III-Nitride Nanowires: Novel Emitters for Lighting
Speaker: George Wang, EFRC Thrust Leader
Date: September 14, 2011
Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials
Abstract: 1D nanostructures, such as nanowires and nanorods, based on the III nitride (AlGaInN) materials system have attracted attention as potential nanoscale building blocks in LEDs, lasers, sensors, photovoltaics, and high power and high speed electronics. Compared to planar films, III-nitride semiconductor nanowires have several potential advantages including higher crystalline quality and reduced strain, which enables growth on arbitrary substrates as well as allowing for a greater range of alloy compositions and hence energies to be achieved. However, before their promise can be fully realized, several challenges must be addressed in the areas of 1) controlled nanowire synthesis; 2) understanding and controlling the nanowire structural, electrical, thermal, and optical properties; and 3) nanowire device integration. Our work seeks to address these areas to lay the scientific and technological foundation for nanowire-based lighting and other energy-related applications. III-nitride nanowires can be fabricated by a variety of techniques, including “bottom-up” approaches and “top-down” lithographic approaches. Bottom-up techniques have been the dominant method and typically involve a nanoscale metal catalyst particle to direct the 1D growth or anisotropic growth conditions. Advantages of using this approach include nanowires free of detrimental crystal defects known as dislocations, and the ability to grow on inexpensive, lattice mismatched substrates, including glass and metal foil, which we have demonstrated in our lab. I will discuss recent results involving the aligned, bottom-up growth of Ni-catalyzed GaN and III-nitride core-shell nanowires, along with extensive results providing insights into the nanowire properties obtained using cutting-edge structural, electrical, and optical nanocharacterization techniques. Some topics I will cover include: in-situ TEM studies of nanowire electrical breakdown and nanomechanics, spatially-resolved cathodoluminescence studies of band-edge and defect luminescence in NWs; and strain-related spatial variation of In incorporation in InGaN shells. I will also discuss the development of an inexpensive, lithography-free technique employing nanowire templates for the growth of high-quality GaN, which could enable more efficient and longer-lifetime visible LEDs. Bottom-up nanowire growth methods do however have the disadvantage of requiring highly specific growth conditions to increase the on-axis growth rate while minimizing lateral growth, which can lead to non-optimal material quality and less flexibility in material design, such as doping and heterostructures. I will describe a new “top-down” approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter. This top-down method allows fabrication of nanorods from high quality, arbitrarily doped films grown by metal-organic chemical vapor deposition using standard, optimized conditions. The fabrication, structure, optical properties, lasing characteristics, and device performance of the nanorods and nanorod LEDs will be discussed.