Dv420a

Surface morphologies of the samples were observed by FE-SEM (Hitachi S-4700). The existence of the graphene was confirmed by Raman spectroscopic analysis (excitation laser with a wavelength of 514 nm and a power of 20 mW; Renishaw 2000). Crystal structures of the samples were analyzed using HR-STEM (JEOL JEM-ARM 200F). For the HR-STEM observations, samples were cross-sectionally milled with a 30-kV–accelerated beam of gallium ions using a focused ion beam machine (FEI Helios NanoLab). The incidence electron beam was directed along the GaN[1 $\overline{2}$ 10]║Al₂O₃[1 $\overline{1}$ 00] to determine the remote heteroepitaxial relationship between GaN MR and Al₂O₃ wafer across the graphene layers. The compositional profiles of the indium contents in the top plane and sidewall InGaN/GaN MQWs were recorded from EDX spectroscopy in the scanning TEM mode to confirm the InGaN QW layers. The electrical and light emission properties were characterized by measuring the I-V characteristics and EL spectra using a source meter (Keithley 2400) and a monochromator with a charge-coupled device detector (Andor iDus DV420A), respectively.

Optical transmission spectra were recorded at room temperature with a standard UV/VIS spectrophotometer (UV-3101PC, Shimadzu, Duisburg, Germany). Optical transmission spectra at higher sample temperature were measured employing a heating stage with the visible spectrophotometer (spectrometer: iDus DV420A, Andor, Belfast, UK; lamp: Xenon lamp XBO150, Osram, Munich, Germany; control unit: Oriel, Newport spectra-physics GmbH, Darmstadt, Germany; heating stage: TS1500, Linkam Scientific Instruments Ltd., Surrey, UK). Transmission spectra in the infrared range were acquired with a FTIR spectrophotometer (IRAffinity, Shimadzu, Duisburg, Germany) at a resolution of 8 cm⁻¹ and 512 scans. If not otherwise stated, the optical measurements were conducted under normal laboratory atmospheric conditions (~25°C, 36% r.h.).