High‑Quality Multi‑Layer Graphene on 4H‑SiC via Joule‑Heat Decomposition: Raman Characterization

Background

Graphene’s exceptional mechanical, electronic, and thermal properties have propelled it to the forefront of nano‑electronics, transparent electrodes, and printable photonics research (see, e.g., Refs. [1,2]). Numerous synthesis routes exist: mechanical cleavage delivers pristine monolayers but in limited size [5]; chemical vapor deposition (CVD) on Ni or Cu yields large‑area films [6,7]; metal‑free CVD on silicon produces small flakes [8]. Thermal decomposition of silicon carbide (SiC) remains the most promising for producing high‑quality, wafer‑scale epitaxial graphene (EG) directly on a semiconductor substrate, thereby avoiding transfer‑related defects [9–11].

While several heating strategies (radio‑frequency induction, laser, and other methods) have been explored for EG growth [12–14], the JHD technique offers rapid, controllable heating via direct current, achieving surface temperatures from ~1230 °C to >1600 °C within seconds. This method allows wafer‑sized graphene films and offers a low‑cost, high‑efficiency growth platform. In this study, we examine how the applied current, resulting temperature, and growth time influence the structural and electrical properties of MLG grown on 4H‑SiC by JHD.

Methods/Experimental

Growth of Graphene on 4H‑SiC

Commercial two‑inch N‑type 4H‑SiC wafers (350 µm thick, ~0.02 Ω cm) were diced into 25 mm × 5 mm pieces. After sonication in methanol, acetone, and ethanol, followed by RCA cleaning, samples were mounted between Mo electrodes on a ceramic platform (see Fig. 1a). The chamber was evacuated to ~10⁻⁶ Torr and a DC current (2.79–3.43 A) was applied, raising the substrate surface to 1230–1600 °C. Growth proceeded for 5 min, after which the samples were cooled in vacuum for over 4 h before characterization.

a Schematic of the JHD platform. Inset: SiC surface during heating. b Raman spectra of SiC and MLG grown at different temperatures (5 min). c Raman spectra of MLG at 1470 °C for 2, 5, and 10 min. d Raman spectra from spots A, B, and C (Fig. 1a inset) on a sample grown at 3.24 A for 5 min.

Sample Characterization

Surface temperatures were monitored with a MI16MB18 infrared thermometer. Raman spectroscopy employed a WITec alpha 300RA confocal microscope (488 nm laser, 600 lines/mm grating, Peltier‑cooled CCD). AFM (SPA‑400) mapped morphology before and after inductively coupled plasma (ICP) etching (30 sccm O₂, 60 s). Au contacts were deposited by thermal evaporation, and the linear transmission line method (LTLM) was used to extract contact resistance, with IV measurements performed by a Keithley 2410 SourceMeter and 6514 electrometer at room temperature.

Results and Discussion

Four MLG samples were synthesized at DCs of 2.79, 3.05, 3.24, and 3.43 A, corresponding to peak surface temperatures of ~1230, 1350, 1470, and 1600 °C, respectively. Raman spectra (Fig. 1b) revealed the characteristic D (~1370 cm⁻¹), G (~1600 cm⁻¹), and 2D (~2750 cm⁻¹) bands of graphene. Compared with micromechanical cleavage graphene (MCG), the MLG G and 2D bands were blue‑shifted by ~20 and ~77 cm⁻¹, respectively, consistent with strain from lattice mismatch [16] and, to a lesser extent, doping [17–19].

At 1230 °C, the high I_D/I_G (~1.01) and absent 2D band indicated a highly defective film. Raising the temperature to 1350 °C reduced I_D/I_G to ~0.38 and produced a symmetric 2D band (FWHM ≈ 72 cm⁻¹), confirming improved crystallinity. Further heating to 1470 °C lowered I_D/I_G to ~0.06 and slightly red‑shifted the 2D band, suggesting strain relaxation and additional layer formation. However, at 1600 °C, I_D/I_G increased to ~0.43, implying defect generation from rapid, out‑of‑equilibrium sublimation [14] and more pronounced strain relief (red‑shift of all Raman peaks).

To probe time effects, three samples were grown at 3.24 A (~1470 °C) for 2, 5, and 10 min (Fig. 1c). The 5‑min film exhibited the lowest I_D/I_G (~0.06) and the fewest defects, while 2‑ and 10‑min films had higher ratios (~0.41 and ~0.29). The 2‑min growth was insufficient for full layer reconstruction; the 10‑min growth introduced defects from residual chamber gases [22]. Raman peak positions remained unchanged across times, indicating a stable strain state and similar layer count (I_G/I_2D ≈ 3.0).

Spatial uniformity was confirmed by Raman mapping of spots A, B, and C (Fig. 1d). Spots A and B, farther from the Mo electrodes, displayed low I_D/I_G and symmetric 2D bands, with consistent I_G/I_2D and I_SiC/I_G ratios, confirming a uniform 45‑layer film over ~12 × 5 mm².

Optical imaging (Fig. 2a) revealed a largely uniform color contrast with isolated dark dots. Raman mapping (Fig. 2b) showed that these dots had higher 2D intensity and slightly red‑shifted G and 2D peaks, implying preferential graphene growth at surface defects or screw dislocations, which accelerate SiC decomposition [23]. The FWHM of the 2D band remained uniform except near these defect sites (Fig. 2d).

AFM after ICP etching (Fig. 3a) produced a clear terrace between graphene and etched SiC. Height profiles (Fig. 3b) yielded an average step height of ~15.46 nm, corresponding to ~45 graphene layers (interlayer spacing ≈ 0.34 nm) [24]. Post‑etching Raman spectra (Fig. 3c) confirmed complete removal of graphene (absence of D, G, 2D bands). RMS roughness increased from 0.84 nm (pre‑etch) to 2.79 nm (post‑etch), reflecting surface roughness induced by graphene growth.

Electrical characterization of the 1470 °C, 5‑min MLG (Fig. 4a) used LTLM to extract sheet resistance (ρ_s = 52.36 Ω / sq) and specific contact resistance (ρ_c = 5.03 × 10⁻⁵ Ω cm²) via the linear relation R_T = (ρ_s/Z)d + 2R_C and ρ_c = ρ_s L_T². These values demonstrate excellent Au–graphene ohmic contact.

Conclusions

We demonstrated a facile JHD technique that produces wafer‑scale, defect‑free, ~45‑layer MLG directly on 4H‑SiC (0001). By optimizing temperature (~1470 °C) and growth time (5 min), we achieved a 12 × 5 mm² film with I_D/I_G ≈ 0.06, ρ_s ≈ 52 Ω / sq, and ρ_c ≈ 5 × 10⁻⁵ Ω cm². Future work will explore epitaxial SiC substrates, inert gas confinement, and other process refinements to further enhance film homogeneity and integrate JHD‑grown graphene into SiC‑based optoelectronic devices.

Abbreviations

AFM:

Atomic force microscope

Al:

Aluminum

C:

Carbon

CVD:

Chemical vapor deposition

DC:

Direct current

EG:

Epitaxial graphene

FWHM:

Full width at half maximum

ICP:

Inductively coupled plasma

I_X:

Intensity of X band

JHD:

Joule heat decomposition

LTLM:

Linear transmission line method

MCG:

Micromechanical cleavage graphene

MLG:

Multi‑layer graphene

Mo:

Molybdenum

SiC:

Silicon carbide