Search

Metal contacts form one of the main limitations for the introduction of 2D materials in next-generation scaled devices. Through ab-initio simulation techniques, we shed light on the fundamental physics and screen several 2D and 3D top and side contact metals. Our findings highlight that a low semiconducting-metal contact resistance can be achieved. By selecting an appropriate 2D metal, we demonstrate both ohmic or small Schottky barrier top and side contacts. This leads to a contact resistance below 100 Ωμm and good device drive performance with currents in ON state up to 1400 μA/μm, i.e., reduced by a mere 25% compared to a reference with perfect ohmic contacts, provided a sufficiently high doping concentration of 1.8×1013 cm−2 is used. Additionally, we show that this doping concentration can be achieved through electrostatic doping with a gate. Finally, we perform a screening of possible 2D–3D top contacts. Finding an ohmic 2D–3D contact without a Schottky barrier has proven difficult, but it is shown that for the case of intermediate interaction strength and a limited Schottky barrier, contact resistances below 100 Ωμm can be achieved.

Chalcogenides are attractive materials for microelectronics applications due to their ability to change physical, optical, and electrical properties under applied electric stimulus. One of the most recent interest in chalcogenides is their use to enable the development of a two-terminal selector device, which is a fast, volatile, resistance switching device. The electrical signature of Ovonic threshold switching (OTS) chalcogenide materials is well suited for such applications. While there are numerous known OTS materials, most of them contain toxic elements. There is hence a need to find environment-friendly OTS materials. For this to happen, we strive to predict electrical device parameters only from atomistic first-principles simulations of the chalcogenide materials, as this can be a faster and less expensive route to screen the performances of chalcogenide candidates. By mapping the experimentally measured set of electrical OTS materials into atomistic models and computing their electronic properties, we were able to identify correlations between computed properties such as the theoretical trap/mobility gaps, the local atomic coordination environments of the elements adopted in the material, and the experimentally measured first-fire/threshold/hold voltages, hold/leakage currents, or extracted trap density. These findings can guide in identifying OTS materials with predefined electronic properties, tailored to the requirements of specific microelectronics applications with only first-principles simulations.

Stress simulations of Si0.5Ge0.5-channel nanowire transistors with typical 5 nm technology-node dimensions are performed to study the effect of layout on the channel stress generated by virtual substrates, by epitaxial mismatch from source/drain epitaxial layers, and by the sacrificial layers between the wires. Dependence of the stress on wire length originates from the wire cut, leading to elastic stress relaxation of the wire/sacrificial layer stack which is more pronounced toward the edge of the structure, and from reduced effectiveness of source/drain stressors at this edge. Consequently, large (>500 MPa) variations in channel stress are observed between long and short wires, between devices at different positions along the structure, as well as between the various wires within the same transistor for multiwire architectures. Besides layout dependence, this work also reports on the effect of nonideal, defective source/drain stressors on channel stress generation in nanowires. For nanowires, these defects may originate from merging of different epitaxial growth fronts coming from segments which start from different wires and the substrate. They can be classified into horizontal (between wires), vertical (between gates), and lateral defects (between neighboring structures). Stress simulations with air gaps at these locations allow estimating the impact of these defects for the case where there is no stress transfer across the defect planes. While horizontal and lateral defects lead to minimal (

With decreasing dimensions and increasing complexity, semiconductor devices are getting more difficult to fabricate. In particular the allowed deposition temperature becomes lower. Amorphous materials, which do not require annealing steps, are therefore becoming more interesting. First principles modelling of amorphous materials is, however, way more complex than modelling crystalline ones. Especially to screen for new materials, a fully ab initio approach is hence too expensive. We take on this challenge by employing a combination of high throughput first principles calculations and artificial intelligence (AI). We construct 4500 atomistic models, each containing 200-atoms, to capture the properties of amorphous phases of primary, X-O, and binary, X-Y-O, metal oxides. For these models, we calculate the relevant properties for a transistor channel. Expanding this exercise to more complex metal oxides would lead to a prohibitively large number of options. We solve this problem by training support vector regression models based on the data generated for the primary and binary oxides to predict the properties of ternary and more complex oxides. By combining the trained models, we construct an objective function that, at its minimum, points to the optimal composition in terms of electronic performances and material stability. After screening a series of objective functions, we identify the Zn-Mg-Al-O metal oxide (Zn and Mg around 40-50 at%, Al below 10 at%) as being the most interesting improvement to the current industry standard a-IGZO for the development of a high charge carrier mobility layer of a thin film transistor compatible with low deposition temperature requirements. It is predicted to combine an improvement in terms of electron mobility and chemical stability with respect to a-IGZO. This method, combining first principles calculated data with AI, is however not restricted to finding new materials for the active layer in a thin film transistor. From a general perspective, this approach can be used for any alloy or compound discovery problem, in which the pivotal material properties can be calculated for, in the order of a thousand, relevant one- and two-element materials.

