III-N ring microlasers on a silicon substrate with InGaN/GaN active layers emitting near 420 nm were investigated. The growth conditions and fabrication steps were optimized to realize stable lasing under optical pumping in cavities with a diameter of 6-10 mu m. Chemically sensitive transmission electron microscopy images indicate that InGaN layers present in form of isolated islands. Between these InGaN islands, large areas of GaN are visible, forming barriers to lateral transport of free charge carriers in the active region and preventing their nonradiative surface recombination. For the first time, temperature stability of InGaN/GaN microring lasers characteristics are studied and lasing up to 100 degrees Celsius is demonstrated with the wavelength shift less than 1 nm. At room temperature, the threshold pump power is as low as 220 kW/cm(2). The obtained results significantly expand the potential areas of application of III-N microlasers.

The high temperature behavior of thin metal films (tungsten and titanium) confined between two off-axis single crystal SiC substrates is investigated. Through the application of transmission and scanning transmission electron microscopy, scanning electron microscopy, and X-ray scattering techniques, we examine the phase and morphology changes induced by high temperature annealing in thin layers consisting of these materials, as well as at their interfaces with SiC. Upon high-temperature annealing, a uniform and continuous W film formed by low-temperature deposition undergoes a transition to an array of discontinuous domains surrounded by a direct SiC/SiC interface. In contrast, a Ti film remains continuous with a strong thickness alteration. In parallel to stepbunching process of the internal SiC surfaces, both materials transform into new crystalline phases which contain Si and/or C atoms and achieve an epitaxial relationship with the SiC structures. The experimental findings are discussed in terms of dewetting phenomena and analyzed in light of potential chemical and structural reactions that may occur during interface reconstructions.

Despite it being a popular sample preparation technique, the optoelectronic effects of thinning bulk samples into lamella using a Ga-based focused ion beam (FIB), as is commonly performed for studies in a transmission electron microscope, have been seldom studied systematically. In this work, we confront this using correlative cathodoluminescence spectroscopy to investigate the optical properties of high In content c-plane InGaN/GaN quantum wells (QWs), fabricated including a growth-interrupting step that forms regions of quantum disk InGaN islands that behave as localized emitters and, furthermore, reduce the strain-induced quantum Stark effect present in the majority of such heterostructures. Using picosecond electrostatically beam-blanked electron pulses, we measured the decay transients as a function of position and wavelength. The sample was studied before and after undergoing preparation as a lamella, demonstrating that FIB preparation affects both the spectral and temporal luminescence properties despite measures undertaken to protect the sample during fabrication. Non-radiative defects introduced by the ion beam quenched the luminescence as well as reduced the lifetime of emission, with the QW luminescence component particularly affected due to the emission being weakly localized and hence allowing carriers to migrate to defect areas. These findings underscore the importance of correlating bulk and lamella properties to accurately interpret optical measurements.

Optical manipulation of ferroelectric polarization is a promising method for potentially ultrafast and remote polarization switching without electrodes. Here, we report optical ferroelastic and ferroelectric switching by UV irradiation in epitaxial BaTiO3 thin films grown on a SrTiO3-buffered Si substrate. The pristine BaTiO3 film is in the tetragonal ferroelectric phase with both in-plane and out-of-plane ferroelectric polarization. After irradiation by a 325 nm UV laser, the polarization is mainly out-of-plane indicating ferroelastic switching. Moreover, all initial downward polarized domains have switched to upward, thus showing ferroelectric 180°-domain switching. After irradiation the film exhibits mainly a single up-oriented polarization and as a result, the irradiated regions exhibit an enhanced piezoelectric response. We propose that the observed ferroelastic and ferroelectric switching is triggered by additional strain/stress fields generated by internal electric fields arising mainly from the spatial charge carrier separation after photoexcitation. These strain/stress fields add up to the Vegard strain field and to local heating, which induce defect motion and a final state with full strain relaxation. This optical switching enables remote manipulation of ferroelastic and ferroelectric domains in BaTiO3 films on silicon. Moreover, UV illumination appears as a potential postdeposition treatment to heal defects and obtain a strain-free epitaxial layer.

