EG Epitaxial graphene ML Monolayer QFS BLG Quasi freestanding bilayer epitaxial graphene XPS X-ray photoelectron spectroscopy SEM Scanning electron microscopy FTIR Fourier transform infrared slm Standard liters per minute CNP Charge neutrality point Graphene surface plasmons are tightly localized electrons oscillating in a collective motion when excited by an incident electromagnetic wave. Micronscale graphene structures confine these oscillations, producing plasmon resonances in the terahertz range and the resonance frequency is tuned electrostatically via an applied gate voltage [1]. In graphene, although the linear dispersion results in a frequency invariant interband absorption of 2.3% in the visible range, radiation in the THz regime is dominated by intraband conductivity which, when coupled to surface plasmons, increases absorption [2]. This latter quality makes graphene attractive for numerous optoelectronic applications in the underdeveloped THz regime [2-5], including detectors [4, 6-8], emitters [9], modulators [2, 10], antennas [3], switches [5], filters [11] and mixers [12] for communications [13], medical [14, 15], astronomical [16] and security applications [3]. Epitaxial graphene (EG), formed by the sublimation of Si from 4H-or 6H-SiC, is attractive for THz
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 523 K for 2 hr under vacuum, which is comparable to state-of-the-art values.
The processing parameters which favour the onset of an impurity band conduction around room temperature with a contemporaneous elevated p-type conductivity in Al+ implanted 4H-SiC are highlighted by comparing original and literature results. In the examined cases, Al is implanted at 300–400 °C, in concentrations from below to above the Al solubility limit in 4H-SiC (2 × 1020 cm−3) and post implantation annealing temperature is ≥1950 °C. Transport measurements feature the onset of an impurity band conduction, appearing at increasing temperature for increasing Al implant dose, until this transport mechanism is enabled around room temperature. This condition appears suitable to guarantee a thermal stability of the electrical properties. In this study, the heaviest doped and less resistive samples (Al implanted concentration of 5 × 1020 cm−3 and resistivity of about 2 × 10−2 Ω cm) show a carrier density above the Al solubility limit, which is consistent with at least a 50% electrical activation for a 15% compensation. The model of Miller and Abrahams well describes the resistivity data of the lower doped sample, whereas a deviation from the behaviour predicted by such a model is observed in the higher doped specimens, consistent with the occurrence of a variable range hopping at low temperature.
A microwave heating technique has been used for the electrical activation of Al+ ions implanted in semi-insulating 4H-SiC. Annealing temperatures in the range of 2000–2100 °C and annealing time of 30 s have been used. The implanted Al concentration has been varied from 5×1019 to 8×1020 cm-3. A minimum resistivity of 2×10-2 Ω·cm and about 70% electrical activation of the implanted Al have been measured at room temperature for an implanted Al concentration of 8×1020 cm-3 and microwave annealing at 2100 °C for 30 s
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