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Soft x-ray synchrotron-radiation photoemission has been used to study the interface between Ge and SrTiO3(100). Mapping as annealing temperature varies reveals that Ge adsorption is strongly dependent on the SrTiO3 surface's initial composition: the richer the surface is in TiOx, the more Ge wets the STO surface. Time-resolved photoemission has enabled us to monitor Ge bonding and desorption as a function of annealing temperature. As temperature increases, the Ge2+, Ge+, and Ge° component evolution reveals that Ge adatoms, initially bonded to two (or more) oxygen atoms gradually aggregate to form Ge clusters. Finally, the bonding is via a single oxygen atom, suggesting (111 )-oriented clusters. [ABSTRACT FROM AUTHOR]

Epitaxial SrTiO3 (STO)/Si templates open a unique opportunity for the integration of ferroelectric oxides, such as BaTiO3 on silicon and for the realization of new devices exploiting ferroelectricity. STO itself has been shown as ferroelectric at room temperature when deposited in thin layers on Si, while bulk STO is tetragonal and, thus, ferroelectric below 105 K. Here, we demonstrate the coexistence, at room temperature, of strained cubic and tetragonal phases in thin STO/Si layers. The tetragonal STO phase presents a pronounced tetragonality for thickneßes up to 24 ML. Above this thickneß, the strained cubic STO phase starts relaxing while the tetragonal STO phase progreßively transits to cubic STO. The origin of the simultaneous formation of these two phases is analyzed and is attributed to oxygen segregation at the early stages of the growth. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the tensile-strained growth of LaAlO3 on SrTiO3(001) substrates by molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) analysis shows that for thicknesses smaller than 3.8nm, LaAlO3 films grow pseudomorphically. Plastic relaxation starts for films thicker than this critical thickness of 3.8nm. The in- and out-of-plane lattice parameters of the strain-relaxed layers have been measured by ex situ X-ray diffraction (XRD) at grazing angles. The relaxation mechanism is discussed on the basis of these experiments. [Copyright &y& Elsevier]

Abstract: In this work, the potentiality of molecular beam epitaxy techniques to prepare epitaxial lanthanum aluminate (LaAlO3) films on Si(001) is explored. We first demonstrate that the direct growth of LaAlO3 on Si(001) is impossible : amorphous layers are obtained at temperatures below 600°C whereas crystalline layers can be grown at higher temperatures but interfacial reactions leading to silicate formation occur. An interface engineering strategy is then developed to avoid these reactions. SrO and SrTiO3 have been studied as buffer for the subsequent growth of LaAlO3. Only partial LaAlO3 epitaxy is obtained on SrO whereas high quality layers are achieved on SrTiO3. However both SrO and SrTiO3 appear to be unstable with respect of Si at the growth temperature of LaAlO3 (700°C). This leads to the formation of relatively thick amorphous interfacial layers. Despite their instability at high temperature, these processes could be used for the fabrication of twins-free LaAlO3 templates on Si, and for the fabrication of complex oxide/Si heterostructures for various applications. [Copyright &y& Elsevier]

The effects of atomic layer epitaxy (ALE) on the surface morphology, the antiphase boundaries (APBs) and the dislocation density of GaAs grown on exactly (0 0 1)-oriented Ge/Si virtual substrates (VS) have been investigated. ALE combined with the insertion of a low temperature (<500°C) GaAs buffer layer led to a drastic reduction of the APBs and of the dislocation density (<5×105 cm−2), and to a reduction of the surface roughness, thus leading to an important improvement of the photoluminescence yield of InGaAs quantum well (QW) based structures. Light emitting diodes (LEDs) containing InGaAs/GaAs QWs have been realised on exactly (0 0 1)-oriented Ge/Si VS. We observed room-temperature continuous wave operation from these LEDs. [Copyright &y& Elsevier]

