Nano-sized Islands Research Articles

Fabrication of nanorod InGaN/GaN multiple quantum wells with self-assembled Ni nano-island masks

The evolution of nano-sized Ni metal islands deposited by electron-beam evaporation on InGaN/GaN multiple quantum wells (MQWs) surface as a function of rapid thermal annealing (RTA) temperature and initial deposited Ni film thickness is reported. It is shown that the dimension and density of self-assembled Ni nano-islands depend critically on the annealing temperature and deposited Ni film thickness. The formation of the islands is described in terms of Ostwald ripening and coarsening mechanisms. Subsequently, the nano-masks are successfully applied to fabricate nanorod InGaN/GaN MQWs by using inductively coupled-plasma (ICP) etching. Uniform etching rate has been obtained by comparing the nanorod height etched for different times. Photoluminescence (PL) investigation shows the nanorod MQWs with optimized light output efficiency could be acquired under particular ICP and RF etching power. Strain relaxation and dislocation reduction effect would contribute to enhanced light output of nanorod InGaN/GaN MQWs compared with the as-grown plane MQWs.

Surface morphology evolution of CeO2/YSZ (001) buffer layers fabricated via magnetron sputtering

Cerium dioxide (CeO2)/yttria-stabilized zirconia (YSZ) has long been proven to be an effective buffer layer architecture for high-temperature superconducting coated conductors. In this study, CeO2 films were deposited on YSZ (001) single crystal substrates via reactive unbalanced magnetron sputtering with varying substrate temperature, sputtering pressure, radio frequency sputtering power, and film thickness. High-quality texture was achieved even at ambient temperature, and deposition parameters were optimized to achieve the best degree of in-plane alignment with (111) ϕ scan full width at half maximum around 1.3°. Atomic force microscopy was utilized to investigate film surface morphology and roughness. At a low sputtering pressure, a flat and uniform film surface comprising nano-sized isotropic islands was observed. The surface islands transited to an anisotropic spindle-like shape at pressure higher than 1.0Pa. The spindle-shaped islands elongated along the CeO2 [110] or [11¯0] directions, constructing an interwoven surface morphology. The distinct surface morphology evolution was correlated with the change in the film strain state attributed to varying sputtering pressure. A possible mechanism for this morphology evolution was discussed.

Large antisymmetric magnetoresistance across chemically etched GaMnAs nanoconstrictions

We report the observation of a highly antisymmetric magnetoresistance in GaMnAs specimens with lateral nano-constrictions. The experiments were carried out on samples with an in-plane easy axis of magnetization, in in-plane applied magnetic field. We attribute this behavior to the formation of isolated nano-sized islands at the nano-constrictions, whose magnetization can undergo out-of-plane reorientation during switching. The change in resistance is up to 50%, which cannot be explained by the normal anisotropic magnetoresistance in GaMnAs. We propose tunneling anisotropic magnetoresistance as the most likely mechanism for this effect.

Electrical and photovoltaic properties of self-assembled Ge nanodomes on Si(001)

SiGe nano-size islands play a key role in novel electronic and optoelectronic devices. Therefore, the understanding of basic electrical properties of individual nanoislands is crucial. Here, the electrical and photovoltaic properties of individual self-assembled Ge nanodomes (NDs) on Si(001) have been studied by conductive and photoconductive atomic force microscopy (AFM). The transition areas between the {113} and {15 3 23} facets turned out to be most conductive whereas the {113} facets exhibit minimum conductivity, which is attributed to a local increase in Si concentration. Local current-to-voltage measurements revealed that the NDs show an ohmic resistance, which is in the M\ensuremath{\Omega} region and scales with the ND-substrate interface area. Upon illumination by the AFM feedback laser at 860 nm, a photovoltage is generated. This photovoltage originates in the $p$-$i$-$n$ structure formed between the $p$-type substrate, the Ge ND, and the $n$-type diamond AFM probe.

Thermal evolution of the morphology of Ni/Ag/Si(111)-√3 × √3 surface

The temperature-driven changes in morphology of the interface formed by room temperature (RT) deposition of Ni atoms onto an Ag/Si(111)-√3×√3 surface were investigated by scanning tunneling microscopy. Roughly 70% of Ni deposition diffused into bulk substrate within the temperature range between RT and 573K. The images as obtained after annealing up to 670K correspond to the formation of nano-sized islands of nickel silicides. Two types of islands, large triangular islands typical of the whole range of applied coverage, and smaller islands of different shapes, coexist at Ni coverage higher than 1 monolayer. Annealing above 870K led to the formation of a 7×7 phase in coexistence with small 5×5 domains at the expense of a complete disappearance of the √3×√3 phase. Also, formation of Ni,Si alloy was observed at the temperature, along with segregation of bulk-dissolved Ni species onto the surface.

