OLEDs) with treated PEDOT:PSS as anode buffer layer had been fabricated. The OLEDs with an appropriately treated PEDOT:PSS as anode buffer layer exhibited significantly enhanced lifetime and decreased driving voltage. The results suggest that the appropriate mild oxygen plasma treatment of PEDOT:PSS layers may be useful for the improvement of the interface with the hole transport layer and enhanced device performance.

Thermodynamic control of germanium quantum dot growth on silicon

Strained epitaxial growth of Ge on Si(0 0 1) produces self-assembled, nanometer scale islands, or quantum dots. We study this growth by atomistic simulation, computing the energy of island structures to determine when and how islanding occurs. The distribution of island sizes on a surface is determined by the relation of island energy to size. Applying the calculated energy per atom to the Boltzmann–Gibbs distribution, we predict size distributions as functions of coverage and temperature. The peak populations around 86,000 atoms (35 nm wide) compare favorably with experiment.

Germanium Island Size Distribution by Atomistic Simulation

ABSTRACTStrained epitaxial growth of Ge on Si(001) produces self-assembled, nanometer scale islands, or quantum dots. We study this growth by atomistic simulation, computing the energy of island structures to determine when and how islanding occurs. The distribution of island sizes on a surface is determined by the relation of island energy to size. Applying the calculated chemical potential to the Boltzmann-Gibbs distribution, we predict size distributions as functions of coverage and temperature. The peak populations around 80 000 atoms (35 nm wide) compare favorably with experiment.

Nanostructures formed on CdSe/ZnSe surfaces

Nanometer scale islands were observed in air on CdSe/ZnSe surfaces grown by molecular beam epitaxy. Measurements by atomic force microscope revealed that the aspect ratio of the island increased first and then decreased with time. Surface analyses by Auger electron spectroscopy indicated clearly that the island was composed in large part of oxides and the outer shell of the island was Zn-enriched and Cd-depleted. These islands showed sharp emissions as quantum dots. This work demonstrates that the islands were formed mainly due to the oxidation of the surface and that it grew in a way similar to that generated from a strained film. In addition, intermixing of ZnSe and Zn segregation during the island formation occurred.

Vertical Distribution of Lattice Strain in Polycrystalline Silicon Films Grown on Silicon Dioxide

The vertical distribution of lattice strains in low-pressure chemical-vapor-deposited polycrystalline silicon (poly-Si) films grown on was studied by energy-dispersive grazing-incidence X-ray diffraction with synchrotron radiation white X-rays. The results revealed that a 5 nm thick poly-Si film consisting of nanometer-scale islands had tensile strain originating from poly- interface stress. Thicker films indicated not only strain relaxation but also compressive strain at thicknesses greater than 20 nm. Transmission electron microscopy showed lattice defects including dislocations. This leads one to conclude that compressive strain is caused by insertion-type dislocations induced by grain-boundary formation and subsequent bending in these boundaries. © 2001 The Electrochemical Society. All rights reserved.

Epitaxially Self-Assembled Quantum Dots

Nanometer-scale islands that form spontaneously on a semiconductor substrate have atomlike properties and potential applications in optical and optoelectronic devices, quantum computing, and information storage.

Features of nanometer scale islands on CdSe/ZnSe surfaces

We examined the early-stage (within a few hours) properties in air of nanometer scale islands formed on a CdSe/ZnSe surface. When a CdSe/ZnSe surface grown in vacuum was exposed to air, some islands were newly formed. The aspect ratio of the island was found to increase first and then decrease. A depletion zone was observed in the perimeter of the island which indicates the intermixing of ZnSe into the island. Surface analyses using Auger electron spectroscopy indicated that the island was composed mainly of oxides and the outer shell was Zn enriched and Cd depleted. This letter shows that surface oxidation plays a key role in the formation of islands on CdSe/ZnSe surfaces in atmosphere.

