Enhanced thermopower of GaN nanowires with transitional metal impurities
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Enhanced thermopower of GaN nanowires with transitional metal impurities G. A. Nemnes1, Camelia Visan2, T. L. Mitran1, Adela Nicolaev1, L. Ion1 and S. Antohe1 1 University of Bucharest, Faculty of Physics, ‘‘Materials and Devices for Electronics and Optoelectronics’’ Research Center, P.O. Box MG-11, 077125 Magurele-Ilfov, Romania 2 ‘‘Horia Hulubei’’ National Institute for Physics and Nuclear Engineering (IFIN-HH), 077126 Magurele-Ilfov, Romania
ABSTRACT The thermopower properties of GaN nanowires with transitional metal impurities are investigated in the framework of constrained spin density functional theory (DFT) calculations. The nanowires are connected to nanoscopic Al[111] electrodes, which ensure a natural coupling to the wurtzite structure of the nanowires. We investigate the thermoelectric properties comparatively for the pristine GaN nanowire and the system with one Mn adatom. Our study points out the predicted qualitative behavior for systems with a peak in the total transmission, as well as the sign change in the thermopower. For the system with the magnetic impurity we find an enhanced conductance, thermopower and figure of merit. The detectable spin current polarization suggests the device structure may be also used in low temperature sensing applications. INTRODUCTION The ongoing efforts for producing efficient thermoelectric devices currently include the investigation of nanowires [1] and nanotubes [2] or even atomic chains [3] and single molecules [4] contacted to nano- or bulk electrodes. Besides generating thermoelectricity, nanostructures of this type constitute the main building blocks of high performance temperature sensors and cooling devices. The focus on atomic sized thermoelectric devices is not only supported by the benefits of scaling, but also by the enhanced thermopower (Seebeck coefficient), which arises from typically sharp variations of the device conductance. According to the Cuttler-Mott formula [5-7], ,
(1)
the Seebeck coefficient, , depends on the derivative of the logarithmic conductance, , with respect to the chemical potential, . Rapid variations of the conductance were found in nanowire systems with attractive potential impurities [8]. The transmission function presents a series of sharp dips in front of each plateau, which gives a relatively high thermopower. Depending on the position of the chemical potential, by raising the temperature it is also possible to obtain a change in sign for the Seebeck coefficient. The thermopower of atomic sized wurtzite AlN nanowires, with Al[111] bulk contacts, has been investigated recently [9]. The group-III nitrides are large bandgap semiconductors.
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It is worth mentioning that the atomic sized device with a transitional metal impurity produces a spin polarized current. It follows that the applied temperature difference can be measured from the polarization of the spin current, which suggests the possibility to consider applications of spintronic devices for high sensitivity temperature sensors. CONCLUSIONS Therm
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