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UU2.08 - Modulation Doped Ge-SiGe Core-Shell Nanowires 
Date/Time:
April 22, 2014   4:30pm - 4:45pm
 
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The characterization of core-shell nanowire (NW) heterostructures has proven challenging by comparison to planar heterostructures due to their reduced dimensions. Consequently, fundamental core-shell heterostructure properties such as elastic strain and energy band offsets, as well as the strain-band structure interplay remain largely unexplored experimentally. In this work, we demonstrate radial modulation doping in coherently strained Ge-SiGe core-shell NWs along with a novel technique to directly measure the valence band offset (Δ) in this heterostructure. We grow modulation doped Ge-SiGe core-shell NWs through a combination of vapor-liquid-solid (VLS) and chemical-vapor deposition (CVD) growth processes. A thin boron modulation doped region is incorporated into the SiGe shell during CVD growth by the addition of BH source gas for a fixed period of time. We then fabricate Ω-gate NW field-effect transistors (FETs) using an AlO top dielectric and highly doped source/drain contacts with Ni electrodes. Electrical measurements show an enhancement in peak hole mobility at low-temperature in the modulation doped devices, along with a clear kink in the drain current () versus gate voltage () characteristics. Finite-element calculations indicate that the kink observed in the - data is a result of the shell becoming populated with holes at large gate overdrive voltage and stems from a decrease in carrier mobility due to charged impurity scattering in the shell. Through a comparison of transport data to finite-element calculations, we extract Δ in Ge-SiGe core-shell NW heterostructures, and its dependence on the shell’s Si composition ().
 


 
 
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