High-density Islands Research Articles

The degree and nature of radiation damage in zircon observed by Si29 nuclear magnetic resonance

A quantitative analysis of Si29 nuclear magnetic resonance spectra of radiation damaged, natural zircons showed that the local structure in crystalline and amorphous regions depend explicitly on radiation dose. Nonpercolating amorphous islands of high density “glass” within the crystalline matrix show a low interconnectivity of SiO4 tetrahedra. This structural state is quite different from that of the high dose, percolating regions of low density glass with more polymerised tetrahedra. A continuous nonlinear dose dependence between the high and low density glass states is reported. A continuous evolution of the local structure of the crystalline phase up to the percolation point is also reported. No phase separation into binary oxides was observed. The total number of permanently displaced atoms per α-recoil event is ∼3800 atoms for low radiation doses and decreases to ∼2000 atoms for 10×1018 α events/g. No indication of partitioning of paramagnetic impurities between crystalline and amorphous regions was found for these natural zircons. The amorphous fractions of the metamict zircons were determined as a function of their accumulated radiation dose. These values coincide closely with those recently determined by x-ray diffraction studies. They are much greater than previously assumed based on density measurements. The dose dependence is consistent with the concept of direct impact amorphization in the atomic cascade following an α-recoil event.

Effect of annealing conditions on the formation of low-dose SIMOX structures implanted at 190 keV

The structure of low-dose SIMOX wafers as a function of annealing conditions has been studied using TEM. A set of samples were implanted at 190 keV with doses of 0.5, 0.83 and 1.8 × 1018cm-2 oxygen followed by annealing treatment at different ramp rates (1~ Cmin-1 or 5~ Cmin-1), temperatures (1310 °C or 1350 °C), and holding times (0 or 5 h). The results show that a higher annealing temperature (∼1350 °C) with a longer holding time improves the quality of the top Si layers and the smoothness of the interfaces in low-dose SIMOX wafers. A slow thermal ramp rate results in sharp interfaces but leads to a high density of Si islands in the buried oxide (BOX) layer. Chemical etching experiment performed on the top Si layer of a low-dose SIMOX shows pipeline structure indicating the inhomogeneous chemical reactivity of the top Si layer. © 2001 Kluwer Academic Publishers

MBE grown InGaN quantum dots and quantum wells: effects of in-plane localization

InGaN/GaN heterostructure samples were grown by molecular beam epitaxy using ammonia as a nitrogen precursor. The growth of InGaN/GaN self-assembled quantum dots was monitored in situ by reflection high energy electron diffraction intensity oscillations. Atomic force microscopy scans showed a very high density of InGaN islands, ∼1×10 11 cm −2, well above the dislocation density. This could explain the increased radiative efficiency of these samples compared to homogeneous quantum wells. Light emitting diodes (LEDs) with InGaN active layers buried in GaN were realized. Electroluminescence and photocurrent spectra of these LEDs evidence a strong Stokes shift that can be attributed to high localization of carriers in InGaN layers.

Structural and optical characterization of InAs nanostructures grown on (001) and high index InP substrates

The effects of InP substrate orientations on self-assembled InAs quantum dots (QDs) have been investigated by molecular beam epitaxy (MBE). A comparison between atomic force microscopy (AFM) and photoluminescence (PL) spectra shows that a high density of smaller InAs islands can be obtained by using such high index substrates. On the other hand, by introducing a lattice-matched underlying In 0.52Al 0.24Ga 0.24As layer, the InAs QDs can be much more uniform in size and have a great improvement in PL properties. More importantly, 1.55-μm luminescence at room temperature (RT) can be realized in InAs QDs deposited on (001) InP substrate with underlying In 0.52Al 0.24Ga 0.24As layer.

Structural and chemical analysis of CdSe islands in a ZnSe matrix by transmission electron microscopy

The structural properties and the Cd-distribution of CdSe layers with a nominal thickness between 0.5 and 3.6 monolayers (ML) in a ZnSe matrix were investigated by plan-view and cross-section transmission electron microscopy. All layers contain a high density of Cd-rich islands with sizes below 10 nm, which are completely embedded in a 3 nm thick CdZnSe wetting layer. Large islands with a density two orders of magnitude below the small island density are additionally observed at t n ⩾3 ML above the critical wetting layer thickness for the transition from the two-dimensional to the three-dimensional growth mode. Composition analyses on a subnanometer scale reveal the Cd-distribution and Cd-content, which increases in the wetting layer and in the small islands with the nominal layer thickness.