Two-dimensional (2D) ferromagnetic materials are considered to be promising candidates for future generations of spintronic devices. Yet, 2D materials with intrinsic ferromagnetism are scarce. High-throughput first-principles simulations are performed in order to screen 2D materials that present a non-magnetic to a ferromagnetic transition upon hole doping. A global evolutionary search is subsequently performed, in order to identify alternative possible atomic structures of the eligible candidates, and 122 materials exhibiting hole-doping induced ferromagnetism are identified. Their energetic and dynamic stability, as well as their magnetic properties under hole doping, are investigated systematically. Half of these 2D materials are metal halides, followed by chalcogenides, oxides, and nitrides, some of them having predicted Curie temperatures above 300 K. The exchange interactions responsible for the ferromagnetic order in these 2D materials are also discussed. This work not only provides theoretical insights into hole-doped 2D ferromagnetic materials but also enriches the family of 2D magnetic materials for possible spintronic applications.

We propose a simple additive approach to simulate X-ray photoelectron spectra (XPS) of macromolecules based on the $GW$ method. Single-shot $GW$ ($G_0W_0$) is a promising technique to compute accurate core-electron binding energies (BEs). However, its application to large molecules is still unfeasible. To circumvent the computational cost of $G_0W_0$, we break the macromolecule into tractable building blocks, such as isolated monomers, and sum up the theoretical spectra of each component, weighted by their molar ratio. In this work, we provide a first proof of concept by applying the method to four test polymers and one copolymer, and show that it leads to an excellent agreement with experiments. The method could be used to retrieve the composition of unknown materials and study chemical reactions, by comparing the simulated spectra with experimental ones.

In this work, IGZO device integration is reported leveraging our 300mm-fab facilities. Our objective is mainly to gain insights into the process and material elements which drive the control of the performance parameters of IGZO nFETs. To control the final doping of the IGZO channel, it is well reported in literature that a final oxygen anneal can be applied to passivate the oxygen vacancies which are formed during the device fabrication. This technique is also used in our 300mm flow. As it might be expected, in front gate IGZO nFETs, the passivation efficiency is limited by the presence of the top metal gate stack. Therefore, it seems important to limit the formation of the oxygen vacancies during the deposition of the gate dielectrics. Three oxides were studied: SiO2, Al2O3 and HfO2 on top of the IGZO channel. After oxygen annealing, only PECVD-SiO2 shows a large recovery while for Al2O3 and HfO2, it remains low. This result challenges the implementation of conventional water-based high-K materials in gate first IGZO integration. Contrary to the use of undoped IGZO in the channel for IOFF control, contacts can rely on maximizing the oxygen vacancies to increase the dopant concentration in the S/D regions. When not done locally, it could be an extra source of variability. This doping is made through oxygen scavenging from IGZO by a thin metal contact barrier. Low specific contact resistivity down to 1x107 Ohm.cm2 is demonstrated when Ti is thinner than 5nm. With thicker barrier, the formation of TiO2 and specific alloys is taking place at the IGZO/Contact barrier interface. This is both confirmed by ab initio simulations and by advanced physical characterization techniques. Regarding the channel, X-ray diffraction techniques are extensively here since this is a nondestructive in-fab technique that provides detailed information about the crystallographic structure of the IGZO material. We show a typical spectrum where clear peaks/humps are seen and attributed to: 1. amorphous IGZO, 2. CAAC-IGZO and 3. a previously not reported, to the best of our knowledge, phase called here s-IGZO. The s-phase is only formed under certain conditions of power, temperature and oxygen flow during material deposition. Thick IGZO (>12nm) back-gated nFETs with active layers submitted to final O2 anneal are used to study the different phases of IGZO and their electrical impact on device parameters. In this configuration, the carrier transport preferentially occurs in the bottom half of the IGZO channel while the top half (SiO2/IGZO interface) mostly drives the electrostatic control of BG transistors. The amorphous IGZO has much reduced spread in VTH-ON and higher ID,LIN (~mobility) than that of CAAC-IGZO. Reliability tests have also been carried out to compare the two phases and the results show the existence of two competing PBTI degradation mechanisms for CAAC-IGZO. It is important to report the degradation over time and different VG stress because a cut line at a specific value could have shown no BTI degradation of CAAC-IGZO and then attributing an unfair benefit of this phase over the a-IGZO phase. Combining these findings to some other results where the sheet resistance of IGZO under hydrogen exposure depends on the phases, we can reasonably conclude that CAAC owns different doping levels in comparison to a-IGZO. Based on the previous learning about n-type dopant location in IGZO, we show that the performance of transistors keeps increasing when the channel length is reduced and scales with the channel width. A measure of the VTH-ON variation has been performed at the 300mm wafer scale from long to ~120nm LCH and down to 200nm WCH dimensions. The figure in attachement shows the Id-Vg curves of >100 Back Gated IGZO-nFETs with no failed devices detected. The standard variation of the VTH-ON across LCH and WCH is often less than 40mV with a minimum of 20mV. In conclusion, we have demonstrated in this study scaled IGZO nFETs with excellent VTH_ON control using an industry compatible 300mm process flow. This has been achieved thanks to a careful mapping of n-type doping in the three dimensions of the IGZO channel. While semi-crystalline IGZO seems to be more robust against hydrogen than that of amorphous-IGZO, a new IGZO phase is found to help boost ION at short channel. The demonstration of back-gate IGZO nFETs with low variability and relatively high drive will benefit to the top-gate architecture development. This will provide new opportunities for the IGZO-based devices like an “on-the-fly” VTH setting for the performance control of advanced applications. Figure 1