Electron transport in discontinuous gold films and the effect of Coulomb blockade and percolation

Understanding the electron transport in disordered assemblies of weakly coupled nano-sized metal clusters is important for many applications. Here, we investigate experimentally and theoretically the electron transport properties of metal cluster assemblies in the form of discontinuous gold films. Discontinuous films of different average island size are produced by sputter deposition, and the resistance and the non-linear current-voltage (I-V) characteristics of the films are measured as a function of temperature. To interpret the experimental electron transport data, a conduction percolation model is employed where broad probability distributions for both the tunnel junction gaps and the Coulomb blockade energies are used. Excellent agreement between experimental data and model calculations is found. In particular, the non-Arrhenius resistive behavior, the I-V power-law behavior, and the I-V characteristics at large bias voltage are all shown to be due to a conduction percolation mechanism governing disordered networks of nano-sized metal islands connected by tunnel junctions.

Microstructure and Magnetic Properties of Nano-Island CoPt Thin Films

Abstract The separated CoPt nano-size islands were fabricated on amorphous glass substrates in this work. The microstructures and magnetic properties of CoPt thin films with thicknesses between 1 and 20 nm after annealing at 700 °C for 30 minutes were investigated. The morphology of CoPt thin film would change from a well-distributed and discontinuous nano-size CoPt islands into a continuous one as increasing the film thickness from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. This nano-size CoPt thin film may be a good candidate for ultra-high density magnetic recording media.

The study of magnetic properties and microstructures of nano-size FePt islands on amorphous carbon film

This work focuses on the formation mechanisms of nano-island FePt film on commercial copper grids covered with an amorphous carbon film. FePt films of different thickness (1–7.5nm) were deposited on amorphous carbon film and then post-annealed at 700°C for 30min. The configuration of the film was changed during the annealing process due to the surface energy difference between the amorphous carbon films and FePt alloy. We have prepared nanometer-size island-shaped FePt films on the amorphous carbon films and investigated their magnetic properties and microstructures. A discontinuous nano-size island magnetic film can reduce the exchange coupling of the media and increase the recording density.

Initial growth of SrTiO3 on Sr/Si(001) studied by scanning tunneling microscope

SrTiO3 ultra-thin film was deposited on the Sr/Si(001) surface using pulsed laser deposition (PLD) at room temperature and studied using scanning tunneling microscopy (STM). After annealing at 660 ℃ for about 60 minutes in ultrahigh vacuum (UHV), nanosize islands were formed on the Sr/Si(001) surface. High resolution STM images and dI/dV mapping of islands on Sr/Si(001) were obtained. The islands can be attributed to TiSi2 islands with C49 and C54 structures. The existence of Sr on Si is not sufficient to prevent the reaction between Si and Ti in preparation of ultra-thin SrTiO3 films.

Effect of Thicknesses on the Microstructure and Magnetic Properties of CoPt Thin Films

The microstructures and magnetic properties of CoPt thin films with thicknesses between 1 and 20 nm deposited on amorphous glass substrate and post-annealing at 600°C for 30 min were investigated. The morphology of CoPt thin film would change from a discontinuous nano-size CoPt islands into a continuous film gradually as the film thickness was increased from 1 to 20 nm. The formation mechanism of the CoPt islands may be due to the surface energy difference between the glass substrate and CoPt alloy. Each CoPt island could be a single domain particle. This discontinuous nano-island CoPt recording film may increase the recording density and enhance the signal to noise ratio while comparing with the continuous film. The as-deposited 5 nm CoPt film revealed the separated islands morphology after annealing at 600°C for 30 min. This nano-size CoPt thin film may be a candidate for ultra-high density magnetic recording media due to its discontinuous islanded nanostructure.