The miniaturization of ferroelectrics with lateral size reduction is crucial for technological advancement but requires an understanding of the fundamental behavior of ferroelectrics at the nanoscale. While much attention has been focused on vertical scaling of perovskite ferroelectrics with thickness reduction, lateral scaling remains less explored. In this study, ferroelectricity is investigated in 20 nm thick single-crystalline BaTiO3 nanodisks with a diameter ranging from approximate to 400 down to 100 nm. They are fabricated by Ne ion milling of a 20 nm BaTiO3 film epitaxially grown on SrTiO3-buffered silicon. The nanodisks are ferroelectric with a Curie temperature in the range 230-270 degrees C as determined by temperature-dependent piezoresponse force microscopy. In 100 nm-diameter nanodisks, the vertical polarization component adopts three distinct patterns in the pristine state, aligning with theoretical predictions. The most prevalent pattern features a uniformly up-oriented vertical component. The rotational invariance of these domain patterns in the plane suggests a combination of center-type and flux-closure domains. Additionally, the up polarization can be switched progressively to down polarization upon application of a pulsed bias of increasing time width. The control of the polarization in nanostructures and of their progressive switching is of particular interest for memory applications.

The realization of perovskite oxide nanostructures with controlled shape and dimensions remains a challenge. Here, we investigate the use of helium and neon focused ion beam (FIB) milling in an ion microscope to fabricate BaTiO3 nanopillars of sub-500 nm in diameter starting from BaTiO3 (001) single crystals. Irradiation of BaTiO3 with He ions induces the formation of nanobubbles inside the material, eventually leading to surface swelling and blistering. Ne-FIB is shown to be suitable for milling without inducing surface swelling. The resulting structures are defect-free single crystal nanopillars, which are enveloped, on the top and lateral sidewalls, by a point defect-rich crystalline region and an outer Ne-rich amorphous layer. The amorphous layer can be selectively etched by dipping in diluted HF. The geometry and beam-induced damage of the milled nanopillars depend strongly on the patterning parameters and can be well controlled. Ne ion milling is shown to be an effective method to rapidly prototype BaTiO3 crystalline nanostructures.

AbstractA fascinating aspect of nanoscale ferroelectric materials is the emergence of topological polar textures, which include various complex and stable polarization configurations. The manipulation of such topological textures through external stimuli like electric fields holds promise for advanced nanoelectronics applications. There are, however, several challenges to reach potential applications, among which reliably creating and controlling these textures at the nanoscale on silicon, and with lead-free compounds. We report the realization of epitaxial BaTiO3 nanoislands on silicon, with a lateral size as small as 30-60 nm, and demonstrate stable center down-convergent polarization domains that can be reversibly switched by an electric field to center up-divergent domains. Piezoresponse force microscopy data reconstruction and phase field modeling give insight into the 3D patterns. The trapezoidal-shape nanoislands give rise to center down-convergent lateral swirling polarization component with respect to the nanoisland axis, which prevents the formation of bound charges on the side walls, therefore minimizing depolarization fields. The texture resembles a swirling vortex of liquid flowing into a narrowing funnel. Chirality emerges from the whirling polarization configurations. The ability to create and electrically manipulate chiral whirling polar textures in BaTiO3 nanostructures grown monolithically on silicon holds promise for applications in future topological nanoelectronics.

