Mass-transport driven growth dynamics of AlGaAs shells deposited around dense GaAs nanowires by metalorganic vapor phase epitaxy

Abstract

Group III-V compound semiconductor nanowires with radial modulation of the materials composition and/ or doping in the form of core-shell and core-multishell nanowire heterostructures show promise as novel and high-performance nano-scale light emitting diodes, lasers, photodetectors and solar cells. Strict control over the growth of such radially heterostructured nanowires is, however, necessary. We report the experimental dependence of AlGaAs shell growth by metalorganic vapor phase epitaxy (MOVPE) around free-standing Au-catalysed GaAs nanowires on the relevant sizes and densities of the nanostructures. A model based on (i) the vapor mass transport of group III species, and (ii) perfect conformality between the nanowires and the substrate of AlGaAs deposition is proposed and validated, describing the observed MOVPE growth dynamics of the shell material around dense ensembles of GaAs nanowires. We predict the complex (non-linear) dependence of the shell growth rate on the initial GaAs nanowire diameters (i.e., initial Au catalyst nanoparticle size), heights, local densities on the substrate, and deposition time, which is in very good agreement with experimental data; in particular, a monotonic decrease of AlGaAs shell thickness is expected and observed with increasing nanowire density.5998 | CrystEngComm, 2015, 17, 5998-6005This journal is

Nanoscale spectroscopic imaging of GaAs-AlGaAs quantum well tube nanowires: correlating luminescence with nanowire size and inner multishell structure

“…This effect was previously reported for AlGaAs shells grown around GaAs cores in dense nanowire arrays and explained as a result of the group III vapor mass-transport limited growth regime holding during high-temperature MOVPE [22]. Given an array of free-standing nanowires on the substrate, the group III precursor flux arriving at the growth front during the shell growth time will divide between the free substrate surface and the nanowire array cumulative lateral surface: the application of the law of mass conservation, under the assumption of (i) homogeneous vapor concentration of group III precursors along the nanowire height and (ii) perfect conformality of material overgrowth between the substrate and nanowires, leads to write a differential equation expressing the shell radial growth rate (namely dh s /dt) as [22]:…”

“…In the following, we demonstrate that this is possible based on the knowledge of the peculiar shell growth mode of present nanowires, which allows to quantitatively predict the thickness of the GaAs QWT within each nanowire, based on their FE-SEM measured core and overall diameters: indeed, as the nanowires in Figure 1D belong to the same sample, their different diameters imply different radial growth rates for QWTs belonging to different nanowires. This effect was previously reported for AlGaAs shells grown around GaAs cores in dense nanowire arrays and explained as a result of the group III vapor mass-transport limited growth regime holding during high-temperature MOVPE [22]. Given an array of free-standing nanowires on the substrate, the group III precursor flux arriving at the growth front during the shell growth time will divide between the free substrate surface and the nanowire array cumulative lateral surface: the application of the law of mass conservation, under the assumption of (i) homogeneous vapor concentration of group III precursors along the nanowire height and (ii) perfect conformality of material overgrowth between the substrate and nanowires, leads to write a differential equation expressing the shell radial growth rate (namely dh s /dt) as [22]:…”

“…Table 1: Comparison between GaAs and AlGaAs shell thickness values, as measured from analysis of HAADF-STEM tomography of the GaAs-AlGaAs core-multishell nanowire in Figure 4, and simulated ones based on the multishell growth model [22,23].…”

“…Further details are reported in Ref. [22]. The shell growth model above can be generalized to the case of multiple shell nanowires [23] and in particular employed to predict the thickness of the GaAs QWT within the nanowire.…”

“…At each slice, the GaAs QWTs were segmented (binarized) by applying a threshold of Z = 31.2 (corresponding to 10% Al content). Table 1: Comparison between GaAs and AlGaAs shell thickness values, as measured from analysis of HAADF-STEM tomography of the GaAs-AlGaAs core-multishell nanowire in Figure 4, and simulated ones based on the multishell growth model [22,23]. Figure 1), whereas QWT thickness are from data in (A).…”