Diffraction studies for stoichiometry effects in BaTiO3 grown by molecular beam epitaxy on Ge(001)

In this work, we present a systematic study of the effect of the stoichiometry of BaTiO3 (BTO) films grown on the Ge(001) substrate by molecular-beam-epitaxy using different characterization methods relying on beam diffraction, including reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and selected-area electron diffraction in transmission electron microscopy. Surprisingly, over a wide range of [Ba]/[Ti] ratios, as measured by the Rutherford backscattering spectrometry, all the BTO layers exhibit the same epitaxial relationship BTO(001)// Ge(001) with the substrate, describing a 45 degrees lattice rotation of the BTO lattice with respect to the Ge lattice. However, varying the [Ba]/[Ti] ratio does change the diffraction behavior. From RHEED patterns, we can derive that excessive [Ba] and [Ti] generate twinning planes and a rougher surface in the non-stoichiometric BTO layers. XRD allows us to follow the evolution of the lattice constants as a function of the [Ba]/[Ti] ratio, providing an option for tuning the tetragonality of the BTO layer. In addition, we found that the intensity ratio of the 3 lowest-order Bragg peaks I-(001)/I-(002), I-(101)/I-(002), and I-(111)/I-(002) derived from omega - 2 theta scans characteristically depend on the BTO stoichiometry. To explain the relation between observed diffraction patterns and the stoichiometry of the BTO films, we propose a model based on diffraction theory explaining how excess [Ba] or [Ti] in the layer influences the diffraction response. Published by AIP Publishing

Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology

Epitaxial growth of Sb-based heterostructures on highly mismatched substrates for field effect transistor applications

La nécessité de diminuer la consommation à la fois des systèmes autonomes communicants à haute fréquence et des circuits CMOS implique l’utilisation de transistors fonctionnant sous faible tension d’alimentation. Les performances des composants à base de silicium se dégradant rapidement dans ce régime de fonctionnement, les semiconducteurs III-V à faible bande interdite sont aujourd’hui envisagés comme une alternative. Parmi ceux-ci, l’InAs paraît le plus prometteur. Dans ce contexte, ce travail a pour but d’ouvrir la voie à l'utilisation d’un canal à base d'InAs pour les systèmes analogiques et numériques. Plus précisément, nous étudions la croissance par épitaxie par jets moléculaires des hétérostructures InAs/AlSb sur des substrats (001) GaAs et GaP par l’intermédiaire d'une couche tampon Ga(Al)Sb. La microscopie à force atomique, la microscopie électronique en transmission et la diffraction d’électrons de haute énergie sont utilisées afin de mettre en évidence l’influence critique des conditions de croissance sur la nucléation des antimoniures. Cette étude sert de base à l’optimisation de canaux InAs à haute mobilité sur ces deux substrats fortement désadaptés en maille. Les résultats obtenus dans le cas de GaP sont ensuite étendus au cas de pseudo-substrats commerciaux GaP/Si de haute qualité cristalline pour l’intégration de matériaux à base d’InAs sur des substrats Si (001) exactement orientés. Des mobilités électroniques atteignant 28 000 cm-2.V-1.s-1 à 300K et supérieures à 100 000 cm-2.V-1.s-1 à 77K sont démontrées.Low power consumption transistors operating at low supply voltage are highly required for both high frequency autonomous communicating systems and CMOS technology. Since the performances of silicon-based devices are strongly degraded upon low voltage operation, low bandgap III-V semiconductors are now considered as alternative active materials. Among them, one of the best candidates is InAs. Therefore, the present work aims on paving the way to the use of InAs in transistor channels for both high-speed analog and digital applications. We particularly investigate the molecular beam epitaxy growth of InAs/AlSb heterostructures on both (001) GaAs and GaP via an antimonide metamorphic buffer layer. Using atomic force microscopy, transmission electron microscopy and reflection high energy electron diffraction, we first show the critical influence of the growth conditions on the III-Sb nucleation. From this study, we then achieve optimized high mobility InAs layers on these two highly mismatched substrates. The results obtained in the GaP case are extended to commercially available high quality GaP/Si platforms for the integration of InAs-based materials on an exactly oriented (001) Si substrate. State of the art mobility of 28 000 cm-2.V-1.s-1 at 300K and higher than 100 000 cm-2.V-1.s-1 at 77K are demonstrated

Growth mode dependence of misfit dislocation configuration at lattice mismatched III-V semiconductor interfaces

In GaSb/GaAs hetero-epitaxy, it is shown that a two-dimensional growth of GaSb promotes the generation of Lomer dislocations and confines the lattice mismatched strain at the hetero-interface. In contrast, 60° dislocations and closely spaced 60° pairs are predominantly generated in the three-dimensional growth mode. Consequently, a 60° dislocation glide model in combination with surface effects is able to account for the formation of Lomer, 60°, and 60° dislocation pair at high or low mismatch at hetero-interface between zinc-blende materials