Preferred orientations in nano-gold/silica/silicon interfaces

Gold in contact with silicon substrates Si(1 0 0), Si(1 1 1), and SiO 2 is studied by thermal evaporation and annealing in N 2 using the modified sphere-plate technique. The final orientation distribution of crystalline Au films grown on Si substrate systems that incorporate a native amorphous oxide layer of silica and Au on amorphous silica (SiO 2 glass) substrates is influenced by preferred orientations and twinning. Experimental evidence suggests that the orientation of Au{1 1 1} close packed planes (multiply twinned) was found to be of low-energy as the annealing temperature was increased to 530 °C and 920 °C. Additional orientations were observed for Au{1 0 0} on Si(1 0 0) substrates and Au{1 0 0}, {1 1 0}, and {3 1 1} on SiO 2 substrates. After annealing at 920 °C the size distribution of the gold particles was determined to be within the range of 20–800 nm while the morphology of gold surface appears spherical to faceted in character. These results show similarities to recent findings for smaller nano-size 1D particles, islands and thin Au films on silicon annealed over lower temperature ranges.

Low-Temperature Scanning Tunneling Microscopy and Spectroscopy Investigation of the Electronic Surface State of Self-Organized Cr Islands on Au(111)

We have investigated the morphologic and electronic properties of self-organized nanosized Cr islands grown on atomically flat Au(111) surfaces by means of low-temperature scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. We observe the existence of an electronic surface state on the Cr islands, which manifests itself through the formation of standing wave patterns within the interior of the atomically flat islands that gain in complexity for increasing bias voltages. The patterns are only weakly observable when compared to similar standing waves formed within self-organized Co islands on Au(111). This difference is attributed to the more irregular shape of the Cr islands. Furthermore, we have found that the presence of the Cr surface state is reflected by the appearance of a distinct electronic state in the -200 mV to -150 mV range, resembling to previous findings for self-organized Co islands on Au(111).

Self-assembled nano-size FePt islands for ultra-high density magnetic recording media

To find a method to form nano-size FePt alloy for ultra-high density magnetic recording media, this work concentrated on the formation mechanisms of nano-island FePt films on amorphous glass substrates. FePt films of different thicknesses (1–10 nm) were deposited on amorphous glass substrates and post-annealed at 700 °C for 10 and 30 min. The configuration of the film changed during the annealing process due to the surface energy difference between the glass substrate and FePt alloy. Investigation of the microstructures and magnetic properties of the ordered L1 0 FePt films revealed that the 1 nm FePt film annealed at 700 °C for 10 min had perpendicular magnetic anisotropy and good reproducibility of forming well-separated FePt nano-size islands for ultra-high density magnetic recording media.

Equilibrium shape and size of supported heteroepitaxial nanoislands

We study the equilibrium shape, shape transitions and optimal size of strained heteroepitaxial nanoislands with a two-dimensional atomistic model using simply adjustable interatomic pair potentials. We map out the global phase diagram as a function of substrate-adsorbate misfit and interaction. This phase diagram reveals all the phases corresponding to different well-known growth modes. In particular, for large enough misfits and attractive substrate there is a Stranski-Krastanow regime, where nano-sized islands grow on top of wetting films. We analyze the various terms contributing to the total island energy in detail, and show how the competition between them leads to the optimal shape and size of the islands. Finally, we also develop an analytic interpolation formula for the various contributions to the total energy of strained nanoislands.

Reconstructed Structures of Nanosized Co Islands on Ag/Ge(111) √3 × √3 Surfaces

Structural evolution of Co/Ag/Ge(111) at high temperatures was studied by using scanning tunneling microscopy and low energy electron diffraction. The mean square root of 3 x mean square root of 3-Ag layer between the substrate Ge( 11) and Co adatoms can avoid the formation of Co-Ge compounds below 800 K. The Co atoms nucleate to form islands where mean square root of 13 x mean square root of 13 or 2 x 2 reconstructions were observed after annealing between 373 K and 737 K. The mean square root of 13 x mean square root of 13 structure with mirror symmetry relative to [-211], [11-2], and [1-21] axes was observed for 1-2 layer Co islands. Co islands with over 2 layers appear 2 x 2 structure. All reconstruction structures of the nano-sized Co islands and substrate Ag/Ge(111) mean square root of 3 x mean square root of 3 surface were analyzed using the atomic hard sphere model. The bright protrusions of these reconstructions all sit in the centers of Ag or Ge trimers, which were predicted to have maximum binding energy.