Simulation and growth of gold on silicon oxide in one-dimensional and quasi-one-dimensional arrays

The granular film deposition of gold atoms by means of thermal evaporation on silicon oxide surfaces is known to produce nanometer-scale islands which show electrical characteristics dominated by Coulomb blockade. Here we report on the growth and the simulation of structures produced by a combination of evaporation and lithography. A lithographic technique is used to define a region of a width of several nm that is surrounded by polymethylmethacrylate (PMMA). The PMMA confines the diffusion of the atoms, and the growth process is characterized by different boundary conditions compared with the case where atoms are deposited on a macroscopic surface. This method enables us to create quasi-one-dimensional (quasi-1D) and 1D structures in which the gold islands are arranged in a single row where the lateral size is only a few nanometers. Such structures offer the possibility of studying Coulomb blockade in 1D arrays and a signature of self-organized growth is observed in such structures.

Photoinduced Deposition of Size-Controlled Gold Nanoclusters onto the Self-Assembled Monolayer of 1,9-Nonanedithiol

Abstract Gold nanoclusters are formed photochemically in organic solvent to give nanometer-scale islands on the self-assembled monolayer (SAM) of 1,9-nonanedithiol. The size and shape are variable by choosing electron-donating organic solvents. In ethanol, the plate-like nanoclusters are produced with layer-by-layer growth to give atomically flat top. The nano-structures are formed in N,N-dimethylformamide to give larger size than those in ethanol under comparable conditions.

On the nature of nanometer-scale islands formed by cadmium selenide deposition on hexagonal cadmium sulfide (0001)A

Using ex situ atomic force microscopy, nanometer-scale islands were observed on CdS(0001)A surfaces to appear after deposition of small amounts of CdSe. The sizes of these islands are similar to those reported in atomic force microscopy (AFM) studies of uncapped CdSe/ZnSe structures. Investigations by force modulation microscopy and photoluminescence spectroscopy show, however, that they are difficult to assign to a CdSe Stranski–Krastanow growth. Instead, AFM points to an onset of the island formation with the exposure of the samples to atmosphere. By use of electron microprobe analysis, it is suggested that these islands consist of selenium.

The formation and properties of adsorbate nanostructures on Cu surfaces

Scanning tunneling microscopy (STM) has been used to observe naturally occurring nanostructures formed on the (110) and (100) faces of single-crystal Cu samples. On the (110) surface, a combination of chemical and thermal treatments result in the formation of a periodic array of Cu - O stripes separated by areas of the clean surface. Deposition of submonolayer coverages of N onto both the (100) and (110) faces of Cu results in nanometer-scale islands. On the (100) surface, periodic arrays of N islands are formed. The feasibility of using these islands to create new nanostructures is demonstrated. On the (110) surface, the ability to combine O- and N-induced nanostructures to form an `atomic-scale tartan' is demonstrated.

Measurement of Glancing Incidence-Exit X-ray Scattering in Reflection High Energy Electron Diffraction and Total-Reflection-Angle X-ray Spectroscopy System

We developed the total-reflection-angle X-ray spectroscopy in reflection high energy electron diffraction (RHEED-TRAXS) experiments method (measuring X-rays at glancing exit angle) by introducing glancing incidence X-rays into a RHEED-TRAXS system so that the measurement of glancing incidence-exit X-ray scattering (GIEXS) has been realized. Nanometer scale islands, deposited on a sample surface in the RHEED-TRAXS system, were excited by an electron beam to generate characteristic X-rays. These X-rays impinge on the sample surface at small glancing angle to play a role of glancing incidence; thus, the TRAXS observation of the X-rays emitted from islands is equivalent to X-ray measurement under glancing incidence-exit geometry. Applying the GIEXS method to surface layer analysis for the SrTiO3 (001) single crystal, we found that the critical angle of total reflection observed by the GIEXS method can be used to determine the composition of the surface atomic layer.

Fabrication of atomic-scale metallic microstructures by retarding-field focused ion beams

Atomic-scale metallic microstructures have been deposited on various semiconductor substrates. A retarding-field method is used to form a low-energy, focused beam of Au ions from a liquid metal alloy source. The ions were deposited with landing energies down to 20 eV to form ultrathin films. The microstructure of the ultrathin films was found to be metallic islands, the smallest of which are 1 nm or less. The microstructure on a GaAs substrate displayed a high degree of size and spacing regularity. The technique was used as a method to fabricate nanometer-scale islands in a multiple-junction Coulomb blockade device.