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A comprehensive quantitative root cause study of defect evolution leading to memory window closure from a charge balance and charge trapping perspective throughout all phases of a Si channel Hf0.5Zr0.5O2 (HZO) ferroelectric field-effect-transistor (FEFET) is reported. Starting with the first write pulse, an excessive SiO2 interlayer field is revealed that triggers the creation of defect levels Dit in excess of 1015 cm−2 eV−1 at the HZO–SiO2 interface screening ferroelectric (FE) polarization while enabling FE switching. Under subsequent early bipolar fatigue cycling (up to 104 cycles), defect creation commences at the SiO2–Si interface due to the high injected hole fluence (0.39 C/m2) during each stress pulse causing negative bias instability (NBI), which shifts the threshold voltage of the erase state VT,ERS by −0.3 V with accrual of permanently captured charge Nit of up to +5 × 10−3 C/m2 (3 × 1012 cm−2). Subsequently, Nit NBI generation at the SiO2–Si interface accelerates reaching levels of +7 × 10−2 C/m2, locking both FEFET program and erase drain current vs gate–source-voltage (ID–VGS) characteristics in the FEFET on-state inducing memory window closure at 105 cycles while FE switching (switched polarization Psw = 0.34 C/m2) remains essentially intact. These findings guide the down-selection toward suitable semiconductor/FE systems for charge balanced, reliable, and high endurance FEFETs. [ABSTRACT FROM AUTHOR]

In this work, we develop an ultrathin epitaxial h-AlN as an interfacial layer (IL) between HfO2 and monolayer WS2 channel by atomic layer deposition (ALD). The growth of ultrathin dielectrics using ALD has been challenging due to the dangling-bond-free surface of 2D materials. By utilizing sub-1 nm h-AlN as an IL and depositing HfO2 high-k dielectric, we were able to form a uniform and atomically flat gate dielectric without voids. The resulting structure exhibits an equivalent oxide thickness as low as 1 nm and ultra-low leakage currents of ∼10−6 A/cm2. The fabricated top-gate WS2 transistors demonstrate on-off ratios of around 106 and subthreshold swing as low as 93 mV/dec. Furthermore, we have verified the feasibility of using h-AlN IL for a gate-all-around structure. Our work presents a CMOS-compatible low-temperature ALD process for integrating gate dielectrics, offering excellent thickness scalability and uniform coverage around monolayer WS2 nanosheets. The combination of high-quality two-dimensional dielectrics and semiconductors will contribute to the development of future high-performance and low-power electronic devices. [ABSTRACT FROM AUTHOR]

Anisotropic growth of two-dimensional (2D) tungsten disulfide (WS2) crystals occurs during atomic layer deposition (ALD) from WS2 seeds at predetermined locations on large area dielectric substrates. The number of ALD reaction cycles determines the lateral dimensions of the WS2 crystals. This 2D synthesis approach is compatible with temperature sensitive structures due to the low deposition temperature and can be extended to other 2D materials and heterostructures thereof. The crystallinity of the seed and the selectivity of ALD precursors toward seeds and underlying starting surface affect the structural quality of the 2D crystals. [ABSTRACT FROM AUTHOR]

In the context of neuromorphic computation, spintronic memristors are investigated for their use as synaptic weights. In this paper, we propose and experimentally demonstrate a resistive synaptic device based on ten magnetic tunnel junctions (MTJs) connected in a serial configuration. Our device exhibits multiple resistance levels that support its use as a synaptic element. It allows for two operating knobs: external magnetic field and voltage pulses (Spin-Transfer Torque). Moreover, it can be operated in different ways. When varying continuously the amplitude of the voltage pulse and/or the magnetic field, eleven resistance states can be reached. In contrast, if the initial state of the chain is reset between every step, a very large number of levels are reached. Ideally, a total of 2 N resistance levels could be accessible. This coincides well with the desired analog-like behavior in ideal memristors. Since this device consists of a scalable number of N MTJs, and MTJ technology is continuously optimized and improved, the proposed memristor shows promise as a scalable synapse solution for neuromorphic hardware implementations. [ABSTRACT FROM AUTHOR]