Grain boundaries between 60° rotated and twinned crystals constitute the dominant type of extended line defects in two-dimensional transition metal dichalcogenides (2D MX2) when grown on a single c...

Ferroelectrics (FEs) are increasingly used in nonvolatile memory applications. However, the impact of the electric dipole switching on its material parameters, in particular on the dielectric response, is not fully understood. In this work, an analytical model, linking the dielectric response to the potential energy curve, is first used to qualitatively illustrate the nonconstant evolution of the dielectric response with applied electric field. Increasing precision, we then show from first-principles density functional theory simulations that defect-free FE materials undergo changes in potential energy near the FE switching that lead to vibrational modes softening, which impacts the dielectric response. In particular, we observe that the dielectric response epsilon of an FE material with antialigned polarization increases as the applied electric field increases toward the coercive field. We incorporate this new insight in the time-dependent NLS-based predictions and demonstrate that this evolution of the dielectric response right before polarization reversal is required for a proper match between experiment and prediction of the capacitance-voltage (CV) characteristics of the metal-ferroelectric-metal (MFM) capacitor.

The resistivity scaling of metals is a crucial limiting factor for further downscaling of interconnects in nanoelectronic devices that affects signal delay, heat production, and energy consumption. Here, we generalize a commonly considered figure of merit for selecting promising candidate metals with highly anisotropic Fermi surfaces in terms of their electronic transport properties at the nanoscale. For this, we introduce a finite-temperature transport tensor, based on band structures obtained from first principles. This transport tensor allows for a straightforward comparison between highly anisotropic metals in nanostructures with different lattice orientations and arbitrary transport directions. By evaluating the temperature dependence of the tensor components, we also assess the validity of a Fermi surface-based evaluation of the transport properties at zero temperature, rather than considering standard operating temperature conditions., 15 pages, 9 figures, updated numerical integration scheme and analysis of temperature dependence

For the last 15 years, two-dimensional (2D) materials are being actively studied as a possible replacement for silicon. Due to their layered nature, the materials can be scaled down to a single atomic layer, favouring extreme device scaling. In addition, surface effects such as roughness, dangling bonds, and interface states are also reduced/eliminated [1],[2]. Besides, these materials confine charge carriers to atomic-thin layers, which provides excellent gate-control and reduces short-channel effects [3]. Graphene (single layer graphite) was the first of the 2D material to be isolated, and very high electron mobility $(\gt 10^{5}$ cm$^{2} /$Vs) in its suspended state was reported [4]. However, it does not have a bandgap, which is a severe limitation for transistor applications (challenging to turn off the device). Still, it initiated the discovery of other 2D materials with a band gap, such as phosphorene (mono- or few-layer black phosphorous) [5] and transition metal dichalcogenides (TMDs) [6]. However, TMDs and phosphorene are found to have lower intrinsic mobility when compared to graphene [7],[8]. This can be attributed to the large electron-phonon scattering present in these materials. Theoretical calculations have predicted that, even in 2D material-based transistors of channel lengths as small as about 5 nm, electron-phonon scattering plays a significant role [9],[10].

