The interface of epitaxial nanographene on GaN by PECVD

Graphene, as the first experimentally cleaved two-dimension material with many unique properties, is promising in semiconductor and other fields.^1–4 Many synthesis methods for graphene scaled-up application in these fields have been explored recently.^5,6 Chemical vapor deposition (CVD) on various catalyst metal substrates is an important technique for large-area graphene growth.⁷ However, the growth temperature, about 1000°C, is a bit high for some semiconductor materials and related devices (e.g. GaN, which decomposition temperature is 850°C). And in the utilization of graphene on metal, the post-transfer process is inevitable.^8–10 To circumvent these problems, direct growth of graphene on semiconductor substrates at relatively low temperature by plasma enhanced chemical vapor deposition (PECVD), which used RF (AC) frequency or DC discharge between two electrodes to create a plasma of the reacting gases (such as methane), is another potential technology.^11–14 G. Y. Zhang, et al. and other groups had reported synthesized nanographene (domain size from few to tens nanometers) by PECVD directly on dielectric materials such as silicon oxide and sapphire.^12–16 

GaN, as the representative third-generation semiconductor material, is widely used in today’s light-emitting diodes (LEDs), laser diodes (LDs), and power devices.^17,18

Utilization of weak van der Waals force between graphene layers, the upper epitaxy GaN devices can be mechanically exfoliated and transferred. Besides, Graphene can be used as a transparent conducting film, positive strain release layer, heat dispersal layer and so on, in GaN optoelectronics such as LED.^19–21 Y. S. Kim, et al.²⁰ successfully synthesized graphene directly on LEDs at a low temperature (600 °C) by PECVD. It was found that ohmic contact had been formed between GaN and graphene. However, they didn’t pay much attention to the interface and the surface of GaN after growth, though they found the carbon atoms diffused into GaN surface nearly 300 nm depth from the secondary ion mass spectrometry result.

In this paper, we have synthesized nanographene on GaN and focused much more on the interface between nanographene and GaN by comparing the samples before and after PECVD. The properties of PECVD epitaxial nanographene on GaN have been characterized by Raman spectroscopy, AFM, TEM, XPS, Van der Pauw four-point probe tester, and so on. The further study of the interface of graphene on GaN will contribute to the heterogeneous integration of graphene and semiconductor.

In our experiment, the nanographene was synthesized on GaN at a low temperature (550°C) by PECVD in a tube furnace. GaN substrates for graphene growth should be ultrasonic cleaning in acetone, alcohol and DI water before loaded in the furnace. And then, the GaN substrates were annealed in a hydrogen atmosphere at 550°C. After annealing, methane as the precursor was introduced into the chamber and decomposed at 110 W. The whole growth period sustained one hour at 550°C.

Raman spectroscopy as a convenient tool was carried out at room temperature. A 532 nm diode pumped solid state green laser was focused on the post-grown GaN substrate through a 100x objective. The excited Raman scattering was collected by Labram HR 800 Raman spectrometer with an 1800 cm^-1 grating. The XPS was carried out in a commercially available Thermo Scientific ESCALAB 250Xi with a K-Alα source. For AFM studies, the Veeco Dimension 3100 AFM worked in tapping mode was used. TEM was performed by Tecnai G2 F20 S-Twin operated at 200 kV. Van der Pauw four-point probe tester was used to characterize the transport properties of this epitaxial nanographene. The MD2000D Spectroscopic Ellipsometer, which worked in the wavelength range from 193 nm to 1690 nm, was used to obtained transmittance spectrum.

In Fig. 1(a), the transmittance of the post-growth GaN substrate slightly declined, which indicated that some carbon atoms were absorbed on GaN. Generally, the Raman spectroscopy of graphene has three dominated peaks such as D bond at 1350 cm^-1, G bond at 1580 cm^-1 and 2D bond at 2700 cm^-1.^22–24 As shown in Fig. 1(b), the strong signal peaks of GaN, the D, G, and 2D bonds were observed after growth. The Raman spectrum with obvious D and D’ versus weak G was caused by the small grain size and abundant edges.^12–14 As a rough estimate, the average interdefect distance, around 4.6 nm, could be calculated through the following formula:²⁵ 

