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. 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. 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 . On InAs NWs we observe a surprising removal of atomic defects during operation.
 E. Hilner et al., Nano Lett., 8 (2008) 3978; M. Hjort et al., ACS Nano 6, 9679 (2012)  M. Hjort et al., Nano Lett., 13, 4492 (2013); M. Hjort et al., ACS Nano, 8 (2014) 12346  J.L. Webb, et al Nano Lett. 15 (2015) 4865
 O. Persson et al., Nano Lett. 15 (2015) 3684
 M. Hjort et al Nano Lett., 17 (2017), 3634
 J.V. Knutsson et al ACS Nano, 11 (2017) 10519
 J.L. Webb et al, Sci. Rep. 7 (2017) 12790