Recent Advances in Semiconductor Nanowire Heterostructures

We review a variety of recent optical experiments which provide unique characterizations of the energy landscape in semiconductor nanowires, both simple nanowires and more complex semiconductor heterostructures.  The optical experiments include the techniques of photoluminescence, time-resolved photoluminescence, photoluminescence excitation spectroscopy, photocurrent spectroscopy, and Raman scattering brought to bear on structures from simple core-shell structures, to axial heterostructures, to core-multiple-shell heterostructures.  The physics that is illuminated ranges from localization of excitons in GaAs quantum well tubes, to strain in GaAs/GaP axial heterostructures,  to carrier relaxation dynamics in both ZB and WZ InP probed by the new technique of transient Rayleigh scattering.  We briefly explore these several examples in what follows.

First we discuss a core-multi-shell structure consisting of a GaAs quantum well bounded by two AlGaAs shells all wrapped around a [111] oriented GaAs nanowire.  Recently our group (Fickenscher et al. Nano Letters 2013, 1016-1022)  has obtained photoluminescence (PL) and photoluminescence excitation (PLE) data which indicate these materials are strong emitters and that the quantum well exhibits strong quantum confinement.  We have modeled these strain-free quantum well tubes (QWT) and find that the electron and hole ground states are strongly confined to the corners of the hexagonally symmetric quantum well. Thus this confinement defines truly quantum wires which run along the length of the nanowires along its corners.  Single nanowire PL and PLE measurements show that for quantum well tube widths larger than 5 nm excited states are observed, but for smaller widths only the ground state is observed.  For a 4 nm QWT sample, we observe narrow emission lines on the high energy side of the luminescence band which suggest highly localized states similar to that observed for quantum dots (see Figs. below).  

The use of semiconductor nanowires for both basic and applied studies takes advantage of the unique opportunities they provide for growing heterostructures even with materials that have quite different lattice constants.  Both radial heterostructures and axial heterostructures have been explored.  We have used Raman scattering to study the spatially-resolved strain and stress in a GaAs/GaP axial heterostructure nanowire.  These two materials have widely different (3.6%) lattice constants.  To characterize the stress or strain of this nanowire heterostructure, we have used high spatial resolution Raman scans along the nanowires which show the GaAs/GaP interface is clearly identifiable.  We interpret the phonon energy shifts in each material as one approaches the interface.  Near the interface we observe both a GaAs TO phonon as well as a GaP TO phonon.  The GaAs phonon is clearly shifted to higher energies due to compression from the GaP which has a smaller lattice constant.  A strain gradient exists across the interface so the GaP phonon energies shift to lower and GaAs phonon shift to higher energies as one approaches the interface from the GaP section of the nanowire.  We observe that the strain does not return to the unstrained value even microns away from the interface.  We note that the stresses reflected in both the GaP and GaAs Raman spectra are of a magnitude which implies that the electronic energy landscape of such structures are significantly modified.

As a third example we briefly describe a promising new technique (Montazeri et al. Nano Letters 2012, 5389-5395), transient Rayleigh scattering spectroscopy [TRS], which enables us to probe the carrier dynamics of single semiconductor nanowires on the picosecond scale.  A 200 fs pump photoexcites a high density of carriers in the nanowire and a delayed probe monitors the change in reflectivity.  The photoexcited carriers modify the real and imaginary parts of the complex index of refraction through band-filling, band gap renormalization, and plasma screening – and thus the measured reflectivity response.  A model calculation enables one to extract the density of electrons and holes and the temperature of the electron-hole plasma as a function of time.  We first explored carrier dynamics in a simple core-shell GaAs/AlGaAs structure.  Very recently we have used TRS spectroscopy to investigate both zincblende and wurtzite InP nanowires.  We see distinctively different behaviors including the A, B, and C excitons in the wurtzite phase, as well as distinctively different time-dependent responses.