E_L is the laser excitation energy, which is 2.3 eV in our experiment. The number 4.3x10³ is an empirical constant. The ratio of I_D/I_G used in the calculation was the average value of I_D/I_G that obtained from several Raman spectra. For this small L_D, the domain size L_α is proportional to $LD2$⁠. And it could be estimated as ∼20 nm by the morphology of nanographene in Fig. S1(b). The selected-area electron diffraction (SAED) with two sets of hexagonally arranged spots in Fig. 1(c) indicated a polycrystalline graphite pattern. And the interatomic spacing could also be calculated as ∼0.14 nm through the distance of two symmetrical spots in (110) (almost 1/14 nm) in the profile line as shown in Fig. 1(d). The high-resolution transmission electron microscopy (HRTEM) image in the inset of Fig. 1(d) showed about three layers nanographene film with the interlayer spacing approximately 0.34 nm, which further indicated its graphene structure.²⁶ As the AFM height images shown in Fig. 2(a) and (b), the surface morphology of the post-growth sample almost kept the same as the original GaN substrate. The step flow of post-growth GaN was obvious and almost same as original substrates. This showed that the surface of GaN was carpeted with nanographene. And, there was a slight influence on the surface of GaN during the graphene growth process. No extra Raman scattering peaks emerged from 300 cm^-1 to 640 cm^-1 for the post-growth GaN, as shown in Fig. S2, also illustrated no defects or other damaged regions being introduced.

For further details, XPS had been used to study the interaction between nanographene film and GaN. The C 1s peak of the sample after growth increased significantly, which could be seen in Fig. 3(a). In Fig. 3(b), the dominated peak at ∼284.4 eV in C 1s core-level XPS spectrum confirmed the sp² C-C consisted of honeycomb structure in nanographene. The other two peaks, C-OH and C-O, were corresponding to absorption -OH or CO_x along the edges.¹⁴ As shown in Fig. 3(c) and (d), there was no shift in high-resolution XPS spectra of Ga 2P_1/2, Ga 2P_3/2 and N 1s for GaN substrate before and after growth, which indicated that there were not any other extra atoms except N around Ga (or Ga around N). So that, there shouldn’t be any other chemical bonds at the interface between epitaxial nanographene film and GaN substrate.

Utilization of the weak van der Waals force between nanographene and GaN substrate, a universal transfer method can be used to transfer graphene to any other substrates.²⁷ After growth, the nanographene film could be separated easily from GaN substrate by oxygen bubble, which was generated by heating the mixture of ammonia and hydrogen peroxide solution. The detailed information about this transfer process can be found in the supplementary material. Here, the synthesized graphene on GaN had been transferred to silicon dioxide. As shown in Fig. 4(a), the characteristic peaks of the nanographene film such as D, G, and 2D had disappeared after transfer, which indicated this simple transferring method was efficient. Fig. 4(b) and (c) respectively showed the Raman mapping of 2D bond of the post-growth GaN before and after transfer. As the nanographene synthesized with a short duration, the intensity of 2D bond for this sample in Fig. 4(b) was weak and inhomogeneous. After the transferring process, the intensity of 2D bond for GaN substrate was obviously declined to nearly zero (background signal of the instrument was inevitable), which indicated that the nanographene film had been separated from the GaN substrate completely by using oxygen bubble to overcome the force of the interface.^27–29 

In Fig. 5(a), the optical transmittance of the post-growth GaN in Fig. 1(a) decreased about 3% relative to bare GaN substrate for the wide spectrum from ultraviolet to near-infrared, especially at the characteristic wavelengths of GaN LEDs (455 nm and 520 nm). It is a little bit better than previous work.^20,30,31 It should be noted that the oscillation of this curve was induced by the optical interference at the interface between GaN and sapphire. As shown in Fig. 5(b), the linear I-V characteristics (blue) of the post-growth GaN raveled ohmic contact for adjacent nanographene films interconnects. However, there was no current but noise of the instrument as the red curves shown while applying voltage on the bare GaN substrate (undoped). The relatively low conductivity of the nanographene compared to CVD graphene on metals may be attributed to its high defect density. Such optical and electrical properties are comparable to reduced graphene oxide or chemical exfoliated graphene as used in transparent and conductive electronics.^31,32

In summary, the nanographene has been synthesized on the GaN by PECVD with methane as the carbon source at a relatively low temperature. It was found that the surface topography of the GaN substrate didn’t change much after growth, which indicated that the direct growth nanographene film would not degrade the properties of GaN-based optoelectronics. Furthermore, the XPS characterization results suggested that there was no chemical bonding between epitaxial nanographene film and GaN. And thanks to the weak van der Waals force between the nanographene film and GaN, the epitaxial nanographene film could be easily separated from the substrate by a universal bubble method. These results may give us some insight into the nanographene on GaN and heterogeneous integration of graphene and GaN.

See supplementary material for other information about the nanographene and transfer process.

This work was supported by the State Key Program of National Natural Science Foundation of China (No. 61734008), the National Natural Science Foundation of China (Nos. 61574097, 61604170), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).