Growth of self-assembled InP quantum islands for red-light-emitting injection lasers

In order to achieve laser emission in the visible part of the spectrum, we have investigated the growth of self-assembled InP quantum dots on GaInP by low-pressure metal-organic vapor phase epitaxy (MOVPE) using the Stranski-Krastanow growth mode. Unlike the well-established InAs-GaAs system, when InP is deposited on GaInP, typically, two types of coherently strained islands with different sizes are formed. A high density of small islands is favored when using growth conditions with a reduced surface diffusion, i.e., low temperatures, high growth rates, and substrates with high misorientation angles. After the deposition of 3.4 monolayers of InP at 580/spl deg/C on GaAs-substrates with a surface angle of 15/spl deg/ to the next [111]B-plane, 2.10/sup 10/ InP dots per square centimeter with an average height of 4 nm were assembled. The emission of these InP islands at 1.72 eV (4.2 K) shows an inhomogeneous broadening of 42 meV because of the size fluctuation of the quantum dots. At 90 K, lasing from self-assembled InP quantum islands was observed above a threshold current density of 288 A/cm/sup 2/. The detected laser line is located at 1.8 eV, about 80 meV higher than is the ground-state transition energy. We attribute this behavior to lasing from excited states in agreement with power-dependent photoluminescence experiments. For temperatures above 150 K, the threshold current density increases dramatically because of a thermally activated escape of carriers up to 4.9 k17/cm/sup 2/ at room temperature, where the characteristic temperature is 35 K. Injection lasers containing stacked InP quantum islands and AlGaInP barrier layers with a higher band offset may exhibit an improved temperature dependence.

Annealing experiments on supercritical Si 1– xGe x layers grown by RPCVD

This paper presents a study of the relaxation mechanisms for SiGe epitaxy performed by reduced pressure chemical vapor deposition (RPCVD) in a commercially available single wafer epitaxial reactor. The samples were grown at 500°C and an overall pressure of 40 Torr, with SiH 2Cl 2 and GeH 4 as precursor gasses. In contrast to the situation for molecular beam epitaxy (MBE), we found that under RPCVD conditions the relaxation by 3D-growth was strongly suppressed. Relaxation then occurred by the development of misfit dislocations even in the case of high Ge-concentrations (>40%). This behavior is attributed to the high hydrogen coverage of the substrate surface during the RPCVD, which reduces the surface migration of the adsorbed species. Furthermore, the relaxation mechanism has been studied in annealing experiments where wave and island formation was observed. The annealing experiments clearly confirm that surface migration plays a significant role for 3D-growth. Using the annealing procedure layers of high island density and strong photoluminescence have been produced.

Molecular beam epitaxial growth of InAs self-assembled quantum dots with light-emission at 1.3 μm

We studied InAs self-assembled island formation as a function of the growth rate and examined the effects of overgrown layers on the light emission features. The InAs islands were grown by a conventional molecular beam epitaxy on GaAs. We found that the island size and density strongly depended on the growth rate. With a reduction in the growth rate, the island size increased and the light emission wavelength correspondingly shifted to the longer side. The emission wavelength reached 1.3 μm when the growth-rate was reduced to 2×10 −3 monolayer (ML)/s, but the island density decreased (5×10 9 cm −2). We also confirmed that the emission wavelength red-shifted when islands were covered by InGaAs alloys, depending on both the thickness and In composition. Using a combined method, growing islands at a growth rate in the middle range and embedding islands with InGaAs layers, we obtained relatively high density islands with a light emission at 1.3 μm. We confirmed room-temperature lasing from these quantum dots with a threshold current of 8 mA under continuous wave conditions.

