Jean Fompeyrine

Doubling the critical temperature of La1.9Sr0.1CuO4 using epitaxial strain

The discovery of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature (T c) of 133 K was observed in the HgBa2Ca2Cu3O8+δ (ref. 2), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T c — up to ∼164 K in the case of HgBa2Ca2Cu3O8+δ (ref. 3). Here we show, by analysing the uniaxial strain and pressure derivatives of T c, that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La1.9Sr0.1CuO4, where we obtain a critical temperature of 49 K in strained single-crystal thin films — roughly double the bulk value of 25 K. Furthermore, the resistive behaviour at low temperatures (but above T c) of the strained samples changes markedly, going from insulating to metallic.

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic–antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.

A strong electro-optically active lead-free ferroelectric integrated on silicon

The development of silicon photonics could greatly benefit from the linear electro-optical properties, absent in bulk silicon, of ferroelectric oxides, as a novel way to seamlessly connect the electrical and optical domain. Of all oxides, barium titanate exhibits one of the largest linear electro-optical coefficients, which has however not yet been explored for thin films on silicon. Here we report on the electro-optical properties of thin barium titanate films epitaxially grown on silicon substrates. We extract a large effective Pockels coefficient of r(eff) = 148 pm V(-1), which is five times larger than in the current standard material for electro-optical devices, lithium niobate. We also reveal the tensor nature of the electro-optical properties, as necessary for properly designing future devices, and furthermore unambiguously demonstrate the presence of ferroelectricity. The integration of electro-optical active films on silicon could pave the way towards power-efficient, ultra-compact integrated devices, such as modulators, tuning elements and bistable switches.

Inversion mode n-channel GaAs field effect transistor with high-k/metal gate

Highly effective passivation of GaAs surface is achieved by a thin amorphous Si (a-Si) cap, deposited by plasma enhanced chemical vapor deposition method. Capacitance voltage measurements show that carrier accumulation or inversion layer is readily formed in response to an applied electrical field when GaAs is passivated with a-Si. High performance inversion mode n-channel GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated with an a-Si/high-k/metal gate stack. Drain current in saturation region of 220mA∕mm with a mobility of 885cm2∕Vs were obtained at a gate overdrive voltage of 3.25V in MOSFETs with 5μm gate length.

High-K dielectrics for the gate stack

This article gives an overview of recent developments in the search for the next-generation dielectric for the complementary metal-oxide semiconductor gate stack. After introducing the main quantities of interest, the paper concentrates on a figure of merit that connects two main properties of the gate stack, namely, the leakage current and the capacitance. This is done for single layers as well as for bilayers consisting of interfacial SiOx and a high-K dielectric. In the case of the bilayers, the impact of the interfacial layer SiOx is enormous, reducing the leakage current by an order of magnitude per monolayer. This extreme dependance makes a good correlation between the leakage and the structural parameters nearly impossible. This is illustrated using numerical examples designed to help the reader evaluate the orders of magnitude involved. The origin of the interfacial layer is traced back by means of thermodynamic considerations. As the estimates put forward in the literature do not correspond to th...

Field-effect transistors with SrHfO3 as gate oxide

The authors demonstrate that the compound SrHfO3 grown epitaxially on Si(100) by molecular-beam epitaxy is a potential gate dielectric to fabricate n- and p-metal-oxide semiconductor field-effect transistors with equivalent oxide thickness (EOT) below 1nm. The electrical properties on capacitors and transistors show low gate leakage and good capacitance and I-V output characteristics. The lower electron and hole mobilities, which are strongly limited by charge trapping, nevertheless fit well with the general trend of channel mobility reduction with decreasing EOT.

Very high-κ ZrO2 with La2O3 (LaGeOx) passivating interfacial layers on germanium substrates

Thin La2O3 (LaGeOx) passivating layers combined with ZrO2 caps form a chemically stable bilayer gate stack on Ge with good electrical properties. The most important observation is that a higher-κ tetragonal zirconia phase coexists with the most commonly observed monoclinic, increasing the κ value of the oxide to about 32, thus benefiting the measured stack equivalent oxide thickness. This indicates that the ZrO2/La2O3 combination could be a promising candidate gate stack for Ge metal-oxide-semiconductor devices in terms of scalability.

Interface formation and defect structures in epitaxial La2Zr2O7 thin films on (111) Si

We have studied the growth of epitaxial La2Zr2O7 thin films on (111) Si. Although the interface structure can be strongly affected by the Si oxidation during the deposition process, epitaxial growth of La2Zr2O7 was obtained. A detailed study by means of transmission electron microscopy reveals two types of structures (pyrochlore and fluorite) with the same average chemical composition but strong differences in reactivity and interface formation. The structural complexity of the ordered pyrochlore structure seems to prevent excess oxygen diffusion and interfacial SiO2 formation.

Germanium-induced stabilization of a very high-k zirconia phase in ZrO2/GeO2 gate stacks

Electrical data on ZrO2/GeO2 stacks prepared by atomic oxygen beam deposition on Ge at 225 °C reveal a relatively weak dependence of the stack equivalent oxide thickness upon the ZrO2 thickness. This trend points to a very high zirconia dielectric permittivity (k) value which is estimated to be around 44. This is indicative of zirconia crystallization into a tetragonal phase which is also supported by x-ray diffraction data. X-ray photoelectron spectroscopy analysis is in line with the assumption that due to a finite GeO2 decomposition, Ge is incorporated into the growing ZrO2, thus, stabilizing the high-k tetragonal phase.

Evidence of electron and hole inversion in GaAs metal-oxide-semiconductor capacitors with HfO2 gate dielectrics and α-Si∕SiO2 interlayers

Evidence of inversion in GaAs metal-oxide-semiconductor capacitors with HfO2 gate dielectrics and α-Si∕SiO2 interlayers is reported. Capacitors formed on n-GaAs with atomic layer-deposited HfO2 displayed C-V characteristics with minimum Dit of 7×1011cm−2∕eV, while capacitors with molecular beam epitaxy-deposited HfO2 on p-GaAs had Dit=3×1012cm−2∕eV. Lateral charge transport was confirmed using illuminated C-V measurements on capacitors fabricated with thick Al electrodes. Under these conditions, capacitors on n-GaAs (p-GaAs) showed “low-frequency” C-V behavior, indicated by a sharp capacitance increase and saturation at negative (positive) gate bias, confirming the presence of mobile charge at the semiconductor/dielectric interface.