Self-assembled Islands Research Articles

The effect of local atomic structure on the optical properties of GeSi self-assembled islandsburied in silicon matrix

The local atomic structure of GeSi self-assembled islands buried in a silicon matrix stronglyinfluences the optical properties of such systems. In the present paper this structurewas determined by x-ray absorption fine-structure (XAFS) spectroscopy andhigh resolution transmission electron microscopy (HRTEM) and used to build aschematic description of the band structure model. Quantitative analysis of theextended XAFS (EXAFS) spectrum was performed for three coordination shellsaround the Ge absorbing atom with multiple scattering taken into account. It wasproved that the coordination number of elements in an alloy resulting from EXAFSanalysis for all three coordination spheres (i.e. ‘mixing degree’ parameters) cannot betaken as the concentration of alloy but can be used together with a proper modelof the alloy unit cell to calculate a realistic concentration. The fraction of Gecalculated in this way is consistent with HRTEM results. The found model of the unitcell was used to generate a x-ray absorption near edge structure spectrum by abinitio calculations. This approach yielded a spectrum in good agreement with theexperimental one. The information gained from XAFS and HRTEM was thenused for calculation of the band structure diagram. Results of the calculation arediscussed and compared with the experimental photoluminescence spectrum.

Effect of tensile-strained Si layer on photoluminescence of Ge(Si) self-assembled islands grown on relaxed SiGe/Si(001) buffer layers

The results of studying the photoluminescence of the structures with Ge(Si) self-assembled islands embedded into tensile-strained Si layer are reported. The structures were grown on smooth relaxed Si{sub 1-x}Ge{sub x}/Si(001) (x = 0.2-0.3) buffer layers. The photoluminescence peak found in the photoluminescence spectra of the studied structures is related to the indirect (in real space) optical transition between the holes localized in the Ge(Si) islands and electrons localized in the tensile-strained Si layers under and above an island. It is shown that one can efficiently control the position of the photoluminescence peak for a specified type of structure by varying the thickness of the strained Si layers. It is found that, at 77 K, the intensity of the photoluminescence signal from the heterostructures with Ge(Si) self-assembled islands contained between the tensile-strained Si layers exceeds by an order of magnitude the intensity of the photoluminescence signal from the GeSi structures with islands formed on the Si(001) substrates.

CdSe quantum dot formation induced by amorphous Se

The mechanism allowing the transition from a two-dimensional strained layer of CdSe on ZnSe to self-assembled islands induced by the use of amorphous selenium is still not fully understood. For a better understanding, atomic force microscopy and transmission electron microscopy studies were performed on CdSe films with a thickness close to that for quantum dot formation. Below this thickness, the sample surface results in undulations along the [1 1 0] crystal direction, while few quantum dots are situated in the wave valleys. Plan view transmission electron microscopy studies reveal a strong anisotropy of the islands and show that the Se desorption conditions are crucial.

Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate–fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er 3N@C 80 and Sc 3N@C 80 on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250–300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.

A rack-and-pinion device at the molecular scale

Molecular machines, and in particular molecular motors with synthetic molecular structures and fuelled by external light, voltage or chemical conversions, have recently been reported. Most of these experiments are carried out in solution with a large ensemble of molecules and without access to one molecule at a time, a key point for future use of single molecular machines with an atomic scale precision. Therefore, to experiment on a single molecule-machine, this molecule has to be adsorbed on a surface, imaged and manipulated with the tip of a scanning tunnelling microscope (STM). A few experiments of this type have described molecular mechanisms in which a rotational movement of a single molecule is involved. However, until now, only uncontrolled rotations or indirect signatures of a rotation have been reported. In this work, we present a molecular rack-and-pinion device for which an STM tip drives a single pinion molecule at low temperature. The pinion is a 1.8-nm-diameter molecule functioning as a six-toothed wheel interlocked at the edge of a self-assembled molecular island acting as a rack. We monitor the rotation of the pinion molecule tooth by tooth along the rack by a chemical tag attached to one of its cogs.

