Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer

The monolithic integration of wurtzite GaN on Si via metal amp; 8722;organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single layer graphene as an atomically thin buffer layer for c axis oriented growth of vertically aligned GaN nanorods mediated by nanometer sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene covered Si 111 as well as Si 100 . High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro Raman, and cathodoluminescence measurements supported by finite difference time domain simulations. Current amp; 8722;voltage characteristics revealed high vertical conduction of the as grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN based heterojunction device concept

Position-Controlled Uniform GaAs Nanowires on Silicon using Nanoimprint Lithography

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO₂ mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As₂ or As₄) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p–n junctions. Finally, NIL positioned GaAs/AlGaAs core–shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core–shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications