One of the main challenges to exploit molybdenum disulfide (MoS) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS surface can be tailored at nanoscale using soft O plasma treatments. The morphological, chemical, and electrical modifications of MoS surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS to a broader distribution (from 0.2 to 0.8 eV) after 600 s O plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS FETs, fabricated by self-aligned deposition of source/drain contacts in the O plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio I/I ≈ 10 and field-effect mobilities of 11.5 and 7.2 cm V s for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O plasma functionalized MoS surface.

In this paper, hydrogen bubbling delamination of graphene (Gr) from copper using a strong electrolyte (KOH) water solution was performed, focusing on the effect of the KOH concentration (CKOH) on the Gr delamination rate. A factor of ∼10 decrease in the time required for the complete Gr delamination from Cu cathodes with the same geometry was found increasing CKOH from ∼0.05 M to ∼0.60 M. After transfer of the separated Gr membranes to SiO2 substrates by a highly reproducible thermo-compression printing method, an accurate atomic force microscopy investigation of the changes in Gr morphology as a function of CKOH was performed. Supported by these analyses, a microscopic model of the delamination process has been proposed, where a key role is played by graphene wrinkles acting as nucleation sites for H2 bubbles at the cathode perimeter. With this approach, the H2 supersaturation generated at the electrode for different electrolyte concentrations was estimated and the inverse dependence of td on CKOH was quantitatively explained. Although developed in the case of Cu, this analysis is generally valid and can be applied to describe the electrolytic delamination of graphene from several metal substrates.

The temperature dependence of the specific resistance ρc in annealed Ti∕Al∕Ni∕Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600°C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (>700°C), with ρc in the low 10−5Ωcm2 range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600°C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21eV after annealing at 600°Cto0.81eV at 800°C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2×1018cm−3 in the as-prepared sample to 4.6×1019cm−3 in the annealed contacts. The electrical properties were correlated to the microstructure of the interfacial region, providing a scenario to explain the transition from Schottky to Ohmic behavior in annealed Ti∕Al∕Ni∕Au contacts.

In this letter, high responsivity 4H-SiC vertical Schottky UV photodiodes based on the pinch-off surface effect, obtained by means of self-aligned Ni2Si interdigit contacts, are demonstrated. The diode area was 1mm2, with a 37% directly exposed to the radiation. The dark current was about 200pA at −50V. Under a 256nm UV illumination, a current increase of more than two orders of magnitude is observed, resulting in a 78% internal quantum efficiency. The vertical photodiodes showed an ultraviolet-visible rejection ratio >7×103 and a responsivity a factor of about 1.8 higher than a conventional planar metal-semiconductor-metal structure.

The results of a new epitaxial process using an industrial 6x2” wafer reactor with the introduction of HCl during the growth have been reported. A complete reduction of silicon nucleation in the gas phase has been observed even for high silicon dilution parameters (Si/H2>0.05) and an increase of the growth rate until about 20 µm/h has been measured. No difference has been observed in terms of defects, doping uniformity (average maximum variation 8%) and thickness uniformity (average maximum variation 1.2 %) with respect to the standard process without HCl.

High-quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. Atomic layer deposition (ALD) is the method of choice to deposit high-κ dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp Gr surface, a chemical prefunctionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper, we report a detailed morphological, structural, and electrical investigation of AlO thin films grown by a two-steps ALD process on a large area Gr membrane residing on an AlO-Si substrate. This process consists of the HO-activated deposition of a AlO seed layer a few nanometers in thickness, performed in situ at 100 °C, followed by ALD thermal growth of AlO at 250 °C. The optimization of the low-temperature seed layer allowed us to obtain a uniform, conformal, and pinhole-free AlO film on Gr by the second ALD step. Nanoscale-resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by the increase of Gr sheet resistance (from ∼300 Ω/sq in pristine Gr to ∼1100 Ω/sq after AlO deposition). Analysis of the transfer characteristics of Gr field-effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (ε = 7.45) and the breakdown electric field (E = 7.4 MV/cm) of the AlO film as well as the transconductance and the holes field-effect mobility (∼1200 cm V s). A special focus has been given to the electrical characterization of the AlO-Gr interface by the analysis of high frequency capacitance-voltage measurements, which allowed us to elucidate the charge trapping and detrapping phenomena due to near-interface and interface oxide traps.

Molybdenum disulphide (MoS2) is currently regarded as a promising material for the next generation of electronic and optoelectronic devices. However, several issues need to be addressed to fully exploit its potential for field effect transistor (FET) applications. In this context, the contact resistance, R C, associated with the Schottky barrier between source/drain metals and MoS2 currently represents one of the main limiting factors for suitable device performance. Furthermore, to gain a deeper understanding of MoS2 FETs under practical operating conditions, it is necessary to investigate the temperature dependence of the main electrical parameters, such as the field effect mobility (μ) and the threshold voltage (V th). This paper reports a detailed electrical characterization of back-gated multilayer MoS2 transistors with Ni source/drain contacts at temperatures from T = 298 to 373 K, i.e., the expected range for transistor operation in circuits/systems, considering heating effects due to inefficient power dissipation. From the analysis of the transfer characteristics (I D−V G) in the subthreshold regime, the Schottky barrier height (ΦB ≈ 0.18 eV) associated with the Ni/MoS2 contact was evaluated. The resulting contact resistance in the on-state (electron accumulation in the channel) was also determined and it was found to increase with T as R C proportional to T 3.1. The contribution of R C to the extraction of μ and V th was evaluated, showing a more than 10% underestimation of μ when the effect of R C is neglected, whereas the effect on V th is less significant. The temperature dependence of μ and V th was also investigated. A decrease of μ proportional to 1/T α with α = 1.4 ± 0.3 was found, indicating scattering by optical phonons as the main limiting mechanism for mobility above room temperature. The value of V th showed a large negative shift (about 6 V) increasing the temperature from 298 to 373 K, which was explained in terms of electron trapping at MoS2/SiO2 interface states.

This paper reports on the behavior of Al/Ti/ p-GaN interfaces as gate contacts for p-GaN/AlGaN/GaN normally off high electron mobility transistor (HEMTs), highlighting the impact of the thermal budget on the metal gate on the device characteristics. In fact, while the devices subjected to an annealing at 800°C show a considerable high leakage current, those with nonannealed Al/Ti gate contacts exhibit a normally off behavior, with a pinch-off voltage V po = +1.1 V and an on/off current ratio of 3 × 10 8 . Temperature-dependent electrical measurements on back-to-back Schottky diodes allowed to determine a Schottky barrier height B of 2.08 and 1.60 eV, for the nonannealed and 800°C annealed gate contacts, respectively. Hence, the increase in the leakage current observed upon annealing at 800°C was attributed to the lowering of the Schottky barrier height B of the metal gate. The interfacial structural characterization explained the barrier lowering induced by the annealing. This scenario was discussed through the simulated band diagram of the heterostructures, considering the experimental values of B . These results provide useful information for the device makers to optimize the fabrication flow of normally off HEMTs with p-GaN gate.Index Terms-AlGaN/GaN heterostructures, enhancement mode operation, high electron mobility transistors (HEMTs), p-GaN, Schottky barrier. 0018-9383