Il continuo down-scaling dei transistor ha permesso il raggiungimento di una eccezionale miniaturizzazione. Cio ha reso la resistenza di contatto parte preponderante della resistenza parassita del dispositivo. In questa tesi si riporta la caratterizzazione layer epitassiali drogati con Fosforo, Arsenico e Co-drogati (Si:P, Si:As, Si:P:As) per NMOS S/D. La Deposizione Chimica da Vapore e stata impiegata per un processo selettivo a 670 °C e uno non-selettivo a una temperature piu bassa di 450 °C. Sono state studiate le proprieta fisiche come morfologia, qualita cristallina, composizione, attivazione dei dopanti ed e stata verificata la stabilita termica sino a 700 °C. Variando la concentrazione dall' 1% al 4% sono stati esplorati scenari differenti. La resistivita del contatto e stata misurata su stacks, realizzate impiegando la Ti-silicidation, per mezzo della tecnica MR-CTLM. I risultati mostrano che per i layer, realizzati alla temperatura di 670 °C a concentrazioni totali elevate (~3%), la concentrazione attiva del Si:As e Si:P:As layer e generalmente piu bassa se comparata al Si:P, inoltre il Si:As mostra minore tensile strain. I layer co-drogati realizzati con il processo non-selettivo e a basse concentrazioni totali (~1.2%) sono caratterizzati da una maggiore concentrazione attiva dovuta a un meccanismo di vacancy trapping piu efficiente, mentre non si osservano miglioramenti tangibili dopo aver applicato laser annealing. La resistivita del contatto misurata su Ti-stack raggiunge 2 · 10−9 Ω .cm2 per differenti condizioni di processo. L'introduzione di As mostra effetti positivi nella resistenza di contatto dei layer as-grown ~3% per il processo selettivo. I layer epitassiali realizzati in Si:As e Si:P:As realizzati a 450 °C mostrano ρc comparabile a quelli drogati con Fosforo (Si:P). Queste proprieta insieme a una riduzione della diffusione per effetto di rapid-thermal-annealing rendono Si:As and Si:P:As materiali promettenti per NMOS S/D.

Imec’s AttoLab is the first industrial laboratory capable of watching the ultrafast dynamics of photoresists following 13.5 nm, EUV exposure, and for emulating high-numerical-aperture (high-NA) exposure on 300-mm wafers using two-beam EUV interference. The two respective beamlines are powered by a laser-based high-harmonic generation EUV source. Its capabilities have recently been proven by imaging 20 nm pitch lines and spaces using Lloyd’s Mirror interference lithography. In parallel, time-averaged and time-resolved techniques for studying the ultrafast dynamics of photoresists after EUV exposure, coherent diffractive imaging to study resist interfaces, and more sophisticated interference lithography techniques for printing sub-22 nm pitches on full 300-mm wafers are being developed. Taking advantage of the bright and short EUV pulses now available at imec, we will be able to contribute to a smooth transition towards next generation high-NA lithography.

The potential of a wide range of layered ternary carbide and nitride MAX phases as conductors in interconnect metal lines in advanced CMOS technology nodes has been evaluated using automated first principles simulations based on density functional theory. The resistivity scaling potential of these compounds, i.e. the sensitivity of their resistivity to reduced line dimensions, has been benchmarked against Cu and Ru by evaluating their transport properties within a semiclassical transport formalism. In addition, their cohesive energy has been assessed as a proxy for the resistance against electromigration and the need for diffusion barriers. The results indicate that numerous MAX phases show promise as conductors in interconnects of advanced CMOS technology nodes., Comment: 26 pages, 4 figures, 4 tables

A first-principles study of native point defects in monoclinic, cubic, two different tetragonal, and five different orthorhombic phases of hafnia (HfO2) is presented. They include vacancy of tri-coordinated and tetra-coordinated oxygen, metal vacancy, interstitial metal, and interstitial oxygen. Defect formation energy, trap depth, and relaxation energy upon optical excitation of defects are listed. The trap depth of oxygen vacancies shows little variation among different phases compared to other defects. Results of the trap depth are compared against measurements and found to have reasonable agreement. ispartof: Journal Of Applied Physics vol:129 issue:8 status: Published online

Interconnect options will be introduced and reviewed targeting tight pitch metal layers at the local levels. Examples include hybrid metallization, semi-damascene interconnects as well as potential new conductor materials.

Identification of two-dimensional (2D) materials with magnetic properties has received strong research attention in the development of advanced spin-based devices. By means of first-principles calculations, we investigate the stability, electronic properties, and the hole-doping-induced magnetic properties of metal oxide ($M\mathrm{O}, M=\mathrm{Ga},\mathrm{In}$) monolayers. They are intrinsically nonmagnetic stable semiconductors, with high energetic, vibrational, and thermal stability. Hole doping can switch them from nonmagnetic to ferromagnetic and turn them into half-metals over a wide range of hole densities. Monte Carlo simulations predict that the highest Curie temperature of the GaO monolayer can reach \ensuremath{\sim}125 K. Our results indicate that monolayer $M\mathrm{O}$ could be eligible candidate materials for 2D spintronic devices.