We have investigated near infrared p–i–n waveguide photodetectors with Ge/Si self-assembled islands as optical active region. Two samples with Ge islands grown by high-pressure chemical vapor deposition were studied. The first sample consists of a p–i–n junction with islands embedded in a silicon-on-insulator waveguide. The second sample consists of a p–i–n junction with Ge island layers embedded in a Si0.98Ge0.02 alloy waveguide. The spectral responsivity of the detectors was measured with a broad-band source and with semiconductor laser diodes. At room temperature both samples exhibit similar responsivities at 0 V applied bias, with 55 (25) mA/W at 1.3 μm and 0.15 (0.25) mA/W at 1.55 μm for the Si0.98Ge0.02 alloy (silicon-on-insulator) waveguide. The photocurrent results are correlated to the interband absorption and to the photoluminescence of the islands. [Copyright &y& Elsevier]

We have investigated the interband and the intraband absorption properties of Ge/Si self-assembled islands. The investigated structure consists of a p–i–n junction containing Ge/Si self-assembled islands embedded in a Si0.98Ge0.02 waveguiding layer. The variation of transmission associated with carrier injection under forward bias is monitored both in the near-infrared and in the mid-infrared spectral ranges. We show that the carrier injection leads to an absorption resonant at 185 meV which is polarized along the growth axis of the islands. This transition corresponds to an intraband optical transition from the island ground states to the two-dimensional wetting layer states. This assignment is supported by a two-dimensional band structure calculation performed in a 14 band k·p formalism. Meanwhile, the carrier injection leads to a bleaching of the interband absorption. We show that this electroabsorption spectroscopy is a useful tool for the study of self-assembled islands that is complementary of standard photoluminescence, electroluminescence or absorption spectroscopies. [Copyright &y& Elsevier]

The epitaxial growth of InP on (001)-oriented SrTiO3 (STO) is analyzed, and evidence is shown of the formation of twins during the early stages of the growth. A detailed crystallographic description of these twins is presented, and the origin of their formation is discussed. The elevated interface energy characteristic of the InP/STO system, combined with the initial three-dimensional growth of the semiconductor and with the existence of a commensurate heterointerface between STO and the twinned InP variant are identified as the main causes of twin formation during the growth of this highly heterogeneous epitaxial system. [ABSTRACT FROM AUTHOR]

Surface morphology and surface chemistry of epitaxial Ir/SrTiO 3 /Si(001) pseudo-substrates were systematically characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) before and after the Bias Enhanced Nucleation (BEN) of diamond. Based on these characterizations, an ellipsometric model was built taking into account both the roughening of the iridium surface and the formation of domains occurring during the BEN process. Sequential spectroscopic ellipsometry (SE) characterizations allowed estimating the roughening of the iridium surface, the thickness of the amorphous carbon layer and the coverage ratio of domains. By annealing the sample in air after BEN, domains were successfully removed without changing the iridium surface morphology. From these analyses, we demonstrate that SE is enough sensitive and discriminative to monitor both the formation of domains and the roughening of iridium surface, distinctly. SE can then be viewed as a powerful in situ tool of critical importance to study, monitor and finally enhance the diamond nucleation and growth on iridium for various advanced applications. Unlabelled Image • The analysis by ellipsometry proved its sensitivity to the formation of domains. • Sequential analysis by FE-SEM, AFM and XPS allows to build the most relevant ellipsometric model keeping a physical meaning. • This study opens the way to in situ monitoring of early stages of heteroepitaxial diamond growth during BEN. [ABSTRACT FROM AUTHOR]

Advanced characterizations with combined analytical tools were carried out at the different stages of diamond heteroepitaxy on Ir/STO/Si (001) substrates. HRTEM and STEM-EELS revealed the presence of epitaxial nanometric diamond crystals after bias enhanced nucleation. UV Raman allowed estimating the diamond film quality and its strain at the early stages of heteroepitaxial growth. The crystalline structure and the strain within thick heteroepitaxial films were determined by XRD and CL investigations. A CL study of the cross-section provided the mapping of the dislocation network along the growth direction. Measurements performed on lateral Schottky diodes fabricated on a thick diamond film showed an excellent reproducibility on the substrate with a Schottky barrier height in good agreement with those obtained on homoepitaxial layers. Unlabelled Image • Characterizations performed at different stages of heteroepitaxy to relate the electrical properties to the structural ones. • After BEN and without CVD growth, diamond nuclei are evidenced by HRTEM and STEM-EELS • Measurements on lateral Schottky diodes showed an excellent reproducibility on the substrate • Schottky barrier height in good agreement with those obtained on homoepitaxial layers • Heteroepitaxial diamond films show similar boron incorporation efficiency as homoepitaxial ones. [ABSTRACT FROM AUTHOR]

