Raffaella Lo Nigro

Mapping the Density of Scattering Centers Limiting the Electron Mean Free Path in Graphene

Recently, giant carrier mobility μ (>10(5) cm(2) V(-1) s(-1)) and micrometer electron mean free path (l) have been measured in suspended graphene or in graphene encapsulated between inert and ultraflat BN layers. Much lower μ values (10000-20000 cm(2) V(-1) s(-1)) are typically reported in graphene on common substrates (SiO(2), SiC) used for device fabrication. The debate on the factors limiting graphene electron mean free path is still open with charged impurities (CI) and resonant scatterers (RS) indicated as the most probable candidates. As a matter of fact, the inhomogeneous distribution of such scattering sources in graphene is responsible of nanoscale lateral inhomogeneities in the electronic properties, which could affect the behavior of graphene nanodevices. Hence, high resolution two-dimensional (2D) mapping of their density is very important. Here, we used scanning capacitance microscopy/spectroscopy to obtain 2D maps of l in graphene on substrates with different dielectric permittivities, that is, SiO(2) (κ(SiO2) = 3.9), 4H-SiC (0001) (κ(SiC) = 9.7) and the very-high-κ perovskite strontium titanate, SrTiO(3) (001), briefly STO (κ(STO) = 330). After measuring l versus the gate bias V(g) on an array of points on graphene, maps of the CI density (N(CI)) have been determined by the neutrality point shift from V(g) = 0 V in each curve, whereas maps of the RS density (N(RS)) have been extracted by fitting the dependence of l on the carrier density (n). Laterally inhomogeneous densities of CI and RS have been found. The RS distribution exhibits an average value ∼3 × 10(10) cm(-2) independently on the substrate. For the first time, a clear correlation between the minima in the l map and the maxima in the N(CI) map is obtained for graphene on SiO(2) and 4H-SiC, indicating that CI are the main source of the lateral inhomogeneity of l. On the contrary, the l and N(CI) maps are uncorrelated in graphene on STO, while a clear correlation is found between l and N(RS) maps. This demonstrates a very efficient dielectric screening of CI in graphene on STO and the role of RS as limiting factor for electron mean free path.

Dielectric properties of Pr2O3 high-k films grown by metalorganic chemical vapor deposition on silicon

Praseodymium oxide (Pr2O3) thin films have been deposited on Si(100) substrates by metalorganic chemical vapor deposition using praseodymium tris-2,2,6,6-tetramethyl-3,5-heptandionate as source material. Film structural, morphological, and compositional characterizations have been carried out. Dielectric properties have been studied as well by capacitance–voltage and current–voltage measurements on metal-oxide-semiconductor capacitors of several areas. The Pr2O3 films have shown a dielectric constant e=23–25 and a leakage current density of 8.8×10−8 A/cm2 at +1 V.

Localized electrical characterization of the giant permittivity effect in CaCu3Ti4O12 ceramics

Nanoscale imaging of the electrical properties of CaCu3Ti4O12 ceramics has been performed using scanning impedance microscopy. Two kinds of electrical inhomogeneity are detected, namely, depletion layers at grain boundaries and calcium titanate inclusions, both of which are more resistive than the bulk of the grains. Energy filtered transmission electron microscopy was used to estimate the chemical composition of the inclusions.

Morphological and structural control of nanostructured oriented CeO2 films grown on random metallic substrates

CeO2 (cubic fluorite) thin films have been deposited on no-rolled Hastelloy C276 substrates by metal-organic chemical vapour deposition (MOCVD) from the Ce(hfa)3·diglyme [Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme = (CH3O(CH2CH2O)2CH3)] precursor. The X-ray patterns of all samples grown in the 350–550 °C temperature range point to the formation of oriented CeO2 films, while at higher deposition temperatures (650–850 °C) random CeO2 films are formed. XRD data indicate that 450 °C is the best deposition temperature. Detailed studies of the influence of all deposition parameters (precursor vaporization temperature, O2 and Ar gas flows, deposition temperature and time) on the CeO2 film growth have been carried out. There is evidence that the deposition process occurs in a mass transport regime. A suitable rationale for the observed textural changes vs. temperature has been proposed and the present columnar grain morphology, depending upon deposition temperatures, has been related to the zone model proposed by Mochvan and Demchishin for physical vapour deposition processes.

