The band gap tunability of Ill-nitrides, added with their novel size dependent properties in the nano-regime, offers great potential for important optoelectronic applications. The motivation behind our GaN nanostructure studies and also the current status of research will be discussed. I shall present our recent results of the MBE growth and multi-technique characterization of GaN thin films on single crystal sapphire and silicon surfaces. Using a high-nitrogen rich condition in a PA-MBE growth, we dissuade adatoms from long range diffusion to promote super-saturation and GaN nucleation at the edge and screw dislocations, formed due to lattice mismatch, between GaN and Al2O3 (0001), Si(111) and (100). I shall show that this kinetically control growth results in the spontaneous formation of oriented, single crystal nanostructures such as nanowall network and nanocolumn forests, due to patterned strain relaxation pathways. The nanostructures show broad band edge emission but no defect emission. The nanowall network formed around voids which are open screw dislocations, demonstrate exotic properties. Cathodoluminiscence mapping shows coherent emission of the band edge emission confined to regions inside the voids between nanowalls. Adsorption of Ag nanoparticles on the nanowall network is seen to strongly influence emission properties that manifest a coupling of the band edge emission with the Ag surface plasmons. Depending on the size and distributions of the Ag nanoparticles (see Fig), we show that we can enhance and tune Photoluminescence emission. The nanowalls also show magnetism at their narrow apex and other interesting transport properties. The nanowall network formed at the edge dislocations promote 1 -D nanocolumn formations in the void between walls in a Frank type spiral growth of m-faceted GaN. These highly dense m-faceted nanocolumns are hexagonally shaped wurtzite, c-oriented structures and are defect controlled. We show that depending on the kinetics of the growth, we can control the twist parameters of the Burger's vector leading to compact and defected rods, Frank-Read growth and nanotubes formation. By FFT image analysis we compute nanocolumn size, shape density and supra-structural arrangement, and used XRD and PL to their structural and optical emission properties.
Jawaharlal Nehru Centre for Advanced Scientific Research
Indo-US Science & Technology Forum
NSF - Office of International Science and Engineering
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MGRN 121, Purdue University, West Lafayette, IN