Recent Advances in Seeded and Seed-Layer-Free Atomic Layer Deposition of High-K Dielectrics on Graphene for Electronics

Graphene (Gr) with its distinctive features is the most studied two-dimensional (2D) material for the new generation of high frequency and optoelectronic devices. In this context, the Atomic Layer Deposition (ALD) of ultra-thin high-k insulators on Gr is essential for the implementation of many electronic devices. However, the lack of out-of-plane bonds in the sp2 lattice of Gr typically hinders the direct ALD growth on its surface. To date, several pre-functionalization and/or seed-layer deposition processes have been explored, to promote the ALD nucleation on Gr. The main challenge of these approaches is achieving ultra-thin insulators with nearly ideal dielectric properties (permittivity, breakdown field), while preserving the structural and electronic properties of Gr. This paper will review recent developments of ALD of high k-dielectrics, in particular Al2O3, on Gr with "in-situ" seed-layer approaches. Furthermore, recent reports on seed-layer-free ALD onto epitaxial Gr on SiC and onto Gr grown by chemical vapor deposition (CVD) on metals will be presented, discussing the role played by Gr interaction with the underlying substrates

Near-surface processing on AlGaN/GaN heterostructures: a nanoscale electrical and structural characterization

The effects of near-surface processing on the properties of AlGaN/GaN heterostructures were studied, combining conventional electrical characterization on high-electron mobility transistors (HEMTs), with advanced characterization techniques with nanometer scale resolution, i.e., transmission electron microscopy, atomic force microscopy (AFM) and conductive atomic force microscopy (C-AFM). In particular, a CHF3-based plasma process in the gate region resulted in a shift of the threshold voltage in HEMT devices towards less negative values. Two-dimensional current maps acquired by C-AFM on the sample surface allowed us to monitor the local electrical modifications induced by the plasma fluorine incorporated in the material

Advanced materials nanocharacterization

This special issue of Nanoscale Research Letters contains scientific contributions presented at the Symposium D "Multidimensional Electrical and Chemical Characterization at the Nanometer-scale of Organic and Inorganic Semiconductors" of the E-MRS Fall Meeting 2010, which was held in Warsaw, Poland from 13th to 17th September, 2010

Active dopant profiling and Ohmic contacts behavior in degenerate n-type implanted silicon carbide

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 8.5 × 1019 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

Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001)

In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range

Proposal for a New Score-Based Approach To Improve Efficiency of Diagnostic Laboratory Workflow for Acute Bacterial Meningitis in Adults

Microbiological tests on cerebrospinal fluid (CSF) utilize a common urgent-care procedure that does not take into account the chemical and cytological characteristics of the CSF, resulting sometimes in an unnecessary use of human and diagnostic resources. The aim of this study was to retrospectively validate a simple scoring system (bacterial meningitis-Careggi score [BM-CASCO]) based on blood and CSF sample chemical/cytological parameters for evaluating the probability of acute bacterial meningitis (ABM) in adults. BM-CASCO (range, 0 to 6) was defined by the following parameters: CSF cell count, CSF protein levels, CSF lactate levels, CSF glucose-to-serum glucose ratio, and peripheral neutrophil count. BM-CASCO was retrospectively calculated for 784 cases of suspected ABM in adult subjects observed during a four-and-a-half-year-period (2010 to 2014) at the emergency department (ED) of a large tertiary-care teaching hospital in Italy. Among the 28 confirmed ABM cases (3.5%), Streptococcus pneumoniae was the most frequent cause (16 cases). All ABM cases showed a BM-CASCO value of ≥3. Most negative cases (591/756) exhibited a BM-CASCO value of ≤1, which was adopted in our laboratory as a cutoff to not proceed with urgent microbiological analysis of CSF in cases of suspected ABM in adults. During a subsequent 1-year follow-up, the introduction of the BM-CASCO in the diagnostic workflow of ABM in adults resulted in a significant decrease in unnecessary microbiological analysis, with no false negatives. In conclusion, BM-CASCO appears to be an accurate and simple scoring system for optimization of the microbiological diagnostic workflow of ABM in adults

Threshold voltage instability by charge trapping effects in the gate region of p-GaN HEMTs

In this work, the threshold voltage instability of normally-off p-GaN high electron mobility transistors (HEMTs) has been investigated by monitoring the gate current density during device on-state. The origin of the gate current variations under stress has been ascribed to charge trapping occurring at the different interfaces in the metal/p-GaN/AlGaN/GaN system. In particular, depending on the stress bias level, electrons (VG 6 V) are trapped, causing a positive or negative threshold voltage shift {DVTH, respectively. By monitoring the gate current variations at different temperatures, the activation energies associated to the electrons and holes trapping could be determined and correlated with the presence of nitrogen (electron traps) or gallium (hole traps) vacancies. Moreover, the electrical measurements suggested the generation of a new electron-trap upon long-time bias stress, associated to the creation of crystallographic dislocation-like defects extending across the different interfaces (p-GaN/AlGaN/GaN) of the gate stack