Breakdown kinetics of Pr2O3 films by conductive-atomic force microscopy

The dielectric breakdown (BD) kinetics of praseodymium thin films has been determined by comparison between current-voltage measurements on large-area (up to 78.54μm2) metal-oxide-semiconductor structures and conductive-atomic force microscopy (C-AFM). C-AFM clearly images the weak BD single spots under constant voltage stresses. The stress time on the single C-AFM tip dot was varied from 2.5×10−3 to 8×10−2s. The density of BD spots, upon increasing the stress time, exhibits an exponential trend. The Weibull slope and the characteristic time of the dielectric BD have been determined by direct measurements at nanometer scale.

Study of the Thermal Properties of Pr(III) Precursors and Their Implementation in the MOCVD Growth of Praseodymium Oxide Films

A praseodymium adduct, Pr(hfa) 3 .diglyme [(H-hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentandione, diglyme = CH 3 O(CH 2 CH 2 O) 2 CH 3 )] has been synthesized. It has been applied as a Pr source for the metallorganic chemical vapor deposition (MOCVD) of praseodymium containing films on silicon substrate and compared with Pr(tmhd) 3 [(H-tmhd = 2,2,6,6-tetramethyl-3,5-heptandione)] precursor. Physical and thermal properties of both Pr(hfa) 3 .diglyme and Pr(tmhd)3 precursors have been fully analyzed and their efficacy as MOCVD precursors for the growth of praseodymium oxide films have been fully tested. Depending on the oxygen partial pressure (p O2 ), different praseodymium oxide phases have been obtained.

Epitaxial NiO gate dielectric on AlGaN/GaN heterostructures

This letter reports on epitaxial nickel oxide (NiO) films grown by metal-organic chemichal vapor deposition on AlGaN/GaN heterostructures. The grown material was epitaxial, free from voids and exhibited a permittivity of 11.7, close to bulk NiO. This approach is advantageous with respect other methods such as the thermal oxidation of Ni films due to a better reproducibility and film quality. A reduction of the leakage current in Schottky diodes with an interfacial NiO layer has been observed and described using the metal-insulator-semiconductor Schottky model. The results indicate that these films are promising as gate dielectric for AlGaN/GaN transistors technology.

MOCVD of CeF3 films on Si(100) substrates: synthesis, characterization and luminescence spectroscopy

CeF3 thin films have been deposited on Si(100) substrates by metal–organic chemical vapor deposition (MOCVD). The Ce(hfa)3·diglyme [Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, diglyme = (CH3O(CH2CH2O)2CH3)] precursor has been adopted as a single source for both Ce and F components. This adduct has proved to be a good and reliable precursor, suitable for evaporation from the liquid phase due to its rather low melting point (75 °C). The structural, compositional, morphological and spectroscopic properties of the films have been investigated by X-ray diffraction (XRD), wavelength dispersion X-ray analysis (WDX), scanning electron microscopy (SEM) and luminescence spectroscopy.

Poole-Frenkel emission in epitaxial nickel oxide on AlGaN/GaN heterostructures

In this letter, the conduction mechanism through epitaxial nickel oxide (NiO) dielectric films grown by metal-organic chemical vapor deposition on AlGaN/GaN heterostructures was investigated. In particular, macroscopic current-voltage measurements carried out at different temperatures allowed to demonstrate that Poole-Frenkel (PF) mechanism rules the conduction through the dielectric layer, with an emission barrier of 0.2 eV. Conductive atomic force microscopic measurements were carried out to directly image the presence of preferential current spots on the NiO surface, which have been correlated to the defects responsible for the PF emission.

Micro- and nanoscale electrical characterization of large-area graphene transferred to functional substrates

Summary Chemical vapour deposition (CVD) on catalytic metals is one of main approaches for high-quality graphene growth over large areas. However, a subsequent transfer step to an insulating substrate is required in order to use the graphene for electronic applications. This step can severely affect both the structural integrity and the electronic properties of the graphene membrane. In this paper, we investigated the morphological and electrical properties of CVD graphene transferred onto SiO2 and on a polymeric substrate (poly(ethylene-2,6-naphthalene dicarboxylate), briefly PEN), suitable for microelectronics and flexible electronics applications, respectively. The electrical properties (sheet resistance, mobility, carrier density) of the transferred graphene as well as the specific contact resistance of metal contacts onto graphene were investigated by using properly designed test patterns. While a sheet resistance R sh ≈ 1.7 kΩ/sq and a specific contact resistance ρc ≈ 15 kΩ·μm have been measured for graphene transferred onto SiO2, about 2.3× higher R sh and about 8× higher ρc values were obtained for graphene on PEN. High-resolution current mapping by torsion resonant conductive atomic force microscopy (TRCAFM) provided an insight into the nanoscale mechanisms responsible for the very high ρc in the case of graphene on PEN, showing a ca. 10× smaller “effective” area for current injection than in the case of graphene on SiO2.