Semiconductor nanowire/graphene hybrids for high-efficiency light emitting diodes

The goal of M-era Net NAGRALED consortium was to develop high-efficiency light emitting diodes (LEDs) using III-nitride semiconductor nanowires grown on graphene. The consortium was composed of three universities (NTNU (Norway), Konkuk/Ewha Womans University (Korea) and Sejong University (Korea)), two companies (CrayoNano (Norway) and Itswell (Korea)) and one research institute (ETRI (Korea)) in Norway and South Korea. Here we summarize the results achieved in the NAGRALED project by the Norwegian partners, NTNU and CrayoNano. NTNU's main task in the project was the growth and characterization of III-nitride nanowires on graphene using molecular beam epitaxy (MBE) as well as on growth of graphene using chemical vapor deposition (CVD). CrayoNano's main task in the project was to work on processing of the CVD graphene, growth of III-nitride nanowires on graphene using metal-organic CVD (MOCVD) as well on LED device fabrication and device characterization. In the beginning of the project we succeeded to grow graphene using CVD method and Pt as catalyst. Graphene with high quality was grown on Pt films as thin as 25 nm and the graphene transfer process was established. Later in the project we also used commercial Cu-catalyzed CVD graphene. Further, we succeeded with the fabrication of hole-patterned masks on the different types of graphene using electron beam lithography (EBL) in NTNU NanoLab. By using this EBL, a working process was developed to fabricate hole arrays with hole diameter as small as 100 nm and pitch of 400 nm in a highly reproducible way. High-density self-assembled AlGaN/GaN nanowire heterostructures were grown by MBE on transferred graphene on amorphous silica glass. Transmission electron microscopy investigations were carried out and the nanowires were found to exhibit a high crystal quality with no observable defects or stacking faults. Further, for visible LEDs, InGaN quantum wells were grown between the bottom n-GaN and top p-GaN segments. The nanowires emitted light near 500 nm from the InGaN quantum wells well suited for visible LED display applications. CrayoNano achieved selective area growth of AlGaN nanopyramids on hole mask patterned single-layer graphene using MOCVD with intense room temperature emission at 365 nm. CrayoNano developed various III-nitride LED prototypes based on both random and positioned growth of nanowires on graphene. We successfully developed the necessary post-processing steps of various top-emitting (visible) and flip-chip (UV) LEDs. The LEDs were planarized using different filling materials with the bottom and top metal contact electrodes deposited on the graphene substrate and the top of nanowires, respectively. NTNU performed detailed current-voltage (I-V) characteristic measurements and electroluminescence measurements from the processed LEDs. NTNU made working flip-chip UV LEDs based on random AlGaN nanowires where graphene was used as both a growth substrate and transparent electrode. Electroluminescence emission was achieved during continuous biasing at a wavelength of 365 nm with no defect-related yellow emission. Based on temperature dependent photoluminescence an internal quantum efficiency of ~46 % was estimated. These results indicate that graphene can be used as a functional substrate and electrode for III-nitride-based device technology. In general, however the performance of these first proof-of-principle III-nitride nanowire/graphene LEDs had low performance and there are still several challenges related to both the graphene quality as well as to the growth and processing of the III-nitride nanowire/graphene LEDs. These challenges need to be solved in future research development before the LEDs can be used in applications.

Results from the NAGRALED project will be beneficial for future research studies in academia as well as for development work in industry to develop future NW/graphene hybrid device systems. Considering the potential of the superior properties in light-efficiency and added functionalities of these devices, new high-volume market areas with large economic benefits can be anticipated. However, further large investments will be needed in both academia, to study their basic properties, as well as in industry to develop and test prototype devices in pilot and full-scale production. The results of the developed III-nitride NW/graphene LEDs in NAGRALED will be beneficial for CrayoNano to open up new applications fields, in particular for future micro LEDs for displays, enabling them to expand their present business activities from UVC LEDs, where they are pioneering this technology for use in e.g. water disinfection.

Since 2010 we have been working on the epitaxial growth of III-V semiconductors on graphene by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) methods, which are two major techniques in commercial III-V semiconductor growth. The current thin film LED technology is relatively well-matured. Light emission efficiency is regarded to be almost in saturation, thus cost efficiency with smart production has become more important factor in commercialization. Our novel III-V semiconductor nanowire (NW) array/graphene hybrid material concept can be a key revolutionizing the present solid-state lighting technology due to several important structural and cost-effective advantages over conventional planar film devices grown on single crystal substrate. In contrast to 2D film growth on thick solid substrates, graphene is one atom layered substrate and the NW growth only needs a low consumption of III-V semiconductor materials. The small size of NW and its small footprint easily release the lattice strain caused by lattice mismatch, thus making it possible to grow them in high crystal quality on graphene. In addition the NWs provide a structural versatility of designing 3D heterostructured architectures with compositional modulation along the axial or radial direction. The higher surface-to-volume ratio of NW structure can increase the light emission efficiency with enlarged active layer area than thin film in an equivalent dice area and the light extraction efficiency as a waveguide, all of which will contribute to energy saving. The NAGRALED consortium will be composed of three universities, two companies and one research institute in Norway and South Korea. The whole process from the first preparation of graphene substrates to the last standardized LED evaluation of devices will be developed and optimized for the realization of high-efficiency InGaN NW array/graphene LEDs.