(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

We report novel luminescent materials based on group III-V compound semiconductor nanowires.  Semiconductor nanowires are essentially one-dimensional rods with length of several microns and diameter below 100 nm.  Hence, nanowires exhibit interesting quantum confinement and carrier transport properties.  Nanowires are grown using metal seed particles by the vapor-liquid-solid (VLS) process in a molecular beam epitaxy or metalorganic chemical vapor deposition system.  By varying the material deposition during growth, axial or radial nanowire heterostructures and p-n junctions may be formed for various device applications including light emitting diodes, lasers, and photodetectors.  Due to the large surface area to volume ratio of a nanowire, lattice mismatch strain may be accommodated by elastic distortion of the nanowire without detrimental misfit dislocations, which gives a much greater ability to perform bandgap engineering in nanowires as compared to thin films.  Hence, unique heterostructures are possible in nanowires that would be impossible in thin films, opening up new device applications and possibilities in condensed matter physics. 

We will report our recent work on the photoluminescence properties of InAsP/InP nanowires.  InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system.   InAs_yP_1-y segments were grown in the middle of the InP nanowires, creating a multiple quantum dot structure or superlattice.  The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). Photoluminescence (PL) from the quantum dot structure could be tuned by the InAs_yP_1-y composition (y), or by the size of the quantum dot via the quantum confinement effect.  Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots.  To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.

The growth mechanism of the quantum dots were inferred from the InAsP and InP segment lengths as a function of nanowire diameter.  Short InAsP segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAsP and InP grew by diffusive transport of adatoms from the nanowire sidewalls.  The present study offers a manner to engineer the lengths of InAsP quantum dots embedded in InP barriers to better control the PL or CL emission.   A novel group III and V gas switching sequence is presented to improve compositional control of the QD.