Tunable thermal conductivity in defect engineered nanowires at low temperatures

Abstract

We measure the thermal conductivity (κ) of individual InAs nanowires (NWs), and find that it is 3 orders of magnitude smaller than the bulk value in the temperature range of 10 to 50 K. We argue that the low κ arises from the strong localization of phonons in the random superlattice of twindefects oriented perpendicular to the axis of the NW. We observe significant electronic contribution arising from the surface accumulation layer which gives rise to tunability of κ with the application of electrostatic gate and magnetic field. Our devices and measurements of κ at different carrier concentrations and magnetic field without introducing structural defects, offer a means to study new aspects of nanoscale thermal transport.Thermal transport measurements on semiconducting nanowires (NWs) have attracted a lot of attention in the last few years. Measurements of thermal transport in nanostructures are important as they provide a platform to test the existing descriptions of phonons in confined structures and across complex interfaces 1 , and have the potential to result in technological applications as thermoelectric systems 1-3 . Different materials are benchmarked using the thermoelectric figure of merit ZT = S 2 T ρκ , where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity and T is the absolute temperature. ZT can be increased by appropriate engineering of nanostructures. One of the ways to increase ZT is by reducing κ without degrading its electrical conductivity and Seebeck coefficient 4 . As a result, an ideal thermoelectric material is a glass for phonons and ordered for electronic transport. Recently, several theoretical models as well as experimental studies have been carried out in different semiconductor NWs like Si, Ge, Bi 2 Te 3 etc.5-9 . It is found that for Si NWs, the value of κ is reduced by two orders of magnitude 5,8,9 compared to bulk values by tuning the roughness of the surface. III-V semiconductors are also known to be good thermoelectric materials, and theoretical studies suggest that InSb and InAs NWs are good candidates for better ZT 10 . InAs NWs have been studied extensively to probe their charge and spin-transport 11-13 . An aspect of InAs NWs that makes studying their thermal transport, hitherto little explored 14,15 , of interest is the ability to tune the density of twin defects and polytypes along its length by varying growth parameters [16][17][18][19][20] . Exploiting this control over crystal structure can help synthesize defect-engineered NWs, whose lattice has aperiodic array of twins along its length that modify phonon behavior, without significantly compromising their electrical properties 20 . Such NWs satisfy the key criteria for a good thermoelectric material -localization of phonons without localizing electrons.In this work we explore the κ of suspended InAs NW field effect transistors (FETs); the NWs have high density of aperiodic twins and polytypes perpendicular to their axis [16][17][18]20 . The random nature of defects suppress...

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