Papers by Author: Vito Raineri

Abstract: The electrical properties of epitaxial graphene (EG) grown on 8° off-axis 4H-SiC (0001) by annealing at 1600 °C in inert gas ambient (Ar) were studied. The sheet resistance of the EG layers (R_sh=740±50Ω/sq) and the specific contact resistance of Ni contacts to EG (ρ_c≈6x10^-5 Ωcm²) were evaluated on micrometer scale by measurements on transmission line model (TLM) structures. Si₃N₄ was evaluated as a gate dielectric, showing excellent coverage to EG and a limited effect on its conductivity. The high n-type doping (~10¹³ cm^-2) of EG, as well as the field effect mobility (μ) dependence on n were determined using top gated field effect transistors (FETs) with Si₃N₄ gate dielectric. Electron mean free path (l_loc) and mobility (μ_loc) were also locally determined, on submicrometer scale, by scanning probe microscopy, showing a broad distribution of μ_loc values, with the most probable value very similar to macroscopic carrier mobility μ.

Abstract: In this paper we study the surface morphology of <11-20> 4° degree off, silicon terminated, 4H Silicon Carbide (4H-SiC) in terms of growth parameters and post growth argon thermal annealing. We find that out-of-equilibrium conditions favor the reduction of the surface roughness. Furthermore, we find preliminary indications that the same growth parameters that lead to the reduction of the surface roughness promote also a reduction of (1,3) and (4,4) stacking faults density.

Abstract: In this paper the structural and electrical evolution of Au/Ni contacts to p-type gallium nitride (GaN) upon annealing in different atmospheres was monitored. Rapid annealing of the contacts in an oxidizing atmosphere (N2/O2) resulted into a lower specific contact resistance (ρc) with respect to annealing in non-reacting ambient (Ar). The formation of a nickel oxide (NiO) layer was observed on the surface of the sample annealed in N2/O2 ,while was not present at the interface with p-GaN. The improvement of the contacts was associated with a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing), as determined by the temperature dependence of the ρc. Local electrical measurements demonstrated the formation of inhomogeneous barriers. The electrical measurements were correlated with the different microstructure of the annealed contacts.

Abstract: This paper reports on the effects of different post-implantation annealings on the electrical properties of interfaces to p-type implanted 4H-SiC. The morphology of p-type implanted 4H-SiC was controlled using a capping layer during post-implantation activation annealing of the dopant. Indeed, the surface roughness of Al-implanted regions strongly depends on the use of the protective capping layer during the annealing. However, while the different morphological conditions do not affect the macroscopical electrical properties of the implanted SiC (such as the sheet resistance), they led to an improvement of the morphology and of the specific contact resistance of Ti/Al Ohmic contacts formed on the implanted regions. These electrical and morphologic improvements were associated with a lowering of Schottky barrier height. Preliminary results showed that the different activation annealing conditions of p-type implanted SiC can affect also the electrical parameters (like threshold voltage and mobility) of lateral MOSFETs.

Abstract: We present a nanoscale morphological and structural characterization of few layers of graphene grown by thermal decomposition of off-axis 4H-SiC (0001). A comparison between transmission electron microscopy (TEM) in cross-section and in plan view allows to fully exploit the potentialities of TEM. Such a comparison was used to get information on the number of graphene layers as well as on the rotational order between the layers and with respect to the substrate. Some peculiar structures observed by TEM (wrinkles) could only be systematically measured by atomic force microscopy (AFM). In particular, the density and the height of the wrinkles in the few layers of graphene was investigated.

Abstract: The formation of good Ohmic contacts to p-type silicon carbide (SiC) and gallium nitride (GaN) is an important physical and technological concern, because of the difficulty to find metals with low Schottky barriers to p-type wide band gap materials, and due to the high ionization energies of p-type dopant impurities. Typically, to overcome these issues, alloyed metallic compounds are used. In this work, the electrical properties of alloyed Ohmic contacts to p-type (Al-implanted) 4H-SiC and p-type (Mg-doped epilayers) GaN are presented and correlated with their microstructure. The impact of the surface preparation and annealing conditions are discussed, reporting the cases of Al/Ti contacts to p-SiC and Au/Ni contacts to p-GaN. The electrical characterization as a function of temperature allowed to define the dominant transport mechanism and to determine the barrier heights.

Abstract: Graphene films were grown on thin polycrystalline Ni using a buried amorphous carbon (a-C) layer as C source. Rapid thermal processes (RTP) at temperatures from 600 to 800°C were used to promote C diffusion into Ni and its subsequent segregation on Ni surface, during the sample cool down. RTP at 800°C was the optimal condition for graphene film formation. Micro-Raman spectroscopy showed that the grown film is mostly composed by multilayers of graphene. Atomic force microscopy showed that the film presents peculiar corrugations (wrinkles), isotropically oriented and with heights ranging from from ~1 to ~15 nm. Selected area diffraction by transmission electron microscopy on the MLG membranes shows a rotational disorder between the stacked graphene layers.

Abstract: In this work the electrical and structural properties of AlGaN/GaN heterostructures grown onto 8°-off-axis 4H-SiC epilayers were investigated. A morphological and structural analysis of the samples showed the presence of “V-shaped” near-surface defects in the AlGaN layer, with a preferential orientation along the miscut direction [11-20]. In the presence of these defects an anisotropy of the current-voltage characteristics of high electron mobility transistors (HEMTs), fabricated with two different orientations, was observed. The sheet carrier density ns and the channel mobility n were determined from the device characteristics. The results were discussed considering the possible implications for AlGaN/GaN HEMT technology.

Abstract: In this study we examined the structural evolution of graphene grown on 8° off-axis 4H-SiC(0001) substrates at temperatures from 1600°C to 1700°C in Ar ambient. Morphological transformation of SiC substrate after annealing was examined by Tapping Mode Atomic Force Microscopy. Moreover, by etching-out graphene layers from graphitized SiC substrates in selective trenches we determined the number of graphene layers. Numbers of graphene layers were then independently confirmed by Transmission Electron Microscopy imaging.

Abstract: Local current transport across graphene/4H-SiC was studied with nanometric scale lateral resolution by Scanning Current Spectroscopy on both graphene grown epitaxially on 4H-SiC(0001) (EG-SiC) and graphene exfoliated from highly oriented pyrolitic graphite and deposited on 4H-SiC(0001) (DG-SiC). The study revealed that the Schottky barrier height (SBH) of EG/4H-SiC(0001) is lowered by ~0.49eV. This is explained in terms of Fermi-level pinning above the Dirac point in EG due to the presence of positively charged states at the interface between Si face of 4H-SiC and carbon-rich buffer layer. Furthermore, Scanning Capacitance Spectroscopy based method allowed evaluating local electron mean free path (lgr) in graphene. lgr in EG-SiC was observed to be, on average, ~0.4 times that in DG-SiC and exhibited large point-to-point variations due to presence of laterally homogeneous positively charged buffer layer at the interface. However, lgr in graphene on SiC was observed to be larger than on standard SiO2 samples (DG-SiO2), which is explained by better dielectric screening of charged impurities and lower surface polar phonon scattering at the graphene/substrate interface.