Hole-doping of GaX, InX (X = S or Se) and SnO monolayers is predicted to induce a stable ferromagnetic order in these two-dimensional materials, making them potentially interesting for nanoscaled spintronic devices. Ferromagnetism in these materials arises from their peculiar Mexican-hat valence band edge structure, which leads to a Stoner instability. We discuss here the results from first-principles simulations on the p-type doping-induced ferromagnetism in these 2D materials. Hole-doping, induced by intrinsic and extrinsic point defects, is considered. Metal vacancies are found to produce shallow spin-polarized gap states near the valence band edge, leading to a p-type behavior. Among the investigated potential extrinsic defects (dopants), As substitution of the oxygen atoms in SnO or Bi substitution of the selenium atoms in InSe appears to be good candidates for the hole-doping of these 2D materials.

Positron annihilation spectroscopy in the Doppler and coincidence Doppler mode was applied on Ge 1 − xSn x epitaxial layers, grown by chemical vapor deposition with different total As concentrations ( ∼ 10 19– 10 21 cm − 3), high active As concentrations ( ∼ 10 19 cm − 3), and similar Sn concentrations (5.9%–6.4%). Positron traps are identified as mono-vacancy complexes. Vacancy-As complexes, V-As i, formed during the growth were studied to deepen the understanding of the electrical passivation of the Ge 1 − xSn x:As epilayers. Larger mono-vacancy complexes, V-As i ( i ≥ 2), are formed as the As doping increases. The total As concentration shows a significant impact on the saturation of the number of As atoms ( i = 4) around the vacancies in the sample epilayers. The presence of V-As i complexes decreases the dopant activation in the Ge 1 − xSn x:As epilayers. Furthermore, the presence of Sn failed to hinder the formation of larger V-As i complexes and thus failed to reduce the donor-deactivation.

This paper benchmarks various epitaxial growth schemes based on n-type group-IV materials as viable source/drain candidates for Ge nMOS devices. Si:P grown at low temperature on Ge, gives an active carrier concentration as high as 3.5 × 1020 cm−3 and a contact resistivity down to 7.5 × 10−9 Ω.cm2. However, Si:P growth is highly defective due to large lattice mismatch between Si and Ge. Within the material stacks assessed, one option for Ge nMOS source/drain stressors would be to stack Si:P, deposited at contact level, on top of a selectively grown n-Si y Ge1−x−y Sn x at source/drain level, in line with the concept of Si passivation of n-Ge surfaces to achieve low contact resistivities as reported in literature (Martens et al. 2011 Appl. Phys. Lett., 98, 013 504). The saturation in active carrier concentration with increasing P (or As)-doping is the major bottleneck in achieving low contact resistivities for as-grown Ge or Si y Ge1−x−y Sn x . We focus on understanding various dopant deactivation mechanisms in P-doped Ge and Ge1−x Sn x alloys. First principles simulation results suggest that P deactivation in Ge and Ge1−x Sn x can be explained both by P-clustering and donor-vacancy complexes. Positron annihilation spectroscopy analysis, suggests that dopant deactivation in P-doped Ge and Ge1−x Sn x is primarily due to the formation of P n -V and Sn m P n -V clusters.

The fine balance between dipole-field energy and anion drift force defines the switching mechanism during polarization reversal: for the first time we show that only Pbcm mechanism obeys the ferroelectric switching physics, whereas P4(2)/nmc (or any other) mechanism does not. However, with lower energy barrier, it represents an important antiferroelectric mechanism. Constraints relaxation can lead to 90 degrees polarization rotation (domain deactivation). Intrinsically, the Si/VO-doping can switch faster than undoped HfO2 or HfZrOx. Theoretical Arrhenius model / intrinsic material switching (DFT) overestimates the switching speed extracted from experiments.

