The Condensed Matter & Surface Sciences Colloquium Series presents Philipp Ebert of Forschungszentrum Jülich GmbH (Germany) on “Band Offsets in III-V Nanowires Determined by Scanning Tunneling Spectroscopy”, on Thursday, Feb. 2, at 4:10 p.m. in Walter 245.
Philipp Ebert
Abstract: Compound semiconductor nanowires (NWs) are promising building blocks for novel (opto)electronic and energy harvesting devices. Their efficiency depend critically on interfaces between different structural or compositional segments within III-V semiconductor NWs. Scanning tunneling spectroscopy (STS) is an ideal tool for the determination of band gaps and band offsets at interfaces, but thus far is mostly wrongly interpreted in literature: The commonly high step density at the sidewalls of III-V semiconductor NWs leads to extrinsic surface states pinning the Fermi energy within the fundamental band gap. Since the pinning level is different on every polytype and material composition, the relative band edge positions between different types of NW segments are extrinsically determined, but not by intrinsic band offsets. The extrinsic band offsets are much larger and likely have a stronger influence on the carrier transport than intrinsic band offsets. This problem is intrinsic to any surface sensitive characterization technique. Thus, we developed a new methodology to accurately determine band offsets between different NW segments by using a thin overgrown shell of a material with a wider band gap which assures that the Fermi level pinning of both segments is identical. Tunneling through the thin shell probes the band edge position of the underlying core materials under defined pinning conditions, providing the intrinsic band offsets. We illustrate the physical effects on interfaces of wurtzite/zincblende GaAs and GaAsSb/GaAs NWs.
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