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The interfacial electrical properties of deposited oxide (SiO2) onto cubic silicon carbide (3C-SiC) were investigated after different post-oxide deposition annealing (PDA) by means of metal–oxide–semiconductor (MOS) capacitors and nanoscale capacitance mapping. The deposited oxides subjected to PDA at 450 °C in either nitrogen or forming gas showed a reduction of the interface and oxide traps, as well as an improved oxide field strength compared to the thermally grown insulating layer. Spatially resolved nanoscale capacitance mapping performed onto the oxide surface revealed that the density of the electrically active stacking faults (SFs) in 3C-SiC is diminished by appropriate PDA. The results pave the way to obtain an ideal SiO2/3C-SiC system suitable for power device applications. [ABSTRACT FROM AUTHOR]

In this paper, the forward and reverse current transport mechanisms in as-deposited and 400 °C annealed tungsten carbide (WC) Schottky contacts on AlGaN/GaN heterostructures have been studied. In particular, under forward bias, the WC/AlGaN Schottky contacts exhibited a deviation from the ideal thermionic emission model due to the occurrence of a tunneling component of the current. From the temperature dependence of the ideality factor, a characteristic tunneling energy E00 in the range of 33–36 meV has been estimated. On the other hand, two different transport mechanisms have been identified under reverse bias. At low reverse bias (VR < 2 V), Poole–Frenkel emission rules the current transport, with an emission barrier ϕt = 0.68 eV in the as-deposited contact, which increases up to ϕt = 0.79 eV upon annealing at 400 °C. This behavior has been correlated with the improvement of the metal/AlGaN electronic properties. At higher reverse bias (VR > 2 V), the leakage current is dominated by a thermally activated process with an activation energy (0.27 eV) that is independent of the Schottky contact fabrication process. In this case, the temperature dependence of the leakage could be well described by a two-dimensional variable range hopping conduction associated with the presence of surface defects in the material. [ABSTRACT FROM AUTHOR]

The presence of crystallographic defects can induce notable effects on the mechanisms ruling the current transport in metal/semiconductor contacts. In this context, in this Letter, the impact of stacking faults (SFs) on the characteristics of 4H-SiC Schottky diodes was investigated under both forward and reverse bias. In particular, in the presence of SFs under the contact, while no significant effect on the ideality factor and barrier height was observed under forward bias, an anomalous increase in the leakage current occurred under reverse bias. The observed behavior of the leakage current could be explained by a space-charge limited current model, consistent with the presence of a distribution of trapping states in the gap of 4H-SiC. An increase in the reverse bias above 30 V leads to a complete trap filling. The weak temperature-dependence of the leakage current observed in this regime suggests the coexistence with a tunneling of the carriers through the barrier. The results can be useful to understand unexpected failures in 4H-SiC Schottky diodes. [ABSTRACT FROM AUTHOR]

The effect of germanium (Ge) on n-type 4H-SiC is experimentally studied by electrical characterization of homoepitaxial layers grown by chemical vapor deposition (CVD). Measurements show that electrical properties of epitaxial layers can be changed by intentional incorporation of germane (GeH4) gas during the deposition process. On the nanoscale, two-dimensional mappings acquired with conductive atomic force microscopy show preferential conductive paths on the surface of Gedoped samples, which are related to the presence of this isoelectronic impurity. Hall effect measurements confirm that also macroscopic electrical properties of n-type 4H-SiC are improved due to incorporation of Ge into SiC during CVD growth. In particular, despite equal free electron concentration, enhanced mobility in a wide temperature range is measured in Ge-doped samples as compared to a pure 4H-SiC layer. Based on our results from Hall effect measurements as well as admittance spectroscopy and deep level transient spectroscopy, it is speculated that Ge influences the generation and annealing of other point defects and thus helps to reduce the total concentration of defects. [ABSTRACT FROM AUTHOR]