2-D material FETs hold the promise of excellent gate control, but the impact of nonidealities on their performance remains poorly understood. This is because of the need, so far, to use computationally intensive nonequilibrium Green’s function (NEGF) simulations. Here, we therefore use a semiclassical model to investigate the role of nonidealities in the scaling of back-gated (BG) and top-gated (TG) monolayer MoS2 FETs. We verify the electrostatics and transport of the semiclassical model with density functional theory-based NEGF simulations and calibrate nonidealities, such as interface traps ( ${D}_{\textsf {it}}$ ) and Schottky contact barrier height ( $\phi _{\textsf {SB}}$ ) to experimental monolayer and bilayer MoS2 FETs. We find that among the nonidealities, ${D}_{\textsf {it}}$ has the strongest subthreshold swing impact with 70 mV/dec obtainable in BG devices for a ${D}_{\textsf {it}}$ of $5\times {10}^{11}$ cm−2eV $^{-1}$ , an equivalent oxide thickness (EOT) of 1 nm, and a channel length ( ${L}_{\textsf {ch}}$ ) of 5 nm. For scaled EOT, $\phi _{\textsf {SB}}$ only strongly impacts ${I}_{ \mathrm{\scriptscriptstyle ON}}$ for the TG case, as the overlapping gate thins the Schottky barriers in the BG case. We show in TG devices that a spacer of only 5 nm results in a 1000-fold drop in ${I}_{ \mathrm{\scriptscriptstyle ON}}$ because of the nonidealities. We propose positive spacer oxide charge as a solution and show that a charge density of above 1013 cm−2 is required to fully recover the device performance.

In this contribution, we report a fundamental study of the factors that set the contact resistivity between metals and highly doped n-type 2D and 3D semiconductors. We investigate the case of n-type doped Si contacted with amorphous TiSi combining first principles calculations with Non-Equilibrium Green functions transport simulations. The evolution of the intrinsic contact resistivity with the doping concentration is found to saturate at similar to 2 x 10(-10) Omega.cm(2) for the case of TiSi and imposes an intrinsic limit to the ultimate contact resistance achievable for n-doped Silamorphous-TiSi (aTiSi). The limit arises from the intrinsic properties of the semiconductors and of the metals such as their electron effective masses and Fermi energies. We illustrate that, in this regime, contacting heavy electron effective mass metals with semiconductor helps reducing the interface intrinsic contact resistivity. This observation seems to hold true regardless of the 3D character of the semiconductor, as illustrated for the case of three 2D semiconducting materials, namely MoS2, ZrS2 and HfS2. (C) The Author(s) 2018. Published by ECS.

Density functional theory predicts an abrupt drop in the stability, in the kinetic barrier for polarization reversal and spontaneous polarization of a ScxAl1-xN wurtzite phase, when the Sc:Al ratio approaches 50:50. The same effect is obtained by the application of a tensile strain. The resulting polarization reversal barrier correlates with experimental coercive fields, and highly textured film measurements exhibit polarization values close to the theoretically predicted ones. Film thickness below 5 nm has a significant impact on the elastic properties.

First-principles calculations are used to systematically investigate the impact of surface effects on the energetic cost to incorporate point defects near the growing surface [Ge(001)] and its consequence on the dopant activation in P-doped Ge layers. We illustrate the formation enthalpy Delta H-f for the incorporation of a mono-vacancy, which is the dominant point defect responsible for the electrically inactive dopant incorporation in P-doped Ge. The calculated values point toward a significant lowering of Delta H-f near the growing surface in comparison to the bulk cubic Ge supercell. The impact of the surface almost vanishes beyond the sixth atomic monolayer located below the surface and the calculated Delta H-f values overlap with the ones computed for a bulk cubic Ge supercell. For epitaxial Ge:P layers, grown at low growth temperatures (

Summary The crystal and defect structure of SnS crystals grown using chemical vapour deposition for application in electronic devices are investigated. The structural analysis shows the presence of two distinct crystal morphologies, that is thin flakes with lateral sizes up to 50 μm and nanometer scale thickness, and much thicker but smaller crystallites. Both show similar Raman response associated with SnS. The structural analysis with transmission electron microscopy shows that the flakes are single crystals of α-SnS with [010] normal to the substrate. Parallel with the surface of the flakes, lamellae with varying thickness of a new SnS phase are observed. High-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), first-principles simulations (DFT) and nanobeam diffraction (NBD) techniques are employed to characterise this phase in detail. DFT results suggest that the phase is a strain stabilised β’ one grown epitaxially on the α-SnS crystals. TEM analysis shows that the crystallites are also α-SnS with generally the [010] direction orthogonal to the substrate. Contrary to the flakes the crystallites consist of two to four grains which are tilted up to 15° relative to the substrate. The various grain boundary structures and twin relations are discussed. Under high-dose electron irradiation, the SnS structure is reduced and β-Sn formed. It is shown that this damage only occurs for SnS in direct contact with SiO2.