Atomic exchange processes and bimodal initial growth of Co/Cu(001)

The initial growth of Co on Cu(001) is atomically identified by scanning tunneling microscopy using CO titration and density-functional theory total-energy calculations. Both reveal that at low coverage Co adatoms occupy substitutional sites in the Cu substrate surface that act as pinning centers for subsequent island nucleation. The interaction with diffusing adatoms is found to be attractive and stronger for nearest-neighbor Co compared to Cu. The atomic substitution process actuates a bimodal growth mode as experimentally found in the island size distributions. This gives rise to a high density of small Co islands and large Co-decorated Cu islands.

The growth of SiC on Si substrates with C 2H 4 and Si 2H 6

SiC was grown on Si (100) substrates oriented and off-oriented by 2–5° towards [011] with simultaneous supply of C 2H 4 and Si 2H 6 at 1050°C. SiC formed during removal of oxide could be removed at 1150°C. Twinned growth occurred on both oriented and off-oriented substrates during carbonization, but fewer twins formed on the off-oriented substrate than that on the oriented substrate. In SiC growth process, twinned growth continued on the off-oriented substrate whereas twinned growth stopped and single crystal SiC with double-domain (2×1) superstructure formed on the oriented substrate. SiC single crystal could grow on a carbonized twinned buffer layer. Obvious SiC LO and TO phonon modes were observed with Raman spectroscopy in the epilayer grown on the oriented substrate. The surface of the epilayer grown on the oriented substrate was smooth, while there was a high density of islands on the epilayer grown on the off-oriented substrate. The film grown on the oriented substrate is superior than that grown on the off-oriented substrate.

Chemical and structural transformation of sapphire (Al2O3) surface by plasma source nitridation

The chemical, morphological, and structural characteristics of nitrogen plasma treated c-plane sapphire substrate surfaces were studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The plasma treatment was carried out by exposing sapphire substrates (Ts=700 °C) to a flow of 35 sccm of activated nitrogen species generated by a constricted-plasma source for 5–60 min. The emergence of the N 1s peak in XPS indicates nitrogen incorporation in sapphire as soon as after 5 min of nitridation. AFM images show that the sapphire contained a high density of islands after 1 h of nitridation. A thin polycrystalline AlN layer was observed on the nitridated sapphire surface by TEM. Both the thickness of the AlN layer and the N 1s photoelectron peak intensity increase nonlinearly with respect to nitridation time. The nonlinear relationship between the thickness of the nitridated layer and the reaction time suggests the growth of the AlN layer follows a diffusion limited growth mechanism.

Wavelength tuning of InAs quantum dots grown on (311)B InP

We report on the synthesis of InAs quantum dots on (311)B InP substrates. It is found that the use of such high index surfaces allows the formation of a high density (5×1010 islands/cm2) of small InAs islands (diameter≈350 Å) on InP. Moreover, a large improvement of the size uniformity is obtained in comparison with deposition on (100) surface. The standard height deviations are ±13% and ±50% for islands grown on (311)B and (100) surfaces, respectively. Then, we show that the modification of the As/P flux sequences, after the island formation, permits the control of the quantum dot emission wavelength. The achievement of quantum dots emitting at 1.55 μm at 300 K indicates that this method is promising for telecom device making.

O-mediated layer growth of Cu on Ru(0001)

