I. INTRODUCTION

Graphene, a 2D material with single-carbon-atom thickness, has been intensively studied owing to its unique physical, chemical, and electrical properties^[1-5]. Since graphene was first obtained by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) in 2004, many approaches for graphene synthesis have been developed, including chemical reduction of graphene oxide, conversion of SiC to graphene via sublimation of Si atoms at high temperatures, and chemical vapor deposition (CVD) on a transition metal catalyst^[2-5]. Among these methods, CVD-graphene produced using a metal catalyst has enabled the fabrication of large-area and highly crystalline graphene, which has been incorporated into many graphene-based electronic devices^[4,5]. Particularly, CVD-graphene has been used to overcome the physical limitations of semiconductor materials by exploiting its high intrinsic carrier mobility and chemical stability. For example, the novel architecture of a graphene transistor called a barristor has been demonstrated by combining CVD-graphene with Si^[6]. In addition, the thermal and mechanical stabilities of semiconductor nanomaterials were enhanced by using a graphene shell^[7]. However, graphene is most commonly synthesized on a catalyst substrate, after which it is transferred to the desired substrate, such as SiO₂, Al₂O₃, or Si^[4,5]. This transfer process results in the inevitable degradation of graphene properties through defect formation, contamination, and wrinkle formation^[8]. Therefore, direct growth of graphene on semiconductor surfaces rather than on a metallic substrate has emerged as a promising process for graphene-based electronic applications.

The silicon-germanium alloy, which is a key material for future CMOS technology, enables high-speed operation with low power consumption^[9]. Furthermore, strain-relaxed SiGe layers can act as buffer layers for the epitaxial growth of III–V materials on Si substrates^[10]. Therefore, SiGe has attracted considerable attention from semiconductor researchers and industry. In this study, we demonstrate the direct growth of graphene on SiGe without using a metal catalyst via conventional low-pressure chemical vapor deposition (LPCVD). This approach eliminated the need for the transfer step and can be directly applied to conventional microelectronics technology.

II. EXPERIMENTAL

1. Direct Growth

Before synthesizing graphene, a p-type SiGe (100) wafer (2 wt% Ge, MTI Korea) was sequentially treated using a general radio corporation of America (RCA) cleaning process, oxygen plasma treatment, and HF dipping to obtain a hydrogen terminated surface. The hydrogen-terminated SiGe wafer was loaded into the center of a reactive tube. Subsequently, the CVD chamber was depressurized to approximately ~10^-6 Torr, and H₂ gas was introduced. At 930 °C, a mixture of CH₄ and H₂ gas was introduced into the chamber for 2 h to synthesize the graphene. A total pressure of 80 Torr was maintained during the growth process. Finally, the CH₄ gas was switched off and the furnace was cooled to room temperature (Fig. 1).

Fig. 1. Schematic illustration of the (a) graphene growth system, (b) graphene growth process.

2. Au-assisted Dry Transfer

The as-grown graphene was transferred onto a SiO₂/Si wafer (or desired substrate) for subsequent analysis and characterization via Au-assisted dry-transfer. A thin Au film was deposited onto the graphene-grown SiGe substrate using a thermal evaporator to form a supporting layer. A poly(methyl methacrylate) (PMMA) layer was then coated onto the Au/graphene/SiGe via spin-coating (3000 rpm, baked at 100 °C for 1 min). Subsequently, the PMMA/Au/graphene/SiGe was adhered to thermal-release-tape (TRT, Haeun Chemtec, RP70N5) to delaminate the graphene from the SiGe substrate. The separated film (TRT/PMMA/Au/graphene) was then placed on a SiO₂/Si substrate (or any desired substrate). The TRT was removed by heating for 2 min at 100 °C. Finally, the PMMA and thin Au film were completely removed by dipping the sample into a mixed acetone and KI/I2 solution (Fig. 2).

Fig. 2. Schematic illustration of graphene direct growth on a SiGe substrate and graphene transfer onto a SiO₂/Si substrate using the dry-transfer method.

