R. L. Myers-Ward

Epitaxial-Graphene RF Field-Effect Transistors on Si-Face 6H-SiC Substrates

We report dc and the first-ever measured small-signal radio-frequency (RF) performance of epitaxial-graphene RF field-effect transistors (FETs), where the epitaxial-graphene layer is formed by graphitization of 2-in-diameter Si-face semi-insulating 6H-SiC (0001) substrates. The gate is processed with a metal gate on top of a high-k Al2 O3 gate dielectric deposited via an atomic-layer-deposition method. With a gate length (Lg) of 2 mum and an extrinsic transconductance of 148 mS/mm, the extrinsic current-gain cutoff frequency (fT) is measured as 4.4 GHz, yielding an extrinsic fT ldr Lg of 8.8 GHz middot mum. This is comparable to that of Si NMOS. With graphene FETs fabricated in a layout similar to those of Si n-MOSFETs, on-state current density increases dramatically to as high as 1.18 A/mm at Vds = 1 V and 3 A/mm at Vds = 5 V. The current drive level is the highest ever observed in any semiconductor FETs.

Hall effect mobility of epitaxial graphene grown on silicon carbide

Epitaxial graphene (EG) films were grown in vacuo by silicon sublimation from the (0001) and (0001¯) faces of 4H-SiC and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ∼150 000 cm2 V−1 s−1 on the (0001¯) face and ∼5800 cm2 V−1 s−1 on the (0001) face.

Top-Gated Epitaxial Graphene FETs on Si-Face SiC Wafers With a Peak Transconductance of 600 mS/mm

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 (I_on) of 0.59 A/mm at V_ds = 1 V and 1.65 A/mm at V_ds = 3 V. I_on/I_off ratios of 12 and 19 were measured at V_ds = 1 and 0.5 V, respectively. A peak extrinsic g_m as high as 600 mS/mm was measured at V_ds = 3.05 V, with a gate length of 2.94 ¿m. The field-effect mobility versus effective electric field (E_eff) 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 E_eff ~ 0.27 MV/cm .

Low-Phase-Noise Graphene FETs in Ambipolar RF Applications

In this letter, we present both the 1/f noise and phase noise performance of top-gated epitaxial graphene field-effect transistors (FETs) in nonlinear circuit applications for the first time. In the case of frequency doublers, the fundamental signal is suppressed by 25 dB below the second harmonic signal. With a phase noise of -110 dBc/Hz measured at a 10-kHz offset, a carrier-to-noise degradation (ΔCNR) of 6 dB was measured for the frequency doubler. This implies noiseless frequency multiplication without additional 1/f noise upconversion during the nonlinear process. The frequency multiplication was demonstrated above the gigahertz range. The 1/f noise of top-gated epitaxial graphene FETs is comparable or lower than that of exfoliated graphene FETs.

Morphology characterization of argon-mediated epitaxial graphene on C-face SiC

Epitaxial graphene layers were grown on the C-face of 4H–SiC and 6H–SiC using an argon-mediated growth process. Variations in growth temperature and pressure were found to dramatically affect the morphological properties of the layers. The presence of argon during growth slowed the rate of graphene formation on the C-face and led to the observation of islanding. The similarity in the morphology of the islands and continuous films indicated that island nucleation and coalescence is the growth mechanism for C-face graphene.

Enhanced Performance in Epitaxial Graphene FETs With Optimized Channel Morphology

This letter reports the impact of surface morphology on the carrier transport and radio-frequency performance of graphene FETs formed on epitaxial graphene synthesized on SiC substrates. Such graphene exhibits long terrace structures with widths between 3-5 μm and steps of 10 ± 2 nm in height. While a carrier mobility value above 3000 cm2/V·s at a carrier density of 1012 cmx2 is obtained in a single graphene terrace, the step edges can result in a step resistance of ~21 kΩ·μm. By orienting the transistor layout so that the entire channel lies within a single graphene terrace and by reducing the access resistance associated with the ungated part of the channel, a cutoff frequency above 200 GHz is achieved for graphene FETs with channel lengths of 210 nm, i.e., the highest value reported on epitaxial graphene thus far.

Basal plane dislocation reduction in 4H-SiC epitaxy by growth interruptions

The paths of basal plane dislocations (BPDs) through 4H-SiC epitaxial layers grown on wafers with an 8° offcut were tracked using ultraviolet photoluminescence imaging. The reduction of BPDs by conversion to threading edge dislocations was investigated at ex situ and in situ growth interrupts. For ex situ interrupts, BPDs are imaged after each of several growths. The wafer remains in the reactor for in situ interrupts and BPDs are imaged after the growth is finished. For in situ interrupts, a combination of temperature, propane flow, and duration has been determined, which achieve a BPD reduction of 98%.

Bilayer graphene grown on 4H-SiC (0001) step-free mesas.

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

High temperature measurements of metal contacts on epitaxial graphene

Electrical characteristics of Cr/Au and Ti/Au metal contacts on epitaxial graphene on 4H-SiC showed significant variations in resistance parameters at 300 K. These parameters decreased substantially as the temperature increased to 673 K. The work function, binding energy, and diffusion energy of the deposited metals were used to explain these observed variations. The quantitative analysis of our data demonstrates that non-reactive metals with higher work functions result in lower contact resistance, which can be further decreased by 70% using appropriate annealing. These results provide important information when considering epitaxial graphene for high temperature applications.

Recombination processes controlling the carrier lifetime in n−4H–SiC epilayers with low Z1/2 concentrations

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped...