Maya Marinova

Ferroelectric control of a Mott insulator.

The electric field control of functional properties is an important goal in oxide-based electronics. To endow devices with memory, ferroelectric gating is interesting, but usually weak compared to volatile electrolyte gating. Here, we report a very large ferroelectric field-effect in perovskite heterostructures combining the Mott insulator CaMnO3 and the ferroelectric BiFeO3 in its “supertetragonal” phase. Upon polarization reversal of the BiFeO3 gate, the CaMnO3 channel resistance shows a fourfold variation around room temperature, and a tenfold change at ~200 K. This is accompanied by a carrier density modulation exceeding one order of magnitude. We have analyzed the results for various CaMnO3 thicknesses and explain them by the electrostatic doping of the CaMnO3 layer and the presence of a fixed dipole at the CaMnO3/BiFeO3 interface. Our results suggest the relevance of ferroelectric gates to control orbital- or spin-ordered phases, ubiquitous in Mott systems, and pave the way toward efficient Mott-tronics devices.

Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator.

The electric field control of functional properties is a crucial goal in oxide-based electronics. Nonvolatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here, we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca(1-x)Ce(x)MnO3, thus providing insight on how interface-engineering can enhance these switching effects.

Sublimation Growth and Structural Characterization of 3C-SiC on Hexagonal and Cubic SiC Seeds

Epitaxial growth of cubic silicon carbide on 6H-SiC substrates, and 6H-SiC substrates with (111) 3C-SiC buffer layer, deposited by vapour liquid solid mechanism, was compared. The morphological details of the grown layers were studied by optical microscopy and their microstructure by transmission electron microscopy. The influence of the substrate on the nucleation of 3C-SiC, the initial homoepitaxial 6H-SiC nucleation before 3C-SiC as well as the formation of defects, are discussed.

Structural and optical characterization of the formation of β-FeSi2 nanocrystallites in an n-type (100) Si matrix

Ion-beam synthesized (IBS) samples, prepared with a low dose of iron ions and subjected to rapid thermal annealing (RTA) were studied. The samples were characterized with cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM) and far infrared transmittance (FIRT) spectroscopy. The formation of ?-FeSi2 nanocrystallites, with various shapes and sizes, in the Si matrix was revealed. The optical constants dispersions of the samples were obtained from the reflectance (R) and transmittance (T) spectra, taken between 0.38 and 1.2?eV. From these dispersions, the energy band diagram of the interface ?-FeSi2/Si was determined, and compared to those reported by other authors.

6H-Type Zigzag Faults in Low-Doped 4H-SiC Epitaxial Layers

A new type of 6H zigzag faults has been identified from high resolution transmission electron microscopy (HRTEM) measurements performed on low-doped 4H-SiC homoepitaxial layer grown on off-axis substrates in a hot-wall CVD reactor. They are made of half unit cells of 6H with corresponding low temperature photoluminescence (LTPL) response ranging from about 3 eV to 2.5 eV at liquid helium temperature.

Searching for Ge Clusters inside 3C-SiC Layers Grown by Vapor-Liquid-Solid Mechanism on 6H-SiC Substrates

The use of Ge very rich Si-Ge liquid phase during the heteroepitaxial growth of 3C-SiC on Si-face, on-axis 6H-SiC(0001) substrate by vapour-liquid-solid mechanism leads to the formation of Ge based precipitates inside the 3C layer. These Ge based features are investigated by TEM and atomic models of the Ge clustering are proposed by means of high resolution TEM image simulation. Conventional TEM shows only a few small precipitates sparsely distributed near the interface, as well as dislocations and stacking faults starting from the interface in an almost regular manner. High resolution TEM shows fine structural imperfections in the form of Guinier Preston zones also near the interface. It is concluded that the high Ge content creates an enlargement of the SiC lattice leading to a misfit with the substrate. This could be the driving force for the formation of all the observed features.

On the Mechanism of Twin Boundary Elimination in 3C-SiC(111) Heteroepitaxial Layers on α-SiC Substrates

This paper deals with the formation and propagation of twin boundaries (TBs) inside 3C-SiC layers grown heteroepitaxially on -SiC substrate. The equivalent probability of nucleating 60° rotated 3C islands on such substrate lead to the systematic formation of TB upon coalescence of these islands. Elimination of these defects should occur by bending of the propagation direction. Bending through incoherent TBs is usually encountered during both VLS and CVD growth and it generates crystalline defects due to high built-in energy. One would prefer coherent TBs, formed by two-by-two annihilation of neighbouring TBs, which do not form new defect except microtwin inclusion at the interface. Such TB annihilation seems to be a specificity of growth by VLS mechanism. The mechanism of such bending is discussed

Influence of the C/Si Ratio on the Dopant Concentration and Defects in CVD Grown 3C‐SiC Homoepitaxial Layers

The study reports on structural and optical investigations of (111) 3C‐SiC layers grown homoepitaxially by Chemical Vapour Deposition (CVD) at different C/Si ratios. The seeds were 3C‐SiC layers grown on on‐axis Si‐face (0001) 6H‐SiC substrates by Vapour‐Liquid‐Solid (VLS) mechanism in Si‐Ge or Si‐Sn melt. Transmission Electron Microscopy (TEM) investigation showed that the main defects reaching the VLS seed surface are dislocations, stacking faults (SFs) and twin boundaries. In the CVD layer the defect density is reduced compared to the VLS layer at low C/Si ratio (in the range of 1–3). The low‐temperature Photoluminescence (LTPL) spectra of all the layers display a defect related peak at 1.98 eV attributed to DI defect, while G bands are observed in the range of 1.82–1.92 eV range at C/Si ratio of 7 and 10. Common defect in CVD layers was multiple twin complex, which appears as a rule at the vicinity of the twinned domains. This multiple twin complex consists of four twins bound by two fully symmetrical...

Defects in (111) 3C‐SiC layers grown at different temperatures by VLS and CVD on 6H‐SiC substrates

In the present work homoepitaxial (111) 3C‐SiC layers, grown by Chemical Vapour Deposition (CVD) on top of 3C‐SiC seeds grown by the Vapour Liquid Solid (VLS) mechanism on Si‐face on‐axis (0001) 6H‐SiC substrates, are investigated by means of Transmission Electron Microscopy (TEM). The CVD process was performed at constant C/Si ratio and the growth temperature was varied from 1450° C to 1650° C. The main defects in the VLS seeds which are directly nucleated on the interface with the 6H‐SiC substrate are microtwins, dislocations and stacking faults (SFs). Within the CVD layers, the main defects appearing are also SFs together with multiple twin complexes which consist of two Σ3 and one Σ9 boundaries. Systematic analysis revealed that the multiple twin complexes disappear with increasing temperature while the trend for SFs is more difficult to follow. Some 3C to 6H polytypic transformation was found to occur at CVD growth temperature above 1550° C.

On the Twin Boundary Propagation in (111) 3C-SiC Layers

The present study reports on the propagation of twin boundaries in (111) 3C-SiC by means of conventional (CTEM) and high resolution transmission electron microscopy (HRTEM). The investigated 3C-SiC layers were homoepitaxially grown by Chemical Vapour Deposition (CVD) on layers previously grown by Vapor Liquid Solid (VLS) mechanism on 6H-SiC substrates. At the initial stages of growth the usual twin boundary that occurs is an incoherent {-211} Σ3 one. It transforms to more energetically favorable cases by several ways: (i) The initial {-211} boundary turns 90º, to a fully coherent (111) interface, forming microtwins; (ii) A step-like interface occurs with facets along the (111) and the {-211} planes; (iii) It transforms in a fourfold twin complex propagating to the surface.