Potential mapping of growth sequence in semiconductor NW p-n junctions

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(Thesis) Ph.D.
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Semiconducting nanowire-based clean energy technologies are promising in terms of efficiency and material consumption. The vapor-solid-liquid (VLS) mechanism using gold (Au) as the catalyst allows for the growth of various compounds. Although dopant impurity control and interface sharpness define the operating principles of these new clean energy technologies, growing and characterizing them is challenging. This complexity increases if one takes into account sequencing and/or binary and ternary compound growth, for instance p- to n-type growth and vice-versa. Measurement of the electrostatic potential gradients due to dopant activation of the various interfaces is of primary importance, since this leads to the understanding of VLS growth parameters that yield working devices. We have confirmed the presence of narrow, degenerately-doped axial silicon nanowire (SiNW) p-n junctions via off-axis electron holography. SiNWs were grown via the VLS - Au catalyst, using silane (SiH 4 ), diborane (B 2 H 6 ) and phosphine (PH 3 ) as the precursors, and hydrochloric acid (HCl) to styabilize the growth. Two types of growth were carried out and in each case we explored growth with both n/p and p/n sequences. In the first type, we abruptly switched the dopant precursors at the desired junction location, and in the second type we slowed the growth rate at the junction to allow the dopants to readily leave the Au catalyst. We demonstrate degenerately-doped p/n and n/p NW segments with abrupt potential profiles of 1.0 ± 0.1 and 0.9 ± 0.1 V, and depletion region widths as narrow as 13 ± 1 nm via EH. The results presented here show that the direct VLS growth of degenerately-doped axial SiNW p-n junctions is feasible, an essential step in the fabrication of more complex SiNW-based devices for electronics and solar energy. Electron Holographic Tomography was used to obtain 3-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite growth sequences: GaInP:Zn/InP:S and InP:Sn/GaInP:Zn, using Zn as the p-type dopant in the GaInP, but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam induced current images obtained using scanning electron microscopy indicated the presence of p-n junctions in both cases and current-voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. EHT measurements confirmed a short depletion width in both cases (21 ± 3 nm), but different built-in potentials, V bi , of 1.0 V for the p-type (Zn) to n-type (S) transition, and iii0.4 V for both were lower than the expected 1.5 V for these junctions, if degenerately-doped. Charging induced by the electron beam was evident in phase images which showed non-linearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the p-type Au contact. We attribute their lower V bi to asymmetric secondary electron emission, beam-induced current biasing and poor grounding contacts.
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Thesis advisor: Kavanagh, Karen
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