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(a) Scanning electron micrograph of a nanowire device with labels indicating heat conduction and generation. Metal-nanowire contacts are circled. (b) Temperature profiles obtained by applying the electrothermal analysis to a ZnO nanowire structure with metal contacts. The parameters used to generate the profiles are k = 20 W/m-K, r = 80 nm, Lw = 10 μm, ρc = 5 × 10−3 Ω-cm2, g = 1.2 W/m-K, and I = 400 nA. The resistivity range is representative of ZnO nanowires measured by other researchers.26
Scanning electron microscope image of ZnO nanowire device (a) before and (b) and (c) after measurement. There is pitting of the contact where the effective electrode width and area of contact with the nanowire decrease, and the metal electrode melts. The rightmost electrode shown in (a) was not used for measurements presented here.
Peak contact temperature changes with electrode width. The minimum width the ZnO nanowire device can tolerate is 720 nm; further electrode narrowing causes the contact to reach the electrode melting point.
Ratio, θ, of the heat generation rate in the nanowire to the heat generation at the contacts varies with electrical contact resistivity of the metal-nanowire contact and indicates the likely device failure point. The figure shows the variation in θ for a ZnO nanowire device operating at 1 μA for which the Peltier coefficient is evaluated at the contact temperature determined from the electrothermal model. The parameters used are Π = 0.17 V, r = 66 nm, Lw = 7 μm, ρw = 1 × 10−2 Ω-cm.
The ratio of nanowire-internal to contact heat generation plotted as a function of reported contact resistivity for Si (▪), GaN (♦), and ZnO (•) nanowire devices from various studies (Si,31,38–42 GaN,33,43–46 ZnO2,47–49). Nanowire material type is designated by marker type. Data from each source are differentiated by color as indicated in the references. Data from this work are circled.
ZnO nanowire parameters determined here from 4- and 2-point measurements.
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