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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 °C. The Schottky barrier height (ΦB) 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 °C annealed sample, to 0.56 eV after annealing at 800 °C. This large reduction of ΦB 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 °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 °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 ΦB and the increased leakage current in the 800 °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. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the electrical behavior of a Ni/Au Schottky barrier on free standing GaN has been studied employing a variety of techniques and correlated with the material and interface quality. The temperature dependence of the ideality factor (n) and of the Schottky barrier height (Φ B) revealed a spatial inhomogeneity of the barrier. This behavior has been described by means of the Tung's model on inhomogeneous Schottky barriers. The origin of the barrier inhomogeneity can be likely associated to the surface quality of the GaN epilayer or to microstructure of the Ni/GaN interface. [ABSTRACT FROM AUTHOR]

The forward current-voltage ( I-V) characteristics of Ni/Au Schottky contacts on AlGaN/GaN heterostructures have been studied in this work. The electrical characteristics exhibited a strongly non-ideal behavior that could not be described by the thermionic emission theory. Hence, we used a 'two diodes model,' considering both the presence of the Ni/AlGaN barrier and of a second barrier height at the AlGaN/GaN heterojunction. Capacitance-voltage ( C-V) measurements enabled us to experimentally determine the properties of the two dimensional electron gas (2DEG) and, hence, of the second barrier at the AlGaN/GaN interface. Following this approach, the anomalous I-V curves could be explained. Moreover, the value of the barrier height at zero-electric field (flat-band barrier height) was introduced and determined with this procedure, and resulted in a good agreement with literature data based on photoemission measurements. This approach provides a valid procedure for an accurate determination of the barrier height in AlGaN/GaN heterostructures, and the results can have useful implications for the fabrication of AlGaN/GaN HEMTs devices. [ABSTRACT FROM AUTHOR]

The transfer characteristics ( ID- VG) of multilayers MoS2 transistors with a SiO2/Si backgate and Ni source/drain contacts have been measured on as-prepared devices and after annealing at different temperatures ( Tann from 150 °C to 200 °C) under a positive bias ramp ( VG from 0 V to +20 V). Larger Tann resulted in a reduced hysteresis of the ID- VG curves (from ∼11 V in the as-prepared sample to ∼2.5 V after Tann at 200 °C). The field effect mobility (∼30 cm2 V-1 s-1) remained almost unchanged after the annealing. On the contrary, the subthreshold characteristics changed from the common n-type behaviour in the as-prepared device to the appearance of a low current hole inversion branch after annealing. This latter effect indicates a modification of the Ni/MoS2 contact that can be explained by the formation of a low density of regions with reduced Schottky barrier height (SBH) for holes embedded in a background with low SBH for electrons. Furthermore, a temperature dependent analysis of the subthreshold characteristics revealed a reduction of the interface traps density from ∼9 × 1011 eV-1cm-2in the as-prepared device to ∼2 × 1011 eV-1cm-2after the 200 °C temperature-bias annealing, which is consistent with the observed hysteresis reduction. [ABSTRACT FROM AUTHOR]

Contacts with MoS 2 are currently the object of many investigations, since current injection through metal/MoS 2 interfaces represents one of the limiting factors to the performance of MoS 2 thin film transistors. In this paper, we employed conductive atomic force microscopy (CAFM) to investigate the current injection mechanisms from a nanometric contact (a Pt coated tip) to the surface of MoS 2 thin films exfoliated on SiO 2 . The analysis of local current–voltage ( I – V ) characteristics on a large array of tip positions provided high spatial resolution information on the lateral homogeneity of the tip/MoS 2 Schottky barrier Φ B and of the ideality factor n . From the histograms of the measured Φ B and n values, an average Schottky barrier height of 297 meV with standard deviation of 22 meV and an average ideality factor of 1.65 with a standard deviation is 0.15 have been estimated. The implications of these lateral variations of Φ B and n in MoS 2 nano-Schottky diodes on the electrical properties of macroscopic contacts to MoS 2 have been discussed also in relation with recent literature results. [ABSTRACT FROM AUTHOR]

Conductive atomic force microscopy (CAFM) is employed to investigate the current injection from a nanometric contact (a Pt coated tip) to the surface of MoS2 thin films. The analysis of local current-voltage characteristics on a large array of tip positions provides high spatial resolution information on the lateral homogeneity of the tip/MoS2 Schottky barrier ΦB and ideality factor n, and on the local resistivity ρloc of the MoS2 region under the tip. Here, ΦB=300±24meV, n=1.60±0.23, and ρloc=2.99±0.68Ωcm are calculated from the distributions of locally measured values. A linear correlation is found between the ρloc and ΦB values at each tip position, indicating a similar origin of the ρloc and ΦB inhomogeneities. These findings are compared with recent literature results on the role of sulfur vacancy clusters on the MoS2 surface as preferential paths for current injection from metal contacts. Furthermore, their implications on the behavior of MoS2 based transistors are discussed. [ABSTRACT FROM AUTHOR]