Intense photoluminescence from Ge(Si) self-assembled islands embedded in a tensile-strained Si layer

Photoluminescence (PL) of structures with Ge(Si) self-assembled islands located between tensile-strained Si layers grown on smooth strain-relaxed Si1−xGex/Si (0 0 1) (x = 0.2–0.3) buffer layers has been studied. The peak observed in the PL spectra of the structures studied is associated with indirect optical recombination of electrons confined in strained Si layers on the heterojunction with islands and holes localized in Ge-rich islands. The redshift of the PL peak with an increase in the strained Si layer thickness confirms the validity of the proposed optical transition. The intensity of the PL signal from Ge(Si) islands located between tensile-strained Si layers at 77 K is one order of magnitude higher than the PL signal from Ge(Si) islands grown on unstrained Si (0 0 1) substrates. The substantial rise of the PL signal is associated with efficient quantum confinement of electrons in the strained Si layer on the heterojunction with Ge(Si) islands.

Si(001)パターン化表面への自己集合Geアイランド形成

Si(001)パターン化表面への自己集合Geアイランド形成

Freezing shape and composition of Ge∕Si(001) self-assembled islands during silicon capping

We use atomic force microscopy, x-ray photoemission spectroscopy, and x-ray absorption spectroscopy to study the effect of the deposition of a Si cap layer by chemical vapor deposition on the morphology and composition of a Ge island layer grown at 600°C. We found that the capping of self-assembled Ge islands under a silicon layer results in high-quality, atomically flat layer only at deposition temperature above 700°C. On the other hand at this temperature Ge–Si intermixing and island coarsening are greatly enhanced, resulting in an increased average island volume. Here we show that the predeposition at low temperature of a thin cap layer preserves island shape, size, and composition when the capped islands undergo a subsequent process at higher temperature up to 750°C. It is shown, therefore, that with a two-step capping process it is possible to combine the benefit of a low temperature capping, which reduces island alloying and coarsening, with that of a high temperature capping which is needed to recover a flat surface.

Two-Dimensional Photonic Crystals Coupled to One-Dimensional Bragg Mirrors

Planar two-dimensional (2-D) photonic crystals can be combined with a one-dimensional (1-D) Bragg mirror to control the quality factor and out-of-plane coupling of optical modes. We have investigated the optical properties of such structures fabricated on silicon. The optical properties are probed by the room-temperature photoluminescence of Ge/Si self-assembled islands as an internal source. We show that the enhancement of the quality factor can be obtained by controlling the thickness of the silicon upper layer in which the 2-D photonic crystal is etched and the air filling factor of the photonic crystal. Quality factors of 2200 around 1100nm are obtained by this method for bulk photonic crystals with a square lattice pattern. The experimental results are supported by three-dimensional (3-D) finite-difference time-domain calculations of the investigated structures

Periodic arrays of epitaxial self-assembled SiGe quantum dot molecules grown on patterned Si substrates

Ex situ focused ion-beam (FIB) patterning of arrays of holes on Si (001) substrates results in the subsequent formation of SiGe quantum dot molecules at each of the patterned sites during heteroepitaxial growth under kinetically limited growth conditions where island formation is constrained. These quantum dot molecules are fourfold self-assembled island nanostructures bound by a central pit. During growth, material is ejected from the patterned sites forming the pits that in turn provide favorable sites for the cooperative nucleation of {105} faceted islands. The degree of order and quality of the resulting structures depend on many factors including growth temperature, ion-beam milling depth, Si buffer thickness, and spacings between FIB exposed sites. This technique provides a method for controlling the lateral placement of semiconductor nanostructures, which could be used in applications such as complex nanoelectronic architectures.

Si-based two-dimensional photonic crystals coupled to one-dimensional Bragg mirrors

Planar two-dimensional photonic crystals can be combined with a one-dimensional Bragg mirror to control the quality factor and out-of-plane coupling of optical Bloch modes. We have investigated the optical properties of such structures fabricated on silicon. The photonic crystals are fabricated in the upper Si layer deposited on top of quarter-wave thick SiO 2-polycrystalline Si layers. The optical properties are probed by the room-temperature photoluminescence of Ge/Si self-assembled islands as an internal source. We show that an enhancement of the quality factor can be obtained by controlling the thickness of the silicon upper layer in which the two-dimensional photonic crystal is etched and by controlling the air filling factor of the photonic crystal. Quality factors of 2200 around 1100 nm are obtained by this method for defect-free photonic crystals with a square lattice pattern. The experimental results are supported by three-dimensional finite-difference time-domain (FDTD) calculations of the radiated modes for the investigated structures.