The film growth of Cu on clean and O-precovered Ru(0001) at different growth temperatures form 300 K to 450 K was investigated by scanning tunnelling microscopy (STM). Cu films on clean Ru(0001) grow in a multilayer mode at these temperatures. By using an O precoverage in the range of 0.1 monolayers (ML) up to a saturation coverage of 0.5 ML on clean Ru(0001), at 400 K different growth regimes are obtained. For ML a multilayer mode is preserved which changes into an O-induced two-dimensional (2D) growth for higher (0.2-0.5 ML). STM reveals the formation of an O/Cu surfactant structure on the surface due to migration of O initially located at the Ru surface. Its surface coverage rises linearly with O precoverage up to ML where it covers the surface completely. By increasing up to 0.5 ML, a drastic change in the morphology and density of the 2D islands occurs, which is accompanied by a change of the O/Cu surfactant structure. The O/Cu surfactant structure displays some order on a local scale for low , which changes into a disordered structure for ML. Structural similarities to oxidized surfaces of Cu(111) and the structures induced by postadsorption on Cu/Ru(0001) are discussed. Different models of surfactant mechanisms are presented to explain the observations. The locally ordered O/Cu surfactant structure (for ML) together with specific Cu film defects induce a heterogeneous nucleation of Cu with a high island density. Different mobilities of migrating Cu adatoms are established on top of the small islands and on the O/Cu structure resulting in enhanced interlayer diffusion explaining the observed 2D growth. The average island density only slightly changes within the temperatures investigated. In contrast, the saturated and disordered O/Cu surfactant structure (for = 0.4 - 0.5 ML) causes homogeneous nucleation. For this structure, the island density strongly depends on temperature and gives rise to an Arrhenius-like behaviour. The observed 2D growth is attributed to a reduction of the interlayer diffusion barrier. Cu growth on a formerly annealed Cu/O/Ru(0001) film system yields an almost perfect layer-by-layer growth caused by heterogeneous nucleation at periodically arranged Cu film defect sites. The relationship of the O/Cu surfactant structures to the ordered O/Cu bilayer on Ru(0001) - interpreted as a disrupted -like oxidized surface - was revealed.

Strain modification in thin Si1−x−yGexCy alloys on (100) Si for formation of high density and uniformly sized quantum dots

The effects of alloying C with Ge and Si and varying the C/Ge ratio during the growth of very thin layers of the ternary alloy SiGeC grown on Si (100) substrates and the resulting strain modification on self-assembled and self-organized quantum dots are examined. During coherent islanded growth, where dislocations are not formed yet to relieve the strain, higher strain energy produced by greater lattice mismatch acts to reduce the island size, increase the density of islands, and significantly narrow the distribution of island sizes to nearly uniformly sized quantum dots. Strain energy can also control the critical thickness for dislocation generation within the three-dimensional islands, which then limits the maximum height which coherent islands can achieve. After the islands relax by misfit dislocations, the island sizes increase and the island size distribution becomes broader with the increase of misfit and strain. The optimal growth for a high density of uniform coherent islands occurred for the Si0.49Ge0.48C0.03 alloy composition grown on (100) Si, at a growth temperature of 600 °C, with an average thickness of 5 nm, resulting in a narrow size distribution (about 42 nm diameter) and high density (about 2×1010 dots/cm2) of quantum dots.

Preparation, properties, and cell attachment/growth behavior of PVA/chitosan-blended hydrogels

Hydrogels were prepared from blends of poly(vinyl alcohol) and chitosan in various ratios. Electron spectroscopy for chemical analysis of the resulting hydrogel membranes showed that the chitosan component was concentrated on the air-surface side of cast membranes, at a nearly constant concentration over the range 10 wt.% to 40 wt.% of chitosan content of the blends. Two-dimensional distribution of the chitosan component in the air-surface side of the sample with 40 wt.% chitosan was uniform, while that with 15 wt.% chitosan was heterogeneous and localized in high-density islands. Although the zeta-potential on the air-surface side of the hydrogels increased to a constant value with increasing chitosan content at pH 7.2, the zeta-potential values in all the hydrogels were negative. The attachment and growth of fibroblast cells (L-929) on the hydrogels was studied by cell culture. On hydrogels with more than 15 wt.% chitosan content, the attached cells were able not only to remain viable, but also to proliferate. The relative cell attachment after incubation for 30 h increased with increasing chitosan content in the hydrogels. The hydrogel with 40 wt.% chitosan content was superior to collagen, a widely-used mammalian cell-culture substrate, for cell attachment and growth. This cell attachment/growth behavior appears to be due not only to the interaction between chitosan molecules and cells, but also to the nature of the two-dimensional distribution of chitosan components on the surface of the hydrogels.

