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Intentionally doped n-type 4H-SiC films were grown on 4° and 8° off-axis substrates to investigate the influence of electron concentration on the incorporation of electron traps Z1/2 and EH6/7. No discernible change was seen in the Z1/2 and EH6/7 trap concentrations for films grown on both orientations with electron concentrations in the range of 1×1014 to 1×1016 cm-3, suggesting that the Z1/2 and EH6/7 traps are not associated with isolated carbon vacancies. The defect concentrations did not correlate with the measured carrier lifetimes, which is consistent with a carrier lifetime controlled by other recombination centers. Observed decreases in lifetime were related to increases in doping levels, with similar trends seen for both orientations. Carrier lifetimes in 8° material were slightly longer than in 4° films for similar doping concentrations, most likely being associated with surface recombination and/or extended defects. [ABSTRACT FROM AUTHOR]

Local recombination properties of threading screw and edge dislocations in 4H–SiC epitaxial layers have been studied using electron beam induced current (EBIC). The minority carrier diffusion length in the vicinity of dislocations was found to vary with dislocation type. Screw dislocations had a more pronounced impact on diffusion length than the edge dislocations, evidencing stronger recombination activity. Temperature dependence of EBIC contrast of dislocations suggests that their recombination activity is controlled by deep energy levels in the vicinity of dislocation cores. This paper shows that the type of dislocation (screw or edge) can be identified from analysis of EBIC contrast. [ABSTRACT FROM AUTHOR]

It is well-known that the performance of graphene electronic devices is often limited by extrinsic scattering related to resist residue from transfer, lithography, and other processes. Here, we report a polymer-assisted fabrication procedure that produces a clean graphene surface following device fabrication by a standard lithography process. The effectiveness of this improved lithography process is demonstrated by examining the temperature dependence of epitaxial graphene-metal contact resistance using the transfer length method for Ti/Au (10 nm/50 nm) metallization. The Landauer-Buttiker model was used to explain carrier transport at the graphene-metal interface as a function of temperature. At room temperature, a contact resistance of 140 Ω-μm was obtained after a thermal anneal at 523K for 2 hr under vacuum, which is comparable to state-of-the-art values. [ABSTRACT FROM AUTHOR]

Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7-5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces. [ABSTRACT FROM AUTHOR]

A simple hydrothermal growth process was developed to deposit conformal manganese oxide nanospheres with a diameter of 10 to 20 nm on mesoporous carbon paper. The coating of nanospheres increased the cyclic-voltammogramic response of carbon paper by a factor of 5 with a slight dependence on the scan rate. For comparison, a related chemistry was also developed to fabricate a dense packing of manganese oxide nanorods with a diameter of 20 to 50 nm and a length of approximately 500 nm. The nanorods also increased the cyclic-voltammogramic response of carbon paper but only by a factor of approximately 3. [ABSTRACT FROM AUTHOR]

In situ growth interrupts were executed during the growth of 8° off-cut 4H-SiC epitaxial layers to determine the dependence of the efficiency for converting basal plane dislocations (BPDs) to threading edge dislocations (TEDs) on the interrupt temperature. Three samples were grown with 15 min growth interrupts executed near the middle of a 30–40 μm thick, n-type doped layer at 1400, 1500, and 1580 °C. Ultraviolet photoluminescence (UVPL) mapping of these layers reveals the BPDs and permits the observation of how these defects extend into and through the epilayer. From the lateral length of the BPDs in the UVPL wafer map, it was determined whether or not the BPD converted into a TED at the growth interrupt interface. The percentage of BPDs that converted for the 1400, 1500, and 1580 °C interrupts were 20%, 28%, and 51%, respectively. The proposed cause for the higher conversion rate at the interrupt temperature of 1580 °C is the higher etching rate that occurs at this temperature, and it is presumed that there is an enhanced etch rate where the BPD intersects the surface which will result in etch pit decorations. It was also observed that no new in-grown stacking faults formed at the growth interrupt interface, as has been observed for ex situ growth interrupts. [ABSTRACT FROM AUTHOR]

The carbon-to-silicon ratio influences the background doping level of silicon carbide grown by hot-wall chemical vapor deposition (HWCVD). A quadrupole mass spectrometer was used to measure the process composition in the exhaust stream of a HWCVD reactor. The 26 amu mass-to-charge acetylene peak showed the strongest dynamic response to intentional changes in the precursor partial pressure at growth temperature. Methane peaks showed a similar but weaker dynamic response. The acetylene peaks have a direct linear correlation to variations in the propane and silane precursors, and thus can be tracked to give real-time in situ measurement of changes in the C/Si ratio. [ABSTRACT FROM AUTHOR]

