Moving spins with heat: spin-Seebeck effect in a ferromagnetic semiconductor and Polarization-induced pn-junctions in wide band gap semiconductor nanowires – Roberto Myers

Date: Mon. October 3rd, 2011, 12:30 pm-1:30 pm
Location: Rockefeller 221

Many proposed spin-based devices require transfer of spin into non-magnetic materials, which is usually accomplished by driving a charge current from a ferromagnet into a non-magnetic material. Heat can also be used to transfer spins into non-magnetic material using the spin-Seebeck effect, as demonstrated by Uchida et al. in permalloy[1]. We also observed this in GaMnAs [2], a ferromagnetic semiconductor. A different orientation of spin is injected into platinum bars on the hot side of the sample as compared to the cold side, and this spatial distribution of spin currents is unaffected by electrical breaks highlighting that the effect is driven by phonons in the substrate. Temperature dependent measurements reveal that the spin-Seebeck coefficient scales with the thermal conductivity showing a peak at the phonon-drag related maximum in thermopower [3]. In the second part of the talk, three-dimensional wide band gap nanowire heterostructures are discussed [4]. We discuss the formation of pn-junctions within wide band gap semiconductor nanowires without requiring the addition of impurity-dopants. Polarization-grading can lead to n or p type conduction by compositionally grading a thin film of a non-symmetric crystal such that a fixed bound polarization charge is uniformly distributed. This can raise or lower the Fermi level with free charges being provided from surface states. By linearly grading the composition of self-assembled, catalyst-free AlGaN nanowires a pn-junction is formed. These polarization-induced diodes are integrated on silicon and exhibit clear rectification persisting to low temperature, as well as room temperature ultraviolet electroluminescence. By inserting quantum wells of various compositions into the pn-junction, the ultraviolet electroluminescence is seen from 3.4 to 5 eV.
[1] K. Uchida et al., Nature 455, 778 (2008).
[2] C. M. Jaworski et al., Nature Materials 9, 898 (2010).
[3] C. M. Jaworski et al., Phys. Rev. Lett. 106 186601 (2011).
[4] S. D. Carnevale et al., Nano Lett. 11 866 (2011).