Self-organized Islands Research Articles

On spherical nanoinhomogeneity embedded in a half-space analyzed with Steigmann–Ogden surface and interface models

This paper determines the elastic fields in a positive half-space embedded with a spherical inhomogeneity under the context of Steigmann–Ogden surface/interface mechanical model. The half-space is loaded by an equal-biaxial far-field tension applied at infinity. While bulk domains are treated as linearly isotropic elastic solids, both the half-space plane boundary and the matrix/inhomogeneity interface are modeled by the Steigmann–Ogden theory. The well-developed method of Boussinesq displacement potentials is used to solve the elastostatic Navier’s equations of equilibrium. In view of the geometry and loading configurations, four sets of cylindrical and spherical harmonic potentials are carefully proposed to solve the problem. Implementation of the nonclassical Steigmann–Ogden boundary conditions at both the half-space free surface and the spherical interface results in a semianalytical solution in the form of infinite Legendre series. Extensive parametric studies are performed with respect to the surface and interface Steigmann–Ogden material parameters, shear moduli ratio between the spherical inhomogeneity and its surrounding matrix, and the inhomogeneity radius-to-depth ratio. Comparison and contrast with the well known Gurtin–Murdoch model reveals the significance of surface flexural rigidities at both boundaries of the considered mechanical model. When compared with the effects of matrix/inhomogeneity interface, the surface tension, Lamé constants and flexural rigidities at the half-space plane boundary are of secondary importance. This work is able to shed some lights on the modeling of self-organized adatoms and islands that is essential in the semiconductor industry.

Acoustic properties of strained SiGe/Si layers in the sub-terahertz frequency range

This work studies the scattering of coherent acoustical phonons within the frequency range of 30−200 GHz in strained SiGe/Si heterostructures with uniform SiGe layers and layers where the initial stage of self-organized islands formation was observed. Coherent phonon pulses reflected by single SiGe layers were detected, and phonon interference in the systems composed of two thin (approximately 10 nm) SiGe layers was observed. Acoustical properties were determined for single SiGe layers, and lateral acoustical inhomogeneity of the layers was estimated in the subterahertz frequency range. The results show that within the range of germanium content of 10%−32% acoustical properties of an approximately 10-nm SiGe layer are insensitive to internal strains governed by lattice mismatch and non-uniformities caused by initial stage of Stranski–Krastanov growth. The sound velocity and wave impedance of SiGe layers can be determined within 5% error, using the corresponding parameters of relaxed SiGe solid solutions with the same germanium content.

Plasmon-enhanced photoelectrochemical water splitting by InGaN/GaN nano-photoanodes

It has been extensively focused on the development of photoelectrochemical (PEC) solar water splitting cells due to the urgent need for clean energy. In this study, InGaN/GaN multiple quantum wells (MQWs) nanorods (NRs) PEs are purposely designed with coupled plasmonic metal (Ag–Au core–shell nanowires) for high efficiency PEC water splitting, aiming for enhancing visible light absorption and carrier kinetic energy. By means of self-organized Nickel island patterning and ICP dry etching with proper depths, the photocurrent density of InGaN/GaN nano-photoanodes in NaCl solution can be significantly enhanced from 0.74 mA cm−2 to 1.4 mA cm−2 at 1.5 V versus RHE (reversible hydrogen electrode). Furthermore, by decorating plasmonic metal into the InGaN/GaN NR arrays, the PEC water splitting performance can reach a maximum photocurrent of 1.77 mA cm−2 at 1.5 versus RHE. Such improvement is mainly attributed to the synergistic effects of the visible light absorption of MQWs and the generation of surface plasmon resonance (SPR). The approach of plasmon-enhanced nanostructure with the III-nitride based nano-PEs will accelerate the developments of visible light photocatalysts for PEC solar cells.