LEEM Measurements of Step Energies at the (001) Surface of Heavily Boron-Doped Silicon

We have used the low energy electron microscope (LEEM) to investigate step faceting and striped phase formation on heavily boron-doped Si(001) surfaces. Measurements of thermal fluctuations along the steps, equilibrium island shapes, and the angle of triangular facets provide quantitative information on the step formation energies. We find that a simple, systematic temperature dependence of the spatial anisotropy of the free energy of isolated steps is sufficient to account for the observed faceting of the B type steps. These results suggest that the proliferation of straight A step edges at lower temperature is caused by the energy of isolated A steps becoming vanishingly small. We show that rapid cooling of the samples leads to the nucleation and growth of a high density of vacancy islands on the surfaces. Based on these results, we propose a qualitative model for mass transport during stripe formation.

A theoretical study of adsorption and the surfactant effect of Sb on the Ag(100) surface

The adsorption and the surfactant effect of Sb on the Ag(100) surface are studied using first-principles calculations. Our calculated results show that the pre-adsorbed Sb atoms act as nucleation centres for the deposited Ag atoms, and surface-substitutional Sb atoms repel on-fcc-site Ag atoms nearby. We suggest that the Sb repulsion network reduces the Ag-adatom mobility and leads to a high density of Ag islands which improve the smoothness of the layer-by-layer growth.

Surface free energy modification of CaF 2 by atomic-height island formation on heteroepitaxy of GaAs on CaF 2

An epitaxial GaAs film with a flat surface was grown on a CaF 2 film by means of surface free energy modification of CaF 2 by atomic-height island formation. An additional low-temperature deposition at a final stage of growth of the CaF 2 film on Si(111) produced high density islands whose height was one monolayer of CaF 2 on the terraces. This surface structure contributed to enhance wettability of overgrown GaAs. As a result, 1.3- μm-thick GaAs film on CaF 2/Si(111) in which rms of surface roughness was 8 nm and electron mobility was 2000 cm 2/Vs in the Si doped layer (2×10 17 cm −3) was obtained in this method.

Characterization of thin copper films on Ni(111) by CO titration

Ultra-thin copper films on Ni(111) ( Θ Cu≤1 ML) were studied by the adsorption of CO followed by high resolution X-ray photoelectron spectroscopy. Saturating the surface at 100 K with CO leads to adsorption on the entire surface, i.e. Cu-covered as well as bare Ni(111) areas. Upon annealing to 220 K the Cu-bonded CO molecules desorb leaving only the Ni-areas covered with CO. High resolution XPS spectra in the O 1s and Cu 2p 3/2 regime have been recorded after adsorption at 100 K and after annealing to 220 K. The total area of Ni-terminated surface regions can be determined from the integrated O 1s XPS intensities after annealing; from the linear decrease of this area versus the amount of adsorbed copper a two-dimensional growth mode can be identified for layers which are annealed to 800 K. In addition, the CO adsorption sites on the Cu-terminated as well as on the Ni-terminated parts of the surface can be determined from the corresponding O 1s binding energies. The changes in the O 1s peak shapes with increasing Cu coverage lead to the conclusion that the adlayers have a very high island density.

Characterization of self-assembled Ge islands on Si(100) by atomic force microscopy and transmission electron microscopy

We present an alternative starting point of the fabrication of nanostructures for electronic devices by using self-assembling structures. One way for the growth of self-assembling structures as quantum dot (QD) arrays is based on the formation of coherently strained Ge islands on Si and requires controlling of a defined island growth (Stranski–Krastanov). For this reason we carried out systematic quantitative investigations of the growth of Ge islands. Stacks of two layers of Ge islands with a Si spacer were grown on a Si buffer and characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). In the first series the Ge layer thickness was varied at a constant growth temperature and in the second series the growth temperature was varied for a constant Ge layer thickness. Many of the results described in this paper confirm the expected growth behavior of Ge islands, i.e. the Stranski–Krastanov growth mode and the increasing island density with decreasing growth temperature. However, two new aspects of the island growth were found. Small Ge islands are already formed from 2.15 monolayers (ML) of Ge at high growth temperatures. At lower growth temperatures (≤645°C, 6.2 ML coverage), few large islands with defects and a high density of small coherent islands are observed simultaneously. TEM studies of cross-sectional and plan-view samples reveal that the small islands are elongated along (100) directions. Reasons for the formation of the two kinds of islands are discussed.