The temperature-induced phase and morphology changes of a thin layer sandwiched between two substrates which it partially wets are investigated using transmission electron microscopy, scanning electron microscopy, and x-ray scattering techniques. For this, SiC wafers were bonded with Si layers of various thicknesses and annealed at temperatures below and above the Si melting point. Below the melting point of Si, solid-state dewetting occurs. It starts with the heterogeneous nucleation of pits at the Si/SiC interfaces and progresses to their partial transformation into voids crossing the whole film. The further growth of voids is accompanied with an increase in the Si film thickness. Final equilibrium is shown to be impacted by Si crystallographic state evolution. Above the Si melting temperature, liquid Si drives SiC interfaces step bunching. When high steps and large terraces are formed over the two SiC surfaces, Si is shown to be trapped within quasi-closed pockets. Eventually, the interface locally closes around these Si inclusions with the creation of SiC/SiC direct contacts. The influence of both annealing temperatures and Si film thickness on all these processes is discussed.

We exploit the three-dimensional (3D) character of the strain field created around InGaN islands formed within the multilayer structures spaced by a less than 1-nm-thick GaN layer for the creation of spatially correlated electronically coupled quantum dots (QDs). The laterally inhomogeneous vertical out-diffusion of In atoms during growth interruption is the basic mechanism for the formation of InGaN islands within as-deposited 2D layers. An anisotropic 3D strain field created in the first layer is sufficient to justify the vertical correlation of the islands formed in the upper layers spaced by a sufficiently thin GaN layer. When the thickness of a GaN spacer exceeds 1 nm, QDs from different layers under the same growth conditions emit independently and in the same wavelength range. When extremely thin (less than 1 nm), a GaN spacer is formed solely by applying short GI, and a double wavelength emission in the blue and green spectral ranges evidences the electromechanical coupling. With k→·p→ calculations including electromechanical fields, we model the optoelectronic properties of a structure with three InGaN lens-shaped QDs embedded in a GaN matrix, with three different configurations of In content. The profiles of the band structures are strongly dependent on the In content arrangement, and the quantum-confined Stark effect is significantly reduced in a structure with an increasing gradient of In content from the top to the bottom QD. This configuration exhibits carrier tunneling through the QDs, an increase of wave functions overlap, and evidence emerges of three distinct peaks in the spectral range.

Silicon carbide (SiC) is a very promising material for power electronics thanks to its wide band gap and high thermal conductivity. Stacking of SiC substrates of different crystalline quality and/or doping levels through wafer bonding is of particular interest for applications. Although a direct SiC/SiC bonding may result in an unstable interface, a layer of intermediate material playing a role of a “glue” can be sandwiched between the SiC substrates to ensure their good attachment. However, the processing of the power electronics devices at a high temperature may degrade the properties of this material which evolution under high temperature annealing must be studied in advance. In this work, we exploited the possibility to use silicon (Si) as a bonding material. We studied the structure evolution of a nanometer-thick polycrystalline Si layer sandwiched between two monocrystalline SiC substrates with a high temperature annealing. We focused our attention on understanding the impact of as-bonded Si layer thickness and the temperature of annealing on the eventual Si layer morphology. Tests were conducted for Si films of as-bonded thickness of 8 nm, 20 nm and 40 nm at temperatures ranging from 950°C to 1400°C, i.e. below Si melting temperature of 1414°C, and above it up to 1700°C. The experiments were realized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and high-resolution X-ray diffraction techniques. Cross-sectional TEM images were used to describe the structure and morphology of the interface. SEM was used to observe the inner surface of one of the SiC substrates once the other was removed by a cleavage. X-ray measurements were carried out to determine Si film thickness and texture as well as the strain in Si and SiC. We show that annealing at temperatures ranging from 950°C to 1400°C results in pitting, i.e. creation of voids, at the Si/SiC interfaces which are observed for any studied as-bonded thickness of a Si layer. We attribute this effect to a solid state dewetting of Si from SiC. This dewetting effect evolves into complete piercing of the silicon film and formation of cylindrical voids within the Si layer which grow in diameter with annealing temperature. We also observe a thickening of the silicon layer. The driving force for a solid state dewetting will be discussed in terms of SiC/Si interface energy, vacancies precipitation and Si atoms diffusion. When annealed to relatively higher temperatures allowing for Si melting followed by its solidification, we put into evidence the formation of step bunching on the internal surfaces of both SiC substrates. Regarding Si, it is concentrated along the SiC steps. The mechanism of the reconstruction of SiC internal surfaces resulting in the creation of step bunching will be discussed in relation to Si liquid state dewetting, carbon atoms dissolution, recondensation and migration within liquid Si. The effect of as-bonded silicon film thickness on the Si dewetting process is not pronounced after annealing at 950°C: all samples contain pits of a similar size and density. When the annealing temperature is slightly increased above 1000°C, the pits in the Si films of a lower initial thickness transform into cylindrical voids piercing the layers. The thicker layers contain both pits and cylindrical voids. At a high enough temperature of 1400°C only cylindrical voids were observed in all samples. The thicker layers present the lowest densities of voids. For the annealing at temperatures higher than 1400°C, the as-bonded thickness of a Si layer had a direct impact on the height of the steps formed on the internal surfaces of the SiC substrates. The eventual steps were higher the thicker the Si layer was used for bonding. The role of the diffusion and interaction of Si and C atoms and vacancies during annealing will be discussed in relation to the dynamics of solid and liquid state Si dewetting, size and density of pits and cylindrical voids and eventual Si layer morphology. Figure 1, on the left, SEM images showing the evolution of Si solid state dewetting with annealing temperature, from pitting to the formation of cylindrical voids. On the right, SEM and cross-sectional TEM images showing the step bunching on the SiC internal surfaces and the Si accumulation at the steps obtained after Si liquid state dewetting. Figure 1