Semi-polar (11–22) GaN epitaxial films with significantly reduced defect densities grown on m-plane sapphire using a sequence of two in situ SiNx interlayers

This letter reports an approach for growing semipolar (11–22) GaN films with significantly reduced defect densities on m-plane sapphire substrates by incorporating a sequence of two in situ SiNx layers. The first SiNx layer is deposited on an initial epitaxial GaN layer and acts as a nanomask for preventing the propagation of extended defects. The second SiNx layer is deposited just after subsequent epitaxial GaN overgrowth begins to form self-organized GaN islands, which encourages further GaN growth to initiate from the exposed island sidewalls while blocking the penetration of the remaining defects. X-ray rocking curve measurements show that our semipolar GaN films provide relatively low full width at half maximum values at 0.119° along both the [11–23] and [10–10] directions. Additionally, transmission electron microscopy analyses confirm that our semipolar GaN films provide a significantly reduced density of threading dislocations down to ∼6 × 108 cm−2, which is two orders of magnitude less than those of conventionally deposited films.

Zinc(II) Tetraphenylporphyrin Adsorption on Au(111): An Interplay Between Molecular Self-Assembly and Surface Stress

The interaction between zinc(II) tetraphenylporphyrin (ZnTPP) molecules and a Au(111) surface is investigated using scanning tunnel microscopy, from initial adsorption sites to monolayer self-assembly, with a particular emphasis on its relation to the surface atomic structure reorganization. At low coverage, no ZnTPP molecules are observed on terraces, and adsorbates appear to only decorate step edges. At intermediate coverage, ZnTPPs adsorb into self-organized islands of flat-lying macrocycles in quasi-registry with the underlying surface reconstruction, in areas delimited by herringbone reconstruction domain walls. At monolayer coverage, the adsorption geometry of the self-organized molecular layer can be fully characterized with respect to the atomic structure of Au(111) surface atoms. Moreover alteration of the Au(111) surface reconstruction domain size is observed, caused by an adsorbate-induced reduction of the Au(111) surface stress anisotropy. This behavior is not ubiquitous to ZnTPP monolayers as...

Formation and nature of InGaN quantum dots in GaN nanowires.

InGaN/GaN disk-in-nanowire heterostructures on silicon substrates have emerged as important gain media for the realization of visible light sources. The nature of quantum confinement in the disks is largely unknown. From the unique nature of the measured temperature dependence of the radiative lifetime and direct transmission electron microscopy, it is evident that such self-organized islands (disks) behave as quantum dots. This is confirmed by the observation of single photon emission from a single disk-in-nanowire and the presence of a sharp minimum in the line width enhancement factor of edge emitting lasers having the InGaN disks as the gain media.

Self-organized 2D Ni particles deposited on titanium oxynitride-coated Si sculpted by a low energy ion beam

Self-ordered Ni nanoparticles grown on TiNxOy-coated crystalline silicon previously sculpted by ion beam bombardment are reported. The samples are obtained following a sequential in situ routine deposition procedure. First, crystalline Si is Xe+ bombarded, generating regular patterns. Second, a thin TiNxOy film is grown on the patterned Si substrate. Immediately, nano-sized nickel particles are deposited by ion beam sputtering and temperature-annealed forming a 2D lattice. The self-organized Ni islands are induced by preferential Ni site nucleation on the coated sculpted Si grooves.

Self-organized metallic islands on nano-patterned silicon substrate

Combining the self-organized growth of nanometer size islands with the use of a pre-patterned substrate, the fabrication of a metallic nanoparticle array with sub-100 nm period is demonstrated. The array pattern is artificially defined through conventional lithography on a Si substrate. Two passivation steps are used first ex situ with a wet hydrogenation followed by the in situ formation under ultrahigh vacuum of an Au-Si wetting layer. This allows for the subsequent nucleation and growth of Au on an atomically clean surface. The long range order of the nanoparticle array is preserved over the whole millimeter-size patterned area.

自组织GaN小岛的InGaN/GaN量子阱白光发射

自组织GaN小岛的InGaN/GaN量子阱白光发射

Site-controlled growth of InP/GaInP islands on periodic hole patterns in GaAs substrates produced by microsphere photolithography

We present microsphere photolithography in combination with wet chemical etching as a fast and low-cost method to produce regular hole arrays in a (100) GaAs surface, which are suitable for controlled nucleation of self-organized InP islands in a metal–organic vapor-phase epitaxy (MOVPE) system. A hexagonal close-packed monolayer of microspheres is used as an array of microlenses to focus UV-light on UV-sensitive photoresist. In this way, regular arrays of holes with 2μm spacing can be realized in the photoresist with controllable feature size in the sub-μm range, which are transferred to a GaAs buffer, using an isotropic etchant. These templates are used to study the site-controlled nucleation of InP islands on a Ga0.51In0.49P barrier. For this purpose the subsequent overgrowth of the templates with a GaAs buffer layer and the GaInP barrier is investigated previous to the additional deposition of the InP. The template quality is monitored during structuring and overgrowth experiments using atomic force microscopy (AFM). Preferred nucleation of InP islands inside the almost filled holes can be observed for uncapped samples. The correlation between the initial patterning and the optical signal of the InP islands is investigated by micro-photoluminescence (micro-PL) measurements. We observe site-controlled nucleation of large, but optically active, InP islands on these templates.

