Eric Frantz

Correlating Raman Spectral Signatures with Carrier Mobility in Epitaxial Graphene: A Guide to Achieving High Mobility on the Wafer Scale

We report a direct correlation between carrier mobility and Raman topography of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall mobility of material on SiC(0001) is highly dependent on thickness and monolayer strain uniformity. Additionally, we achieve high mobility epitaxial graphene (18100 cm(2)/(V s) at room temperature) on SiC(0001) and show that carrier mobility depends strongly on the graphene layer stacking.

Nucleation of epitaxial graphene on SiC(0001).

A promising route for the synthesis of large-area graphene, suitable for standard device fabrication techniques, is the sublimation of silicon from silicon carbide at elevated temperatures (>1200 degrees C). Previous reports suggest that graphene nucleates along the (110n) plane, known as terrace step edges, on the silicon carbide surface. However, to date, a fundamental understanding of the nucleation of graphene on silicon carbide is lacking. We provide the first direct evidence that nucleation of epitaxial graphene on silicon carbide occurs along the (110n) plane and show that the nucleated graphene quality improves as the synthesis temperature is increased. Additionally, we find that graphene on the (110n) plane can be significantly thicker than its (0001) counterpart and appears not to have a thickness limit. Finally, we find that graphene along the (110n) plane can contain a high density of structural defects, often the result of the underlying substrate, which will undoubtedly degrade the electronic properties of the material. Addressing the presence of non-uniform graphene that may contain structural defects at terrace step edges will be key to the development of a large-scale graphene technology derived from silicon carbide.

Characterization of high temperature piezoelectric crystals with an ordered langasite structure

Piezoelectric single crystals with the ordered langasite structure A3BC3D2O14, including Sr3TaGa3Si2O14, Sr3NbGa3Si2O14, Ca3TaGa3Si2O14, and Ca3TaAl3Si2O14 (CTAS), were studied as a function of temperature, up to 900 °C. The dielectric permittivity e11 and piezoelectric coefficient d11 of the ordered crystals were found to be on the orders of 12–16 and 4–5 pC/N, respectively, slightly lower than langasite (La3Ga5SiO14-LGS) or langanite (La3Ga5.5Nb0.5O14) crystals which possess a disordered structure. The mechanical quality factor Q and electrical resistivity ρ, however, were found to be greatly improved at elevated temperatures ≥500 °C, being one to two orders of magnitude higher, due to cation ordering. Of particular interest is the CTAS crystal, in which, the Ga cations are totally replaced by low cost Al cations. Together with its thermally stable piezoelectric properties and high electrical resistivity, CTAS crystals offer a competitive material for high temperature sensing applications.

Characterization of piezoelectric single crystal YCa4O(BO3)3 for high temperature applications

Operation at temperatures well above ambient is desired for applications such as smart structures integrated within aircraft and space vehicles. Piezoelectric yttrium calcium oxyborate single crystal YCa4O(BO3)3 (YCOB) was found to exhibit no phase transition until its melting temperature around ∼1500°C. The temperature characteristics of the resonance frequency, electromechanical coupling, and dielectric permittivity were studied in the temperature range of 30–950°C for different orientations. The electrical resistivity at 800°C was found to be greater than 2×108Ωcm. Together with its temperature independent electromechanical coupling factor (∼12%) and engineered resonance frequency behavior, these make YCOB crystals excellent candidates for sensing applications at ultra high temperatures.

Gadolinium calcium oxyborate piezoelectric single crystals for ultrahigh temperature (>1000 °C) applications

ReCa4O(BO3)3 oxyborate crystals (ReCOB, where Re is a rare earth element such as Gd) were grown using the Czochralski pulling technique. The crystals belong to Cm space group and the relationships of the as-grown crystal morphology with crystallographic and physical coordinates were determined. The optimum length extensional and thickness shear vibrations of GdCOB were found for (ZYl)40° and (YXt)33° cuts, with electromechanical coupling factors k32 and k26, being on the order of 17.5% and 25% and piezoelectric coefficients d32 and d26 around −4.5 and 11.7 pC/N, respectively. Of particular significance is the nearly temperature independent behavior up to >1000 °C. Together with its high resistivity (∼5×106 Ω cm at 1000 °C) and high mechanical quality factor (∼4000 at 1000 °C) make GdCOB and/or ReCOB crystals promising candidates for the next generation sensing applications at ultrahigh temperature (>1000 °C).

High-temperature piezoelectric single crystal ReCa 4 O(BO 3 ) 3 for sensor applications

Large-size and high-quality ReCa4O(BO3)3 (ReCOB, Re = rare earth) single crystals were grown by the Czochralski pulling method. In this work, the electrical properties were investigated at room temperature and elevated temperature for YCa4O(BO3)3 (YCOB). The dielectric permittivity, piezoelectric strain coefficient, and electromechanical coupling were found to be on the order of 11, 6.5 pC/N, and 12.5%, respectively, with a high piezoelectric voltage coefficient around 0.067 Vm/N. The electrical resistivity of YCOB was found to be 2 times 108 Ohmmiddotcm at 800degC, with Q values of > 4,500 at 950degC. The frequency/temperature coefficient of YCOB was found to be -75 to -85ppm/K in the temperature range of 30 to 950degC, depending on the crystal orientations. Together with their temperature-independent properties, ReCOB crystals are promising candidates for sensing applications at elevated temperatures.

Epitaxial Graphene Growth on SiC Wafers

An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz.

Large-Area Epitaxial Graphene: Effect of Strain and Thickness on Electronic Properties

The recent success of graphene transistor operation in the giga-hertz range has solidified the potential of this material for high speed electronic applications. Realization of a graphene technology on the production scale; however, requires the ability to synthesize large area graphene, and rapidly characterize the materials structural and electronic quality. We report a direct link between carrier mobility and Raman topography of epitaxial graphene grown on silicon carbide. We have examined epitaxial graphene with mobility values of 25 - 1100 cm2/V-s, and show that the Hall mobility of epitaxial graphene on the Si-face of SiC (SiC(0001)) is not only highly dependent on thickness uniformity, but also on mono-layer strain uniformity. It is not until the thickness and strain uniformity is approaches 50% of the device width that one is able to achieve mobility values higher than 1000 cm2/V-s.

High temperature piezoelectric single crystal ReCa 4 O(BO 3 ) 3 for sensors

Large size and high quality ReCa4O(BO3)3 single crystals were grown by the Czochralski pulling method. In this work, the electrical properties were investigated at room temperature and elevated temperature for YCa4O(BO3)3 (YCOB). The dielectric permittivity, piezoelectric strain coefficient and electromechanical coupling were found to be on the order of 11, 6 pC/N and 12.5%, respectively, with a high piezoelectric voltage coefficient around 0.067 Vm/N. The electrical resistivity of YCOB was found to be 2times108 Ohm.cm at 800degC, with Q values of >4,500 at 950degC. The frequency/temperature coefficient of YCOB was found to be -75 ~ -85 ppm/K in the temperature range of 30-950degC, depending on the crystal orientation. Together with its temperature independent properties, ReCOB crystals are promising candidates for sensing applications at elevated temperatures.