© 2017 Elsevier B.V. Several native point defects in MoS 2 are theoretically studied, using first-principles simulations. The isotropic g-values of these defects are computed, and compared to recently reported experimental results, obtained from electron spin resonance experiments performed on MoS 2 layers synthesized by the sulfurization of Mo films. We tentatively assign the observed electron spin resonance signal with isotropic g-value of 2.002 to a sulfur antisite defect. This defect presents energy levels in the MoS 2 band-gap, and could hamper the proper functioning of MoS 2 -based field effect transistors. We next study the interaction of H 2 molecules with the sulfur antisite defect, using first-principles molecular dynamics simulations and the nudged elastic band method. Our results predict that this defect can be passivated by hydrogen, with a computed activation energy of about 1.5 eV. ispartof: Applied Surface Science vol:416 pages:853-857 status: published

Two dimensional (2D) materials are currently attracting considerable interest due to their promising applications in future nanoelectronic devices. Magnetic 2D materials are also gaining attention, for their possible use in novel spintronic devices [1]. Recently, hole-doping in monolayer SnO has been theoretically predicted to induce a paramagnetic to ferromagnetic phase transition in this 2D material, for a typical hole density of about 5x1013/cm2 [2]. Magnetism in monolayer SnO arises from an exchange splitting of electronic states at the top of the valence band, where the density of states exhibits a sharp van Hove singularity, resulting in a so-called Stoner (magnetic) instability [2]. This behavior is typical of materials with “Mexican-hat” energy band edges (as predicted for the valence band edge of monolayer SnO), and has also been predicted in e.g. monolayer GaSe [3]. In this work, the possibility to induce hole doping in monolayer SnO by intrinsic and extrinsic defects is investigated, using first-principles simulations, based on density functional theory. It is found that Sn vacancies generate spin-polarized gap states near the valence band edge of monolayer SnO, and that these point defects are behaving like acceptors. A typical density of 5x1013/cm2 of Sn vacancies induces a paramagnetic to ferromagnetic phase transition in the material, with a magnetic moment density of 1 Bohr magneton/hole. Substitutional doping of Sn atoms by e.g. In or Zn atoms is also predicted to result in hole-doping of monolayer SnO, and also leads to a ferromagnetic order in the 2D material. The possibility to induce a ferromagnetic phase transition in monolayer SnO by electrostatic doping will also be discussed. [1] W. Han, APL Mater. 4, 032401 (2016). [2] L. Seixas et al., Phys. Rev. Lett. 116, 206803 (2016). [3] T. Cao et al., Phys. Rev. Lett. 114, 236602 (2015).

In recent CMOS technology, extreme shrinking of contact area at source/drain regions raises serious concerns of high metal/semiconductor contact resistance. Confronting this problem, we introduce a precontact amorphization implantation plus Ti silicidation technique (PCAI + TiSi x ) and achieve ultralow contact resistivity ( $\rho _{c}$ ) of (1.3 – 1.5) $\times 10^{-9} ~\Omega \cdot \text {cm}^{2}$ on Si:P. This PCAI + TiSi x technique utilizes light amorphization (low-energy implantation), thin Ti and TiSi x film, and moderate thermal budget (500 °C –550 °C): these features are compatible with modern CMOS manufacturing. Moreover, the PCAI + TiSi x -induced $\rho _{c}$ reduction is proved universal on both n- and p-Si. With additional characterizations, we find that the silicidation-induced $\rho _{c}$ variation is not merely a Schottky barrier height tuning effect. The electrical and physical characterizations suggest that the low $\rho _{c}$ is strongly correlated with the formation of interfacial TiSi x crystallites between amorphous TiSi alloy and Si.

A state-of-the-art DFT-NEGF based ATOmistic - MOdelling Solver (ATOMOS) was developed and used to assess the physics and fundamental-performance potential of various scaled mono-layer transition-metal-dichalcogenides and blackphosphorus (BP) MOSFETs down to a gate length of 5 nm, including the effect of electron-phonon scattering. Our study highlights the good scalability and drive-current potential of HfS2 and the impact of optical-phonon scattering for BP.

The contact resistance at two-dimensional graphene/MoS2 lateral heterojunctions is theoretically studied, using first-principles simulations based on density functional theory and the nonequilibrium Green's function method. The computed contact resistance lies in the range of 102 to 104 Ω μm, depending on the contact edge symmetry (armchair or zigzag) and termination (Mo and/or S terminated). This large variation in the contact resistance arises from the variation in the interface barrier height, which is sensitive to the presence of polar C-Mo bonds or sulfur dangling bonds at the interface. These results highlight that the control of the edge symmetry and/or edge termination is crucial to achieve a low contact resistance (in the range of a few hundred ohms micrometer) at graphene/MoS2 lateral heterojunctions for 2D material-based field-effect devices. Part of this work was financially supported by the KU Leuven Research Funds, Project No. C14/17/080, as well as the 2DFun project, an ERA-NET project in the framework of the Graphene Flagship. The authors acknowledge the support from Flanders Innovation and Entrepreneurship.

