Seminar by Anders Mikkelsen – University of Copenhagen

Seminar by Anders Mikkelsen

Department of Physics and NanoLund, Lund University, Sweden

III-V nanowire electronic and geometric surface structure studied to the atomic scale even during device operation.

The III-V nanowire (NW) technology platform has reached a level of advancement that allows atomic scale control of crystal structure and surface morphology as well as flexible device integration. However, to fully realize the potential of NWs we need a high degree of control of their surfaces, presumably to the atomic scale. We have been working for a number of years to solve this issue using a variety of surface sensitive techniques both locally and at the MAX IV synchrotron in Lund.

In the present talk I will focus on atomically resolved Scanning Tunneling Microscopy/Spectroscopy (STM/S) that we have previously used on a wide variety of III-V Nanowires (NWs) systems and on operational NW devices [1-4]. We now use these methods for studying atomic scale crystal phase changes, the impact on local electronic properties and demonstrating full atomic resolution STM during device operation [5-7]. We explore the surface alloying of Sb into GaAs NWs with controlled axial stacking of Wurtzite (Wz) and Zincblende (Zb) crystal phases[5]. We find that Sb preferentially incorporates into the surface layer of the Zb segments rather than the Wz and explain the mechanism using DFT calculations. This demonstrate a simple processing-free route to compositional control at the monolayer level. Using <10K STM/S we measure local density of states of Zb crystal segments in Wz InAs NWs down to the smallest possible atomic scale crystal lattice change[6]. We find that the general Zb electronic structure is preserved locally in even the small possible segments and that signatures of confined states can be observed in them. We demonstrate a novel device platform allowing STM/S with atomic scale resolution across a III-V NW device simultaneously with full electrical operation as well as high temperature processing in reactive gases [7]. On InAs NWs we observe a surprising removal of atomic defects during operation.

[1] E. Hilner et al., Nano Lett., 8 (2008) 3978; M. Hjort et al., ACS Nano 6, 9679 (2012) [2] M. Hjort et al., Nano Lett., 13, 4492 (2013); M. Hjort et al., ACS Nano, 8 (2014) 12346 [3] J.L. Webb, et al Nano Lett. 15 (2015) 4865
[4] O. Persson et al., Nano Lett. 15 (2015) 3684
[5] M. Hjort et al Nano Lett., 17 (2017), 3634
[6] J.V. Knutsson et al ACS Nano, 11 (2017) 10519
[7] J.L. Webb et al, Sci. Rep. 7 (2017) 12790