(Color online) Raman spectrum from a HPSI 4H(0001) SiC wafer after graphene formation and patterning. The spectrum was taken in the middle of a graphene Hall bar. The SiC background, recorded on the same wafer in an area where the graphene had been removed, was subtracted. The dashed line indicates the expected position of the D peak.
(Color online) [(a) and (b)] Tapping mode AFM images from 4H(0001) SiC wafer after graphene formation using the original growth process. (a) Height image. (b) Phase image from the same area of graphene as in (a) light gray (yellow) vs dark gray (brown) regions: 1 ML graphene vs 2 ML graphene (see discussion in text). One continuous graphene layer grown conformally over steps and pits. [(c) and (d)] Tapping mode AFM images from 6H(0001) SiC wafer after graphene formation using the modified growth process. (c) Height image. (d) Phase image from the same area of graphene as in (c) light gray (yellow) vs dark gray (brown) regions are observed that as in (b) they should correspond to 1 ML graphene vs 2 ML, respectively; however, in this sample, the dark gray (brown) region covers the majority of the surface.
HRTEM cross sections. (a) Graphene is the dark stripe between the thick bright stripes, which are attributed to the spacing between graphene and adjacent layers in this focus condition. One continuous graphene layer and possibly a second graphene layer can be observed (arrows). (b) Graphene layer conformally covering a two-bilayer step on the SiC surface.
Drawing of photolithographically patterned graphene Hall bar device.
Plot of the voltage difference between electrodes and of the Hall bar vs the magnetic field . A perfectly linear plot proves the validity of the assumptions used in the standard Hall mobility measurement method for one to two layer graphene grown on the Si-face of SiC (e.g., dominance of a single type of majority carriers, and constant with varying magnetic field).
(Color online) Hall and field-effect mobility vs gate voltage measured from two top-gated Hall bar devices.
(a) FET output characteristics for a device with gate length and channel width . The drain current is normalized to channel width. (b) Plot of transconductance , normalized to channel width, as a function of top-gate voltage at for this rf-FET.
De-embedded current gain vs frequency from the same rf-FET as in Fig. 7. Cutoff frequency is determined to be 16 and 24 GHz for of 1.0 and 2.0 V, respectively. The current gain exhibits the frequency dependence expected for a well-behaved FET.
De-embedded current gain vs frequency for a rf-FET with , exhibiting the maximum cutoff frequency that we measured from devices of this gate length. The current gain exhibits the frequency dependence expected for a well-behaved FET.
(Color online) (a) Devices fabricated on epitaxial graphene on the Si-face of a 2 in. HPSI SiC 4H(0001) wafer. (b) SEM micrograph of a rf-FET device. line denotes the direction of the cross-section drawing depicted in (c). (c) Three-dimensional cross-sectional slice drawing of epitaxial graphene RF-FET device. (d) De-embedded current gain vs frequency for three rf-FETs with exhibiting a range of cutoff frequencies between 60 and 100 GHz. All three devices were tested at a drain bias of 2 V, and the device that showed the highest at 2 V was also tested at resulting in . (Some of these data were reported in Ref. 14.) The current gain exhibits the frequency dependence expected for a well-behaved FET.
(a) Raman spectrum of as-grown graphene on SiC 6H(0001) [sample corresponding to Table II and Figs. 2(c) and 2(d)] after SiC background subtraction. (b) Plot of the 2D peak intensity of the Raman spectrum of the same sample for three different positions on a quarter of a 2 in. wafer (close to the center, intermediate position between center and edge, and close to the edge). For each position, several spectra separated by about from each other were recorded.
(Color online) Plot of room-temperature Hall mobility vs top-gate voltage from a device built on the sample corresponding to Fig. 11.
Room-temperature Hall measurement results from as-prepared graphene on HPSI 4H(0001) SiC (ungated Hall bars with size ).
Room-temperature Hall measurement results from graphene on semi-insulating 6H(0001) SiC (ungated Hall bars with size ). Devices (4,3), (5,3), and (5,5) were measured 6 months after the first three devices.
Room-temperature Hall mobilities (published and from this work) from graphene grown on the Si-face of hexagonal SiC
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