In this paper, the electrical behavior of tungsten carbide (WC) Schottky barrier on 4H-SiC was investigated. First, a statistical current-voltage (I-V) analysis in forward bias, performed on a set of equivalent diodes, showed a symmetric Gaussian-like distribution of the barrier heights after annealing at 700 degrees C, where a low Schottky barrier height (phi(B) = 1.05 eV) and an ideality factor n = 1.06 were measured. The low value of the barrier height makes such a WC contact an interesting candidate to reduce the conduction losses in 4H-SiC Schottky diodes. A deeper characterization has been carried out, by monitoring the temperature dependence of the I-V characteristics and the behavior of the relevant parameters phi(B) and n. The increase of the barrier height and decrease of the ideality factor with increasing temperature indicated a lateral inhomogeneity of the WC/4H-SiC Schottky contact, which was described by invoking Tung's model. Interestingly, the temperature dependence of the leakage current under reverse bias could be described by considering in the thermionic field emission model the temperature dependent barrier height related to the inhomogeneity. These results can be useful to predict the behavior of WC/4H-SiC Schottky diodes under operative conditions.

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 degrees C. After optimization of the C(3)H(8)flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a similar to 2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp(2)/sp(3)hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size similar to 7 nm) and compressively strained. A Gr sheet resistance of similar to 15.8 k omega sq(-1)was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

An electronic device comprising : a semiconductor body of silicon carbide , SiC , having a first and a second face , opposite to one another along a first direction , which pres ents positive - charge carriers at said first face that form a positive interface charge ; a first conduction terminal , which extends at the first face of the semiconductor body ; a second conduction terminal , which extends on the second face of the semiconductor body ; a channel region in the semicon ductor body , configured to house , in use , a flow of electrons between the first conduction terminal and the second con duction terminal ; and a trapping layer , of insulating material , which extends in electrical contact with the semiconductor body at said channel region and is designed so as to present electron - trapping states that generate a negative charge such as to balance , at least in part , said positive interface charge .

A normally - off HEMT transistor includes a heterostructure including a channel layer and a barrier layer on the channel layer ; a 2DEG layer in the heterostructure ; an insulation layer in contact with a first region of the barrier layer ; and a gate electrode through the whole thickness of the insula tion layer , terminating in contact with a second region of the barrier layer . The barrier layer and the insulation layer have a mismatch of the lattice constant ( " lattice mismatch " ) , which generates a mechanical stress solely in the first region of the barrier layer , giving rise to a first concentration of electrons in a first portion of the two - dimensional conduc tion channel which is under the first region of the barrier layer which is greater than a second concentration of elec trons in a second portion of the two - dimensional conduction channel which is under the second region of the barrier layer.

This chapter reviews the current technologies for normally-off GaN-based HEMTs. First, the HEMT "cascode" approach is briefly described, highlighting advantages and limitation of this design.Then, after illustrating the recessed gate HEMT and the fluorinate gate approach, the focus is put on the recessed gate hybrid metal insulator semiconductor high electron mobility transistor (MISHEMT) and on the p-GaN gate HEMT. These are today the most promising and robust approaches for normally-off GaN HEMTs. The most critical issues of these technologies (e.g. heterostructure design, gate dielectrics, and metal gates) are discussed in this chapter.

This chapter is a general introduction to the properties and applications of GaN and related materials. After an historical background on the relevant milestones of nitrides research, special emphasis will be put on InGaN quantum wells and AlGaN/GaN heterostructures, which are important systems for light-emitting diodes (LEDs), laser diodes (LDs), and high electron mobility transistors (HEMTs). The main applications of nitride materials for both optoelectronic devices and power- and high-frequency electronics are also described, anticipating some of the most critical issues that are illustrated in detail in the rest of the book.

The book "Nitride Semiconductor Technology" provides an overview of nitride semiconductors and their uses in optoelectronics and power electronics devices. It explains the physical properties of those materials as well as their growth methods. Their applications in high electron mobility transistors, vertical power devices, LEDs, laser diodes, and vertical-cavity surface-emitting lasers are discussed in detail. The book further examines reliability issues in these materials and puts forward perspectives of integrating them with 2D materials for novel high-frequency and high-power devices. In summary, it covers nitride semiconductor technology from materials to devices and provides the basis for further research.

