Hysteresis associated with intrinsic-oxide traps in gate-tunable tetrahedral CVD-MoS2 memristor. - In: IEEE Xplore digital library, ISSN 2473-2001, (2022), S. 527-530
We introduce back gated memristor based on CVD-grown 30-40 nm thick MoS2 channel. The device demonstrates bipolar behaviour and the measurements are consistent with the simulations performed within the intrinsic-oxide traps model. This confirms the theory that the source of hysteresis in thin-film MoS2 memristors is charge trapping on MoS2/SiO2 interface and the grain boundaries. The impact of back gate voltage bias, voltage sweep range and channel area on memristive effect was studied and quantified using hysteresis area. Hysteresis in bipolar memristors can be tuned by back gate voltage, which makes these devices promising for neuromorphic computing.
Formation and characterization of three-dimensional tetrahedral MoS2 thin films by chemical vapor deposition. - In: Crystal growth & design, ISSN 1528-7505, Bd. 22 (2022), 9, S. 5229-5238
A method to synthesize the three-dimensional arrangement of bulk tetrahedral MoS2 thin films by solid source chemical vapor deposition of MoO3 and S is presented. The developed synthesizing recipe uses a temperature ramping with a constant N2 gas flow in the deposition process to grow tetrahedral MoS2 thin film layers. The study analyses the time-dependent growth morphologies, and the results are combined and presented in a growth model. A combination of optical, electron, atomic force microscopy, Raman spectroscopy, and X-ray diffraction are used to study the morphological and structural features of the tetrahedral MoS2 thin layers. The grown MoS2 is c-axis oriented 2H-MoS2. Additionally, the synthesized material is further used to fabricate back-gated field-effect transistors (FETs). The fabricated FET devices on the tetrahedral MoS2 show on/off current ratios of 10^6 and mobility up to ∼56 cm^2 V^-1 s^-1 with an estimated carrier concentration of 4 × 10^16 cm-3 for VGS = 0 V.
The angle dependent ΔE effect in TiN/AlN/Ni micro cantilevers. - In: Sensors and actuators, ISSN 1873-3069, Bd. 345 (2022), 113784
In this work, magnetoelectric MEMS sensors based on a TiN/AlN/Ni laminate are investigated for the first time in regards of the anisotropic elastic properties when using hard magnetic Nickel as magnetostrictive layer. The implications of crystalline, uniaxial and shape anisotropy are analysed arising from the anisotropic ΔE effect in differently oriented cantilevers with 25 µm length and 15˚ spacing. The ΔE effect is derived analytically to consider the angular dependency of the different anisotropies within the sensors. In the measured frequency spectra complex profiles are observable consisting of contributions from neighbouring structures which are connected by a common electrode. The crosstalk effect is strongly depending on the cantilever orientation and reflects the anisotropic mechanical properties of the material stack. The intensity of the crosstalk effect is increasing for shortened cantilevers and narrowing distance between structures. The ΔE effect is investigated based on cantilevers of different angular spacing and of a single cantilever that is rotated in the magnetic field. The derived peak sensitivities are reaching values of 1.15 and 1.31T-1. The angular dependency of the sensitivity is found to be approximately constant for differently oriented cantilevers. In contrast, for a singly rotated cantilever an angular dependency of the 4th order is observed.
Silicon carbide formation in reactive silicon-carbon multilayers. - In: Materials science forum, ISSN 1662-9752, Bd. 1062 (2022), S. 44-48
An alternative low thermal budget silicon carbide syntheses route is presented. The method is based on self-propagating high-temperature synthesis of binary silicon-carbon-based reactive multilayers. With this technique, it is possible to obtain cubic polycrystalline silicon carbide at relatively low annealing temperatures by a solid state reaction. The reaction starts above 600 ˚C. The transformation process proceeds in a four-step process. The reaction enthalpy was determined to be (-70 ± 4) kJ/mol.