The gold-assisted exfoliation is a very effective method to produce large-area (cm2-scale) membranes of molybdenum disulfide (MoS2) for electronics. However, the strong MoS2/Au interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of MoS2. Here, we report an atomic force microscopy, micro-Raman, and micro-photoluminescence (μ-PL) investigation of 2H-MoS2 with variable thickness exfoliated on Au and subsequently transferred on an Al2O3/Si substrate. The E2g–A1g vibrational mode separation Δω (typically used to estimate MoS2 thickness) exhibits an anomalous large value (Δω ≈ 21.2 cm−1) for monolayer (1L) MoS2 on Au as compared to the typical one (Δω ≈ 18.5 cm−1) measured on 1L MoS2 on Al2O3. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the MoS2/Au interaction, were found to gradually decrease while increasing the number of MoS2 layers. Furthermore, μ-PL spectra for 1L MoS2 on Au exhibit a strong quenching and an overall redshift of the main emission peak at 1.79 eV, compared to the 1.84 eV peak for 1L MoS2 on Al2O3. After PL spectra deconvolution, such redshift was explained in terms of a higher trion/exciton intensity ratio, probably due to the higher polarizability of the metal substrate, as well as to the smaller equilibrium distance at the MoS2/Au interface. [ABSTRACT FROM AUTHOR]

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 °C. After optimization of the C3H8 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 ∼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 sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ 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. [ABSTRACT FROM AUTHOR]

In this paper, the nanoscale structural and electrical evolution of Ta- and Ti-based contacts was investigated employing several analytical techniques. A correlation between the improvement of the electrical quality of the contacts and the formation of Al-alloyed phases (TaAl3 or TiAl3) during annealing was observed. However, while for the Ti/Al contacts an Ohmic behavior with a contact resistance Rc ∼ 1.8 Ω mm has been achieved after annealing at 500 °C, Ta/Al contacts exhibited a higher contact resistance (Rc ∼ 36.3 Ω·mm) even after annealing at 700 °C. The different electrical behaviour has been explained considering the different interface and the homogeneity of the current transport at a nanoscale level. [ABSTRACT FROM AUTHOR]

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]

We report here a detailed study about the formation and self-organization of nanoscale structures during ion beam implantation at room temperature of 300 keV Ge+ in Ge as a function of the ion fluence in the range between 1×1014 to 4×1016 cm-2. “Microexplosions” characterize the morphology of the swelled material; a random cellular structure consisting of cells surrounded by amorphous Ge ripples has been observed and studied in details by combining atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. [ABSTRACT FROM AUTHOR]

In this paper, the influence of a high-temperature (900 °C) selective oxidation process on the electrical properties of AlGaN/GaN heterostructures was investigated. In particular, electrical measurements performed on appropriate devices and test patterns demonstrated that the current flow through the two-dimensional electron gas (2DEG) was suppressed, even if the thickness of the local oxide did not reach the AlGaN/GaN interface. The combination of macroscopic current-voltage and capacitance-voltage measurements with depth-resolved scanning capacitance microscopy elucidated the doping dependence and the compositional stability of the material during high-temperature oxidation. The reduction in the 2DEG sheet carrier density and the variation of the threshold voltage of simple high electron mobility transistor structures upon high-temperature annealing were also discussed. [ABSTRACT FROM AUTHOR]