Investigation of Multilayer Silicon Structures with Buried Iron Silicide Nanocrystallites: Growth, Structure, and Properties

The growth of nanosize islands of iron silicides on Si(100) substrates and epitaxial silicon overgrowth atop them have been studied by low energy electron diffraction and reflectance high energy electron diffraction methods. The near optimal formation conditions of iron silicide islands with high density and minimal sizes have been determined by using of atomic force microscopy. Multilayer (8-10) monolithic structures with buried iron silicide nanocrystallites have been grown after the definition of monocrystalline burying conditions of iron silicides nanocrystallites in silicon lattice. The structure of buried nanocrystallites has been studied in multilayer monolithic heterostructures by high resolution transmission electron microscopy. It was established that in multilayer samples the majority of nanocrystallites have beta-FeSi2 structure, but some of them have gamma-FeSi2 structure. It was observed an influence of additional annealing at 850 degrees C on the morphology and structure of nanocrystallites. By means of deep level transient spectroscopy data one and two trap levels have been observed in multilayer structures (without and with additional annealing, respectively). Photoluminescence spectra have been studied at 4.2 K and the causes of its absence from buried beta-FeSi, NC have been analyzed.

Self-Organization of CrSi2 Nanoislands on Si(111) and Growth of Monocrystalline Silicon with Buried Multilayers of CrSi2 Nanocrystallites

The process of self-organization of CrSi2 nanosize islands on Si(111)7 x 7 surface has been investigated during reactive deposition (RDE) of Cr at 500 degrees C by methods of low energy diffraction (LEED) and differential reflectance spectroscopy (DRS). Morphology of grown samples has been studied by atomic force microscopy (AFM). DRS data have demonstrated the semiconductor nature of silicide islands from the beginning of Cr deposition at 500 degreesC. The optimal temperature (750 0C) and optimal Si thickness (50 nm) have been determined for silicon molecular epitaxy (MBE) growth atop CrSi2 nanosize islands. Monolithic silicon-silicide heterostructures with multilayers of CrSi2 nanosize crystallites have been grown.

Silicon layers atop iron silicide nanoislands on Si(100) substrate: Island formation, silicon growth, morphology and structure

Iron silicide island growth on Si(100)2 × 1 surface, silicon growth over iron silicide nanosize islands and structure of silicon and buried iron silicide nanocrystallites have been studied by low electron energy diffraction, atomic force microscopy and high resolution transmission electron microscopy. The best crystal quality of the continuous monocrystal silicon layer and minimal roughness have been observed for the silicon growth temperature T = 700 °C and silicon layer thickness 100 nm. Nanocrystallites of two types have been observed: small (5–6 nm) and large (30–50 nm). A model of silicon growth atop Si(100) substrate with nanosize iron silicide islands at different substrate temperatures has been proposed. The crystal structure and sizes have been determined for β-FeSi 2 and γ-FeSi 2 nanocrystallites.

Critical current enhancement in PLD YBa2Cu3O7−x films using artificial pinning centers

We introduced high-density columnar defects as artificial pinning centers (APCs) for quantized vortices into YBa2Cu3O7−x (YBCO) films during the film deposition procedure. APCs were introduced perpendicular to the film surface using nano-sized Y2O3 islands prepared on SrTiO3(100) substrates. Varying the deposition parameters and the substrate annealing conditions allowed strong changes in the shape and density of the Y2O3 islands to be induced. Compared to a pure YBCO film, the best performance among the APCs samples was obtained on the Y2O3-deposited substrate with 5 laser pulses, corresponding to a nominal thickness of 0.2nm. Even when the APC films were prepared using the same deposition conditions, the columnar defects enhanced Jc at 77K from 1.8 to 2.7MA/cm2 (self field) and from 0.06 to 0.10MA/cm2 (H=5T, H∥c-axis).

Lifecycle studies of field emission of BN thin films

Amorphous BN films were prepared by the RF magnetron sputtering method and their field emission behaviors were characterized by I– V curves and F– N plots. Changes of electric field were carried out by increase/decrease in order to test the lifetime of the cascade material. In the meantime AFM and SEM monitored the change in the films’ morphologies and compositions. We find that the nano-sized islands can act as the FE emitters and essentially determine the FE performance of the films. We also report the existence of an activation process, after which the cascade performs more satisfactorily in higher emission current. It is also supposed that the field-induced evaporation plays an important role in the exhaust of the cascade material.