Nucleation Mechanism during WS2 Plasma Enhanced Atomic Layer Deposition on Amorphous Al2O3 and Sapphire Substrates

The structure, crystallinity and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in absence of a template, are polycrystalline or amorphous. Little is known about their nucleation mechanisms. Therefore, we investigate the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma and H2S at 300 °C on amorphous ALD Al2O3 starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al2O3 starting surface promotes fast closure of the WS2 layer. The WS2 layers are fully continuous at WS2 content corresponding to only 1.2 WS2 monolayers. On amorphous Al2O3, (0002) textured and polycrystalline WS2 layers form with grain size of 5 nm to 20 nm due to high nucleation density (~1014 nuclei/cm2). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surface to three-dimensional (Volmer-Weber) growth after WS2 layer closure. Further growth proceeds from both WS2 basal planes in register with the underlying WS2 grain, and from or over grain boundaries of the underlying WS2 layer with different in-plane orientation. In contrast, on monocrystalline sapphire, WS2 crystal grains can locally align along a preferred in-plane orientation. Epitaxial seeding occurs locally albeit a large portion of crystals remain randomly oriented, presumably due to the low deposition temperature. The WS2 sheet resistance is 168 MΩµm suggesting that charge transport in the WS2 layers is limited by grain boundaries.status: publishe

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

The flora phenotype ontology (FLOPO):tool for integrating morphological traits and phenotypes of vascular plants

Background: The systematic analysis of a large number of comparable plant trait data can support investigations into phylogenetics and ecological adaptation, with broad applications in evolutionary biology, agriculture, conservation, and the functioning of ecosystems. Floras, i.e., books collecting the information on all known plant species found within a region, are a potentially rich source of such plant trait data. Floras describe plant traits with a focus on morphology and other traits relevant for species identification in addition to other characteristics of plant species, such as ecological affinities, distribution, economic value, health applications, traditional uses, and so on. However, a key limitation in systematically analyzing information in Floras is the lack of a standardized vocabulary for the described traits as well as the difficulties in extracting structured information from free text. Results: We have developed the Flora Phenotype Ontology (FLOPO), an ontology for describing traits of plant species found in Floras. We used the Plant Ontology (PO) and the Phenotype And Trait Ontology (PATO) to extract entity-quality relationships from digitized taxon descriptions in Floras, and used a formal ontological approach based on phenotype description patterns and automated reasoning to generate the FLOPO. The resulting ontology consists of 25,407 classes and is based on the PO and PATO. The classified ontology closely follows the structure of Plant Ontology in that the primary axis of classification is the observed plant anatomical structure, and more specific traits are then classified based on parthood and subclass relations between anatomical structures as well as subclass relations between phenotypic qualities. Conclusions: The FLOPO is primarily intended as a framework based on which plant traits can be integrated computationally across all species and higher taxa of flowering plants. Importantly, it is not intended to replace established vocabularies or ontologies, but rather serve as an overarching framework based on which different application- and domain-specific ontologies, thesauri and vocabularies of phenotypes observed in flowering plants can be integrated

Calibration of Models for III-V TFET Performance Prediction

The Tunneling Field-Effect Transistor (TFET) is a promising candidate for future low-power logic applications. The carrier injection mechanism of the TFET is quantum mechanical Band-To-Band Tunneling (BTBT), which is accompanied by an energy filtering mechanism. This filtering allows switching from the off-state to the on-state using a lower supply voltage than the MOSFET, and hence reducing the power consumption of integrated circuits. To identify favorable III-V TFET configurations and guide TFET fabrication, semi-classical and quantum mechanical simulations are crucial. However, there is uncertainty on the accuracy of the models relevant to TFET and on their input parameters. The topic of this thesis is the experimental calibration of the models for the desired BTBT as well as the ones of relevant parasitic mechanisms, like Shockley-Read-Hall and trap-assisted tunneling due to bulk traps. We also characterize Field-Induced Quantum Confinement (FIQC) and the energy band alignment of heterojunction TFET. We achieve this using tunnel diodes and MOS capacitors, which are easier to fabricate and characterize than complete TFETs. Our work enables improved understanding of experimental TFET data and more accurate performance prediction of III-V heterojunction TFET.nrpages: 259status: publishe

Calibration of the high-doping induced ballistic band-tails tunneling current in In0.53Ga0.47As Esaki diodes

© 2017 IEEE. I. Introduction The growing demand for power efficient devices has accelerated the research into the use of the tunnel field-effect transistor (TFET) in future ultra-low power applications because of its promising potential for sub-60 mV/dec subthreshold swing achieved through quantum mechanical band-To-band tunneling (BTBT) [1]-[3]. Unfortunately, a significant gap between theoretical predictions and experiments remains to be bridged [2]. Considerable efforts are being made to develop models for some of the main causes of suboptimal TFET performance such as trap-Assisted tunneling (TAT) [4], [5], phonon-Assisted tunneling (PAT) [6], and Auger generated leakage currents [7]. However, aside from qualitative analyses [8] and purely predictive work on the device impact of tunneling transitions involving high-doping induced band-Tails states in InSb nanowire TFETs [9] and 2D-TFETs [10], no attempts have been made to calibrate these contributions. This work aims to fill this gap by developing and calibrating an approximate ballistic semi-classical (SC) model for high-doping induced band-Tails using the experimental I-V data of In0.53Ga0.47As p-i-n Esaki diodes [11]. The hypothesis is posited that the mismatch between measurement and simulation in the negative differential resistance regime (see Fig. 1), which cannot be explained by SC TAT models, is caused by ballistic band-Tails tunneling. The calibration thus gives an upper limit to the band-Tails current. Lastly, the impact of band-Tails on the performance of a p-n-i-n TFET is investigated.status: publishe

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

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