Spontaneous formation of core‐shell
G
a
A
s
P
nanowires with enhanced electrical conductivity
Authors: Sun Wen , Huang Yang , Guo Yanan , Liao Zhiming , Gao Qiang , Tan Hark Hoe , Jagadish Chennupati , Liao Xiaozhou , Zou Jin 
Published: December 2016

The growth of GaAsP nanowires on GaAs (111)
B
substrates exhibit a core‐shell heterosturture with P‐enriched cores, which is attributed to Au catalysts enhancement of the local decomposition of PH
3
. These core–shell GaAsP nanowires exhibit enhanced electrical conductivity when compared with uniform GaAsP nanowires. This study provides an approach to enhance the electrical conductivity of III–V semiconductor nanowires.

Introduction:
Ternary III‐V epitaxial nanowires allow a continuous tuning of the bandgap, and they are also are required for fabricating complex radial/axial heterostructures devices.
1
During the Au‐catalyzed growth of III–V nanowires, group III and group V elements take different pathways to incorporate into the nanowires: group III elements through alloying with the Au catalyst, while group V elements through the triple phase line.
2
In this study, the composition distribution between two group V elements was studied, and the electrical properties of these core–shell ternary GaAsP nanowires were investigated.

Experiment:
GaAsP nanowires were epitaxially grown on the GaAs (111)
B
substrates using a horizontal flow MOCVD reactor at the pressure of 100 mbar with ultrahigh purity H
2
as the carrier gas. Trimethylgallium (TMG) and was used as the group‐III source, while PH
3
and AsH
3
were used as the group‐V sources. Nanowires were grown at 500 °C and 420 °C for 30 min with a V/III ratio of 39.3 and a PH
3
/( PH
3
+ AsH
3
) ratio of 0.98. Electron microscopies were used to investigate the characteristics and transport measurements of grown nanowires.
3

Results and discussion:
SEM study showed that most nanowires synthesized at 500 °C grew vertically on the GaAs {111}
B
substrate with a tapered morphology. The nanowire quantitative analyses of EDS spectra taken from different sections indicate that the compositional distribution along the nanowire is uneven. The As/(P + As) ratio increases from 19 at% at the top to 22 at% at the middle and to 25 at% at the bottom of the nanowire. The tapered nanowire morphology suggests that lateral growth took place during the nanowire growth at 500 °C,
3
which would lead to the formation of a shell. To clarify whether our tapered nanowires have a core–shell structure, TEM investigations were carried out on cross‐sections of individual nanowires sliced from the tip, the middle and the bottom regions. Fig. 1(a) is a typical example of cross‐section obtained from the bottom region of a nanowire, and shows a truncated‐triangular shaped cross‐section. Fig. 1(b) is a corresponding SAED. It should be noted that the As concentration in the centre (Fig. 1c) is similar to that of tip region of nanowire, indicating that the As concentration in the nanowire core is uniform along the nanowires. The EDS maps in Fig. 1(f, g) clearly identify the enriched P core and the enriched As shell. The formation of the core‐shell GaAsP nanowires was attributed to two facts: (1) Since Au catalysts can enhance the decomposition of PH
3
.
4
the P concentration around the nanowire catalysts should be higher than the general environment, leading to a relatively higher P concentration in the nanowire core; (2) the sticking coefficient of As adatoms is higher than that of P adatoms,
5
and therefore, it is possible that As is preferentially incorporated on the sidewall of the formally formed core, leading to the higher concentration of As in the nanowire shells.

Vertically grown GaAsP nanowires at a temperature of 420 °C are uniform in their lateral dimension, as shown in Fig. 2(a). The compositional distribution along the nanowire is uniform with an As/(P + As) ratio of approximately 48 at% at the top (Fig. 2(c)) and approximately 47 at% at the bottom (Fig. 2(d)) of the nanowire. EDS maps in Fig. 2(f) and (g) shows the evenly distributed As and P across the nanowire cross‐section, indicating that the GaAsP nanowires grown at 420 °C are homogeneous nanowires. Fig. 3 shows the I–V characteristics of the GaAsP nanowires synthesis at different temperatures. The homogeneous GaAsP nanowire grown at 420 °C show no electrical conductivity, while the core–shell GaAsP nanowires are capable of enhanced conductivity. It is likely that the band offset
6
in core–shell nanowires could lead to the accumulation of carrier gas at core–shell interfaces, and in turn, the enhanced conductivity in undoped nanowires.