In this work, the desorption of a submonolayer deposit of Ge on SrTiO3(001) is studied by reflection high energy electron diffraction. The results are compared to those of a previous experiment done using soft x-ray photoelectron spectroscopy. Combining these techniques allows for correlating interface chemistry and crystal orientation, and for bringing clarifying elements concerning the competition between (111) and (001) crystal orientation typical for the semiconductor/perovskite epitaxial systems. Despite poor interface matching, (111)-oriented islands are stabilized at the expense of (001)-oriented islands due to the relatively low energy of their free facets. Such 'surface energy driven' crystallographic orientation of the deposit is enhanced by the low adhesion energy characteristic of the Ge/SrTiO3 system. [ABSTRACT FROM AUTHOR]

• Epitaxial BaTiO3 film was grown on buffered silicon wafer by magnetron sputtering. • Low defects BaTiO3 film with (100) out-of-plane orientation were obtained. • BaTiO3 bulk orthorhombic-rhombohedral phase transition is not observed. • A strong influence of the Si substrate (clamping effect) is observed on the film. BaTiO 3 thin films (60 nm-thick) grown on SrTiO 3 /Si templates have been characterized for their structural and electrical properties. The epitaxy of the BaTiO 3 film on silicon was confirmed by X-ray diffraction with good crystallinity. The temperature-dependent structural properties were checked by infrared spectroscopy in absorption mode. The films were found to remain in a single ferroelectric phase over a temperature range from 5 to 385 K. Low-temperature orthorhombic-rhombohedral phase transitions characteristic of bulk BaTiO 3 are absent in the films due to the clamping effect from the Si substrate. [ABSTRACT FROM AUTHOR]

We report here an interface accommodation mechanism observed by using in situ grazing incidence X-ray diffraction in the very early stages of Ge epitaxial growth on SrTiO3. This mechanism leads to interface-localized misfit accommodation and involves two regimes: very early dislocation emergence followed by a damped collective oscillatory lattice parameter evolution. We show that this behavior is compatible with the simplest nonlinear Frenkel-Kontorova model assuming the weak elastic-chain/substrate interaction. [ABSTRACT FROM AUTHOR]

Abstract: We report lateral orderings of III–V nanostructures by the stress field of a periodic dislocation network (DN) shallowly buried and parallel to the surface. These nanostructures consist of thickness modulations of GaAs and InGaAs layers grown by metalorganic vapor-phase epitaxy. We study here two samples for which the growth was performed at two different temperatures. The DNs of these two samples are composed of a one-dimensional network of mixed dislocations and of a one-dimensional network of screw dislocations. These DNs are grain boundaries which form, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals. Both these misorientations are imposed in a controlled manner. The dimensions and orientation of the nanostructures formed at the surfaces of the samples are identical to those of the cells defined by the underlying DNs. Moreover, we demonstrate that by reducing the growth temperature, we favor the formation of less flat nanostructures which approach the aspect of conventional quantum dots. [Copyright &y& Elsevier]

Abstract: Shallowly buried dislocation networks (DNs) can be used to organize the formation of epitaxial semiconductor quantum dots, via the periodic strain field induced at the growth front. We fabricate such DNs by bonding epitaxially two GaAs crystals twisted and tilted with respect to each other and studied them by transmission electron microscopy. The DNs differ greatly from the mere superposition of a square screw DN and of a one-dimensional mixed DN accommodating, respectively twist and tilt. We present a detailed quantitative analysis of these DNs and explain how their properties relate to the crystal disorientations. Dislocation interaction generates a surface pattern of dilatational and compressive strain favouring the ordered growth of (Ga)InAs nanostructures. [Copyright &y& Elsevier]