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

A method for the reciprocal space treatment of high-resolution transmission electron microscopy (HR-TEM) and high-resolution scanning transmission electron microscopy (HR-STEM) images has been developed. Named "Absolute strain" (AbStrain), it allows for quantification and mapping of interplanar distances and angles, displacement fields and strain tensor components with reference to a user-defined Bravais lattice and with their corrections from the image distortions specific to HR-TEM and HR-STEM imaging. We provide the corresponding mathematical formalism. AbStrain goes beyond the restriction of the existing method known as geometric phase analysis by enabling direct analysis of the area of interest without the need for reference lattice fringes of a similar crystal structure on the same field of view. In addition, for the case of a crystal composed of two or more types of atoms, each with its own sub-structure constraint, we developed a method named "Relative displacement" for extracting sub-lattice fringes associated to one type of atom and measuring atomic columns displacements associated to each sub-structure with reference to a Bravais lattice or to another sub-structure. The successful application of AbStrain and Relative displacement to HR-STEM images of functional oxide ferroelectric heterostructures is demonstrated.

We develop a model of an epitaxial self-organized InGaAs quantum dot buried in GaAs, which takes into account experimentally determined indium distribution inside the QD, its geometry and crystallography. The problem of solid mechanics was solved to determine the stress-strain field. Then, the parameters of the electron and hole ground states were evaluated by solving the problem of the quantum mechanics on the same mesh. The results of calculations appeared to be reasonably well consistent with experimentally recorded optical emission spectra for the QDs in the same sample. The experimentally-verified modeling reveals a bagel-like shape of the hole wave function at the ground state, which should considerably impact the optical and magnetic properties of the QDs. Such shape of the wave function is beyond the predictions of simplified models with uniform indium distribution.

Light-emitting III–N heterostructures are grown by gas-phase epitaxy from organometallic compounds on SiC/Si (111) templates (substrates) formed using matched atomic substitution. Investigations of the optical and structural properties of heterostructures are carried out in order to reveal the formation of defects in the structures. It is shown that such heterostructures exhibit specific features of the growth of the (Al,Ga)N buffer layer associated with the presence of pores in Si under the SiC/Si interface. The use of an optimized buffer layer design makes it possible to significantly reduce the dislocation density and form an active region with good structural quality.