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Structural and Raman characterization of III-nitride layers on CVD graphene. (a) Integrated Raman intensity maps across a section of the sample for (1) AlN epilayer peak (2) the graphene 2D band and (3) silicon substrate peak indicating the presence of the graphene layer after III-nitride growth. (b) A representative Raman spectrum taken at the point marked with a + in the intensity maps. (c) Cross-sectional TEM image of the AlN buffer region showing lattice planes with a spacing of 2.38 Å. (d) FE-SEM image showing the surface morphology of GaN deposited directly on graphene where large chunks of GaN and Ga droplets are seen. (e) FE-SEM image showing conformal coverage of GaN grown at 1040 °C on AlN buffer layers on graphene. (f) Powder x-ray diffraction profile of film in (d) showing preferential orientation.
Photoluminescence from different III-nitride MQWs on CVD graphene. (a) RT and LT PL from a 20-period GaN/Al0.5Ga0.5N multiquantum well stack grown on graphene (inset shows the cross sectional TEM image of the MQW region). (b) RT PL from a series of three Al x Ga1− x N/Al0.5Ga0.5N 20 period MQWs of varying aluminium content in the QW. (c) RT and LT PL from a 7-period GaN/InGaN MQW stack grown on graphene. (d) RT PL from two InGaN/GaN 7-period MQW stacks grown on graphene at 725 °C and 775 °C, respectively. Reducing the growth temperature of the InGaN QW enhances indium incorporation into the well and causes a red-shift of the emission wavelength.
Release and transfer process for the III-nitride layer on graphene. (a)–(e) show the schematic process steps: (a) CVD graphene is transferred on SiO2-coated Si substrate, (b) III-nitride film is grown on that substrate, (c) PMMA is spin-coated on the III-nitride film on graphene, (d) the underlying SiO2 layer is etched in a buffered HF solution, (e) the graphene-bound nitride layer floats up to the surface, image shown in part (f). After appropriate rinsing, it can be transferred to a flexible substrate like (g) a plastic (Kapton) film. (h) An image of the RT PL from the free-standing film after transfer. (i) The spectra before and after transfer shows that there is no degradation in the optical characteristics through the transfer process.
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