Gallium nitride (GaN) and its AlGaN/GaN heterostructures grown on large area Si substrates are promising systems to fabricate power devices inside the existing Si CMOS lines. For this purpose, however, Au-free metallizations are required to avoid cross contaminations. In this paper, the mechanisms of current transport in Au-free metallization on AlGaN/GaN heterostructures are studied, with a focus on non-recessed Ti/Al/Ti Ohmic contacts. In particular, an Ohmic behavior of Ti/Al/Ti stacks was observed after an annealing at moderate temperature (600°C). The values of the specific contact resistance ?c decreased from 1.6×10 ?cm to 7×10 ?cm with increasing the annealing time from 60 to 180s. The temperature dependence of ?c indicated that the current flow is ruled by a thermionic field emission (TFE) mechanism, with barrier height values of 0.58 eV and 0.52 eV, respectively. Finally, preliminary results on the forward and reverse bias characterization of Au-free tungsten carbide (WC) Schottky contacts are presented. This contact exhibited a barrier height value of 0.82 eV.

In this work, the origin of the dielectric breakdown of 4H-SiC power MOSFETs was studied at the nanoscale, analyzing devices that failed after extremely long (three months) of high temperature reverse bias (HTRB) stress. A one-to-one correspondence between the location of the breakdown event and a threading dislocation propagating through the epitaxial layer was found. Scanning probe microscopy (SPM) revealed the conductive nature of the threading dislocation and a local modification of the minority carriers concentration. Basing on these results, the role of the threading dislocation on the failure of 4H-SiC MOSFETs could be clarified.

This Letter reports on the active dopant profiling and Ohmic contact behavior in degenerate P-implanted silicon carbide (4H-SiC) layers. Hall measurements showed a nearly temperature-independent electron density, corresponding to an electrical activation of about 80% of the total implanted dose. Using the Hall result as calibration, the depth resolved active P-profile was extracted by scanning capacitance microscopy (SCM). Such information on the active P-profile permitted to elucidate the current injection mechanism at the interface of annealed Ni Ohmic contacts with the degenerate n-type 4H-SiC layer. Modeling the temperature dependence of the specific contact resistance with the thermionic field emission mechanism allowed extracting a doping concentration of 8.5x10(19)cm(-3) below the metal/4H-SiC interface, in excellent agreement with the value independently obtained by the SCM depth profiling. The demonstrated active dopant profiling methodology can have important implications in the 4H-SiC device technology.

In this article, electron trapping in aluminum oxide (Al2O3) thin films grown by plasma enhanced atomic layer deposition on AlGaN/GaN heterostructures has been studied and a correlation with the presence of oxygen defects in the film has been provided. Capacitance-voltage measurements revealed the occurrence of a negative charge trapping effect upon bias stress, able to fill an amount of charge traps in the bulk Al2O3 in the order of 5 x 10(12) cm(-2). A structural analysis based on electron energy-loss spectroscopy demonstrated the presence of low-coordinated Al cations in the Al2O3 film, which is an indication of oxygen vacancies, and can explain the electrical behavior of the film. These charge trapping effects were used for achieving thermally stable (up to 100 degrees C) enhancement mode operation in AlGaN/GaN transistors, by controlling the two-dimensional electron gas depletion.

Tungsten carbide (WC) contacts have been investigated as an original gold-free Schottky metallization for AlGaN/GaN heterostructures. The evolution of the electrical and structural/compositional properties of the WC/AlGaN contact has been monitored as a function of the annealing temperature in the range from 400 to 800 degrees C. The Schottky barrier height (phi(B)) at the WC/AlGaN interface, extracted from the forward current-voltage characteristics of the diode, decreased from 0.82-0.85 eV in the as-deposited and 400 degrees C annealed sample, to 0.56 eV after annealing at 800 degrees C. This large reduction of phi(B)was accompanied by a corresponding increase of the reverse leakage current. Transmission electron microscopy coupled with electron energy loss spectroscopy analyses revealed the presence of oxygen (O) uniformly distributed in the WC layer, both in the as-deposited and 400 degrees C annealed sample. Conversely, oxygen accumulation in a 2-3 nm thin W-O-C layer at the interface with AlGaN was observed after the annealing at 800 degrees C, as well as the formation of W2C grains within the film (confirmed by x-ray diffraction analyses). The formation of this interfacial W-O-C layer is plausibly the main origin of the decreased phi(B)and the increased leakage current in the 800 degrees C annealed Schottky diode, whereas the decreased O content inside the WC film can explain the reduced resistivity of the metal layer. The results provide an assessment of the processing conditions for the application of WC as Schottky contact for AlGaN/GaN heterostructures.