Size and density control of InAs quantum dot ensembles on self-assembled nanostructured templates

New morphologies of InAs quantum dot (QD) ensembles forming on self-assembled GaAs nano-holed island templates are demonstrated. Droplet homoepitaxy (GaAs/GaAs) is used to generate holed nanoscale-sized mounds that appear to elongate along . Depending on the InAs monolayer (ML) coverages, subsequent InAs deposition forms different sizes and shapes of QD ensembles. While we initially observe the formation of the QDs at the hole sites when less InAs is deposited, QDs form around the edges of the mounds with greater InAs deposition. By varying the InAs depositions and growth temperatures, we demonstrate an ability to control the size and density of QDs. The observed decrease in the necessary critical thickness required for the InAs 2D–3D transition may be due to the higher density of monolayer steps on the sidewalls of the holes and on the edges of the mounds. This hybrid growth approach overcomes some limitations of typical QD growth on planar GaAs surfaces and may find applications in optoelectronics.

Mechanism of the nanoscale localization of Ge quantum dot nucleation on focused ionbeam templated Si(001) surfaces

We investigate the fundamental mechanism by which self-assembled Ge islands can benucleated at specific sites on Si(001) using ultra-low-dose focused ion beam (FIB)pre-patterning. Island nucleation is controlled by a nanotopography that forms after theimplantation of Ga ions during subsequent thermal annealing of the substrate. Thisnanotopography evolves during the annealing stage, changing from a nanoscale annulardepression associated with each focused ion beam spot to a nanoscale pit, and eventuallydisappearing (planarizing). The correspondence of Ge quantum dot nucleationsites to the focused ion beam features requires a growth surface upon which thenanotopography is preserved. A further key observation is that the Ge wetting layerthickness is reduced in patterned regions, allowing the formation of islands on thetemplated regions without nucleation elsewhere. These results provide routes to thegreatly enhanced design and control of quantum dot distributions and dimensions.

Self-assembled magnetic nanostructures: Epitaxial Ni nanodots on TiN/Si (001) surface

Systems containing single domain magnetic particles are of great interest in view of their possible applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this work is plan-view STEM Z-contrast imaging study of the self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer–Weber (V–W) mode. The growth was carried out using pulsed laser deposition (PLD) technique with epitaxial titanium nitride film as the template, which was in turn grown on silicon (001) substrate via domain matching epitaxy. Our results show that the base of nickel islands is rectangular with the two principal edges parallel to two orthogonal 〈110〉 directions, which is [110] and [ $$\bar{1}10$$ ] for [001] oriented growth. The size distribution of the islands is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski–Krastanow (S–K) mode. A certain degree of self-organization was also found in the lateral distribution of islands: island chains were observed along the directions close to 〈011〉, which are also the edge directions. The interaction between neighboring islands through the island edge-induced strain field is believed to be responsible for the size uniformity and the lateral ordering.

Spontaneous Ge island ordering promoted by partial silicon capping

In this paper, we show that lateral arrangement of Ge/Si(0 0 1) self-assembled islands in a square array oriented along the [1 0 0]–[0 1 0] directions can be obtained through the lateral displacement of the islands themselves. We found that when the deposited islands are exposed to an external silicon flux, the impinging silicon atoms induce Ge–Si intermixing resulting in island shape transformation from domes to large pyramids. This transformation leads to an array of closely spaced islands interacting elastically among themselves through the substrate. By means of atomistic simulations we demonstrate that this elastic repulsion drives a net flux of Ge and Si atoms from one side to the other side of the islands, leading to a lateral displacement of the whole island. This displacement ends when the two islands are sufficiently far away, or when another island is approached during the motion. In a dense ensemble of islands, this mechanism drives the tendency to order observed experimentally.

Strain distribution in a transistor using self-assembled SiGe islands in source and drain regions

We propose to improve a p-channel metal-oxide-semiconductor field-effect transistor using laterally closely spaced double self-assembled SiGe∕Si islands as drain and source to create a high hole mobility channel. The strain distribution in and around the channel is calculated for two realistic island geometries with various distances between the islands. A compressive strain of more than 1% in the channel can be achieved for SiGe islands and small distance between these two islands. We demonstrate that the proposed double SiGe∕Si island structure can be realized by epitaxial growth on patterned substrates designed for static random access memory cell.