In this work, we report on the sensitivity of photo-electrical detection of magnetic resonances (PDMR) for magnetometry measurement using low density nitrogen vacancy (NV) ensembles in CVD-grown diamond. We demonstrate that the selection of the laser excitation wavelength is of importance for achieving optimal magnetic field sensitivity. The PDMR sensitivity obtained using a yellow-green (561 nm) laser surpasses the performances of a green laser (532 nm), by suppressing the photoionization of defects other than NV centers (such as P1 centers). It consequently allows to carry out the PDMR measurements at lower laser powers with increased magnetic resonance contrast. Noticeably for both the green and the yellow-green illuminations, PDMR leads to an improved sensitivity to magnetic fields in the selected conditions compared to optically detected magnetic resonance. [ABSTRACT FROM AUTHOR]

The functionality of a cross-shaped Spin Torque Majority Gate (STMG) is primarily limited by the pinning of a domain wall (DW) at the center of the device. Here, an analytical model is built to calculate the conditions for such a pinning and to deduce the operating range. The assumptions of the model and the conclusions are validated by micromagnetic simulations. The total magnetic energy of the DW state is derived. By minimizing this energy with respect to two degrees of freedom, the DW stability condition is obtained. We find that the lateral length of the STMG is the critical dimension: it must be smaller than about five times the DW width. This result is confirmed by micromagnetic simulations with a high accuracy. In process, we solved a more fundamental problem: the macrospin limit of a finite ferromagnet containing one pinning site. We found the correction of the usual DW width expression due to finite length of wires. [ABSTRACT FROM AUTHOR]

Perpendicular Magnetic Anisotropy (PMA) is a key requirement for state of the art Magnetic Random Access Memories (MRAM). Currently, PMA has been widely reported in standard Magnetic Tunnel Junction material stacks using MgO as a dielectric. In this contribution, we present the first report of PMA at the interface with a high-j dielectric grown by Atomic Layer Deposition, HfO2. The PMA appears after annealing a HfO2\Co\Pt\Ru stack in N2 with the Keff of 0.25 mJ/m² as determined by Vibrating Sample Magnetometry. X-Ray Diffraction and Transmission Electron Microscopy show that the appearance of PMA coincides with interdiffusion and the epitaxial ordering of the Co\Pt bilayer. High-κ dielectrics are especially interesting for Voltage Control of Magnetic Anisotropy applications and are of potential interest for low-power MRAM and spintronics technologies. [ABSTRACT FROM AUTHOR]

Spin wave modes in magnetic waveguides with the width down to 320 nm have been studied by electrical propagating spin-wave spectroscopy and micromagnetic simulations for both longitudinal and transverse magnetic bias fields. For longitudinal bias fields, a 1.3 GHz wide spin-wave band was observed in agreement with analytical dispersion relations for uniform magnetization. However, the transverse bias field led to several distinct bands, corresponding to different quantized width modes, with both negative and positive slopes. Micromagnetic simulations showed that, in this geometry, the magnetization was nonuniform and tilted due to the strong shape anisotropy of the waveguides. Simulations of the quantized spin-wave modes in such nonuniformly magnetized waveguides resulted in spin wave dispersion relations in good agreement with the experiments. [ABSTRACT FROM AUTHOR]

