Michael A. Capano

Top-gated graphene field-effect-transistors formed by decomposition of SiC

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400cm2∕Vs, respectively, which are much larger than the corresponding values from conventional SiC or silicon.

Atomic-layer-deposited nanostructures for graphene-based nanoelectronics

Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108cm∕s and a room-temperature mobility of >15000cm2∕Vs. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2O3 and HfO2 nanoribbons, with dimensions of 5–200nm in width and >50μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250°C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations.

Growth and characterization of 4H-SiC epilayers on substrates with different off-cut angles

In this paper, epitaxial growth on 4H-SiC (0001) substrates with 4° and 8° off-cut angles is investigated. Both growth rate and nitrogen doping show a dependence on the C∕Si ratio and off-cut angle. Good surface morphologies are obtained on the 8° off-angle substrates over a wide range of C∕Si ratios, while macrostep bunching is observed on the 4° off-angle substrates depending on C∕Si ratios. Step bunching behaviors on both substrate orientations are investigated by Nomarski microscopy and atomic force microscopy, and possible step bunching mechanisms are proposed to explain the results. Basal plane dislocation (BPD) densities are examined by means of molten KOH etching. Low off-cut angle is found to enhance the conversion of BPDs into threading edge dislocations. A BPD density of 2.6cm−2 is achieved on the 4° off-angle substrate under optimized growth conditions. The effects of changing the C∕Si ratio on BPD densities are also investigated.

Ionization energies and electron mobilities in phosphorus- and nitrogen-implanted 4H-silicon carbide

Comparisons are made between the carrier concentrations, ionization energies, and electron mobilities in 4H–SiC samples implanted with similar doses of nitrogen or phosphorus and annealed at 1300 or 1700 °C for 10 min in argon. The objective of the research is to determine which element may yield lower resistance 4H–SiC. Ionization energies of 53 and 93 meV are measured from phosphorus-implanted 4H–SiC, and are assigned to the hexagonal and cubic lattice positions in 4H–SiC, respectively. The corresponding ionization energies for nitrogen-implanted 4H–SiC are 42 and 84 meV, respectively. Phosphorus is not activated to the same extent that nitrogen is, and the carrier concentrations are about a factor of five lower for phosphorus-implanted 4H–SiC annealed at 1300 °C than for nitrogen-implanted 4H–SiC annealed at the same temperature. A higher mobility for phosphorus-implanted 4H–SiC is observed, but is not sufficiently high to offset the lower carrier concentration of this material. For the doses considere...

Mechanism of ohmic behavior of Al/Ti contacts to p-type 4H-SiC after annealing

We report on experiments to determine the mechanism of ohmic behavior of Al/Ti contacts to p-type SiC after thermal annealing. After ruling out heavy doping of the SiC surface due to diffusion of aluminum, and electric field enhancement due to surface morphology modification, we propose that the only remaining explanation is alloy formation at the metal–semiconductor interface. We present evidence from x-ray diffraction studies identifying these alloys as Ti3SiC2 and Al4C3, and review corroborating transmission electron microscopy studies. An alloy-assisted ohmic contact mechanism is presented and discussed.

Carbon Nanotube Array Thermal Interfaces for High-Temperature Silicon Carbide Devices

Multiwalled carbon nanotube (MWCNT) arrays have been directly synthesized from templated Fe2O3 nanoparticles on the C-face of 4H-SiC substrates by microwave plasma chemical vapor deposition (MPCVD), and the room-temperature thermal resistances of SiC-MWCNT-Ag interfaces at 69–345 kPa as well as the thermal resistances of SiC-MWCNT-Ag interfaces up to 250°C (at 69 kPa) have been measured using a photoacoustic technique. The SiC-MWCNT-Ag interfaces with MWCNTs grown on the C-face of SiC achieved thermal resistances less than 10 mm2 K/W, which is lower than the resistances of MWCNT interfaces grown using the same catalysis and growth methods on the Si-face of SiC and Ti-coated SiC. The thermal resistances of the SiC-MWCNT-Ag interfaces exhibit weak temperature dependence in the measured range, indicating that the interfaces are suitable for high-temperature power electronics applications.

Residual strains in cubic silicon carbide measured by Raman spectroscopy correlated with x-ray diffraction and transmission electron microscopy

Cubic (3C) silicon carbide (SiC) epilayers grown on Si substrates by chemical vapor deposition, characterized using transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXRD), and Raman spectroscopy, reveal the presence of biaxial in-plane strain. Defect (stacking faults, twins, dislocations) distributions revealed by TEM are correlated with peak widths obtained from HRXRD measurements and Raman shifts of the zone center longitudinal optical phonon line. TEM showed defect densities decreasing with increasing distance from SiC∕Si interface as the lattice mismatch stress is relaxed. Structural defect densities show the most significant reduction within the first 2μm of the epilayer. TEM observations were correlated with a monotonic decrease in HRXRD peak width (full width at half maximum) from 780arcsec (1.5μm thick epilayer) to 350arcsec (10μm thick epilayer). Raman spectroscopy indicates that the residual biaxial in-plane strain decreases with increasing epilayer thickness initially...

High-voltage UMOSFETs in 4H SiC

We report the design and fabrication of UMOSFETs on 4H-SiC with specific on-resistances of 105.1 m/spl Omega/ cm/sup 2/ at a current density of 100 A/cm/sup 2/ and blocking voltage of 5,050 V. These devices incorporate an extended self-aligned p-bottom implant to protect the gate oxide from high electric fields. A novel shadow implant mask was developed to shield the sidewalls from any unwanted p-bottom implant. These UMOSFETs are also the first to incorporate a post-oxidation anneal (POA) in nitric oxide (NO) to improve the inversion layer electron mobility. A single zone JTE (junction termination extension) with peak blocking voltage of 7,500 V was implemented in the device design.

Nanomanipulation of ridges in few-layer epitaxial graphene grown on the carbon face of 4H-SiC

The atomic force microscope (AFM) is used to study the morphology of graphene grown on 4H-SiC. A mesh-like network of ridges with high curvature is revealed that bound atomically flat, tile-like facets of few-layer graphene (FLG). To further study the structural properties of the ridge network, nanomanipulation experiments are performed using an AFM tip to deform the ridges in both the vertical and lateral directions. From these experiments, evidence is obtained that the ridges can be displaced in both the vertical and lateral directions. In some instances, ridges are found to return to their original shape after deformation. Cross-section transmission electron microscopy (TEM) studies show that the ridges are formed by the delamination of FLG from the SiC substrate.

Time‐of‐flight analysis of the plume dynamics of laser‐ablated 6H‐silicon carbide

The mass and velocity distributions of neutral species, ejected from a laser‐ablated 6H‐SiC target, are measured by time‐of‐flight mass spectroscopy. The laser‐generated plume is shown to consist of atomic silicon and carbon and small neutral clusters. Measured values of the mean kinetic energies of neutral Si and SiC2 species are 1.1 and 1.5 eV, respectively, when a 6H‐SiC target is irradiated with 248 nm radiation at a power density of 1×108 W/cm2. The kinetic energy distribution of Si+ ions is also measured for comparison with the neutral species, and shows a peak in the distribution at 38 eV. These data illustrate that an electronic, and not a thermal, mechanism for particle emission is active during the ablation of a 6H‐SiC target. The relationship between the dynamics of the photoablation process and the properties of pulsed‐laser‐deposited films is also briefly discussed.