We have observe a metal-on-metal growth mode in Co/Ru(0001) at elevated temperature, where self-assembled 3D dots and antidots in rather regular truncated pyramidal shapes are mainly attributed to a stress-driven mechanism. While good lattice match has been one of the major criteria to guide epitaxial growth, it is possible that well-chosen lattice mismatched systems could be utilized to fabricate strain-engineered regular magnetic nanostructure arrays with different sizes and periodicity. A linear alignment of self-assembled Co dots and stripes can be created along grooves on a Ru(0001) substrate. Our observations suggest that it may be possible to direct the alignment and positioning in self-assembly of complex patterns by means of substrate templating, which should be of general applicability beyond the Co/Ru system. Magnetically, the dots are ferromagnetic with in-plane single-domain state, while the thick films with antidots exhibit perpendicular stripe domains.

X-ray diffraction and x-ray standing waves (XSW) have been used to investigate the quality of epitaxial ultra-thin Ge films grown on Si(001) with and without Te as a surfactant. The efficacy of Te as a surfactant in this application has been debated. We measured samples between 1 and 10 ML in thickness and our results clearly indicate that Ge films grown with Te are superior to those grown without Te. The coherent positions and coherent fract ons determined from XSW analysis agree well with those predicted by linear elasticity theory for Ge/Si(001). Furthermore, grazing incidence diffraction measurements (GIXD) suggests that 9 ML Ge grown on Si(001) with Te is strained in-plane while the same film grown without Te is relaxed.

We study the formation and self-organization of “ripples” and “dots” spontaneously appearing during uniform irradiation of Si, Ge, and GaSb with energetic ion beams. Features have been produced both with sub-keV unfocused Ar+ ions and with a 30 keV Ga+ Focused Ion Beam. We follow the evolution of features from small amplitude to “nanospikes” with increasing ion dose. It appears that the edge of the sputtered region influences the patterns formed, an effect that may make it possible to guide the self-organization by the imposition of lateral boundary conditions on the sputter instability.

In this work, we demonstrate for the first time lateral composition modulation in GaAs/GaSb short period superlattices (SPS). Several different structures were investigated in order to study the role of Group V overpressure on the final composition and uniformity of the structure. Cross sectional transmission electron diffraction and xray diffraction reciprocal space maps reveal that the film is indeed phase separated. The composition modulation observed in these films is a result of the SPS growth and not due to spinodal decomposition, as evidenced by the fact that an alloy grown at the same conditions results in a homogeneous layer.

A variational approach to the modelling of growth and relaxation in elastically strained epitaxial thin films and islands is presented. The continuum model is derived for a twocomponent material system evolving via the kinetic mechanisms of evaporation/condensation and surface and lattice diffusion under the influence of elastic mismatch strain, surface tension and combinational phase energy. Phenomena such as surface segregation and interdiffusion between the film and the substrate are incorporated into the model. This approach is illustrated by a 2D simulation of the onset of instabilities during the growth of epitaxial thin films.

Self assembled quantum dots of Ge were obtained by magnetron sputter epitaxy of seven monolayers of Ge on a 33nm thick undoped Si buffer grown on top of a p-doped (100) Si substrate. The samples obtained in this manner were then capped with an increasing number of silicon layers in order to study the effect of Si deposition on the strain and the morphology of the dots. They were characterized “ex situ” by spectroscopic ellipsometry and Raman spectroscopy. The optical experiments revealed well defined differences between the capped and uncapped samples and among samples with different cap thicknesses.

By monitoring the energy and the splitting of the E0', E1 and E2 interband optical transitions of Ge and the Ge-Si vibrational mode, the optical measurements evidence strain effects as well as the formation of SiGe alloy, in agreement with the ``in situ'' STM measurements.

We report on real time stress measurements by a sensitive cantilever beam technique of Ge and SiGe Alloy Films on Si(001) in combination with structural investigations by in situ STM (scanning tunneling microscopy) and ex situ AFM (atomic force microscopy). Characteristic features in the stress curves provide detailed insight into the development and relief of the misfit strain as well as the respective growth mode. For the Stranski-Krastanow system Ge/Si(001) the strain relaxation proceeds mainly in two steps: (i) by the formation of 3D islands on top of the Ge wetting layer and (ii) via misfit dislocations in larger 3D islands and upon their percolation. Co-deposition of Si influences the stress behavior drastically. The growth mode changes from Stranski-Krastanow to a kinetic 3D island mode at Si concentrations of about 20% leading to the so far smallest quantum dots of the Ge/Si system.

A series of ZnO films with various thicknesses were prepared on (0001) sapphire substrate by pulsed laser deposition (PLD). Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis were utilized to investigate the effects of misfit strain on the surface morphology and the crystallinity. The electrical and optical properties of the films were also investigated as a function of the film thickness. It was found that the crystalline quality, electrical and optical properties of the films depended on the film thickness and were improved with increasing the film thickness. This is attributed to the fact that the films thinner than 400 nm are under the severe misfit strain, which decreases as the film thickness increases further.