Spin-on-diffussant (SoD) processing as a doping technique in two-dimensional semiconductors shows a general process compatibility with materials such as MoS2 and WSe2. The characteristic Raman modes in chemical vapor deposition-grown MoS2 are retained after a phosphorus-based SoD processing at temperatures up to 1000 °C; the evolution of the characteristic Raman peaks with SoD processing indicates a reduction in the intrinsic electron concentration. Electrical measurements show a corresponding p-type shift in the MoS2 MOSFET transfer characteristics and indicate possible oxidation of MoS2 by the SoD processing. Both these effects correlate with XPS measurements which confirm the diffusion of phosphorus atoms into MoS2 and increased atomic percentage of Mo oxide after SoD processing. The p-type shifts in the electrical characteristics correspond to a maximum of 2.4 × 1012 cm–2 change in the intrinsic carrier concentration. Repeating the experiment on WSe2 flake devices leads to similar trends: an increased p-type and a decreased n-type conduction indicating p-type doping and a significant increase in the OFF-state leakage current. [ABSTRACT FROM AUTHOR]

The integration of van der Waals materials in nanoelectronic devices requires the deposition of few-layered MX2 films with excellent quality crystals covering a large area. In recent years, astonishing progress in the monolayer growth of WS2 and MoS2 was demonstrated, but multilayer growth resulted often in separated triangular or hexagonal islands. These polycrystalline films cannot fully employ the specific MX2 properties since they are not connected in-plane to the other domains. To coalesce separated islands, ultrahigh-temperature postdeposition anneals in H2S are applied, which are not compatible with bare silicon substrates. Starting from the deposition of stoichiometric short-ordered films, the present work studies different options for subsequent high-temperature annealing in an inert atmosphere to form crystalline films with large grains from stoichiometric films with small grains. The rapid thermal annealing, performed over a few seconds, is compared to excimer laser annealing in the nanosecond range, which are both able to crystallize the thin WS2. The WS2 recrystallization temperature can be lowered using metallic crystallization promoters (Co and Ni). The best result is obtained using a Co cap, due to the circumvention of Co and S binary phase formation below the eutectic temperature. The recrystallization above a critical temperature is accompanied by sulfur loss and 3D regrowth. These undesired effects can be suppressed by the application of a dielectric capping layer prior to annealing. A SiO2 cap can suppress the sulfur loss successfully during annealing and reveals improved material quality in comparison to noncapped films. [ABSTRACT FROM AUTHOR]

Ultra-thin MoS2 film doping through surface functionalization with physically adsorbed species is of great interest due to its ability to dope the film without reduction in the carrier mobility. However, there is a need for understanding how the thickness of the MoS2 film is related to the induced surface doping for improved electrical performance. In this work, we report on the relation of MoS2 film thickness with the doping effect induced by the n-dopant adsorbate poly(vinyl-alcohol). Field effect transistors built using MoS2 films of different thicknesses were electrically characterized, and it was observed that the ION/OFF ratio after doping in thin films is more than four orders of magnitudes greater when compared with thick films. Additionally, a semi-classical model tuned with the experimental devices was used to understand the spatial distribution of charge in the channel and explain the observed behavior. From the simulation results, it was revealed that the two-dimensional carrier density induced by the adsorbate is distributed rather uniformly along the complete channel for thin films (<5.2 nm) contrary to what happens for thicker films. [ABSTRACT FROM AUTHOR]

Spin Torque Majority Gate (STMG) is a logic concept that inherits the non-volatility and the compact size of MRAM devices. In the original STMG design, the operating range was restricted to very small size and anisotropy, due to the exchange-driven character of domain expansion. Here, we propose an improved STMG concept where the domain wall is driven with current. Thus, input switching and domain wall propagation are decoupled, leading to higher energy efficiency and allowing greater technological optimization. To ensure majority operation, pinning sites are introduced. We observe through micromagnetic simulations that the new structure works for all input combinations, regardless of the initial state. Contrary to the original concept, the working condition is only given by threshold and depinning currents. Moreover, cascading is now possible over long distances and fan-out is demonstrated. Therefore, this improved STMG concept is ready to build complete Boolean circuits in absence of external magnetic fields. [ABSTRACT FROM AUTHOR]