High-reflectance group III-nitride distributed Bragg reflectors (DBRs) were deposited by metal-organic chemical-vapor deposition on Si(111) substrates. A reflectance greater than 96% was demonstrated for an AlN/GaN DBR with a stop band centered in the blue-green range of the visible spectrum. Crack-free GaN cap layers were grown on the DBR structures to demonstrate the opportunity to build III-nitride optoelectronic devices in this material. The DBR structure was under significant strain due to growth on a mismatched substrate, although the GaN cap layer was shown to be strain-free. [ABSTRACT FROM AUTHOR]

In this letter, we present state-of-the-art performance of top-gated graphene n-FETs and p-FETs fabricated with epitaxial graphene layers grown on Si-face 6H-SiC substrates on 50-mm wafers. The current--voltage characteristics of these devices show excellent saturation with ON-state current densities (Ion) of 0.59 A/mm at Vds = 1 V and 1.65 A/mm at Vds = 3 V. Ion/Ioff ratios of 12 and 19 were measured at Vds = 1 and 0.5 V, respectively. A peak extrinsic gm as high as 600 mS/mm was measured at Vds = 3.05 V, with a gate length of 2.94 µm. The field-effect mobility versus effective electric field (Eeff) was measured for the first time in epitaxial graphene FETs, where record field-effect mobilities of 6000 cm²/V ⋅ s for electrons and 3200 cm²/V ⋅ s for holes were obtained at Eeff ~ 0.27 MV/cm. [ABSTRACT FROM AUTHOR]

GaN nanowires were irradiated using a cyclotron at 5 MeV energy with a fluency of up to 3.38×1015/cm2 protons. The resistance of the GaN was increased by 95% at a dose of 1.69×1015/cm2 protons and then 116% at a dose of 3.38×1015/cm2 protons because of the damage induced by the high energy protons. Cathodoluminescence of the GaN nanowires found a slight broadening of near band-edge emission and a dramatic decrease in the intensity of midgap transitions. These GaN-based nanomaterials have a potential in space technology because of their strong bonding energy compared to other material systems such as silicon and GaAs. Furthermore, the relatively small decrease in resistivity confirms the predicted robustness to proton irradiation of GaN nanowires compared to a GaN thin film. [ABSTRACT FROM AUTHOR]

The article examines the success of silicon carbide as a material used within high-voltage, high-current devices that can be operated at elevated temperatures. Developments in crystal growth have made large-area, high-quality silicon carbide substrates available for applications such as solid-state white lighting and high-frequency transmitters. It has also been suggested as a material for bolstering green energy, by replacing less energy-efficient switches that are based on silicon technology. The author examines many aspects of the material, including the choice of a semiconductor for switching electrical currents on and off, atomic ratios, polycrystallinity, and lattice dynamics.

A direct correlation between local threading screw dislocation densities in an aluminium gallium nitride/gallium nitride (AlGaN/GaN) heterostructure and the mobility of electrons in the two-dimensional electron gas (2-DEG) formed at the interface of that heterostructure is presented. Threading screw dislocations are directly imaged through open areas of Hall-effect test structures using electron channelling contrast imaging. The dislocation density measured for a given test structure is correlated to the mobility of the same structure. The results show a direct, negative correlation between mobility and screw/mixed dislocation density. [ABSTRACT FROM AUTHOR]

A long-standing goal of GaN device research has been the development of a reliable, well-controlled process for p-GaN formation by ion implantation. Results to date have indicated an activation of 1% or less using high-temperature rapid thermal annealing (RTA) techniques and coimplantation. Although Mg is a relatively deep acceptor, this is still much less than the theoretically achievable value (8.2% based on the 160 meV acceptor level). A multicycle RTA process is presented that is capable of achieving up to 8% activation of the Mg-implanted GaN. This approaches the theoretical value, and represents a significant step in GaN device research. [ABSTRACT FROM AUTHOR]

The article reports on researchers at the U.S. Naval Research Laboratory implanting magnesium (Mg) in galium nitride (GaN) and employed a novel high temperature annealing technique. The method has realised electrical activation of implanted material without co-implantation. It also showed an increase in the activation ratios for in situ Mg-doped GaN from around 1% to a record 10%.