A method based on cyclic gate bias stress followed by a single point drain current measurement is used to probe the interface or near-interface traps in the SiO2/4H-SiC system over the whole 4H-SiC bandgap. The temperature-dependent instability of the threshold voltage in lateral MOSFETs is investigated, and two separated trapping mechanisms were found. The experimental results corroborate the hypothesis that one mechanism is nearly temperature independent and it is correlated with the presence of near-interface oxide traps that are trapped via tunneling from the semiconductor. The second mechanism, having an activation energy of 0.1eV, has been correlated with the presence of intrinsic defects at the SiO2/4H-SiC interface.

The nucleation and growth mechanism of aluminum oxide (AlO) in the early stages of atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM), conductive-atomic force microscopy (C-AFM) and Raman spectroscopy. Differently than for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by AlO island growth in the very early stages, followed by the formation of a continuous AlO film (~2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. The electrical insulating properties of the deposited ultrathin AlO films were demonstrated by nanoscale current mapping with C-AFM. Raman spectroscopy analyses showed low impact of the ALD process on the defect's density of EG. The EG strain was also almost unaffected by the deposition in the regime of island growth and coalescence, whereas a significant increase was observed after the formation of a compact AlO film. The obtained results can have important implications for device applications of epitaxial graphene requiring ultra-thin high-k insulators.

Nickel silicidation reactions were activated on 4H-SiC using laser annealing at wavelength of 308 nm to study interaction and reaction of the involved atomic species. With this intent, the deposited nickel layer thickness was scaled from 100 nm to 10 nm and the laser fluence was spanned from 2.2 to 4.2 J/cm(2). A combination of structural characterization by X-ray diffraction, Energy-Dispersive X-ray spectroscopy, Raman Spectroscopy, Transmission Electron Microscopy and morphological investigations through Scanning Electron Microscopy with theoretical predictions as a function of the applied laser fluence, have unveiled that the starting nickel thickness plays a main role, especially above the threshold for nickel melting (2.8 J/cm(2)). As a general paradigm, sufficient silicon release from the substrate occurs above this threshold that is available for silicidation, with amount increasing with the laser fluence. This addresses stoichiometry and morphology of the silicided contact that indeed depend on the available Nickel atoms (i.e., the Ni layer thickness) and on the thermal profile, as tested at a fixed fluence of 3.8 J/cm(2). In addition, a layer-by-layer variable stoichiometry is established in each sample through the contact, with the deepest silicide being relatively more Si-rich. All those findings have impact on the electrical parameters of testing diodes. Based on data cross-linking, NiSi2 contacting layers and C-free interfaces provide a convenience in reducing resistance contributions.

Emerging wide bandgap semiconductor devices such as the ones built with SiC have the potential to revolutionize the power electronics industry through faster switching speeds, lower losses, and higher blocking voltages, which are superior to standard silicon-based devices. The current epitaxial technology enables more controllable and less defective large area substrate growth for the hexagonal polymorph of SiC (4H-SiC) with respect to the cubic counterpart (3C-SiC). However, the cubic polymorph exhibits superior physical properties in comparison to its hexagonal counterpart, such as a narrower bandgap (2.3 eV), possibility to be grown on a silicon substrate, a reduced density of states at the SiC/SiO2 interface, and a higher channel mobility, characteristics that are ideal for its incorporation in metal oxide semiconductor field effect transistors. The most critical issue that hinders the use of 3C-SiC for electronic devices is the high number of defects in bulk and epilayers, respectively. Their origin and evolution are not understood in the literature to date. In this manuscript, we combine ab initio calibrated Kinetic Monte Carlo calculations with transmission electron microscopy characterization to evaluate the evolution of extended defects in 3C-SiC. Our study pinpoints the atomistic mechanisms responsible for extended defect generation and evolution, and establishes that the antiphase boundary is the critical source of other extended defects such as single stacking faults with different symmetries and sequences. This paper showcases that the eventual reduction of these antiphase boundaries is particularly important to achieve good quality crystals, which can then be incorporated in electronic devices.