Coherently strained Ge islands were grown at a substrate temperature of 550°C by magnetron sputter epitaxy on Si (001) and studied by scanning tunnelling microscopy (STM). The shape changes induced by exposure to a Si-flux at 450°C were investigated as a function of the Sicoverage. During Si-capping, multifaceted domes were found to flatten and to transform into {105}-faceted pyramids and subsequently into stepped mounds through intermediate shapes. The observed sequence of morphological changes is induced by Si-Ge intermixing and is shown to be the inverse of that occurring during Ge or Si1-xGex growth on Si (001). The results are interpreted with a model in which the stable shape of an island mainly depends on its volume and composition.

Strain relaxed Si1−xGex buffer layers are of great importance as virtual substrates for Si1−xGex/Si quantum well structures and devices. We apply He+ ion implantation and subsequent annealing on pseudomorphic, MBE-grown Si1−xGex/Si(100) heterostructures with an implantation depth of about 100 nm below the Si1−xGex/Si interface. A narrow defect band is generated inducing the formation of strain relieving misfit dislocations during subsequent thermal annealing. Efficient strain relaxation was demonstrated for Si1−xGex layers with Ge fractions up to 30 at. %. The variation of the implantation dose and the annealing conditions changes the dislocation configuration and the He bubble structure. At a dose of 2×1016 cm−2 a high degree of relaxation is accompanied by a low density of threading dislocations of about 107 cm−2 for a Ge content of 30%. An additional increase of the Ge content can be achieved by annealing in oxygen. The oxidation of Si1−xGex leads to the formation of SiO2 while the Ge atoms are rejected from the oxide leading to a pile-up of Ge below the oxidation front. The heterostructures were analyzed using X-ray diffraction, Rutherford backscattering/channeling spectrometry and transmission electron microscopy.

We have studied infrared photoluminescence (PL) and x-ray diffraction (XRD) of 400 nm and 1500 nm thick InAs epilayers on GaAs, and 4 nm thick InAs on graded InGaAs layer with total thickness of 300 nm grown by molecular beam epitaxy. The PL peak positions of 400 nm, 1500 nm and 4 nm InAs epilayer measured at 10 K are blue-shifted from that of InAs bulk by 6.5, 4.5, and 6 meV, respectively, which can be largely explained by the residual strain in the epilayer. The residual strain caused by the lattice mismatch between InAs and GaAs or graded InGaAs/GaAs was observed from XRD measurements. While the PL peak position of 400 nm thick InAs layer is linearly shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of 4 nm InAs epilayer on InGaAs and 1500 nm InAs layer on GaAs is gradually blue-shifted and then, saturated above a power of 75 mW. These results suggest that adopting a graded InGaAs layer between InAs and GaAs can efficiently reduce the strain due to lattice mismatch in the structure of InAs/GaAs.

Metal ceramic interfaces are important in applications as diverse as magnetic storage media to support catalysts. Understanding the correlation between the films structure and its properties is very important for applications. For example the lattice mismatch between substrate and epilayer during heteroepitaxial growth induces strain as well as defects on the structure of the films that in turn affect the magnetic properties.

We have previously shown that it is possible to obtain epitaxial (001) and (111) Ni films grown on MgO substrates. [1] In particular we observed that the crystalline quality of the films improved considerably after 10 nm of film growth and annealing of the films indicated significant reduction of the surface roughness. In addition, after annealing (001) Ni films we observed nano-patterning of the surface through self-assembly of periodic “stripes”.

We will now present our analysis of the magnetic properties of these films, in particular the azimuthal dependence of the magnetization reversal, using MOKE, and we will attempt to correlate our findings with the surface characterization obtained with STM.

Stress relaxation mechanisms have been investigated during growth of SiGe/Si heterostructures in a regime where misfit dislocations and surface morphological evolution strongly interact. Stress evolution was measured in real time using in situ wafer curvature measurements during molecular beam epitaxy of Ge0.3Si0.7 on Si (001). This real-time data has been combined with detailed analysis of the evolving surface morphology and misfit dislocation density using atomic force microscopy and transmission electron microscopy. Several distinct stages of microstructural evolution were observed during epitaxial growth of Ge0.3Si0.7alloys at intermediate temperatures (550°C) and relatively high deposition rate (0.9 Å/s). Initial planar, coherent growth was followed by pyramidal pit formation in the metastable wetting layer for 15nm thick films. Subsequent cooperative nucleation of island ridges occurs along <100> directions surrounding the pits. Dislocations are next introduced into the film abruptly at 50nm film thickness and relieve 40% of the film stress by 100nm of growth. Cross-hatch features aligned along the <110> directions accompany the introduction of misfit dislocations. As the dislocation density increases in the film, the coherent island/pit structures begin to disappear and the film morphology is dominated by the evolving cross-hatch. Implications for competitive and/or correlated relaxation of strain by misfit dislocations and surface morphology based upon this data will be discussed. This work demonstrates that combining real-time stress measurements with microscopy enables mechanistic and quantitative understanding of coupled relaxation modes in strained layer heteroepitaxy.