Thin nanostructured gold films were deposited on SiO2 by the sputtering technique at room temperature. Films of different thicknesses were deposited ranging from 2 to 16 nm. The film morphology as a function of the thickness was analyzed by microscopic techniques such as atomic force microscopy and transmission electron microscopy. These analyses allowed us to clarify the growth mechanism of the gold nanograins forming the film: in a first stage of growth (2–6 nm) normal grain growth proceeds; then (8–16 nm) the grain surface energy anisotropy drives the growth of abnormal large gold grains by annihilation of the normal ones. During the abnormal growth other normal grain continue to growth. The normal grain size distribution is showed to be a monomodal log-normal distribution that evolves toward larger mean grain radius continuously following a scaling law. By determination of the grain growth exponent, the kinetic mechanism responsible of the grain growth is demonstrated to be the gold atomic diffusion on grain boundaries. By fitting the experimental data using established theoretical models, the room-temperature gold grain boundary coefficient diffusion and mobility were derived. The abnormal grain grows, manifest itself as a bimodal grain size distribution: with the log-normal distribution of the normal grain size, a second Gaussian grain size distribution rises, shifting toward lower size increasing the film thickness. The abnormal grain growth continues until all the abnormal grain boundaries meet each other. [ABSTRACT FROM AUTHOR]

Very thin Au layer was deposited on Si(100) using the sputtering technique. By annealing at 873 K Au/Si nanodroplets were formed and their self-organization was induced changing the annealing time. The evolution of droplet size distribution, center-to-center distance distribution, and droplet density as a function of the annealing time at 873 K was investigated by Rutherford backscattering spectrometry, atomic force microscopy (AFM), and scanning electron microscopy. As a consequence of such study, the droplet clustering is shown to be a ripening process of hemispherical three-dimensional structures limited by the Au surface diffusion. The application of the ripening theory allowed us to calculate the surface diffusion coefficient and all other parameters needed to describe the entire process. Furthermore, the AFM measurements allowed us to study the roughness evolution of the sputtered Au thin film and compare the experimental data with the dynamic scaling theories of growing interfaces. [ABSTRACT FROM AUTHOR]

Very thin Au layers were deposited on SiC hexagonal and SiO2 substrates by sputtering. The Au surface diffusion, clustering, and self-organization of Au nanoclusters on these substrates, induced by thermal processes, were investigated by Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. On both types of substrates, clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory allowed us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process. The system Au nanoclusters/SiC and Au nanoclusters/SiO2 are proposed as nanostructured materials for nanoelectronic and nanotechnology applications. [ABSTRACT FROM AUTHOR]

In this paper, the effects of ion irradiation on Schottky barriers formed on silicon carbide are discussed. After Si-ion irradiation at the near-interface region in Ti/4H-SiC contacts an increase of the Schottky barrier height from 1.05 to 1.21 eV was observed, accompanied by a lowering of the reverse leakage current. The combination of several methods allowed us to determine the physical properties of the Schottky barrier and to explain the mechanism responsible for the barrier height changes. In particular, the structural and electrical modifications of the interfacial region, both of Ti layer and SiC (i.e., different orientation of the Ti layer, irradiation-induced defects in the epilayer, dopant deactivation, and the consequent reduction of the surface electric field) are responsible for the increase of the Schottky barrier height and the reduction of the leakage current. The electrical characterization of the contacts at different temperatures also suggested that ion irradiation induced modifications in the inhomogeneous nature of the Ti Schottky barrier. [ABSTRACT FROM AUTHOR]

Scanning capacitance microscopy (SCM) was carried out in the angle beveling configuration on B doped, very narrow quantum wells (QWs) of Si0.75Ge0.25 layers strained between Si films. The majority carrier concentration profiles were calculated from the SCM raw data measured on QWs with a minimum width of 5 nm, doped with different B concentrations ranging from 2×1016 to 6×1018 cm-3. The equilibrium carrier distribution in the heterostructures has been calculated by different simulation approaches, which will be discussed. Moreover, the effect of the biased tip-sample interaction was studied by accurate simulations of the dC/dV vs V characteristics for different positions of the tip moving on the beveled sample surface. The agreement between the experimental and simulated SCM profiles is very good. Thus, a spatial SCM resolution of at least 5 nm was demonstrated on angle beveled samples, not only in terms of signal sensitivity, but also in terms of quantitative majority carrier profiling. [ABSTRACT FROM AUTHOR]

