Carrier trapping and activation at short-period wurtzite/zinc-blende stacking sequences in polytypic InAs nanowires

Abstract

We explore the effects of random and short-period crystal-phase intermixing in InAs nanowires (NW) on the carrier trapping and thermal activation behavior using correlated optical and electrical transport spectroscopy. The polytypic InAs NWs are grown by catalyst-free molecular beam epitaxy under different temperatures, resulting in different fractions of wurtzite (WZ) and zincblende (ZB) and variable short-period (\ensuremath{\sim}1--4 nm) WZ/ZB stacking sequences. Temperature-dependent microphotoluminescence $(\ensuremath{\mu}\text{PL})$ studies reveal that variations in the WZ/ZB stacking govern the emission energy and carrier confinement properties. The optical transition energies are modeled for a wide range of WZ/ZB stacking sequences using a Kronig-Penney type effective mass approximation, while comparison with experimental results suggests that polarization sheet charges on the order of \ensuremath{\sim}0.0016--0.08 C/m along the WZ/ZB interfaces need to be considered to best describe the data. The thermal activation characteristics of carriers trapped inside the short-period WZ/ZB structure are directly reproduced in the temperature-dependent carrier density evolution (4--300 K) probed by four-terminal (4T) NW-field effect transistor measurements. In particular, we find that activation of carriers in-between $\ensuremath{\sim}{10}^{16}\ensuremath{-}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ follows a two-step process, with activation at low temperature attributed to WZ/ZB traps and activation at high temperature being linked to surface states and electron accumulation at the InAs NW surface.