Si:C and Si:C:P alloys are potential candidates for source-drain stressor applications in n-type Fin Field Effect Transistors (FinFETs). Increasing the C content to achieve high strain results in the arrangement of C atoms as third nearest neighbors (3nn) in the Si: C lattice. During thermal annealing, the presence of C atoms as 3nn may promote clustering at the interstitial sites, causing loss of stress. The concentration of C atoms as 3nn is reduced by the incorporation of a small amount of Ge atoms during the growth, whereas in-situ P doping does not influence this 3nn distribution [J Solid State Sci. Technol vol 6, p 755, 2017]. Small amounts of Ge are provided during low temperature selective epitaxial growth scheme, which are based on cyclic deposition and etching (CDE). In this work, we aim to provide physical insights into the aforementioned phenomena, to understand the behavior of 3nn C atoms and the types of defects that are formed in the annealed Si: C films. Using ab-initio simulations, the Ge-C interaction in the Si matrix is investigated and this insight is used to explain how the Ge incorporation leads to a reduced 3nn distribution of the C atoms. The interaction between C and P in the Si: C: P films is also investigated to explain why the P incorporation has not led to a reduction in the 3nn distribution. We then report on the Raman characterization of Si: C layers subjected to post epi annealing. As the penetration depth of the laser is dependent on the wavelength, Raman measurements at two different wavelengths enable us to probe the depth distribution of 3nn C atoms after applying different annealing conditions. We observed a homogeneous loss in 3nn C throughout the layer. Whereas in the kinematic modeling of high resolution X-ray diffraction spectra, a gradient in the substitutional C loss was observed close to the epitaxial layer/substrate interface. This gradient can be due to the out diffusion of C atoms into the Si substrate or to the formation of interstitial C clusters, which cannot be distinguished in HR-XRD. Deep Level Transient Spectroscopy indicated that the prominent out-diffusing species was interstitial CO complex while the interstitial C defects were also prevalent in the epi layer. (c) 2019 The Electrochemical Society.

The E centers (dopant-vacancy pairs) play a significant role in dopant deactivation in semiconductors. In order to gain insight into dopant-defect interactions during epitaxial growth of in situ phosphorus doped Ge, positron annihilation spectroscopy, which is sensitive to open-volume defects, was performed on Ge layers grown by chemical vapor deposition with different concentrations of phosphorus (similar to 1 x 10(18)-1 x 10(20) cm(-3)). Experimental results supported by first-principles calculations based on the two component density-functional theory gave evidence for the existence of mono-vacancies decorated by several phosphorus atoms as the dominant defect type in the epitaxial Ge. The concentration of vacancies increases with the amount of P-doping. The number of P atoms around the vacancy also increases, depending on the P concentration. The evolution of P-n-V clusters in Ge contributes significantly to the dopant deactivation. Published under license by AIP Publishing.

For the first time, we demonstrate, using an atomistic description of a 30nm diameter spin-transfer-torque magnetic random access memories (STT-MRAM), that the difference in mechanical properties of its sub-nanometer layers induces a high compressive strain in the magnetic tunnel junction (MTJ) and leads to a detrimental magnetostrictive effect. Our model explains the issues met in engineering the electrical and magnetic performances in scaled STT-MRAM devices. The resulting high compressive strain built in the stack, particularly in the MgO tunnel barrier (t-MgO), and its associated non-uniform atomic displacements, impacts on the quality of the MTJ interface and leads to strain relieve mechanisms such as surface roughness and adhesion issues. We illustrate that the strain gradient induced by the different materials and their thicknesses in the stacks has a negative impact on the tunnel magneto-resistance (TMR), on the magnetic nucleation process and on the STT-MRAM performance.

To keep Moore's law alive, 2D materials are considered as a replacement for Si in advanced nodes due to their atomic thickness, which offers superior performance at nm dimensions. In addition, 2D materials are natural candidates for monolithic integration which opens the door for density scaling along the 3rd dimension at reasonable cost. This paper highlights the obstacles and paths to a scaled 2D CMOS solution. The baseline requirements to challenge the advanced Si nodes are defined both with a physical compact model and TCAD analysis, which allows us to identify the most promising 2D material and device design. For different key challenges, possible integrated solutions are benchmarked and discussed. Finally we report on the learning from our first lab to fab vehicle designed to bridge the lab and IMEC's 300mm pilot line.