Band-edge alignment ofSiGe∕Siquantum wells andSiGe∕Siself-assembled islands

We report on the energy band gap and band lineup of $\mathrm{SiGe}∕\mathrm{Si}$ heterostructures either in the case of coherently strained quantum wells or in the case of $\mathrm{SiGe}∕\mathrm{Si}$ self-assembled islands. We take into account the strain field and the quantum confinement effects through an accurate description of the conduction band including the $\ensuremath{\Delta}$ and $L$ bands. The strain field is calculated using a microscopic valence force field theory. The conduction-band diagram and energies are obtained from a 30-band $\mathbf{k}∙\mathbf{p}$ Hamiltonian accounting for the strain through the Bir-Pikus Hamiltonian. The band-edge description is first given for biaxially strained pseudomorphic $\mathrm{SiGe}$ layers. In $\mathrm{SiGe}$ quantum wells grown on relaxed silicon, the band line-up switches from type I to type II depending on the value of the average valence band offset. Applying the 30-band formalism to the case of heterostructures grown on relaxed silicon germanium buffer layers indicates that a better agreement with experimental data is obtained for a valence-band offset value $\mathrm{\ensuremath{\Delta}}{E}_{v}=0.54x$ where $x$ is the Ge composition. For this parameter, a type-II band lineup is thus expected for all compositions of pseudomorphic $\mathrm{SiGe}$/relaxed Si heterostructures. For $\mathrm{GeSi}∕\mathrm{Si}$ islands, we take into account the strain relaxation in the surrounding Si matrix. A type-II band lineup is predicted for all Ge compositions. The near-infrared interband recombination energy of the islands is calculated as a function of their $\mathrm{SiGe}$ composition.

Effect of surface states on carrier dynamics in InGaAsP/InP stressor quantumdots

The carrier dynamics in strain-induced InGaAsP/InP quantum dots (QDs) is investigatedby time-resolved photoluminescence and continuous-wave photoluminescence. The stressorQDs are fabricated by depositing self-assembled InAs islands (or stressor islands) on top ofa near-surface InGaAsP/InP quantum well (QW). The temporal behaviour of theQD photoluminescence transients are observed to exhibit a two-phase decay.Rate equation analyses reveal that by increasing the distance of the QW fromthe surface the surface capture time constant is increased considerably whilethe capture time constant of an electron from the QW to the QD is decreased.

Growth behavior and microstructure of Ge self-assembled islands on nanometer-scale patterned Si substrate

We investigate the growth behavior and microstructure of Ge self-assembled islands of nanometer dimension on Si (0 0 1) substrate patterned with hexagonally ordered holes of ∼25 nm depth, ∼30 nm diameter, and ∼7×10 10 cm −2 density. At 9 Å Ge coverage and 650 °C growth temperature, Ge islands preferentially nucleate inside the holes, starting at the bottom perimeter. Approximately 14% of the holes are filled by Ge islands. Moiré fringe analysis reveals partial strain relaxation of about 72% on average, which is not uniform even within a single island. Crystalline defects such as dislocation are observed from islands smaller than 30 nm. Increased Ge coverage to 70 Å forms larger aggregates of many interconnected islands with slightly increased filling factor of about 17% of the holes. Reducing the growth temperature to 280 °C results in much higher density of islands with a filling factor of about 80% and with some aggregates. The results described in this report represent a potential approach for fabricating semiconductor quantum dots via epitaxy with higher than 10 10 cm −2 density.

Dendritic Structures of Poly(Ethylene Glycol) on Silicon Nitride and Gold Surfaces

A hydrophilic silicon nitride surface was grafted with poly(ethylene glycol) monomethyl ether (average formula weight of 5000 Da) in a one-step protocol. The domains of stable dendritic structures of self-assembled monolayer islands on a silicon nitride surface were observed with atomic force microscopy. The moduli of elasticity of these dendritic structures in air and in KCl aqueous solution were compared. The value of the Young's modulus of these structures is reduced by more than 3 orders of magnitude, from approximately 12 GPa measured in air to approximately 5 MPa in KCl solution. This dramatic reduction in elasticity was attributed to the swelling of the dendritic structures in aqueous solution, which was verified by the increased film thickness. These dendritic structures were not stable in the aqueous environment and could be removed by soaking in water for 22 h because of the hydrolysis of the silicate bonds. This fact was confirmed by the reduction of the C1s signal in the X-ray photoelectron spectroscopy experiments. These morphologies are not unique to silicon nitride substrate; similar features were also observed for thiolated poly(ethylene glycol) monomethyl ether molecules absorbed on a gold surface.