The electrical behaviour of graphene (Gr) contacts to AlxGa1-xN/GaN heterostructures has been investigated, focusing, in particular, on the impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface. Two Al0.25Ga0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, as evaluated by atomic force microscopy (AFM) and transmission electron microscopy, were compared in this study, i.e., a uniform and defect-free sample and a sample with a high density of typical V-defects, which locally cause a reduction of the AlGaN thickness. Nanoscale resolution current voltage (I-V) measurements by an Au coated conductive AFM tip were carried out at several positions both on the bare and Gr-coated AlGaN surfaces. Rectifying contacts were found onto both bare AlGaN surfaces, but with a more inhomogeneous and lower Schottky barrier height (ΦB ≈ 0.6 eV) for AlGaN with V-defects, with respect to the case of the uniform AlGaN (ΦB ≈ 0.9 eV). Instead, very different electrical behaviours were observed in the presence of the Gr interlayer between the Au tip and AlGaN, i.e., a Schottky contact with reduced barrier height (ΦB ≈ 0.4 eV) for the uniform AlGaN and an Ohmic contact for the AlGaN with V-defects. Interestingly, excellent lateral uniformity of the local I-V characteristics was found in both cases and can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities. Due to the locally reduced AlGaN layer thickness, V defect act as preferential current paths from Gr to the 2DEG and can account for the peculiar Ohmic behaviour of Gr contacts on defective AlGaN. [ABSTRACT FROM AUTHOR]

[Display omitted] • This work demonstrates the feasibility of fabricating vertical Schottky diodes on 3C-SiC material obtained by an innovative method combining sublimation epitaxy and chemical vapor deposition. • Morphological, and structural analyses demonstrated the good quality of the epilayers grown with this method. • Vertical Pt/3C-SiC Schottky diodes exhibited an ideality factor of 1.21 and a barrier height of 0.6 eV. • The temperature dependent forward characteristics suggested the formation of an inhomogeneous Schottky barrier, attributed to the presence of near-surface defects. • The density of these conductive defects determined by C-AFM was much lower than in the 3C-SiC material grown on silicon substrate. • The results pave the way for the application of this polytype for medium-voltage power devices. In this paper, we demonstrate the feasibility of fabricating vertical Schottky diodes on bulk cubic silicon carbide (3C-SiC) material obtained by combining sublimation epitaxy and chemical vapor deposition, starting from 4°-off axis 4H-SiC. First, the good quality of the epilayers grown with this method was demonstrated by morphological and structural analyses. Then, fabricated vertical Pt/3C-SiC Schottky diodes exhibited an ideality factor of 1.21 and a barrier height of 0.6 eV, as determined by thermionic emission model. The temperature dependent forward current analysis indicated the formation of an inhomogeneous barrier, which has been related with the presence of conductive surface defects, detected by nanoscale local current measurements. On the other hand, the reverse leakage current could be described by thermionic field emission model, including image force lowering. These findings demonstrate the viability of the proposed approach for bulk 3C-SiC growth for device fabrication. The material quality and the feasibility of fabricating vertical diodes based on 3C-SiC with a low barrier pave the way for the application of this polytype for medium-voltage power devices. [ABSTRACT FROM AUTHOR]

Abstract: Silicon carbide (SiC) and gallium nitride (GaN) are excellent wide band gap materials for power electronics. In spite of the significant progresses achieved in the last years, there are several issues limiting the performances of the developed devices. As an example, interfaces in SiC and GaN-based structures represent still one of the major concerns. Among them, metal/p-type SiC (or GaN) interfaces and metal-oxide-semiconductor (MOS) interfaces are fundamental building blocks for the performances of both diodes and transistors. In this context, the improvement of the related technology requires further efforts to better understand some physical aspects related to these interfaces. This paper reviews some of our recent results related to metal/semiconductor (ohmic contacts) and metal-oxide-semiconductor interfaces to p-type SiC and GaN. Firstly, the impact of the morphology of p-type implanted SiC, annealed under different conditions, on the properties of Ti/Al contacts will be discussed. The influence of different annealing conditions on the channel mobility in 4H–SiC MOSFETs will be also addressed. In the second part of the paper, the evolution of microstructure and Schottky barrier height in Ni/Au contacts to p-type GaN will be presented. All these aspects will be discussed in the present context of SiC and GaN research, considering the practical implications for devices. [Copyright &y& Elsevier]