The localization of voids in thin Si1-xGex layers after He+ implantation and thermal annealing is reported. A Si/Si1-xGex multilayer grown onto (001) Si was implanted with He+ in the 10-30 keV range, with fluences from 7×1015 up to 1×1016 cm-2, and annealed at 800 °C for 1 h. Samples were analyzed by transmission electron microscopy, showing void formation only within the two layers containing Ge or at the film/substrate interface. Our results support the idea that the compressive strain in the Si1-xGex layers induces the nucleation of small cavities and the growth of voids by a mechanism where vacancies are stabilized by He. [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]

Metal contacts (Schottky or Ohmic) to p-type wide band gap (WBG) semiconductors SiC and GaN are important for power devices technologies. This work reports on the properties of Ti/Al-based contacts to p-type 4H-SiC and p-type GaN, monitored using different techniques and test-patterns. In particular, in the case of p-type SiC, Ti/Al and Ti/Al/W contacts showed a superior Ohmic behavior after annealing at 900-1100 °C (with ρc ≈ 1.5-6 × 10−4 Ωcm2), attributed to the formation of Ti- and Al-containing phases at the interface and in the stack. Due to the lower ρc with respect to Ni-based contacts, Ti/Al allowed a reduction of the forward voltage drop in 4H-SiC p-n junctions. On the other hand, in the case of p-type GaN, Ti/Al contacts exhibited a higher barrier height (2.08 eV) with respect to Ni contacts. Hence, they can be promising as Schottky gates in high electron mobility transistors (HEMTs). However, temperature-dependent electrical measurements revealed a decrease of the barrier height down to 1.60 eV upon annealing at 800 °C, with an increase of the leakage current in test-diodes. This result provided an important indication for the practical use of Ti/Al gate electrodes for the fabrication of normally off HEMTs with a p-GaN gate. [ABSTRACT FROM AUTHOR]

Group III nitride semiconductors (III-N), including GaN, AlN, InN, and their alloys, are currently the materials of choice for many applications in optoelectronics (light-emitting diodes, laser diodes), and high-power and high-frequency transistors. Due to its attractive electrical, optical, mechanical, and thermal properties, graphene (Gr) integration with III-N technology has been considered in the last few years, in order to address some of the major issues which still limit the performances of GaN-based devices. To date, most of the studies have been focused on the use of Gr as transparent conductive electrode (TCE) to improve current spreading from top electrodes and light extraction in GaN-LEDs. This paper will review recent works evaluating the benefits of Gr integration with III-N for high power and high frequency electronics. From the materials side, recent progresses in the growth of high quality GaN layers on Gr templates and in the deposition of Gr on III-N substrates and templates will be presented. From the applications side, strategies to use Gr for thermal management in high-power AlGaN/GaN transistors will be discussed. Finally, recent proposals of implementing new ultra-high-frequency (THz) transistors, such as the Gr base hot electron transistor (GBHET), by Gr integration with III-N will be highlighted. [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]

We study the mechanisms of CH3NH3PbI3 degradation and its transformation to PbI2 by means of X-ray diffraction and the density functional theory. The experimental analysis shows that the material can degrade in both air and vacuum conditions, with humidity and temperature-annealing strongly accelerating such process. Based on ab initio calculations, we argue that even in the absence of humidity, a decomposition of the perovskite structure can take place through the statistical formation of molecular defects with a non-ionic character, whose volatility at surfaces should break the thermodynamic defect equilibria. We finally discuss the strategies that can limit such phenomenon and subsequently prolong the lifetime of the material. [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]