Tunable diode laser absorption spectroscopy (TDLAS) is used to measure the 6s 2 1 S-5d6p 3 D 0 absorption line profile of a Ba atomic beam produced in a molecular beam epitaxy (MBE) reactor. Despite the noisy MBE environment, a signal-to-noise ratio up to 100 is obtained thanks to a thorough optimization of the measurement setup. A model that realistically describes this absorption profile is presented, taking into account the angular distribution of atomic concentration in the atomic beam as well as the reactor and setup geometry. Our results highlight the influence of the latter on the Doppler line broadening, and allows to estimate the angular distribution of concentration in the beam emitted by the effusion cell. The model can be adapted to estimate the absorption profile for any element in any reactor geometry, making it a useful tool to design MBE dedicated atomic absorption sensors such as optical flux monitoring sensors, or to characterize the beam emitted by any effusion cell.

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5 th and 6 th generation telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO 3 films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO 3 layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO 3 , LiNbO 3 and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO 3 /SrTiO 3 /Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm -1 as expected for the nearly stoichiometric LiNbO 3 . According to the theoretical calculations, 36°Y-X LiNbO 3 films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr 1- x Ti x O 3 films., (© 2024 IOP Publishing Ltd.)

Crystallization temperature is a critical parameter in the stabilization of the metastable ferroelectric phase of HfO 2 . The optimal crystallization temperature used for polycrystalline films is too low to grow epitaxial films. We have developed a new growth strategy, based on the use of an ultrathin seed layer, to obtain high-quality epitaxial films of orthorhombic Hf 0.5 Zr 0.5 O 2 at a lower temperature. The threshold temperature for epitaxy is reduced from about 750 °C to about 550 °C using a seed layer. Epitaxial films deposited at low temperatures exhibit highly enhanced endurance, and films grown at 550-600 °C show high polarization, no wake-up effect, and greatly reduced fatigue and improved endurance in comparison with the films deposited at high temperatures without a seed layer. We propose that the endurance enhancement is due to a positive effect of the defects, which limits the propagation of pinned ferroelectric domains.

The growth of crystalline Li-based oxide thin films on silicon substrates is essential for the integration of next-generation solid-state lithionic and electronic devices including on-chip microbatteries, memristors, and sensors. However, growing crystalline oxides directly on silicon typically requires high temperatures and oxygen partial pressures, which leads to the formation of undesired chemical species at the interface compromising the crystal quality of the films. In this work, we employ a 2 nm gamma-alumina (γ-Al 2 O 3 ) buffer layer on Si substrates in order to grow crystalline thin films of Li 4 Ti 5 O 12 (LTO), a well-known active material for lithium-ion batteries. The ultrathin γ-Al 2 O 3 layer enables the formation of a stable heterostructure with sharp interfaces and drastically improves the LTO crystallographic and electrochemical properties. Long-term galvanostatic cycling of 50 nm LTO films in liquid-based half-cells demonstrates a high capacity retention of 91% after 5000 cycles at 100 C. Rate capability tests showcase a specific charge of 56 mA h g -1 at an exceptional C-rate of 5000 C (15 mA cm -2 ). Moreover, with sub-millisecond current pulse tests, the reported thin-film heterostructure exhibits rapid Li-ion (de)intercalation, which could lead to fast switching timescales in resistive memory devices and electrochemical transistors.

We have studied electronic transport in undoped GaAs/SrTiO 3 core-shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/ μ m. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO 3 shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO 3 shells as gate insulators., (© 2022 IOP Publishing Ltd.)

We theoretically and experimentally demonstrate a strong and tunable optical anisotropy in epitaxially-grown VO 2 thin films. Using a combination of temperature-dependent X-ray diffraction, spectroscopic ellipsometry measurements and first-principle calculations, we reveal that these VO 2 thin films present an ultra-large birefringence (Δ n > 0.9). Furthermore, leveraging the insulator-to-metal transition of VO 2 , we demonstrate a dynamic reconfiguration of optical properties from birefringent to hyperbolic, which are two distinctive regimes of anisotropy. Such a naturally birefringent and dynamically switchable platform paves the way for multi-functional devices exploiting tunable anisotropy and hyperbolic dispersion., (© 2022 the author(s), published by De Gruyter, Berlin/Boston.)

