Emergence of localized states in narrow GaAs/AlGaAs nanowire quantum well tubes

We use low-temperature photoluminescence, photoluminescenceexcitation, and photoluminescence imaging spectroscopyto explore the optical and electronic properties of GaAs/AlGaAsquantum well tube (QWT) heterostructured nanowires (NWs). Wefind that GaAs QWTs with widths >5 nm have electronic states whichare delocalized and continuous along the length of the NW. As theNW QWT width decreases from 5 to 1.5 nm, only a single electronstate is bound to the well, and no optical excitations to a confinedexcited state are present. Simultaneously, narrow emission lines(fwhm <600 ueV) appear which are localized to single spatial pointsalong the length of the NW. We find that these quantum-dot-likestates broaden at higher temperatures and quench at temperaturesabove 80 K. The lifetimes of these localized states are observed to varyfrom dot to dot from 160 to 400 ps. The presence of delocalized states and then localized states as the QWTs become moreconfined suggests both opportunities and challenges for possible incorporation into quantum-confined device structures.