In this paper, hydrogen bubbling delamination of graphene (Gr) from copper using a strong electrolyte (KOH) water solution was performed, focusing on the effect of the KOH concentration (CKOH) on the Gr delamination rate. A factor of ~10 decrease in the time required for the complete Gr delamination from Cu cathodes with the same geometry was found increasing CKOH from ~0.05M to ~0.60 M. After transfer of the separated Gr membranes to SiO2 substrates by a highly reproducible thermo-compression printing method, an accurate atomic force microscopy investigation of the changes in Gr morphology as a function of CKOH was performed. Supported by these analyses, a microscopic model of the delamination process has been proposed, where a key role is played by graphene wrinkles acting as nucleation sites for H2 bubbles at the cathode perimeter. With this approach, the H2 supersaturation generated at the electrode for different electrolyte concentrations was estimated and the inverse dependence of td on CKOH was quantitatively explained. Although developed in the case of Cu, this analysis is generally valid and can be applied to describe the electrolytic delamination of graphene from several metal substrates. [ABSTRACT FROM AUTHOR]

This paper compares the behavior of the gate oxide in 4H-SiC lateral MOSFETs subjected to post-deposition annealing (PDA) in N2O and POCl3. A significantly higher channel mobility was measured in 4H-SiC MOSFETs subjected to PDA in POCl3 (108 cm2 V-1s-1) with respect to NO (19 cm2 V-1s-1), accompanying a reduction of the interface traps density. Hence, a different temperature coefficient of the mobility and of the threshold voltage was observed in the two cases. According to structural analysis, the gate oxide subjected to PDA in POCl3 showed a different surface morphology than that treated in N2O, as a consequence of the strong incorporation of phosphorous inside the SiO2 matrix during annealing. This latter explained the instability of the electrical behavior of MOS capacitors annealed in POCl3. [ABSTRACT FROM AUTHOR]

The scattering mechanisms limiting electronic transport in substrate-supported graphene have been investigated by mapping the electron mean free path (l) in graphene on substrates with different dielectric permittivities, i.e. SiO2(κSiO2=3.9), 4H-SiC (0001) (κSiC=9.7) and SrTiO3(001) (κSrTiO3=330). From the analysis of the local l versus gate bias curves, histograms and nanoscale maps of the densities of charged impurities (NCI) have been extracted. l was found to increase from (107±6) nm to (218±11) nm moving from SiO2 to SiC substrate, whereas a decrease to (179±5)nm was found for gra- phene on SrTiO3. Furthermore, a clear correlation between the minima in the l maps and the maxima in the NCI maps was obtained for graphene on SiO2 and SiC, whereas the two maps are uncorrelated in graphene on SrTiO3. These results indicate that charged impurity scattering is the main source of the lateral inhomogeneity of l on lower κ substrates, like SiO2 and SiC, whereas a different scattering mechanism (probably resonant scattering) must be invoked as the origin of the nanoscale variations of l in graphene on high-κ substrates like SrTiO3. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

This letter reports on the electrical properties of SiO2/4H-SiC interfaces after post-oxidation annealing (POA) in N2O and POCl3. The notably higher channel mobility measured in 4H-SiC MOSFETs subjected to POA in POCl3 was ascribed both to a reduction of the interface traps density and to an increase of donor concentration incorporated in SiC. Scanning spreading resistance microscopy on a SiC surface directly exposed to POA revealed that the incorporation of P-related shallow donors upon POA in POCl3 is more efficient than N-shallow donors incorporation during N2O treatments which subsequently explains the significantly enhanced channel conductivity of the MOSFETs. [ABSTRACT FROM AUTHOR]

Summary: The research on graphene is probably the topic in condensed matter physics having the fastest and most widespread development nowadays. Impressive results have been obtained during the last six years in the field of material synthesis and in the exploration of fundamental physical properties. This paper reviews the state of the art of graphene synthesis methods. Furthermore, a thorough experimental investigation of the electronic transport properties is presented and the various scattering mechanisms limiting electron mean free path and mobility are discussed. Employing novel scanning probe methods to measure the nanoscale lateral variations of the electron mean free path in graphene on different substates, the role played by electron scattering by charged impurities and by resonant scatterers is clarified. [ABSTRACT FROM AUTHOR]