We have followed by Atomic Force Microscopy (AFM) the epitaxial growth of InAs on GaAs(001) starting from the initial formation of a strained two-dimensional wetting layer up to the self-assembled nucleation and growth of 3D nanoparticles. In this work we underline many aspects of the morphology of this system, which substantiate the role either of kinetics on thermodynamics in the process of growth as well as the role of surface instabilities in controlling lateral ordering of the nanoaggregates.

Epitaxial growth of La1-x(SrorCa)xMnO3 manganites (LSMO or LCMO) has been successfully achieved on SrTiO3 (001) and MgO(001) substrates. We report the influence of the deposition conditions, in particular growth rate, annealing and nature of the substrate, on the state of strain in the manganite layers. Fully strained layers are reported at very slow growth rate on SrTiO3 substrates while misfit dislocations are clearly observed in layers grown on MgO and alsoin layers grown on SrTiO3, after annealing at high temperature. Besides, evidence is given for a rhombohedral (respectively orthorhombic) distortion relaxation in LSMO (respectively LCMO) layers. The microstructure of the manganite layers is analyzed taking into account their non-isomorphic growth on the cubic substrates.

An overview is given of the quantum-dot cellular automata (QCA) architecture, along with a summary of experimental demonstrations of QCA devices. QCA is a transistorless computation paradigm that can provide a solution to such challenging issues as device and power density. The basic building blocks of the QCA architecture, such as AND, OR gates and clocked cells have been demonstrated and will be presented here. The quantum dots used in the experiments to date are metal islands that are coupled by capacitors and tunnel junctions, and devices operate only at very low temperatures. For QCA to be used in practical devices, the operating temperature must be raised, and issues such as background charge must be addressed. An introduction will be given to these issues and possible solutions.

Scanning Probe Microscopy (SPM) in situ is used to study the evolution of Ge islands grown by Physical Vapor Deposition on Si(111) 7×7 reconstructed surfaces. Large 3D islands form on the Wetting Layer (WL), with average lateral dimension in the range 200 - 500 nm. The statistical distribution of the island shapes has been analyzed, showing that three types of shapes coexist under certain conditions: strained, partially relaxed and ripened (atoll-like) islands. We measured the contact angles of the island facets, and observed the depletion of the substrate around the ripened islands. These features are attributed to the misfit strain, which is partially relieved by interdiffusion of Si into the Ge layers.

We have studied by Scanning Tunneling Microscopy (STM) the effect of step bunching on Ge/Si(111) epitaxy. We have verified that self-organization of Ge islands is greatly influenced by “step bunching” which arises from the flash-annealing procedure used to reconstruct the Si surface. Two different growth regimes arise: initially islands nucleate and evolve only at steps, up to complete ripening; subsequently the same evolution is observed on flat areas of the sample. The average distance between islands and steps is nearly constant, originating a single row of equally spaced islands, followed by other rows of islands in between. The exploitation of this phenomenon, which is governed by the surface diffusion length of Ge on Si (estimated from our data) and by the terrace width, constitutes one possible path to achieve self-organization of quantum dots.

We demonstrate the use of low-energy electron microscopy (LEEM) as a tool for studying dis-location formation in low-Ge-content SiGe films on Si(001) and silicon-on-insulator. Compared to TEM, sample preparation for LEEM consists only of conventional surface cleaning. Yet, because of its sensitivity to local variations in surface strain on Si(001), LEEM can detect dislocations at the earliest stages of strain relaxation. In identically prepared SiGe films, the typical dislocation extends over the entire viewable region of several hundred microns in SiGe/Si, but is less than 100 microns in SiGe/SOI. In addition, dislocation cross-slip and threading segments are common in SiGe/SOI, but virtually non-existent in SiGe/Si. We have also observed dislocation formation in real-time during high temperature annealing. Preliminary results appear to demonstrate dislocation multiplication and blocking at a perpendicular glide plane. The applicability of LEEM to strain relaxation in other Si-based systems will be discussed.

In this letter we report the transition from self-assembled InAs quantum-wires to quantumdots grown on (100) InP substrates. This transition is obtained when the wires are annealed at the growth temperature. Our results suggest that the quantum-wires are a metastable shape originated from the anisotropic diffusion over the InP buffer layer during the formation of the first InAs monolayer. The wires evolve to a more stable shape (dot) during the annealing. The driving force for the transition is associated with variations in the elastic energy and hence in the chemical potential produced by height fluctuations along the wire. The regions along the wires with no height variations are more stable allowing the formation of complex, self-assembled nanostructures such as dots interconnected by wires.