III-V nanowire/graphene hybrid structures and devices

In the last year we have developed CVD growth of graphene on Pt as an alternative to Cu, made studies of nanowire/graphene contacts, as well as made electrical noise studies of GaAsSb nanowire Schottky diodes. We have investigated the growth of graphene on various types of Pt substrates using low-pressure thermal CVD. It was found that high temperature sputtering is crucial for successful growth of single layer graphene on Pt thin films, which is attributed to the highly (111)-oriented crystallized structure and high stability of the Pt films without significant dewetting during graphene growth. This enabled us to grow single layer graphene on Pt films down to 25 nm thickness. In contrast to this, e-beam deposited Pt films show large dewetting of the Pt due to its weak adhesion to the SiO2/Si substrates resulting in the growth of defective graphene. Graphene growth on polycrystalline Pt foil results in single layer graphene decorated with micro-sized multilayer graphene islands, which were not observed in the graphene grown on high temperature sputtered Pt films. This suggests that C segregations and precipitations from the Pt bulk can be minimized leading to the growth of homogeneous single layer graphene on the thin Pt films. Raman and EBSD mappings on as-grown graphene show a shift of the Raman G and 2D bands correlated with the surface facet orientations of the Pt substrates. We have also developed a process of single GaAs nanowire/graphene hybrid devices with a planar junction configuration. A position-controlled micro transfer and imprinting technique that enables us to choose and transfer a single nanowire (or graphene) selectively on a target graphene (or a single nanowire) for high quality junction was developed. Both GaAs nanowire/graphene bottom- and top-contact devices have been made. The opto-electrical measurement results of the fabricated single GaAs nanowire/graphene hybrid devices were also studied. The electrical properties of a GaAsSb nanowire Schottky diode have been investigated by using static and low-frequency noise analysis. The GaAsSb nanowire device exhibited 1/f dependency in the noise spectrum. These results show that our GaAsSb nanowire Schottky diode has decent quality and interface trap density, compared with other nanowire based devices. In addition, through the low frequency measurements, we also found that the GaAsSb nanowire Schottky diode has a NEP of sub-pW levels. Our results can serve as an indicator of material quality, interface property, and photo-sensitivity of GaAsSb nanowire based devices.

Using epitaxial growth methods, novel three dimensional axial and/or radial heterostructured III-V nanowires (NWs) can be fabricated with a huge degree of freedom in band structure engineering through composition, crystal phase and strain control. This ma kes III-V semiconductor NWs one of the most promising materials for future nano-optoelectronic device applications. Graphene (GP), a newly discovered nanomaterial, on the other hand, has many superior characteristics that are believed to revolutionize fut ure nano-electronic systems. The topic in this project is to combine heterostructured III-V NWs, grown by metal-catalyzed molecular beam epitaxy, with GP into a new hybrid material structure. By combining such materials into a hybrid structure, one has t he potential to go beyond what is possible with any of them alone. In order to realize such hybrid systems, we will attempt to grow epitaxial III-V NWs directly on GP, and study the fundamental physical properties of III-V NW/GP contacts. Detailed studies of NW growth on GP, with various preparations of GP, will be made in order to get a fundamental insight to the general nucleation and growth mechanism needed to achieve epitaxial NWs on GP. The electro-optical properties of various types of GP/NW contact s will be investigated at the nano-scale using micro photoluminescence and scanning near field optical microscopy. The optical transparency of GP will enable us to visualize the electro-optics properties of single NW/GP contacts. The structural properties will be investigated by (cross-sectional) high resolution transmission electron microscopy and related techniques. Apart from the fundamental scientific interest, the results from this study will have a great interest from an application point of view, i n order to realize future III-V NW/GP hybrid device systems.