Silicon carbide - graphene nano-gratings on 4H and 6H semi-insulating SiC. - In: Materials science forum, ISSN 1662-9752, Bd. 1062 (2022), S. 170-174
A technical methodology of fabrication of hierarchically scaled multitude graphene nanogratings with varying pitches ranging from the micrometer down to sub 40 nm scale combined with sub 10 nm step heights on 4H and 6H semi-insulating SiC for length scale measurements is proposed. The nanogratings were fabricated using electron-beam lithography combined with dry etching of graphene, incorporating a standard semiconductor processing technology. A scientific evaluation of critical dimension, etching step heights, and surface characterization of graphene nanograting on both polytypes were compared and evaluated.
Designing MoS2 channel properties for analog memory in neuromorphic applications. - In: Journal of vacuum science & technology, ISSN 2166-2754, Bd. 40 (2022), 3, S. 030602-1-030602-5
In this paper, we introduce analog nonvolatile random access memory cells for neuromorphic computing. The analog memory cell MoS2 channel is designed based on the simulation model including Fowler-Nordheim tunneling through a charge-trapping stack, trapping process, and transfer characteristics to describe a full write/read circle. 2D channel materials provide scaling to higher densities as well as preeminent modulation of the conductance by the accumulated space charge from the oxide trapping layer. In this paper, the main parameters affecting the distribution of memory states and their total number are considered. The dependence of memory state distribution on channel doping concentration and the number of layers is given. In addition, how the nonlinearity of memory state distribution can be overcome by variation of operating conditions and by applying pulse width modulation to the bottom gate voltage is also shown.
Size dependent properties of reactive materials. - In: Inorganics, ISSN 2304-6740, Bd. 10 (2022), 4, 56, S. 1-19
The nature of the self-sustained reaction of reactive materials is dependent on the physical, thermal, and mechanical properties of the reacting materials. These properties behave differently at the nano scale. Low-dimensional nanomaterials have various unusual size dependent transport properties. In this review, we summarize the theoretical and experimental reports on the size effect on melting temperature, heat capacity, reaction enthalpy, and surface energy of the materials at nano scale because nanomaterials possess a significant change in large specific surface area and surface effect than the bulk materials. According to the theoretical analysis of size dependent thermodynamic properties, such as melting temperature, cohesive energy, thermal conductivity and specific heat capacity of metallic nanoparticles and ultra-thin layers varies linearly with the reciprocal of the critical dimension. The result of this scaling relation on the material properties can affect the self-sustained reaction behavior in reactive materials. Resultant, powder compacts show lower reaction propagation velocities than bilayer system, if the particle size of the reactants and the void density is decreased an increase of the reaction propagation velocity due to an enhanced heat transfer in reactive materials can be achieved. Standard theories describing the properties of reactive material systems do not include size effects.
Metal oxide semiconducting nanomaterials for air quality gas sensors: operating principles, performance, and synthesis techniques. - In: Microchimica acta, ISSN 1436-5073, Bd. 189 (2022), 5, 196, S. 1-22
To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring.
Electric bias-induced edge degradation of few-layer MoS2 devices. - In: Materials today, ISSN 2214-7853, Bd. 53 (2022), 2, S. 281-284
In this work, we experimentally investigate the effects of electric bias on the degradation of few-layer MoS2 back-gated field-effect transistors in ambient air. The devices were fabricated using mechanically exfoliated MoS2 flakes, which were transferred to a Si/SiO2 substrate by a PDMS-based transfer. We report an accelerated electric bias-induced degradation of the devices under investigation and used optical and scanning electron microscopy (SEM) to monitor changes of the morphology of the MoS2 channel. In particular, we found a linear dependency of the degradation on the electric field between the Ti/Au source and drain contacts. In addition, we identify four regions in which morphological changes occur, of which the edges of the MoS2 channel are most affected.
Nanoscale surface morphology modulation of graphene - i-SiC heterostructures. - In: Materials today, ISSN 2214-7853, Bd. 53 (2022), 2, S. 289-292
A multitude gratings design consists of gratings with different pitches ranging from the micrometre down to sub 40 nm scale combined with sub 10 nm step heights modulating the surface morphology for length scale measurements is proposed. The surface morphology modulation was performed using electron beam lithography incorporating a standard semiconductor processing technology. The critical dimension, edge roughness, step heights and line morphology in dependence on the grating pitch is studied.