Nanodot to nanowire: A strain-driven shape transition in self-organized endotaxial CoSi2 on Si(100)

We report a phenomenon of strain-driven shape transition in the growth of nanoscale self-organized endotaxial CoSi2 islands on Si(100) substrates. Nanodots of CoSi2 grow in the square shape following the four fold symmetry of the Si(100) substrate, up to a critical size of 67 × 67 nm2, where a shape transition takes place. Larger islands grow as nanowires with ever increasing length and the width decreasing to an asymptotic value of ∼25 nm. This produces long nanowires of nearly constant width. The endotaxial nanostructures grow into the Si substrate with a small extension above the surface.

Structural investigations of silicon nanostructures grown by self-organized island formation for photovoltaic applications

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) lay- ers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by elec- tron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nan- odot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional in- vestigations reveal a single-crystalline structure of the nan- odots. This characteristic is observed as long as the thick-

Anisotropy Driven Interrupted Coarsening of the Asaro-Tiller-Grinfeld Instability

We investigate the effects of surface energy anisotropy on the coarsening dynamics of the Asaro-Tiller-Grinfeld instability at stake in thin semiconductor films. We consider a continuum model which accounts for wetting interactions between the film and its substrate, elasticity driven mass currents and surface energy anisotropy. We derive an explicit non-linear, non-local evolution equation for the film height that we solve numerically. Anisotropy, which dictates the island shapes, impacts the growth kinetics by weakening the possible elastic relaxation, which can lead during annealing to an interruption of Ostwald ripening. The resulting stationary state is characterized by square-based pyramids separated by a wetting layer. It is found that the instability onset is delayed when the film thickness decreases above the critical thickness for the instability to occur. We characterize the influence of the growing flux used for the film deposition on the stationary state reached during subsequent annealing, and find that the density of the resulting self-organized islands increases with the flux.

The influence of a Si cap on self-organized SiGe islands and the underlying wetting layer

For the prototypical SiGe/Si(001) Stranski-Krastanow (SK) growth system, the influence of intermixing caused by the deposition of a Si cap layer at temperatures Tcap between 300°C and 700°C is studied both for the SiGe wetting layer (WL) and the SiGe islands. Systematic growth experiments were carried out with an ultrahigh resolution of down to 0.005 monolayers (ML) of deposited Ge. The properties of the samples were investigated via photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy. We studied in detail the influence of Tcap in the three main coverage regions of SiGe SK growth, which are (i) the WL build-up regime, (ii) the island nucleation regime, where most of the Ge is supplied via material transfer from the WL, and (iii) the saturation regime, where the WL thickness remains initially stable. At Tcap = 300°C, we found that both the WL and the island are essentially preserved in composition and shape, whereas at 500°C the WL becomes heavily alloyed during capping, and at 700°C the islands also become alloyed. At Tcap = 500°C we found enhanced WL intermixing in the presence of dome-shaped islands, whereas at Tcap = 700°C the WL properties become dominated by the dissolution of pyramid-shaped islands upon capping. At Ge coverages above ≈6 ML, we found an unexpected thickening of the WL, almost independently of Tcap. This finding suggests that the density and the volume of the dome-shaped islands have an upper limit, beyond which excess Ge from the external source again becomes incorporated into the WL. Finally, we compared PL spectra with AFM-based evaluations of the integral island volumes in order to determine in a straightforward manner the average composition of the SiGe islands.

