A fundamental reason for the usefulness of semiconductors in electronics is the possibility of controlling the conductivity and carrier type by adding impurities. In this way, a semiconductor can be doped to conduct with electrons or holes, enabling p-n junctions and transistors to be formed. Semiconductor nanowires are expected to be important components in future nano-electronics devices [1, 2]. However, it is not yet fully understood how dopants are incorporated into the nanowire using metal particle-assisted growth of nanowires. In addition, many growth techniques use carbon-containing precursors that may introduce unintentional carbon impurities into the nanowire.
Here we present an atomic scale investigation of structural and electronic properties of III-V heterostructure nanowires grown in the <111>B direction on GaAs(001) using cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (XSTS). Axial heterostructures, where GaAs, InGaAs and AlGaAs were varied in the growth direction as well as radial heterostructures that were formed around the nanowire, were grown by MOVPE. AlGaAs, a wider bandgap material was used to cap the nanowire, for passivation and as an additional means of altering the energy structure within the nanowire. Finally, these free-standing nanowires were embedded in GaAs [3]. By cleaving such samples, we have been able to directly probe the structure and electronic properties inside exposed heterostructures incorporated into nanowires.
We performed a detailed investigation of the structure of different segments of nanowire heterostructures as well as on the interfaces between different segments. We made extensive investigations of the electronic structure of the nanowires and the incorporated heterostructures by systematically obtaining atomically resolved XSTS in both axial and radial directions of the nanowires. Our XSTS measurements indicate a p-doped character in the nanowires as well as in the embedding GaAs, which we relate to the specifics of the growth procedure.
[1] L. Samuelson, Mater. Today 6, 22–31 (2003).
[2] C. Thelander et al., Mater. Today 9, 28-35 (2006).
[3] A. Mikkelsen et al., Nature Mat. 3, 519-523 (2004).
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