III. RESULTS AND DISCUSSION

After direct synthesis of the graphene under the optimized growth conditions as described above, Raman analysis was performed to characterize the as-grown graphene on the SiGe substrate. Strong Raman peaks at 1350, 1580, and 2700 cm^-1 corresponding to the D, G, and 2D Raman modes were detected over the whole area. These peaks indicated that the graphene was successfully synthesized on the SiGe substrate (Fig. 3) ^[11]. However, the intensity of the obtained Raman spectra varied depending on the position (Fig. 3(a) and Fig. 3(b)). According to the surface morphology analysis of the SiGe substrate after graphene growth, large numbers of unknown particles were present on the flat SiGe substrate. Thus, the intensity fluctuation of the Raman mapping images was caused by the out-of-focus of the laser light on the particles (Fig. 4).

Fig. 3. (a)-(b) Raman mapping images of the G and 2D peaks obtained from the as-grown graphene on the SiGe substrate, (c) Raman spectra of the graphene at the points marked in (b).

Fig. 4. (a) AFM image of the SiGe substrate after the graphene growth process, (b) Line-profile of the corresponding red line drawn in (a).

To avoid artifacts from the SiGe substrate during Raman analysis of the graphene and to visualize the graphene more clearly, the as-grown graphene was transferred onto a 300-nm SiO₂/Si substrate via Au-assisted dry-transfer. The optical microscopy (OM) image in Fig. 5(a) shows that the graphene was fully grown on the SiGe substrate, but the number of layers was not uniform. The Raman spectra of the selected regions of the OM image shown in Fig. 5(a) show D, G, and 2D peaks^[12]. The integrated intensity ratio of the 2D and G peaks (I(2D)/I(G)) at the three marked positions yielded values of 3.42, 1.63, and 1.24, indicating the presence of mono-, bi-, and few-layer graphene, respectively (Fig. 5(b))^[11]. On the other hand, the I(D)/I(G) value was similar (0.30, 0.25, and 0.34) at all positions, although these are smaller than those of the graphene on the Si substrate [12]. This indicated that the incorporated Ge atoms may exhibit catalytic activity for sp² hybridization to a larger extent than a pristine Si substrate^[13].

Fig. 5. (a) Optical microscopy, (b) Raman spectra of the transferred graphene on the 300 nm SiO₂/Si substrate.

Because the carbon solid solubility of Si and Ge is considerably lower than that of copper and nickel, we expect that graphene would be grown on the SiGe via a self-limiting growth mechanism, where a monolayer of graphene is dominant^[4,5,14]. However, the coexistence of different graphene layers is clear from the SEM and AFM images (Fig. 6(a) and Fig. 6(b)). The dark circular patterns were identified as multilayer graphene and partially cover the bright background area consisting of mono- and bi-layer graphene.

Fig. 6. (a) SEM, (b) AFM images of graphene on the SiO2/Si substrate.

For further examination, we determined the chemical composition of the unknown particles on the SiGe substrate via SEM and EDS analysis after the graphene growth process. It was confirmed the unknown particles were primarily Ge-enriched particles (77 wt% of Ge; Fig. 7(a) and Fig. 7(b)). Because the Ge atoms could diffuse out from the SiGe substrate under high-temperature and low-pressure conditions, the Ge-enriched particles formed and spread throughout the surface of the SiGe substrate^[15,16]. The diffused Ge atoms likely provided additional nucleation sites for multilayer graphene growth. It should be noted that the size and distribution of the Ge-enriched particles coincided with those of the multilayer graphene spots on the transferred graphene film. Therefore, we expect that suppressing out-diffusion of the Ge atoms in the SiGe alloy represents a promising strategy to synthesize more uniform graphene on SiGe substrates.

Fig. 7. (a) SEM image of the as-grown graphene on the SiGe substrate, (b) Energy dispersive spectroscopy (EDS) spectra of the rectangular regions indicated in (a).

IV. CONCLUSIONS

We demonstrated the direct growth of graphene on a SiGe substrate using the conventional LPCVD method. The obtained graphene was composed of mono-, bi-, and few-layered graphene and exhibited better crystallinity compared to that of graphene synthesized on a Si surface. We believe that the SiGe-catalyzed graphene growth method is applicable to current CMOS-based semiconductor manufacturing processes and to next-generation graphene-semiconductor-based nanoelectronics.