Graphene is traditionally prepared by exfoliation of graphite, and to fabricate large area single layer graphene is challenging. Chemical vapor deposition (CVD) of graphene on transition metals is recently developed for this purpose. Since 2009, we in Chalmers have grown single layer graphene on metal foils (Cu, Pt, Ta, etc.), or evaporated metallic thin films on silicon substrate. [1-9] The graphene is grown in a cold-wall commercial Aixtron system with CH4 or C2H2 precursors. The graphene can be transferred onto other substrates such as SiO2/Si by wet chemically etching away the copper catalyst or, more environmentally friendly, by electrochemical bubbling delamination.[10] The carrier mobility for electrons and holes is about 3000 cm2/(Vs), measured through both the field effect and the Hall effect. Some devices show mobilities ~5000 cm2/(Vs). 

We also grow graphene directly on insulators without metal catalysts in CVD.[1-9] The as-deposited graphene is nanocrystalline, large area and uniform. Despite the lower mobility (40 cm2/(Vs)) compared to catalyzed graphene, its transparency (97%) and conductivity (1-a few kΩ/□ without intentional doping) is similar to standard graphene, making such transfer-free graphene very promising in applications of transparent electronics and molecular electronics. The graphene can be grown on arbitrary dielectrics that withstand high temperature. We have proposed a novel noncatalytic CVD (as opposed to catalytic graphene CVD on metals) mechanism to explain our experimental findings. [3] Both the catalyzed and noncatalyzed graphene can be suspended, which is promising for nanoelectromechanical systems (NEMS).[7,8]

The CVD graphene finds its applications in GaN based optoelectronics. GaN compounds are widely used in light emitting diodes (LEDs) covering the spectrum from yellow to ultraviolet. We first study ordered and dense GaN light emitting nanorods with graphene grown by CVD as suspended transparent electrodes. [11] As the substitute of indium tin oxide (ITO), the graphene avoids complex processing to fill up the gaps between nanorods and subsequent surface flattening and offers high conductivity to improve the carrier injection. The as fabricated devices have 32% improvement in light output power compared to conventional planar GaN-graphene diodes, mainly due to the much more enlarged light emitting areas. [11] 

Nevertheless, although the graphene is well conducting and transparent, due to its Fermi level mismatch with the GaN, their electrical contact is not ohmic, leading to an unacceptably high work voltage of the device for real applications. To understand this issue further, CVD graphene is used in (planar) GaN LEDs as transparent electrodes, where 7–10 nm ITO contact layer is inserted between the graphene and p-GaN to enhance hole injection. [12] Devices with forward voltage and transparency comparable to those using traditional 240 nm ITO are achieved with better ultraviolet performances. [12] This result indicates that the large turn-on voltage can be indeed attributed to the poor graphene-GaN contact, which can be solved by the thin ITO interlayer.

However, the ITO interlayer is not a sustainable solution due to the scarcity of indium resources. Therefore, we suggest depositing graphene directly on GaN by CVD. The graphene-GaN interface is produced in high temperature and high vacuum CVD chamber, resulting in improved electrical properties. Furthermore, in situ doping of graphene can be carried out which will tune the Fermi level further to match that of p-GaN. Also, it is a reproducible and scalable technique, getting rid of all the uncertainty and irreproducibility associated with the complex transfer process of CVD graphene. Some preliminary results will be presented regarding the direct growth method, [13] indicating its promising future in real industrial applications of GaN optoelectronics.

Graphene is traditionally prepared by exfoliation of graphite, and to fabricate large area single layer graphene is challenging. Chemical vapor deposition (CVD) of graphene on transition metals is recently developed for this purpose. Since 2009, we in Chalmers have grown single layer graphene on metal foils (Cu, Pt, Ta, etc.), or evaporated metallic thin films on silicon substrate. [1-9] The graphene is grown in a cold-wall commercial Aixtron system with CH4 or C2H2 precursors. The graphene can be transferred onto other substrates such as SiO2/Si by wet chemically etching away the copper catalyst or, more environmentally friendly, by electrochemical bubbling delamination.[10] The carrier mobility for electrons and holes is about 3000 cm2/(Vs), measured through both the field effect and the Hall effect. Some devices show mobilities ~5000 cm2/(Vs). 

We also grow graphene directly on insulators without metal catalysts in CVD.[1-9] The as-deposited graphene is nanocrystalline, large area and uniform. Despite the lower mobility (40 cm2/(Vs)) compared to catalyzed graphene, its transparency (97%) and conductivity (1-a few kΩ/□ without intentional doping) is similar to standard graphene, making such transfer-free graphene very promising in applications of transparent electronics and molecular electronics. The graphene can be grown on arbitrary dielectrics that withstand high temperature. We have proposed a novel noncatalytic CVD (as opposed to catalytic graphene CVD on metals) mechanism to explain our experimental findings. [3] Both the catalyzed and noncatalyzed graphene can be suspended, which is promising for nanoelectromechanical systems (NEMS).[7,8]

The CVD graphene finds its applications in GaN based optoelectronics. GaN compounds are widely used in light emitting diodes (LEDs) covering the spectrum from yellow to ultraviolet. We first study ordered and dense GaN light emitting nanorods with graphene grown by CVD as suspended transparent electrodes. [11] As the substitute of indium tin oxide (ITO), the graphene avoids complex processing to fill up the gaps between nanorods and subsequent surface flattening and offers high conductivity to improve the carrier injection. The as fabricated devices have 32% improvement in light output power compared to conventional planar GaN-graphene diodes, mainly due to the much more enlarged light emitting areas. [11] 

Nevertheless, although the graphene is well conducting and transparent, due to its Fermi level mismatch with the GaN, their electrical contact is not ohmic, leading to an unacceptably high work voltage of the device for real applications. To understand this issue further, CVD graphene is used in (planar) GaN LEDs as transparent electrodes, where 7–10 nm ITO contact layer is inserted between the graphene and p-GaN to enhance hole injection. [12] Devices with forward voltage and transparency comparable to those using traditional 240 nm ITO are achieved with better ultraviolet performances. [12] This result indicates that the large turn-on voltage can be indeed attributed to the poor graphene-GaN contact, which can be solved by the thin ITO interlayer.

However, the ITO interlayer is not a sustainable solution due to the scarcity of indium resources. Therefore, we suggest depositing graphene directly on GaN by CVD. The graphene-GaN interface is produced in high temperature and high vacuum CVD chamber, resulting in improved electrical properties. Furthermore, in situ doping of graphene can be carried out which will tune the Fermi level further to match that of p-GaN. Also, it is a reproducible and scalable technique, getting rid of all the uncertainty and irreproducibility associated with the complex transfer process of CVD graphene. Some preliminary results will be presented regarding the direct growth method, [13] indicating its promising future in real industrial applications of GaN optoelectronics.