Dynamic scaling behavior has been observed during the room-temperature growth of sputtered Au films on SiO2 using the atomic force microscopy technique. By the analyses of the dependence of the roughness, σ, of the surface roughness power, P(f), and of the correlation length, ξ, on the film thickness, h, the roughness exponent, α = 0.9 ± 0.1, the growth exponent, β = 0.3 ± 0.1, and the dynamic scaling exponent, z = 3.0 ± 0.1 were independently obtained. These values suggest that the sputtering deposition of Au on SiO2 at room temperature belongs to a conservative growth process in which the Au grain boundary diffusion plays a dominant role. [ABSTRACT FROM AUTHOR]

The current transport through Schottky contacts on wide band gap semiconductors (GaN and SiC) was studied on nanoscale by conductive atomic force microscopy. Two very different metal-semiconductor systems were investigated: (i) a uniform (∼5 nm thick) Pt contact on GaN, and (ii) a discontinuous contact formed by self-assembled Au nanoclusters on SiC. The local current-voltage (I-V) measurements allowed to demonstrate the “laterally inhomogeneous” electrical behavior of the Pt/GaN contact, which was formed by a distribution of nanoscale patches with different barrier heights. This behavior was explained in terms of the inhomogeneities of the Pt/GaN interface and/or of the electrically active defects present in the GaN epilayer. The standard deviation of the local barrier height histogram (σ[uppercase_phi_synonym]) was correlated with the dependence of the ideality factor (n) on temperature, deduced from conventional I-V measurements at variable temperatures on macroscopic Pt/GaN diodes. The local Schottky barrier height at the interface between the single metal nanoparticle and the semiconductor was determined in the system of self-assembled Au nanoclusters on SiC. The histogram of the Schottky barrier heights was measured on samples with different cluster size distributions and the dependence of the barrier height on the cluster size was demonstrated. [ABSTRACT FROM AUTHOR]

Scanning capacitance microscopy (SCM) is used to probe on nanoscale the capacitive behavior of graphene deposited on a SiO2/Si n+ substrate (with SiO2 thickness of 300 or 100 nm). The SCM tip provides the contact on graphene, while the Si n+ substrate acts as the backgate contact in the graphene/SiO2/Si capacitor. The authors studied the screening by the graphene two-dimensional electron gas on the modulating potential applied between the backgate and the SCM tip. In particular, they determined the effect of the oxide thickness on the lateral distribution (i.e., screening length) and the density of the screening charge in graphene. Experimental results indicate that thinner oxide leads to higher screening charge density in the graphene sheet and to higher effectively biased area on graphene. This correspondingly increases the total capacitance (Ctot) of the graphene/SiO2/Si capacitor. We evaluated the dependence on the dielectric thickness of the classical metal-oxide-semiconductor (MOS) capacitance (CMOS) and of the quantum capacitance (Cq) contributions to Ctot. [ABSTRACT FROM AUTHOR]

We report on the calculations of the cohesive energy, melting temperature and vacancy formation energy for Au nanocrystals with different size supported on and embedded in SiO2. The calculations are performed crossing our previous data on the surface free energy of the supported and embedded nanocrystals with the theoretical surface-area-difference model developed by W. H. Qi for the description of the size-dependent thermodynamics properties of low-dimensional solid-state systems. Such calculations are employed as a function of the nanocrystals size and surface energy. For nanocrystals supported on SiO2, as results of the calculations, we obtain, for a fixed nanocrystal size, an almost constant cohesive energy, melting temperature and vacancy formation energy as a function of their surface energy; instead, for those embedded in SiO2, they decreases when the nanocrystal surface free energy increases. Furthermore, the cohesive energy, melting temperature and vacancy formation energy increase when the nanocrystal size increases: for the nanocrystals on SiO2, they tend to the values of the bulk Au; for the nanocrystals in SiO2 in correspondence to sufficiently small values of their surface energy, they are greater than the bulk values. In the case of the melting temperature, this phenomenon corresponds to the experimentally well-known superheating process. [ABSTRACT FROM AUTHOR]

