title: Mixed polarity in polarization-induced p-n junction nanowire light-emitting diodes.
abstract: Polarization-induced nanowire light emitting diodes (PINLEDs) are fabricated by grading the Al composition along the c-direction of AlGaN nanowires grown on Si substrates by plasma-assisted molecular beam epitaxy (PAMBE). Polarization-induced charge develops with a sign that depends on the direction of the Al composition gradient with respect to the [0001] direction. By grading from GaN to AlN then back to GaN, a polarization-induced p-n junction is formed. The orientation of the p-type and n-type sections depends on the material polarity of the nanowire (i.e., Ga-face or N-face). Ga-face material results in an n-type base and a p-type top, while N-face results in the opposite. The present work examines the polarity of catalyst-free nanowires using multiple methods: scanning transmission electron microscopy (STEM), selective etching, conductive atomic force microscopy (C-AFM), and electroluminescence (EL) spectroscopy. Selective etching and STEM measurements taken in annular bright field (ABF) mode demonstrate that the preferred orientation for catalyst-free nanowires grown by PAMBE is N-face, with roughly 10% showing Ga-face orientation. C-AFM and EL spectroscopy allow electrical and optical differentiation of the material polarity in PINLEDs since the forward bias direction depends on the p-n junction orientation and therefore on nanowire polarity. Specifically, C-AFM reveals that the direction of forward bias for individual nanowire LEDs changes with the polarity, as expected, due to reversal of the sign of the polarization-induced charge. Electroluminescence measurements of mixed polarity PINLEDs wired in parallel show ambipolar emission due to the mixture of p-n and n-p oriented PINLEDs. These results show that, if catalyst-free III-nitride nanowires are to be used to form polarization-doped heterostructures, then it is imperative to understand their mixed polarity and to design devices using these nanowires accordingly.
authors.affiliation: Department of Materials Science and Engineering, ‡Department of Electrical and Computer Engineering, and §Department of Physics, Ohio State University, Columbus, Ohio 43210, United States.
authors.forename: Santino D
authors.initials: SD
authors.lastname: Carnevale
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Kent
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Phillips
authors.forename: A T M G
authors.initials: AT
authors.lastname: Sarwar
authors.forename: Camelia
authors.initials: C
authors.lastname: Selcu
authors.forename: Robert F
authors.initials: RF
authors.lastname: Klie
authors.forename: Roberto C
authors.initials: RC
authors.lastname: Myers