With the rapid progress of spintronic devices, spin-logic concepts hold promises of energy-delay conscious computation for efficient logic gate operations. We report on the electrical characterization of domain walls in interconnected magnetic tunnel junctions. By means of spin-transfer torque effect, domains walls are produced at the common free layer and its propagation towards the output pillar sensed by tunneling magneto-resistance. Domain pinning conditions are studied quasi-statically showing a strong dependence on pillar size, ferromagnetic free layer width and inter-pillar distance. Addressing pinning conditions are detrimental for cascading and fan-out of domain walls across nodes, enabling the realization of domain-wall-based logic technology. [ABSTRACT FROM AUTHOR]

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 the absence of a template are polycrystalline or amorphous. Little is known about their nucleation mechanisms. Therefore, the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma and H2S at 300 °C is investigated 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 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. [ABSTRACT FROM AUTHOR]

We study the excitation of spin waves in scaled magnetic waveguides using the magnetoelastic effect. In uniformly magnetized systems, normal strains parallel or perpendicular to the magnetization direction do not lead to spin wave excitation since the magnetoelastic torque is zero. Using micromagnetic simulations, we show that the nonuniformity of the magnetization in submicron waveguides due to the effect of the demagnetizing field leads to the excitation of spin waves for oscillating normal strains both parallel and perpendicular to the magnetization. The excitation by biaxial normal in-plane strain was found to be much more efficient than that by uniaxial normal out-of-plane strain. For narrow waveguides with a width of 200 nm, the excitation efficiency of biaxial normal in-plane strain was comparable to that of shear strain. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic anisotropy (PMA) is an essential condition for CoFe thin films used in magnetic random access memories. Until recently, interfacial PMA was mainly known to occur in materials stacks with MgO\CoFe(B) interfaces or using an adjacent crystalline heavy metal film. Here, PMA is reported in a CoFeB\Ta bilayer deposited on amorphous high-κ dielectric (relative permittivity κ=20) HfO2, grown by atomic layer deposition (ALD). PMA with interfacial anisotropy energy Ki up to 0.49 mJ/m2 appears after annealing the stacks between 200°C and 350°C, as shown with vibrating sample magnetometry. Transmission electron microscopy shows that the decrease of PMA starting from 350°C coincides with the onset of interdiffusion in the materials. High-κ dielectrics are potential enablers for giant voltage control of magnetic anisotropy (VCMA). The absence of VCMA in these experiments is ascribed to a 0.6 nm thick magnetic dead layer between HfO2 and CoFeB. The results show PMA can be easily obtained on ALD high-κ dielectrics. [ABSTRACT FROM AUTHOR]

We report on the electrical characterization of synthetic, large-area MoS2 layers obtained by the sulfurization technique. The effects of dielectric encapsulation and localized defect states on the intrinsic transport properties are explored with the aid of temperature-dependent measurements. We study the effect of dielectric environment by transferring as-grown MoS2 films into different dielectrics such as SiO2, Al2O3, HfO2, and ZrO2 with increasing dielectric permittivity. Electrical data are collected on a statistically-relevant device ensemble and allow to assess device performances on a large scale assembly. Our devices show relative in-sensitiveness of mobility with respect to dielectric encapsulation. We conclude that the device behavior is strongly affected by several scattering mechanisms of different origin that can completely mask any effect related to dielectric mismatch. At low temperatures, conductivity of the devices is thermally activated, a clear footprint of the existence of a mobility edge separating extended states in the conduction band from impurity states in the band-gap. [ABSTRACT FROM AUTHOR]

The impact of the scaling of the channel length on the performances of metal-oxide-semiconductor field effect transistors, based on two-dimensional (2D) channel materials, is theoretically investigated, using density functional theory combined with the non-equilibrium Green's function method. It is found that the scaling of the channel length below 10nm leads to strong device performance degradations. Our simulations reveal that this degradation is essentially due to the tunneling current flowing between the source and the drain in these aggressively scaled devices. It is shown that this electron tunneling process is modulated by the effective mass of the 2D channel material, and sets the limit of the scaling in future transistor designs. [ABSTRACT FROM AUTHOR]