Epitaxial thin films of HfO 2 doped with La have been grown on SrTiO 3 (001) and Si(001), and the impact of the La concentration on the stabilization of the ferroelectric phase has been determined. Films with 2-5 at. % La doping present the least amount of paraelectric monoclinic and cubic phases and exhibit the highest polarization, having a remanent polarization above 20 μC/cm 2 . The dopant concentration results in an important effect on the coercive field, which is reduced with increasing La content. Combined high polarization, high retention, and high endurance of at least 10 10 cycles is obtained in 5 at. % La-doped films., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

Epitaxial PbZr 0.52 Ti 0.48 O 3 (PZT) layers were integrated on Si(001) with single PZT {001} orientation, mosaïcity below 1° and a majority of a-oriented ferroelectric domains (∼65%). Ferroelectric and pyroelectric properties are determined along both the out-of-plane and in-plane directions through parallel-plate capacitor and coplanar interdigital capacitor along the <100> PZT direction. A large anisotropy in these properties is observed. The in-plane remnant polarization (21.5 µC.cm -2 ) is almost twice larger than that measured along the out-of-plane direction (13.5 µC.cm -2 ), in agreement with the domain orientation. Oppositely, the in-plane pyroelectric coefficient (-285 µC.m -2 .K -1 ) is much lower than that measured out-of-plane (-480 µC.m -2 .K -1 ). The pyroelectric anisotropy is explicated in term of degree of structural freedom with temperature. In particular, the low in-plane pyroelectric coefficient is explained by a two-dimensional clamping of the layers on the substrate which induces tensile stress (from thermal expansion), competing with the decreasing tetragonality of a-domains (shortening of the polar c-axis lattice parameter). Temperature-dependent XRD measurements have revealed an increased fraction of a-domains with temperature, attesting the occurrence of a partial two-dimensional clamping. These observed properties are of critical importance for integrated pyroelectric devices.

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water-based chemical-solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n-doping and nanostructuring a BaTiO 3 thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO 3- δ /La 0.7 Sr 0.3 MnO 3 /SrTiO 3 /Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO 3- δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

High-quality thermoelectric La 0.2 Sr 0.8 TiO 3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO 3 (001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10 -4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO 3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately -60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.

We propose an arsenic-capping/decapping method, allowing the growth of an epitaxial shell around the GaAs nanowire (NW) core which is exposed to an ambient atmosphere, and without the introduction of impurities. Self-catalyzed GaAs NW arrays were firstly grown on Si(111) substrates by solid-source molecular beam epitaxy. Aiming for protecting the active surface of the GaAs NW core, the arsenic-capping/decapping method has been applied. To validate the effect of this method, different core/shell NWs have been fabricated. Analyses highlight the benefit of the As capping-decapping method for further epitaxial shell growth: an epitaxial shell with a smooth surface is achieved in the case of As-capped-decapped GaAs NWs, comparable to the in situ grown GaAs/AlGaAs NWs. This As capping method opens a way for the epitaxial growth of heterogeneous material shells such as functional oxides using different reactors.

We have studied the growth of a SrTiO3 shell on self-catalyzed GaAs nanowires grown by vapor-liquid-solid assisted molecular beam epitaxy on Si(111) substrates. To control the growth of the SrTiO3 shell, the GaAs nanowires were protected using an arsenic capping/decapping procedure in order to prevent uncontrolled oxidation and/or contamination of the nanowire facets. Reflection high energy electron diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were performed to determine the structural, chemical, and morphological properties of the heterostructured nanowires. Using adapted oxide growth conditions, it is shown that most of the perovskite structure SrTiO3 shell appears to be oriented with respect to the GaAs lattice. These results are promising for achieving one-dimensional epitaxial semiconductor core/functional oxide shell nanostructures.

Continuous-wave operation, to as high as 7 degrees C, of 1.5-microm optically pumped vertical-external-cavity surface-emitting lasers is reported. The epitaxial structure, monolithically grown on InP by metal-organic chemical vapor deposition, consists of an InAlAs/GaInAlAs Bragg reflector, an InGaAs/InGaAsP active region, and an InP capping layer. The threshold incident pump intensity is <9 kW/cm2.