Marilyne Sousa

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.

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.

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.

Optical properties of epitaxial SrHfO3 thin films grown on Si

The perovskite SrHfO3 can be a potential candidate among the high-permittivity materials for gate oxide replacement in future metal-oxide semiconductor field-effect transistor technology. Thin films of SrHfO3 were grown by molecular beam epitaxy and compared with SrTiO3 films. Their optical properties were investigated using spectroscopic ellipsometry and analyzed with respect to their structural properties characterized by x-ray diffractometry, atomic force microscopy, and transmission electron microscopy. A band gap of Eg=6.1±0.1eV is measured optically, which renders this material better suited for gate dielectric applications than SrTiO3 with Eg∼3.4eV. At similar equivalent oxide thickness, SrHfO3 also exhibits lower gate leakage current than SrTiO3 does.

H plasma cleaning and a-Si passivation of GaAs for surface channel device applications

We discuss GaAs(001) cleaning and surface passivation for metal-oxide-semiconductor capacitors and field effect transistors fabricated with HfO2 as high-κ gate oxide. An amorphous-Si passivating layer is deposited by molecular beam deposition on a 2×1 reconstructed GaAs surface cleaned using a remote rf H plasma. The H plasma effectively removes C contaminants from the surface, but a progressive Ga enrichment and the presence of Ga–O bonds are observed. The capacitance-voltage measurements on capacitors under peripheral illumination show inversion, which is an indication of a passivated interface. The Dit distribution as function of energy in the band gap is extracted by using the conductance technique at high and low temperatures and is reported for HfO2/a-Si gate stacks on H-cleaned GaAs. The observed Dit distribution is asymmetric. Values as low as 7×1011 eV−1 cm−1 are found in the upper half of the band gap. One clear peak at 0.7 eV and a tail at 0.2 eV above the valence band maximum, which can be par...

An integration path for gate-first UTB III-V-on-insulator MOSFETs with silicon, using direct wafer bonding and donor wafer recycling

In this work we demonstrate for the first time that the excellent thermal stability of ultra-thin body (UTB) III-V heterostructures on silicon provides a path for the cointegration of self-aligned In0.53Ga0.47As MOSFETs with silicon. We first demonstrate that the transfer of high-quality InGaAs / InAlAs heterostructures (tch <; 10 nm) can be achieved by direct wafer bonding and hydrogen-induced thermal splitting, and that the donor wafer can be recycled for a cost-effective process. The thermal stability of the bonded layer enables to integrate UTB III-V MOSFETs at 500 nm pitch using a gate-first flow featuring raised source/drain (S/D) grown at 600oC. The expected benefit of an UTB structure is benchmarked by comparing sub-threshold slope (SS) and drain-induced-barrier-lowering (DIBL) against state-of-the-art III-V-o-I or Tri-Gate FET data.

Thermal stability of the SrTiO3∕(Ba,Sr)O stacks epitaxially grown on Si

The growth of epitaxial SrTiO3 on silicon relies on the preparation of a template layer consisting of a mixture of barium oxide and strontium oxide, (Ba,Sr)O. In this letter, the limited thermal stability of this template layer is demonstrated. X-ray photoemission spectroscopy measurements reveal that both SrTiO3∕(Ba,Sr)O and (Ba,Sr)O∕Si interfaces are susceptible to chemical reactions upon thermal treatment to an extent that is correlated with the thermal budget. These results have strong implications on the overall viability of (Ba,Sr)O as template for the growth of crystalline oxides on Si.

Co-integration of InGaAs n- and SiGe p-MOSFETs into digital CMOS circuits using hybrid dual-channel ETXOI substrates

We demonstrate for the first time a dense co-integration of co-planar nano-scaled SiGe p-FETs and InGaAs n-FETs. This result is based on hybrid substrates containing extremely-thin SiGe and InGaAs layers on insulators (ETXOI). We first show that such hybrid substrates can be fabricated by direct wafer bonding with stacked high-mobility layers thinner than 8nm. A process flow is presented that allows us to fabricate n- and p-channel field effect transistors with ultra-thin body and BOX (UTBB-FET) on the same wafer. Gate lengths down to 40nm produced at sub-μm gate-pitch are achieved. Working CMOS inverters are obtained using a common front-end which confirms the viability of this integration scheme for hybrid high-mobility dual-channel CMOS. We also highlight that back-biasing technique for Vt tuning can still be used despite the dualchannel structure, as implemented in standard ETSOI circuits.

Controlling tetragonality and crystalline orientation in BaTiO3 nano-layers grown on Si

A hybrid growth process was developed in order to epitaxially integrate nano-layers of the multi-functional perovskite BaTiO₃ onto Si(001) substrates. In particular, we combined molecular beam epitaxy (MBE) with radio-frequency sputtering. Due to its strong influence on the functional properties, the crystalline structure of the layers was thoroughly investigated throughout our study. MBE-grown seed layers are tetragonal and c-axis oriented up to a thickness of 20 nm. A transition into a-axis films is visible for thicker layers. When the seed layer thickness exceeds 6 nm, subsequently sputtered BaTiO₃ films are epitaxial. However, their crystalline structure, their orientation with respect to the substrate, and their morphology are strongly dependent on the deposition and post-deposition thermal budget. Consistently with their crystalline symmetry, thin MBE BaTiO₃ films are piezo- and ferroelectric with a spontaneous polarization perpendicular to the surface. Also for thick films, the functional response, as determined via piezo-force microscopy, is in good agreement with the structural properties.

Nanoelectrical analysis of single molecules and atomic-scale materials at the solid/liquid interface

Evaluating the built-in functionality of nanomaterials under practical conditions is central for their proposed integration as active components in next-generation electronics. Low-dimensional materials from single atoms to molecules have been consistently resolved and manipulated under ultrahigh vacuum at low temperatures. At room temperature, atomic-scale imaging has also been performed by probing materials at the solid/liquid interface. We exploit this electrical interface to develop a robust electronic decoupling platform that provides precise information on molecular energy levels recorded using in situ scanning tunnelling microscopy/spectroscopy with high spatial and energy resolution in a high-density liquid environment. Our experimental findings, supported by ab initio electronic structure calculations and atomic-scale molecular dynamics simulations, reveal direct mapping of single-molecule structure and resonance states at the solid/liquid interface. We further extend this approach to resolve the electronic structure of graphene monolayers at atomic length scales under standard room-temperature operating conditions.