Conventional junctionless (JL) multi-gate (MuG) field-effect transistors(FETs) require extremely scaled channels to deliver high on-state current with low short-channel effect related leakage. In this letter, using ultra-thin 2D materials (e.g., monolayer MoS2), we present comparison of short-channel effects in JL, and inversion-mode (IM) FETs. We show that JL FETs exhibit better sub-threshold slope (S.S.) and drain-induced-barrier-lowering (DIBL) in comparison to IM FETs due to reduced peak electric field at the junctions. But, threshold voltage (VT) roll-off with channel length downscaling is found to be significantly higher in JL FETs than IM FETs, due to higher source/drain controlled charges (dE/dx) in the channel. Further, we show that although VT roll-off in JL FETs improves by increasing the gate control, i.e., by scaling the oxide, or channel thickness, the sensitivity of threshold voltage on structural parameters is found out to be high. [ABSTRACT FROM AUTHOR]

Achieving low resistance contacts to graphene is a common concern for graphene device performance and hybrid graphene/metal interconnects. In this work, we have used the circular Transfer Length Method (cTLM) to electrically characterize Ag, Au, Ni, Ti, and Pd as contact metals to graphene. The consistency of the obtained results was verified with the characterization of up to 72 cTLM structures per metal. Within our study, the noble metals Au, Ag and Pd, which form a weaker bond with graphene, are shown to result in lower contact resistance (Rc) values compared to the more reactive Ni and Ti. X-ray Photo Electron Spectroscopy and Transmission Electron Microscopy characterization for the latter have shown the formation of Ti and Ni carbides. Graphene/Pd contacts show a distinct intermediate behavior. The weak carbide formation signature and the low Rc values measured agree with theoretical predictions of an intermediate state of weak chemisorption of Pd on graphene. [ABSTRACT FROM AUTHOR]

Perpendicular Magnetic Anisotropy (PMA) is a key requirement for state of the art Magnetic Random Access Memories (MRAM). Currently, PMA has been widely reported in standard Magnetic Tunnel Junction material stacks using MgO as a dielectric. In this contribution, we present the first report of PMA at the interface with a high-j dielectric grown by Atomic Layer Deposition, HfO2. The PMA appears after annealing a HfO2\Co\Pt\Ru stack in N2 with the Keff of 0.25 mJ/m² as determined by Vibrating Sample Magnetometry. X-Ray Diffraction and Transmission Electron Microscopy show that the appearance of PMA coincides with interdiffusion and the epitaxial ordering of the Co\Pt bilayer. High-κ dielectrics are especially interesting for Voltage Control of Magnetic Anisotropy applications and are of potential interest for low-power MRAM and spintronics technologies. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic anisotropy (PMA) is an essential condition for CoFe thin films used in magnetic random access memories. Until recently, interfacial PMA was mainly known to occur in materials stacks with MgO\CoFe(B) interfaces or using an adjacent crystalline heavy metal film. Here, PMA is reported in a CoFeB\Ta bilayer deposited on amorphous high-κ dielectric (relative permittivity κ=20) HfO2, grown by atomic layer deposition (ALD). PMA with interfacial anisotropy energy Ki up to 0.49 mJ/m2 appears after annealing the stacks between 200°C and 350°C, as shown with vibrating sample magnetometry. Transmission electron microscopy shows that the decrease of PMA starting from 350°C coincides with the onset of interdiffusion in the materials. High-κ dielectrics are potential enablers for giant voltage control of magnetic anisotropy (VCMA). The absence of VCMA in these experiments is ascribed to a 0.6 nm thick magnetic dead layer between HfO2 and CoFeB. The results show PMA can be easily obtained on ALD high-κ dielectrics. [ABSTRACT FROM AUTHOR]