Alcohol-Based Digital Etch for III–V Vertical Nanowires With Sub-10 nm Diameter

This letter introduces a novel alcohol-based digital etch technique for III-V FinFET and nanowire MOSFET fabrication. The new technique addresses the limitations of the conventional water-based approach in enabling structures with sub-10-nm 3-D features. Using the same oxidation step, the new technique shows an etch rate of 1 nm/cycle, identical to the conventional approach. Sub-10 nm fins and nanowires with a high mechanical yield have been achieved. InGaAs nanowires with a diameter of 5 nm and an aspect ratio greater than 40 have been demonstrated. The new technique has also been successfully applied to InGaSb-based heterostructures, the first demonstration of digital etch in this material system. Vertical InGaAs nanowire gate-all-around MOSFETs with a subthreshold swing of 70 mV/decade at V DS = 50 mV have been obtained at a nanowire diameter of 40 nm, demonstrating the good interfacial quality that the new technique provides

Importance of the substrate's surface evolution during the MOVPE growth of 2D-transition metal dichalcogenides

In this paper, we explore the impact of changing the growth conditions on the substrate surface during the metal-organic vapor phase epitaxy of 2D-transition metal dichalcogenides. We particularly study the growth of molybdenum disulfide (MoS2) on sapphire substrates at different temperatures. We show that a high temperature leads to a perfect epitaxial alignment of the MoS2 layer with respect to the sapphire substrate underneath, whereas a low temperature growth induces a 30° epitaxial alignment. This behavior is found to be related to the different sapphire top surface re-arrangement under H2S environment at different growth temperatures. Structural analyses conducted on the different samples confirm an improved layer quality at high temperatures. MoS2 channel-based metal-oxide-semiconductor field-effect transistors are fabricated showing improved device performance with channel layers grown at high temperature.status: publishe

Epitaxial registry and crystallinity of MoS2 via molecular beam and metalorganic vapor phase van der Waals epitaxy

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Calibration of the effective tunneling bandgap in GaAsSb/InGaAs for improved TFET performance prediction

© 2016 IEEE. The effective bandgap for heterojunction band-to-band tunneling (Eg,eff) is a crucial design parameter for a heterojunction tunneling FET (TFET). However, there is significant uncertainty on Eg,eff, especially for In0.53Ga0.47As/ GaAs0.5Sb0.5. This makes the prediction of TFET performance difficult. We calibrate Eg,eff by fabricating heterojunction p+/i/n+ diodes, comparing the simulated and the measured current-voltage and capacitance-voltage curves, while taking Eg,eff as a fitting parameter. Our calibration significantly reduces the uncertainty on Eg,eff compared with the range found in the literature. The comparison with the previous work on highly doped heterojunction diodes suggests that dopant-dependent bandgap narrowing reduces Eg,eff and therefore significantly impacts the performance of highly doped TFET.status: publishe

Demonstration of Direction Dependent Conduction through MoS2 Films Prepared by Tunable Mass Transport Fabrication

© The Author(s) 2016. Published by ECS. All rights reserved. This work focuses on the synthesis of MoS2 films grown by sulfurization of pre-deposited ultra-thin Mo films in the presence of a sulfur-containing gaseous precursor. The growth of high-quality TMDs requires careful control of specific thermodynamic and kinetic parameters. A deep understanding of mass transport mechanisms is achieved through a systematic investigation of the structural properties of MoS2 films grown using different sulfurization conditions. Mass transport smoothing is used to reduce surface roughness and fabricate MoS2 films in conformal contact with the underlying substrate. Oriented MoS2 films are obtained by combining kinetically controlled sulfurization conditions and the use of single crystal templates. Good uniformity and excellent conformity of layers over large area has enabled the synthesis of patternedMoS2 films which show anisotropic conduction properties.status: publishe

Material-Selective Doping of 2D TMDC through AlxOy Encapsulation

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and comprehension of its mechanisms. Low-temperature atomic layer deposition of aluminum oxide is found to n-dope MoS2 and ReS2 but not WS2. Based on electrical, optical, and chemical analyses, we propose and validate a hypothesis to explain the doping mechanism. Doping is ascribed to donor states in the band gap of Al x O y , which donate electrons or not, based on the alignment of the electronic bands of the 2D TMDC. Through systematic experimental characterization, incorporation of impurities (e.g., carbon) is identified as the likely cause of such states. By modulating the carbon concentration in the capping oxide, doping can be controlled. Through systematic and comprehensive experimental analysis, this study correlates, for the first time, 2D TMDC doping to the carbon incorporation on dielectric encapsulation layers. We highlight the possibility to engineer dopant layers to control the material selectivity and doping concentration in 2D TMDC.status: publishe