III-V Nanowires for Optoelectronics

How the light emission of (In,Ga)As shell quantum wells in nanowires depends on the crystal structure

When III-V compound semiconductors are grown as nanowires instead of planar thin films, crystal structures may form that otherwise would only be metastable. In consequence, changes in crystal structure can occur along the growth axis. This polytypism offers the chance to study the properties of metastable crystal polytypes, but may affect the performance of devices based on such nanowires. Previous studies focused mostly on binary nanowires, eminently GaAs. In this work, we employ a unique combination of correlated, highly spatially-resolved measurement techniques to elucidate the differences between wurtzite (WZ) and zincblende (ZB) ternary (In,Ga)As shell quantum wells around GaAs nanowires. Such heterostructures are particularly relevant for nanowire-based optoelectronic applications.

Figure 1. (a) Birds-eye view scanning electron micrograph of the GaAs-(In,Ga)As core-shell nanowire ensemble. Superimposed are monochromatic CL maps of the low-temperature quantum well emission at 1.35 and 1.39 eV. (b) Sketch of the axial nanowire cross-section highlighting how the shell quantum well is grown on the core segments with different crystal structure, as deduced from EBSD and nXRD. (c) Two-dimensional cut through the APT data for one side facet of a NW with Ga atoms in blue, and In atoms in yellow. The outline of the quantum well is highlighted by the isosurface for a mole fraction of 8%. In the right panel, the isosurface for a mole fraction of 13% In is mapped and filled in yellow highlighting the reduced In content in the WZ segment.

The GaAs core of our nanowires exhibits mostly the ZB polytype but a WZ segment is formed at the top towards the end of the core growth. The crystal structure of the core is adopted by the (In,Ga)As shell quantum well grown subsequently. Extended polytype segments are evidenced independently by electron backscatter diffraction (EBSD) and nanoprobe X-ray diffraction (nXRD). For the same individual nanowires, cathodoluminescence (CL) imaging along the axis reveals a blueshift of the quantum well emission energy by about 75 meV in the WZ segment. Using atom probe tomography (APT), again directly correlated to CL measurements for the same nanowires, we find a 30% drop in In mole fraction going from the ZB to the WZ segment. These insights into the crystal structure and composition of the core-shell heterostructures enable k · p electronic band structure calculations for the specific sample geometry. We learn that the change in In composition is not the only reason for the observed blueshift of the quantum well emission. Equally important is the compressive strain imposed by the GaAs core on the (In,Ga)As shell quantum well, which has a much stronger impact on the hole ground state in the WZ than in the ZB segment. Our results highlight the role of the crystal structure in tuning the emission of (In,Ga)As quantum wells and pave the way to exploit the possibilities of three-dimensional bandgap engineering in core-shell nanowire heterostructures. At the same time, we have demonstrated an advanced characterization toolkit for the investigation of semiconductor nanostructures.

1 Author J. Lähnemann , M. O. Hill , J. Herranz , O. Marquardt , G. Gao , A. Al Hassan , A. Davtyan , S. O. Hruszkewycz , M. V. Holt , C. Huang , I. Calvo-Almazán , U. Jahn , U. Pietsch , L. J. Lauhon , L. Geelhaar
Title

Correlated nanoscale analysis of the emission from wurtzite versus zincblende (In,Ga)As/GaAs nanowire core-shell quantum wells

Source Nano Lett. , 19 , 4448 ( 2019 )
DOI : 10.1021/acs.nanolett.9b01241 | Download arXiv: 1903.07372 | 3094 Cite : Bibtex RIS
J. Lähnemann, M. O. Hill, J. Herranz, O. Marquardt, G. Gao, A. Al Hassan, A. Davtyan, S. O. Hruszkewycz, M. V. Holt, C. Huang, I. Calvo-Almazán, U. Jahn, U. Pietsch, L. J. Lauhon, and L. Geelhaar

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