InGaN/GaN quantum wells on self-organized faceted GaN islands: Growth and luminescence studies

The growth behavior and luminescence properties of InGaN/GaN quantum wells (QW) on in situ self-organized GaN islands of various distinct smooth sidewall faceting are simultaneously investigated and directly compared. The QW thickness is found to be specific polar angle dependent, leading to variations in QW thickness on multifaceting islands. As a result, by color tuning through island shaping and modifications of the InGaN/GaN QWs on the faceted islands, polychromatic emissions are achieved.

Selective sensitivity of ellipsometry to magnetic nanostructures

Magneto-optic (MO) ellipsometry of ferromagnetic materials is extremely sensitive to ultra-thin films, multilayers, and nanostructures. It gives a possibility to measure all components of the magnetization vector in the frame of the magneto-optic vector magnetometry and enables us to separate magnetic contributions from different depths and materials in nanostructures, which is reviewed in this article. The method is based on ellipsometric separation using the selective MO Kerr effect. The figure of merit used to quantify the ellipsometric selectivity to magnetic nanostructures is defined on the basis of linear matrix algebra. We show that the method can be also used to separate MO contributions from areas of the same ferromagnetic materials deposited on different buffer layers. The method is demonstrated using both: (i) modeling of the MO ellipsometry response and (ii) MO measurement of ultra-thin Co islands epitaxially grown on self-organized gold islands on Mo/Al2O3 buffer layer prepared using the molecular beam epitaxy at elevated temperatures. The system is studied using longitudinal (in-plane) and polar (perpendicular) MO Kerr effects.

Ellipsometric Selective Sensitivity to Magnetic Nanostructures

Magneto-optic (MO) ellipsometry of ferromagnetic materials is extremely sensitive to ultra-thin films, multilayers, and nanostructures. It gives a possibility to measure all components of the magnetization vector in the frame of the magneto-optic vector magnetometry and enable us to separate magnetic contributions from different depths and materials in nanostructures, which is reviewed in this article. The method is based on ellipsometric separation using the selective MO Kerr effect. The figure of merit used to quantify the ellipsometric selectivity to magnetic nano structures is defined on the basis of linear matrix algebra. W e show that the method can be also used to separate MO contributions from areas of the same ferromagnetic materials deposited on different buffer layers. The method is demonstrated using both: (i) modeling of the MO ellipsometry response and (ii) MO measurement of ultra-thin Co islands epitaxially grown on self-organized gold islands on Mo/Al2O3 buffer layer prepared using the molecular beam epitaxy at elevated temperatures. The system is studied using longitudinal (in-plane) and polar (perpendicular) MO Kerr effects.

Nanogrids and Beehive-Like Nanostructures Formed by Plasma Etching the Self-Organized SiGe Islands.

A lithography-free method for fabricating the nanogrids and quasi-beehive nanostructures on Si substrates is developed. It combines sequential treatments of thermal annealing with reactive ion etching (RIE) on SiGe thin films grown on (100)-Si substrates. The SiGe thin films deposited by ultrahigh vacuum chemical vapor deposition form self-assembled nanoislands via the strain-induced surface roughening (Asaro-Tiller-Grinfeld instability) during thermal annealing, which, in turn, serve as patterned sacrifice regions for subsequent RIE process carried out for fabricating nanogrids and beehive-like nanostructures on Si substrates. The scanning electron microscopy and atomic force microscopy observations confirmed that the resultant pattern of the obtained structures can be manipulated by tuning the treatment conditions, suggesting an interesting alternative route of producing self-organized nanostructures.

Mechanics of Quantum-Dot Self-Organization by Epitaxial Growth on Small Areas

Energetic arguments are used to understand the mechanics of Stranski–Krastanow epitaxial systems constrained to grow on a finite area of a substrate. Examples include selective area epitaxy and growth on patterned substrate features as raised mesa and etched pits. Accounting only for strain energy, (isotropic) surface energy, wetting layer potential energy, and geometric constraints, a rich behavior is obtained, whereby equilibrium configurations consist of a single island, multiple islands, or no islands, depending on the size of the growth area. It is shown that island formation is completely suppressed in the case of growth on a sufficiently small area. These behaviors are in stark contrast to growth on an indefinitely large area, where the same model suggests that the minimum free energy configuration of systems beyond the wetting layer transition thickness is a single island atop a wetting layer. The constraint of growing on a finite area can suppress island coarsening and produce minimum energy configurations with multiple self-organized islands of uniform size and shape.

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).