Electron transport in InAs-InAlAs core-shell nanowires

Abstract

Evidence is given for the effectiveness of InAs surface passivation by the growth of an epitaxial In 0.8 Al 0.2 As shell. The electron mobility is measured as a function of temperature for both core-shell and unpassivated nanowires, with the core-shell nanowires showing a monotonic increase in mobility as temperature is lowered, in contrast to a turnover in mobility seen for the unpassivated nanowires. We argue that this signifies a reduction in low temperature ionized impurity scattering for the passivated nanowires, implying a reduction in surface states. Core-shell nanowires were grown in a gas source molecular beam epitaxy system using Au seed particles 16 . First, InAs cores were grown axially by the Au assisted vapour liquid solid mechanism on a p-GaAs (111)B substrate at a growth temperature of 420This was followed by the inclusion of Al to facilitate radial growth of the In 0.8 Al 0.2 As shell 17 .The nanowires were characterized using transmission electron microscopy (TEM).As-grown nanowires were sonicated and suspended in ethanol, dispersed onto TEM grids have an inner core and an outer shell structure. In general, the nanowires had a core diameter of 20-50 nm and a shell that was 12-15 nm thick, independent of core diameter. The chemical composition of the nanowires was analyzed by energy-dispersive x-ray spectroscopy (EDS). As shown in Figure 1a, the EDS line scan analysis along the radial direction showsIn and As in the core region and In, As and Al in the shell region. High-resolution TEM (HRTEM) image of a representative unetched nanowire in Figure 1b clearly shows lattice 3 fringes of a single-crystal nanowire along the [2110] zone axis. Both core and shell exhibit wurtzite crystal structure, evidenced by ABAB... stacking, and confirmed by selected area diffraction. HRTEM and electron diffraction data are both consistent with a dislocation-free core-shell interface for these nanowires. However, at higher Al concentrations, dislocations due to relaxation of the core-shell interface are observed 17 . The nanowires studied here had low stacking fault densities, achieved by using sufficiently low growth rates 17,18 . Below, we also show data from unpassivated InAs nanowires that were grown under nominally identical conditions to the growths of the nanowire cores mentioned previously, except that the axial growth rate was 0.25 µm/hr and 0.5 µm/hr for the core-shell and bare nanowires, respectively.FET devices were fabricated using a standard e-beam lithography technique. As-grown nanowires were mechanically deposited onto a 175 nm thick SiO 2 layer above a n ++ -Si substrate. Selected nanowires with diameters 40−80nm were located relative to pre-existing fiducial markers by scanning electron microscopy (SEM), with care taken to minimize the electron dose. The contact areas were etched with citric acid to remove the shell material, followed by room temperature sulfur passivation to prevent oxide regrowth 19 during the sample transfer to an e-beam metal evaporator. Ni/Au (30nm/50nm) metal contacts were...

Temperature-dependent electron mobility in InAs nanowires

“…charging effects. This might provide an alternate explanation for the observed mobility drop below 50 K. However, we have recently observed an opposite trend in InAs-In 0.8 Al 0.2 As core-shell nanowires [49], in which the mobility continues to increase as temperature is lowered, despite the fact that strong, qualitatively similar Coulomb oscillations appear below ∼ 10 K in both types of nanowire. We ascribe the difference in mobility behaviour to a reduction of InAs surface states by the epitaxial passivation.…”

InAs Nanowire with Epitaxial Aluminum as a Single-Electron Transistor with Fixed Tunnel Barriers

“…This intermediate GaAs layer is expected to reduce the stress at the surface of the InAs and improve the intrinsic properties of the NW (like e.g. carrier mobility), as already observed in various NWs with cover shells [30][31][32][33][34]. A sketch of the cross section of the wires is shown Fig.…”

Nb/InAs nanowire proximity junctions from Josephson to quantum dot regimes

“…Ar [26,27]. Devices d3 and d5 were on the same chip; otherwise each device is representative of a distinct fabrication run.…”