A bottom-up methodology to fabricate a nanostructured material by Au nanoclusters on 6H-SiC surface is illustrated. Furthermore, a methodology to control its structural properties by thermal-induced self-organization of the Au nanoclusters is demonstrated. To this aim, the self-organization kinetic mechanisms of Au nanoclusters on SiC surface were experimentally studied by scanning electron microscopy, atomic force microscopy, Rutherford backscattering spectrometry and theoretically modelled by a ripening process. The fabricated nanostructured materials were used to probe, by local conductive atomic force microscopy analyses, the electrical properties of nano-Schottky contact Au nanocluster/SiC. Strong efforts were dedicated to correlate the structural and electrical characteristics: the main observation was the Schottky barrier height dependence of the nano-Schottky contact on the cluster size. Such behavior was interpreted considering the physics of few electron quantum dots merged with the concepts of ballistic transport and thermoionic emission finding a satisfying agreement between the theoretical prediction and the experimental data. The fabricated Au nanocluster/SiC nanocontact is suggested as a prototype of nano-Schottky diode integrable in complex nanoelectronic circuits. [ABSTRACT FROM AUTHOR]

We demonstrate that He can be a powerful tool to control B diffusion both in crystalline (c-Si) and preamorphized Si (PA-Si). By means of positron annihilation spectroscopy (PAS), we showed in He-implanted c-Si the formation after annealing of large open-volume defects at the implant projected range Rp of He (voids) and of smaller vacancy-type defects toward the surface (nanovoids). In particular, these nanovoids locally suppress the amount of self-interstitials (Is) generated by B implantation, as verified by PAS, eventually reducing B diffusion and leading to a boxlike shape of the B-implanted profile. On the other hand, for B implantation in PA-Si, the authors demonstrated that if He-induced voids are formed between the end-of-range (EOR) defects and the surface, they act as a diffusion barrier for Is coming from the EOR defects. Indeed, this barrier strongly reduces diffusion of B placed in proximity of the surface. [ABSTRACT FROM AUTHOR]

We report on the equilibrium shape and atomic structure of thermally-processed Au nanocrystals (NCs) as determined by high resolution transmission electron microscopy (TEM). The NCs were either deposited on SiO2 surface or embedded in SiO2 layer. Quantitative data on the NCs surface free energy were obtained via the inverse Wulff construction. Nanocrystals inside the SiO2 layer are defect-free and maintain a symmetrical equilibrium shape during the growth. Nanocrystals on SiO2 surface exhibit asymmetrical equilibrium shape that is characterized by the introduction of twins and more complex atomic defects above a critical size. The observed differences in the equilibrium shape and atomic structure evolution of growing NCs in and on SiO2 is explained in terms of evolution in isotropic/anisotropic environment making the surface free energy function angular and/or radial symmetric/asymmetric affecting the rotational/translational invariance of the surface stress tensor. [ABSTRACT FROM AUTHOR]

We addressed the issues related to quantitative carrier profiling by scanning capacitance microscopy (SCM) on doped layers with different dimensions, starting from thick (∼5 μm) uniformly B-doped Si layers, down to Si/Si1-xGex/Si quantum wells with nanometric width. We preliminarly discussed the influence of the SCM hardware on the quantification, by comparing the analyses performed on Si calibration standards with two different atomic force microscopes, i.e., DI3100 by Veeco and XE-100 by PSIA, equipped with different SCM sensors. Furthermore, both concentration and spatial resolution are demonstrated by measurements on specially designed samples containing B-doped quantum wells of Si0.75Ge0.25 layers strained between Si films. Measurements were taken both on cross-sectioned samples and on beveled ones. A SCM spatial resolution of 1 nm with a concentration sensitivity from 5% to 10% was demonstrated by quantitative majority carrier profiling on the B-doped Si/Si0.75Ge0.25/Si heterostructures. [ABSTRACT FROM AUTHOR]