In spite of its great promise for energy-efficient power conversion, the electronic quality of cubic silicon carbide (3C-SiC) on silicon is currently limited by the presence of a variety of extended defects in the heteroepitaxial material. However, the specific role of the different defects on the electronic transport is still under debate. A macro- and nanoscale characterization of Schottky contacts on 3C-SiC/Si is carried out to elucidate the impact of the anti-phase boundaries (APBs) and stacking faults (SFs) on the forward and reverse current-voltage characteristics of these devices. Current mapping of 3C-SiC by conductive atomic force microscopy directly shows the role of APBs as the main defects responsible of the reverse bias leakage, while both APBs and SFs are shown to work as preferential current paths under forward polarization. Distinct differences between these two types of defects are also confirmed by electronic transport simulations of a front-to-back contacted SF and APB. These experimental and simulation results provide a picture of the role played by different types of extended defects on the electrical transport in vertical or quasi-vertical devices based on 3C-SiC/Si, and can serve as a guide for improving material quality by defects engineering.

Due to its excellent physical properties and availability directly on a semiconductor substrate, epitaxial graphene (EG) grown on the (0001) face of hexagonal silicon carbide is a material of choice for advanced applications in electronics, metrology and sensing. The deposition of ultrathin high-k insulators on its surface is a key requirement for the fabrication of EG-based devices, and, in this context, atomic layer deposition (ALD) is the most suitable candidate to achieve uniform coating with nanometric thickness control. This paper presents an overview of the research on ALD of high-k insulators on EG, with a special emphasis on the role played by the peculiar electrical/structural properties of the EG/SiC (0001) interface in the nucleation step of the ALD process. The direct deposition of AlO thin films on the pristine EG surface will be first discussed, demonstrating the critical role of monolayer EG uniformity to achieve a homogeneous AlO coverage. Furthermore, the ALD of several high-k materials on EG coated with different seeding layers (oxidized metal films, directly deposited metal-oxides and self-assembled organic monolayers) or subjected to various prefunctionalization treatments (e.g., ozone or fluorine treatments) will be presented. The impact of the pretreatments and of thermal ALD growth on the defectivity and electrical properties (doping and carrier mobility) of the underlying EG will be discussed.

Semiconducting transition metal dichalcogenides (TMDs) are promising materials for future electronic and optoelectronic applications. However, their electronic properties are strongly affected by peculiar nanoscale defects/inhomogeneities (point or complex defects, thickness fluctuations, grain boundaries, etc.), which are intrinsic of these materials or introduced during device fabrication processes. This paper reviews recent applications of conductive atomic force microscopy (C-AFM) to the investigation of nanoscale transport properties in TMDs, discussing the implications of the local phenomena in the overall behavior of TMD-based devices. Nanoscale resolution current spectroscopy and mapping by C-AFM provided information on the Schottky barrier uniformity and shed light on the mechanisms responsible for the Fermi level pinning commonly observed at metal/TMD interfaces. Methods for nanoscale tailoring of the Schottky barrier in MoS for the realization of ambipolar transistors are also illustrated. Experiments on local conductivity mapping in monolayer MoS grown by chemical vapor deposition (CVD) on SiO substrates are discussed, providing a direct evidence of the resistance associated to the grain boundaries (GBs) between MoS domains. Finally, C-AFM provided an insight into the current transport phenomena in TMD-based heterostructures, including lateral heterojunctions observed within MoWSe alloys, and vertical heterostructures made by van der Waals stacking of different TMDs (e.g., MoS/WSe) or by CVD growth of TMDs on bulk semiconductors.

The dielectric breakdown (BD) of thermal oxide (SiO) grown on cubic silicon carbide (3C-SiC) was investigated comparing the electrical behavior of macroscopic metal-oxidesemiconductor (MOS) capacitors with nanoscale current and capacitance mapping using conductive atomic force (C-AFM) and scanning capacitance microscopy (SCM). Spatially resolved statistics of the oxide BD events by C-AFM revealed that the extrinsic premature BD is correlated to the presence of peculiar extended defects, the anti-phase boundaries (APBs), in the 3C-SiC layer. SCM analyses showed a larger carrier density at the stacking faults (SFs) the 3C-SiC, that can be explained by a locally enhanced density of states in the conduction band. On the other hand, a local increase of minority carriers concentration was deduced for APBs, indicating that they behave as conducting defects having also the possibility to trap positive charges. The results were explained with the local electric field enhancement in correspondence of positively charged defects.