Showing papers by "Sudha Mokkapati published in 2013"

Optically pumped room-temperature GaAs nanowire lasers

Abstract: Near-infrared lasers are important for optical data communication, spectroscopy and medical diagnosis. Semiconductor nanowires offer the possibility of reducing the footprint of devices for three-dimensional device integration and hence are being extensively studied in the context of optoelectronic devices1, 2. Although visible and ultraviolet nanowire lasers have been demonstrated widely3, 4, 5, 6, 7, 8, 9, 10, 11, progress towards room-temperature infrared nanowire lasers has been limited because of material quality issues and Auger recombination12, 13. (Al)GaAs is an important material system for infrared lasers that is extensively used for conventional lasers. GaAs has a very large surface recombination velocity, which is a serious issue for nanowire devices because of their large surface-to-volume ratio14, 15. Here, we demonstrate room-temperature lasing in core–shell–cap GaAs/AlGaAs/GaAs nanowires by properly designing the Fabry–Perot cavity, optimizing the material quality and minimizing surface recombination. Our demonstration is a major step towards incorporating (Al)GaAs nanowire lasers into the design of nanoscale optoelectronic devices operating at near-infrared wavelengths.

Enhanced minority carrier lifetimes in GaAs/AlGaAs core-shell nanowires through shell growth optimization.

Abstract: The effects of AlGaAs shell thickness and growth time on the minority carrier lifetime in the GaAs core of GaAs/AlGaAs core–shell nanowires grown by metal–organic chemical vapor deposition are investigated. The carrier lifetime increases with increasing AlGaAs shell thickness up to a certain value as a result of reducing tunneling probability of carriers through the AlGaAs shell, beyond which the carrier lifetime reduces due to the diffusion of Ga–Al and/or impurities across the GaAs/AlGaAs heterointerface. Interdiffusion at the heterointerface is observed directly using high-angle annular dark field scanning transmission electron microscopy. We achieve room temperature minority carrier lifetimes of 1.9 ns by optimizing the shell growth with the intention of reducing the effect of interdiffusion.

III–V semiconductor nanowires for optoelectronic device applications

Abstract: III-V semiconductor nanowires are promising candidates for optoelectronic device applications due to their unique one dimensional geometry. High quantum efficiency, defined as QE=τnr/(τnr+τr), where τnr is the non-radiative lifetime and τr is the radiative lifetime of minority carriers in the nanowires is necessary for device applications. Due to the large surface area to volume ratio in the nanowires, non-radiative recombination associated with surface states often results in low quantum efficiency. The quantum efficiency of the nanowires can be increased either by increasing τnr or by reducing τr. We present experimental results on these two different approaches to increase the quantum efficiency of semiconductor nanowires.

Effect of Nanoparticle Size Distribution on the Performance of Plasmonic Thin-Film Solar Cells: Monodisperse Versus Multidisperse Arrays

Abstract: The effect of the silver nanoparticle size distribution on the performance of plasmonic polycrystalline Si thin-film solar cells is studied. Monodisperse particle arrays are fabricated using nanoimprint lithography. Multidispersed particle arrays are fabricated using thermal evaporation followed by annealing. The short-circuit current enhancement for the cells without a back reflector is 24% and 18% with the multidisperse array and the monodispersed array, respectively. For the cells with a back reflector, the current enhancement increases to 34% and 30%, respectively, compared with 13% enhancement due to the reflector alone. Better performance of multidisperse Ag nanoparticle arrays is attributed to a broader scattering cross section of the array owing to a broad particle size distribution and a higher nanoparticle coverage.

Periodic dielectric structures for light-trapping in InGaAs/GaAs quantum well solar cells.

Abstract: We study dielectric diffraction gratings for light-trapping in quantum well solar cells and compare their performance with plasmonic and Lambertian light-trapping structures. The optimum structural parameters are identified for symmetric uni-periodic, symmetric bi-periodic and asymmetric bi-periodic gratings. The enhancement in short-circuit current density from the quantum well region with respect to a reference cell with no diffraction grating is calculated. The ratio of this enhancement to the maximum achievable enhancement (i.e. no transmission losses) is 33%, 75% and 74%, respectively for these structures. The optimum asymmetric and symmetric bi-periodic structures perform closest to Lambertian light-trapping, while all three optimum grating structures outperform optimum plasmonic light-trapping. We show that the short-circuit current density from the quantum well region is further enhanced by incorporating a rear reflector.

Design considerations for semiconductor nanowire-plasmonic nanoparticle coupled systems for high quantum efficiency nanowires.

Abstract: The optimal geometries for reducing the radiative recombination lifetime and thus enhancing the quantum efficiency of III–V semiconductor nanowires by coupling them to plasmonic nanoparticles are established. The quantum efficiency enhancement factor due to coupling to plasmonic nanoparticles reduces as the initial quality of the nanowire increases. Significant quantum efficiency enhancement is observed for semiconductors only within about 15 nm from the nanoparticle. It is also identified that the modes responsible for resonant enhancement in the quantum efficiency of an emitter in the nanowire are geometric resonances of surface plasmon polariton modes supported at the nanowire/nanoparticle interface.

Nanostructure photovoltaics based on III-V compound semiconductors

Abstract: In this work, solar cells based on III-V compound semiconductor quantum dots and nanowires are demonstrated. Experimental results based on material and device studies will be presented and discussed for high efficiency photovoltaic applications.

Multi-colour emission from GaAs core-AlGaAs shell photonic nanowires

Abstract: Semiconductor nanowires grown via the vapour-liquid-solid (VLS) mechanism are promising for miniaturisation of optoelectronic devices. Efficient optoelectronic devices require these nanowires to have high quantum efficiency. While optimizing the growth process to eliminate bulk defects and achieve perfect surface passivation is one approach to increase the quantum efficiency of nanowires1, coupling the nanowires to resonant plasmonic structures to reduce the radiative lifetime of carriers in the semiconductor is an alternative approach. In this paper, we demonstrate increase in quantum efficiency of AlGaAs shell of GaAs core-AlGaAs shell-GaAs cap nanowires (Figure 1(a)) by coupling the nanowires to plasmonic nanoparticles deposited on their surface. Increase in quantum efficiency of the AlGaAs shell leads to multi-colour emission from the nanowires (from the band-edge of GaAs core and the AlGaAs shell). This approach to achieve multi-colour emission from the nanowires does not rely on the growth of quantum confined layers like quantum wells or quantum dots in the nanowires. In addition to achieving multi-colour emission, our approach also provides independent control on the polarization response of emission from the GaAs core and the AlGaAs shell of the nanowires2.